Deploy-Prozess für Linux und Windows ergänzen

.gitignore

@@ -1,7 +1,14 @@
 Notizen.txt
+
 # Build-Verzeichnis
 /build/
 
+# Distributions-Verzeichnis
+/dist/
+
+# Temporäre Arbeitsdateien von fyne-cross
+/fyne-cross/
+
 # Lokale Build-Binaries
 /rs2322tcp-client
 /rs2322tcp-server

CHANGELOG.md

@@ -2,58 +2,350 @@
 
 Alle wesentlichen Änderungen am Projekt werden in dieser Datei dokumentiert.
 
-## [Unreleased]
+## [0.0.11] - 2026-08-25
+
+### Hinzugefügt
+
+- Windows-Client erfolgreich mit Fyne-GUI gestartet.
+- Übertragung der konfigurierten `startup_bytes` an das zugeordnete
+  Remote-Gerät beim Verbindungsaufbau getestet.
+
+### Geändert
+
+- Windows-spezifische Behandlung virtueller serieller Ports angepasst.
+- Öffnen und Verwaltung virtueller serieller Schnittstellen unter Windows
+  überarbeitet.
+
+### Getestet
+
+- Minimaler Fyne-Test unter Windows erfolgreich gestartet.
+- `rs2322tcp-client` unter Windows erfolgreich mit Fyne-GUI gestartet.
+- Verbindung des Windows-Clients zum `rs2322tcp-server` erfolgreich aufgebaut.
+- Rotor-Startcode erfolgreich vom Windows-Client bis zum Server übertragen.
+
+## [0.0.10] - 2026-08-21
+
+### Behoben
+
+- Linux-Portverwaltung nach der Windows-Client-Erweiterung korrigiert.
+- Doppelte Linux-Definitionen der virtuellen Portverwaltung entfernt.
+- Plattformübergreifende Trennung der virtuellen Port-Erzeugung korrigiert.
+- Dauerhafte Installations-Symlinks für virtuelle Linux-Ports bleiben beim Schließen eines Ports erhalten.
+- Gesamter Go-Testlauf unter Linux erfolgreich.
+
+## [0.0.9] - 2026-08-21
 
 ### Added
-- Linux-PTY-Unterstützung für die clientseitige virtuelle serielle Schnittstelle
+
+- Windows-Unterstützung für den rs2322tcp-Client
+- Plattformabhängige Verwaltung virtueller serieller Schnittstellen
+- Windows-COM-Port-Unterstützung über `go.bug.st/serial`
+- Unterstützung des virtuellen Windows-Portbereichs COM100 bis COM199
+- Windows-spezifische Portvalidierung und Portpfadverwaltung
+- Technischer Windows-Client unter `cmd/rs2322tcp-client-test`
+- Plattformabhängige Implementierungen für das Öffnen und Verwalten
+  virtueller serieller Schnittstellen
+- Windows-spezifische Implementierung von `VirtualSerial`
+
+### Changed
+
+- Virtuelle Portverwaltung für Linux und Windows auf eine gemeinsame
+  Schnittstelle umgestellt
+- Clientseitige Portvalidierung für die unterschiedlichen Plattformen
+  erweitert
+- Client-Konfiguration für den Windows-Betrieb angepasst
+
+### Tests
+
+- Windows 11 / amd64 erfolgreich getestet
+- Windows-COM-Port COM100 über com0com erfolgreich geöffnet
+- com0com-Portpaar `COM10 ↔ COM100` erfolgreich getestet
+- Verbindung des Windows-Clients über Tailscale zum Raspberry-Pi-Server
+  erfolgreich getestet
+- Bidirektionale Datenübertragung zwischen Windows-Client und
+  Raspberry-Pi-Server erfolgreich getestet
+- Bidirektionale Datenübertragung mit einem realen SPID Rot2Prog
+  erfolgreich getestet
+- Rot2Prog mit `P5 = 59` (SPID) und Function Mode `A` (Auto) erfolgreich
+  betrieben
+- Tatsächliche Antwort des Rot2Prog erfolgreich über den vollständigen
+  Datenweg übertragen:
+
+```text
+57 07 01 06 00 01 03 06 02 00 01 20
+```
+
+- `go test ./internal/client` erfolgreich
+
+### Status
+
+Der Windows-Client kann über einen virtuellen Windows-COM-Port mit dem
+rs2322tcp-Server auf einem Raspberry Pi kommunizieren.
+
+Der vollständige Datenweg wurde mit einem realen SPID Rot2Prog
+bidirektional erfolgreich getestet:
+
+```text
+COM10
+   │
+   ▼
+com0com
+   │
+   ▼
+COM100
+   │
+   ▼
+rs2322tcp-client (Windows)
+   │
+   ▼
+TCP / Tailscale
+   │
+   ▼
+rs2322tcp-server (Raspberry Pi)
+   │
+   ▼
+RS232 / SPID Rot2Prog
+```
+
+Der technische Windows-Client bleibt als separates Testprogramm erhalten.
+
+Die grafische Fyne-Oberfläche des Windows-Clients ist noch nicht
+vollständig abgeschlossen.
+
+
+## [0.0.8] - 2026-08-12
+
+### Added
+
+- Prüfung der lokal vorhandenen virtuellen Ports beim Client-Start
+- Abgleich der vorhandenen symbolischen Links mit den in `client.json`
+  definierten virtuellen Schnittstellen
+- Erkennung von in `client.json` definierten, aber lokal nicht vorhandenen
+  virtuellen Ports
+- Erkennung von lokal vorhandenen, aber nicht in `client.json` definierten
+  virtuellen Ports
+- Verständlicher Fehlerdialog bei einer abweichenden virtuellen
+  Portkonfiguration
+- Beenden des Clients nach Bestätigung einer fehlerhaften
+  Portkonfiguration
+
+### Changed
+
+- Virtuelle Ports werden entsprechend der in `client.json` festgelegten
+  Portnummern gezielt geöffnet
+- Die Verwaltung virtueller Ports arbeitet weiterhin ohne Root-Rechte
+- Der Fehlerdialog verwendet einen einzelnen OK-Button
+- Allgemeine Schriftgröße der grafischen Benutzeroberfläche auf 22 erhöht
+- Hauptfenster des Clients auf 800 × 600 Pixel vergrößert
+
+### Tests
+
+- Prüfung vorhandener und konfigurierter virtueller Ports erfolgreich getestet
+- Fehlender virtueller Port erfolgreich erkannt und im Fehlerdialog angezeigt
+- Zusätzlicher, nicht konfigurierter virtueller Port erfolgreich erkannt
+- Clientstart nach Korrektur der virtuellen Portkonfiguration erfolgreich getestet
+- Gerätezuordnung nach Neustart des Clients erfolgreich aus `client.json`
+  geladen
+- `go test ./...` erfolgreich
+- `go test -race ./internal/client` erfolgreich
+- `go test -race ./internal/gui` erfolgreich
+- `git diff --check` erfolgreich
+
+## [0.0.7] - 2026-08-12
+
+### Added
+
+- Pflege der Gerätezuordnung über die grafische Benutzeroberfläche
+- Zuordnung der vom Server angebotenen Geräte zu den konfigurierten
+  virtuellen seriellen Schnittstellen
+- Unterstützung des Zustands „nicht verbunden“
+- Freie virtuelle Schnittstellen werden bei der Gerätezuordnung automatisch
+  neu angeboten
+- Verhinderung der gleichzeitigen Zuordnung einer virtuellen Schnittstelle
+  zu mehreren Geräten
+- Bereits zugeordnete Geräte können nur auf ihre aktuelle Schnittstelle oder
+  „nicht verbunden“ gesetzt werden
+- Ungespeicherte Änderungen werden beim Schließen des Hauptfensters erkannt
+  und dem Anwender angezeigt
+- Änderungen werden beim Schließen des Gerätezuordnungsfensters in der
+  Client-Konfiguration gespeichert
+- Virtuelle Schnittstellen bleiben auch ohne Gerätezuordnung in der
+  Client-Konfiguration erhalten
+- Ein leerer Wert für `remote_device` kennzeichnet eine freie virtuelle
+  Schnittstelle
+- `Application.SaveConfig()` zum zentralen Speichern der Client-Konfiguration
+
+### Changed
+
+- Validierung der Client-Konfiguration erlaubt nun virtuelle Schnittstellen
+  ohne aktuelle Gerätezuordnung
+- Die Gerätezuordnung arbeitet auf Basis der vorhandenen virtuellen
+  Schnittstellen und verändert bei einer Trennung nur deren
+  `remote_device`-Zuordnung
+
+### Tests
+
+- Gerätezuordnung mit mehreren Server-Geräten praktisch getestet
+- Freigabe einer virtuellen Schnittstelle und anschließende erneute Auswahl
+  erfolgreich getestet
+- Speicherung einer Gerätezuordnung über die GUI erfolgreich getestet
+- Speicherung einer getrennten Gerätezuordnung mit leerem
+  `remote_device` erfolgreich getestet
+- `go test ./internal/config` erfolgreich
+- `go test ./internal/gui` erfolgreich
+- `go test -race ./internal/gui` erfolgreich
+- `git diff --check` erfolgreich
+## [0.0.6] - 2026-08-10
+
+### Added
+
+- Clientseitige Runtime zur Verwaltung mehrerer virtueller serieller Ports
+- Verbindung konfigurierter virtueller Ports mit den vom Server bereitgestellten
+  Remote-Geräten
+- Clientseitige TCP-Data-Connections zu den dynamischen Data-Ports des Servers
+- Bidirektionale Datenübertragung über eine `Bridge`
+- Verwaltung des Lebenszyklus von virtuellen Ports, Bridges und Client-Verbindung
+- Integrationstest für den vollständigen Datenweg zwischen virtuellem seriellen
+  Port und Server
+- Tests für Verbindungsaufbau, Gerätezuordnung und Runtime-Lebenszyklus
+
+### Tests
+
+- `go test ./...` erfolgreich
+- `go test -race ./...` erfolgreich
+- Bidirektionale Übertragung über Runtime, Bridge, TCP und PTY erfolgreich
+- Race Conditions in Client-Control-Verbindung, PTY und virtuellen Ports behoben
+## [0.0.5] - 2026-08-10
+
+### Added
+
+- Konfigurierbarer Bereich für lokale virtuelle serielle Schnittstellen
+- Standardbereich für virtuelle Ports von `/dev/ttyUSB100` bis
+  `/dev/ttyUSB199`
+- Abweichender Portbereich kann über `virtual_port_range` in der
+  Client-Konfiguration festgelegt werden
+- Rückwärtskompatibilität für bestehende Client-Konfigurationen ohne
+  `virtual_port_range`
+- Verwaltung virtueller Ports über `VirtualPortManager`
+- Automatische Auswahl des nächsten freien virtuellen Ports
+- Freigabe reservierter Ports bei deren Schließen
+- Linux-spezifische Verwaltung der virtuellen Port-Symlinks unter
+  `~/.rs2322tcp/virtual/`
+- Verknüpfung der virtuellen `/dev/ttyUSBxxx`-Schnittstellen mit den
+  vom Client erzeugten PTYs
+- Einführung von `VirtualPort` und `ManagedVirtualPort`
+- Umfangreiche Tests für Portauswahl, Portfreigabe, Symlink-Verwaltung
+  und Linux-PTY-Integration
+
+### Tests
+
+- `go test ./...` erfolgreich
+- `go test -race ./...` erfolgreich
+- Bidirektionale Kommunikation über `/dev/ttyUSB100` erfolgreich getestet
+- Externe Software kann ausschließlich `/dev/ttyUSB100` verwenden
+- Übertragung vom virtuellen `/dev/ttyUSB100` zum Client erfolgreich getestet
+- Übertragung vom Client zum virtuellen `/dev/ttyUSB100` erfolgreich getestet
+- Zweistufige Symlink-Kette von `/dev/ttyUSB100` über
+  `~/.rs2322tcp/virtual/ttyUSB100` bis zum PTY erfolgreich getestet
+
+---
+
+## [0.0.4] - 2026-08-10
+
+### Added
+
+- Interaktiver Geräte-Simulator `rs2322tcp-device-sim`
+- Senden beliebiger Bytefolgen über eine interaktive Hex-Eingabe
+- Ausgabe empfangener Daten als Hex-Dump
+- Unterstützung von Hex-Eingaben mit und ohne Leerzeichen
+- Fehlerbehandlung für ungültige Hex-Eingaben
+- Automatisierte Tests für die Hex-Konvertierung
+- Erweiterte Dokumentation des Geräte-Simulators und des PTY-Testaufbaus
+- Möglichkeit, den Simulator als virtuelle serielle Gegenstelle für
+  externe Software zu verwenden
+
+### Tests
+
+- Byte-orientierte Übertragung mit beliebigen Binärdaten getestet
+- Übertragung aller Bytewerte `00` bis `FF` getestet
+- Übertragung vom PTY-Slave zum PTY-Master getestet
+- Übertragung vom PTY-Master zum PTY-Slave getestet
+- `go test ./...` erfolgreich
+- `go test -race ./...` erfolgreich
+
+---
+
+## [0.0.3] - 2026-08-10
+
+### Added
+
+- Linux-PTY-Unterstützung für die clientseitige virtuelle serielle
+  Schnittstelle
 - Abstraktion der virtuellen seriellen Schnittstelle über `VirtualSerial`
 - Automatisierter bidirektionaler PTY-Test mit allen 256 möglichen Bytewerten
-- Eigenständiges Programm `rs2322tcp-device-sim` als Simulator für ein angeschlossenes serielles Gerät
+- Eigenständiges Programm `rs2322tcp-device-sim` als Simulator für ein
+  angeschlossenes serielles Gerät
 - Hexadezimale Darstellung der vom Geräte-Simulator empfangenen Rohdaten
+
+### Tests
+
+- PTY-Slave und PTY-Master erfolgreich miteinander getestet
+- Bidirektionale Übertragung von Binärdaten getestet
+- Bytefolgen mit Werten von `00` bis `FF` erfolgreich übertragen
+- Empfang beliebiger Datenblöcke über den PTY-Master getestet
+
+---
+
+## [0.0.2] - 2026-08-09
+
+### Added
+
 - Implementierung der seriellen Schnittstellenschicht unter `internal/serial`
 - Verwendung von `go.bug.st/serial v1.7.1` für die Ansteuerung der seriellen Schnittstellen
 - Konfigurationsunterstützung für Baudrate, Datenbits, Parität und Stopbits
-- Öffnen und Schließen serieller Schnittstellen über die interne Serial-Abstraktion
+- Serielle Geräte können über Konfiguration definiert werden
-- Lesen und Schreiben von RS232-Daten über die Serial-Abstraktion
+- Server-Control-Verbindung
-- Dynamische TCP-Data-Listener für konfigurierte Geräte
+- Session-Verwaltung
-- Session-Verwaltung für dynamische Data-Listener
+- Dynamische Data-Ports pro Gerät
-- Session-Verwaltung für aktive DataConnections
+- Serverseitiger Aufbau der TCP-Data-Verbindung
-- Bidirektionale Datenübertragung zwischen TCP und RS232
+- `DataConnection` für die bidirektionale Datenübertragung
-- Öffnen der seriellen Schnittstelle erst beim Aufbau einer tatsächlichen Data-Verbindung
+- `DataListener` zur Annahme von Data-Verbindungen
-- Begrenzung auf eine aktive Data-Verbindung pro konfiguriertem Gerät
+- TCP → RS232 Datenübertragung
-- Integrationstest mit virtuellen seriellen Schnittstellen über `socat`
+- RS232 → TCP Datenübertragung
-- Tests für TCP → RS232 und RS232 → TCP
-- Tests für Session-Reconnect und Ressourcenverwaltung
 - Client-Control-Verbindung unter `internal/client`
 - Clientseitige Verarbeitung von `HELLO`
 - Clientseitige Abfrage der konfigurierten Geräte über `GET_DEVICES`
-- Clientseitige Verarbeitung der vom Server gelieferten Geräteinformationen einschließlich dynamischem Data-Port
+- Clientseitige Verarbeitung der vom Server gelieferten Geräteinformationen
+  einschließlich dynamischem Data-Port
 - Clientseitiger Aufbau der TCP-Data-Verbindung
 - Integrationstest für die vollständige Client-/Server-Kommunikation
 - End-to-End-Test der TCP-/RS232-Datenübertragung in beide Richtungen
 
 ### Changed
 
-- Go-Version des Projektes auf Go `1.25.0` aktualisiert
+- Serial-Kommunikation vollständig über `go.bug.st/serial`
-- `go.bug.st/serial` wird in Version `v1.7.1` verwendet
+- Server öffnet serielle Schnittstellen erst bei aktiver Data-Verbindung
-- `golang.org/x/sys v0.43.0` wird als indirekte Abhängigkeit verwendet
+- Pro Gerät wird nur eine aktive Data-Verbindung zugelassen
-- Die Serial-Bibliothek `go.bug.st/serial` wurde zusätzlich auf dem privaten Git-Server des Projektes als Ausfallsicherung gespiegelt:
+- Session- und Ressourcenverwaltung überarbeitet
-  `git.lang-dieter.de/third-party/go-serial`
+- Race Condition im `DataListener` behoben
-- Der `DataListener` wurde nebenläufigkeitssicher implementiert
-- Das Schließen eines `DataListener` kann gleichzeitig mit einem laufenden `Accept()` erfolgen
 
 ### Tests
 
-- `go test ./...` erfolgreich
+- Unit-Tests für Serial
-- `go test -race ./...` erfolgreich
+- Unit-Tests für Session
-- Race Condition im `DataListener` erkannt und behoben
+- Unit-Tests für DataConnection
+- Unit-Tests für DataListener
+- Integrationstest mit virtuellen seriellen Schnittstellen über `socat`
+- Tests für TCP → RS232 und RS232 → TCP
+- Tests für Session-Reconnect und Ressourcenverwaltung
 - Client-/Server-Integrationstest erfolgreich
 - Race-Test des Client-/Server-Integrationstests erfolgreich
-- Automatisierter bidirektionaler PTY-Test mit einem vollständigen Bytebereich von `0x00` bis `0xFF`
+
-- Race-Detection für die neue Linux-PTY-Komponente erfolgreich
 ### Status
 
-Die grundlegende TCP-/RS232-Datenübertragung ist serverseitig implementiert und der Client kann inzwischen die Control-Verbindung sowie die zugehörige Data-Verbindung aufbauen.
+Die grundlegende TCP-/RS232-Datenübertragung ist serverseitig implementiert
+und der Client kann die Control-Verbindung sowie die zugehörige
+Data-Verbindung aufbauen.
 
 Der aktuelle Datenpfad ist:
 
@@ -103,49 +395,19 @@ Der Geräte-Simulator ist als eigenständiges Testprogramm vorhanden und kann
 eine serielle Schnittstelle öffnen und empfangene Rohdaten hexadezimal
 anzeigen.
 
-Die TX-Funktion des Geräte-Simulators sowie die vollständige Integration der
+Die interaktive TX-Funktion des Geräte-Simulators sowie die vollständige
-virtuellen seriellen Schnittstelle in den Client-Datenpfad sind noch nicht
+Integration der virtuellen seriellen Schnittstelle in den produktiven
-implementiert.
+Client-Datenpfad sind noch nicht abgeschlossen.
+
 ---
 
 ## [0.0.1] - 2026-08-09
 
 ### Added
 
-- Grundstruktur des Projektes `rs2322tcp`
+- Initiale Projektstruktur
-- Go-Modul `git.lang-dieter.de/rs2322tcp`
+- Go-Modul und Build-Grundlagen
-- separates Programmgerüst für `rs2322tcp-client`
+- Basis für Server und Client
-- separates Programmgerüst für `rs2322tcp-server`
+- Konfigurationsgrundlagen
-- gemeinsame Versionierungsinformationen
+- Versionsverwaltung und Build-Metadaten
-- Git-basierte Build-Informationen
+- Erste Dokumentation
-- zentrales Build-Skript `scripts/build.sh`
-- automatische Quellcode-Formatierung beim Build
-- automatische Tests beim Build
-- SHA256-Prüfsummen der erzeugten Binärdateien
-- `.gitignore` für Build- und Entwicklungsdateien
-- erste Projekt- und Architekturdokumentation
-
-### Build-Ziele
-
-Der erste Build unterstützt:
-
-- Linux amd64 – Client und Server
-- Linux arm64 – Server
-- Windows amd64 – Client
-
-### Architekturentscheidungen
-
-- TCP wird für die Übertragung der RS232-Daten verwendet.
-- Windows und Linux werden als gleichberechtigte Client-Plattformen betrachtet.
-- Der Server ist zunächst für den Raspberry Pi 5 vorgesehen.
-- Tailscale wird als vertrauenswürdiges Netzwerk verwendet.
-- Eine zusätzliche TLS-Schicht ist zunächst nicht vorgesehen.
-- Für eine spätere Audioübertragung ist ein separates UDP-basiertes Transportkonzept vorgesehen.
-- `socat` kann für Entwicklung und Diagnose eingesetzt werden.
-- Eine Sniffer-/Logging-Funktion für den übertragenen Datenverkehr ist vorgesehen.
-
-### Status
-
-Version `0.0.1` enthält ausschließlich die Projektgrundlage.
-
-Die eigentliche TCP-/RS232-Kommunikation und die virtuelle serielle Schnittstelle sind noch nicht implementiert.

cmd/com0com-test/main.go

@@ -0,0 +1,239 @@
+//go:build windows
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: cmd/com0com-test/main.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Direkter Test eines com0com-Portpaares.
+ *
+ * Der Test öffnet COM100 und COM10 gleichzeitig und überprüft die
+ * Datenübertragung in beide Richtungen:
+ *
+ *     COM100 -> COM10
+ *     COM10  -> COM100
+ *
+ * Dadurch kann die Funktion von com0com und go.bug.st/serial unabhängig
+ * vom rs2322tcp-Client, TCP-Server und PTY-Testaufbau geprüft werden.
+ * ============================================================================
+ */
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"time"
+
+	"go.bug.st/serial"
+)
+
+func main() {
+	fmt.Println("rs2322tcp com0com Direkt-Test")
+	fmt.Println()
+
+	mode := &serial.Mode{
+		BaudRate: 600,
+		DataBits: 8,
+		Parity:   serial.NoParity,
+		StopBits: serial.OneStopBit,
+	}
+
+	com10, err := serial.Open("COM10", mode)
+	if err != nil {
+		log.Fatalf("COM10 öffnen: %v", err)
+	}
+	defer com10.Close()
+
+	fmt.Println("COM10 geöffnet.")
+
+	com100, err := serial.Open("COM100", mode)
+	if err != nil {
+		log.Fatalf("COM100 öffnen: %v", err)
+	}
+	defer com100.Close()
+
+	fmt.Println("COM100 geöffnet.")
+	fmt.Println()
+
+	// ------------------------------------------------------------------------
+	// Test 1: COM100 -> COM10
+	// ------------------------------------------------------------------------
+
+	request := []byte{
+		0x57,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x1F,
+		0x20,
+	}
+
+	fmt.Printf(
+		"COM100 TX: %d Bytes\n",
+		len(request),
+	)
+	printHex(request)
+
+	n, err := com100.Write(request)
+	if err != nil {
+		log.Fatalf("COM100 schreiben: %v", err)
+	}
+
+	if n != len(request) {
+		log.Fatalf(
+			"COM100 schreiben: %d von %d Bytes geschrieben",
+			n,
+			len(request),
+		)
+	}
+
+	fmt.Println("COM100 schreiben erfolgreich.")
+
+	received := make([]byte, len(request))
+
+	if err := com10.SetReadTimeout(2 * time.Second); err != nil {
+		log.Fatalf("COM10 ReadTimeout setzen: %v", err)
+	}
+
+	n, err = com10.Read(received)
+	if err != nil {
+		log.Fatalf("COM10 lesen: %v", err)
+	}
+
+	fmt.Printf(
+		"COM10 RX: %d Bytes\n",
+		n,
+	)
+	printHex(received[:n])
+
+	if n != len(request) {
+		log.Fatalf(
+			"COM10: %d Bytes empfangen, erwartet %d",
+			n,
+			len(request),
+		)
+	}
+
+	if !equalBytes(request, received[:n]) {
+		log.Fatal("COM10: empfangene Daten stimmen nicht mit den gesendeten Daten überein")
+	}
+
+	fmt.Println("Test COM100 -> COM10 erfolgreich.")
+	fmt.Println()
+
+	// ------------------------------------------------------------------------
+	// Test 2: COM10 -> COM100
+	// ------------------------------------------------------------------------
+
+	response := []byte{
+		0x57,
+		0x07,
+		0x01,
+		0x06,
+		0x00,
+		0x01,
+		0x03,
+		0x06,
+		0x02,
+		0x00,
+		0x01,
+		0x20,
+	}
+
+	fmt.Printf(
+		"COM10 TX: %d Bytes\n",
+		len(response),
+	)
+	printHex(response)
+
+	n, err = com10.Write(response)
+	if err != nil {
+		log.Fatalf("COM10 schreiben: %v", err)
+	}
+
+	if n != len(response) {
+		log.Fatalf(
+			"COM10 schreiben: %d von %d Bytes geschrieben",
+			n,
+			len(response),
+		)
+	}
+
+	fmt.Println("COM10 schreiben erfolgreich.")
+
+	received = make([]byte, len(response))
+
+	if err := com100.SetReadTimeout(2 * time.Second); err != nil {
+		log.Fatalf("COM100 ReadTimeout setzen: %v", err)
+	}
+
+	n, err = com100.Read(received)
+	if err != nil {
+		log.Fatalf("COM100 lesen: %v", err)
+	}
+
+	fmt.Printf(
+		"COM100 RX: %d Bytes\n",
+		n,
+	)
+	printHex(received[:n])
+
+	if n != len(response) {
+		log.Fatalf(
+			"COM100: %d Bytes empfangen, erwartet %d",
+			n,
+			len(response),
+		)
+	}
+
+	if !equalBytes(response, received[:n]) {
+		log.Fatal("COM100: empfangene Daten stimmen nicht mit den gesendeten Daten überein")
+	}
+
+	fmt.Println("Test COM10 -> COM100 erfolgreich.")
+	fmt.Println()
+
+	fmt.Println("==========================================")
+	fmt.Println("Alle com0com-Direkt-Tests erfolgreich.")
+	fmt.Println("==========================================")
+}
+
+// printHex gibt einen Byte-Block hexadezimal aus.
+func printHex(data []byte) {
+	for i, b := range data {
+		if i > 0 {
+			fmt.Print(" ")
+		}
+
+		fmt.Printf("%02X", b)
+	}
+
+	fmt.Println()
+}
+
+// equalBytes compares two byte slices.
+func equalBytes(a, b []byte) bool {
+	if len(a) != len(b) {
+		return false
+	}
+
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+
+	return true
+}

cmd/rs2322tcp-client-test/main.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"git.lang-dieter.de/rs2322tcp/internal/client"
+)
+
+func main() {
+	const configFile = "./configs/client.json"
+
+	application, err := client.NewApplication(configFile)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "NewApplication: %v\n", err)
+		os.Exit(1)
+	}
+
+	if err := application.Start(); err != nil {
+		fmt.Fprintf(os.Stderr, "Application.Start: %v\n", err)
+		os.Exit(1)
+	}
+
+	fmt.Println("rs2322tcp technical client started")
+	fmt.Println("Press Ctrl+C to stop.")
+
+	signals := make(chan os.Signal, 1)
+	signal.Notify(
+		signals,
+		os.Interrupt,
+		syscall.SIGTERM,
+	)
+
+	<-signals
+
+	fmt.Println("Stopping client...")
+
+	if err := application.Close(); err != nil {
+		fmt.Fprintf(os.Stderr, "Application.Close: %v\n", err)
+		os.Exit(1)
+	}
+}

cmd/rs2322tcp-client/main.go

@@ -1,23 +1,140 @@
 /*
-Package main implements the rs2322tcp client application.
+ * ============================================================================
-
+ * Projekt.....: rs2322tcp
-The client will provide a local virtual serial interface and
+ * Datei.......: cmd/rs2322tcp-client/main.go
-transparently transport serial data over TCP to the rs2322tcp server.
+ * Copyright (C) 2026 Dieter Lang
-
+ *
-At this stage the application only provides the basic program
+ * SPDX-License-Identifier: GPL-3.0-or-later
-structure and version information.
+ *
-
+ * Beschreibung:
-Project: rs2322tcp
+ * Einstiegspunkt für den grafischen rs2322tcp-Client.
-Module:  git.lang-dieter.de/rs2322tcp
+ *
-*/
+ * Der Client wird als eigenständige Desktop-Anwendung ausgeführt. Die
+ * grafische Oberfläche basiert auf Fyne und bildet sowohl die Statusanzeige
+ * als auch später die Konfiguration der lokalen virtuellen seriellen
+ * Ports ab.
+ *
+ * Die eigentliche Client- und Runtime-Logik bleibt in den internen
+ * Packages gekapselt. Diese Datei ist bewusst auf den Programmstart
+ * und die Übergabe an die GUI beschränkt.
+ * ============================================================================
+ */
 package main
 
 import (
 	"fmt"
+	"image/color"
 
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/app"
+	"fyne.io/fyne/v2/theme"
+
+	"git.lang-dieter.de/rs2322tcp/internal/gui"
 	"git.lang-dieter.de/rs2322tcp/internal/version"
 )
 
-func main() {
+///////////////////////////////////////////////////////////////////////////////
-	fmt.Printf("rs2322tcp-client %s\n", version.Version)
+// Application theme
+///////////////////////////////////////////////////////////////////////////////
+
+// applicationTheme keeps the standard Fyne theme and increases the general
+// text size for better readability.
+//
+// All other colors, fonts, icons and dimensions remain unchanged.
+type applicationTheme struct{}
+
+// Ensure applicationTheme implements fyne.Theme.
+var _ fyne.Theme = (*applicationTheme)(nil)
+
+// Color returns the standard Fyne theme color.
+func (t *applicationTheme) Color(
+	name fyne.ThemeColorName,
+	variant fyne.ThemeVariant,
+) color.Color {
+	return theme.DefaultTheme().Color(name, variant)
+}
+
+// Font returns the standard Fyne theme font.
+func (t *applicationTheme) Font(
+	style fyne.TextStyle,
+) fyne.Resource {
+	return theme.DefaultTheme().Font(style)
+}
+
+// Icon returns the standard Fyne theme icon.
+func (t *applicationTheme) Icon(
+	name fyne.ThemeIconName,
+) fyne.Resource {
+	return theme.DefaultTheme().Icon(name)
+}
+
+// Size returns the standard Fyne theme size, except for normal text.
+//
+// Normal text is increased from the Fyne default to 22 px for better
+// readability.
+func (t *applicationTheme) Size(
+	name fyne.ThemeSizeName,
+) float32 {
+	if name == theme.SizeNameText {
+		return 22
+	}
+
+	return theme.DefaultTheme().Size(name)
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Main
+///////////////////////////////////////////////////////////////////////////////
+
+func main() {
+	fmt.Printf(
+		"rs2322tcp-client %s\n",
+		version.Version,
+	)
+
+	application := app.NewWithID(
+		"git.lang-dieter.de.rs2322tcp.client",
+	)
+
+	// Use the standard Fyne theme with a larger general text size.
+	application.Settings().SetTheme(
+		&applicationTheme{},
+	)
+
+	guiApplication, err := gui.NewApp(
+		application,
+		"./configs/client.json",
+	)
+	if err != nil {
+		fmt.Printf(
+			"GUI konnte nicht gestartet werden: %v\n",
+			err,
+		)
+
+		return
+	}
+
+	// SetOnClosed gehört zum Fyne-Fenster, nicht zur Fyne-Anwendung.
+	//
+	// Beim Schließen des Hauptfensters wird die technische Client-
+	// Anwendung beendet. Dadurch werden Runtime, Bridges, virtuelle
+	// Ports und die Serververbindung sauber geschlossen.
+	guiApplication.Window().SetOnClosed(
+		func() {
+			guiApplication.Close()
+		},
+	)
+
+	// Zuerst das GUI-Fenster erzeugen und anzeigen.
+	//
+	// Dadurch wird ein möglicher Fehler beim Start von Fyne/GLFW erkannt,
+	// bevor die technische Client-Kommunikation gestartet wird.
+	guiApplication.Window().Show()
+
+	// Erst nachdem das Fenster erfolgreich erzeugt wurde, wird das
+	// technische Client-System asynchron gestartet.
+	guiApplication.Start()
+
+	// Den Fyne-Eventloop starten.
+	application.Run()
 }

cmd/rs2322tcp-device-sim/main.go

@@ -7,24 +7,37 @@
  * SPDX-License-Identifier: GPL-3.0-or-later
  *
  * Beschreibung:
- * Einfacher Simulator für ein serielles Endgerät.
+ * Interaktiver Simulator für ein serielles Endgerät.
  *
  * Der Simulator öffnet eine angegebene serielle Schnittstelle und protokolliert
  * alle vom angeschlossenen System empfangenen Bytes als Hexdump.
  *
+ * Zusätzlich können über die Standardeingabe hexadezimal angegebene Bytefolgen
+ * an das serielle Gerät gesendet werden. Dadurch kann der Simulator sowohl als
+ * passiver Sniffer als auch als einfaches interaktives Testgerät verwendet
+ * werden.
+ *
  * Der Simulator stellt bewusst keine eigene PTY-Erzeugung bereit. Für
  * Entwicklung und Integrationstests kann ihm beispielsweise ein unter Linux
  * erzeugter PTY-Slave (/dev/pts/N) übergeben werden. Im späteren Testaufbau
  * kann derselbe Simulator jedoch auch direkt mit einer echten seriellen
  * Schnittstelle verbunden werden.
  *
- * Der Simulator dient zunächst als passives Gegenstück zum
+ * Der Simulator dient zunächst als externes Gegenstück zum
- * rs2322tcp-server. Er soll insbesondere ermöglichen, die vom Server an ein
+ * rs2322tcp-server. Er ermöglicht insbesondere, die vom Server an ein
- * simuliertes Gerät übertragenen Bytes sichtbar zu machen.
+ * simuliertes Gerät übertragenen Bytes sichtbar zu machen und umgekehrt
+ * definierte Bytefolgen an den Server zurückzusenden.
  *
- * Zukünftige Erweiterungen können unter anderem das manuelle Senden von
+ * Eingabeformat:
- * Bytes, automatische Geräteantworten, Protokollsimulation und das Aufzeichnen
+ *
- * bzw. Wiedergeben von Datenströmen umfassen.
+ *     TX> 46 41 00 10 0D
+ *
+ * Die Leerzeichen zwischen den Bytes sind optional. Groß- und Kleinschreibung
+ * der Hex-Ziffern spielt keine Rolle.
+ *
+ * Zukünftige Erweiterungen können unter anderem automatische Geräteantworten,
+ * Protokollsimulation und das Aufzeichnen bzw. Wiedergeben von Datenströmen
+ * umfassen.
  *
  * Aufruf:
  *
@@ -36,12 +49,16 @@
  *     Port: /dev/pts/7
  *     Warte auf Daten ...
  *
- * Empfangene Daten werden mit Zeitstempel, Richtung und Byteanzahl ausgegeben:
+ * Empfangene und gesendete Daten werden mit Zeitstempel, Richtung und
+ * Byteanzahl ausgegeben:
  *
  *     12:34:01.123 RX  5 Bytes
  *               46 41 00 10 0D
  *
- * Der Simulator interpretiert die empfangenen Daten nicht. Die Darstellung
+ *     12:34:05.456 TX  5 Bytes
+ *               46 41 01 10 0D
+ *
+ * Der Simulator interpretiert die seriellen Daten nicht. Die Darstellung
  * erfolgt ausschließlich als Rohdaten in hexadezimaler Form.
  *
  * ============================================================================
@@ -50,10 +67,14 @@
 package main
 
 import (
+	"bufio"
+	"encoding/hex"
 	"flag"
 	"fmt"
 	"io"
 	"log"
+	"os"
+	"strings"
 	"time"
 
 	"golang.org/x/sys/unix"
@@ -92,6 +113,13 @@ func main() {
 	fmt.Printf("Port: %s\n", *port)
 	fmt.Println("Warte auf Daten ...")
 	fmt.Println()
+	fmt.Println("Hex-Bytes senden mit:")
+	fmt.Println("TX> 46 41 00 10 0D")
+	fmt.Println()
+
+	// Die Standardeingabe wird parallel zur seriellen Schnittstelle gelesen.
+	// Dadurch kann der Simulator gleichzeitig Daten empfangen und senden.
+	go readTX(fd)
 
 	// Der Simulator arbeitet byteorientiert. Die Größe des Puffers ist
 	// bewusst ausreichend groß gewählt, damit auch größere Datenblöcke
@@ -135,15 +163,130 @@ func main() {
 	}
 }
 
+// readTX liest Zeilen von stdin und interpretiert sie als Folge von
+// hexadezimal angegebenen Bytes.
+//
+// Die Eingabe wird bewusst zeilenorientiert verarbeitet. Dadurch kann eine
+// komplette Bytefolge zuerst geprüft werden, bevor sie an die serielle
+// Schnittstelle gesendet wird.
+func readTX(fd int) {
+	scanner := bufio.NewScanner(os.Stdin)
+
+	fmt.Print("TX> ")
+
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+
+		if line == "" {
+			fmt.Print("TX> ")
+			continue
+		}
+
+		data, err := parseHexBytes(line)
+		if err != nil {
+			fmt.Printf("Ungültige Hex-Eingabe: %v\n", err)
+			fmt.Print("TX> ")
+			continue
+		}
+
+		if len(data) == 0 {
+			fmt.Print("TX> ")
+			continue
+		}
+
+		if err := writeAll(fd, data); err != nil {
+			fmt.Fprintf(os.Stderr, "Senden: %v\n", err)
+			return
+		}
+
+		printTX(data)
+		fmt.Print("TX> ")
+	}
+
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintf(os.Stderr, "Lesen von stdin: %v\n", err)
+	}
+
+	// Bei EOF (Ctrl+D) wird der Dateideskriptor geschlossen. Dadurch wird
+	// der blockierende Lesevorgang im Haupt-Goroutine beendet und der
+	// Simulator kann sauber verlassen werden.
+	_ = unix.Close(fd)
+}
+
+// parseHexBytes wandelt eine zeilenorientierte Hex-Eingabe in einen
+// Byte-Block um.
+//
+// Leerzeichen zwischen den Bytes sind erlaubt und werden entfernt. Dadurch
+// werden beispielsweise sowohl
+//
+//	46 41 00 10 0D
+//
+// als auch
+//
+//	464100100D
+//
+// akzeptiert. Die Funktion selbst enthält keine Ausgabe und ist deshalb
+// unabhängig vom interaktiven Terminalbetrieb testbar.
+func parseHexBytes(line string) ([]byte, error) {
+	fields := strings.Fields(line)
+	hexText := strings.Join(fields, "")
+
+	if hexText == "" {
+		return nil, nil
+	}
+
+	data, err := hex.DecodeString(hexText)
+	if err != nil {
+		return nil, err
+	}
+
+	return data, nil
+}
+
+// writeAll schreibt alle Bytes des angegebenen Datenblocks auf den seriellen
+// Dateideskriptor. unix.Write() darf weniger Bytes als angefordert schreiben.
+func writeAll(fd int, data []byte) error {
+	for len(data) > 0 {
+		n, err := unix.Write(fd, data)
+		if err != nil {
+			if err == unix.EINTR {
+				continue
+			}
+
+			return err
+		}
+
+		if n == 0 {
+			return io.ErrShortWrite
+		}
+
+		data = data[n:]
+	}
+
+	return nil
+}
+
 // printRX gibt einen empfangenen Datenblock als Hexdump aus.
 //
 // Die Daten werden absichtlich nicht als Text interpretiert. Dadurch bleiben
 // auch nicht druckbare Bytes und Bytes oberhalb von 0x7F sichtbar und können
 // bei der Analyse serieller Protokolle eindeutig nachvollzogen werden.
 func printRX(data []byte) {
+	printHexDump("RX", data)
+}
+
+// printTX gibt einen gesendeten Datenblock als Hexdump aus.
+func printTX(data []byte) {
+	printHexDump("TX", data)
+}
+
+// printHexDump gibt einen Datenblock mit Zeitstempel, Richtung und Byteanzahl
+// als Hexdump aus.
+func printHexDump(direction string, data []byte) {
 	fmt.Printf(
-		"%s RX  %d Bytes\n",
+		"%s %s  %d Bytes\n",
 		time.Now().Format("15:04:05.000"),
+		direction,
 		len(data),
 	)
 

cmd/rs2322tcp-device-sim/main_test.go

@@ -0,0 +1,92 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: main_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Unit-Tests für die Hex-Eingabe des Geräte-Simulators.
+ *
+ * Getestet werden gültige Bytefolgen, optionale Leerzeichen, Groß- und
+ * Kleinschreibung sowie typische ungültige Eingaben.
+ *
+ * ============================================================================
+ */
+
+package main
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestParseHexBytes(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		want    []byte
+		wantErr bool
+	}{
+		{
+			name:  "mehrere Bytes mit Leerzeichen",
+			input: "46 41 00 10 0D",
+			want:  []byte{0x46, 0x41, 0x00, 0x10, 0x0D},
+		},
+		{
+			name:  "mehrere Bytes ohne Leerzeichen",
+			input: "464100100D",
+			want:  []byte{0x46, 0x41, 0x00, 0x10, 0x0D},
+		},
+		{
+			name:  "gemischte Groß- und Kleinschreibung",
+			input: "0a FF 01 b7",
+			want:  []byte{0x0A, 0xFF, 0x01, 0xB7},
+		},
+		{
+			name:  "ein Byte",
+			input: "00",
+			want:  []byte{0x00},
+		},
+		{
+			name:  "leere Eingabe",
+			input: "",
+			want:  nil,
+		},
+		{
+			name:  "nur Leerzeichen",
+			input: "   ",
+			want:  nil,
+		},
+		{
+			name:    "ungültiges Hex-Zeichen",
+			input:   "GG",
+			wantErr: true,
+		},
+		{
+			name:    "ungerade Hex-Länge",
+			input:   "4",
+			wantErr: true,
+		},
+		{
+			name:    "dreistellige Eingabe",
+			input:   "100",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := parseHexBytes(tt.input)
+
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("Fehlerstatus: got %v, wantErr %v", err, tt.wantErr)
+			}
+
+			if !bytes.Equal(got, tt.want) {
+				t.Fatalf("Ergebnis: got % X, want % X", got, tt.want)
+			}
+		})
+	}
+}

cmd/rs2322tcp-pty-test/main.go

@@ -0,0 +1,348 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: cmd/rs2322tcp-pty-test/main.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Entwicklungs- und Testwerkzeug zum Bereitstellen eines einzelnen
+ * bidirektionalen Linux-PTYs als simuliertes serielles Endgerät.
+ *
+ * Der erzeugte PTY-Slave wird vom rs2322tcp-server wie eine echte serielle
+ * Schnittstelle verwendet. Der Helper arbeitet auf dem zugehörigen
+ * PTY-Master als Gegenstelle.
+ *
+ * Alle vom Server empfangenen Daten werden als RX-Hexdump ausgegeben.
+ *
+ * Bei der bekannten Rotor-Anfrage wird automatisch die konfigurierte
+ * Rotor-Antwort zurückgesendet. Alle anderen empfangenen Daten werden
+ * unverändert als Echo zurückgesendet.
+ *
+ * Der PTY-Slave bleibt während der gesamten Laufzeit des Helpers geöffnet.
+ * Dadurch bleibt die virtuelle serielle Gegenstelle auch dann verfügbar,
+ * wenn der Server den Port erst nach dem Start des Helpers öffnet.
+ *
+ * Beim Beenden werden Master und Slave geschlossen. Der Test-Port
+ * verschwindet anschließend wieder aus /dev/pts.
+ * ============================================================================
+ */
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+var (
+	rotorRequest = []byte{
+		0x57,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x00,
+		0x1F,
+		0x20,
+	}
+
+	rotorResponse = []byte{
+		0x57,
+		0x07,
+		0x01,
+		0x06,
+		0x00,
+		0x01,
+		0x03,
+		0x06,
+		0x02,
+		0x00,
+		0x01,
+		0x20,
+	}
+)
+
+func main() {
+	masterFD, slaveFD, slavePath, err := createPTY()
+	if err != nil {
+		fmt.Fprintf(
+			os.Stderr,
+			"PTY-Erzeugung fehlgeschlagen: %v\n",
+			err,
+		)
+		os.Exit(1)
+	}
+
+	defer unix.Close(masterFD)
+	defer unix.Close(slaveFD)
+
+	fmt.Println("rs2322tcp-pty-test")
+	fmt.Printf("Port: %s\n", slavePath)
+	fmt.Println("Modus: Rotor-Simulation mit Echo-Fallback")
+	fmt.Println("Warte auf Daten ...")
+
+	signals := make(chan os.Signal, 1)
+	signal.Notify(
+		signals,
+		os.Interrupt,
+		syscall.SIGTERM,
+	)
+	defer signal.Stop(signals)
+
+	done := make(chan struct{})
+
+	go func() {
+		handleSerial(masterFD)
+		close(done)
+	}()
+
+	select {
+	case <-signals:
+		fmt.Println()
+		fmt.Println("PTY-Testport wird geschlossen.")
+
+	case <-done:
+		fmt.Println()
+		fmt.Println("PTY-Gegenstelle wurde geschlossen.")
+	}
+}
+
+func handleSerial(fd int) {
+	buf := make([]byte, 4096)
+
+	for {
+		n, err := unix.Read(fd, buf)
+		if err != nil {
+			if err == unix.EINTR {
+				continue
+			}
+
+			if err == unix.EIO || err == io.EOF {
+				return
+			}
+
+			fmt.Fprintf(
+				os.Stderr,
+				"Lesen vom PTY-Master: %v\n",
+				err,
+			)
+			return
+		}
+
+		if n == 0 {
+			continue
+		}
+
+		received := append([]byte(nil), buf[:n]...)
+
+		printHexDump("RX", received)
+
+		response := responseFor(received)
+
+		if err := writeAll(fd, response); err != nil {
+			fmt.Fprintf(
+				os.Stderr,
+				"Schreiben zum PTY-Master: %v\n",
+				err,
+			)
+			return
+		}
+
+		printHexDump("TX", response)
+	}
+}
+
+func responseFor(received []byte) []byte {
+	if bytes.Equal(received, rotorRequest) {
+		return rotorResponse
+	}
+
+	return received
+}
+
+func createPTY() (int, int, string, error) {
+	masterFD, err := unix.Open(
+		"/dev/ptmx",
+		unix.O_RDWR|unix.O_NOCTTY,
+		0,
+	)
+	if err != nil {
+		return -1, -1, "", fmt.Errorf(
+			"open PTY master: %w",
+			err,
+		)
+	}
+
+	closeMasterOnError := true
+	defer func() {
+		if closeMasterOnError {
+			_ = unix.Close(masterFD)
+		}
+	}()
+
+	if err := unix.IoctlSetPointerInt(
+		masterFD,
+		unix.TIOCSPTLCK,
+		0,
+	); err != nil {
+		return -1, -1, "", fmt.Errorf(
+			"unlock PTY slave: %w",
+			err,
+		)
+	}
+
+	ptsNumber, err := unix.IoctlGetInt(
+		masterFD,
+		unix.TIOCGPTN,
+	)
+	if err != nil {
+		return -1, -1, "", fmt.Errorf(
+			"get PTY number: %w",
+			err,
+		)
+	}
+
+	slavePath := fmt.Sprintf(
+		"/dev/pts/%d",
+		ptsNumber,
+	)
+
+	slaveFD, err := unix.Open(
+		slavePath,
+		unix.O_RDWR|unix.O_NOCTTY,
+		0,
+	)
+	if err != nil {
+		return -1, -1, "", fmt.Errorf(
+			"open PTY slave %s: %w",
+			slavePath,
+			err,
+		)
+	}
+
+	closeSlaveOnError := true
+	defer func() {
+		if closeSlaveOnError {
+			_ = unix.Close(slaveFD)
+		}
+	}()
+
+	if err := configurePTY(slaveFD); err != nil {
+		return -1, -1, "", err
+	}
+
+	closeMasterOnError = false
+	closeSlaveOnError = false
+
+	return masterFD, slaveFD, slavePath, nil
+}
+
+func configurePTY(fd int) error {
+	termios, err := unix.IoctlGetTermios(
+		fd,
+		unix.TCGETS,
+	)
+	if err != nil {
+		return fmt.Errorf(
+			"get PTY termios: %w",
+			err,
+		)
+	}
+
+	termios.Iflag &^= unix.IGNBRK |
+		unix.BRKINT |
+		unix.PARMRK |
+		unix.ISTRIP |
+		unix.INLCR |
+		unix.IGNCR |
+		unix.ICRNL |
+		unix.IXON
+
+	termios.Oflag &^= unix.OPOST
+
+	termios.Lflag &^= unix.ECHO |
+		unix.ECHONL |
+		unix.ICANON |
+		unix.ISIG |
+		unix.IEXTEN
+
+	termios.Cflag &^= unix.CSIZE | unix.PARENB
+	termios.Cflag |= unix.CS8
+
+	termios.Cc[unix.VMIN] = 1
+	termios.Cc[unix.VTIME] = 0
+
+	if err := unix.IoctlSetTermios(
+		fd,
+		unix.TCSETS,
+		termios,
+	); err != nil {
+		return fmt.Errorf(
+			"set PTY raw mode: %w",
+			err,
+		)
+	}
+
+	return nil
+}
+
+func writeAll(fd int, data []byte) error {
+	for len(data) > 0 {
+		n, err := unix.Write(fd, data)
+		if err != nil {
+			if err == unix.EINTR {
+				continue
+			}
+
+			return err
+		}
+
+		if n == 0 {
+			return io.ErrShortWrite
+		}
+
+		data = data[n:]
+	}
+
+	return nil
+}
+
+func printHexDump(direction string, data []byte) {
+	fmt.Printf(
+		"%s %s  %d Bytes\n",
+		time.Now().Format("15:04:05.000"),
+		direction,
+		len(data),
+	)
+
+	for offset := 0; offset < len(data); offset += 16 {
+		end := offset + 16
+		if end > len(data) {
+			end = len(data)
+		}
+
+		fmt.Print("          ")
+
+		for _, b := range data[offset:end] {
+			fmt.Printf("%02X ", b)
+		}
+
+		fmt.Println()
+	}
+}

cmd/rs2322tcp-server/main.go

@@ -1,12 +1,9 @@
 /*
 Package main implements the rs2322tcp server application.
 
-The server will provide the network endpoint for rs2322tcp clients
+The server provides the network endpoint for rs2322tcp clients
-and connect incoming connections to configured physical RS232
+and connects incoming data connections to configured physical
-interfaces.
+RS232 interfaces.
-
-At this stage the application only provides the basic program
-structure and version information.
 
 Project: rs2322tcp
 Module:  git.lang-dieter.de/rs2322tcp
@@ -14,11 +11,56 @@ Module:  git.lang-dieter.de/rs2322tcp
 package main
 
 import (
+	"flag"
 	"fmt"
+	"log"
 
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/server"
 	"git.lang-dieter.de/rs2322tcp/internal/version"
 )
 
 func main() {
-	fmt.Printf("rs2322tcp-server %s\n", version.Version)
+	configFile := flag.String(
+		"config",
+		"./configs/server.json",
+		"Pfad zur Server-Konfigurationsdatei",
+	)
+
+	flag.Parse()
+
+	cfg, err := config.LoadServer(*configFile)
+	if err != nil {
+		log.Fatalf("Server-Konfiguration laden: %v", err)
+	}
+
+	controlServer, err := server.NewControlServer(cfg)
+	if err != nil {
+		log.Fatalf("Control-Server erstellen: %v", err)
+	}
+
+	if err := controlServer.Listen(); err != nil {
+		log.Fatalf("Control-Server starten: %v", err)
+	}
+
+	defer controlServer.Close()
+
+	fmt.Printf(
+		"rs2322tcp-server %s\n",
+		version.Version,
+	)
+
+	fmt.Printf(
+		"Control-Server gestartet auf %s\n",
+		controlServer.Addr(),
+	)
+
+	fmt.Printf(
+		"Geräte: %d\n",
+		len(cfg.Devices),
+	)
+
+	if err := controlServer.Serve(); err != nil {
+		log.Fatalf("Control-Server beendet: %v", err)
+	}
 }

configs/client.json

@@ -1,16 +1,18 @@
 {
     "server": {
-        "address": "100.64.0.10",
+        "address": "127.0.0.1",
         "port": 5000
     },
+    "virtual_port_range": {
+        "first": 100,
+        "last": 199
+    },
     "virtual_ports": [
         {
-            "port": "COM7",
+            "port": "/dev/ttyUSB100",
-            "remote_device": "radio"
+            "remote_device": "rotor",
-        },
+			"startup_bytes": "57 00 00 00 00 00 00 00 00 00 00 1F 20"
-        {
-            "port": "COM8",
-            "remote_device": "rotor"
         }
+
     ]
 }

configs/server-test.json

@@ -0,0 +1,41 @@
+{
+    "listen": {
+        "address": "0.0.0.0",
+        "port": 5000
+    },
+
+    "hardware_error_response": "ERROR - HARDWARE NOT AVAILABLE",
+
+    "serial_monitor": false,
+
+    "devices": [
+        {
+            "id": "radio",
+            "name": "Funkgerät",
+            "serial_port": "/dev/ttyUSB1",
+            "baud_rate": 9600,
+            "data_bits": 8,
+            "parity": "none",
+            "stop_bits": 1
+        },
+        {
+            "id": "rotor",
+            "name": "Antennenrotor",
+            "serial_port": "/dev/pts/1",
+            "baud_rate": 600,
+            "data_bits": 8,
+            "parity": "none",
+            "stop_bits": 1
+        },
+        {
+            "id": "gps",
+            "name": "GPS-Testgerät",
+            "serial_port": "/dev/ttyUSB3",
+            "baud_rate": 600,
+            "data_bits": 8,
+            "parity": "none",
+            "stop_bits": 1
+        }
+
+    ]
+}

configs/server.json

@@ -3,11 +3,16 @@
         "address": "0.0.0.0",
         "port": 5000
     },
+
+    "hardware_error_response": "ERROR - HARDWARE NOT AVAILABLE",
+
+    "serial_monitor": true,
+
     "devices": [
         {
             "id": "radio",
             "name": "Funkgerät",
-            "serial_port": "/dev/ttyUSB0",
+            "serial_port": "/dev/ttyUSB1",
             "baud_rate": 9600,
             "data_bits": 8,
             "parity": "none",
@@ -16,11 +21,21 @@
         {
             "id": "rotor",
             "name": "Antennenrotor",
-            "serial_port": "/dev/ttyUSB1",
+            "serial_port": "/dev/ttyUSB0",
-            "baud_rate": 4800,
+            "baud_rate": 600,
+            "data_bits": 8,
+            "parity": "none",
+            "stop_bits": 1
+        },
+        {
+            "id": "gps",
+            "name": "GPS-Testgerät",
+            "serial_port": "/dev/ttyUSB3",
+            "baud_rate": 600,
             "data_bits": 8,
             "parity": "none",
             "stop_bits": 1
         }
+
     ]
 }

go.mod

@@ -2,6 +2,46 @@ module git.lang-dieter.de/rs2322tcp
 
 go 1.25.0
 
-require go.bug.st/serial v1.7.1
+require (
+	go.bug.st/serial v1.7.1
+	golang.org/x/sys v0.43.0
+)
 
-require golang.org/x/sys v0.43.0 // indirect
+require github.com/FyshOS/fancyfs v0.0.1 // indirect
+
+require (
+	fyne.io/fyne/v2 v2.8.0
+	fyne.io/systray v1.12.2 // indirect
+	github.com/BurntSushi/toml v1.6.0 // indirect
+	github.com/anthonynsimon/bild v0.14.0 // indirect
+	github.com/clipperhouse/uax29/v2 v2.2.0 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/fredbi/uri v1.1.1 // indirect
+	github.com/fsnotify/fsnotify v1.9.0 // indirect
+	github.com/fyne-io/gl-js v0.2.1-0.20260315212741-029c47fd27e8 // indirect
+	github.com/fyne-io/glfw-js v0.4.0 // indirect
+	github.com/fyne-io/image v0.1.1 // indirect
+	github.com/fyne-io/oksvg v0.2.0 // indirect
+	github.com/go-gl/gl v0.0.0-20260331235117-4566fea9a276 // indirect
+	github.com/go-gl/glfw/v3.4/glfw v0.1.0-pre.1.0.20260707082822-2a407d02d01a // indirect
+	github.com/go-text/render v0.2.1 // indirect
+	github.com/go-text/typesetting v0.3.4 // indirect
+	github.com/godbus/dbus/v5 v5.2.2 // indirect
+	github.com/hack-pad/go-indexeddb v0.3.2 // indirect
+	github.com/hack-pad/safejs v0.1.0 // indirect
+	github.com/jeandeaual/go-locale v0.0.0-20250612000132-0ef82f21eade // indirect
+	github.com/jsummers/gobmp v0.0.0-20230614200233-a9de23ed2e25 // indirect
+	github.com/mattn/go-runewidth v0.0.24 // indirect
+	github.com/nfnt/resize v0.0.0-20180221191011-83c6a9932646 // indirect
+	github.com/nicksnyder/go-i18n/v2 v2.5.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/rymdport/portal v0.4.2 // indirect
+	github.com/srwiley/oksvg v0.0.0-20221011165216-be6e8873101c // indirect
+	github.com/srwiley/rasterx v0.0.0-20220730225603-2ab79fcdd4ef // indirect
+	github.com/stretchr/testify v1.11.1 // indirect
+	github.com/yuin/goldmark v1.8.2 // indirect
+	golang.org/x/image v0.24.0 // indirect
+	golang.org/x/net v0.35.0 // indirect
+	golang.org/x/text v0.22.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

go.sum

@@ -1,4 +1,89 @@
+fyne.io/fyne/v2 v2.8.0 h1:KNUdIk1eKsXSPy/wU6MdiR1hppAPvyzbjPbtJ8h6EUQ=
+fyne.io/fyne/v2 v2.8.0/go.mod h1:tLJK7CVtUBOnMiSDR+J88t/quiGuEhwGs09tIVM1RXg=
+fyne.io/systray v1.12.2 h1:Y8DZxgLHsVQt6rY9Zrkkg+j67S7vv/1F2viOWKPpVeA=
+fyne.io/systray v1.12.2/go.mod h1:RVwqP9nYMo7h5zViCBHri2FgjXF7H2cub7MAq4NSoLs=
+github.com/BurntSushi/toml v1.6.0 h1:dRaEfpa2VI55EwlIW72hMRHdWouJeRF7TPYhI+AUQjk=
+github.com/BurntSushi/toml v1.6.0/go.mod h1:ukJfTF/6rtPPRCnwkur4qwRxa8vTRFBF0uk2lLoLwho=
+github.com/FyshOS/fancyfs v0.0.1 h1:kgvm7VvwOMLkYTqSflplp62SlMVWQ2uAoHw9CXwXHYg=
+github.com/FyshOS/fancyfs v0.0.1/go.mod h1:S5SHVz/5R72iCXOxCqdcyTPSlg3JxNd0gaHyGBSrY8A=
+github.com/anthonynsimon/bild v0.14.0 h1:IFRkmKdNdqmexXHfEU7rPlAmdUZ8BDZEGtGHDnGWync=
+github.com/anthonynsimon/bild v0.14.0/go.mod h1:hcvEAyBjTW69qkKJTfpcDQ83sSZHxwOunsseDfeQhUs=
+github.com/clipperhouse/uax29/v2 v2.2.0 h1:ChwIKnQN3kcZteTXMgb1wztSgaU+ZemkgWdohwgs8tY=
+github.com/clipperhouse/uax29/v2 v2.2.0/go.mod h1:EFJ2TJMRUaplDxHKj1qAEhCtQPW2tJSwu5BF98AuoVM=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/felixge/fgprof v0.9.3 h1:VvyZxILNuCiUCSXtPtYmmtGvb65nqXh2QFWc0Wpf2/g=
+github.com/felixge/fgprof v0.9.3/go.mod h1:RdbpDgzqYVh/T9fPELJyV7EYJuHB55UTEULNun8eiPw=
+github.com/fredbi/uri v1.1.1 h1:xZHJC08GZNIUhbP5ImTHnt5Ya0T8FI2VAwI/37kh2Ko=
+github.com/fredbi/uri v1.1.1/go.mod h1:4+DZQ5zBjEwQCDmXW5JdIjz0PUA+yJbvtBv+u+adr5o=
+github.com/fsnotify/fsnotify v1.9.0 h1:2Ml+OJNzbYCTzsxtv8vKSFD9PbJjmhYF14k/jKC7S9k=
+github.com/fsnotify/fsnotify v1.9.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
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+github.com/go-gl/glfw/v3.4/glfw v0.1.0-pre.1.0.20260707082822-2a407d02d01a/go.mod h1:T5Dn0JwIJOX1euPZ/iT4tq6nFYtmukjcYa7937HuYK8=
+github.com/go-text/render v0.2.1 h1:qwHhxqGUjjg4L0XyJWj7M7bpY75NZM+kBpv2Yfw5mcg=
+github.com/go-text/render v0.2.1/go.mod h1:HCCAq8MUlm/WRcXshBb4K/n+IkjeXQ1c2Ba+yICSm0A=
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+github.com/jeandeaual/go-locale v0.0.0-20250612000132-0ef82f21eade/go.mod h1:ZDXo8KHryOWSIqnsb/CiDq7hQUYryCgdVnxbj8tDG7o=
+github.com/jsummers/gobmp v0.0.0-20230614200233-a9de23ed2e25 h1:YLvr1eE6cdCqjOe972w/cYF+FjW34v27+9Vo5106B4M=
+github.com/jsummers/gobmp v0.0.0-20230614200233-a9de23ed2e25/go.mod h1:kLgvv7o6UM+0QSf0QjAse3wReFDsb9qbZJdfexWlrQw=
+github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
+github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/mattn/go-runewidth v0.0.24 h1:cpokDiIn0MGnhdHwuWnJBITySJ20QyNGnY2kR/ay2DU=
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internal/client/application.go

@@ -0,0 +1,347 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/application.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Gemeinsame Anwendungsschicht für den rs2322tcp-Client.
+ *
+ * Die Anwendungsschicht verbindet die Konfiguration, die Control-Verbindung
+ * zum Server und die clientseitige Runtime. Sie stellt damit einen einfachen
+ * Lebenszyklus für die spätere grafische Benutzeroberfläche bereit.
+ *
+ * Der technische Client wird bei Start und Reconnect vollständig neu
+ * initialisiert:
+ *
+ *     Konfiguration laden
+ *             |
+ *             v
+ *     Lokale virtuelle Ports prüfen
+ *             |
+ *             v
+ *     Server verbinden
+ *             |
+ *             v
+ *     Geräteliste abfragen
+ *             |
+ *             v
+ *     Runtime starten
+ *
+ * Ein Reconnect beendet zunächst die bestehende Runtime und Control-
+ * Verbindung und führt anschließend denselben Startablauf erneut aus.
+ *
+ * Die Anwendungsschicht enthält bewusst keine GUI-Logik und keine
+ * Fyne-Abhängigkeit.
+ * ============================================================================
+ */
+package client
+
+import (
+	"fmt"
+	"sync"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/transport"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Application
+///////////////////////////////////////////////////////////////////////////////
+
+// Application represents one running rs2322tcp client application.
+//
+// Application owns the current client connection and Runtime. The
+// configuration file is the persistent source of the client configuration.
+//
+// The Application is intended to be used by the command-line entry point
+// as well as by the later graphical user interface.
+type Application struct {
+	mu sync.Mutex
+
+	configFile string
+
+	config  *config.ClientConfig
+	client  *Client
+	runtime *Runtime
+	devices []transport.RemoteDeviceInfo
+
+	runtimeDone chan error
+}
+
+// NewApplication creates a new client Application.
+//
+// The client configuration is loaded immediately. No network connection
+// is established. Start must be called before the application connects to
+// the server or starts the Runtime.
+func NewApplication(configFile string) (*Application, error) {
+	if configFile == "" {
+		return nil, fmt.Errorf("configuration file is empty")
+	}
+
+	cfg, err := config.LoadClient(configFile)
+	if err != nil {
+		return nil, fmt.Errorf("load client configuration: %w", err)
+	}
+
+	return &Application{
+		configFile: configFile,
+		config:     cfg,
+	}, nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Start
+///////////////////////////////////////////////////////////////////////////////
+
+// Start loads the client configuration, validates the locally available
+// virtual ports, connects to the server and starts the client Runtime.
+//
+// The Runtime is started asynchronously because Runtime.Run blocks while
+// the configured virtual serial connections are active.
+func (a *Application) Start() error {
+	if a == nil {
+		return fmt.Errorf("application is nil")
+	}
+
+	a.mu.Lock()
+
+	if a.runtime != nil || a.client != nil {
+		a.mu.Unlock()
+
+		return fmt.Errorf("application is already running")
+	}
+
+	a.mu.Unlock()
+
+	cfg, err := config.LoadClient(a.configFile)
+	if err != nil {
+		return fmt.Errorf("load client configuration: %w", err)
+	}
+
+	// Before connecting to the server, make sure that the virtual ports
+	// configured in client.json match the locally installed virtual ports.
+	//
+	// This check is deliberately performed before New(address), so an
+	// invalid local virtual-port configuration prevents the client from
+	// starting at all.
+	if err := validateLocalVirtualPorts(cfg); err != nil {
+		return err
+	}
+
+	address := fmt.Sprintf(
+		"%s:%d",
+		cfg.Server.Address,
+		cfg.Server.Port,
+	)
+
+	client, err := New(address)
+	if err != nil {
+		return err
+	}
+
+	devices, err := client.GetDevices()
+	if err != nil {
+		_ = client.Close()
+
+		return fmt.Errorf("get remote devices: %w", err)
+	}
+
+	manager := NewVirtualPortManager(
+		cfg.VirtualPortRange,
+	)
+
+	runtime, err := NewRuntime(
+		client,
+		manager,
+	)
+	if err != nil {
+		_ = client.Close()
+
+		return fmt.Errorf("create runtime: %w", err)
+	}
+
+	runtimeDone := make(chan error, 1)
+
+	a.mu.Lock()
+
+	a.config = cfg
+	a.client = client
+	a.runtime = runtime
+	a.devices = append(
+		[]transport.RemoteDeviceInfo(nil),
+		devices...,
+	)
+	a.runtimeDone = runtimeDone
+
+	a.mu.Unlock()
+
+	go func() {
+		runtimeDone <- runtime.Run(*cfg)
+	}()
+
+	return nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Reconnect
+///////////////////////////////////////////////////////////////////////////////
+
+// Reconnect stops the current client connection and starts it again.
+//
+// The configuration file is loaded again during Start. This is intentional:
+// changes made by the GUI are therefore picked up automatically.
+//
+// Reconnect is also useful when the server connection has been lost or the
+// user explicitly requests a new connection.
+func (a *Application) Reconnect() error {
+	if a == nil {
+		return fmt.Errorf("application is nil")
+	}
+
+	if err := a.Close(); err != nil {
+		return fmt.Errorf("close current connection: %w", err)
+	}
+
+	return a.Start()
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Close
+///////////////////////////////////////////////////////////////////////////////
+
+// Close stops the Runtime and closes the client control connection.
+//
+// Calling Close more than once is safe.
+func (a *Application) Close() error {
+	if a == nil {
+		return nil
+	}
+
+	a.mu.Lock()
+
+	runtime := a.runtime
+	client := a.client
+
+	a.runtime = nil
+	a.client = nil
+	a.devices = nil
+	a.runtimeDone = nil
+
+	a.mu.Unlock()
+
+	var firstErr error
+
+	if runtime != nil {
+		if err := runtime.Close(); err != nil {
+			firstErr = err
+		}
+	} else if client != nil {
+		if err := client.Close(); err != nil {
+			firstErr = err
+		}
+	}
+
+	return firstErr
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Configuration
+///////////////////////////////////////////////////////////////////////////////
+
+// Config returns a copy of the currently loaded client configuration.
+//
+// The configuration is available after NewApplication, even when the
+// server connection has not yet been established.
+func (a *Application) Config() *config.ClientConfig {
+	if a == nil {
+		return nil
+	}
+
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	if a.config == nil {
+		return nil
+	}
+
+	cfg := *a.config
+
+	cfg.VirtualPorts = append(
+		[]config.VirtualPortConfig(nil),
+		a.config.VirtualPorts...,
+	)
+
+	return &cfg
+}
+
+// SaveConfig saves the supplied client configuration to the configuration
+// file used by this Application.
+//
+// After a successful save the supplied configuration also becomes the
+// current in-memory configuration.
+//
+// The running Runtime is not restarted automatically. A subsequent
+// Reconnect or application restart will use the saved configuration.
+func (a *Application) SaveConfig(
+	cfg *config.ClientConfig,
+) error {
+	if a == nil {
+		return fmt.Errorf("application is nil")
+	}
+
+	if cfg == nil {
+		return fmt.Errorf("client configuration is nil")
+	}
+
+	if err := config.SaveClient(a.configFile, cfg); err != nil {
+		return err
+	}
+
+	a.mu.Lock()
+	a.config = cfg
+	a.mu.Unlock()
+
+	return nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Devices
+///////////////////////////////////////////////////////////////////////////////
+
+// Devices returns a copy of the remote devices received from the server.
+//
+// The returned slice belongs to the caller and can therefore be modified
+// without changing the Application state.
+func (a *Application) Devices() []transport.RemoteDeviceInfo {
+	if a == nil {
+		return nil
+	}
+
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	return append(
+		[]transport.RemoteDeviceInfo(nil),
+		a.devices...,
+	)
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Runtime state
+///////////////////////////////////////////////////////////////////////////////
+
+// Running reports whether the application currently owns a Runtime and a
+// client connection.
+func (a *Application) Running() bool {
+	if a == nil {
+		return false
+	}
+
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	return a.runtime != nil && a.client != nil
+}

internal/client/application_test.go

@@ -0,0 +1,344 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/application_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für die Anwendungsschicht des rs2322tcp-Clients.
+ *
+ * Die Tests prüfen den Lebenszyklus der Application und die lokale
+ * Konfigurationsverwaltung, ohne dafür eine echte Serververbindung
+ * aufbauen zu müssen.
+ *
+ * Insbesondere wird sichergestellt, dass eine neu erzeugte Application
+ * ihre lokale Konfiguration bereits kennt, obwohl noch keine Verbindung
+ * zum Server besteht.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Test helpers
+///////////////////////////////////////////////////////////////////////////////
+
+// writeTestConfig creates a temporary client configuration file.
+func writeTestConfig(t *testing.T) string {
+	t.Helper()
+
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "client.json")
+
+	content := `{
+		"server": {
+			"address": "127.0.0.1",
+			"port": 5000
+		},
+		"virtual_port_range": {
+			"first": 100,
+			"last": 199
+		},
+		"virtual_ports": [
+			{
+				"port": "/dev/ttyUSB100",
+				"remote_device": "radio"
+			},
+			{
+				"port": "/dev/ttyUSB101",
+				"remote_device": "rotor"
+			}
+		]
+	}`
+
+	if err := os.WriteFile(
+		filename,
+		[]byte(content),
+		0600,
+	); err != nil {
+		t.Fatalf("write test configuration: %v", err)
+	}
+
+	return filename
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// NewApplication
+///////////////////////////////////////////////////////////////////////////////
+
+// TestNewApplicationRejectsEmptyConfigFile verifies that an empty
+// configuration-file path is rejected.
+func TestNewApplicationRejectsEmptyConfigFile(t *testing.T) {
+	t.Helper()
+
+	application, err := NewApplication("")
+	if err == nil {
+		t.Fatal("NewApplication with empty config file returned no error")
+	}
+
+	if application != nil {
+		t.Fatal("NewApplication with empty config file returned an application")
+	}
+}
+
+// TestNewApplication verifies that a valid configuration-file path creates
+// an application and loads the local configuration without starting the
+// network connection.
+func TestNewApplication(t *testing.T) {
+	t.Helper()
+
+	configFile := writeTestConfig(t)
+
+	application, err := NewApplication(configFile)
+	if err != nil {
+		t.Fatalf("NewApplication: %v", err)
+	}
+
+	if application == nil {
+		t.Fatal("NewApplication returned nil application")
+	}
+
+	if application.Running() {
+		t.Fatal("new application reports running")
+	}
+
+	cfg := application.Config()
+	if cfg == nil {
+		t.Fatal("new application returned no configuration")
+	}
+
+	if len(cfg.VirtualPorts) != 2 {
+		t.Fatalf(
+			"new application returned %d virtual ports, want 2",
+			len(cfg.VirtualPorts),
+		)
+	}
+
+	if cfg.VirtualPorts[0].Port != "/dev/ttyUSB100" {
+		t.Errorf(
+			"virtual port 0 = %q, want %q",
+			cfg.VirtualPorts[0].Port,
+			"/dev/ttyUSB100",
+		)
+	}
+
+	if cfg.VirtualPorts[0].RemoteDevice != "radio" {
+		t.Errorf(
+			"remote device 0 = %q, want %q",
+			cfg.VirtualPorts[0].RemoteDevice,
+			"radio",
+		)
+	}
+
+	if cfg.VirtualPorts[1].Port != "/dev/ttyUSB101" {
+		t.Errorf(
+			"virtual port 1 = %q, want %q",
+			cfg.VirtualPorts[1].Port,
+			"/dev/ttyUSB101",
+		)
+	}
+
+	if cfg.VirtualPorts[1].RemoteDevice != "rotor" {
+		t.Errorf(
+			"remote device 1 = %q, want %q",
+			cfg.VirtualPorts[1].RemoteDevice,
+			"rotor",
+		)
+	}
+
+	if devices := application.Devices(); len(devices) != 0 {
+		t.Fatalf(
+			"new application returned %d devices, want 0",
+			len(devices),
+		)
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Configuration copy
+///////////////////////////////////////////////////////////////////////////////
+
+// TestApplicationConfigReturnsCopy verifies that modifying the returned
+// configuration does not modify the Application's internal configuration.
+func TestApplicationConfigReturnsCopy(t *testing.T) {
+	t.Helper()
+
+	configFile := writeTestConfig(t)
+
+	application, err := NewApplication(configFile)
+	if err != nil {
+		t.Fatalf("NewApplication: %v", err)
+	}
+
+	cfg := application.Config()
+	if cfg == nil {
+		t.Fatal("Config returned nil")
+	}
+
+	cfg.VirtualPorts[0].Port = "/dev/ttyUSB999"
+	cfg.VirtualPorts[0].RemoteDevice = "changed"
+
+	current := application.Config()
+	if current == nil {
+		t.Fatal("second Config returned nil")
+	}
+
+	if current.VirtualPorts[0].Port != "/dev/ttyUSB100" {
+		t.Errorf(
+			"internal port changed to %q",
+			current.VirtualPorts[0].Port,
+		)
+	}
+
+	if current.VirtualPorts[0].RemoteDevice != "radio" {
+		t.Errorf(
+			"internal remote device changed to %q",
+			current.VirtualPorts[0].RemoteDevice,
+		)
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Close
+///////////////////////////////////////////////////////////////////////////////
+
+// TestApplicationCloseBeforeStart verifies that Close can safely be called
+// before the application has been started and that the local configuration
+// remains available.
+func TestApplicationCloseBeforeStart(t *testing.T) {
+	t.Helper()
+
+	configFile := writeTestConfig(t)
+
+	application, err := NewApplication(configFile)
+	if err != nil {
+		t.Fatalf("NewApplication: %v", err)
+	}
+
+	if err := application.Close(); err != nil {
+		t.Fatalf("Close before Start: %v", err)
+	}
+
+	if application.Running() {
+		t.Fatal("application reports running after Close")
+	}
+
+	if cfg := application.Config(); cfg == nil {
+		t.Fatal("configuration was lost after Close")
+	}
+}
+
+// TestApplicationCloseIsIdempotent verifies that Close can be called more
+// than once without producing an error and without losing the local
+// configuration.
+func TestApplicationCloseIsIdempotent(t *testing.T) {
+	t.Helper()
+
+	configFile := writeTestConfig(t)
+
+	application, err := NewApplication(configFile)
+	if err != nil {
+		t.Fatalf("NewApplication: %v", err)
+	}
+
+	if err := application.Close(); err != nil {
+		t.Fatalf("first Close: %v", err)
+	}
+
+	if err := application.Close(); err != nil {
+		t.Fatalf("second Close: %v", err)
+	}
+
+	if application.Running() {
+		t.Fatal("application reports running after repeated Close")
+	}
+
+	if cfg := application.Config(); cfg == nil {
+		t.Fatal("configuration was lost after repeated Close")
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Invalid configuration
+///////////////////////////////////////////////////////////////////////////////
+
+// TestNewApplicationRejectsInvalidConfiguration verifies that an invalid
+// configuration file is rejected immediately.
+func TestNewApplicationRejectsInvalidConfiguration(t *testing.T) {
+	t.Helper()
+
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "invalid.json")
+
+	if err := os.WriteFile(
+		filename,
+		[]byte(`{"invalid": true}`),
+		0600,
+	); err != nil {
+		t.Fatalf("write invalid configuration: %v", err)
+	}
+
+	application, err := NewApplication(filename)
+	if err == nil {
+		t.Fatal("NewApplication with invalid configuration returned no error")
+	}
+
+	if application != nil {
+		t.Fatal(
+			"NewApplication with invalid configuration returned an application",
+		)
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Nil receiver
+///////////////////////////////////////////////////////////////////////////////
+
+// TestNilApplication verifies that the public lifecycle methods behave
+// safely when called on a nil Application receiver.
+//
+// This is deliberately a small defensive test. The GUI should normally
+// never operate on a nil Application.
+func TestNilApplication(t *testing.T) {
+	t.Helper()
+
+	var application *Application
+
+	if err := application.Start(); err == nil {
+		t.Fatal("nil Application Start returned no error")
+	}
+
+	if err := application.Reconnect(); err == nil {
+		t.Fatal("nil Application Reconnect returned no error")
+	}
+
+	if err := application.Close(); err != nil {
+		t.Fatalf("nil Application Close: %v", err)
+	}
+
+	if application.Running() {
+		t.Fatal("nil Application reports running")
+	}
+
+	if application.Config() != nil {
+		t.Fatal("nil Application returned a configuration")
+	}
+
+	if devices := application.Devices(); devices != nil {
+		t.Fatal("nil Application returned devices")
+	}
+}
+
+// Keep config imported explicitly so the test documents that the temporary
+// file represents the normal ClientConfig structure.
+var _ config.ClientConfig

internal/client/bridge.go

@@ -0,0 +1,130 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/bridge.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Bidirektionale Daten-Bridge zwischen der virtuellen seriellen
+ * Schnittstelle des Clients und der TCP-Data-Verbindung.
+ *
+ * Die Bridge transportiert den Byte-Strom in beide Richtungen:
+ *
+ *     VirtualSerial ─────► TCP
+ *     VirtualSerial ◄───── TCP
+ *
+ * Die Bridge kennt weder das Control-Protokoll noch die konkrete
+ * serielle Hardware. Sie verbindet ausschließlich zwei io.ReadWriter-
+ * Endpunkte miteinander.
+ * ============================================================================
+ */
+package client
+
+import (
+	"fmt"
+	"io"
+	"net"
+	"sync"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Bridge
+///////////////////////////////////////////////////////////////////////////////
+
+// Bridge verbindet eine virtuelle serielle Schnittstelle mit einer
+// TCP-Data-Verbindung.
+//
+// Beide Datenrichtungen werden gleichzeitig bedient. Dadurch bleibt der
+// serielle Datenstrom vollständig bidirektional.
+type Bridge struct {
+	serial VirtualSerial
+	conn   net.Conn
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Constructor
+///////////////////////////////////////////////////////////////////////////////
+
+// NewBridge creates a new bidirectional data bridge.
+//
+// The bridge does not take ownership of the endpoints until Run is called.
+func NewBridge(serial VirtualSerial, conn net.Conn) (*Bridge, error) {
+	if serial == nil {
+		return nil, fmt.Errorf("virtual serial is nil")
+	}
+
+	if conn == nil {
+		return nil, fmt.Errorf("TCP connection is nil")
+	}
+
+	return &Bridge{
+		serial: serial,
+		conn:   conn,
+	}, nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Run
+///////////////////////////////////////////////////////////////////////////////
+
+// Run starts the bidirectional byte transfer and blocks until one of the
+// directions terminates.
+//
+// Closing the endpoints after termination releases the opposite blocked
+// transfer as well.
+func (b *Bridge) Run() error {
+	if b == nil || b.serial == nil {
+		return fmt.Errorf("bridge is not initialized")
+	}
+
+	if b.conn == nil {
+		return fmt.Errorf("TCP connection is nil")
+	}
+
+	type result struct {
+		name string
+		err  error
+	}
+
+	results := make(chan result, 2)
+
+	var wg sync.WaitGroup
+	wg.Add(2)
+
+	go func() {
+		defer wg.Done()
+
+		_, err := io.Copy(b.conn, b.serial)
+		results <- result{
+			name: "serial to TCP",
+			err:  err,
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+
+		_, err := io.Copy(b.serial, b.conn)
+		results <- result{
+			name: "TCP to serial",
+			err:  err,
+		}
+	}()
+
+	first := <-results
+
+	// Beide Richtungen sollen beendet werden. Das Schließen der Endpunkte
+	// unterbricht insbesondere einen eventuell noch blockierenden Read.
+	_ = b.conn.Close()
+	_ = b.serial.Close()
+
+	wg.Wait()
+
+	if first.err != nil {
+		return fmt.Errorf("%s: %w", first.name, first.err)
+	}
+
+	return nil
+}

internal/client/bridge_test.go

@@ -0,0 +1,271 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/bridge_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für die bidirektionale Daten-Bridge zwischen VirtualSerial und
+ * TCP-Verbindung.
+ *
+ * Die Tests verwenden net.Pipe und eine kleine In-Memory-Implementierung
+ * von VirtualSerial. Dadurch wird ausschließlich die Daten-Bridge getestet,
+ * ohne echte TCP-Ports oder eine reale PTY-Schnittstelle zu benötigen.
+ * ============================================================================
+ */
+package client
+
+import (
+	"bytes"
+	"io"
+	"net"
+	"sync"
+	"testing"
+	"time"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Test VirtualSerial
+///////////////////////////////////////////////////////////////////////////////
+
+type testVirtualSerial struct {
+	mu     sync.Mutex
+	reader *bytes.Reader
+	writes [][]byte
+	closed bool
+}
+
+func newTestVirtualSerial(data []byte) *testVirtualSerial {
+	return &testVirtualSerial{
+		reader: bytes.NewReader(data),
+	}
+}
+
+func (s *testVirtualSerial) Path() string {
+	return "test-serial"
+}
+
+func (s *testVirtualSerial) Read(p []byte) (int, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if s.closed {
+		return 0, io.EOF
+	}
+
+	return s.reader.Read(p)
+}
+
+func (s *testVirtualSerial) Write(p []byte) (int, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if s.closed {
+		return 0, io.ErrClosedPipe
+	}
+
+	cp := append([]byte(nil), p...)
+	s.writes = append(s.writes, cp)
+
+	return len(p), nil
+}
+
+func (s *testVirtualSerial) Written() []byte {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	var result []byte
+	for _, p := range s.writes {
+		result = append(result, p...)
+	}
+	return result
+}
+
+func (s *testVirtualSerial) Close() error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.closed = true
+	return nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Blocking test VirtualSerial
+///////////////////////////////////////////////////////////////////////////////
+
+type blockingVirtualSerial struct {
+	mu     sync.Mutex
+	writes [][]byte
+	readCh chan struct{}
+	closed bool
+}
+
+func newBlockingVirtualSerial() *blockingVirtualSerial {
+	return &blockingVirtualSerial{
+		readCh: make(chan struct{}),
+	}
+}
+
+func (s *blockingVirtualSerial) Path() string {
+	return "blocking-test-serial"
+}
+
+func (s *blockingVirtualSerial) Read([]byte) (int, error) {
+	<-s.readCh
+	return 0, io.EOF
+}
+
+func (s *blockingVirtualSerial) Write(p []byte) (int, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if s.closed {
+		return 0, io.ErrClosedPipe
+	}
+
+	s.writes = append(s.writes, append([]byte(nil), p...))
+	return len(p), nil
+}
+
+func (s *blockingVirtualSerial) Written() []byte {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	var result []byte
+	for _, p := range s.writes {
+		result = append(result, p...)
+	}
+	return result
+}
+
+func (s *blockingVirtualSerial) Close() error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if !s.closed {
+		s.closed = true
+		close(s.readCh)
+	}
+
+	return nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestNewBridge(t *testing.T) {
+	serial := newTestVirtualSerial(nil)
+	connA, connB := net.Pipe()
+	defer connA.Close()
+	defer connB.Close()
+
+	if _, err := NewBridge(nil, connA); err == nil {
+		t.Fatal("expected error for nil VirtualSerial")
+	}
+
+	if _, err := NewBridge(serial, nil); err == nil {
+		t.Fatal("expected error for nil TCP connection")
+	}
+
+	bridge, err := NewBridge(serial, connA)
+	if err != nil {
+		t.Fatalf("NewBridge: %v", err)
+	}
+
+	if bridge.serial != serial {
+		t.Fatal("bridge serial endpoint mismatch")
+	}
+
+	if bridge.conn != connA {
+		t.Fatal("bridge TCP endpoint mismatch")
+	}
+}
+
+func TestBridgeSerialToTCP(t *testing.T) {
+	serial := newTestVirtualSerial([]byte("hello"))
+
+	clientConn, bridgeConn := net.Pipe()
+
+	bridge, err := NewBridge(serial, bridgeConn)
+	if err != nil {
+		t.Fatalf("NewBridge: %v", err)
+	}
+
+	runDone := make(chan error, 1)
+	go func() {
+		runDone <- bridge.Run()
+	}()
+
+	got := make([]byte, 5)
+	if _, err := io.ReadFull(clientConn, got); err != nil {
+		t.Fatalf("read TCP data: %v", err)
+	}
+
+	if string(got) != "hello" {
+		t.Fatalf("received %q, want %q", got, "hello")
+	}
+
+	_ = clientConn.Close()
+
+	select {
+	case err := <-runDone:
+		if err != nil {
+			t.Fatalf("Bridge.Run: %v", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("Bridge.Run did not terminate")
+	}
+}
+
+func TestBridgeTCPToSerial(t *testing.T) {
+	serial := newBlockingVirtualSerial()
+
+	clientConn, bridgeConn := net.Pipe()
+
+	bridge, err := NewBridge(serial, bridgeConn)
+	if err != nil {
+		t.Fatalf("NewBridge: %v", err)
+	}
+
+	runDone := make(chan error, 1)
+	go func() {
+		runDone <- bridge.Run()
+	}()
+
+	want := []byte{0x46, 0x41, 0x00, 0x10, 0x0D}
+
+	if _, err := clientConn.Write(want); err != nil {
+		t.Fatalf("write TCP data: %v", err)
+	}
+
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		if bytes.Equal(serial.Written(), want) {
+			break
+		}
+
+		if time.Now().After(deadline) {
+			t.Fatalf(
+				"serial received % X, want % X",
+				serial.Written(),
+				want,
+			)
+		}
+
+		time.Sleep(1 * time.Millisecond)
+	}
+
+	_ = clientConn.Close()
+
+	select {
+	case err := <-runDone:
+		if err != nil {
+			t.Fatalf("Bridge.Run: %v", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("Bridge.Run did not terminate")
+	}
+}

internal/client/connection.go

@@ -0,0 +1,188 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/connection.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Aufbau einer Verbindung zwischen einem lokalen virtuellen seriellen Port
+ * und dem zugehörigen entfernten Gerät.
+ *
+ * Die konfigurierte lokale virtuelle Schnittstelle wird dabei exakt
+ * übernommen.
+ *
+ * Optional können nach dem Aufbau der Datenverbindung konfigurierte
+ * Startbytes an das entfernte Gerät gesendet werden.
+ * ============================================================================
+ */
+package client
+
+import (
+	"encoding/hex"
+	"fmt"
+	"net"
+	"strings"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/transport"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Virtual port connection
+///////////////////////////////////////////////////////////////////////////////
+
+// connectVirtualPort opens the configured local virtual port and the
+// corresponding remote TCP data connection and returns both endpoints.
+//
+// The caller owns the returned virtual port and TCP connection.
+//
+// The local virtual port is opened using exactly the port number specified
+// in virtualPortConfig.Port. This preserves the user's configured mapping
+// between local virtual interfaces and remote devices.
+//
+// If StartupBytes are configured, they are sent once immediately after the
+// TCP data connection has been established. Invalid startup bytes do not
+// abort the connection setup. A warning is printed and the connection
+// continues normally.
+func connectVirtualPort(
+	c *Client,
+	manager *VirtualPortManager,
+	virtualPortConfig config.VirtualPortConfig,
+	devices []transport.RemoteDeviceInfo,
+) (*ManagedVirtualPort, net.Conn, error) {
+	if c == nil || c.conn == nil {
+		return nil, nil, fmt.Errorf("client is not connected")
+	}
+
+	if manager == nil {
+		return nil, nil, fmt.Errorf("virtual port manager is nil")
+	}
+
+	device, err := findRemoteDevice(virtualPortConfig, devices)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	portNumber, err := virtualPortNumber(virtualPortConfig.Port)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	virtualPort, err := manager.OpenSpecific(portNumber)
+	if err != nil {
+		return nil, nil, fmt.Errorf(
+			"open virtual port %q for %q: %w",
+			virtualPortConfig.Port,
+			virtualPortConfig.RemoteDevice,
+			err,
+		)
+	}
+
+	conn, err := c.OpenDataConnection(device)
+	if err != nil {
+		_ = virtualPort.Close()
+
+		return nil, nil, fmt.Errorf(
+			"open data connection for %q: %w",
+			device.ID,
+			err,
+		)
+	}
+
+	if err := sendStartupBytes(conn, virtualPortConfig); err != nil {
+		fmt.Printf(
+			"warning: startup bytes for %q were not sent: %v\n",
+			virtualPortConfig.RemoteDevice,
+			err,
+		)
+	}
+
+	return virtualPort, conn, nil
+}
+
+// sendStartupBytes sends the optional configured startup byte sequence over
+// the already established TCP data connection.
+//
+// An empty StartupBytes configuration does not send anything and is not an
+// error.
+//
+// The configured value may contain spaces between hexadecimal bytes, for
+// example:
+//
+//	57 00 00 00 00 00 00 00 00 00 00 1F 20
+func sendStartupBytes(
+	conn net.Conn,
+	virtualPortConfig config.VirtualPortConfig,
+) error {
+	startupBytes := strings.TrimSpace(virtualPortConfig.StartupBytes)
+	if startupBytes == "" {
+		return nil
+	}
+
+	hexText := strings.Join(strings.Fields(startupBytes), "")
+
+	data, err := hex.DecodeString(hexText)
+	if err != nil {
+		return fmt.Errorf("invalid hexadecimal byte sequence: %w", err)
+	}
+
+	if len(data) == 0 {
+		return nil
+	}
+
+	written, err := conn.Write(data)
+	if err != nil {
+		return fmt.Errorf("send startup bytes: %w", err)
+	}
+
+	if written != len(data) {
+		return fmt.Errorf(
+			"send startup bytes: wrote %d of %d bytes",
+			written,
+			len(data),
+		)
+	}
+
+	return nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Bridge connection
+///////////////////////////////////////////////////////////////////////////////
+
+// connectBridge creates the local virtual port, opens the corresponding
+// remote TCP data connection and creates the data bridge.
+//
+// The caller owns the returned bridge and its endpoints.
+func connectBridge(
+	c *Client,
+	manager *VirtualPortManager,
+	virtualPortConfig config.VirtualPortConfig,
+	devices []transport.RemoteDeviceInfo,
+) (*Bridge, *ManagedVirtualPort, net.Conn, error) {
+	virtualPort, conn, err := connectVirtualPort(
+		c,
+		manager,
+		virtualPortConfig,
+		devices,
+	)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	bridge, err := NewBridge(virtualPort, conn)
+	if err != nil {
+		_ = conn.Close()
+		_ = virtualPort.Close()
+
+		return nil, nil, nil, fmt.Errorf(
+			"create bridge for %q: %w",
+			virtualPortConfig.RemoteDevice,
+			err,
+		)
+	}
+
+	return bridge, virtualPort, conn, nil
+}

internal/client/connection_test.go

@@ -0,0 +1,525 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/connection_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für den Aufbau einer Verbindung zwischen einem lokalen virtuellen
+ * seriellen Port und dem zugehörigen entfernten TCP-Gerät.
+ *
+ * Die Tests prüfen insbesondere die Auswahl des Remote-Geräts, die Erzeugung
+ * des virtuellen Ports, den Aufbau der TCP-Datenverbindung und die Verbindung
+ * dieser Endpunkte über eine Bridge.
+ * ============================================================================
+ */
+package client
+
+import (
+	"net"
+	"strconv"
+	"strings"
+	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/transport"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Test connection
+///////////////////////////////////////////////////////////////////////////////
+
+// testRemoteAddrConn wraps a net.Conn and provides a TCP-style remote address.
+//
+// net.Pipe() is used for the control connection in these tests. Its default
+// RemoteAddr() is "pipe", which cannot be used by OpenDataConnection() to
+// determine the server host. This wrapper supplies a normal TCP address.
+type testRemoteAddrConn struct {
+	net.Conn
+	remoteAddr net.Addr
+}
+
+// RemoteAddr returns the TCP-style remote address configured for the test.
+func (c *testRemoteAddrConn) RemoteAddr() net.Addr {
+	return c.remoteAddr
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Test helpers
+///////////////////////////////////////////////////////////////////////////////
+
+// startTestDataServer starts a local TCP listener for a simulated remote
+// device.
+//
+// The listener accepts exactly one connection. The accepted connection is
+// returned through the channel so the test can verify that the client has
+// successfully established the data connection.
+func startTestDataServer(t *testing.T) (string, <-chan net.Conn) {
+	t.Helper()
+
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen for test data server: %v", err)
+	}
+
+	t.Cleanup(func() {
+		_ = listener.Close()
+	})
+
+	accepted := make(chan net.Conn, 1)
+
+	go func() {
+		conn, err := listener.Accept()
+		if err != nil {
+			return
+		}
+
+		accepted <- conn
+	}()
+
+	return listener.Addr().String(), accepted
+}
+
+// remoteDeviceFromAddress creates a RemoteDeviceInfo whose DataPort is
+// taken from the supplied local TCP address.
+func remoteDeviceFromAddress(
+	t *testing.T,
+	id string,
+	address string,
+) transport.RemoteDeviceInfo {
+	t.Helper()
+
+	_, portText, err := net.SplitHostPort(address)
+	if err != nil {
+		t.Fatalf("split test server address %q: %v", address, err)
+	}
+
+	port, err := strconv.Atoi(portText)
+	if err != nil {
+		t.Fatalf("parse test server port %q: %v", portText, err)
+	}
+
+	return transport.RemoteDeviceInfo{
+		ID:       id,
+		Name:     id,
+		DataPort: port,
+	}
+}
+
+// newTestClient creates a Client whose control connection uses net.Pipe()
+// but reports a normal TCP-style remote address.
+func newTestClient(t *testing.T) *Client {
+	t.Helper()
+
+	controlClient, serverConn := net.Pipe()
+
+	t.Cleanup(func() {
+		_ = controlClient.Close()
+		_ = serverConn.Close()
+	})
+
+	return &Client{
+		conn: &testRemoteAddrConn{
+			Conn: controlClient,
+			remoteAddr: &net.TCPAddr{
+				IP:   net.ParseIP("127.0.0.1"),
+				Port: 5000,
+			},
+		},
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestConnectVirtualPortNilClient(t *testing.T) {
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	cfg := config.VirtualPortConfig{
+		Port:         virtualPortPath(100),
+		RemoteDevice: "radio",
+	}
+
+	devices := []transport.RemoteDeviceInfo{
+		{
+			ID:       "radio",
+			DataPort: 50123,
+		},
+	}
+
+	_, _, err := connectVirtualPort(
+		nil,
+		manager,
+		cfg,
+		devices,
+	)
+	if err == nil {
+		t.Fatal("expected error for nil client")
+	}
+
+	if !strings.Contains(err.Error(), "client is not connected") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestConnectVirtualPortNilManager(t *testing.T) {
+	cfg := config.VirtualPortConfig{
+		Port:         virtualPortPath(100),
+		RemoteDevice: "radio",
+	}
+
+	devices := []transport.RemoteDeviceInfo{
+		{
+			ID:       "radio",
+			DataPort: 50123,
+		},
+	}
+
+	_, _, err := connectVirtualPort(
+		&Client{},
+		nil,
+		cfg,
+		devices,
+	)
+	if err == nil {
+		t.Fatal("expected error for nil manager")
+	}
+
+	if !strings.Contains(err.Error(), "client is not connected") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestConnectVirtualPortUnknownDevice(t *testing.T) {
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	client := newTestClient(t)
+
+	cfg := config.VirtualPortConfig{
+		Port:         virtualPortPath(100),
+		RemoteDevice: "unknown",
+	}
+
+	devices := []transport.RemoteDeviceInfo{
+		{
+			ID:       "radio",
+			DataPort: 50123,
+		},
+	}
+
+	_, _, err := connectVirtualPort(
+		client,
+		manager,
+		cfg,
+		devices,
+	)
+	if err == nil {
+		t.Fatal("expected error for unknown device")
+	}
+
+	if !strings.Contains(
+		err.Error(),
+		`remote device "unknown" not found`,
+	) {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestConnectVirtualPortClosesPortWhenDataConnectionFails(t *testing.T) {
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  100,
+		},
+	)
+
+	client := newTestClient(t)
+
+	cfg := config.VirtualPortConfig{
+		Port:         virtualPortPath(100),
+		RemoteDevice: "radio",
+	}
+
+	devices := []transport.RemoteDeviceInfo{
+		{
+			ID:       "radio",
+			DataPort: 1,
+		},
+	}
+
+	port, conn, err := connectVirtualPort(
+		client,
+		manager,
+		cfg,
+		devices,
+	)
+
+	if err == nil {
+		if conn != nil {
+			_ = conn.Close()
+		}
+
+		if port != nil {
+			_ = port.Close()
+		}
+
+		t.Fatal("expected data connection error")
+	}
+
+	if port != nil {
+		t.Fatal(
+			"virtual port must not be returned after connection failure",
+		)
+	}
+
+	if conn != nil {
+		t.Fatal(
+			"TCP connection must not be returned after connection failure",
+		)
+	}
+
+	// Port 100 must have been released after the failed connection.
+	portNumber, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("Reserve after cleanup: %v", err)
+	}
+
+	if portNumber != 100 {
+		t.Fatalf("reserved port = %d, want 100", portNumber)
+	}
+
+	manager.Release(portNumber)
+}
+
+func TestConnectVirtualPortSuccess(t *testing.T) {
+	dataAddress, accepted := startTestDataServer(t)
+
+	device := remoteDeviceFromAddress(
+		t,
+		"radio",
+		dataAddress,
+	)
+
+	client := newTestClient(t)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	cfg := config.VirtualPortConfig{
+		Port:         virtualPortPath(100),
+		RemoteDevice: "radio",
+	}
+
+	virtualPort, dataConn, err := connectVirtualPort(
+		client,
+		manager,
+		cfg,
+		[]transport.RemoteDeviceInfo{device},
+	)
+	if err != nil {
+		t.Fatalf("connectVirtualPort: %v", err)
+	}
+
+	if virtualPort == nil {
+		t.Fatal("virtual port is nil")
+	}
+
+	if dataConn == nil {
+		t.Fatal("data connection is nil")
+	}
+
+	defer virtualPort.Close()
+	defer dataConn.Close()
+
+	if virtualPort.Path() != virtualPortPath(100) {
+		t.Fatalf(
+			"virtual port path = %q, want %q",
+			virtualPort.Path(),
+			virtualPortPath(100),
+		)
+	}
+
+	// The test data server must receive the connection created by
+	// OpenDataConnection().
+	select {
+	case conn := <-accepted:
+		_ = conn.Close()
+
+	case <-t.Context().Done():
+		t.Fatal("test context cancelled while waiting for data connection")
+	}
+}
+
+func TestConnectVirtualPortUsesConfiguredPort(t *testing.T) {
+	dataAddress, accepted := startTestDataServer(t)
+
+	device := remoteDeviceFromAddress(
+		t,
+		"radio",
+		dataAddress,
+	)
+
+	client := newTestClient(t)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	cfg := config.VirtualPortConfig{
+		Port:         virtualPortPath(100),
+		RemoteDevice: "radio",
+	}
+
+	virtualPort, dataConn, err := connectVirtualPort(
+		client,
+		manager,
+		cfg,
+		[]transport.RemoteDeviceInfo{device},
+	)
+	if err != nil {
+		t.Fatalf("connectVirtualPort: %v", err)
+	}
+
+	if virtualPort == nil {
+		t.Fatal("virtual port is nil")
+	}
+
+	if dataConn == nil {
+		t.Fatal("data connection is nil")
+	}
+
+	defer virtualPort.Close()
+	defer dataConn.Close()
+
+	if virtualPort.Path() != virtualPortPath(100) {
+		t.Fatalf(
+			"virtual port path = %q, want %q",
+			virtualPort.Path(),
+			virtualPortPath(100),
+		)
+	}
+
+	// Port 100 is the configured port and therefore must be reserved.
+	if !manager.used[100] {
+		t.Fatal("configured port 100 is not reserved")
+	}
+
+	// Port 101 must still be available. This proves that the connection
+	// did not simply take another free port.
+	number, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("Reserve() after connection: %v", err)
+	}
+
+	if number != 101 {
+		t.Fatalf(
+			"first free port after connection = %d, want 101",
+			number,
+		)
+	}
+
+	manager.Release(number)
+
+	select {
+	case conn := <-accepted:
+		_ = conn.Close()
+
+	case <-t.Context().Done():
+		t.Fatal("test context cancelled while waiting for data connection")
+	}
+}
+
+func TestConnectBridgeSuccess(t *testing.T) {
+	dataAddress, accepted := startTestDataServer(t)
+
+	device := remoteDeviceFromAddress(
+		t,
+		"radio",
+		dataAddress,
+	)
+
+	client := newTestClient(t)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	cfg := config.VirtualPortConfig{
+		Port:         virtualPortPath(100),
+		RemoteDevice: "radio",
+	}
+
+	bridge, virtualPort, dataConn, err := connectBridge(
+		client,
+		manager,
+		cfg,
+		[]transport.RemoteDeviceInfo{device},
+	)
+	if err != nil {
+		t.Fatalf("connectBridge: %v", err)
+	}
+
+	if bridge == nil {
+		t.Fatal("bridge is nil")
+	}
+
+	if virtualPort == nil {
+		t.Fatal("virtual port is nil")
+	}
+
+	if dataConn == nil {
+		t.Fatal("data connection is nil")
+	}
+
+	defer virtualPort.Close()
+	defer dataConn.Close()
+
+	if bridge.serial != virtualPort {
+		t.Fatal("bridge serial endpoint does not match virtual port")
+	}
+
+	if bridge.conn != dataConn {
+		t.Fatal("bridge TCP endpoint does not match data connection")
+	}
+
+	if virtualPort.Path() != virtualPortPath(100) {
+		t.Fatalf(
+			"virtual port path = %q, want %q",
+			virtualPort.Path(),
+			virtualPortPath(100),
+		)
+	}
+
+	// The test data server must receive the TCP data connection created by
+	// connectBridge().
+	select {
+	case conn := <-accepted:
+		_ = conn.Close()
+
+	case <-t.Context().Done():
+		t.Fatal("test context cancelled while waiting for data connection")
+	}
+}

internal/client/control.go

@@ -1,7 +1,7 @@
 /*
  * ============================================================================
  * Projekt.....: rs2322tcp
- * Datei.......: control.go
+ * Datei.......: internal/client/control.go
  * Copyright (C) 2026 Dieter Lang
  *
  * SPDX-License-Identifier: GPL-3.0-or-later
@@ -18,6 +18,7 @@ import (
 	"bufio"
 	"fmt"
 	"net"
+	"sync"
 
 	"git.lang-dieter.de/rs2322tcp/internal/transport"
 )
@@ -28,6 +29,8 @@ import (
 
 // Client represents a connection to an rs2322tcp server.
 type Client struct {
+	mu sync.Mutex
+
 	conn   net.Conn
 	reader *bufio.Reader
 }
@@ -79,6 +82,9 @@ func (c *Client) Conn() net.Conn {
 		return nil
 	}
 
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
 	return c.conn
 }
 
@@ -87,12 +93,21 @@ func (c *Client) Conn() net.Conn {
 ///////////////////////////////////////////////////////////////////////////////
 
 func (c *Client) hello() error {
-	if c == nil || c.conn == nil {
+	if c == nil {
+		return fmt.Errorf("client is not connected")
+	}
+
+	c.mu.Lock()
+	conn := c.conn
+	reader := c.reader
+	c.mu.Unlock()
+
+	if conn == nil {
 		return fmt.Errorf("client is not connected")
 	}
 
 	if err := transport.WriteMessage(
-		c.conn,
+		conn,
 		transport.NewHello(),
 	); err != nil {
 		return fmt.Errorf("send hello: %w", err)
@@ -101,7 +116,7 @@ func (c *Client) hello() error {
 	var response transport.HelloResponseMessage
 
 	if err := transport.ReadMessage(
-		c.reader,
+		reader,
 		&response,
 	); err != nil {
 		return fmt.Errorf("read hello response: %w", err)
@@ -125,24 +140,39 @@ func (c *Client) hello() error {
 //
 // The returned list contains the session-specific dynamic TCP data ports.
 func (c *Client) GetDevices() ([]transport.RemoteDeviceInfo, error) {
-	if c == nil || c.conn == nil {
+	if c == nil {
+		return nil, fmt.Errorf("client is not connected")
+	}
+
+	c.mu.Lock()
+	conn := c.conn
+	reader := c.reader
+	c.mu.Unlock()
+
+	if conn == nil {
 		return nil, fmt.Errorf("client is not connected")
 	}
 
 	if err := transport.WriteMessage(
-		c.conn,
+		conn,
 		transport.NewGetDevices(),
 	); err != nil {
-		return nil, fmt.Errorf("send get-devices request: %w", err)
+		return nil, fmt.Errorf(
+			"send get-devices request: %w",
+			err,
+		)
 	}
 
 	var response transport.DeviceListMessage
 
 	if err := transport.ReadMessage(
-		c.reader,
+		reader,
 		&response,
 	); err != nil {
-		return nil, fmt.Errorf("read device list: %w", err)
+		return nil, fmt.Errorf(
+			"read device list: %w",
+			err,
+		)
 	}
 
 	if response.Type != transport.MessageDeviceList {
@@ -160,13 +190,25 @@ func (c *Client) GetDevices() ([]transport.RemoteDeviceInfo, error) {
 ///////////////////////////////////////////////////////////////////////////////
 
 // Close closes the client control connection.
+//
+// The connection is removed from the Client while holding the mutex.
+// The actual network close is performed afterwards so that another caller
+// cannot race with the state change.
 func (c *Client) Close() error {
-	if c == nil || c.conn == nil {
+	if c == nil {
 		return nil
 	}
 
-	err := c.conn.Close()
+	c.mu.Lock()
+
+	conn := c.conn
 	c.conn = nil
 
-	return err
+	c.mu.Unlock()
+
+	if conn == nil {
+		return nil
+	}
+
+	return conn.Close()
 }

internal/client/device.go

@@ -0,0 +1,70 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/device.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Zuordnung eines lokalen virtuellen Ports zu einem vom Server gemeldeten
+ * Remote-Gerät.
+ * ============================================================================
+ */
+package client
+
+import (
+	"fmt"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/transport"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Remote device lookup
+///////////////////////////////////////////////////////////////////////////////
+
+// findRemoteDevice finds the server-provided device information matching
+// the remote device ID configured for a local virtual port.
+//
+// The caller uses the returned RemoteDeviceInfo to open the session-specific
+// TCP data connection.
+func findRemoteDevice(
+	virtualPort config.VirtualPortConfig,
+	devices []transport.RemoteDeviceInfo,
+) (transport.RemoteDeviceInfo, error) {
+	if virtualPort.RemoteDevice == "" {
+		return transport.RemoteDeviceInfo{}, fmt.Errorf(
+			"remote device is empty for virtual port %q",
+			virtualPort.Port,
+		)
+	}
+
+	var found *transport.RemoteDeviceInfo
+
+	for i := range devices {
+		device := &devices[i]
+
+		if device.ID != virtualPort.RemoteDevice {
+			continue
+		}
+
+		if found != nil {
+			return transport.RemoteDeviceInfo{}, fmt.Errorf(
+				"remote device %q is ambiguous",
+				virtualPort.RemoteDevice,
+			)
+		}
+
+		found = device
+	}
+
+	if found == nil {
+		return transport.RemoteDeviceInfo{}, fmt.Errorf(
+			"remote device %q not found",
+			virtualPort.RemoteDevice,
+		)
+	}
+
+	return *found, nil
+}

internal/client/device_test.go

@@ -0,0 +1,159 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/device_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für die Zuordnung lokaler virtueller Ports zu den vom Server
+ * gemeldeten Remote-Geräten.
+ * ============================================================================
+ */
+package client
+
+import (
+	"strings"
+	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/transport"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Test data
+///////////////////////////////////////////////////////////////////////////////
+
+func testRemoteDevices() []transport.RemoteDeviceInfo {
+	return []transport.RemoteDeviceInfo{
+		{
+			ID:       "radio",
+			Name:     "Radio",
+			BaudRate: 9600,
+			DataBits: 8,
+			Parity:   "none",
+			StopBits: 1,
+			DataPort: 50123,
+		},
+		{
+			ID:       "rotor",
+			Name:     "Rotor",
+			BaudRate: 4800,
+			DataBits: 8,
+			Parity:   "none",
+			StopBits: 1,
+			DataPort: 50124,
+		},
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestFindRemoteDevice(t *testing.T) {
+	devices := testRemoteDevices()
+
+	virtualPort := config.VirtualPortConfig{
+		Port:         "/dev/ttyUSB100",
+		RemoteDevice: "radio",
+	}
+
+	got, err := findRemoteDevice(virtualPort, devices)
+	if err != nil {
+		t.Fatalf("findRemoteDevice: %v", err)
+	}
+
+	if got.ID != "radio" {
+		t.Fatalf("ID = %q, want %q", got.ID, "radio")
+	}
+
+	if got.DataPort != 50123 {
+		t.Fatalf("DataPort = %d, want %d", got.DataPort, 50123)
+	}
+}
+
+func TestFindRemoteDeviceRotor(t *testing.T) {
+	devices := testRemoteDevices()
+
+	virtualPort := config.VirtualPortConfig{
+		Port:         "/dev/ttyUSB101",
+		RemoteDevice: "rotor",
+	}
+
+	got, err := findRemoteDevice(virtualPort, devices)
+	if err != nil {
+		t.Fatalf("findRemoteDevice: %v", err)
+	}
+
+	if got.ID != "rotor" {
+		t.Fatalf("ID = %q, want %q", got.ID, "rotor")
+	}
+
+	if got.DataPort != 50124 {
+		t.Fatalf("DataPort = %d, want %d", got.DataPort, 50124)
+	}
+}
+
+func TestFindRemoteDeviceEmptyID(t *testing.T) {
+	devices := testRemoteDevices()
+
+	virtualPort := config.VirtualPortConfig{
+		Port: "/dev/ttyUSB100",
+	}
+
+	_, err := findRemoteDevice(virtualPort, devices)
+	if err == nil {
+		t.Fatal("expected error for empty remote device")
+	}
+
+	if !strings.Contains(err.Error(), "remote device is empty") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestFindRemoteDeviceNotFound(t *testing.T) {
+	devices := testRemoteDevices()
+
+	virtualPort := config.VirtualPortConfig{
+		Port:         "/dev/ttyUSB100",
+		RemoteDevice: "unknown",
+	}
+
+	_, err := findRemoteDevice(virtualPort, devices)
+	if err == nil {
+		t.Fatal("expected error for unknown remote device")
+	}
+
+	if !strings.Contains(err.Error(), `remote device "unknown" not found`) {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestFindRemoteDeviceAmbiguous(t *testing.T) {
+	devices := []transport.RemoteDeviceInfo{
+		{
+			ID:       "radio",
+			DataPort: 50123,
+		},
+		{
+			ID:       "radio",
+			DataPort: 50125,
+		},
+	}
+
+	virtualPort := config.VirtualPortConfig{
+		Port:         "/dev/ttyUSB100",
+		RemoteDevice: "radio",
+	}
+
+	_, err := findRemoteDevice(virtualPort, devices)
+	if err == nil {
+		t.Fatal("expected error for ambiguous remote device")
+	}
+
+	if !strings.Contains(err.Error(), `remote device "radio" is ambiguous`) {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}

internal/client/integration_test.go

@@ -29,6 +29,16 @@ import (
 	"git.lang-dieter.de/rs2322tcp/internal/server"
 )
 
+///////////////////////////////////////////////////////////////////////////////
+// Test configuration
+///////////////////////////////////////////////////////////////////////////////
+
+const testHardwareErrorResponse = "TEST ERROR RESPONSE"
+
+///////////////////////////////////////////////////////////////////////////////
+// Virtual serial pair
+///////////////////////////////////////////////////////////////////////////////
+
 func startVirtualSerialPair(t *testing.T) (string, string, func()) {
 	t.Helper()
 
@@ -79,6 +89,10 @@ func startVirtualSerialPair(t *testing.T) (string, string, func()) {
 	}
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// Read helper
+///////////////////////////////////////////////////////////////////////////////
+
 func readExactWithTimeout(
 	t *testing.T,
 	reader io.Reader,
@@ -105,6 +119,10 @@ func readExactWithTimeout(
 	}
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// Integration test
+///////////////////////////////////////////////////////////////////////////////
+
 func TestClientServerSerialIntegration(t *testing.T) {
 	serialA, serialB, cleanup := startVirtualSerialPair(t)
 	defer cleanup()
@@ -130,6 +148,7 @@ func TestClientServerSerialIntegration(t *testing.T) {
 			Address: "127.0.0.1",
 			Port:    controlPort,
 		},
+		HardwareErrorResponse: testHardwareErrorResponse,
 		Devices: []config.DeviceConfig{
 			{
 				ID:         "radio",

internal/client/pty_linux.go

@@ -1,19 +1,51 @@
 //go:build linux
 
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/pty_linux.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Linux-spezifische Implementierung der virtuellen seriellen Schnittstelle
+ * über ein PTY-Paar.
+ *
+ * Der PTY-Master wird intern vom Client verwendet. Der zugehörige PTY-Slave
+ * wird über den virtuellen Port als serielle Schnittstelle bereitgestellt.
+ * ============================================================================
+ */
+
 package client
 
 import (
 	"fmt"
+	"sync/atomic"
 
 	"golang.org/x/sys/unix"
 )
 
+///////////////////////////////////////////////////////////////////////////////
+// ptySerial
+///////////////////////////////////////////////////////////////////////////////
+
+// ptySerial implements VirtualSerial using a Linux PTY pair.
+//
+// The file descriptors are stored atomically because Read/Write may execute
+// concurrently with Close. Close must be able to invalidate the descriptors
+// without racing with a concurrent Read or Write.
 type ptySerial struct {
-	masterFD int
+	masterFD atomic.Int64
-	slaveFD  int
+	slaveFD  atomic.Int64
 	path     string
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// Constructor
+///////////////////////////////////////////////////////////////////////////////
+
+// newVirtualSerial creates a new Linux PTY pair.
 func newVirtualSerial() (VirtualSerial, error) {
 	masterFD, err := unix.Open(
 		"/dev/ptmx",
@@ -60,7 +92,11 @@ func newVirtualSerial() (VirtualSerial, error) {
 		0,
 	)
 	if err != nil {
-		return nil, fmt.Errorf("open PTY slave %s: %w", path, err)
+		return nil, fmt.Errorf(
+			"open PTY slave %s: %w",
+			path,
+			err,
+		)
 	}
 
 	closeSlave := true
@@ -74,16 +110,23 @@ func newVirtualSerial() (VirtualSerial, error) {
 		return nil, err
 	}
 
+	serial := &ptySerial{
+		path: path,
+	}
+
+	serial.masterFD.Store(int64(masterFD))
+	serial.slaveFD.Store(int64(slaveFD))
+
 	closeMaster = false
 	closeSlave = false
 
-	return &ptySerial{
+	return serial, nil
-		masterFD: masterFD,
-		slaveFD:  slaveFD,
-		path:     path,
-	}, nil
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// PTY configuration
+///////////////////////////////////////////////////////////////////////////////
+
 func configurePTY(fd int) error {
 	termios, err := unix.IoctlGetTermios(
 		fd,
@@ -134,33 +177,80 @@ func configurePTY(fd int) error {
 	return nil
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// VirtualSerial implementation
+///////////////////////////////////////////////////////////////////////////////
+
+// Path returns the PTY slave path.
 func (p *ptySerial) Path() string {
 	return p.path
 }
 
+// Read reads raw bytes from the PTY master.
 func (p *ptySerial) Read(b []byte) (int, error) {
-	return unix.Read(p.masterFD, b)
+	fd := int(p.masterFD.Load())
+
+	if fd < 0 {
+		return 0, fmt.Errorf("PTY master is closed")
+	}
+
+	return unix.Read(fd, b)
 }
 
+// Write writes raw bytes to the PTY master.
 func (p *ptySerial) Write(b []byte) (int, error) {
-	return unix.Write(p.masterFD, b)
+	fd := int(p.masterFD.Load())
+
+	if fd < 0 {
+		return 0, fmt.Errorf("PTY master is closed")
+	}
+
+	fmt.Printf(
+		"PTY TX [%s]: % X\n",
+		p.path,
+		b,
+	)
+
+	n, err := unix.Write(fd, b)
+
+	if err != nil {
+		fmt.Printf(
+			"PTY TX Fehler [%s]: %v\n",
+			p.path,
+			err,
+		)
+		return n, err
+	}
+
+	fmt.Printf(
+		"PTY TX erfolgreich [%s]: %d Bytes\n",
+		p.path,
+		n,
+	)
+
+	return n, nil
 }
 
+// Close closes the PTY master and slave.
+//
+// The descriptors are invalidated atomically before closing them so that
+// concurrent Read or Write calls do not access the descriptor fields
+// concurrently with Close.
 func (p *ptySerial) Close() error {
 	var firstErr error
 
-	if p.masterFD >= 0 {
+	masterFD := int(p.masterFD.Swap(-1))
-		if err := unix.Close(p.masterFD); err != nil {
+	if masterFD >= 0 {
+		if err := unix.Close(masterFD); err != nil {
 			firstErr = err
 		}
-		p.masterFD = -1
 	}
 
-	if p.slaveFD >= 0 {
+	slaveFD := int(p.slaveFD.Swap(-1))
-		if err := unix.Close(p.slaveFD); err != nil && firstErr == nil {
+	if slaveFD >= 0 {
+		if err := unix.Close(slaveFD); err != nil && firstErr == nil {
 			firstErr = err
 		}
-		p.slaveFD = -1
 	}
 
 	return firstErr

internal/client/runtime.go

@@ -0,0 +1,220 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/runtime.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Clientseitige Laufzeitverwaltung für die konfigurierten virtuellen
+ * seriellen Schnittstellen.
+ *
+ * Die Runtime verbindet die in der Client-Konfiguration angegebenen
+ * virtuellen Ports mit den zugehörigen entfernten Geräten.
+ *
+ * Für jedes konfigurierte virtuelle Gerät wird eine eigene Bridge gestartet:
+ *
+ *     /dev/ttyUSBxxx ─── Bridge ─── TCP Data Connection
+ *
+ * Die Control-Verbindung zum Server wird gemeinsam von allen Bridges
+ * verwendet.
+ * ============================================================================
+ */
+package client
+
+import (
+	"fmt"
+	"sync"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Runtime
+///////////////////////////////////////////////////////////////////////////////
+
+// Runtime manages the client-side virtual serial connections.
+//
+// A Runtime owns the Client control connection and the VirtualPortManager.
+// Each configured virtual port is represented by one running Bridge.
+type Runtime struct {
+	client  *Client
+	manager *VirtualPortManager
+
+	mu      sync.Mutex
+	bridges []*Bridge
+}
+
+// NewRuntime creates a new client Runtime.
+//
+// The Runtime does not establish a server connection and does not open any
+// virtual ports until Run is called.
+func NewRuntime(
+	client *Client,
+	manager *VirtualPortManager,
+) (*Runtime, error) {
+	if client == nil {
+		return nil, fmt.Errorf("client is nil")
+	}
+
+	if manager == nil {
+		return nil, fmt.Errorf("virtual port manager is nil")
+	}
+
+	return &Runtime{
+		client:  client,
+		manager: manager,
+	}, nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Run
+///////////////////////////////////////////////////////////////////////////////
+
+// Run starts all configured virtual serial connections.
+//
+// The supplied configuration determines which local virtual ports are
+// assigned to which remote devices.
+//
+// Run blocks until one bridge terminates or an error occurs while creating
+// one of the configured bridges.
+func (r *Runtime) Run(
+	cfg config.ClientConfig,
+) error {
+	if r == nil || r.client == nil {
+		return fmt.Errorf("runtime is not initialized")
+	}
+
+	if r.manager == nil {
+		return fmt.Errorf("virtual port manager is nil")
+	}
+
+	if len(cfg.VirtualPorts) == 0 {
+		return nil
+	}
+	devices, err := r.client.GetDevices()
+	if err != nil {
+		return fmt.Errorf("get remote devices: %w", err)
+	}
+
+	for _, virtualPortConfig := range cfg.VirtualPorts {
+		if virtualPortConfig.RemoteDevice == "" {
+			continue
+		}
+
+		bridge, _, _, err := connectBridge(
+			r.client,
+			r.manager,
+			virtualPortConfig,
+			devices,
+		)
+		if err != nil {
+			r.Close()
+
+			return err
+		}
+
+		r.mu.Lock()
+		r.bridges = append(r.bridges, bridge)
+		r.mu.Unlock()
+	}
+
+	return r.waitForBridge()
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Bridge lifecycle
+///////////////////////////////////////////////////////////////////////////////
+
+// waitForBridge waits until one of the running bridges terminates.
+//
+// A bridge terminating is considered the end of the Runtime. The Bridge
+// itself closes its serial and TCP endpoints when Run returns.
+func (r *Runtime) waitForBridge() error {
+	results := make(chan error, len(r.bridges))
+
+	r.mu.Lock()
+	bridges := append([]*Bridge(nil), r.bridges...)
+	r.mu.Unlock()
+
+	for _, bridge := range bridges {
+		go func(b *Bridge) {
+			results <- b.Run()
+		}(bridge)
+	}
+
+	err := <-results
+
+	r.Close()
+
+	return err
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Close
+///////////////////////////////////////////////////////////////////////////////
+
+// Close closes all resources owned by the Runtime.
+//
+// Closing the bridge endpoints causes running Bridge.Run calls to terminate.
+// The client control connection is closed afterwards.
+//
+// Calling Close more than once is safe.
+
+func (r *Runtime) Close() error {
+	if r == nil {
+		return nil
+	}
+
+	r.mu.Lock()
+	bridges := append([]*Bridge(nil), r.bridges...)
+	r.bridges = nil
+	r.mu.Unlock()
+
+	var firstErr error
+
+	for _, bridge := range bridges {
+		if bridge == nil {
+			continue
+		}
+
+		if bridge.serial != nil {
+			if err := bridge.serial.Close(); err != nil && firstErr == nil {
+				firstErr = err
+			}
+		}
+
+		if bridge.conn != nil {
+			if err := bridge.conn.Close(); err != nil && firstErr == nil {
+				firstErr = err
+			}
+		}
+	}
+
+	if r.client != nil {
+		if err := r.client.Close(); err != nil && firstErr == nil {
+			firstErr = err
+		}
+	}
+
+	return firstErr
+}
+
+// Client returns the Client used by the Runtime.
+func (r *Runtime) Client() *Client {
+	if r == nil {
+		return nil
+	}
+
+	return r.client
+}
+
+// Manager returns the VirtualPortManager used by the Runtime.
+func (r *Runtime) Manager() *VirtualPortManager {
+	if r == nil {
+		return nil
+	}
+
+	return r.manager
+}

internal/client/runtime_integration_test.go

@@ -0,0 +1,281 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/runtime_integration_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Integrationstest für die clientseitige Runtime mit einem echten
+ * rs2322tcp-Server und einem virtuellen seriellen Client-Port.
+ *
+ * Der Test verwendet die vorhandene virtuelle serielle Test-Schnittstelle
+ * aus integration_test.go und prüft den vollständigen Datenweg:
+ *
+ *     /dev/ttyUSB100
+ *            ↕
+ *         Bridge
+ *            ↕
+ *     TCP Data Connection
+ *            ↕
+ *     rs2322tcp Server
+ *            ↕
+ *       serielle PTY
+ *
+ * Beide Übertragungsrichtungen werden geprüft.
+ * ============================================================================
+ */
+package client_test
+
+import (
+	"net"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"git.lang-dieter.de/rs2322tcp/internal/client"
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/server"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestClientRuntimeSerialIntegration(t *testing.T) {
+	serialA, serialB, cleanup := startVirtualSerialPair(t)
+	defer cleanup()
+
+	const virtualPortPath = "/dev/ttyUSB100"
+
+	home, err := os.UserHomeDir()
+	if err != nil {
+		t.Fatalf("get user home directory: %v", err)
+	}
+
+	internalPortPath := filepath.Join(
+		home,
+		".rs2322tcp",
+		"virtual",
+		"ttyUSB100",
+	)
+
+	////////////////////////////////////////////////////////////////////////////
+	// Server
+	////////////////////////////////////////////////////////////////////////////
+
+	controlProbe, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("find free control port: %v", err)
+	}
+
+	controlPort := controlProbe.Addr().(*net.TCPAddr).Port
+
+	if err := controlProbe.Close(); err != nil {
+		t.Fatalf("close control probe: %v", err)
+	}
+
+	serverConfig := &config.ServerConfig{
+		Listen: config.ListenConfig{
+			Address: "127.0.0.1",
+			Port:    controlPort,
+		},
+		HardwareErrorResponse: testHardwareErrorResponse,
+		Devices: []config.DeviceConfig{
+			{
+				ID:         "radio",
+				Name:       "Funkgerät",
+				SerialPort: serialA,
+				BaudRate:   9600,
+				DataBits:   8,
+				Parity:     "none",
+				StopBits:   1,
+			},
+		},
+	}
+
+	srv, err := server.NewControlServer(serverConfig)
+	if err != nil {
+		t.Fatalf("NewControlServer() failed: %v", err)
+	}
+
+	if err := srv.Listen(); err != nil {
+		t.Fatalf("server Listen() failed: %v", err)
+	}
+	defer srv.Close()
+
+	go func() {
+		_ = srv.Serve()
+	}()
+
+	////////////////////////////////////////////////////////////////////////////
+	// Client and Runtime
+	////////////////////////////////////////////////////////////////////////////
+
+	c, err := client.New(srv.Addr().String())
+	if err != nil {
+		t.Fatalf("client.New() failed: %v", err)
+	}
+	defer c.Close()
+
+	manager := client.NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	runtime, err := client.NewRuntime(c, manager)
+	if err != nil {
+		t.Fatalf("NewRuntime() failed: %v", err)
+	}
+	defer runtime.Close()
+
+	clientConfig := config.ClientConfig{
+		Server: config.ServerConnectionConfig{
+			Address: "127.0.0.1",
+			Port:    controlPort,
+		},
+		VirtualPorts: []config.VirtualPortConfig{
+			{
+				Port:         virtualPortPath,
+				RemoteDevice: "radio",
+			},
+		},
+	}
+
+	////////////////////////////////////////////////////////////////////////////
+	// Start Runtime
+	////////////////////////////////////////////////////////////////////////////
+
+	runtimeDone := make(chan error, 1)
+
+	go func() {
+		runtimeDone <- runtime.Run(clientConfig)
+	}()
+
+	////////////////////////////////////////////////////////////////////////////
+	// Open client-side virtual serial endpoint
+	////////////////////////////////////////////////////////////////////////////
+
+	var virtualSerial *os.File
+
+	deadline := time.Now().Add(2 * time.Second)
+
+	for {
+		virtualSerial, err = os.OpenFile(
+			internalPortPath,
+			os.O_RDWR,
+			0,
+		)
+		if err == nil {
+			break
+		}
+
+		if time.Now().After(deadline) {
+			t.Fatalf(
+				"timeout opening virtual serial port %s: %v",
+				internalPortPath,
+				err,
+			)
+		}
+
+		time.Sleep(10 * time.Millisecond)
+	}
+
+	defer virtualSerial.Close()
+
+	////////////////////////////////////////////////////////////////////////////
+	// Client virtual serial -> Bridge -> Server -> serial
+	////////////////////////////////////////////////////////////////////////////
+
+	clientMessage := []byte("hello from runtime")
+
+	if _, err := virtualSerial.Write(clientMessage); err != nil {
+		t.Fatalf(
+			"write to virtual serial port failed: %v",
+			err,
+		)
+	}
+
+	serialReceived := make([]byte, len(clientMessage))
+
+	serialPeer, err := os.OpenFile(
+		serialB,
+		os.O_RDWR,
+		0,
+	)
+	if err != nil {
+		t.Fatalf(
+			"open server serial peer: %v",
+			err,
+		)
+	}
+	defer serialPeer.Close()
+
+	readExactWithTimeout(
+		t,
+		serialPeer,
+		serialReceived,
+		2*time.Second,
+	)
+
+	if string(serialReceived) != string(clientMessage) {
+		t.Fatalf(
+			"serial received %q, want %q",
+			string(serialReceived),
+			string(clientMessage),
+		)
+	}
+
+	////////////////////////////////////////////////////////////////////////////
+	// Server serial -> Bridge -> Client virtual serial
+	////////////////////////////////////////////////////////////////////////////
+
+	serverMessage := []byte("hello from server")
+
+	if _, err := serialPeer.Write(serverMessage); err != nil {
+		t.Fatalf(
+			"write to server serial peer failed: %v",
+			err,
+		)
+	}
+
+	clientReceived := make([]byte, len(serverMessage))
+
+	readExactWithTimeout(
+		t,
+		virtualSerial,
+		clientReceived,
+		2*time.Second,
+	)
+
+	if string(clientReceived) != string(serverMessage) {
+		t.Fatalf(
+			"virtual serial received %q, want %q",
+			string(clientReceived),
+			string(serverMessage),
+		)
+	}
+
+	///////////////////////////////////////////////////////////////////////////////
+	// Terminate Runtime
+	///////////////////////////////////////////////////////////////////////////////
+
+	if err := virtualSerial.Close(); err != nil {
+		t.Fatalf("close virtual serial port: %v", err)
+	}
+
+	if err := runtime.Close(); err != nil {
+		t.Fatalf("close runtime: %v", err)
+	}
+
+	select {
+	case <-runtimeDone:
+		// Runtime termination is expected after Runtime.Close().
+	case <-time.After(2 * time.Second):
+		t.Fatal("runtime did not terminate after Runtime.Close()")
+	}
+}

internal/client/runtime_test.go

@@ -0,0 +1,82 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/runtime_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für die clientseitige Runtime.
+ *
+ * Die Tests prüfen die Initialisierung der Runtime und den grundlegenden
+ * Lebenszyklus ohne einen externen rs2322tcp-Server vorauszusetzen.
+ * ============================================================================
+ */
+package client
+
+import (
+	"strings"
+	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestNewRuntimeNilClient(t *testing.T) {
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	_, err := NewRuntime(nil, manager)
+	if err == nil {
+		t.Fatal("expected error for nil client")
+	}
+
+	if !strings.Contains(err.Error(), "client is nil") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestNewRuntimeNilManager(t *testing.T) {
+	client := newTestClient(t)
+
+	_, err := NewRuntime(client, nil)
+	if err == nil {
+		t.Fatal("expected error for nil manager")
+	}
+
+	if !strings.Contains(err.Error(), "virtual port manager is nil") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestRuntimeRunWithoutVirtualPorts(t *testing.T) {
+	client := newTestClient(t)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	runtime, err := NewRuntime(client, manager)
+	if err != nil {
+		t.Fatalf("NewRuntime: %v", err)
+	}
+
+	cfg := config.ClientConfig{
+		VirtualPorts: nil,
+	}
+
+	if err := runtime.Run(cfg); err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+}

internal/client/virtual_port.go

@@ -0,0 +1,70 @@
+/*
+Package client contains the client-side components of rs2322tcp.
+
+Project: rs2322tcp
+Module:  git.lang-dieter.de/rs2322tcp
+*/
+package client
+
+import "io"
+
+///////////////////////////////////////////////////////////////////////////////
+// Virtual port
+///////////////////////////////////////////////////////////////////////////////
+
+// VirtualPort represents one local virtual serial port.
+//
+// The visible path is the device name presented to the external application,
+// for example /dev/ttyUSB100.
+//
+// The underlying VirtualSerial remains an internal implementation detail and
+// may use a PTY such as /dev/pts/2.
+type VirtualPort struct {
+	path   string
+	serial VirtualSerial
+}
+
+// NewVirtualPort creates a virtual port with the specified visible path and
+// underlying virtual serial device.
+func NewVirtualPort(path string, serial VirtualSerial) *VirtualPort {
+	return &VirtualPort{
+		path:   path,
+		serial: serial,
+	}
+}
+
+// Path returns the device path visible to the external application.
+func (p *VirtualPort) Path() string {
+	if p == nil {
+		return ""
+	}
+
+	return p.path
+}
+
+// Read reads data from the underlying virtual serial device.
+func (p *VirtualPort) Read(b []byte) (int, error) {
+	if p == nil || p.serial == nil {
+		return 0, io.ErrClosedPipe
+	}
+
+	return p.serial.Read(b)
+}
+
+// Write writes data to the underlying virtual serial device.
+func (p *VirtualPort) Write(b []byte) (int, error) {
+	if p == nil || p.serial == nil {
+		return 0, io.ErrClosedPipe
+	}
+
+	return p.serial.Write(b)
+}
+
+// Close closes the underlying virtual serial device.
+func (p *VirtualPort) Close() error {
+	if p == nil || p.serial == nil {
+		return nil
+	}
+
+	return p.serial.Close()
+}

internal/client/virtual_port_linux.go

@@ -0,0 +1,149 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_linux.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Linux-spezifische Verwaltung der internen Symlinks für virtuelle
+ * rs2322tcp-Seriellschnittstellen.
+ *
+ * Der öffentliche Gerätename /dev/ttyUSBxxx wird bei der Installation
+ * vorbereitet. Der Client verwaltet ausschließlich den benutzerbezogenen
+ * Link unter ~/.rs2322tcp/virtual/.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+const (
+	virtualDirectoryName = ".rs2322tcp/virtual"
+)
+
+// virtualPortDirectory returns the directory in the current user's home
+// directory used for the internal virtual-port links.
+func virtualPortDirectory() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", fmt.Errorf("get user home directory: %w", err)
+	}
+
+	return filepath.Join(home, virtualDirectoryName), nil
+}
+
+// virtualPortLinkPath returns the path of the internal link for a virtual
+// serial port.
+func virtualPortLinkPath(portPath string) (string, error) {
+	directory, err := virtualPortDirectory()
+	if err != nil {
+		return "", err
+	}
+
+	return filepath.Join(directory, filepath.Base(portPath)), nil
+}
+
+// ensureVirtualPortDirectory creates the internal virtual-port directory
+// if it does not already exist.
+func ensureVirtualPortDirectory() error {
+	directory, err := virtualPortDirectory()
+	if err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(directory, 0755); err != nil {
+		return fmt.Errorf(
+			"create virtual port directory %q: %w",
+			directory,
+			err,
+		)
+	}
+
+	return nil
+}
+
+// setVirtualPortLink creates or replaces the internal link for a virtual
+// port.
+//
+// The link itself is located below ~/.rs2322tcp/virtual/ and points to the
+// current PTY, for example /dev/pts/2.
+//
+// The public /dev/ttyUSBxxx installation link is deliberately never
+// modified here.
+func setVirtualPortLink(portPath string, target string) error {
+	if portPath == "" {
+		return fmt.Errorf("virtual port path is empty")
+	}
+
+	if target == "" {
+		return fmt.Errorf("virtual port target is empty")
+	}
+
+	if err := ensureVirtualPortDirectory(); err != nil {
+		return err
+	}
+
+	linkPath, err := virtualPortLinkPath(portPath)
+	if err != nil {
+		return err
+	}
+
+	// Replace only the user-owned internal link.
+	//
+	// The installation-owned /dev/ttyUSBxxx link is deliberately never
+	// touched here.
+	if err := os.Remove(linkPath); err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf(
+			"remove existing virtual port link %q: %w",
+			linkPath,
+			err,
+		)
+	}
+
+	if err := os.Symlink(target, linkPath); err != nil {
+		return fmt.Errorf(
+			"create virtual port link %q -> %q: %w",
+			linkPath,
+			target,
+			err,
+		)
+	}
+
+	return nil
+}
+
+// removeVirtualPortLink removes the user-owned internal link for a virtual
+// port.
+//
+// This function is retained for explicit link-management operations and
+// tests. It is NOT called by ManagedVirtualPort.Close(), because the
+// installation-owned symlink chain must remain available.
+func removeVirtualPortLink(portPath string) error {
+	if portPath == "" {
+		return fmt.Errorf("virtual port path is empty")
+	}
+
+	linkPath, err := virtualPortLinkPath(portPath)
+	if err != nil {
+		return err
+	}
+
+	if err := os.Remove(linkPath); err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf(
+			"remove existing virtual port link %q: %w",
+			linkPath,
+			err,
+		)
+	}
+
+	return nil
+}

internal/client/virtual_port_linux_test.go

@@ -0,0 +1,348 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_linux_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für die Linux-spezifische Verwaltung der internen Symlinks
+ * virtueller rs2322tcp-Seriellschnittstellen.
+ * ============================================================================
+ */
+package client
+
+import (
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+func TestVirtualPortLinkPath(t *testing.T) {
+	path, err := virtualPortLinkPath("/dev/ttyUSB100")
+	if err != nil {
+		t.Fatalf("virtualPortLinkPath() failed: %v", err)
+	}
+
+	wantSuffix := filepath.Join(
+		".rs2322tcp",
+		"virtual",
+		"ttyUSB100",
+	)
+
+	if !filepath.IsAbs(path) {
+		t.Fatalf("path is not absolute: %q", path)
+	}
+
+	if !hasPathSuffix(path, wantSuffix) {
+		t.Errorf(
+			"virtualPortLinkPath() = %q, want suffix %q",
+			path,
+			wantSuffix,
+		)
+	}
+}
+
+func TestSetAndRemoveVirtualPortLink(t *testing.T) {
+	tempHome := t.TempDir()
+
+	t.Setenv("HOME", tempHome)
+
+	portPath := "/dev/ttyUSB100"
+	target := "/dev/pts/2"
+
+	if err := setVirtualPortLink(portPath, target); err != nil {
+		t.Fatalf("setVirtualPortLink() failed: %v", err)
+	}
+
+	linkPath, err := virtualPortLinkPath(portPath)
+	if err != nil {
+		t.Fatalf("virtualPortLinkPath() failed: %v", err)
+	}
+
+	got, err := os.Readlink(linkPath)
+	if err != nil {
+		t.Fatalf("Readlink() failed: %v", err)
+	}
+
+	if got != target {
+		t.Errorf(
+			"link target = %q, want %q",
+			got,
+			target,
+		)
+	}
+
+	if err := removeVirtualPortLink(portPath); err != nil {
+		t.Fatalf("removeVirtualPortLink() failed: %v", err)
+	}
+
+	if _, err := os.Lstat(linkPath); !os.IsNotExist(err) {
+		t.Fatalf(
+			"link still exists after removal: %q",
+			linkPath,
+		)
+	}
+}
+
+func TestSetVirtualPortLinkReplacesExistingLink(t *testing.T) {
+	tempHome := t.TempDir()
+
+	t.Setenv("HOME", tempHome)
+
+	portPath := "/dev/ttyUSB100"
+
+	if err := setVirtualPortLink(
+		portPath,
+		"/dev/pts/2",
+	); err != nil {
+		t.Fatalf(
+			"first setVirtualPortLink() failed: %v",
+			err,
+		)
+	}
+
+	if err := setVirtualPortLink(
+		portPath,
+		"/dev/pts/7",
+	); err != nil {
+		t.Fatalf(
+			"second setVirtualPortLink() failed: %v",
+			err,
+		)
+	}
+
+	linkPath, err := virtualPortLinkPath(portPath)
+	if err != nil {
+		t.Fatalf("virtualPortLinkPath() failed: %v", err)
+	}
+
+	got, err := os.Readlink(linkPath)
+	if err != nil {
+		t.Fatalf("Readlink() failed: %v", err)
+	}
+
+	if got != "/dev/pts/7" {
+		t.Errorf(
+			"link target = %q, want %q",
+			got,
+			"/dev/pts/7",
+		)
+	}
+}
+
+func TestRemoveVirtualPortLinkMissing(t *testing.T) {
+	tempHome := t.TempDir()
+
+	t.Setenv("HOME", tempHome)
+
+	if err := removeVirtualPortLink("/dev/ttyUSB100"); err != nil {
+		t.Fatalf(
+			"removeVirtualPortLink() failed for missing link: %v",
+			err,
+		)
+	}
+}
+
+func TestVirtualPortLinkPathUsesBaseName(t *testing.T) {
+	path, err := virtualPortLinkPath("/dev/ttyUSB123")
+	if err != nil {
+		t.Fatalf("virtualPortLinkPath() failed: %v", err)
+	}
+
+	if filepath.Base(path) != "ttyUSB123" {
+		t.Errorf(
+			"filepath.Base() = %q, want %q",
+			filepath.Base(path),
+			"ttyUSB123",
+		)
+	}
+}
+
+func TestExistingVirtualPortLinksNone(t *testing.T) {
+	tempDir := t.TempDir()
+
+	cfg := config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  102,
+	}
+
+	got, err := existingVirtualPortLinksInDirectory(
+		cfg,
+		tempDir,
+	)
+	if err != nil {
+		t.Fatalf(
+			"existingVirtualPortLinksInDirectory() failed: %v",
+			err,
+		)
+	}
+
+	if len(got) != 0 {
+		t.Fatalf(
+			"existingVirtualPortLinksInDirectory() = %v, want empty list",
+			got,
+		)
+	}
+}
+
+func TestExistingVirtualPortLinksFindsSymlinks(t *testing.T) {
+	tempDir := t.TempDir()
+
+	cfg := config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  102,
+	}
+
+	for _, number := range []int{100, 102} {
+		path := filepath.Join(
+			tempDir,
+			"ttyUSB"+itoa(number),
+		)
+
+		if err := os.Symlink(
+			"/dev/pts/"+itoa(number),
+			path,
+		); err != nil {
+			t.Fatalf(
+				"create test symlink %q: %v",
+				path,
+				err,
+			)
+		}
+	}
+
+	got, err := existingVirtualPortLinksInDirectory(
+		cfg,
+		tempDir,
+	)
+	if err != nil {
+		t.Fatalf(
+			"existingVirtualPortLinksInDirectory() failed: %v",
+			err,
+		)
+	}
+
+	want := []string{
+		filepath.Join(tempDir, "ttyUSB100"),
+		filepath.Join(tempDir, "ttyUSB102"),
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf(
+			"existingVirtualPortLinksInDirectory() = %v, want %v",
+			got,
+			want,
+		)
+	}
+}
+
+func TestExistingVirtualPortLinksIgnoresRegularFile(t *testing.T) {
+	tempDir := t.TempDir()
+
+	cfg := config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  102,
+	}
+
+	path := filepath.Join(tempDir, "ttyUSB100")
+
+	if err := os.WriteFile(
+		path,
+		[]byte("not a symlink"),
+		0644,
+	); err != nil {
+		t.Fatalf(
+			"create regular test file %q: %v",
+			path,
+			err,
+		)
+	}
+
+	got, err := existingVirtualPortLinksInDirectory(
+		cfg,
+		tempDir,
+	)
+	if err != nil {
+		t.Fatalf(
+			"existingVirtualPortLinksInDirectory() failed: %v",
+			err,
+		)
+	}
+
+	if len(got) != 0 {
+		t.Fatalf(
+			"regular file was detected as virtual port: %v",
+			got,
+		)
+	}
+}
+
+func TestExistingVirtualPortLinksIgnoresOutsideRange(t *testing.T) {
+	tempDir := t.TempDir()
+
+	cfg := config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  102,
+	}
+
+	path := filepath.Join(tempDir, "ttyUSB103")
+
+	if err := os.Symlink(
+		"/dev/pts/103",
+		path,
+	); err != nil {
+		t.Fatalf(
+			"create outside-range symlink %q: %v",
+			path,
+			err,
+		)
+	}
+
+	got, err := existingVirtualPortLinksInDirectory(
+		cfg,
+		tempDir,
+	)
+	if err != nil {
+		t.Fatalf(
+			"existingVirtualPortLinksInDirectory() failed: %v",
+			err,
+		)
+	}
+
+	if len(got) != 0 {
+		t.Fatalf(
+			"outside-range symlink was detected: %v",
+			got,
+		)
+	}
+}
+
+func itoa(number int) string {
+	const digits = "0123456789"
+
+	if number == 0 {
+		return "0"
+	}
+
+	result := ""
+
+	for number > 0 {
+		result = string(digits[number%10]) + result
+		number /= 10
+	}
+
+	return result
+}
+
+func hasPathSuffix(path string, suffix string) bool {
+	return path == suffix ||
+		len(path) > len(suffix) &&
+			path[len(path)-len(suffix):] == suffix
+}

internal/client/virtual_port_manager.go

@@ -0,0 +1,183 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_manager.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Verwaltung und Auswahl der für rs2322tcp reservierten virtuellen
+ * seriellen Ports.
+ *
+ * Der VirtualPortManager verwaltet den für rs2322tcp vorgesehenen Bereich
+ * virtueller serieller Schnittstellen.
+ *
+ * Die konkrete Darstellung eines virtuellen Ports ist plattformabhängig.
+ * Unter Linux werden beispielsweise /dev/ttyUSBxxx-Schnittstellen verwendet,
+ * während unter Windows COMxxx-Schnittstellen verwendet werden.
+ *
+ * Die eigentliche Erzeugung beziehungsweise Bereitstellung des seriellen
+ * Ports erfolgt ebenfalls plattformabhängig und ist nicht Bestandteil dieses
+ * Managers.
+ *
+ * Ports können entweder automatisch über den nächsten freien Port oder
+ * gezielt über eine vorgegebene Portnummer reserviert werden.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+	"sync"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// VirtualPortManager
+///////////////////////////////////////////////////////////////////////////////
+
+// VirtualPortManager verwaltet den für rs2322tcp reservierten Bereich
+// virtueller serieller Schnittstellen.
+//
+// Der reservierte Portbereich und die Belegungstabelle werden durch einen
+// Mutex geschützt, da mehrere Bridges beziehungsweise Goroutinen gleichzeitig
+// virtuelle Ports öffnen und freigeben können.
+//
+// Die konkrete Darstellung des sichtbaren Portnamens wird über
+// virtualPortPath() von der jeweiligen Plattform bestimmt.
+type VirtualPortManager struct {
+	mu sync.Mutex
+
+	first int
+	last  int
+	used  map[int]bool
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Constructor
+///////////////////////////////////////////////////////////////////////////////
+
+// NewVirtualPortManager erzeugt einen VirtualPortManager aus dem
+// konfigurierten Portbereich.
+func NewVirtualPortManager(
+	cfg config.VirtualPortRangeConfig,
+) *VirtualPortManager {
+	return &VirtualPortManager{
+		first: cfg.First,
+		last:  cfg.Last,
+		used:  make(map[int]bool),
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Port path
+///////////////////////////////////////////////////////////////////////////////
+
+// PortPath erzeugt den für die externe Software sichtbaren Gerätenamen
+// für eine Portnummer.
+//
+// Die konkrete Darstellung ist plattformabhängig:
+//
+//	Linux   /dev/ttyUSB100
+//	Windows COM100
+//
+// Die Plattformimplementierung befindet sich in
+// virtual_port_path_linux.go beziehungsweise virtual_port_path_windows.go.
+func (m *VirtualPortManager) PortPath(number int) string {
+	if m == nil {
+		return ""
+	}
+
+	return virtualPortPath(number)
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Reservation
+///////////////////////////////////////////////////////////////////////////////
+
+// Reserve reserviert den nächsten freien virtuellen Port.
+//
+// Es wird immer mit dem kleinsten freien Port im konfigurierten Bereich
+// begonnen.
+//
+// Diese Methode bleibt für Fälle erhalten, in denen keine bestimmte
+// Portnummer vorgegeben ist.
+func (m *VirtualPortManager) Reserve() (int, error) {
+	if m == nil {
+		return 0, fmt.Errorf("virtual port manager is nil")
+	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	for number := m.first; number <= m.last; number++ {
+		if m.used[number] {
+			continue
+		}
+
+		m.used[number] = true
+
+		return number, nil
+	}
+
+	return 0, fmt.Errorf(
+		"no free virtual port in range %d-%d",
+		m.first,
+		m.last,
+	)
+}
+
+// ReserveSpecific reserviert exakt den angegebenen virtuellen Port.
+//
+// Der Port muss innerhalb des konfigurierten Bereichs liegen und darf
+// momentan nicht bereits reserviert sein.
+//
+// Diese Methode wird für konfigurierte Zuordnungen verwendet, bei denen
+// die Portnummer aus client.json verbindlich ist.
+func (m *VirtualPortManager) ReserveSpecific(number int) error {
+	if m == nil {
+		return fmt.Errorf("virtual port manager is nil")
+	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if number < m.first || number > m.last {
+		return fmt.Errorf(
+			"virtual port %d is outside configured range %d-%d",
+			number,
+			m.first,
+			m.last,
+		)
+	}
+
+	if m.used[number] {
+		return fmt.Errorf(
+			"virtual port %d is already reserved",
+			number,
+		)
+	}
+
+	m.used[number] = true
+
+	return nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Release
+///////////////////////////////////////////////////////////////////////////////
+
+// Release gibt einen zuvor reservierten Port wieder frei.
+func (m *VirtualPortManager) Release(number int) {
+	if m == nil {
+		return
+	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	delete(m.used, number)
+}

internal/client/virtual_port_manager_linux.go

@@ -0,0 +1,80 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_manager_linux.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Linux-spezifische Erzeugung eines virtuellen rs2322tcp-Ports.
+ *
+ * Die gemeinsame Verwaltung von ManagedVirtualPort und VirtualPortManager
+ * befindet sich in virtual_port_manager_open.go.
+ *
+ * Unter Linux wird der PTY über newVirtualSerial() erzeugt und der interne
+ * Link unter ~/.rs2322tcp/virtual/ auf diesen PTY gesetzt.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+)
+
+// openReserved creates the actual Linux virtual serial port for an already
+// reserved port number.
+//
+// The caller must reserve the port before calling this method.
+//
+// The public /dev/ttyUSBxxx link is not modified here. The installation-owned
+// link remains untouched. Only the user-owned internal link below
+// ~/.rs2322tcp/virtual/ is created or updated to point to the newly created
+// PTY.
+func openReserved(
+	m *VirtualPortManager,
+	number int,
+) (*ManagedVirtualPort, error) {
+	if m == nil {
+		return nil, fmt.Errorf("virtual port manager is nil")
+	}
+
+	portPath := m.PortPath(number)
+
+	serial, err := newVirtualSerial()
+	if err != nil {
+		m.Release(number)
+
+		return nil, fmt.Errorf(
+			"create virtual serial for %s: %w",
+			portPath,
+			err,
+		)
+	}
+
+	if err := setVirtualPortLink(
+		portPath,
+		serial.Path(),
+	); err != nil {
+		_ = serial.Close()
+		m.Release(number)
+
+		return nil, fmt.Errorf(
+			"bind %s to %s: %w",
+			portPath,
+			serial.Path(),
+			err,
+		)
+	}
+
+	port := NewVirtualPort(portPath, serial)
+
+	return &ManagedVirtualPort{
+		port:    port,
+		manager: m,
+		number:  number,
+	}, nil
+}

internal/client/virtual_port_manager_linux_test.go

@@ -0,0 +1,296 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_manager_linux_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Linux-spezifische Tests für die Erzeugung und Verwaltung virtueller
+ * rs2322tcp-Ports.
+ * ============================================================================
+ */
+package client
+
+import (
+	"os"
+	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestVirtualPortManagerOpen(t *testing.T) {
+	tempHome := t.TempDir()
+	t.Setenv("HOME", tempHome)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	port, err := manager.Open()
+	if err != nil {
+		t.Fatalf("Open() failed: %v", err)
+	}
+
+	if port == nil {
+		t.Fatal("Open() returned nil")
+	}
+
+	if port.Path() != "/dev/ttyUSB100" {
+		t.Errorf(
+			"Path() = %q, want %q",
+			port.Path(),
+			"/dev/ttyUSB100",
+		)
+	}
+
+	linkPath, err := virtualPortLinkPath(port.Path())
+	if err != nil {
+		t.Fatalf("virtualPortLinkPath() failed: %v", err)
+	}
+
+	target, err := os.Readlink(linkPath)
+	if err != nil {
+		t.Fatalf("Readlink() failed: %v", err)
+	}
+
+	if target == "" {
+		t.Fatal("internal virtual port link has empty target")
+	}
+
+	if err := port.Close(); err != nil {
+		t.Fatalf("Close() failed: %v", err)
+	}
+
+	targetAfterClose, err := os.Readlink(linkPath)
+	if err != nil {
+		t.Fatalf(
+			"internal link does not exist after Close(): %q: %v",
+			linkPath,
+			err,
+		)
+	}
+
+	if targetAfterClose != target {
+		t.Errorf(
+			"internal link target after Close() = %q, want %q",
+			targetAfterClose,
+			target,
+		)
+	}
+}
+
+func TestVirtualPortManagerOpenSpecific(t *testing.T) {
+	tempHome := t.TempDir()
+	t.Setenv("HOME", tempHome)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  199,
+		},
+	)
+
+	port, err := manager.OpenSpecific(150)
+	if err != nil {
+		t.Fatalf("OpenSpecific(150) failed: %v", err)
+	}
+
+	if port == nil {
+		t.Fatal("OpenSpecific(150) returned nil")
+	}
+
+	if port.Path() != "/dev/ttyUSB150" {
+		t.Errorf(
+			"Path() = %q, want %q",
+			port.Path(),
+			"/dev/ttyUSB150",
+		)
+	}
+
+	linkPath, err := virtualPortLinkPath(port.Path())
+	if err != nil {
+		t.Fatalf("virtualPortLinkPath() failed: %v", err)
+	}
+
+	target, err := os.Readlink(linkPath)
+	if err != nil {
+		t.Fatalf("Readlink() failed: %v", err)
+	}
+
+	if target == "" {
+		t.Fatal("internal virtual port link has empty target")
+	}
+
+	if err := port.Close(); err != nil {
+		t.Fatalf("Close() failed: %v", err)
+	}
+
+	targetAfterClose, err := os.Readlink(linkPath)
+	if err != nil {
+		t.Fatalf(
+			"internal link does not exist after Close(): %q: %v",
+			linkPath,
+			err,
+		)
+	}
+
+	if targetAfterClose != target {
+		t.Errorf(
+			"internal link target after Close() = %q, want %q",
+			targetAfterClose,
+			target,
+		)
+	}
+}
+
+func TestVirtualPortManagerOpenUsesNextPort(t *testing.T) {
+	tempHome := t.TempDir()
+	t.Setenv("HOME", tempHome)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  101,
+		},
+	)
+
+	first, err := manager.Open()
+	if err != nil {
+		t.Fatalf("first Open() failed: %v", err)
+	}
+
+	second, err := manager.Open()
+	if err != nil {
+		_ = first.Close()
+		t.Fatalf("second Open() failed: %v", err)
+	}
+
+	if first.Path() != "/dev/ttyUSB100" {
+		t.Errorf(
+			"first.Path() = %q, want %q",
+			first.Path(),
+			"/dev/ttyUSB100",
+		)
+	}
+
+	if second.Path() != "/dev/ttyUSB101" {
+		t.Errorf(
+			"second.Path() = %q, want %q",
+			second.Path(),
+			"/dev/ttyUSB101",
+		)
+	}
+
+	if err := first.Close(); err != nil {
+		t.Fatalf("first Close() failed: %v", err)
+	}
+
+	if err := second.Close(); err != nil {
+		t.Fatalf("second Close() failed: %v", err)
+	}
+}
+
+func TestVirtualPortManagerOpenReusesReleasedPort(t *testing.T) {
+	tempHome := t.TempDir()
+	t.Setenv("HOME", tempHome)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  100,
+		},
+	)
+
+	first, err := manager.Open()
+	if err != nil {
+		t.Fatalf("first Open() failed: %v", err)
+	}
+
+	if first.Path() != "/dev/ttyUSB100" {
+		t.Fatalf(
+			"first.Path() = %q, want %q",
+			first.Path(),
+			"/dev/ttyUSB100",
+		)
+	}
+
+	if err := first.Close(); err != nil {
+		t.Fatalf("first Close() failed: %v", err)
+	}
+
+	second, err := manager.Open()
+	if err != nil {
+		t.Fatalf("second Open() failed: %v", err)
+	}
+
+	if second.Path() != "/dev/ttyUSB100" {
+		t.Errorf(
+			"second.Path() = %q, want %q",
+			second.Path(),
+			"/dev/ttyUSB100",
+		)
+	}
+
+	if err := second.Close(); err != nil {
+		t.Fatalf("second Close() failed: %v", err)
+	}
+}
+
+func TestVirtualPortManagerOpenNoFreePort(t *testing.T) {
+	tempHome := t.TempDir()
+	t.Setenv("HOME", tempHome)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  100,
+		},
+	)
+
+	port, err := manager.Open()
+	if err != nil {
+		t.Fatalf("first Open() failed: %v", err)
+	}
+
+	defer port.Close()
+
+	if _, err := manager.Open(); err == nil {
+		t.Fatal("second Open() succeeded, want no-free-port error")
+	}
+}
+
+func TestManagedVirtualPortDataPath(t *testing.T) {
+	tempHome := t.TempDir()
+	t.Setenv("HOME", tempHome)
+
+	manager := NewVirtualPortManager(
+		config.VirtualPortRangeConfig{
+			First: 100,
+			Last:  100,
+		},
+	)
+
+	port, err := manager.Open()
+	if err != nil {
+		t.Fatalf("Open() failed: %v", err)
+	}
+
+	defer port.Close()
+
+	data := []byte{0x46, 0x41, 0x00, 0x10, 0x0D}
+
+	if _, err := port.Write(data); err != nil {
+		t.Fatalf("Write() failed: %v", err)
+	}
+}

internal/client/virtual_port_manager_open.go

@@ -0,0 +1,163 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_manager_open.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Plattformneutrale Verwaltung geöffneter virtueller Ports.
+ *
+ * Die konkrete Erzeugung eines virtuellen seriellen Ports erfolgt über
+ * openVirtualPort() in der jeweiligen Plattformimplementierung.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+	"sync"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Managed virtual port
+///////////////////////////////////////////////////////////////////////////////
+
+// ManagedVirtualPort verbindet einen reservierten Port mit einer
+// VirtualSerial-Instanz und verwaltet deren Lebenszyklus.
+type ManagedVirtualPort struct {
+	mu sync.Mutex
+
+	port    *VirtualPort
+	manager *VirtualPortManager
+	number  int
+}
+
+// Path returns the device path visible to the external application.
+func (p *ManagedVirtualPort) Path() string {
+	if p == nil {
+		return ""
+	}
+
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	if p.port == nil {
+		return ""
+	}
+
+	return p.port.Path()
+}
+
+// Read reads data from the underlying virtual serial device.
+func (p *ManagedVirtualPort) Read(b []byte) (int, error) {
+	if p == nil {
+		return 0, fmt.Errorf("managed virtual port is nil")
+	}
+
+	p.mu.Lock()
+	port := p.port
+	p.mu.Unlock()
+
+	if port == nil {
+		return 0, fmt.Errorf("managed virtual port is closed")
+	}
+
+	return port.Read(b)
+}
+
+// Write writes data to the underlying virtual serial device.
+func (p *ManagedVirtualPort) Write(b []byte) (int, error) {
+	if p == nil {
+		return 0, fmt.Errorf("managed virtual port is nil")
+	}
+
+	p.mu.Lock()
+	port := p.port
+	p.mu.Unlock()
+
+	if port == nil {
+		return 0, fmt.Errorf("managed virtual port is closed")
+	}
+
+	return port.Write(b)
+}
+
+// Close closes the managed virtual port and releases its reservation.
+func (p *ManagedVirtualPort) Close() error {
+	if p == nil {
+		return nil
+	}
+
+	p.mu.Lock()
+
+	port := p.port
+	p.port = nil
+
+	manager := p.manager
+	number := p.number
+
+	p.mu.Unlock()
+
+	if port == nil {
+		return nil
+	}
+
+	err := port.Close()
+
+	if manager != nil {
+		manager.Release(number)
+	}
+
+	return err
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Open
+///////////////////////////////////////////////////////////////////////////////
+
+// Open creates a new virtual serial port using the next free port number.
+func (m *VirtualPortManager) Open() (*ManagedVirtualPort, error) {
+	if m == nil {
+		return nil, fmt.Errorf("virtual port manager is nil")
+	}
+
+	number, err := m.Reserve()
+	if err != nil {
+		return nil, err
+	}
+
+	port, err := openVirtualPort(m, number)
+	if err != nil {
+		m.Release(number)
+
+		return nil, err
+	}
+
+	return port, nil
+}
+
+// OpenSpecific creates a new virtual serial port using exactly the specified
+// port number.
+func (m *VirtualPortManager) OpenSpecific(
+	number int,
+) (*ManagedVirtualPort, error) {
+	if m == nil {
+		return nil, fmt.Errorf("virtual port manager is nil")
+	}
+
+	if err := m.ReserveSpecific(number); err != nil {
+		return nil, err
+	}
+
+	port, err := openVirtualPort(m, number)
+	if err != nil {
+		m.Release(number)
+
+		return nil, err
+	}
+
+	return port, nil
+}

internal/client/virtual_port_manager_test.go

@@ -0,0 +1,304 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_manager_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für die Auswahl und Verwaltung virtueller serieller Ports.
+ *
+ * Die Tests decken sowohl die automatische Reservierung über Reserve()
+ * als auch die gezielte Reservierung eines bestimmten Ports über
+ * ReserveSpecific() ab.
+ * ============================================================================
+ */
+package client
+
+import (
+	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestNewVirtualPortManager(t *testing.T) {
+	cfg := config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	}
+
+	manager := NewVirtualPortManager(cfg)
+
+	if manager == nil {
+		t.Fatal("NewVirtualPortManager() returned nil")
+	}
+
+	if manager.first != 100 {
+		t.Errorf("first = %d, want 100", manager.first)
+	}
+
+	if manager.last != 199 {
+		t.Errorf("last = %d, want 199", manager.last)
+	}
+}
+
+func TestVirtualPortManagerReserveFirst(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	number, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("Reserve() failed: %v", err)
+	}
+
+	if number != 100 {
+		t.Errorf("Reserve() = %d, want 100", number)
+	}
+}
+
+func TestVirtualPortManagerReserveNextFree(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	first, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("first Reserve() failed: %v", err)
+	}
+
+	second, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("second Reserve() failed: %v", err)
+	}
+
+	if first != 100 {
+		t.Errorf("first Reserve() = %d, want 100", first)
+	}
+
+	if second != 101 {
+		t.Errorf("second Reserve() = %d, want 101", second)
+	}
+}
+
+func TestVirtualPortManagerRelease(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	number, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("Reserve() failed: %v", err)
+	}
+
+	manager.Release(number)
+
+	reused, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("Reserve() after Release() failed: %v", err)
+	}
+
+	if reused != 100 {
+		t.Errorf(
+			"Reserve() after Release() = %d, want 100",
+			reused,
+		)
+	}
+}
+
+func TestVirtualPortManagerSkipsReservedPorts(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	for i := 0; i < 3; i++ {
+		if _, err := manager.Reserve(); err != nil {
+			t.Fatalf("Reserve() %d failed: %v", i+1, err)
+		}
+	}
+
+	number, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("fourth Reserve() failed: %v", err)
+	}
+
+	if number != 103 {
+		t.Errorf("fourth Reserve() = %d, want 103", number)
+	}
+}
+
+func TestVirtualPortManagerCustomRange(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 200,
+		Last:  299,
+	})
+
+	number, err := manager.Reserve()
+	if err != nil {
+		t.Fatalf("Reserve() failed: %v", err)
+	}
+
+	if number != 200 {
+		t.Errorf("Reserve() = %d, want 200", number)
+	}
+}
+
+func TestVirtualPortManagerNoFreePort(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  102,
+	})
+
+	for i := 0; i < 3; i++ {
+		if _, err := manager.Reserve(); err != nil {
+			t.Fatalf("Reserve() %d failed: %v", i+1, err)
+		}
+	}
+
+	if _, err := manager.Reserve(); err == nil {
+		t.Fatal("Reserve() succeeded, want no-free-port error")
+	}
+}
+
+func TestVirtualPortManagerPortPath(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	tests := []struct {
+		number int
+		want   string
+	}{
+		{
+			number: 100,
+			want:   virtualPortPath(100),
+		},
+		{
+			number: 123,
+			want:   virtualPortPath(123),
+		},
+		{
+			number: 199,
+			want:   virtualPortPath(199),
+		},
+	}
+
+	for _, test := range tests {
+		got := manager.PortPath(test.number)
+
+		if got != test.want {
+			t.Errorf(
+				"PortPath(%d) = %q, want %q",
+				test.number,
+				got,
+				test.want,
+			)
+		}
+	}
+}
+
+func TestVirtualPortManagerReserveSpecific(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	if err := manager.ReserveSpecific(150); err != nil {
+		t.Fatalf("ReserveSpecific(150) failed: %v", err)
+	}
+
+	if !manager.used[150] {
+		t.Fatal("ReserveSpecific(150) did not reserve port 150")
+	}
+}
+
+func TestVirtualPortManagerReserveSpecificOutsideRange(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	tests := []int{
+		99,
+		200,
+	}
+
+	for _, number := range tests {
+		if err := manager.ReserveSpecific(number); err == nil {
+			t.Errorf(
+				"ReserveSpecific(%d) succeeded, want range error",
+				number,
+			)
+		}
+
+		if manager.used[number] {
+			t.Errorf(
+				"ReserveSpecific(%d) marked out-of-range port as used",
+				number,
+			)
+		}
+	}
+}
+
+func TestVirtualPortManagerReserveSpecificAlreadyReserved(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	if err := manager.ReserveSpecific(100); err != nil {
+		t.Fatalf("first ReserveSpecific(100) failed: %v", err)
+	}
+
+	if err := manager.ReserveSpecific(100); err == nil {
+		t.Fatal("second ReserveSpecific(100) succeeded, want already-reserved error")
+	}
+}
+
+func TestVirtualPortManagerReserveSpecificAfterRelease(t *testing.T) {
+	manager := NewVirtualPortManager(config.VirtualPortRangeConfig{
+		First: 100,
+		Last:  199,
+	})
+
+	if err := manager.ReserveSpecific(150); err != nil {
+		t.Fatalf("first ReserveSpecific(150) failed: %v", err)
+	}
+
+	manager.Release(150)
+
+	if err := manager.ReserveSpecific(150); err != nil {
+		t.Fatalf(
+			"ReserveSpecific(150) after Release() failed: %v",
+			err,
+		)
+	}
+}
+
+func TestVirtualPortManagerNil(t *testing.T) {
+	var manager *VirtualPortManager
+
+	if path := manager.PortPath(100); path != "" {
+		t.Errorf("nil PortPath() = %q, want empty string", path)
+	}
+
+	if _, err := manager.Reserve(); err == nil {
+		t.Fatal("nil Reserve() succeeded, want error")
+	}
+
+	if err := manager.ReserveSpecific(100); err == nil {
+		t.Fatal("nil ReserveSpecific() succeeded, want error")
+	}
+
+	manager.Release(100)
+}

internal/client/virtual_port_number_linux.go

@@ -0,0 +1,59 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_number_linux.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Linux-spezifische Umwandlung eines konfigurierten virtuellen
+ * seriellen Ports in dessen numerische Portnummer.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Virtual port number
+///////////////////////////////////////////////////////////////////////////////
+
+// virtualPortNumber converts a configured /dev/ttyUSBxxx path into its
+// numeric virtual port number.
+func virtualPortNumber(path string) (int, error) {
+	const prefix = "/dev/ttyUSB"
+
+	if !strings.HasPrefix(path, prefix) {
+		return 0, fmt.Errorf(
+			"invalid virtual port %q: expected /dev/ttyUSB<number>",
+			path,
+		)
+	}
+
+	value := strings.TrimPrefix(path, prefix)
+
+	if value == "" {
+		return 0, fmt.Errorf(
+			"invalid virtual port %q: port number is missing",
+			path,
+		)
+	}
+
+	number, err := strconv.Atoi(value)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"invalid virtual port %q: invalid port number",
+			path,
+		)
+	}
+
+	return number, nil
+}

internal/client/virtual_port_number_windows.go

@@ -0,0 +1,62 @@
+//go:build windows
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_number_windows.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Windows-spezifische Umwandlung eines konfigurierten virtuellen
+ * seriellen COM-Ports in dessen numerische Portnummer.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Virtual port number
+///////////////////////////////////////////////////////////////////////////////
+
+// virtualPortNumber converts a configured COMxxx path into its numeric
+// virtual port number.
+func virtualPortNumber(path string) (int, error) {
+	const prefix = "COM"
+
+	if !strings.HasPrefix(
+		strings.ToUpper(path),
+		prefix,
+	) {
+		return 0, fmt.Errorf(
+			"invalid virtual port %q: expected COM<number>",
+			path,
+		)
+	}
+
+	value := path[len(prefix):]
+
+	if value == "" {
+		return 0, fmt.Errorf(
+			"invalid virtual port %q: port number is missing",
+			path,
+		)
+	}
+
+	number, err := strconv.Atoi(value)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"invalid virtual port %q: invalid port number",
+			path,
+		)
+	}
+
+	return number, nil
+}

internal/client/virtual_port_open_linux.go

@@ -0,0 +1,77 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_open_linux.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Linux-spezifische Erzeugung eines verwalteten virtuellen rs2322tcp-Ports.
+ *
+ * Die virtuelle serielle Schnittstelle wird über ein PTY-Paar erzeugt.
+ * Der öffentliche /dev/ttyUSBxxx-Eintrag wird über den bereits vorhandenen
+ * internen Link auf den PTY-Slave abgebildet.
+ * ============================================================================
+ */
+
+package client
+
+import "fmt"
+
+///////////////////////////////////////////////////////////////////////////////
+// Open virtual port
+///////////////////////////////////////////////////////////////////////////////
+
+// openVirtualPort creates and initializes a Linux virtual serial port.
+//
+// The port number must already have been reserved by the
+// VirtualPortManager.
+//
+// On failure, the caller remains responsible for releasing the reservation.
+func openVirtualPort(
+	manager *VirtualPortManager,
+	number int,
+) (*ManagedVirtualPort, error) {
+	if manager == nil {
+		return nil, fmt.Errorf("virtual port manager is nil")
+	}
+
+	portPath := manager.PortPath(number)
+
+	serial, err := newVirtualSerial()
+	if err != nil {
+		return nil, fmt.Errorf(
+			"create virtual serial for %s: %w",
+			portPath,
+			err,
+		)
+	}
+
+	if err := setVirtualPortLink(
+		portPath,
+		serial.Path(),
+	); err != nil {
+		_ = serial.Close()
+
+		return nil, fmt.Errorf(
+			"bind %s to %s: %w",
+			portPath,
+			serial.Path(),
+			err,
+		)
+	}
+
+	port := NewVirtualPort(
+		portPath,
+		serial,
+	)
+
+	return &ManagedVirtualPort{
+		port:    port,
+		manager: manager,
+		number:  number,
+	}, nil
+}

internal/client/virtual_port_open_windows.go

@@ -0,0 +1,87 @@
+//go:build windows
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_open_windows.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Windows-spezifische Erzeugung eines verwalteten virtuellen rs2322tcp-Ports.
+ *
+ * Der konfigurierte virtuelle COM-Port wird einmal geöffnet und anschließend
+ * über dasselbe Handle für Lese- und Schreiboperationen verwendet.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+
+	bugserial "go.bug.st/serial"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Open virtual port
+///////////////////////////////////////////////////////////////////////////////
+
+// openVirtualPort creates and initializes a Windows virtual serial port.
+//
+// The port number must already have been reserved by the
+// VirtualPortManager.
+//
+// The visible COM port is provided by the platform-specific PortPath()
+// implementation. The actual COM-port pair configuration is managed
+// externally by the Windows virtual COM-port driver.
+//
+// The COM port is opened exactly once. The resulting handle is used for
+// parallel read and write operations.
+func openVirtualPort(
+	manager *VirtualPortManager,
+	number int,
+) (*ManagedVirtualPort, error) {
+	if manager == nil {
+		return nil, fmt.Errorf("virtual port manager is nil")
+	}
+
+	portPath := manager.PortPath(number)
+	if portPath == "" {
+		return nil, fmt.Errorf(
+			"virtual port path is empty for port %d",
+			number,
+		)
+	}
+
+	mode := &bugserial.Mode{
+		BaudRate: 9600,
+		DataBits: 8,
+		Parity:   bugserial.NoParity,
+		StopBits: bugserial.OneStopBit,
+	}
+
+	port, err := bugserial.Open(portPath, mode)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"open virtual serial port %s: %w",
+			portPath,
+			err,
+		)
+	}
+
+	virtualPort := NewVirtualPort(
+		portPath,
+		&windowsSerial{
+			port: port,
+			path: portPath,
+		},
+	)
+
+	return &ManagedVirtualPort{
+		port:    virtualPort,
+		manager: manager,
+		number:  number,
+	}, nil
+}

internal/client/virtual_port_path_linux.go

@@ -0,0 +1,43 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_path_linux.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Linux-spezifische Darstellung eines für rs2322tcp reservierten
+ * virtuellen seriellen Ports.
+ *
+ * Unter Linux werden die für rs2322tcp sichtbaren virtuellen Ports als
+ *
+ *     /dev/ttyUSBxxx
+ *
+ * dargestellt.
+ *
+ * Die eigentliche Bereitstellung des virtuellen seriellen Ports erfolgt
+ * unabhängig davon in der Linux-spezifischen VirtualSerial-Implementierung.
+ * ============================================================================
+ */
+
+package client
+
+import "fmt"
+
+///////////////////////////////////////////////////////////////////////////////
+// Virtual port path
+///////////////////////////////////////////////////////////////////////////////
+
+// virtualPortPath erzeugt den unter Linux sichtbaren Gerätenamen für eine
+// virtuelle rs2322tcp-Schnittstelle.
+//
+// Beispiel:
+//
+//	number = 100
+//	result = /dev/ttyUSB100
+func virtualPortPath(number int) string {
+	return fmt.Sprintf("/dev/ttyUSB%d", number)
+}

internal/client/virtual_port_path_windows.go

@@ -0,0 +1,48 @@
+//go:build windows
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_path_windows.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Windows-spezifische Darstellung eines für rs2322tcp reservierten
+ * virtuellen seriellen Ports.
+ *
+ * Unter Windows werden die für rs2322tcp sichtbaren virtuellen Ports als
+ *
+ *     COMxxx
+ *
+ * dargestellt.
+ *
+ * Die eigentliche Bereitstellung des virtuellen COM-Ports erfolgt durch die
+ * Windows-spezifische serielle Infrastruktur beziehungsweise später durch
+ * den für rs2322tcp vorgesehenen virtuellen COM-Port-Treiber.
+ *
+ * Diese Datei enthält ausschließlich die plattformabhängige Darstellung des
+ * Portnamens. Die gemeinsame Portverwaltung bleibt in
+ * virtual_port_manager.go erhalten.
+ * ============================================================================
+ */
+
+package client
+
+import "fmt"
+
+///////////////////////////////////////////////////////////////////////////////
+// Virtual port path
+///////////////////////////////////////////////////////////////////////////////
+
+// virtualPortPath erzeugt den unter Windows sichtbaren Gerätenamen für eine
+// virtuelle rs2322tcp-Schnittstelle.
+//
+// Beispiel:
+//
+//	number = 10
+//	result = COM10
+func virtualPortPath(number int) string {
+	return fmt.Sprintf("COM%d", number)
+}

internal/client/virtual_port_test.go

@@ -0,0 +1,117 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für die lokale virtuelle seriellen Schnittstelle.
+ * ============================================================================
+ */
+package client
+
+import "testing"
+
+///////////////////////////////////////////////////////////////////////////////
+// Tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestNewVirtualPort(t *testing.T) {
+	serial := newTestVirtualSerial(nil)
+
+	port := NewVirtualPort("/dev/ttyUSB100", serial)
+
+	if port == nil {
+		t.Fatal("NewVirtualPort() returned nil")
+	}
+
+	if port.Path() != "/dev/ttyUSB100" {
+		t.Errorf("Path() = %q, want %q",
+			port.Path(), "/dev/ttyUSB100")
+	}
+}
+
+func TestVirtualPortRead(t *testing.T) {
+	expected := []byte{0x46, 0x41, 0x00, 0x10, 0x0D}
+
+	serial := newTestVirtualSerial(expected)
+	port := NewVirtualPort("/dev/ttyUSB100", serial)
+
+	buffer := make([]byte, len(expected))
+
+	n, err := port.Read(buffer)
+	if err != nil {
+		t.Fatalf("Read() failed: %v", err)
+	}
+
+	if n != len(expected) {
+		t.Fatalf("Read() returned %d bytes, want %d",
+			n, len(expected))
+	}
+
+	for i := range expected {
+		if buffer[i] != expected[i] {
+			t.Errorf("buffer[%d] = %02X, want %02X",
+				i, buffer[i], expected[i])
+		}
+	}
+}
+
+func TestVirtualPortWrite(t *testing.T) {
+	serial := newTestVirtualSerial(nil)
+	port := NewVirtualPort("/dev/ttyUSB100", serial)
+
+	data := []byte{0x46, 0x41, 0x00, 0x10, 0x0D}
+
+	n, err := port.Write(data)
+	if err != nil {
+		t.Fatalf("Write() failed: %v", err)
+	}
+
+	if n != len(data) {
+		t.Fatalf("Write() returned %d, want %d",
+			n, len(data))
+	}
+
+	written := serial.Written()
+
+	if len(written) != len(data) {
+		t.Fatalf("underlying serial received %d bytes, want %d",
+			len(written), len(data))
+	}
+
+	for i := range data {
+		if written[i] != data[i] {
+			t.Errorf("written[%d] = %02X, want %02X",
+				i, written[i], data[i])
+		}
+	}
+}
+
+func TestVirtualPortClose(t *testing.T) {
+	serial := newTestVirtualSerial(nil)
+	port := NewVirtualPort("/dev/ttyUSB100", serial)
+
+	if err := port.Close(); err != nil {
+		t.Fatalf("Close() failed: %v", err)
+	}
+
+	// Ein zweiter Close darf keinen Fehler erzeugen.
+	if err := port.Close(); err != nil {
+		t.Fatalf("second Close() failed: %v", err)
+	}
+}
+
+func TestVirtualPortNil(t *testing.T) {
+	var port *VirtualPort
+
+	if port.Path() != "" {
+		t.Errorf("nil Path() = %q, want empty string", port.Path())
+	}
+
+	if err := port.Close(); err != nil {
+		t.Errorf("nil Close() failed: %v", err)
+	}
+}

internal/client/virtual_port_validation.go

@@ -0,0 +1,252 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_validation.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Plattformneutrale Prüfung der in client.json definierten virtuellen Ports
+ * gegen die tatsächlich vorhandenen virtuellen Schnittstellen.
+ *
+ * Die Ermittlung der vorhandenen Ports erfolgt über die
+ * plattformabhängige Funktion existingVirtualPortLinks().
+ * ============================================================================
+ */
+package client
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Virtual port validation
+///////////////////////////////////////////////////////////////////////////////
+
+// VirtualPortValidationResult contains the differences between configured
+// and locally available virtual ports.
+type VirtualPortValidationResult struct {
+	Missing []string
+	Extra   []string
+}
+
+// compareVirtualPorts compares the configured virtual ports with the
+// locally available virtual ports.
+//
+// Missing contains ports configured in client.json that are not available
+// locally.
+//
+// Extra contains locally available ports that are not configured in
+// client.json.
+//
+// The order of both input lists is irrelevant.
+func compareVirtualPorts(
+	configured []string,
+	available []string,
+) VirtualPortValidationResult {
+	configuredSet := make(map[string]struct{}, len(configured))
+
+	for _, port := range configured {
+		configuredSet[port] = struct{}{}
+	}
+
+	availableSet := make(map[string]struct{}, len(available))
+
+	for _, port := range available {
+		availableSet[port] = struct{}{}
+	}
+
+	result := VirtualPortValidationResult{
+		Missing: make([]string, 0),
+		Extra:   make([]string, 0),
+	}
+
+	for port := range configuredSet {
+		if _, ok := availableSet[port]; ok {
+			continue
+		}
+
+		result.Missing = append(result.Missing, port)
+	}
+
+	for port := range availableSet {
+		if _, ok := configuredSet[port]; ok {
+			continue
+		}
+
+		result.Extra = append(result.Extra, port)
+	}
+
+	sort.Strings(result.Missing)
+	sort.Strings(result.Extra)
+
+	return result
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Local virtual-port validation
+///////////////////////////////////////////////////////////////////////////////
+
+// validateLocalVirtualPorts compares the virtual ports defined in
+// client.json with the virtual ports currently available on the local
+// computer.
+//
+// The detection of available ports is performed by the platform-specific
+// existingVirtualPortLinks() implementation.
+//
+// A mismatch prevents the client from starting.
+func validateLocalVirtualPorts(
+	cfg *config.ClientConfig,
+) error {
+	if cfg == nil {
+		return fmt.Errorf("client configuration is nil")
+	}
+
+	configured := make(
+		[]string,
+		0,
+		len(cfg.VirtualPorts),
+	)
+
+	for _, virtualPort := range cfg.VirtualPorts {
+		configured = append(
+			configured,
+			virtualPort.Port,
+		)
+	}
+
+	available, err := existingVirtualPortLinks(
+		cfg.VirtualPortRange,
+	)
+	if err != nil {
+		return fmt.Errorf(
+			"prüfen der virtuellen Ports fehlgeschlagen: %w",
+			err,
+		)
+	}
+
+	result := compareVirtualPorts(
+		configured,
+		available,
+	)
+
+	if len(result.Missing) == 0 &&
+		len(result.Extra) == 0 {
+		return nil
+	}
+
+	var message strings.Builder
+
+	////////////////////////////////////////////////////////////////////////////
+	// Missing ports
+	////////////////////////////////////////////////////////////////////////////
+
+	if len(result.Missing) > 0 &&
+		len(result.Extra) == 0 {
+
+		message.WriteString(
+			"Virtuelle Ports nicht eingerichtet",
+		)
+
+		message.WriteString(
+			"\n\nFolgende Ports sind in client.json " +
+				"eingetragen, aber auf diesem Rechner " +
+				"nicht eingerichtet:\n\n",
+		)
+
+		for _, port := range result.Missing {
+			message.WriteString("  ")
+			message.WriteString(port)
+			message.WriteByte('\n')
+		}
+
+		message.WriteString(
+			"\nBitte richten Sie diese virtuellen Ports " +
+				"entsprechend der Installationsanleitung ein.",
+		)
+
+		return fmt.Errorf(
+			"%s",
+			strings.TrimSpace(message.String()),
+		)
+	}
+
+	////////////////////////////////////////////////////////////////////////////
+	// Extra ports
+	////////////////////////////////////////////////////////////////////////////
+
+	if len(result.Missing) == 0 &&
+		len(result.Extra) > 0 {
+
+		message.WriteString(
+			"Virtuelle Ports nicht konfiguriert",
+		)
+
+		message.WriteString(
+			"\n\nFolgende virtuelle Ports sind auf diesem " +
+				"Rechner eingerichtet, aber nicht in " +
+				"client.json eingetragen:\n\n",
+		)
+
+		for _, port := range result.Extra {
+			message.WriteString("  ")
+			message.WriteString(port)
+			message.WriteByte('\n')
+		}
+
+		message.WriteString(
+			"\nBitte ergänzen Sie die entsprechenden " +
+				"Einträge in client.json.",
+		)
+
+		return fmt.Errorf(
+			"%s",
+			strings.TrimSpace(message.String()),
+		)
+	}
+
+	////////////////////////////////////////////////////////////////////////////
+	// Missing and extra ports
+	////////////////////////////////////////////////////////////////////////////
+
+	message.WriteString(
+		"Virtuelle Portkonfiguration stimmt nicht überein",
+	)
+
+	message.WriteString(
+		"\n\nIn client.json eingetragen, aber nicht " +
+			"eingerichtet:\n\n",
+	)
+
+	for _, port := range result.Missing {
+		message.WriteString("  ")
+		message.WriteString(port)
+		message.WriteByte('\n')
+	}
+
+	message.WriteString(
+		"\nEingerichtet, aber nicht in client.json " +
+			"eingetragen:\n\n",
+	)
+
+	for _, port := range result.Extra {
+		message.WriteString("  ")
+		message.WriteString(port)
+		message.WriteByte('\n')
+	}
+
+	message.WriteString(
+		"\nBitte korrigieren Sie die virtuelle " +
+			"Portkonfiguration.",
+	)
+
+	return fmt.Errorf(
+		"%s",
+		strings.TrimSpace(message.String()),
+	)
+}

internal/client/virtual_port_validation_linux.go

@@ -0,0 +1,106 @@
+//go:build linux
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_validation_linux.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Linux-spezifische Ermittlung der lokal vorhandenen virtuellen
+ * rs2322tcp-Schnittstellen.
+ *
+ * Die öffentliche /dev/ttyUSBxxx-Schnittstelle wird ausschließlich gelesen.
+ * Der Client verändert diese Schnittstellen hier nicht.
+ * ============================================================================
+ */
+package client
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Existing virtual ports
+///////////////////////////////////////////////////////////////////////////////
+
+// existingVirtualPortLinks returns all public virtual-port paths that
+// currently exist as symbolic links.
+//
+// Only ports inside the configured virtual-port range are considered.
+// The public /dev/ttyUSBxxx links are only inspected; they are never
+// created, changed, or removed by this function.
+//
+// This check only requires read access and therefore does not require
+// root privileges.
+func existingVirtualPortLinks(
+	cfg config.VirtualPortRangeConfig,
+) ([]string, error) {
+	return existingVirtualPortLinksInDirectory(
+		cfg,
+		"/dev",
+	)
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Existing virtual ports in directory
+///////////////////////////////////////////////////////////////////////////////
+
+// existingVirtualPortLinksInDirectory returns all virtual-port paths in
+// directory that currently exist as symbolic links.
+//
+// This helper is deliberately separated from existingVirtualPortLinks so
+// the filesystem inspection can be tested without modifying /dev.
+func existingVirtualPortLinksInDirectory(
+	cfg config.VirtualPortRangeConfig,
+	directory string,
+) ([]string, error) {
+	if directory == "" {
+		return nil, fmt.Errorf("virtual port directory is empty")
+	}
+
+	manager := NewVirtualPortManager(cfg)
+	if manager == nil {
+		return nil, fmt.Errorf("virtual port manager is nil")
+	}
+
+	links := make([]string, 0)
+
+	for number := cfg.First; number <= cfg.Last; number++ {
+		portName := filepath.Base(
+			manager.PortPath(number),
+		)
+
+		portPath := filepath.Join(directory, portName)
+
+		info, err := os.Lstat(portPath)
+		if err != nil {
+			if os.IsNotExist(err) {
+				continue
+			}
+
+			return nil, fmt.Errorf(
+				"inspect virtual port %q: %w",
+				portPath,
+				err,
+			)
+		}
+
+		if info.Mode()&os.ModeSymlink == 0 {
+			continue
+		}
+
+		links = append(
+			links,
+			filepath.Join(directory, portName),
+		)
+	}
+
+	return links, nil
+}

internal/client/virtual_port_validation_test.go

@@ -0,0 +1,283 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_validation_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für den Vergleich der konfigurierten und tatsächlich vorhandenen
+ * virtuellen rs2322tcp-Schnittstellen.
+ * ============================================================================
+ */
+package client
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestCompareVirtualPortsIdentical(t *testing.T) {
+	configured := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+	}
+
+	available := []string{
+		"/dev/ttyUSB101",
+		"/dev/ttyUSB100",
+	}
+
+	got := compareVirtualPorts(
+		configured,
+		available,
+	)
+
+	if len(got.Missing) != 0 {
+		t.Errorf(
+			"Missing = %v, want empty",
+			got.Missing,
+		)
+	}
+
+	if len(got.Extra) != 0 {
+		t.Errorf(
+			"Extra = %v, want empty",
+			got.Extra,
+		)
+	}
+}
+
+func TestCompareVirtualPortsMissing(t *testing.T) {
+	configured := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+		"/dev/ttyUSB102",
+	}
+
+	available := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+	}
+
+	got := compareVirtualPorts(
+		configured,
+		available,
+	)
+
+	wantMissing := []string{
+		"/dev/ttyUSB102",
+	}
+
+	if !reflect.DeepEqual(got.Missing, wantMissing) {
+		t.Errorf(
+			"Missing = %v, want %v",
+			got.Missing,
+			wantMissing,
+		)
+	}
+
+	if len(got.Extra) != 0 {
+		t.Errorf(
+			"Extra = %v, want empty",
+			got.Extra,
+		)
+	}
+}
+
+func TestCompareVirtualPortsExtra(t *testing.T) {
+	configured := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+	}
+
+	available := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+		"/dev/ttyUSB103",
+	}
+
+	got := compareVirtualPorts(
+		configured,
+		available,
+	)
+
+	if len(got.Missing) != 0 {
+		t.Errorf(
+			"Missing = %v, want empty",
+			got.Missing,
+		)
+	}
+
+	wantExtra := []string{
+		"/dev/ttyUSB103",
+	}
+
+	if !reflect.DeepEqual(got.Extra, wantExtra) {
+		t.Errorf(
+			"Extra = %v, want %v",
+			got.Extra,
+			wantExtra,
+		)
+	}
+}
+
+func TestCompareVirtualPortsMissingAndExtra(t *testing.T) {
+	configured := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+		"/dev/ttyUSB102",
+	}
+
+	available := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+		"/dev/ttyUSB103",
+	}
+
+	got := compareVirtualPorts(
+		configured,
+		available,
+	)
+
+	wantMissing := []string{
+		"/dev/ttyUSB102",
+	}
+
+	wantExtra := []string{
+		"/dev/ttyUSB103",
+	}
+
+	if !reflect.DeepEqual(got.Missing, wantMissing) {
+		t.Errorf(
+			"Missing = %v, want %v",
+			got.Missing,
+			wantMissing,
+		)
+	}
+
+	if !reflect.DeepEqual(got.Extra, wantExtra) {
+		t.Errorf(
+			"Extra = %v, want %v",
+			got.Extra,
+			wantExtra,
+		)
+	}
+}
+
+func TestCompareVirtualPortsEmpty(t *testing.T) {
+	got := compareVirtualPorts(
+		nil,
+		nil,
+	)
+
+	if len(got.Missing) != 0 {
+		t.Errorf(
+			"Missing = %v, want empty",
+			got.Missing,
+		)
+	}
+
+	if len(got.Extra) != 0 {
+		t.Errorf(
+			"Extra = %v, want empty",
+			got.Extra,
+		)
+	}
+}
+
+func TestCompareVirtualPortsConfiguredOnly(t *testing.T) {
+	configured := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+	}
+
+	got := compareVirtualPorts(
+		configured,
+		nil,
+	)
+
+	wantMissing := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+	}
+
+	if !reflect.DeepEqual(got.Missing, wantMissing) {
+		t.Errorf(
+			"Missing = %v, want %v",
+			got.Missing,
+			wantMissing,
+		)
+	}
+
+	if len(got.Extra) != 0 {
+		t.Errorf(
+			"Extra = %v, want empty",
+			got.Extra,
+		)
+	}
+}
+
+func TestCompareVirtualPortsAvailableOnly(t *testing.T) {
+	available := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+	}
+
+	got := compareVirtualPorts(
+		nil,
+		available,
+	)
+
+	if len(got.Missing) != 0 {
+		t.Errorf(
+			"Missing = %v, want empty",
+			got.Missing,
+		)
+	}
+
+	wantExtra := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB101",
+	}
+
+	if !reflect.DeepEqual(got.Extra, wantExtra) {
+		t.Errorf(
+			"Extra = %v, want %v",
+			got.Extra,
+			wantExtra,
+		)
+	}
+}
+
+func TestCompareVirtualPortsIgnoresDuplicates(t *testing.T) {
+	configured := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB100",
+	}
+
+	available := []string{
+		"/dev/ttyUSB100",
+		"/dev/ttyUSB100",
+	}
+
+	got := compareVirtualPorts(
+		configured,
+		available,
+	)
+
+	if len(got.Missing) != 0 {
+		t.Errorf(
+			"Missing = %v, want empty",
+			got.Missing,
+		)
+	}
+
+	if len(got.Extra) != 0 {
+		t.Errorf(
+			"Extra = %v, want empty",
+			got.Extra,
+		)
+	}
+}

internal/client/virtual_port_validation_windows.go

@@ -0,0 +1,132 @@
+//go:build windows
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_port_validation_windows.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Windows-spezifische Ermittlung der lokal vorhandenen seriellen
+ * Schnittstellen für rs2322tcp.
+ *
+ * Unter Windows werden virtuelle Ports als COMxxx-Schnittstellen
+ * bereitgestellt.
+ *
+ * Diese Implementierung prüft den konfigurierten Portbereich und liefert
+ * die tatsächlich vorhandenen COM-Schnittstellen zurück.
+ *
+ * Die Funktion verändert keine Geräte und installiert keine Treiber.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+
+	"golang.org/x/sys/windows/registry"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Existing virtual ports
+///////////////////////////////////////////////////////////////////////////////
+
+// existingVirtualPortLinks returns all COM ports in the configured range
+// that are currently registered as serial ports in Windows.
+//
+// The Windows serial-port information is obtained from the Plug-and-Play
+// device registry. No device is created, modified, or removed.
+func existingVirtualPortLinks(
+	cfg config.VirtualPortRangeConfig,
+) ([]string, error) {
+	key, err := registry.OpenKey(
+		registry.LOCAL_MACHINE,
+		`HARDWARE\DEVICEMAP\SERIALCOMM`,
+		registry.READ,
+	)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"open Windows serial-port registry: %w",
+			err,
+		)
+	}
+	defer key.Close()
+
+	values, err := key.ReadValueNames(0)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"read Windows serial-port registry: %w",
+			err,
+		)
+	}
+
+	available := make([]string, 0)
+
+	for _, valueName := range values {
+		portName, _, err := key.GetStringValue(valueName)
+		if err != nil {
+			continue
+		}
+
+		number, ok := windowsCOMPortNumber(portName)
+		if !ok {
+			continue
+		}
+
+		if number < cfg.First || number > cfg.Last {
+			continue
+		}
+
+		available = append(
+			available,
+			portName,
+		)
+	}
+
+	return available, nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// COM port number
+///////////////////////////////////////////////////////////////////////////////
+
+// windowsCOMPortNumber extracts the numeric part of a Windows COM port name.
+//
+// Examples:
+//
+//	COM1  -> 1
+//	COM10 -> 10
+//
+// The comparison is case-insensitive.
+func windowsCOMPortNumber(portName string) (int, bool) {
+	if len(portName) < 4 {
+		return 0, false
+	}
+
+	if (portName[0] != 'C' && portName[0] != 'c') ||
+		(portName[1] != 'O' && portName[1] != 'o') ||
+		(portName[2] != 'M' && portName[2] != 'm') {
+		return 0, false
+	}
+
+	number := 0
+
+	for _, char := range portName[3:] {
+		if char < '0' || char > '9' {
+			return 0, false
+		}
+
+		number = number*10 + int(char-'0')
+	}
+
+	if number == 0 {
+		return 0, false
+	}
+
+	return number, true
+}

internal/client/virtual_serial.go

@@ -5,8 +5,8 @@ import "io"
 // VirtualSerial repräsentiert eine vom Client bereitgestellte
 // virtuelle serielle Schnittstelle.
 //
-// Path liefert den Namen der für die Anwendung sichtbaren
+// Path liefert den Pfad des internen virtuellen seriellen Geräts,
-// seriellen Schnittstelle, zum Beispiel /dev/pts/1.
+// beispielsweise /dev/pts/2.
 //
 // Das io.ReadWriteCloser-Interface repräsentiert die interne
 // Byte-Verbindung zum virtuellen seriellen Gerät.

internal/client/virtual_serial_windows.go

@@ -0,0 +1,210 @@
+//go:build windows
+
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/client/virtual_serial_windows.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Windows-spezifische Implementierung der virtuellen seriellen Schnittstelle
+ * über go.bug.st/serial.
+ *
+ * Ein COM-Port wird unter Windows nur einmal geöffnet. Dasselbe Handle wird
+ * parallel für Lese- und Schreiboperationen verwendet. Damit kann eine
+ * Goroutine blockierend lesen, während eine andere Goroutine Daten schreibt.
+ * ============================================================================
+ */
+
+package client
+
+import (
+	"fmt"
+	"sync"
+
+	bugserial "go.bug.st/serial"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// windowsSerial
+///////////////////////////////////////////////////////////////////////////////
+
+// windowsSerial implements VirtualSerial using one Windows COM-port handle
+// for both reading and writing.
+type windowsSerial struct {
+	mu sync.Mutex
+
+	port bugserial.Port
+	path string
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Path
+///////////////////////////////////////////////////////////////////////////////
+
+// Path returns the Windows COM-port name.
+func (p *windowsSerial) Path() string {
+	if p == nil {
+		return ""
+	}
+
+	return p.path
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Read
+///////////////////////////////////////////////////////////////////////////////
+
+// Read reads raw bytes from the Windows COM-port handle.
+func (p *windowsSerial) Read(b []byte) (int, error) {
+	if p == nil {
+		return 0, fmt.Errorf("virtual serial port is nil")
+	}
+
+	p.mu.Lock()
+	port := p.port
+	path := p.path
+	p.mu.Unlock()
+
+	if port == nil {
+		return 0, fmt.Errorf("virtual serial port is closed")
+	}
+
+	n, err := port.Read(b)
+
+	if n > 0 {
+		fmt.Printf(
+			"WINDOWS SERIAL RX [%s]: %d Bytes\n% X\n",
+			path,
+			n,
+			b[:n],
+		)
+	}
+
+	if err != nil {
+		fmt.Printf(
+			"WINDOWS SERIAL RX FEHLER [%s]: nach %d Bytes: %v\n",
+			path,
+			n,
+			err,
+		)
+	}
+
+	return n, err
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Write
+///////////////////////////////////////////////////////////////////////////////
+
+// Write writes raw bytes to the Windows COM-port handle.
+func (p *windowsSerial) Write(b []byte) (int, error) {
+	if p == nil {
+		return 0, fmt.Errorf("virtual serial port is nil")
+	}
+
+	p.mu.Lock()
+	port := p.port
+	path := p.path
+	p.mu.Unlock()
+
+	if port == nil {
+		return 0, fmt.Errorf("virtual serial port is closed")
+	}
+
+	fmt.Printf(
+		"WINDOWS SERIAL TX [%s]: %d Bytes\n% X\n",
+		path,
+		len(b),
+		b,
+	)
+
+	fmt.Printf(
+		"WINDOWS SERIAL TX [%s]: Rufe port.Write() auf ...\n",
+		path,
+	)
+
+	n, err := port.Write(b)
+
+	fmt.Printf(
+		"WINDOWS SERIAL TX [%s]: port.Write() zurückgekehrt: %d von %d Bytes\n",
+		path,
+		n,
+		len(b),
+	)
+
+	if err != nil {
+		fmt.Printf(
+			"WINDOWS SERIAL TX FEHLER [%s]: %v\n",
+			path,
+			err,
+		)
+
+		return n, err
+	}
+
+	if n != len(b) {
+		fmt.Printf(
+			"WINDOWS SERIAL TX UNVOLLSTÄNDIG [%s]: %d von %d Bytes\n",
+			path,
+			n,
+			len(b),
+		)
+	} else {
+		fmt.Printf(
+			"WINDOWS SERIAL TX ERFOLGREICH [%s]: %d Bytes\n",
+			path,
+			n,
+		)
+	}
+
+	return n, nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Close
+///////////////////////////////////////////////////////////////////////////////
+
+// Close closes the Windows COM-port handle.
+func (p *windowsSerial) Close() error {
+	if p == nil {
+		return nil
+	}
+
+	p.mu.Lock()
+
+	port := p.port
+	path := p.path
+
+	p.port = nil
+
+	p.mu.Unlock()
+
+	if port == nil {
+		return nil
+	}
+
+	fmt.Printf(
+		"WINDOWS SERIAL [%s]: Schließe COM-Port ...\n",
+		path,
+	)
+
+	if err := port.Close(); err != nil {
+		fmt.Printf(
+			"WINDOWS SERIAL [%s]: Fehler beim Schließen des COM-Ports: %v\n",
+			path,
+			err,
+		)
+
+		return err
+	}
+
+	fmt.Printf(
+		"WINDOWS SERIAL [%s]: COM-Port geschlossen.\n",
+		path,
+	)
+
+	return nil
+}

internal/config/config.go

@@ -2,8 +2,8 @@
 Package config provides configuration types and JSON handling for rs2322tcp.
 
 The server configuration describes the physical serial devices available
-on the remote system. The client configuration describes the connection
+on the remote system. The client configuration describes the connection to
-to the server and the user's assignment of virtual serial ports to remote
+the server and the user's assignment of virtual serial ports to remote
 devices.
 
 Project: rs2322tcp
@@ -23,8 +23,10 @@ import (
 
 // ServerConfig contains the complete server configuration.
 type ServerConfig struct {
-	Listen  ListenConfig   `json:"listen"`
+	Listen                ListenConfig   `json:"listen"`
-	Devices []DeviceConfig `json:"devices"`
+	HardwareErrorResponse string         `json:"hardware_error_response"`
+	SerialMonitor         bool           `json:"serial_monitor"`
+	Devices               []DeviceConfig `json:"devices"`
 }
 
 // ListenConfig contains the network listener configuration.
@@ -50,8 +52,9 @@ type DeviceConfig struct {
 
 // ClientConfig contains the complete client configuration.
 type ClientConfig struct {
-	Server       ServerConnectionConfig `json:"server"`
+	Server           ServerConnectionConfig `json:"server"`
-	VirtualPorts []VirtualPortConfig    `json:"virtual_ports"`
+	VirtualPortRange VirtualPortRangeConfig `json:"virtual_port_range"`
+	VirtualPorts     []VirtualPortConfig    `json:"virtual_ports"`
 }
 
 // ServerConnectionConfig contains the connection information for the
@@ -61,11 +64,47 @@ type ServerConnectionConfig struct {
 	Port    int    `json:"port"`
 }
 
+// VirtualPortRangeConfig defines the reserved range of /dev/ttyUSB device
+// numbers that may be used for virtual rs2322tcp ports.
+type VirtualPortRangeConfig struct {
+	First int `json:"first"`
+	Last  int `json:"last"`
+}
+
 // VirtualPortConfig describes one local virtual serial port and the
 // remote device assigned to it.
+//
+// RemoteDevice may be empty. An empty RemoteDevice means that the virtual
+// port exists but is currently not assigned to a remote device.
 type VirtualPortConfig struct {
 	Port         string `json:"port"`
 	RemoteDevice string `json:"remote_device"`
+	StartupBytes string `json:"startup_bytes,omitempty"`
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Defaults
+///////////////////////////////////////////////////////////////////////////////
+
+const (
+	// DefaultVirtualPortFirst is the first virtual USB serial port number
+	// used when no virtual port range is specified.
+	DefaultVirtualPortFirst = 100
+
+	// DefaultVirtualPortLast is the last virtual USB serial port number
+	// used when no virtual port range is specified.
+	DefaultVirtualPortLast = 199
+
+	// Fehlermeldung wenn Hardware nicht erreichbar ist
+	DefaultHardwareErrorResponse = "ERROR - HARDWARE NOT AVAILABLE"
+)
+
+// DefaultVirtualPortRange returns the default virtual port range.
+func DefaultVirtualPortRange() VirtualPortRangeConfig {
+	return VirtualPortRangeConfig{
+		First: DefaultVirtualPortFirst,
+		Last:  DefaultVirtualPortLast,
+	}
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -80,6 +119,10 @@ func LoadServer(filename string) (*ServerConfig, error) {
 		return nil, err
 	}
 
+	if cfg.HardwareErrorResponse == "" {
+		cfg.HardwareErrorResponse = DefaultHardwareErrorResponse
+	}
+
 	if err := cfg.Validate(); err != nil {
 		return nil, err
 	}
@@ -95,6 +138,12 @@ func LoadClient(filename string) (*ClientConfig, error) {
 		return nil, err
 	}
 
+	// A missing virtual_port_range is intentionally supported for backwards
+	// compatibility. Apply the documented default before validation.
+	if cfg.VirtualPortRange.First == 0 && cfg.VirtualPortRange.Last == 0 {
+		cfg.VirtualPortRange = DefaultVirtualPortRange()
+	}
+
 	if err := cfg.Validate(); err != nil {
 		return nil, err
 	}
@@ -125,6 +174,10 @@ func SaveClient(filename string, cfg *ClientConfig) error {
 		return fmt.Errorf("client configuration is nil")
 	}
 
+	if cfg.VirtualPortRange.First == 0 && cfg.VirtualPortRange.Last == 0 {
+		cfg.VirtualPortRange = DefaultVirtualPortRange()
+	}
+
 	if err := cfg.Validate(); err != nil {
 		return err
 	}
@@ -180,6 +233,9 @@ func (cfg *ServerConfig) Validate() error {
 }
 
 // Validate checks the client configuration for basic errors.
+//
+// A virtual port may exist without a remote-device assignment. This is a
+// valid state and means that the virtual port is currently free.
 func (cfg *ClientConfig) Validate() error {
 	if cfg == nil {
 		return fmt.Errorf("client configuration is nil")
@@ -189,21 +245,37 @@ func (cfg *ClientConfig) Validate() error {
 		return fmt.Errorf("invalid server port: %d", cfg.Server.Port)
 	}
 
+	if cfg.VirtualPortRange.First < DefaultVirtualPortFirst {
+		return fmt.Errorf(
+			"virtual port range first must be >= %d: %d",
+			DefaultVirtualPortFirst,
+			cfg.VirtualPortRange.First,
+		)
+	}
+
+	if cfg.VirtualPortRange.Last < cfg.VirtualPortRange.First {
+		return fmt.Errorf(
+			"invalid virtual port range: %d-%d",
+			cfg.VirtualPortRange.First,
+			cfg.VirtualPortRange.Last,
+		)
+	}
+
 	ports := make(map[string]bool)
 
 	for i, virtualPort := range cfg.VirtualPorts {
 		if virtualPort.Port == "" {
-			return fmt.Errorf("virtual port %d: port is empty", i)
+			return fmt.Errorf(
-		}
+				"virtual port %d: port is empty",
-
+				i,
-		if virtualPort.RemoteDevice == "" {
+			)
-			return fmt.Errorf("virtual port %q: remote device is empty",
-				virtualPort.Port)
 		}
 
 		if ports[virtualPort.Port] {
-			return fmt.Errorf("duplicate virtual port: %q",
+			return fmt.Errorf(
-				virtualPort.Port)
+				"duplicate virtual port: %q",
+				virtualPort.Port,
+			)
 		}
 
 		ports[virtualPort.Port] = true
@@ -219,11 +291,19 @@ func (cfg *ClientConfig) Validate() error {
 func loadJSON(filename string, target interface{}) error {
 	data, err := os.ReadFile(filename)
 	if err != nil {
-		return fmt.Errorf("read configuration %q: %w", filename, err)
+		return fmt.Errorf(
+			"read configuration %q: %w",
+			filename,
+			err,
+		)
 	}
 
 	if err := json.Unmarshal(data, target); err != nil {
-		return fmt.Errorf("parse configuration %q: %w", filename, err)
+		return fmt.Errorf(
+			"parse configuration %q: %w",
+			filename,
+			err,
+		)
 	}
 
 	return nil
@@ -238,7 +318,11 @@ func saveJSON(filename string, value interface{}) error {
 	data = append(data, '\n')
 
 	if err := os.WriteFile(filename, data, 0644); err != nil {
-		return fmt.Errorf("write configuration %q: %w", filename, err)
+		return fmt.Errorf(
+			"write configuration %q: %w",
+			filename,
+			err,
+		)
 	}
 
 	return nil

internal/config/config_test.go

@@ -8,11 +8,13 @@ package config_test
 
 import (
 	"encoding/json"
-	"git.lang-dieter.de/rs2322tcp/internal/config"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
 )
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -50,14 +52,24 @@ func TestLoadServer(t *testing.T) {
 		t.Fatalf("LoadServer() failed: %v", err)
 	}
 
+	if cfg.HardwareErrorResponse != config.DefaultHardwareErrorResponse {
+		t.Errorf(
+			"HardwareErrorResponse = %q, want %q",
+			cfg.HardwareErrorResponse,
+			config.DefaultHardwareErrorResponse,
+		)
+	}
+
 	if cfg.Listen.Address != "0.0.0.0" {
-		t.Errorf("Listen.Address = %q, want %q",
+		t.Errorf(
-			cfg.Listen.Address, "0.0.0.0")
+			"Listen.Address = %q, want %q",
+			cfg.Listen.Address,
+			"0.0.0.0",
+		)
 	}
 
 	if cfg.Listen.Port != 5000 {
-		t.Errorf("Listen.Port = %d, want %d",
+		t.Errorf("Listen.Port = %d, want %d", cfg.Listen.Port, 5000)
-			cfg.Listen.Port, 5000)
 	}
 
 	if len(cfg.Devices) != 1 {
@@ -67,18 +79,19 @@ func TestLoadServer(t *testing.T) {
 	device := cfg.Devices[0]
 
 	if device.ID != "radio" {
-		t.Errorf("Device.ID = %q, want %q",
+		t.Errorf("Device.ID = %q, want %q", device.ID, "radio")
-			device.ID, "radio")
 	}
 
 	if device.SerialPort != "/dev/ttyUSB0" {
-		t.Errorf("Device.SerialPort = %q, want %q",
+		t.Errorf(
-			device.SerialPort, "/dev/ttyUSB0")
+			"Device.SerialPort = %q, want %q",
+			device.SerialPort,
+			"/dev/ttyUSB0",
+		)
 	}
 
 	if device.BaudRate != 9600 {
-		t.Errorf("Device.BaudRate = %d, want %d",
+		t.Errorf("Device.BaudRate = %d, want %d", device.BaudRate, 9600)
-			device.BaudRate, 9600)
 	}
 }
 
@@ -113,30 +126,146 @@ func TestLoadClient(t *testing.T) {
 	}
 
 	if cfg.Server.Address != "100.64.0.10" {
-		t.Errorf("Server.Address = %q, want %q",
+		t.Errorf(
-			cfg.Server.Address, "100.64.0.10")
+			"Server.Address = %q, want %q",
+			cfg.Server.Address,
+			"100.64.0.10",
+		)
 	}
 
 	if cfg.Server.Port != 5000 {
-		t.Errorf("Server.Port = %d, want %d",
+		t.Errorf("Server.Port = %d, want %d", cfg.Server.Port, 5000)
-			cfg.Server.Port, 5000)
+	}
+
+	if cfg.VirtualPortRange.First != config.DefaultVirtualPortFirst {
+		t.Errorf(
+			"VirtualPortRange.First = %d, want %d",
+			cfg.VirtualPortRange.First,
+			config.DefaultVirtualPortFirst,
+		)
+	}
+
+	if cfg.VirtualPortRange.Last != config.DefaultVirtualPortLast {
+		t.Errorf(
+			"VirtualPortRange.Last = %d, want %d",
+			cfg.VirtualPortRange.Last,
+			config.DefaultVirtualPortLast,
+		)
 	}
 
 	if len(cfg.VirtualPorts) != 1 {
-		t.Fatalf("len(VirtualPorts) = %d, want 1",
+		t.Fatalf(
-			len(cfg.VirtualPorts))
+			"len(VirtualPorts) = %d, want 1",
+			len(cfg.VirtualPorts),
+		)
 	}
 
 	virtualPort := cfg.VirtualPorts[0]
 
 	if virtualPort.Port != "COM7" {
-		t.Errorf("VirtualPort.Port = %q, want %q",
+		t.Errorf(
-			virtualPort.Port, "COM7")
+			"VirtualPort.Port = %q, want %q",
+			virtualPort.Port,
+			"COM7",
+		)
 	}
 
 	if virtualPort.RemoteDevice != "radio" {
-		t.Errorf("VirtualPort.RemoteDevice = %q, want %q",
+		t.Errorf(
-			virtualPort.RemoteDevice, "radio")
+			"VirtualPort.RemoteDevice = %q, want %q",
+			virtualPort.RemoteDevice,
+			"radio",
+		)
+	}
+
+	if virtualPort.StartupBytes != "" {
+		t.Errorf(
+			"VirtualPort.StartupBytes = %q, want empty",
+			virtualPort.StartupBytes,
+		)
+	}
+}
+
+func TestLoadClientWithStartupBytes(t *testing.T) {
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "client.json")
+
+	data := `{
+		"server": {
+			"address": "100.64.0.10",
+			"port": 5000
+		},
+		"virtual_ports": [
+			{
+				"port": "COM7",
+				"remote_device": "radio",
+				"startup_bytes": "57 00 00 1F 20"
+			}
+		]
+	}`
+
+	if err := os.WriteFile(filename, []byte(data), 0644); err != nil {
+		t.Fatalf("write test configuration: %v", err)
+	}
+
+	cfg, err := config.LoadClient(filename)
+	if err != nil {
+		t.Fatalf("LoadClient() failed: %v", err)
+	}
+
+	if len(cfg.VirtualPorts) != 1 {
+		t.Fatalf(
+			"len(VirtualPorts) = %d, want 1",
+			len(cfg.VirtualPorts),
+		)
+	}
+
+	if cfg.VirtualPorts[0].StartupBytes != "57 00 00 1F 20" {
+		t.Errorf(
+			"VirtualPort.StartupBytes = %q, want %q",
+			cfg.VirtualPorts[0].StartupBytes,
+			"57 00 00 1F 20",
+		)
+	}
+}
+
+func TestLoadClientCustomVirtualPortRange(t *testing.T) {
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "client.json")
+
+	data := `{
+		"server": {
+			"address": "100.64.0.10",
+			"port": 5000
+		},
+		"virtual_port_range": {
+			"first": 200,
+			"last": 299
+		},
+		"virtual_ports": []
+	}`
+
+	if err := os.WriteFile(filename, []byte(data), 0644); err != nil {
+		t.Fatalf("write test configuration: %v", err)
+	}
+
+	cfg, err := config.LoadClient(filename)
+	if err != nil {
+		t.Fatalf("LoadClient() failed: %v", err)
+	}
+
+	if cfg.VirtualPortRange.First != 200 {
+		t.Errorf(
+			"VirtualPortRange.First = %d, want 200",
+			cfg.VirtualPortRange.First,
+		)
+	}
+
+	if cfg.VirtualPortRange.Last != 299 {
+		t.Errorf(
+			"VirtualPortRange.Last = %d, want 299",
+			cfg.VirtualPortRange.Last,
+		)
 	}
 }
 
@@ -153,10 +282,15 @@ func TestSaveAndLoadClient(t *testing.T) {
 			Address: "100.64.0.10",
 			Port:    5000,
 		},
+		VirtualPortRange: config.VirtualPortRangeConfig{
+			First: 200,
+			Last:  299,
+		},
 		VirtualPorts: []config.VirtualPortConfig{
 			{
 				Port:         "COM7",
 				RemoteDevice: "radio",
+				StartupBytes: "57 00 00 1F 20",
 			},
 			{
 				Port:         "COM8",
@@ -178,10 +312,16 @@ func TestSaveAndLoadClient(t *testing.T) {
 		t.Errorf("loaded Server differs from original")
 	}
 
+	if loaded.VirtualPortRange != original.VirtualPortRange {
+		t.Errorf("loaded VirtualPortRange differs from original")
+	}
+
 	if len(loaded.VirtualPorts) != len(original.VirtualPorts) {
-		t.Fatalf("len(VirtualPorts) = %d, want %d",
+		t.Fatalf(
+			"len(VirtualPorts) = %d, want %d",
 			len(loaded.VirtualPorts),
-			len(original.VirtualPorts))
+			len(original.VirtualPorts),
+		)
 	}
 
 	for i := range original.VirtualPorts {
@@ -228,6 +368,7 @@ func TestClientValidationDuplicateVirtualPort(t *testing.T) {
 		Server: config.ServerConnectionConfig{
 			Port: 5000,
 		},
+		VirtualPortRange: config.DefaultVirtualPortRange(),
 		VirtualPorts: []config.VirtualPortConfig{
 			{
 				Port:         "COM7",
@@ -262,6 +403,7 @@ func TestClientValidationInvalidPort(t *testing.T) {
 		Server: config.ServerConnectionConfig{
 			Port: 0,
 		},
+		VirtualPortRange: config.DefaultVirtualPortRange(),
 	}
 
 	if err := cfg.Validate(); err == nil {
@@ -269,6 +411,54 @@ func TestClientValidationInvalidPort(t *testing.T) {
 	}
 }
 
+func TestClientValidationVirtualPortRangeBelowReservedArea(t *testing.T) {
+	cfg := &config.ClientConfig{
+		Server: config.ServerConnectionConfig{
+			Port: 5000,
+		},
+		VirtualPortRange: config.VirtualPortRangeConfig{
+			First: 99,
+			Last:  199,
+		},
+	}
+
+	if err := cfg.Validate(); err == nil {
+		t.Fatal("Validate() succeeded, want range-below-reserved-area error")
+	}
+}
+
+func TestClientValidationVirtualPortRangeReversed(t *testing.T) {
+	cfg := &config.ClientConfig{
+		Server: config.ServerConnectionConfig{
+			Port: 5000,
+		},
+		VirtualPortRange: config.VirtualPortRangeConfig{
+			First: 200,
+			Last:  100,
+		},
+	}
+
+	if err := cfg.Validate(); err == nil {
+		t.Fatal("Validate() succeeded, want reversed-range error")
+	}
+}
+
+func TestClientValidationVirtualPortRangeValid(t *testing.T) {
+	cfg := &config.ClientConfig{
+		Server: config.ServerConnectionConfig{
+			Port: 5000,
+		},
+		VirtualPortRange: config.VirtualPortRangeConfig{
+			First: 200,
+			Last:  299,
+		},
+	}
+
+	if err := cfg.Validate(); err != nil {
+		t.Fatalf("Validate() failed: %v", err)
+	}
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Example configurations
 ///////////////////////////////////////////////////////////////////////////////
@@ -279,8 +469,8 @@ func TestExampleServerConfig(t *testing.T) {
 		t.Fatalf("LoadServer() failed: %v", err)
 	}
 
-	if len(cfg.Devices) != 2 {
+	if len(cfg.Devices) != 3 {
-		t.Fatalf("len(Devices) = %d, want 2", len(cfg.Devices))
+		t.Fatalf("len(Devices) = %d, want 3", len(cfg.Devices))
 	}
 }
 
@@ -290,8 +480,35 @@ func TestExampleClientConfig(t *testing.T) {
 		t.Fatalf("LoadClient() failed: %v", err)
 	}
 
-	if len(cfg.VirtualPorts) != 2 {
+	if len(cfg.VirtualPorts) != 1 {
-		t.Fatalf("len(VirtualPorts) = %d, want 2", len(cfg.VirtualPorts))
+		t.Fatalf(
+			"len(VirtualPorts) = %d, want 1",
+			len(cfg.VirtualPorts),
+		)
+	}
+
+	expectedPort := "COM100"
+
+	if runtime.GOOS == "linux" {
+		expectedPort = "/dev/ttyUSB100"
+	}
+
+	port := cfg.VirtualPorts[0]
+
+	if port.Port != expectedPort {
+		t.Errorf(
+			"VirtualPorts[0].Port = %q, want %q",
+			port.Port,
+			expectedPort,
+		)
+	}
+
+	if port.RemoteDevice != "rotor" {
+		t.Errorf(
+			"VirtualPorts[0].RemoteDevice = %q, want %q",
+			port.RemoteDevice,
+			"rotor",
+		)
 	}
 }
 
@@ -313,33 +530,51 @@ func TestDeviceRemoteDevice(t *testing.T) {
 	remote := device.RemoteDevice()
 
 	if remote.ID != "radio" {
-		t.Errorf("RemoteDevice.ID = %q, want %q",
+		t.Errorf(
-			remote.ID, "radio")
+			"RemoteDevice.ID = %q, want %q",
+			remote.ID,
+			"radio",
+		)
 	}
 
 	if remote.Name != "Funkgerät" {
-		t.Errorf("RemoteDevice.Name = %q, want %q",
+		t.Errorf(
-			remote.Name, "Funkgerät")
+			"RemoteDevice.Name = %q, want %q",
+			remote.Name,
+			"Funkgerät",
+		)
 	}
 
 	if remote.BaudRate != 9600 {
-		t.Errorf("RemoteDevice.BaudRate = %d, want %d",
+		t.Errorf(
-			remote.BaudRate, 9600)
+			"RemoteDevice.BaudRate = %d, want %d",
+			remote.BaudRate,
+			9600,
+		)
 	}
 
 	if remote.DataBits != 8 {
-		t.Errorf("RemoteDevice.DataBits = %d, want %d",
+		t.Errorf(
-			remote.DataBits, 8)
+			"RemoteDevice.DataBits = %d, want %d",
+			remote.DataBits,
+			8,
+		)
 	}
 
 	if remote.Parity != "none" {
-		t.Errorf("RemoteDevice.Parity = %q, want %q",
+		t.Errorf(
-			remote.Parity, "none")
+			"RemoteDevice.Parity = %q, want %q",
+			remote.Parity,
+			"none",
+		)
 	}
 
 	if remote.StopBits != 1 {
-		t.Errorf("RemoteDevice.StopBits = %d, want %d",
+		t.Errorf(
-			remote.StopBits, 1)
+			"RemoteDevice.StopBits = %d, want %d",
+			remote.StopBits,
+			1,
+		)
 	}
 }
 
@@ -378,28 +613,41 @@ func TestServerRemoteDevices(t *testing.T) {
 	}
 
 	if list.Devices[0].ID != "radio" {
-		t.Errorf("Devices[0].ID = %q, want %q",
+		t.Errorf(
-			list.Devices[0].ID, "radio")
+			"Devices[0].ID = %q, want %q",
+			list.Devices[0].ID,
+			"radio",
+		)
 	}
 
 	if list.Devices[1].ID != "rotor" {
-		t.Errorf("Devices[1].ID = %q, want %q",
+		t.Errorf(
-			list.Devices[1].ID, "rotor")
+			"Devices[1].ID = %q, want %q",
+			list.Devices[1].ID,
+			"rotor",
+		)
 	}
 
 	if list.Devices[0].Name != "Funkgerät" {
-		t.Errorf("Devices[0].Name = %q, want %q",
+		t.Errorf(
-			list.Devices[0].Name, "Funkgerät")
+			"Devices[0].Name = %q, want %q",
+			list.Devices[0].Name,
+			"Funkgerät",
+		)
 	}
 
 	if list.Devices[0].BaudRate != 9600 {
-		t.Errorf("Devices[0].BaudRate = %d, want %d",
+		t.Errorf(
-			list.Devices[0].BaudRate, 9600)
+			"Devices[0].BaudRate = %d, want 9600",
+			list.Devices[0].BaudRate,
+		)
 	}
 
 	if list.Devices[1].BaudRate != 4800 {
-		t.Errorf("Devices[1].BaudRate = %d, want %d",
+		t.Errorf(
-			list.Devices[1].BaudRate, 4800)
+			"Devices[1].BaudRate = %d, want 4800",
+			list.Devices[1].BaudRate,
+		)
 	}
 }
 
@@ -449,11 +697,17 @@ func TestRemoteDeviceJSON(t *testing.T) {
 	}
 
 	if len(decoded.Devices) != 1 {
-		t.Fatalf("len(Devices) = %d, want 1", len(decoded.Devices))
+		t.Fatalf(
+			"len(decoded.Devices) = %d, want 1",
+			len(decoded.Devices),
+		)
 	}
 
 	if decoded.Devices[0].ID != "radio" {
-		t.Errorf("Devices[0].ID = %q, want %q",
+		t.Errorf(
-			decoded.Devices[0].ID, "radio")
+			"Devices[0].ID = %q, want %q",
+			decoded.Devices[0].ID,
+			"radio",
+		)
 	}
 }

internal/gui/app.go

@@ -0,0 +1,729 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/gui/app.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Grafische Benutzeroberfläche des rs2322tcp-Clients.
+ *
+ * Die eigentliche Kommunikation mit dem Server sowie die Verwaltung der
+ * virtuellen seriellen Ports bleiben vollständig in internal/client.
+ *
+ * Die Gerätezuordnung wird in einem separaten Fenster bearbeitet.
+ *
+ * Änderungen an der Gerätezuordnung werden zunächst ausschließlich im
+ * Arbeitsspeicher des Zuordnungsfensters gehalten.
+ *
+ * Beim Schließen des Zuordnungsfensters werden vorhandene Änderungen
+ * gespeichert.
+ *
+ * Wird dagegen das Hauptfenster geschlossen, während das Zuordnungsfenster
+ * noch ungespeicherte Änderungen enthält, wird der Anwender gefragt, ob
+ * diese Änderungen verworfen werden sollen.
+ * ============================================================================
+ */
+package gui
+
+import (
+	"fmt"
+	"sort"
+
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/container"
+	"fyne.io/fyne/v2/dialog"
+	"fyne.io/fyne/v2/layout"
+	"fyne.io/fyne/v2/widget"
+
+	"git.lang-dieter.de/rs2322tcp/internal/client"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// App
+///////////////////////////////////////////////////////////////////////////////
+
+// App represents the graphical rs2322tcp client application.
+//
+// App owns the Fyne window and the technical client application. The GUI
+// does not access the Runtime or the Client directly. All technical client
+// operations are performed through client.Application.
+type App struct {
+	fyneApp fyne.App
+	window  fyne.Window
+
+	clientApplication *client.Application
+
+	statusLabel      *widget.Label
+	errorLabel       *widget.Label
+	reconnectButton  *widget.Button
+	assignmentButton *widget.Button
+
+	assignmentWindow fyne.Window
+	assignmentEditor *AssignmentEditor
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Construction
+///////////////////////////////////////////////////////////////////////////////
+
+// NewApp creates the graphical rs2322tcp client application.
+//
+// No network connection is established by NewApp. Start must be called
+// separately.
+func NewApp(
+	fyneApp fyne.App,
+	configFile string,
+) (*App, error) {
+	if fyneApp == nil {
+		return nil, fmt.Errorf("fyne application is nil")
+	}
+
+	application, err := client.NewApplication(configFile)
+	if err != nil {
+		return nil, err
+	}
+
+	window := fyneApp.NewWindow("rs2322tcp Client")
+	window.Resize(fyne.NewSize(800, 600))
+
+	app := &App{
+		fyneApp:           fyneApp,
+		window:            window,
+		clientApplication: application,
+		statusLabel:       widget.NewLabel("Server: nicht verbunden"),
+		errorLabel:        widget.NewLabel(""),
+	}
+
+	app.reconnectButton = widget.NewButton(
+		"Server neu verbinden",
+		app.reconnect,
+	)
+
+	app.assignmentButton = widget.NewButton(
+		"Gerätezuordnung",
+		app.showAssignments,
+	)
+
+	app.buildContent()
+
+	// Das Schließen des Hauptfensters wird abgefangen, damit ein noch
+	// geöffnetes Zuordnungsfenster mit ungespeicherten Änderungen nicht
+	// stillschweigend verworfen wird.
+	window.SetCloseIntercept(
+		app.handleMainWindowClose,
+	)
+
+	return app, nil
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Main window
+///////////////////////////////////////////////////////////////////////////////
+
+// buildContent creates the main-window content.
+func (a *App) buildContent() {
+	title := widget.NewLabel("rs2322tcp Client")
+
+	content := container.NewVBox(
+		title,
+		a.statusLabel,
+		a.errorLabel,
+		a.assignmentButton,
+		a.reconnectButton,
+	)
+
+	a.window.SetContent(content)
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Client lifecycle
+///////////////////////////////////////////////////////////////////////////////
+
+// Start starts the technical client application in the background.
+//
+// The Fyne window remains responsive while the connection to the server is
+// established.
+func (a *App) Start() {
+	if a == nil {
+		return
+	}
+
+	a.statusLabel.SetText(
+		"Server: Verbindung wird aufgebaut ...",
+	)
+	a.errorLabel.SetText("")
+	a.reconnectButton.Disable()
+
+	go func() {
+		err := a.clientApplication.Start()
+
+		if err != nil {
+			fyne.Do(func() {
+				a.statusLabel.SetText(
+					"Server: nicht verbunden",
+				)
+				a.errorLabel.SetText("")
+
+				a.showStartupError(err)
+			})
+
+			return
+		}
+
+		devices := a.clientApplication.Devices()
+
+		fyne.Do(func() {
+			a.statusLabel.SetText(
+				fmt.Sprintf(
+					"Server: verbunden – %d Geräte verfügbar",
+					len(devices),
+				),
+			)
+
+			a.errorLabel.SetText("")
+			a.reconnectButton.Enable()
+		})
+	}()
+}
+
+// reconnect starts a complete reconnect of the technical client.
+func (a *App) reconnect() {
+	if a == nil {
+		return
+	}
+
+	a.statusLabel.SetText(
+		"Server: Verbindung wird neu aufgebaut ...",
+	)
+	a.errorLabel.SetText("")
+	a.reconnectButton.Disable()
+
+	go func() {
+		err := a.clientApplication.Reconnect()
+
+		if err != nil {
+			fyne.Do(func() {
+				a.statusLabel.SetText(
+					"Server: nicht verbunden",
+				)
+				a.errorLabel.SetText("")
+
+				a.showStartupError(err)
+			})
+
+			return
+		}
+
+		devices := a.clientApplication.Devices()
+
+		fyne.Do(func() {
+			a.statusLabel.SetText(
+				fmt.Sprintf(
+					"Server: verbunden – %d Geräte verfügbar",
+					len(devices),
+				),
+			)
+
+			a.errorLabel.SetText("")
+			a.reconnectButton.Enable()
+		})
+	}()
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Startup error
+///////////////////////////////////////////////////////////////////////////////
+
+// showStartupError displays a modal error dialog for a client startup error.
+//
+// The dialog intentionally has only an "OK" button. A client configuration
+// error must not be bypassed because starting the client with inconsistent
+// virtual-port configuration could result in communication being directed
+// to the wrong or missing local interface.
+//
+// After the user acknowledges the error, the technical client is closed and
+// the complete Fyne application is terminated.
+func (a *App) showStartupError(err error) {
+	if a == nil || a.window == nil || err == nil {
+		return
+	}
+
+	title := widget.NewLabelWithStyle(
+		"Die virtuellen Ports konnten nicht geprüft werden.",
+		fyne.TextAlignLeading,
+		fyne.TextStyle{
+			Bold: true,
+		},
+	)
+
+	message := widget.NewLabel(
+		err.Error(),
+	)
+
+	message.Wrapping = fyne.TextWrapWord
+
+	content := container.NewVBox(
+		title,
+		widget.NewSeparator(),
+		message,
+	)
+
+	confirm := dialog.NewCustomWithoutButtons(
+		"Virtuelle Ports",
+		container.NewPadded(content),
+		a.window,
+	)
+
+	okButton := widget.NewButton(
+		"OK",
+		func() {
+			confirm.Dismiss()
+
+			_ = a.clientApplication.Close()
+
+			a.window.SetCloseIntercept(nil)
+
+			a.fyneApp.Quit()
+		},
+	)
+
+	confirm.SetButtons(
+		[]fyne.CanvasObject{
+			okButton,
+		},
+	)
+
+	confirm.Show()
+
+	confirm.Resize(
+		fyne.NewSize(650, 420),
+	)
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Device assignment
+///////////////////////////////////////////////////////////////////////////////
+
+// showAssignments opens the editable device-assignment window.
+func (a *App) showAssignments() {
+	if a == nil {
+		return
+	}
+
+	// Nur ein Zuordnungsfenster gleichzeitig.
+	if a.assignmentWindow != nil {
+		a.assignmentWindow.Show()
+		return
+	}
+
+	cfg := a.clientApplication.Config()
+	devices := a.clientApplication.Devices()
+
+	editor := NewAssignmentEditor(
+		cfg,
+		devices,
+	)
+
+	window := a.fyneApp.NewWindow(
+		"Gerätezuordnung",
+	)
+
+	window.Resize(
+		fyne.NewSize(650, 450),
+	)
+
+	a.assignmentWindow = window
+	a.assignmentEditor = editor
+
+	// Beim Schließen über das Fenstersymbol wird dieselbe Logik wie beim
+	// Button "Schließen" verwendet.
+	window.SetCloseIntercept(
+		func() {
+			a.handleAssignmentWindowClose(
+				window,
+				editor,
+			)
+		},
+	)
+
+	window.SetOnClosed(func() {
+		if a.assignmentWindow == window {
+			a.assignmentWindow = nil
+			a.assignmentEditor = nil
+		}
+	})
+
+	a.refreshAssignmentWindow(
+		window,
+		editor,
+	)
+
+	window.Show()
+}
+
+// refreshAssignmentWindow rebuilds all assignment Select widgets.
+//
+// This is intentional. When a device is disconnected, its former port
+// becomes free and must immediately become selectable for the other
+// currently unconnected devices.
+func (a *App) refreshAssignmentWindow(
+	window fyne.Window,
+	editor *AssignmentEditor,
+) {
+	if a == nil || window == nil || editor == nil {
+		return
+	}
+
+	deviceIDs := editorDeviceIDs(editor)
+
+	objects := make(
+		[]fyne.CanvasObject,
+		0,
+		len(deviceIDs)*2,
+	)
+
+	objects = append(
+		objects,
+		widget.NewLabelWithStyle(
+			"Server-Gerät",
+			fyne.TextAlignLeading,
+			fyne.TextStyle{Bold: true},
+		),
+	)
+
+	objects = append(
+		objects,
+		widget.NewLabelWithStyle(
+			"Lokale Schnittstelle",
+			fyne.TextAlignLeading,
+			fyne.TextStyle{Bold: true},
+		),
+	)
+
+	for _, deviceID := range deviceIDs {
+		deviceID := deviceID
+
+		deviceLabel := widget.NewLabel(deviceID)
+
+		selectBox := widget.NewSelect(
+			editor.Options(deviceID),
+			nil,
+		)
+
+		currentPort := editor.CurrentPort(deviceID)
+
+		if currentPort == "" {
+			selectBox.SetSelected(
+				NotConnected,
+			)
+		} else {
+			selectBox.SetSelected(
+				currentPort,
+			)
+		}
+
+		selectBox.OnChanged = func(selected string) {
+			if !editor.Set(
+				deviceID,
+				selected,
+			) {
+				return
+			}
+
+			// Nach jeder Änderung werden sämtliche Selectboxen neu
+			// aufgebaut. Dadurch werden gerade freigegebene Ports
+			// sofort bei allen anderen Geräten sichtbar.
+			a.refreshAssignmentWindow(
+				window,
+				editor,
+			)
+		}
+
+		objects = append(
+			objects,
+			deviceLabel,
+			selectBox,
+		)
+	}
+
+	grid := container.New(
+		layout.NewFormLayout(),
+		objects...,
+	)
+
+	info := widget.NewLabel(
+		"Die Änderungen werden beim Schließen gespeichert.",
+	)
+
+	closeButton := widget.NewButton(
+		"Schließen",
+		func() {
+			a.handleAssignmentWindowClose(
+				window,
+				editor,
+			)
+		},
+	)
+
+	footer := container.NewVBox(
+		info,
+		closeButton,
+	)
+
+	window.SetContent(
+		container.NewBorder(
+			nil,
+			footer,
+			nil,
+			nil,
+			container.NewVScroll(grid),
+		),
+	)
+}
+
+// editorDeviceIDs returns all server-device IDs known by the assignment
+// editor in deterministic order.
+func editorDeviceIDs(
+	editor *AssignmentEditor,
+) []string {
+	if editor == nil {
+		return nil
+	}
+
+	deviceIDs := make(
+		[]string,
+		0,
+		len(editor.Current),
+	)
+
+	for deviceID := range editor.Current {
+		deviceIDs = append(
+			deviceIDs,
+			deviceID,
+		)
+	}
+
+	sort.Strings(deviceIDs)
+
+	return deviceIDs
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Assignment saving
+///////////////////////////////////////////////////////////////////////////////
+
+// handleAssignmentWindowClose handles closing of the assignment window.
+//
+// The "Schließen" button saves changed assignments. If there are no changes,
+// the window is simply closed.
+func (a *App) handleAssignmentWindowClose(
+	window fyne.Window,
+	editor *AssignmentEditor,
+) {
+	if a == nil || window == nil || editor == nil {
+		return
+	}
+
+	if !editor.Dirty() {
+		a.closeAssignmentWindowWithoutPrompt(
+			window,
+		)
+
+		return
+	}
+
+	a.saveAssignmentsAndClose(
+		window,
+		editor,
+	)
+}
+
+// saveAssignmentsAndClose converts the editor state into a client
+// configuration, saves it through client.Application and closes the
+// assignment window only after a successful save.
+func (a *App) saveAssignmentsAndClose(
+	window fyne.Window,
+	editor *AssignmentEditor,
+) {
+	if a == nil || window == nil || editor == nil {
+		return
+	}
+
+	cfg := a.clientApplication.Config()
+	if cfg == nil {
+		dialog.ShowError(
+			fmt.Errorf(
+				"Client-Konfiguration ist nicht verfügbar",
+			),
+			window,
+		)
+
+		return
+	}
+
+	updatedCfg := editor.ApplyToConfig(cfg)
+	if updatedCfg == nil {
+		dialog.ShowError(
+			fmt.Errorf(
+				"Gerätezuordnung konnte nicht übernommen werden",
+			),
+			window,
+		)
+
+		return
+	}
+
+	if err := a.clientApplication.SaveConfig(
+		updatedCfg,
+	); err != nil {
+		dialog.ShowError(
+			fmt.Errorf(
+				"Client-Konfiguration konnte nicht gespeichert werden: %w",
+				err,
+			),
+			window,
+		)
+
+		return
+	}
+
+	// Erst nach erfolgreichem Speichern schließen.
+	a.closeAssignmentWindowWithoutPrompt(
+		window,
+	)
+}
+
+// closeAssignmentWindowWithoutPrompt closes the assignment window without
+// invoking its close intercept again.
+func (a *App) closeAssignmentWindowWithoutPrompt(
+	window fyne.Window,
+) {
+	if window == nil {
+		return
+	}
+
+	window.SetCloseIntercept(nil)
+	window.Close()
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Main window closing
+///////////////////////////////////////////////////////////////////////////////
+
+// handleMainWindowClose handles closing of the main window.
+//
+// If the assignment window is open and contains unsaved changes, the user
+// must explicitly decide whether those changes should be discarded.
+func (a *App) handleMainWindowClose() {
+	if a == nil {
+		return
+	}
+
+	if a.assignmentEditor == nil ||
+		!a.assignmentEditor.Dirty() {
+
+		a.finishMainWindowClose()
+
+		return
+	}
+
+	confirm := dialog.NewConfirm(
+		"Ungespeicherte Änderungen",
+		"Die Gerätezuordnung enthält ungespeicherte "+
+			"Änderungen.\n\n"+
+			"Wenn das Hauptfenster geschlossen wird, "+
+			"gehen diese Änderungen verloren.\n\n"+
+			"Möchtest du die Änderungen verwerfen und "+
+			"den Client schließen?",
+		func(discard bool) {
+			if !discard {
+				return
+			}
+
+			a.finishMainWindowClose()
+		},
+		a.window,
+	)
+
+	confirm.SetConfirmText(
+		"Änderungen verwerfen",
+	)
+
+	confirm.SetDismissText(
+		"Abbrechen",
+	)
+
+	confirm.Show()
+}
+
+// finishMainWindowClose closes the assignment window first and then the
+// main application window.
+//
+// The close intercepts are removed because the user has already confirmed
+// the operation.
+func (a *App) finishMainWindowClose() {
+	if a == nil {
+		return
+	}
+
+	if a.assignmentWindow != nil {
+		window := a.assignmentWindow
+
+		window.SetCloseIntercept(nil)
+		window.Close()
+
+		a.assignmentWindow = nil
+		a.assignmentEditor = nil
+	}
+
+	a.window.SetCloseIntercept(nil)
+	a.window.Close()
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Public lifecycle
+///////////////////////////////////////////////////////////////////////////////
+
+// Close closes the technical client application.
+//
+// This method is also used by cmd/rs2322tcp-client after the Fyne main
+// window has actually closed. Any remaining assignment window is closed
+// without prompting because the main-window close handling has already
+// taken place.
+func (a *App) Close() {
+	if a == nil {
+		return
+	}
+
+	if a.assignmentWindow != nil {
+		window := a.assignmentWindow
+
+		window.SetCloseIntercept(nil)
+		window.Close()
+
+		a.assignmentWindow = nil
+		a.assignmentEditor = nil
+	}
+
+	_ = a.clientApplication.Close()
+}
+
+// ShowAndRun displays the main window and starts the Fyne event loop.
+func (a *App) ShowAndRun() {
+	if a == nil {
+		return
+	}
+
+	a.window.ShowAndRun()
+}
+
+// Window returns the main application window.
+func (a *App) Window() fyne.Window {
+	if a == nil {
+		return nil
+	}
+
+	return a.window
+}

internal/gui/assignment.go

@@ -0,0 +1,485 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/gui/assignment.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * GUI-unabhängige Aufbereitung und Bearbeitung der Zuordnung zwischen den
+ * vom rs2322tcp-Server angebotenen Geräten und den lokalen virtuellen
+ * seriellen Schnittstellen.
+ *
+ * Die Datei enthält bewusst keine Fyne-Abhängigkeiten.
+ *
+ * Die Änderungen an der Gerätezuordnung werden zunächst ausschließlich im
+ * Arbeitsspeicher des Zuordnungsfensters gehalten. Das Speichern in die
+ * Client-Konfiguration erfolgt beim Schließen des Fensters.
+ *
+ * Regeln:
+ *
+ *   - Ein Server-Gerät kann höchstens einer lokalen Schnittstelle zugeordnet
+ *     werden.
+ *
+ *   - Eine lokale Schnittstelle kann höchstens einem Server-Gerät zugeordnet
+ *     werden.
+ *
+ *   - "nicht verbunden" ist ein gültiger Zustand.
+ *
+ *   - Ist ein Gerät bereits verbunden, kann es nur auf seinen aktuellen Port
+ *     oder auf "nicht verbunden" gesetzt werden.
+ *
+ *   - Ist ein Gerät nicht verbunden, kann es auf "nicht verbunden" oder auf
+ *     einen momentan freien konfigurierten virtuellen Port gesetzt werden.
+ *
+ *   - Wird ein Gerät getrennt, bleibt der virtuelle Port erhalten und wird
+ *     lediglich frei.
+ * ============================================================================
+ */
+package gui
+
+import (
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/transport"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Constants
+///////////////////////////////////////////////////////////////////////////////
+
+const (
+	// NotConnected is the GUI representation of an unassigned server device.
+	NotConnected = "nicht verbunden"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Device assignment
+///////////////////////////////////////////////////////////////////////////////
+
+// DeviceAssignment beschreibt eine bereits in der Client-Konfiguration
+// vorhandene Zuordnung zwischen einer lokalen virtuellen Schnittstelle
+// und einem entfernten Gerät.
+//
+// Available gibt an, ob das konfigurierte Remote-Gerät momentan in der
+// vom Server gelieferten Geräteliste vorhanden ist.
+type DeviceAssignment struct {
+	LocalPort    string
+	RemoteDevice string
+	Available    bool
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Assignment editor
+///////////////////////////////////////////////////////////////////////////////
+
+// AssignmentEditor enthält den momentan im GUI bearbeiteten
+// Zuordnungszustand.
+//
+// Original enthält den Zustand beim Öffnen des Fensters.
+//
+// Current enthält den momentan vom Anwender bearbeiteten Zustand.
+//
+// Der Schlüssel ist jeweils die stabile Remote-Device-ID. Ein leerer
+// Port bedeutet "nicht verbunden".
+//
+// Ports enthält alle konfigurierten virtuellen Ports. Diese Ports bleiben
+// auch dann erhalten, wenn sie momentan keinem Remote-Gerät zugeordnet sind.
+type AssignmentEditor struct {
+	Original map[string]string
+	Current  map[string]string
+	Ports    []string
+}
+
+// NewAssignmentEditor erzeugt einen bearbeitbaren Zuordnungszustand aus
+// der bestehenden Client-Konfiguration und der aktuellen Geräteliste.
+//
+// Es werden ausschließlich bereits konfigurierte virtuelle Ports betrachtet.
+// Das Anlegen neuer virtueller Ports erfolgt in einem späteren
+// Entwicklungsschritt.
+func NewAssignmentEditor(
+	cfg *config.ClientConfig,
+	devices []transport.RemoteDeviceInfo,
+) *AssignmentEditor {
+	editor := &AssignmentEditor{
+		Original: make(map[string]string),
+		Current:  make(map[string]string),
+		Ports:    make([]string, 0),
+	}
+
+	if cfg != nil {
+		for _, virtualPort := range cfg.VirtualPorts {
+			if virtualPort.Port == "" {
+				continue
+			}
+
+			if !containsString(
+				editor.Ports,
+				virtualPort.Port,
+			) {
+				editor.Ports = append(
+					editor.Ports,
+					virtualPort.Port,
+				)
+			}
+
+			if virtualPort.RemoteDevice == "" {
+				continue
+			}
+
+			editor.Current[virtualPort.RemoteDevice] =
+				virtualPort.Port
+		}
+	}
+
+	for _, device := range devices {
+		if _, exists := editor.Current[device.ID]; !exists {
+			editor.Current[device.ID] = ""
+		}
+	}
+
+	editor.Original = cloneAssignments(
+		editor.Current,
+	)
+
+	return editor
+}
+
+// CurrentPort returns the currently selected local port for a server device.
+//
+// An empty string means "nicht verbunden".
+func (e *AssignmentEditor) CurrentPort(
+	deviceID string,
+) string {
+	if e == nil {
+		return ""
+	}
+
+	return e.Current[deviceID]
+}
+
+// Options returns the currently valid Selectbox options for one server
+// device.
+//
+// For an already connected device:
+//
+//   - current port
+//   - nicht verbunden
+//
+// For an unconnected device:
+//
+//   - nicht verbunden
+//   - all currently free configured virtual ports
+func (e *AssignmentEditor) Options(
+	deviceID string,
+) []string {
+	if e == nil {
+		return []string{
+			NotConnected,
+		}
+	}
+
+	currentPort := e.Current[deviceID]
+
+	if currentPort != "" {
+		return []string{
+			currentPort,
+			NotConnected,
+		}
+	}
+
+	options := []string{
+		NotConnected,
+	}
+
+	for _, port := range e.Ports {
+		if e.portUsedByOtherDevice(
+			port,
+			deviceID,
+		) {
+			continue
+		}
+
+		options = append(
+			options,
+			port,
+		)
+	}
+
+	return options
+}
+
+// Set changes the current assignment of one server device.
+//
+// An empty port or NotConnected disconnects the device.
+//
+// A non-empty port is accepted only when it is a configured virtual port
+// and is not currently used by another server device.
+//
+// The function returns true when the current state was changed.
+func (e *AssignmentEditor) Set(
+	deviceID string,
+	port string,
+) bool {
+	if e == nil {
+		return false
+	}
+
+	if port == NotConnected {
+		port = ""
+	}
+
+	currentPort := e.Current[deviceID]
+
+	if currentPort == port {
+		return false
+	}
+
+	if port != "" {
+		if !containsString(
+			e.Ports,
+			port,
+		) {
+			return false
+		}
+
+		if e.portUsedByOtherDevice(
+			port,
+			deviceID,
+		) {
+			return false
+		}
+	}
+
+	e.Current[deviceID] = port
+
+	return true
+}
+
+// Dirty reports whether the current assignment state differs from the
+// state that was present when the editor was opened.
+func (e *AssignmentEditor) Dirty() bool {
+	if e == nil {
+		return false
+	}
+
+	if len(e.Original) != len(e.Current) {
+		return true
+	}
+
+	for deviceID, originalPort := range e.Original {
+		if e.Current[deviceID] != originalPort {
+			return true
+		}
+	}
+
+	return false
+}
+
+// ApplyToConfig applies the current assignment state to a copy of the
+// supplied client configuration.
+//
+// Existing virtual ports are always preserved. Only their RemoteDevice
+// value is changed.
+//
+// An empty RemoteDevice means that the virtual port currently has no
+// server-device assignment.
+//
+// The original configuration is not modified.
+func (e *AssignmentEditor) ApplyToConfig(
+	cfg *config.ClientConfig,
+) *config.ClientConfig {
+	if e == nil || cfg == nil {
+		return nil
+	}
+
+	result := *cfg
+
+	result.VirtualPorts = make(
+		[]config.VirtualPortConfig,
+		len(cfg.VirtualPorts),
+	)
+
+	for i, virtualPort := range cfg.VirtualPorts {
+		result.VirtualPorts[i] = virtualPort
+
+		// Every existing virtual port remains in the configuration.
+		// It becomes unassigned unless the current GUI state assigns it
+		// to a server device.
+		result.VirtualPorts[i].RemoteDevice = ""
+
+		for deviceID, port := range e.Current {
+			if port == virtualPort.Port {
+				result.VirtualPorts[i].RemoteDevice = deviceID
+				break
+			}
+		}
+	}
+
+	return &result
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Build assignments
+///////////////////////////////////////////////////////////////////////////////
+
+// BuildAssignments erzeugt die Zuordnungsansicht aus der bestehenden
+// Client-Konfiguration und der aktuell vom Server gelieferten Geräteliste.
+//
+// Es werden ausschließlich die in der Client-Konfiguration vorhandenen
+// lokalen virtuellen Ports berücksichtigt.
+func BuildAssignments(
+	cfg *config.ClientConfig,
+	devices []transport.RemoteDeviceInfo,
+) []DeviceAssignment {
+	if cfg == nil {
+		return nil
+	}
+
+	assignments := make(
+		[]DeviceAssignment,
+		0,
+		len(cfg.VirtualPorts),
+	)
+
+	for _, virtualPort := range cfg.VirtualPorts {
+		assignments = append(
+			assignments,
+			DeviceAssignment{
+				LocalPort:    virtualPort.Port,
+				RemoteDevice: virtualPort.RemoteDevice,
+				Available: remoteDeviceExists(
+					virtualPort.RemoteDevice,
+					devices,
+				),
+			},
+		)
+	}
+
+	return assignments
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Unassigned devices
+///////////////////////////////////////////////////////////////////////////////
+
+// UnassignedDevices liefert die vom Server angebotenen Geräte, die aktuell
+// keiner lokalen virtuellen Schnittstelle zugeordnet sind.
+func UnassignedDevices(
+	cfg *config.ClientConfig,
+	devices []transport.RemoteDeviceInfo,
+) []transport.RemoteDeviceInfo {
+	assigned := assignedRemoteDevices(
+		cfg,
+	)
+
+	unassigned := make(
+		[]transport.RemoteDeviceInfo,
+		0,
+	)
+
+	for _, device := range devices {
+		if assigned[device.ID] {
+			continue
+		}
+
+		unassigned = append(
+			unassigned,
+			device,
+		)
+	}
+
+	return unassigned
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Internal helpers
+///////////////////////////////////////////////////////////////////////////////
+
+// portUsedByOtherDevice prüft, ob ein Port momentan einem anderen
+// Server-Gerät zugeordnet ist.
+func (e *AssignmentEditor) portUsedByOtherDevice(
+	port string,
+	deviceID string,
+) bool {
+	for otherDeviceID, assignedPort := range e.Current {
+		if otherDeviceID == deviceID {
+			continue
+		}
+
+		if assignedPort == port {
+			return true
+		}
+	}
+
+	return false
+}
+
+// assignedRemoteDevices erzeugt eine Menge der Remote-Geräte, die bereits
+// einer lokalen virtuellen Schnittstelle zugeordnet sind.
+func assignedRemoteDevices(
+	cfg *config.ClientConfig,
+) map[string]bool {
+	assigned := make(
+		map[string]bool,
+	)
+
+	if cfg == nil {
+		return assigned
+	}
+
+	for _, virtualPort := range cfg.VirtualPorts {
+		if virtualPort.RemoteDevice == "" {
+			continue
+		}
+
+		assigned[virtualPort.RemoteDevice] = true
+	}
+
+	return assigned
+}
+
+// remoteDeviceExists prüft, ob ein Remote-Gerät mit der angegebenen ID in
+// der aktuellen Geräteliste des Servers vorhanden ist.
+func remoteDeviceExists(
+	id string,
+	devices []transport.RemoteDeviceInfo,
+) bool {
+	for _, device := range devices {
+		if device.ID == id {
+			return true
+		}
+	}
+
+	return false
+}
+
+// cloneAssignments erzeugt eine unabhängige Kopie einer Zuordnungskarte.
+func cloneAssignments(
+	assignments map[string]string,
+) map[string]string {
+	clone := make(
+		map[string]string,
+		len(assignments),
+	)
+
+	for deviceID, port := range assignments {
+		clone[deviceID] = port
+	}
+
+	return clone
+}
+
+// containsString prüft, ob ein String in einer Liste enthalten ist.
+func containsString(
+	values []string,
+	value string,
+) bool {
+	for _, current := range values {
+		if current == value {
+			return true
+		}
+	}
+
+	return false
+}

internal/gui/assignment_test.go

@@ -0,0 +1,374 @@
+/*
+ * ============================================================================
+ * Projekt.....: rs2322tcp
+ * Datei.......: internal/gui/assignment_test.go
+ * Copyright (C) 2026 Dieter Lang
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Beschreibung:
+ * Tests für die GUI-unabhängige Aufbereitung der Zuordnung zwischen lokalen
+ * virtuellen seriellen Schnittstellen und den vom rs2322tcp-Server
+ * angebotenen Geräten.
+ * ============================================================================
+ */
+package gui
+
+import (
+	"testing"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+	"git.lang-dieter.de/rs2322tcp/internal/transport"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// Test helpers
+///////////////////////////////////////////////////////////////////////////////
+
+// testClientConfig creates a client configuration containing the supplied
+// local-to-remote assignments.
+func testClientConfig(
+	assignments ...config.VirtualPortConfig,
+) *config.ClientConfig {
+	return &config.ClientConfig{
+		VirtualPorts: assignments,
+	}
+}
+
+// testRemoteDevices creates a list of remote devices from their IDs.
+func testRemoteDevices(ids ...string) []transport.RemoteDeviceInfo {
+	devices := make([]transport.RemoteDeviceInfo, 0, len(ids))
+
+	for _, id := range ids {
+		devices = append(devices, transport.RemoteDeviceInfo{
+			ID:   id,
+			Name: id,
+		})
+	}
+
+	return devices
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// BuildAssignments tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestBuildAssignments(t *testing.T) {
+	cfg := testClientConfig(
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB100",
+			RemoteDevice: "radio",
+		},
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB101",
+			RemoteDevice: "rotor",
+		},
+	)
+
+	devices := testRemoteDevices(
+		"radio",
+		"rotor",
+		"gps",
+	)
+
+	assignments := BuildAssignments(cfg, devices)
+
+	if len(assignments) != 2 {
+		t.Fatalf(
+			"BuildAssignments() returned %d assignments, want 2",
+			len(assignments),
+		)
+	}
+
+	if assignments[0].LocalPort != "/dev/ttyUSB100" {
+		t.Errorf(
+			"assignments[0].LocalPort = %q, want %q",
+			assignments[0].LocalPort,
+			"/dev/ttyUSB100",
+		)
+	}
+
+	if assignments[0].RemoteDevice != "radio" {
+		t.Errorf(
+			"assignments[0].RemoteDevice = %q, want %q",
+			assignments[0].RemoteDevice,
+			"radio",
+		)
+	}
+
+	if !assignments[0].Available {
+		t.Error("assignments[0].Available = false, want true")
+	}
+
+	if assignments[1].LocalPort != "/dev/ttyUSB101" {
+		t.Errorf(
+			"assignments[1].LocalPort = %q, want %q",
+			assignments[1].LocalPort,
+			"/dev/ttyUSB101",
+		)
+	}
+
+	if assignments[1].RemoteDevice != "rotor" {
+		t.Errorf(
+			"assignments[1].RemoteDevice = %q, want %q",
+			assignments[1].RemoteDevice,
+			"rotor",
+		)
+	}
+
+	if !assignments[1].Available {
+		t.Error("assignments[1].Available = false, want true")
+	}
+}
+
+func TestBuildAssignmentsUnavailableDevice(t *testing.T) {
+	cfg := testClientConfig(
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB100",
+			RemoteDevice: "radio",
+		},
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB101",
+			RemoteDevice: "rotor",
+		},
+	)
+
+	devices := testRemoteDevices("rotor")
+
+	assignments := BuildAssignments(cfg, devices)
+
+	if len(assignments) != 2 {
+		t.Fatalf(
+			"BuildAssignments() returned %d assignments, want 2",
+			len(assignments),
+		)
+	}
+
+	if assignments[0].RemoteDevice != "radio" {
+		t.Errorf(
+			"assignments[0].RemoteDevice = %q, want %q",
+			assignments[0].RemoteDevice,
+			"radio",
+		)
+	}
+
+	if assignments[0].Available {
+		t.Error("radio assignment marked available, want unavailable")
+	}
+
+	if assignments[1].RemoteDevice != "rotor" {
+		t.Errorf(
+			"assignments[1].RemoteDevice = %q, want %q",
+			assignments[1].RemoteDevice,
+			"rotor",
+		)
+	}
+
+	if !assignments[1].Available {
+		t.Error("rotor assignment marked unavailable, want available")
+	}
+}
+
+func TestBuildAssignmentsNilConfig(t *testing.T) {
+	devices := testRemoteDevices("radio")
+
+	assignments := BuildAssignments(nil, devices)
+
+	if assignments != nil {
+		t.Fatalf(
+			"BuildAssignments(nil, ...) = %#v, want nil",
+			assignments,
+		)
+	}
+}
+
+func TestBuildAssignmentsEmptyConfig(t *testing.T) {
+	cfg := testClientConfig()
+
+	devices := testRemoteDevices(
+		"radio",
+		"rotor",
+	)
+
+	assignments := BuildAssignments(cfg, devices)
+
+	if len(assignments) != 0 {
+		t.Fatalf(
+			"BuildAssignments() returned %d assignments, want 0",
+			len(assignments),
+		)
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// UnassignedDevices tests
+///////////////////////////////////////////////////////////////////////////////
+
+func TestUnassignedDevices(t *testing.T) {
+	cfg := testClientConfig(
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB100",
+			RemoteDevice: "radio",
+		},
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB101",
+			RemoteDevice: "rotor",
+		},
+	)
+
+	devices := testRemoteDevices(
+		"radio",
+		"rotor",
+		"gps",
+		"tnc",
+	)
+
+	unassigned := UnassignedDevices(cfg, devices)
+
+	if len(unassigned) != 2 {
+		t.Fatalf(
+			"UnassignedDevices() returned %d devices, want 2",
+			len(unassigned),
+		)
+	}
+
+	if unassigned[0].ID != "gps" {
+		t.Errorf(
+			"unassigned[0].ID = %q, want %q",
+			unassigned[0].ID,
+			"gps",
+		)
+	}
+
+	if unassigned[1].ID != "tnc" {
+		t.Errorf(
+			"unassigned[1].ID = %q, want %q",
+			unassigned[1].ID,
+			"tnc",
+		)
+	}
+}
+
+func TestUnassignedDevicesAllAssigned(t *testing.T) {
+	cfg := testClientConfig(
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB100",
+			RemoteDevice: "radio",
+		},
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB101",
+			RemoteDevice: "rotor",
+		},
+	)
+
+	devices := testRemoteDevices(
+		"radio",
+		"rotor",
+	)
+
+	unassigned := UnassignedDevices(cfg, devices)
+
+	if len(unassigned) != 0 {
+		t.Fatalf(
+			"UnassignedDevices() returned %d devices, want 0",
+			len(unassigned),
+		)
+	}
+}
+
+func TestUnassignedDevicesEmptyConfig(t *testing.T) {
+	cfg := testClientConfig()
+
+	devices := testRemoteDevices(
+		"radio",
+		"rotor",
+	)
+
+	unassigned := UnassignedDevices(cfg, devices)
+
+	if len(unassigned) != 2 {
+		t.Fatalf(
+			"UnassignedDevices() returned %d devices, want 2",
+			len(unassigned),
+		)
+	}
+}
+
+func TestUnassignedDevicesNilConfig(t *testing.T) {
+	devices := testRemoteDevices(
+		"radio",
+		"rotor",
+	)
+
+	unassigned := UnassignedDevices(nil, devices)
+
+	if len(unassigned) != 2 {
+		t.Fatalf(
+			"UnassignedDevices(nil, ...) returned %d devices, want 2",
+			len(unassigned),
+		)
+	}
+}
+
+func TestUnassignedDevicesEmptyServerList(t *testing.T) {
+	cfg := testClientConfig(
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB100",
+			RemoteDevice: "radio",
+		},
+	)
+
+	unassigned := UnassignedDevices(cfg, nil)
+
+	if len(unassigned) != 0 {
+		t.Fatalf(
+			"UnassignedDevices() returned %d devices, want 0",
+			len(unassigned),
+		)
+	}
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Existing configuration does not change
+///////////////////////////////////////////////////////////////////////////////
+
+func TestAssignmentFunctionsDoNotModifyConfig(t *testing.T) {
+	cfg := testClientConfig(
+		config.VirtualPortConfig{
+			Port:         "/dev/ttyUSB100",
+			RemoteDevice: "radio",
+		},
+	)
+
+	devices := testRemoteDevices(
+		"radio",
+		"rotor",
+		"gps",
+	)
+
+	_ = BuildAssignments(cfg, devices)
+	_ = UnassignedDevices(cfg, devices)
+
+	if len(cfg.VirtualPorts) != 1 {
+		t.Fatalf(
+			"configuration contains %d virtual ports, want 1",
+			len(cfg.VirtualPorts),
+		)
+	}
+
+	if cfg.VirtualPorts[0].Port != "/dev/ttyUSB100" {
+		t.Errorf(
+			"configuration port = %q, want %q",
+			cfg.VirtualPorts[0].Port,
+			"/dev/ttyUSB100",
+		)
+	}
+
+	if cfg.VirtualPorts[0].RemoteDevice != "radio" {
+		t.Errorf(
+			"configuration remote device = %q, want %q",
+			cfg.VirtualPorts[0].RemoteDevice,
+			"radio",
+		)
+	}
+}

internal/serial/serial.go

@@ -16,6 +16,7 @@ package serial
 import (
 	"fmt"
 	"io"
+	"sync"
 
 	bugserial "go.bug.st/serial"
 
@@ -28,6 +29,7 @@ import (
 
 // Connection represents an opened serial connection.
 type Connection struct {
+	mu   sync.Mutex
 	port bugserial.Port
 }
 
@@ -126,20 +128,36 @@ func createMode(device config.DeviceConfig) (*bugserial.Mode, error) {
 
 // Read reads data from the serial connection.
 func (c *Connection) Read(p []byte) (int, error) {
-	if c == nil || c.port == nil {
+	if c == nil {
 		return 0, io.ErrClosedPipe
 	}
 
-	return c.port.Read(p)
+	c.mu.Lock()
+	port := c.port
+	c.mu.Unlock()
+
+	if port == nil {
+		return 0, io.ErrClosedPipe
+	}
+
+	return port.Read(p)
 }
 
 // Write writes data to the serial connection.
 func (c *Connection) Write(p []byte) (int, error) {
-	if c == nil || c.port == nil {
+	if c == nil {
 		return 0, io.ErrClosedPipe
 	}
 
-	return c.port.Write(p)
+	c.mu.Lock()
+	port := c.port
+	c.mu.Unlock()
+
+	if port == nil {
+		return 0, io.ErrClosedPipe
+	}
+
+	return port.Write(p)
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -148,12 +166,21 @@ func (c *Connection) Write(p []byte) (int, error) {
 
 // Close closes the serial connection.
 func (c *Connection) Close() error {
-	if c == nil || c.port == nil {
+	if c == nil {
 		return nil
 	}
 
-	err := c.port.Close()
+	c.mu.Lock()
+
+	if c.port == nil {
+		c.mu.Unlock()
+		return nil
+	}
+
+	port := c.port
 	c.port = nil
 
-	return err
+	c.mu.Unlock()
+
+	return port.Close()
 }

internal/server/control.go

@@ -336,6 +336,7 @@ func (s *ControlServer) runDataListener(
 		dataConnection, err := NewDataConnection(
 			tcpConn,
 			serialConn,
+			s.config.HardwareErrorResponse,
 		)
 		if err != nil {
 			log.Printf(
@@ -351,6 +352,11 @@ func (s *ControlServer) runDataListener(
 			continue
 		}
 
+		dataConnection.SetSerialMonitor(
+			s.config.SerialMonitor,
+			device.SerialPort,
+		)
+
 		if err := session.AddDataConnection(
 			device.ID,
 			dataConnection,
@@ -362,6 +368,7 @@ func (s *ControlServer) runDataListener(
 				err,
 			)
 
+			_ = dataConnection.Close()
 			continue
 		}
 

internal/server/control_test.go

@@ -40,6 +40,7 @@ func testServerConfig() *config.ServerConfig {
 			Address: "127.0.0.1",
 			Port:    5000,
 		},
+		HardwareErrorResponse: testHardwareErrorResponse,
 		Devices: []config.DeviceConfig{
 			{
 				ID:         "radio",
@@ -451,6 +452,7 @@ func TestControlServerDataConnection(t *testing.T) {
 			Address: "127.0.0.1",
 			Port:    5000,
 		},
+		HardwareErrorResponse: testHardwareErrorResponse,
 		Devices: []config.DeviceConfig{
 			{
 				ID:         "radio",

internal/server/data_connection.go

@@ -1,7 +1,7 @@
 /*
  * ============================================================================
  * Projekt.....: rs2322tcp
- * Datei.......: data_connection.go
+ * Datei.......: internal/server/data_connection.go
  * Copyright (C) 2026 Dieter Lang
  *
  * SPDX-License-Identifier: GPL-3.0-or-later
@@ -16,6 +16,7 @@ package server
 import (
 	"fmt"
 	"io"
+	"log"
 	"net"
 	"sync"
 )
@@ -31,12 +32,20 @@ import (
 //	TCP  ->  Serial
 //	TCP  <-  Serial
 //
+// If writing TCP data to the serial device fails, the configured hardware
+// error response is sent back over the TCP connection before the connection
+// is terminated.
+//
 // The serial side is represented by an io.ReadWriteCloser so that this
 // server component does not depend directly on the concrete serial
 // implementation.
 type DataConnection struct {
-	tcp    net.Conn
+	tcp                   net.Conn
-	serial io.ReadWriteCloser
+	serial                io.ReadWriteCloser
+	hardwareErrorResponse string
+
+	serialMonitor bool
+	serialPort    string
 
 	closeOnce sync.Once
 	closeErr  error
@@ -50,6 +59,7 @@ type DataConnection struct {
 func NewDataConnection(
 	tcp net.Conn,
 	serial io.ReadWriteCloser,
+	hardwareErrorResponse string,
 ) (*DataConnection, error) {
 	if tcp == nil {
 		return nil, fmt.Errorf("TCP connection is nil")
@@ -59,9 +69,14 @@ func NewDataConnection(
 		return nil, fmt.Errorf("serial connection is nil")
 	}
 
+	if hardwareErrorResponse == "" {
+		return nil, fmt.Errorf("hardware error response is empty")
+	}
+
 	return &DataConnection{
-		tcp:    tcp,
+		tcp:                   tcp,
-		serial: serial,
+		serial:                serial,
+		hardwareErrorResponse: hardwareErrorResponse,
 	}, nil
 }
 
@@ -87,6 +102,153 @@ func (c *DataConnection) SerialConn() io.ReadWriteCloser {
 	return c.serial
 }
 
+// SetSerialMonitor enables or disables the data monitor.
+//
+// If enabled, transmitted and received data is written to the server log.
+// serialPort is used only for identifying the physical interface in the
+// monitor output.
+func (c *DataConnection) SetSerialMonitor(
+	enabled bool,
+	serialPort string,
+) {
+	if c == nil {
+		return
+	}
+
+	c.serialMonitor = enabled
+	c.serialPort = serialPort
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Data monitor
+///////////////////////////////////////////////////////////////////////////////
+
+// logTCPData writes TCP data to the server log.
+func (c *DataConnection) logTCPData(
+	direction string,
+	data []byte,
+) {
+	if c == nil || !c.serialMonitor || len(data) == 0 {
+		return
+	}
+
+	log.Printf(
+		"%s % X",
+		direction,
+		data,
+	)
+}
+
+// logSerialData writes serial data to the server log.
+func (c *DataConnection) logSerialData(
+	direction string,
+	data []byte,
+) {
+	if c == nil || !c.serialMonitor || len(data) == 0 {
+		return
+	}
+
+	log.Printf(
+		"%s [SERIAL %s] % X",
+		direction,
+		c.serialPort,
+		data,
+	)
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Data transfer
+///////////////////////////////////////////////////////////////////////////////
+
+// copyTCPToSerial transfers data from the TCP connection to the serial
+// device.
+//
+// If writing to the serial device fails, the configured hardware error
+// response is sent back to the TCP client before the transfer terminates.
+func (c *DataConnection) copyTCPToSerial() error {
+	if c == nil {
+		return fmt.Errorf("data connection is nil")
+	}
+
+	buffer := make([]byte, 32*1024)
+
+	for {
+		n, err := c.tcp.Read(buffer)
+
+		if n > 0 {
+			c.logTCPData("TCP RX", buffer[:n])
+
+			written, writeErr := c.serial.Write(buffer[:n])
+
+			if writeErr != nil {
+				_, _ = io.WriteString(
+					c.tcp,
+					c.hardwareErrorResponse,
+				)
+
+				return writeErr
+			}
+
+			if written != n {
+				_, _ = io.WriteString(
+					c.tcp,
+					c.hardwareErrorResponse,
+				)
+
+				return io.ErrShortWrite
+			}
+
+			c.logSerialData("SERIAL TX", buffer[:n])
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+}
+
+// copySerialToTCP transfers data from the serial device to the TCP
+// connection.
+func (c *DataConnection) copySerialToTCP() error {
+	if c == nil {
+		return fmt.Errorf("data connection is nil")
+	}
+
+	buffer := make([]byte, 32*1024)
+
+	for {
+		n, err := c.serial.Read(buffer)
+
+		if n > 0 {
+			c.logSerialData("SERIAL RX", buffer[:n])
+
+			written := 0
+
+			for written < n {
+				count, writeErr := c.tcp.Write(
+					buffer[written:n],
+				)
+
+				written += count
+
+				if writeErr != nil {
+					return writeErr
+				}
+
+				if count == 0 {
+					return io.ErrShortWrite
+				}
+			}
+
+			c.logTCPData("TCP TX", buffer[:n])
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Run
 ///////////////////////////////////////////////////////////////////////////////
@@ -96,6 +258,10 @@ func (c *DataConnection) SerialConn() io.ReadWriteCloser {
 // Run blocks until one of the two transfer directions terminates.
 // The other direction is then stopped and both connections are closed.
 //
+// If writing data from TCP to the serial device fails, the configured
+// hardware error response is sent to the TCP client before the connection
+// is closed.
+//
 // The first non-EOF transfer error is returned.
 func (c *DataConnection) Run() error {
 	if c == nil {
@@ -111,15 +277,13 @@ func (c *DataConnection) Run() error {
 	go func() {
 		defer wg.Done()
 
-		_, err := io.Copy(c.serial, c.tcp)
+		errCh <- c.copyTCPToSerial()
-		errCh <- err
 	}()
 
 	go func() {
 		defer wg.Done()
 
-		_, err := io.Copy(c.tcp, c.serial)
+		errCh <- c.copySerialToTCP()
-		errCh <- err
 	}()
 
 	err := <-errCh

internal/server/data_connection_test.go

@@ -14,6 +14,7 @@ package server_test
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"net"
 	"sync"
@@ -31,6 +32,8 @@ type testSerialConnection struct {
 	reader *bytes.Reader
 	writer bytes.Buffer
 
+	writeErr error
+
 	mu     sync.Mutex
 	closed bool
 
@@ -45,6 +48,14 @@ func newTestSerialConnection(data []byte) *testSerialConnection {
 	}
 }
 
+func newFailingTestSerialConnection(writeErr error) *testSerialConnection {
+	return &testSerialConnection{
+		reader:   bytes.NewReader(nil),
+		writeErr: writeErr,
+		closeCh:  make(chan struct{}),
+	}
+}
+
 func (s *testSerialConnection) Read(p []byte) (int, error) {
 	s.mu.Lock()
 
@@ -76,6 +87,10 @@ func (s *testSerialConnection) Write(p []byte) (int, error) {
 		return 0, io.ErrClosedPipe
 	}
 
+	if s.writeErr != nil {
+		return 0, s.writeErr
+	}
+
 	return s.writer.Write(p)
 }
 
@@ -116,6 +131,7 @@ func TestNewDataConnection(t *testing.T) {
 	connection, err := server.NewDataConnection(
 		tcpServer,
 		serial,
+		testHardwareErrorResponse,
 	)
 	if err != nil {
 		t.Fatalf("NewDataConnection() failed: %v", err)
@@ -136,6 +152,7 @@ func TestNewDataConnectionRejectsNilTCP(t *testing.T) {
 	connection, err := server.NewDataConnection(
 		nil,
 		serial,
+		testHardwareErrorResponse,
 	)
 
 	if err == nil {
@@ -155,6 +172,7 @@ func TestNewDataConnectionRejectsNilSerial(t *testing.T) {
 	connection, err := server.NewDataConnection(
 		tcpServer,
 		nil,
+		testHardwareErrorResponse,
 	)
 
 	if err == nil {
@@ -179,6 +197,7 @@ func TestDataConnectionTCPToSerial(t *testing.T) {
 	connection, err := server.NewDataConnection(
 		tcpServer,
 		serial,
+		testHardwareErrorResponse,
 	)
 	if err != nil {
 		t.Fatalf("NewDataConnection() failed: %v", err)
@@ -223,6 +242,60 @@ func TestDataConnectionTCPToSerial(t *testing.T) {
 	}
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// TCP -> Serial error
+///////////////////////////////////////////////////////////////////////////////
+
+func TestDataConnectionTCPToSerialWriteError(t *testing.T) {
+	tcpServer, tcpClient := net.Pipe()
+	defer tcpClient.Close()
+
+	writeErr := errors.New("hardware write failed")
+
+	serial := newFailingTestSerialConnection(writeErr)
+
+	connection, err := server.NewDataConnection(
+		tcpServer,
+		serial,
+		testHardwareErrorResponse,
+	)
+	if err != nil {
+		t.Fatalf("NewDataConnection() failed: %v", err)
+	}
+
+	done := make(chan error, 1)
+
+	go func() {
+		done <- connection.Run()
+	}()
+
+	testData := []byte("hello unavailable hardware")
+
+	if _, err := tcpClient.Write(testData); err != nil {
+		t.Fatalf("TCP Write() failed: %v", err)
+	}
+
+	errorResponse := make([]byte, len(testHardwareErrorResponse))
+
+	if _, err := io.ReadFull(tcpClient, errorResponse); err != nil {
+		t.Fatalf("TCP Read() failed: %v", err)
+	}
+
+	if string(errorResponse) != testHardwareErrorResponse {
+		t.Fatalf(
+			"error response = %q, want %q",
+			string(errorResponse),
+			testHardwareErrorResponse,
+		)
+	}
+
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("DataConnection.Run() did not terminate")
+	}
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Serial -> TCP
 ///////////////////////////////////////////////////////////////////////////////
@@ -238,6 +311,7 @@ func TestDataConnectionSerialToTCP(t *testing.T) {
 	connection, err := server.NewDataConnection(
 		tcpServer,
 		serial,
+		testHardwareErrorResponse,
 	)
 	if err != nil {
 		t.Fatalf("NewDataConnection() failed: %v", err)
@@ -285,6 +359,7 @@ func TestDataConnectionClose(t *testing.T) {
 	connection, err := server.NewDataConnection(
 		tcpServer,
 		serial,
+		testHardwareErrorResponse,
 	)
 	if err != nil {
 		t.Fatalf("NewDataConnection() failed: %v", err)

internal/server/data_handler.go

@@ -38,8 +38,9 @@ type SerialFactory func() (io.ReadWriteCloser, error)
 // DataHandler accepts TCP data connections and connects them to a serial
 // device.
 type DataHandler struct {
-	listener      *DataListener
+	listener              *DataListener
-	serialFactory SerialFactory
+	serialFactory         SerialFactory
+	hardwareErrorResponse string
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -53,6 +54,7 @@ type DataHandler struct {
 func NewDataHandler(
 	listener *DataListener,
 	serialFactory SerialFactory,
+	hardwareErrorResponse string,
 ) (*DataHandler, error) {
 	if listener == nil {
 		return nil, fmt.Errorf("data listener is nil")
@@ -62,9 +64,14 @@ func NewDataHandler(
 		return nil, fmt.Errorf("serial factory is nil")
 	}
 
+	if hardwareErrorResponse == "" {
+		return nil, fmt.Errorf("hardware error response is empty")
+	}
+
 	return &DataHandler{
-		listener:      listener,
+		listener:              listener,
-		serialFactory: serialFactory,
+		serialFactory:         serialFactory,
+		hardwareErrorResponse: hardwareErrorResponse,
 	}, nil
 }
 
@@ -114,6 +121,7 @@ func (h *DataHandler) handleConnection(tcpConn net.Conn) {
 	dataConnection, err := NewDataConnection(
 		tcpConn,
 		serialConn,
+		h.hardwareErrorResponse,
 	)
 	if err != nil {
 		log.Printf("create data connection: %v", err)

internal/server/data_handler_test.go

@@ -24,6 +24,12 @@ import (
 	"git.lang-dieter.de/rs2322tcp/internal/server"
 )
 
+///////////////////////////////////////////////////////////////////////////////
+// Test constants
+///////////////////////////////////////////////////////////////////////////////
+
+const testHardwareErrorResponse = "TEST ERROR RESPONSE"
+
 ///////////////////////////////////////////////////////////////////////////////
 // Test serial connection
 ///////////////////////////////////////////////////////////////////////////////
@@ -119,6 +125,7 @@ func TestNewDataHandler(t *testing.T) {
 		func() (io.ReadWriteCloser, error) {
 			return serial, nil
 		},
+		testHardwareErrorResponse,
 	)
 	if err != nil {
 		t.Fatalf("NewDataHandler() failed: %v", err)
@@ -135,6 +142,7 @@ func TestNewDataHandlerRejectsNilListener(t *testing.T) {
 		func() (io.ReadWriteCloser, error) {
 			return newHandlerTestSerial(nil), nil
 		},
+		testHardwareErrorResponse,
 	)
 
 	if err == nil {
@@ -156,6 +164,7 @@ func TestNewDataHandlerRejectsNilFactory(t *testing.T) {
 	handler, err := server.NewDataHandler(
 		listener,
 		nil,
+		testHardwareErrorResponse,
 	)
 
 	if err == nil {
@@ -185,6 +194,7 @@ func TestDataHandlerTCPToSerial(t *testing.T) {
 		func() (io.ReadWriteCloser, error) {
 			return serial, nil
 		},
+		testHardwareErrorResponse,
 	)
 	if err != nil {
 		t.Fatalf("NewDataHandler() failed: %v", err)
@@ -255,6 +265,7 @@ func TestDataHandlerSerialToTCP(t *testing.T) {
 		func() (io.ReadWriteCloser, error) {
 			return serial, nil
 		},
+		testHardwareErrorResponse,
 	)
 	if err != nil {
 		t.Fatalf("NewDataHandler() failed: %v", err)
@@ -311,6 +322,7 @@ func TestDataHandlerSerialFactoryError(t *testing.T) {
 		func() (io.ReadWriteCloser, error) {
 			return nil, io.ErrClosedPipe
 		},
+		testHardwareErrorResponse,
 	)
 	if err != nil {
 		t.Fatalf("NewDataHandler() failed: %v", err)

scripts/build.sh

@@ -2,30 +2,86 @@
 #
 # ============================================================================
 # Projekt.....: rs2322tcp
-# Datei.......: build.sh
+# Datei.......: scripts/build.sh
 #
 # Copyright (C) 2026 Dieter Lang
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 #
 # Beschreibung:
-#   Erstellt die Linux- und Windows-Builds des Projektes.
+#   Erstellt die Release-Builds des Projekts rs2322tcp.
 #
-#   Die Versionsinformationen werden aus Git ermittelt und beim Build
+#   Server:
-#   über den Go-Linker in die Binärdateien eingebettet.
+#     - Linux amd64
+#     - Linux arm64
+#     - Linux 386
+#     - Linux arm
 #
-#   Gebaut werden:
+#   Client:
-#     - Linux amd64: Client und Server
+#     - Linux amd64
-#     - Linux arm64: Server
+#     - Linux arm64
-#     - Windows amd64: Client
+#     - Windows amd64
+#     - Windows 386
+#
+#   Der Server wird CGO-frei direkt mit Go gebaut.
+#
+#   Der Fyne-basierte Client wird für Linux amd64 direkt gebaut.
+#   Die Cross-Builds für Linux arm64 und Windows werden mit fyne-cross
+#   in Docker erzeugt.
+#
+# Voraussetzungen:
+#     - Go
+#     - Git
+#     - Docker
+#     - fyne-cross
+#
+# Installation von fyne-cross:
+#
+#     go install github.com/fyne-io/fyne-cross@latest
+#
+# Windows XP wird derzeit nicht unterstützt.
 # ============================================================================
 #
 
 set -e
 
-PROJECT=$(basename "$(pwd)")
+##############################################################################
-BUILDDIR="build"
+# Projektverzeichnis
-MODULE=$(go list -m)
+##############################################################################
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_DIR="$(cd "${SCRIPT_DIR}/.." && pwd)"
+
+cd "${PROJECT_DIR}"
+
+PROJECT="$(basename "${PROJECT_DIR}")"
+BUILD_DIR="build"
+FYNE_CROSS_DIR="fyne-cross"
+
+##############################################################################
+# fyne-cross
+##############################################################################
+
+FYNE_CROSS="${HOME}/go/bin/fyne-cross"
+
+if [ ! -x "${FYNE_CROSS}" ]; then
+    echo
+    echo "FEHLER: fyne-cross wurde nicht gefunden:"
+    echo "        ${FYNE_CROSS}"
+    echo
+    echo "Installation:"
+    echo "        go install github.com/fyne-io/fyne-cross@latest"
+    echo
+    exit 1
+fi
+
+FYNE_APP_ID="de.lang-dieter.rs2322tcp"
+
+##############################################################################
+# Modul
+##############################################################################
+
+MODULE="$(go list -m)"
 
 ##############################################################################
 # Farben
@@ -41,20 +97,22 @@ RESET="\033[0m"
 # Zeitmessung
 ##############################################################################
 
-START=$(date +%s)
+START="$(date +%s)"
 
 ##############################################################################
 # Projektinformationen
 ##############################################################################
 
-VERSION=$(git describe --tags --always 2>/dev/null || echo "dev")
+VERSION="$(git describe --tags --always 2>/dev/null || echo "dev")"
-BUILD=$(git rev-list --count HEAD 2>/dev/null || echo "0")
+BUILD="$(git rev-list --count HEAD 2>/dev/null || echo "0")"
-BRANCH=$(git branch --show-current 2>/dev/null || echo "unknown")
+BRANCH="$(git branch --show-current 2>/dev/null || echo "unknown")"
-COMMIT=$(git rev-parse --short HEAD 2>/dev/null || echo "unknown")
+COMMIT="$(git rev-parse --short HEAD 2>/dev/null || echo "unknown")"
-GO_VERSION=$(go version | awk '{print $3}')
+GO_VERSION="$(go version | awk '{print $3}')"
-HOST=$(hostname)
+HOST="$(hostname)"
-DATE=$(date "+%Y-%m-%dT%H:%M:%S")
+DATE="$(date "+%Y-%m-%dT%H:%M:%S")"
 
+##############################################################################
+# Kopf
 ##############################################################################
 
 echo
@@ -64,14 +122,14 @@ echo " ${PROJECT} Build Script"
 echo "============================================================"
 echo -e "${RESET}"
 
-printf "%-12s %s\n" "Projekt:"     "$PROJECT"
+printf "%-12s %s\n" "Projekt:" "${PROJECT}"
-printf "%-12s %s\n" "Version:"     "$VERSION"
+printf "%-12s %s\n" "Version:" "${VERSION}"
-printf "%-12s %s\n" "Build:"       "$BUILD"
+printf "%-12s %s\n" "Build:" "${BUILD}"
-printf "%-12s %s\n" "Branch:"      "$BRANCH"
+printf "%-12s %s\n" "Branch:" "${BRANCH}"
-printf "%-12s %s\n" "Commit:"      "$COMMIT"
+printf "%-12s %s\n" "Commit:" "${COMMIT}"
-printf "%-12s %s\n" "Go:"          "$GO_VERSION"
+printf "%-12s %s\n" "Go:" "${GO_VERSION}"
-printf "%-12s %s\n" "Host:"        "$HOST"
+printf "%-12s %s\n" "Host:" "${HOST}"
-printf "%-12s %s\n" "Datum:"       "$DATE"
+printf "%-12s %s\n" "Datum:" "${DATE}"
 
 echo
 
@@ -97,119 +155,192 @@ echo -e "${YELLOW}Bereinige Module ...${RESET}"
 go mod tidy
 
 ##############################################################################
-# Build-Verzeichnis
+# Build-Verzeichnisse
 ##############################################################################
 
 echo -e "${YELLOW}Erzeuge Build-Verzeichnis ...${RESET}"
 
-rm -rf "${BUILDDIR}"
+rm -rf "${BUILD_DIR}"
-mkdir -p "${BUILDDIR}"
+mkdir -p "${BUILD_DIR}"
 
 ##############################################################################
 # Linker-Optionen
 ##############################################################################
 
 LDFLAGS="-s -w \
--X '${MODULE}/internal/version.Version=${VERSION}' \
+-X ${MODULE}/internal/version.Version=${VERSION} \
--X '${MODULE}/internal/version.Build=${BUILD}' \
+-X ${MODULE}/internal/version.Build=${BUILD} \
--X '${MODULE}/internal/version.Commit=${COMMIT}' \
+-X ${MODULE}/internal/version.Commit=${COMMIT} \
--X '${MODULE}/internal/version.Date=${DATE}'"
+-X ${MODULE}/internal/version.Date=${DATE}"
 
 ##############################################################################
-# Buildfunktion
+# Direkter Go-Build
 ##############################################################################
 
 build_target() {
 
-    GOOS=$1
+    local GOOS="$1"
-    GOARCH=$2
+    local GOARCH="$2"
-    TARGET=$3
+    local TARGET="$3"
-    PROGRAM=$4
+    local PROGRAM="$4"
-    OUTPUT=$5
+    local OUTPUT="$5"
+    local CGO="$6"
 
     echo
     echo -e "${BLUE}Baue ${PROGRAM} für ${TARGET}${RESET}"
 
-    mkdir -p "${BUILDDIR}/${TARGET}"
+    mkdir -p "${BUILD_DIR}/${TARGET}"
 
     GOOS="${GOOS}" \
     GOARCH="${GOARCH}" \
-    CGO_ENABLED=0 \
+    CGO_ENABLED="${CGO}" \
     go build \
         -trimpath \
         -ldflags="${LDFLAGS}" \
-        -o "${BUILDDIR}/${TARGET}/${OUTPUT}" \
+        -o "${BUILD_DIR}/${TARGET}/${OUTPUT}" \
         "./cmd/${PROGRAM}"
 
     (
-        cd "${BUILDDIR}/${TARGET}"
+        cd "${BUILD_DIR}/${TARGET}"
         sha256sum "${OUTPUT}" > "${OUTPUT}.sha256"
     )
 
-    SIZE=$(stat -c%s "${BUILDDIR}/${TARGET}/${OUTPUT}")
+    local SIZE
-    SIZE_MB=$(awk "BEGIN {printf \"%.2f\", ${SIZE}/1024/1024}")
+    local SIZE_MB
+
+    SIZE="$(stat -c%s "${BUILD_DIR}/${TARGET}/${OUTPUT}")"
+    SIZE_MB="$(awk "BEGIN {printf \"%.2f\", ${SIZE}/1024/1024}")"
 
     echo -e "${GREEN}OK${RESET}"
     echo "Größe : ${SIZE_MB} MB"
 }
 
 ##############################################################################
-# Linux amd64
+# Fyne-Cross-Build
+##############################################################################
+
+build_fyne_target() {
+
+    local PLATFORM="$1"
+    local ARCH="$2"
+    local TARGET="$3"
+    local OUTPUT="$4"
+    local SOURCE
+    local TARGET_DIR
+
+    echo
+    echo -e "${BLUE}Baue rs2322tcp-client für ${TARGET} mit fyne-cross${RESET}"
+
+    "${FYNE_CROSS}" "${PLATFORM}" \
+        -arch="${ARCH}" \
+        -app-id="${FYNE_APP_ID}" \
+        -output="rs2322tcp-client" \
+        "./cmd/rs2322tcp-client"
+
+    if [ "${PLATFORM}" = "windows" ]; then
+        SOURCE="${FYNE_CROSS_DIR}/bin/${PLATFORM}-${ARCH}/rs2322tcp-client.exe"
+    else
+        SOURCE="${FYNE_CROSS_DIR}/bin/${PLATFORM}-${ARCH}/rs2322tcp-client"
+    fi
+
+    TARGET_DIR="${BUILD_DIR}/${TARGET}"
+
+    if [ ! -f "${SOURCE}" ]; then
+        echo
+        echo -e "${RED}FEHLER: fyne-cross-Builddatei wurde nicht gefunden:${RESET}"
+        echo "        ${SOURCE}"
+        echo
+        exit 1
+    fi
+
+    mkdir -p "${TARGET_DIR}"
+
+    cp "${SOURCE}" "${TARGET_DIR}/${OUTPUT}"
+
+    (
+        cd "${TARGET_DIR}"
+        sha256sum "${OUTPUT}" > "${OUTPUT}.sha256"
+    )
+
+    local SIZE
+    local SIZE_MB
+
+    SIZE="$(stat -c%s "${TARGET_DIR}/${OUTPUT}")"
+    SIZE_MB="$(awk "BEGIN {printf \"%.2f\", ${SIZE}/1024/1024}")"
+
+    echo -e "${GREEN}OK${RESET}"
+    echo "Größe : ${SIZE_MB} MB"
+}
+
+##############################################################################
+# Server-Builds
+#
+# Der Server ist CGO-frei und kann direkt cross-kompiliert werden.
 ##############################################################################
 
 build_target linux amd64 linux-amd64 \
-    rs2322tcp-client rs2322tcp-client
+    rs2322tcp-server rs2322tcp-server 0
-
-build_target linux amd64 linux-amd64 \
-    rs2322tcp-server rs2322tcp-server
-
-##############################################################################
-# Linux arm64
-##############################################################################
 
 build_target linux arm64 linux-arm64 \
-    rs2322tcp-server rs2322tcp-server
+    rs2322tcp-server rs2322tcp-server 0
+
+build_target linux 386 linux-386 \
+    rs2322tcp-server rs2322tcp-server 0
+
+build_target linux arm linux-arm \
+    rs2322tcp-server rs2322tcp-server 0
 
 ##############################################################################
+# Client-Builds
+##############################################################################
+
+# Linux amd64
+build_target linux amd64 linux-amd64 \
+    rs2322tcp-client rs2322tcp-client 1
+
+# Linux arm64
+build_fyne_target linux arm64 linux-arm64 \
+    rs2322tcp-client
+
 # Windows amd64
-##############################################################################
+build_fyne_target windows amd64 windows-amd64 \
+    rs2322tcp-client.exe
 
-build_target windows amd64 windows-amd64 \
+# Windows 386
-    rs2322tcp-client rs2322tcp-client.exe
+build_fyne_target windows 386 windows-386 \
+    rs2322tcp-client.exe
 
 ##############################################################################
 # Abschluss
 ##############################################################################
 
-END=$(date +%s)
+END="$(date +%s)"
-RUNTIME=$((END-START))
+RUNTIME="$((END - START))"
 
-TARGETS=4
+FILES="$(find "${BUILD_DIR}" -type f | wc -l)"
-FILES=$(find "${BUILDDIR}" -type f | wc -l)
 
 echo
 echo -e "${BLUE}"
 echo "============================================================"
-echo "Zusammenfassung"
+echo " Zusammenfassung"
 echo "============================================================"
 echo -e "${RESET}"
 
-printf "%-12s %s\n" "Targets:"     "$TARGETS"
+printf "%-12s %s\n" "Dateien:" "${FILES}"
-printf "%-12s %s\n" "Dateien:"     "$FILES"
+printf "%-12s %s\n" "Builddauer:" "${RUNTIME} Sekunden"
-printf "%-12s %s\n" "Builddauer:"  "${RUNTIME} Sekunden"
 
 echo
 echo -e "${BLUE}"
 echo "============================================================"
-echo "Erzeugte Build-Dateien"
+echo " Erzeugte Build-Dateien"
 echo "============================================================"
 echo -e "${RESET}"
 
-find "${BUILDDIR}" -type f | sort
+find "${BUILD_DIR}" -type f | sort
 
 echo
 echo -e "${GREEN}"
 echo "============================================================"
-echo "Alle Builds erfolgreich abgeschlossen."
+echo " Alle Builds erfolgreich abgeschlossen."
 echo "============================================================"
 echo -e "${RESET}"

scripts/deploy.sh

@@ -0,0 +1,184 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+PROJECT_NAME="rs2322tcp"
+PROJECT_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+BUILD_DIR="${PROJECT_ROOT}/build"
+DIST_DIR="${PROJECT_ROOT}/dist"
+CONFIG_DIR="${PROJECT_ROOT}/configs"
+
+VERSION="$(git -C "${PROJECT_ROOT}" describe --tags --always --dirty 2>/dev/null || echo "unknown")"
+BUILD_DATE="$(date -u +"%Y-%m-%dT%H:%M:%SZ")"
+
+echo
+echo "============================================================"
+echo " rs2322tcp Deploy Script"
+echo "============================================================"
+echo
+echo "Projekt:     ${PROJECT_NAME}"
+echo "Version:     ${VERSION}"
+echo "Datum:       ${BUILD_DATE}"
+echo
+
+if [[ ! -d "${BUILD_DIR}" ]]; then
+    echo "FEHLER: Build-Verzeichnis nicht gefunden:"
+    echo "        ${BUILD_DIR}"
+    echo
+    echo "Bitte zuerst ./scripts/build.sh ausführen."
+    exit 1
+fi
+
+if [[ ! -f "${CONFIG_DIR}/server.json" ]]; then
+    echo "FEHLER: Konfigurationsdatei nicht gefunden:"
+    echo "        ${CONFIG_DIR}/server.json"
+    exit 1
+fi
+
+if [[ ! -f "${CONFIG_DIR}/client.json" ]]; then
+    echo "FEHLER: Konfigurationsdatei nicht gefunden:"
+    echo "        ${CONFIG_DIR}/client.json"
+    exit 1
+fi
+
+echo "Bereinige Distributions-Verzeichnis ..."
+rm -rf "${DIST_DIR}"
+mkdir -p "${DIST_DIR}"
+
+create_linux_package() {
+    local architecture="$1"
+    local program="$2"
+    local config_file="$3"
+    local package_name="$4"
+
+    local source_dir="${BUILD_DIR}/${architecture}"
+    local package_dir="${DIST_DIR}/${architecture}/${package_name}"
+
+    if [[ ! -f "${source_dir}/${program}" ]]; then
+        echo "Überspringe ${package_name} für ${architecture}"
+        echo "Build-Datei nicht gefunden: ${source_dir}/${program}"
+        echo
+        return
+    fi
+
+    echo "Erzeuge ${package_name} für ${architecture}"
+
+    rm -rf "${package_dir}"
+    mkdir -p "${package_dir}"
+
+    cp "${source_dir}/${program}" "${package_dir}/"
+    cp "${CONFIG_DIR}/${config_file}" "${package_dir}/"
+
+    tar -C "${DIST_DIR}/${architecture}" \
+        -czf "${DIST_DIR}/${architecture}/${package_name}-${VERSION}.tar.gz" \
+        "${package_name}"
+
+    (
+        cd "${DIST_DIR}/${architecture}"
+        sha256sum "${package_name}-${VERSION}.tar.gz" \
+            > "${package_name}-${VERSION}.tar.gz.sha256"
+    )
+
+    echo "OK"
+    echo
+}
+
+echo "Erzeuge Linux-Server-Pakete ..."
+echo
+
+create_linux_package \
+    "linux-amd64" \
+    "rs2322tcp-server" \
+    "server.json" \
+    "rs2322tcp-server"
+
+create_linux_package \
+    "linux-arm64" \
+    "rs2322tcp-server" \
+    "server.json" \
+    "rs2322tcp-server"
+
+create_linux_package \
+    "linux-386" \
+    "rs2322tcp-server" \
+    "server.json" \
+    "rs2322tcp-server"
+
+create_linux_package \
+    "linux-arm" \
+    "rs2322tcp-server" \
+    "server.json" \
+    "rs2322tcp-server"
+
+echo "Erzeuge Linux-Client-Pakete ..."
+echo
+
+create_linux_package \
+    "linux-amd64" \
+    "rs2322tcp-client" \
+    "client.json" \
+    "rs2322tcp-client"
+
+create_linux_package \
+    "linux-arm64" \
+    "rs2322tcp-client" \
+    "client.json" \
+    "rs2322tcp-client"
+
+echo "Erzeuge Windows-Client-Pakete ..."
+echo
+
+create_windows_package() {
+    local architecture="$1"
+    local package_name="$2"
+
+    local source_dir="${BUILD_DIR}/${architecture}"
+    local package_dir="${DIST_DIR}/${architecture}/${package_name}"
+
+    if [[ ! -f "${source_dir}/rs2322tcp-client.exe" ]]; then
+        echo "Überspringe ${package_name} für ${architecture}"
+        echo "Build-Datei nicht gefunden: ${source_dir}/rs2322tcp-client.exe"
+        echo
+        return
+    fi
+
+    echo "Erzeuge ${package_name} für ${architecture}"
+
+    rm -rf "${package_dir}"
+    mkdir -p "${package_dir}"
+
+    cp "${source_dir}/rs2322tcp-client.exe" "${package_dir}/"
+    cp "${CONFIG_DIR}/client.json" \
+        "${package_dir}/client-config.json"
+
+    (
+        cd "${DIST_DIR}/${architecture}"
+        zip -qr "${package_name}-${VERSION}.zip" "${package_name}"
+
+        sha256sum "${package_name}-${VERSION}.zip" \
+            > "${package_name}-${VERSION}.zip.sha256"
+    )
+
+    echo "OK"
+    echo
+}
+
+create_windows_package \
+    "windows-amd64" \
+    "rs2322tcp-client"
+
+create_windows_package \
+    "windows-386" \
+    "rs2322tcp-client"
+
+echo "============================================================"
+echo " Erzeugte Distributions-Dateien"
+echo "============================================================"
+echo
+
+find "${DIST_DIR}" -type f | sort | sed "s|${PROJECT_ROOT}/||"
+
+echo
+echo "============================================================"
+echo " Deploy erfolgreich abgeschlossen."
+echo "============================================================"

scripts/test-rotor-com10.bat

@@ -0,0 +1,12 @@
+@echo off
+cd /d "%~dp0.."
+
+echo ==========================================
+echo rs2322tcp COM10 Testsequenz
+echo ==========================================
+echo.
+
+powershell.exe -NoProfile -ExecutionPolicy Bypass -File ".\scripts\test-rotor-com10.ps1"
+
+echo.
+pause

scripts/test-rotor-com10.ps1

@@ -0,0 +1,80 @@
+$port = New-Object System.IO.Ports.SerialPort COM10,600,None,8,one
+
+try {
+    $port.Open()
+
+    [byte[]]$request = 0x57,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x1F,0x20
+
+    $port.Write($request, 0, $request.Length)
+
+    $response = New-Object System.Collections.Generic.List[byte]
+
+    $timeout = [DateTime]::UtcNow.AddSeconds(2)
+
+    while ([DateTime]::UtcNow -lt $timeout) {
+
+        while ($port.BytesToRead -gt 0) {
+            $buffer = New-Object byte[] $port.BytesToRead
+            $count = $port.Read($buffer, 0, $buffer.Length)
+
+            for ($i = 0; $i -lt $count; $i++) {
+                $response.Add($buffer[$i])
+            }
+        }
+
+        # Auf das Startbyte 57 synchronisieren.
+        $start = $response.IndexOf([byte]0x57)
+
+        if ($start -ge 0 -and ($response.Count - $start) -ge 12) {
+            $response = [System.Collections.Generic.List[byte]](
+                $response.GetRange($start, 12)
+            )
+            break
+        }
+
+        Start-Sleep -Milliseconds 20
+    }
+
+    if ($response.Count -eq 12) {
+        $actual = ($response |
+            ForEach-Object { "{0:X2}" -f $_ }) -join " "
+
+        $expected = "57 07 01 06 00 01 03 06 02 00 01 20"
+
+        Write-Host "Bytes empfangen: $($response.Count)"
+        Write-Host "Antwort: $actual"
+
+        if ($actual -eq $expected) {
+            Write-Host "TEST ERFOLGREICH"
+        }
+        else {
+            Write-Host "TEST FEHLGESCHLAGEN"
+            Write-Host "Erwartet: $expected"
+        }
+    }
+    else {
+        Write-Host "Bytes empfangen: $($response.Count)"
+
+        if ($response.Count -gt 0) {
+            $actual = ($response |
+                ForEach-Object { "{0:X2}" -f $_ }) -join " "
+            Write-Host "Antwort: $actual"
+        }
+        else {
+            Write-Host "Keine Antwort"
+        }
+
+        Write-Host "TEST FEHLGESCHLAGEN - keine vollstaendige Antwort gefunden"
+    }
+}
+finally {
+    if ($port.IsOpen) {
+        $port.Close()
+    }
+
+    $port.Dispose()
+    $port = $null
+
+    [GC]::Collect()
+    [GC]::WaitForPendingFinalizers()
+}

scripts/test-windows-client.bat

@@ -0,0 +1,20 @@
+@echo off
+cd /d "%~dp0.."
+
+echo ==========================================
+echo rs2322tcp Windows Client Test
+echo ==========================================
+echo.
+echo Starte normalen rs2322tcp-Client...
+echo COM100 wird vom Client verwendet.
+echo Die konfigurierte Startsequenz wird nach
+echo erfolgreichem Verbindungsaufbau automatisch gesendet.
+echo.
+echo Zum Beenden: Ctrl+C
+echo.
+
+go run .\cmd\rs2322tcp-client
+
+echo.
+echo Client beendet.
+pause