Add configurable local virtual serial ports

CHANGELOG.md

@@ -2,6 +2,41 @@
 
 Alle wesentlichen Änderungen am Projekt werden in dieser Datei dokumentiert.
 
+## [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

configs/client.json

@@ -3,13 +3,17 @@
         "address": "100.64.0.10",
         "port": 5000
     },
+    "virtual_port_range": {
+        "first": 100,
+        "last": 199
+    },
     "virtual_ports": [
         {
-            "port": "COM7",
+            "port": "/dev/ttyUSB100",
             "remote_device": "radio"
         },
         {
-            "port": "COM8",
+            "port": "/dev/ttyUSB101",
             "remote_device": "rotor"
         }
     ]

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 +1,140 @@
+//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.
+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
+	}
+
+	// Remove an existing link. The installation-owned outer /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 internal link for a virtual port.
+//
+// The public /dev/ttyUSBxxx link is never modified.
+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 virtual port link %q: %w",
+			linkPath,
+			err,
+		)
+	}
+
+	return nil
+}

internal/client/virtual_port_linux_test.go

@@ -0,0 +1,153 @@
+//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"
+	"testing"
+)
+
+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 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 +1,85 @@
+/*
+ * ============================================================================
+ * 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.
+ * ============================================================================
+ */
+package client
+
+import (
+	"fmt"
+
+	"git.lang-dieter.de/rs2322tcp/internal/config"
+)
+
+///////////////////////////////////////////////////////////////////////////////
+// VirtualPortManager
+///////////////////////////////////////////////////////////////////////////////
+
+// VirtualPortManager verwaltet den reservierten Bereich virtueller
+// /dev/ttyUSBxxx-Schnittstellen.
+type VirtualPortManager struct {
+	first int
+	last  int
+	used  map[int]bool
+}
+
+// NewVirtualPortManager erzeugt einen VirtualPortManager aus der
+// konfigurierten Portbereich.
+func NewVirtualPortManager(cfg config.VirtualPortRangeConfig) *VirtualPortManager {
+	return &VirtualPortManager{
+		first: cfg.First,
+		last:  cfg.Last,
+		used:  make(map[int]bool),
+	}
+}
+
+// PortPath erzeugt den für die externe Software sichtbaren Gerätenamen
+// für eine Portnummer.
+func (m *VirtualPortManager) PortPath(number int) string {
+	if m == nil {
+		return ""
+	}
+
+	return fmt.Sprintf("/dev/ttyUSB%d", number)
+}
+
+// Reserve reserviert den nächsten freien virtuellen Port.
+// Es wird immer mit dem kleinsten freien Port im konfigurierten Bereich
+// begonnen.
+func (m *VirtualPortManager) Reserve() (int, error) {
+	if m == nil {
+		return 0, fmt.Errorf("virtual port manager is nil")
+	}
+
+	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,
+	)
+}
+
+// Release gibt einen zuvor reservierten Port wieder frei.
+func (m *VirtualPortManager) Release(number int) {
+	if m == nil {
+		return
+	}
+
+	delete(m.used, number)
+}

internal/client/virtual_port_manager_linux.go

@@ -0,0 +1,137 @@
+//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 und Verwaltung virtueller rs2322tcp-Ports.
+ *
+ * Der öffentliche /dev/ttyUSBxxx-Eintrag wird bei der Installation
+ * bereitgestellt. Der Client verwaltet ausschließlich den internen Link
+ * unter ~/.rs2322tcp/virtual/.
+ * ============================================================================
+ */
+package client
+
+import "fmt"
+
+///////////////////////////////////////////////////////////////////////////////
+// Managed virtual port
+///////////////////////////////////////////////////////////////////////////////
+
+// ManagedVirtualPort verbindet einen reservierten Port mit einer
+// VirtualSerial-Instanz und verwaltet deren Lebenszyklus.
+type ManagedVirtualPort struct {
+	port    *VirtualPort
+	manager *VirtualPortManager
+	number  int
+}
+
+// Path returns the device path visible to the external application.
+func (p *ManagedVirtualPort) Path() string {
+	if p == nil || 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 || p.port == nil {
+		return 0, fmt.Errorf("managed virtual port is nil")
+	}
+
+	return p.port.Read(b)
+}
+
+// Write writes data to the underlying virtual serial device.
+func (p *ManagedVirtualPort) Write(b []byte) (int, error) {
+	if p == nil || p.port == nil {
+		return 0, fmt.Errorf("managed virtual port is nil")
+	}
+
+	return p.port.Write(b)
+}
+
+// Close removes the internal link, closes the PTY and releases the
+// reservation.
+func (p *ManagedVirtualPort) Close() error {
+	if p == nil || p.port == nil {
+		return nil
+	}
+
+	var firstErr error
+
+	if err := removeVirtualPortLink(p.port.Path()); err != nil {
+		firstErr = err
+	}
+
+	if err := p.port.Close(); err != nil && firstErr == nil {
+		firstErr = err
+	}
+
+	if p.manager != nil {
+		p.manager.Release(p.number)
+	}
+
+	p.port = nil
+
+	return firstErr
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Open
+///////////////////////////////////////////////////////////////////////////////
+
+// Open creates a new virtual serial port using the next free port number.
+//
+// The internal PTY is created by newVirtualSerial(). The internal
+// ~/.rs2322tcp/virtual/ttyUSBxxx link is then pointed at the PTY.
+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
+	}
+
+	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 +1,221 @@
+//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 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)
+	}
+
+	if _, err := os.Lstat(linkPath); !os.IsNotExist(err) {
+		t.Fatalf(
+			"internal link still exists after Close(): %q",
+			linkPath,
+		)
+	}
+}
+
+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_test.go

@@ -0,0 +1,216 @@
+/*
+ * ============================================================================
+ * 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.
+ * ============================================================================
+ */
+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:   "/dev/ttyUSB100",
+		},
+		{
+			number: 123,
+			want:   "/dev/ttyUSB123",
+		},
+		{
+			number: 199,
+			want:   "/dev/ttyUSB199",
+		},
+	}
+
+	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 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")
+	}
+
+	manager.Release(100)
+}

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_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/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
@@ -50,8 +50,9 @@ 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
@@ -61,6 +62,13 @@ 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.
 type VirtualPortConfig struct {
@@ -68,6 +76,28 @@ type VirtualPortConfig struct {
 	RemoteDevice string `json:"remote_device"`
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// 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
+)
+
+// DefaultVirtualPortRange returns the default virtual port range.
+func DefaultVirtualPortRange() VirtualPortRangeConfig {
+	return VirtualPortRangeConfig{
+		First: DefaultVirtualPortFirst,
+		Last:  DefaultVirtualPortLast,
+	}
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // JSON loading
 ///////////////////////////////////////////////////////////////////////////////
@@ -95,6 +125,12 @@ 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 +161,10 @@ 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
 	}
@@ -189,6 +229,16 @@ 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 {

internal/config/config_test.go

@@ -122,6 +122,16 @@ func TestLoadClient(t *testing.T) {
 			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",
 			len(cfg.VirtualPorts))
@@ -140,6 +150,42 @@ func TestLoadClient(t *testing.T) {
 	}
 }
 
+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)
+	}
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Save and reload
 ///////////////////////////////////////////////////////////////////////////////
@@ -153,6 +199,10 @@ func TestSaveAndLoadClient(t *testing.T) {
 			Address: "100.64.0.10",
 			Port:    5000,
 		},
+		VirtualPortRange: config.VirtualPortRangeConfig{
+			First: 200,
+			Last:  299,
+		},
 		VirtualPorts: []config.VirtualPortConfig{
 			{
 				Port:         "COM7",
@@ -178,6 +228,10 @@ 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",
 			len(loaded.VirtualPorts),
@@ -228,6 +282,7 @@ func TestClientValidationDuplicateVirtualPort(t *testing.T) {
 		Server: config.ServerConnectionConfig{
 			Port: 5000,
 		},
+		VirtualPortRange: config.DefaultVirtualPortRange(),
 		VirtualPorts: []config.VirtualPortConfig{
 			{
 				Port:         "COM7",
@@ -262,6 +317,7 @@ func TestClientValidationInvalidPort(t *testing.T) {
 		Server: config.ServerConnectionConfig{
 			Port: 0,
 		},
+		VirtualPortRange: config.DefaultVirtualPortRange(),
 	}
 
 	if err := cfg.Validate(); err == nil {
@@ -269,6 +325,54 @@ 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
 ///////////////////////////////////////////////////////////////////////////////
@@ -393,13 +497,13 @@ func TestServerRemoteDevices(t *testing.T) {
 	}
 
 	if list.Devices[0].BaudRate != 9600 {
-		t.Errorf("Devices[0].BaudRate = %d, want %d",
+		t.Errorf("Devices[0].BaudRate = %d, want 9600",
-			list.Devices[0].BaudRate, 9600)
+			list.Devices[0].BaudRate)
 	}
 
 	if list.Devices[1].BaudRate != 4800 {
-		t.Errorf("Devices[1].BaudRate = %d, want %d",
+		t.Errorf("Devices[1].BaudRate = %d, want 4800",
-			list.Devices[1].BaudRate, 4800)
+			list.Devices[1].BaudRate)
 	}
 }
 
@@ -449,7 +553,7 @@ 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" {