Detect interfaces per platform; add macOS and single-interface support (closes #2)

Linux runs exactly as before when both bridge interfaces exist. When they do
not, the lone default-route interface is monitored, and a single interface now
draws a single UI pane instead of an empty second one.

macOS is newly supported: a running VPN tunnel (utun) is monitored as the
primary pane alongside the physical default-route interface, or the physical
interface alone when no VPN is up.

Detection lives in internal/netdetect: interface/route data types, pure
selection logic keyed on OS name, and route parsers, all unit-tested on Linux
for both platforms. Only the real route query and the per-platform TCP dial
binding (source address on Linux, IP_BOUND_IF on macOS) are build-tagged. Ping
argument construction is a pure, OS-keyed function. NewMonitor now takes a list
of interfaces.

Model: opus-4-8

README.md

@@ -1,33 +1,70 @@
 # rtnetmon
 
-Real-time network monitoring dashboard for Linux systems with dual-interface support.
+Real-time network monitoring dashboard for Linux and macOS, with one- or
+two-interface support.
 
 ## Overview
 
 rtnetmon is a terminal-based network monitoring tool that provides real-time
-visibility into network health, packet loss, and latency across two network
+visibility into network health, packet loss, and latency across one or two
-interfaces simultaneously. It's designed for Linux systems and uses ncurses
+network interfaces simultaneously. It runs on Linux and macOS and uses a
-for a clean, real-time dashboard interface.
+terminal dashboard interface.
 
 ## Features
 
-- **Dual Interface Monitoring**: Monitor two network interfaces simultaneously
+- **Interface Monitoring**: Monitor one or two network interfaces simultaneously
 - **Real-time Updates**: Live dashboard with sub-second updates
 - **Comprehensive Metrics**:
-  - ICMP reachability tests
+    - ICMP reachability tests
-  - Packet loss percentage
+    - Packet loss percentage
-  - TCP connection latency
+    - TCP connection latency
-  - Interface health status
+    - Interface health status
-- **Visual Indicators**: Color-coded status, spinners, and meters for quick status assessment
+- **Visual Indicators**: Color-coded status, spinners, and meters for quick
-  - Packet loss meter uses reverse coloring (empty/green = good, full/red = bad)
+  status assessment
+    - Packet loss meter uses reverse coloring (empty/green = good, full/red =
+      bad)
 - **Detailed Logging**: Optional logging to file for debugging and analysis
 
 ## Requirements
 
-- Linux operating system
+- Linux or macOS
 - Go 1.21 or later
 - Root/sudo access (for raw ICMP packets)
 - `ping` command available in PATH
+- On Linux: `ip` (with `/proc/net/route` as a fallback)
+- On macOS: `netstat`
+
+## Platform support and interface detection
+
+rtnetmon monitors either one or two interfaces, chosen automatically for the
+platform it runs on. When only one interface is detected, the dashboard shows a
+single pane.
+
+**Linux.** The two named interfaces (`--ifaceA`/`--ifaceB`, default
+`gu0`/`backhaul0`) are used when both exist — this is the original dual-bridge
+setup, unchanged. When neither exists, the single default-route interface is
+monitored instead.
+
+**macOS.** The physical internet interface is found from the default route. When
+a VPN client is running (Mullvad and similar clients create a `utun` tunnel that
+carries a default route or holds a routable address), that tunnel is monitored
+as the primary pane alongside the physical interface. With no VPN running, only
+the physical interface is monitored. Interface names are detected on macOS; the
+`--ifaceA`/`--ifaceB` flags are not used there, but `--labelA`/`--labelB` still
+set the pane labels.
+
+### Supported matrix
+
+| OS    | Interfaces monitored                                        |
+| ----- | ----------------------------------------------------------- |
+| Linux | `gu0` + `backhaul0` when both exist (two panes)             |
+| Linux | the single default-route interface otherwise (one pane)     |
+| macOS | VPN tunnel + physical default-route interface (two panes)   |
+| macOS | the physical default-route interface with no VPN (one pane) |
+
+Anything outside this matrix — on Linux, only one of the named pair present, or
+no/multiple default routes when neither is present; on macOS, no default route
+or more than one physical default route — exits with a clear error.
 
 ## Installation
 
@@ -65,11 +102,17 @@ rtnetmon/
 ├── internal/
 │   ├── cli/           # Command-line interface using Cobra
 │   │   └── root.go
+│   ├── netdetect/     # Per-platform interface/route detection and selection
+│   │   ├── netdetect.go     # Selection logic and route parsers (pure)
+│   │   ├── routes_linux.go  # Linux default-route query (build-tagged)
+│   │   └── routes_darwin.go # macOS default-route query (build-tagged)
 │   └── monitor/       # Core monitoring functionality
-│       ├── monitor.go # Main monitoring types and functions
+│       ├── monitor.go     # Main monitoring types and functions
-│       ├── loops.go   # Monitoring loops (reachability, loss, TCP)
+│       ├── loops.go       # Monitoring loops (reachability, loss, TCP)
-│       ├── styles.go  # Terminal color styles
+│       ├── styles.go      # Terminal color styles
-│       └── ui.go      # User interface rendering
+│       ├── ui.go          # User interface rendering
+│       ├── dial_linux.go  # TCP source binding (build-tagged)
+│       └── dial_darwin.go # TCP IP_BOUND_IF binding (build-tagged)
 ├── go.mod
 ├── go.sum
 ├── Makefile
@@ -103,8 +146,11 @@ The monitor package provides an object-oriented API for programmatic use:
 ```go
 import "git.eeqj.de/sneak/rtnetmon/internal/monitor"
 
-// Create a new monitor
+// Create a new monitor for one or two interfaces
-mon := monitor.NewMonitor("eth0", "Primary", "wlan0", "Backup", "/tmp/monitor.log")
+mon := monitor.NewMonitor([]monitor.IfaceSpec{
+    {Name: "eth0", Label: "Primary"},
+    {Name: "wlan0", Label: "Backup"},
+}, "/tmp/monitor.log")
 
 // Configure timing parameters (optional - defaults are sensible)
 mon.ICMPTimeout = 1 * time.Second

cmd/rtnetmon/main.go

@@ -1,7 +1,4 @@
-//go:build linux
+// netmon – network dashboard (curses) for Linux and macOS
-// +build linux
-
-// netmon – dual-interface network dashboard (curses)
 // WTFPL – 2025-05-16  sneak@sneak.berlin
 package main
 

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/gdamore/tcell/v2 v2.8.1
 	github.com/spf13/cobra v1.8.0
 	github.com/spf13/viper v1.18.2
+	golang.org/x/sys v0.29.0
 )
 
 require (
@@ -29,7 +30,6 @@ require (
 	go.uber.org/atomic v1.9.0 // indirect
 	go.uber.org/multierr v1.9.0 // indirect
 	golang.org/x/exp v0.0.0-20230905200255-921286631fa9 // indirect
-	golang.org/x/sys v0.29.0 // indirect
 	golang.org/x/term v0.28.0 // indirect
 	golang.org/x/text v0.21.0 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect

internal/cli/root.go

@@ -1,18 +1,17 @@
-//go:build linux
-// +build linux
-
 package cli
 
 import (
 	"context"
 	"os"
 	"os/signal"
+	"runtime"
 	"syscall"
 
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
 
 	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
+	"git.eeqj.de/sneak/rtnetmon/internal/netdetect"
 )
 
 // Config holds the application configuration
@@ -73,12 +72,17 @@ func init() {
 	_ = viper.BindPFlag("logfile", rootCmd.Flags().Lookup("logfile"))
 }
 
-func runMonitor(cmd *cobra.Command, args []string) error {
+func runMonitor(cmd *cobra.Command, _ []string) error {
 	monitor.Logf(cfg.LogFile, "Starting rtnetmon")
-	monitor.Logf(cfg.LogFile, "Monitoring interfaces %s and %s", cfg.IfaceA, cfg.IfaceB)
+
+	// Detect which interfaces to monitor for this platform.
+	specs, err := detectInterfaces(cmd)
+	if err != nil {
+		return err
+	}
 
 	// Create the monitor
-	mon := monitor.NewMonitor(cfg.IfaceA, cfg.LabelA, cfg.IfaceB, cfg.LabelB, cfg.LogFile)
+	mon := monitor.NewMonitor(specs, cfg.LogFile)
 
 	// Add reachability hosts
 	for _, host := range cfg.Hosts {
@@ -112,6 +116,43 @@ func runMonitor(cmd *cobra.Command, args []string) error {
 	return mon.Run(ctx)
 }
 
+// detectInterfaces enumerates the host, reads its default routes, and applies
+// the platform selection rules to decide which interfaces to monitor.
+func detectInterfaces(cmd *cobra.Command) ([]monitor.IfaceSpec, error) {
+	ifaces, err := netdetect.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+
+	routes, err := netdetect.DefaultRoutes()
+	if err != nil {
+		return nil, err
+	}
+
+	flags := netdetect.Flags{
+		IfaceA:    cfg.IfaceA,
+		LabelA:    cfg.LabelA,
+		IfaceB:    cfg.IfaceB,
+		LabelB:    cfg.LabelB,
+		LabelASet: cmd.Flags().Changed("labelA"),
+		LabelBSet: cmd.Flags().Changed("labelB"),
+	}
+
+	panes, err := netdetect.Select(runtime.GOOS, ifaces, routes, flags)
+	if err != nil {
+		return nil, err
+	}
+
+	specs := make([]monitor.IfaceSpec, len(panes))
+	for i, p := range panes {
+		specs[i] = monitor.IfaceSpec{Name: p.Name, Label: p.Label}
+	}
+
+	monitor.Logf(cfg.LogFile, "Monitoring %d interface(s)", len(specs))
+
+	return specs, nil
+}
+
 // Execute runs the root command
 func Execute() error {
 	return rootCmd.Execute()

internal/cli/root_test.go

@@ -1,6 +1,3 @@
-//go:build linux
-// +build linux
-
 package cli
 
 import (

internal/monitor/dial_darwin.go

@@ -0,0 +1,36 @@
+//go:build darwin
+
+package monitor
+
+import (
+	"net"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// bindControl returns a socket control hook that pins the connection to the
+// given interface with IP_BOUND_IF. On macOS a bound source address is not
+// enough: without this the kernel still routes the packets over the VPN's
+// default route, so traffic would not leave the interface we are measuring.
+func bindControl(iface string) func(network, address string, c syscall.RawConn) error {
+	ifi, err := net.InterfaceByName(iface)
+	if err != nil {
+		return nil
+	}
+
+	idx := ifi.Index
+
+	return func(_, _ string, c syscall.RawConn) error {
+		var setErr error
+
+		ctrlErr := c.Control(func(fd uintptr) {
+			setErr = unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_BOUND_IF, idx)
+		})
+		if ctrlErr != nil {
+			return ctrlErr
+		}
+
+		return setErr
+	}
+}

internal/monitor/dial_linux.go

@@ -0,0 +1,12 @@
+//go:build linux
+
+package monitor
+
+import "syscall"
+
+// bindControl returns no socket control hook on Linux: binding the dialer's
+// local source address (as tcpDuration already does) is enough to send
+// traffic out of the chosen interface.
+func bindControl(_ string) func(network, address string, c syscall.RawConn) error {
+	return nil
+}

internal/monitor/loops.go

@@ -1,6 +1,3 @@
-//go:build linux
-// +build linux
-
 package monitor
 
 import (

internal/monitor/monitor.go

@@ -1,6 +1,3 @@
-//go:build linux
-// +build linux
-
 package monitor
 
 import (
@@ -11,6 +8,7 @@ import (
 	"net"
 	"os"
 	"os/exec"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -53,9 +51,8 @@ type Monitor struct {
 	packetLossHosts   []string
 	tcpHosts          []string
 
-	// Interfaces
+	// Interfaces to monitor (one or two)
-	interfaceA *InterfaceStatus
+	interfaces []*InterfaceStatus
-	interfaceB *InterfaceStatus
 
 	// Logging
 	logFile string
@@ -68,9 +65,16 @@ type Monitor struct {
 	mu sync.RWMutex
 }
 
-// NewMonitor creates a new Monitor instance with default settings
+// IfaceSpec names one interface to monitor and its display label.
-func NewMonitor(ifaceA, labelA, ifaceB, labelB, logFile string) *Monitor {
+type IfaceSpec struct {
-	return &Monitor{
+	Name  string
+	Label string
+}
+
+// NewMonitor creates a new Monitor instance with default settings, monitoring
+// the given interfaces (one or two).
+func NewMonitor(ifaces []IfaceSpec, logFile string) *Monitor {
+	m := &Monitor{
 		// Default timing configuration
 		ICMPTimeout:      500 * time.Millisecond,
 		TCPTimeout:       500 * time.Millisecond,
@@ -94,15 +98,17 @@ func NewMonitor(ifaceA, labelA, ifaceB, labelB, logFile string) *Monitor {
 		packetLossHosts:   []string{},
 		tcpHosts:          []string{},
 
-		// Initialize interfaces
-		interfaceA: NewInterfaceStatus(ifaceA, labelA),
-		interfaceB: NewInterfaceStatus(ifaceB, labelB),
-
 		// Logging
 		logFile: logFile,
 
 		startTime: time.Now(),
 	}
+
+	for _, spec := range ifaces {
+		m.interfaces = append(m.interfaces, NewInterfaceStatus(spec.Name, spec.Label))
+	}
+
+	return m
 }
 
 // AddReachabilityHost adds a host for reachability monitoring
@@ -159,12 +165,11 @@ func (m *Monitor) Run(ctx context.Context) error {
 	tcpHosts := append([]string{}, m.tcpHosts...)
 	m.mu.RUnlock()
 
-	go m.reachLoop(ctx, m.interfaceA, reachHosts)
+	for _, st := range m.interfaces {
-	go m.reachLoop(ctx, m.interfaceB, reachHosts)
+		go m.reachLoop(ctx, st, reachHosts)
-	go m.lossLoop(ctx, m.interfaceA, lossHosts)
+		go m.lossLoop(ctx, st, lossHosts)
-	go m.lossLoop(ctx, m.interfaceB, lossHosts)
+		go m.tcpLoop(ctx, st, tcpHosts)
-	go m.tcpLoop(ctx, m.interfaceA, tcpHosts)
+	}
-	go m.tcpLoop(ctx, m.interfaceB, tcpHosts)
 
 	// Run UI loop
 	m.logf("Starting UI loop")
@@ -252,19 +257,45 @@ func fetchIPInfo(iface string) string {
 	return fmt.Sprintf("%s [%s] %s", r.IP, r.Hostname, r.Org)
 }
 
+// pingArgs builds the arguments for a single reachability ping on the given
+// OS. Linux binds the interface with -I and takes -W in seconds; macOS binds
+// with -b and takes -W in milliseconds.
+func pingArgs(goos, iface, host string) []string {
+	if goos == "darwin" {
+		return []string{"-b", iface, "-c1", "-W1000", host}
+	}
+
+	return []string{"-I", iface, "-c1", "-W1", host}
+}
+
+// lossArgs builds the arguments for a packet-loss ping burst of count pings.
+func lossArgs(goos, iface, host string, count int) []string {
+	c := strconv.Itoa(count)
+	if goos == "darwin" {
+		return []string{"-q", "-i", "0.05", "-c", c, "-W1000", "-b", iface, host}
+	}
+
+	return []string{"-q", "-i", "0.05", "-c", c, "-W1", "-I", iface, host}
+}
+
 // pingOnce performs a single ping
 func (m *Monitor) pingOnce(iface, host string) bool {
 	ctx, cancel := context.WithTimeout(context.Background(), m.ICMPTimeout)
 	defer cancel()
-	return exec.CommandContext(ctx, "ping", "-I", iface, "-c1", "-W1", host).Run() == nil
+
+	args := pingArgs(runtime.GOOS, iface, host)
+
+	return exec.CommandContext(ctx, "ping", args...).Run() == nil
 }
 
 // lossPercent calculates packet loss percentage
 func (m *Monitor) lossPercent(iface, host string) float64 {
 	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
 	defer cancel()
-	out, err := exec.CommandContext(ctx, "ping", "-q", "-i", "0.05",
+
-		"-c", fmt.Sprint(m.PacketLossPings), "-W1", "-I", iface, host).CombinedOutput()
+	args := lossArgs(runtime.GOOS, iface, host, m.PacketLossPings)
+
+	out, err := exec.CommandContext(ctx, "ping", args...).CombinedOutput()
 	if err != nil {
 		return 1.0
 	}
@@ -302,7 +333,7 @@ func (m *Monitor) tcpDuration(iface, hp string) time.Duration {
 	if err != nil {
 		return m.TCPTimeout
 	}
-	d := net.Dialer{Timeout: m.TCPTimeout, LocalAddr: la}
+	d := net.Dialer{Timeout: m.TCPTimeout, LocalAddr: la, Control: bindControl(iface)}
 	st := time.Now()
 	c, err := d.Dial("tcp", hp)
 	if err != nil {

internal/monitor/monitor_test.go

@@ -1,6 +1,3 @@
-//go:build linux
-// +build linux
-
 package monitor
 
 import (
@@ -10,27 +7,26 @@ import (
 // TestNewMonitor tests the creation of a new Monitor
 func TestNewMonitor(t *testing.T) {
 	// Test that we can create a monitor without errors
-	mon := NewMonitor("test0", "Test Interface A", "test1", "Test Interface B", "/tmp/test.log")
+	mon := NewMonitor([]IfaceSpec{
+		{Name: "test0", Label: "Test Interface A"},
+		{Name: "test1", Label: "Test Interface B"},
+	}, "/tmp/test.log")
 
 	if mon == nil {
 		t.Fatal("NewMonitor returned nil")
 	}
 
 	// Check interfaces
-	if mon.interfaceA == nil {
+	if len(mon.interfaces) != 2 {
-		t.Fatal("interfaceA is nil")
+		t.Fatalf("Expected 2 interfaces, got %d", len(mon.interfaces))
 	}
 
-	if mon.interfaceB == nil {
+	if mon.interfaces[0].Name != "test0" {
-		t.Fatal("interfaceB is nil")
+		t.Errorf("Expected interfaces[0].Name to be 'test0', got '%s'", mon.interfaces[0].Name)
 	}
 
-	if mon.interfaceA.Name != "test0" {
+	if mon.interfaces[1].Name != "test1" {
-		t.Errorf("Expected interfaceA.Name to be 'test0', got '%s'", mon.interfaceA.Name)
+		t.Errorf("Expected interfaces[1].Name to be 'test1', got '%s'", mon.interfaces[1].Name)
-	}
-
-	if mon.interfaceB.Name != "test1" {
-		t.Errorf("Expected interfaceB.Name to be 'test1', got '%s'", mon.interfaceB.Name)
 	}
 
 	// Check default configuration
@@ -56,9 +52,25 @@ func TestNewMonitor(t *testing.T) {
 	}
 }
 
+// TestNewMonitorSingle verifies a monitor can be built with one interface.
+func TestNewMonitorSingle(t *testing.T) {
+	mon := NewMonitor([]IfaceSpec{{Name: "eth0", Label: "default route"}}, "")
+
+	if len(mon.interfaces) != 1 {
+		t.Fatalf("Expected 1 interface, got %d", len(mon.interfaces))
+	}
+
+	if mon.interfaces[0].Name != "eth0" {
+		t.Errorf("Expected interfaces[0].Name to be 'eth0', got '%s'", mon.interfaces[0].Name)
+	}
+}
+
 // TestAddHosts tests adding hosts to the monitor
 func TestAddHosts(t *testing.T) {
-	mon := NewMonitor("test0", "Test A", "test1", "Test B", "")
+	mon := NewMonitor([]IfaceSpec{
+		{Name: "test0", Label: "Test A"},
+		{Name: "test1", Label: "Test B"},
+	}, "")
 
 	// Test adding reachability hosts
 	mon.AddReachabilityHost("8.8.8.8")

internal/monitor/ping_test.go

@@ -0,0 +1,48 @@
+package monitor
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestPingArgs(t *testing.T) {
+	tests := []struct {
+		goos string
+		want []string
+	}{
+		{"linux", []string{"-I", "eth0", "-c1", "-W1", "8.8.8.8"}},
+		{"darwin", []string{"-b", "eth0", "-c1", "-W1000", "8.8.8.8"}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.goos, func(t *testing.T) {
+			got := pingArgs(tt.goos, "eth0", "8.8.8.8")
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("pingArgs(%q) = %v, want %v", tt.goos, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestLossArgs(t *testing.T) {
+	tests := []struct {
+		goos string
+		want []string
+	}{
+		{"linux", []string{
+			"-q", "-i", "0.05", "-c", "20", "-W1", "-I", "eth0", "8.8.8.8",
+		}},
+		{"darwin", []string{
+			"-q", "-i", "0.05", "-c", "20", "-W1000", "-b", "eth0", "8.8.8.8",
+		}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.goos, func(t *testing.T) {
+			got := lossArgs(tt.goos, "eth0", "8.8.8.8", 20)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("lossArgs(%q) = %v, want %v", tt.goos, got, tt.want)
+			}
+		})
+	}
+}

internal/monitor/styles.go

@@ -1,6 +1,3 @@
-//go:build linux
-// +build linux
-
 package monitor
 
 import tcell "github.com/gdamore/tcell/v2"

internal/monitor/styles_test.go

@@ -1,6 +1,3 @@
-//go:build linux
-// +build linux
-
 package monitor
 
 import (

internal/monitor/ui.go

@@ -1,6 +1,3 @@
-//go:build linux
-// +build linux
-
 package monitor
 
 import (
@@ -345,10 +342,11 @@ func (m *Monitor) uiLoop(ctx context.Context) {
 		// Draw the runtime
 		Put(m.screen, 0, 3, "Runtime: "+time.Since(m.startTime).Round(time.Second).String(), CDefault)
 
-		// Draw interfaces
+		// Draw interfaces (one pane each; a single interface draws one pane)
 		y := 5
-		y = m.DrawInterface(m.screen, y, w, m.interfaceA)
+		for _, st := range m.interfaces {
-		_ = m.DrawInterface(m.screen, y, w, m.interfaceB)
+			y = m.DrawInterface(m.screen, y, w, st)
+		}
 
 		// Show the screen
 		m.screen.Show()

internal/netdetect/netdetect.go

@@ -0,0 +1,359 @@
+// Package netdetect chooses which network interfaces rtnetmon should monitor.
+//
+// The host-specific queries (enumerating interfaces, reading the routing
+// table) live behind small data types so the selection logic and the route
+// parsers are pure functions that can be unit-tested on any OS with fake
+// data. Only the real routing-table query is build-tagged per platform.
+package netdetect
+
+import (
+	"fmt"
+	"net"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// Interface is a network interface reduced to what detection needs.
+type Interface struct {
+	Name string
+	Up   bool
+	IPv4 []string
+}
+
+// Route is one routing-table entry reduced to what detection needs.
+type Route struct {
+	Iface   string
+	Gateway string
+	Default bool
+}
+
+// Pane names one interface to display, with its label.
+type Pane struct {
+	Name  string
+	Label string
+}
+
+// Flags carries the user's --iface/--label choices and whether each label was
+// set explicitly on the command line.
+type Flags struct {
+	IfaceA    string
+	LabelA    string
+	IfaceB    string
+	LabelB    string
+	LabelASet bool
+	LabelBSet bool
+}
+
+// Interfaces enumerates the host's interfaces and their IPv4 addresses. This
+// uses the standard library and is the same on every platform.
+func Interfaces() ([]Interface, error) {
+	ifs, err := net.Interfaces()
+	if err != nil {
+		return nil, fmt.Errorf("listing interfaces: %w", err)
+	}
+
+	out := make([]Interface, 0, len(ifs))
+	for _, ifi := range ifs {
+		var v4 []string
+
+		addrs, _ := ifi.Addrs()
+		for _, a := range addrs {
+			if n, ok := a.(*net.IPNet); ok && n.IP.To4() != nil {
+				v4 = append(v4, n.IP.String())
+			}
+		}
+
+		out = append(out, Interface{
+			Name: ifi.Name,
+			Up:   ifi.Flags&net.FlagUp != 0,
+			IPv4: v4,
+		})
+	}
+
+	return out, nil
+}
+
+// Select decides which one or two interfaces to monitor for the given OS.
+// See the README's supported matrix for the exact rules.
+func Select(goos string, ifaces []Interface, routes []Route, f Flags) ([]Pane, error) {
+	switch goos {
+	case "linux":
+		return selectLinux(ifaces, routes, f)
+	case "darwin":
+		return selectDarwin(ifaces, routes, f)
+	default:
+		return nil, fmt.Errorf("unsupported operating system %q", goos)
+	}
+}
+
+// selectLinux keeps today's behavior: if both configured interfaces exist,
+// monitor them as two panes; if neither exists, monitor the single
+// default-route interface; anything else is an error.
+func selectLinux(ifaces []Interface, routes []Route, f Flags) ([]Pane, error) {
+	haveA := hasInterface(ifaces, f.IfaceA)
+	haveB := hasInterface(ifaces, f.IfaceB)
+
+	switch {
+	case haveA && haveB:
+		return []Pane{
+			{Name: f.IfaceA, Label: f.LabelA},
+			{Name: f.IfaceB, Label: f.LabelB},
+		}, nil
+	case !haveA && !haveB:
+		name, err := onlyDefaultRoute(routes)
+		if err != nil {
+			return nil, err
+		}
+
+		return []Pane{{Name: name, Label: singleLabel(f)}}, nil
+	default:
+		return nil, fmt.Errorf(
+			"found only one of the configured interfaces (%q, %q); "+
+				"rtnetmon needs both, or neither (default route only)",
+			f.IfaceA, f.IfaceB)
+	}
+}
+
+// selectDarwin monitors the physical default-route interface, plus a VPN
+// tunnel as the primary pane when one is running.
+func selectDarwin(ifaces []Interface, routes []Route, f Flags) ([]Pane, error) {
+	vpn := findVPN(ifaces, routes)
+
+	phys, err := onlyPhysicalDefaultRoute(routes, vpn)
+	if err != nil {
+		return nil, err
+	}
+
+	if vpn == "" {
+		return []Pane{{Name: phys, Label: singleLabel(f)}}, nil
+	}
+
+	return []Pane{
+		{Name: vpn, Label: labelOr(f.LabelA, f.LabelASet, "VPN")},
+		{Name: phys, Label: labelOr(f.LabelB, f.LabelBSet, "default route")},
+	}, nil
+}
+
+// findVPN returns the name of a VPN tunnel interface, or "" if none is
+// running. A tunnel counts as a running VPN when it carries a default route,
+// or when it is up with a routable (non-link-local) IPv4 address. Idle system
+// tunnels have only a link-local IPv6 address and are skipped.
+func findVPN(ifaces []Interface, routes []Route) string {
+	routeIfaces := map[string]bool{}
+	for _, r := range routes {
+		if r.Default {
+			routeIfaces[r.Iface] = true
+		}
+	}
+
+	sorted := append([]Interface(nil), ifaces...)
+	sort.Slice(sorted, func(i, j int) bool { return sorted[i].Name < sorted[j].Name })
+
+	for _, ifi := range sorted {
+		if !isTunnel(ifi.Name) {
+			continue
+		}
+		if routeIfaces[ifi.Name] {
+			return ifi.Name
+		}
+		if ifi.Up && hasRoutableIPv4(ifi) {
+			return ifi.Name
+		}
+	}
+
+	return ""
+}
+
+// onlyDefaultRoute returns the single default-route interface, or an error if
+// there is not exactly one.
+func onlyDefaultRoute(routes []Route) (string, error) {
+	names := defaultRouteIfaces(routes, "")
+
+	switch len(names) {
+	case 1:
+		return names[0], nil
+	case 0:
+		return "", fmt.Errorf(
+			"no default route found; rtnetmon needs one internet interface")
+	default:
+		return "", fmt.Errorf(
+			"multiple default-route interfaces found (%s); rtnetmon supports only one",
+			strings.Join(names, ", "))
+	}
+}
+
+// onlyPhysicalDefaultRoute returns the single non-tunnel default-route
+// interface (ignoring the VPN), or an error if there is not exactly one.
+func onlyPhysicalDefaultRoute(routes []Route, vpn string) (string, error) {
+	names := defaultRouteIfaces(routes, vpn)
+
+	switch len(names) {
+	case 1:
+		return names[0], nil
+	case 0:
+		return "", fmt.Errorf(
+			"no physical default-route interface found; " +
+				"rtnetmon needs one internet interface")
+	default:
+		return "", fmt.Errorf(
+			"multiple physical default-route interfaces found (%s); "+
+				"rtnetmon supports only one",
+			strings.Join(names, ", "))
+	}
+}
+
+// defaultRouteIfaces returns the sorted, unique interface names that carry a
+// default route, excluding the named VPN interface and any other tunnel.
+func defaultRouteIfaces(routes []Route, vpn string) []string {
+	seen := map[string]bool{}
+
+	var names []string
+
+	for _, r := range routes {
+		if !r.Default || r.Iface == vpn || isTunnel(r.Iface) || seen[r.Iface] {
+			continue
+		}
+
+		seen[r.Iface] = true
+		names = append(names, r.Iface)
+	}
+
+	sort.Strings(names)
+
+	return names
+}
+
+// hasInterface reports whether an interface with the given name exists.
+func hasInterface(ifaces []Interface, name string) bool {
+	for _, ifi := range ifaces {
+		if ifi.Name == name {
+			return true
+		}
+	}
+
+	return false
+}
+
+// isTunnel reports whether the interface name is a macOS userspace tunnel
+// (utunN), which is what Mullvad and other WireGuard/OpenVPN clients use.
+func isTunnel(name string) bool {
+	return strings.HasPrefix(name, "utun")
+}
+
+// hasRoutableIPv4 reports whether the interface has an IPv4 address that is
+// neither loopback nor link-local.
+func hasRoutableIPv4(ifi Interface) bool {
+	for _, s := range ifi.IPv4 {
+		ip := net.ParseIP(s)
+		if ip == nil || ip.IsLoopback() || ip.IsLinkLocalUnicast() {
+			continue
+		}
+
+		return true
+	}
+
+	return false
+}
+
+// singleLabel is the label for a lone pane: the explicit --labelA if given,
+// otherwise a plain description.
+func singleLabel(f Flags) string {
+	return labelOr(f.LabelA, f.LabelASet, "default route")
+}
+
+// labelOr returns value when it was set explicitly, otherwise fallback.
+func labelOr(value string, set bool, fallback string) string {
+	if set {
+		return value
+	}
+
+	return fallback
+}
+
+// parseIPRoute reads `ip -4 route show default` output. Every printed line is
+// a default route.
+func parseIPRoute(out string) []Route {
+	var routes []Route
+
+	for _, line := range strings.Split(out, "\n") {
+		fields := strings.Fields(line)
+		if len(fields) == 0 || fields[0] != "default" {
+			continue
+		}
+
+		r := Route{Default: true}
+		for i := 0; i < len(fields)-1; i++ {
+			switch fields[i] {
+			case "dev":
+				r.Iface = fields[i+1]
+			case "via":
+				r.Gateway = fields[i+1]
+			}
+		}
+
+		if r.Iface != "" {
+			routes = append(routes, r)
+		}
+	}
+
+	return routes
+}
+
+// parseProcNetRoute reads /proc/net/route. A default route has destination
+// 00000000; the gateway is a little-endian hex IPv4 address.
+func parseProcNetRoute(out string) []Route {
+	var routes []Route
+
+	for i, line := range strings.Split(out, "\n") {
+		if i == 0 { // column header
+			continue
+		}
+
+		fields := strings.Fields(line)
+		if len(fields) < 3 || fields[1] != "00000000" {
+			continue
+		}
+
+		routes = append(routes, Route{
+			Iface:   fields[0],
+			Gateway: hexToIP(fields[2]),
+			Default: true,
+		})
+	}
+
+	return routes
+}
+
+// parseNetstat reads `netstat -rn -f inet` output (macOS). Rows whose
+// destination is "default" are default routes; the Netif column (field 4)
+// names the interface.
+func parseNetstat(out string) []Route {
+	var routes []Route
+
+	for _, line := range strings.Split(out, "\n") {
+		fields := strings.Fields(line)
+		if len(fields) < 4 || fields[0] != "default" {
+			continue
+		}
+
+		routes = append(routes, Route{
+			Iface:   fields[3],
+			Gateway: fields[1],
+			Default: true,
+		})
+	}
+
+	return routes
+}
+
+// hexToIP converts a little-endian hex IPv4 address (as found in
+// /proc/net/route) to dotted-quad form.
+func hexToIP(h string) string {
+	v, err := strconv.ParseUint(h, 16, 32)
+	if err != nil {
+		return ""
+	}
+
+	return fmt.Sprintf("%d.%d.%d.%d", v&0xff, (v>>8)&0xff, (v>>16)&0xff, (v>>24)&0xff)
+}

internal/netdetect/netdetect_test.go

@@ -0,0 +1,238 @@
+package netdetect
+
+import (
+	"reflect"
+	"testing"
+)
+
+// gu and backhaul are the Linux bridge interfaces rtnetmon was built for.
+func linuxFlags() Flags {
+	return Flags{
+		IfaceA: "gu0", LabelA: "gu LAN - VPN outbound",
+		IfaceB: "backhaul0", LabelB: "Cox cable direct",
+	}
+}
+
+func TestSelect(t *testing.T) {
+	tests := []struct {
+		name    string
+		goos    string
+		ifaces  []Interface
+		routes  []Route
+		flags   Flags
+		want    []Pane
+		wantErr bool
+	}{
+		{
+			name: "linux both bridge interfaces present",
+			goos: "linux",
+			ifaces: []Interface{
+				{Name: "gu0", Up: true},
+				{Name: "backhaul0", Up: true},
+				{Name: "eth0", Up: true},
+			},
+			routes: []Route{{Iface: "eth0", Default: true}},
+			flags:  linuxFlags(),
+			want: []Pane{
+				{Name: "gu0", Label: "gu LAN - VPN outbound"},
+				{Name: "backhaul0", Label: "Cox cable direct"},
+			},
+		},
+		{
+			name:   "linux no bridge interfaces, single default route",
+			goos:   "linux",
+			ifaces: []Interface{{Name: "eth0", Up: true}, {Name: "lo", Up: true}},
+			routes: []Route{{Iface: "eth0", Default: true}},
+			flags:  linuxFlags(),
+			want:   []Pane{{Name: "eth0", Label: "default route"}},
+		},
+		{
+			name:    "linux only one bridge interface present is an error",
+			goos:    "linux",
+			ifaces:  []Interface{{Name: "gu0", Up: true}, {Name: "eth0", Up: true}},
+			routes:  []Route{{Iface: "eth0", Default: true}},
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name:    "linux no bridge, no default route is an error",
+			goos:    "linux",
+			ifaces:  []Interface{{Name: "eth0", Up: true}},
+			routes:  nil,
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name:    "linux no bridge, multiple default routes is an error",
+			goos:    "linux",
+			ifaces:  []Interface{{Name: "eth0", Up: true}, {Name: "eth1", Up: true}},
+			routes:  []Route{{Iface: "eth0", Default: true}, {Iface: "eth1", Default: true}},
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name: "linux single default route keeps explicit label",
+			goos: "linux",
+			ifaces: []Interface{
+				{Name: "wlan0", Up: true},
+			},
+			routes: []Route{{Iface: "wlan0", Default: true}},
+			flags: Flags{
+				IfaceA: "gu0", LabelA: "Home WiFi", LabelASet: true,
+				IfaceB: "backhaul0", LabelB: "Cox cable direct",
+			},
+			want: []Pane{{Name: "wlan0", Label: "Home WiFi"}},
+		},
+		{
+			name: "macos vpn tunnel plus physical default route",
+			goos: "darwin",
+			ifaces: []Interface{
+				{Name: "en0", Up: true, IPv4: []string{"192.168.1.20"}},
+				{Name: "utun4", Up: true, IPv4: []string{"10.64.0.2"}},
+				{Name: "utun0", Up: true, IPv4: nil},
+			},
+			routes: []Route{
+				{Iface: "utun4", Default: true},
+				{Iface: "en0", Default: true},
+			},
+			flags: linuxFlags(),
+			want: []Pane{
+				{Name: "utun4", Label: "VPN"},
+				{Name: "en0", Label: "default route"},
+			},
+		},
+		{
+			name: "macos vpn detected by routable address without its own route",
+			goos: "darwin",
+			ifaces: []Interface{
+				{Name: "en0", Up: true, IPv4: []string{"192.168.1.20"}},
+				{Name: "utun6", Up: true, IPv4: []string{"10.2.0.2"}},
+			},
+			routes: []Route{{Iface: "en0", Default: true}},
+			flags:  linuxFlags(),
+			want: []Pane{
+				{Name: "utun6", Label: "VPN"},
+				{Name: "en0", Label: "default route"},
+			},
+		},
+		{
+			name: "macos no vpn, single physical default route",
+			goos: "darwin",
+			ifaces: []Interface{
+				{Name: "en0", Up: true, IPv4: []string{"192.168.1.20"}},
+				{Name: "utun0", Up: true, IPv4: nil},
+				{Name: "utun1", Up: true, IPv4: []string{"169.254.1.1"}},
+			},
+			routes: []Route{{Iface: "en0", Default: true}},
+			flags:  linuxFlags(),
+			want:   []Pane{{Name: "en0", Label: "default route"}},
+		},
+		{
+			name: "macos vpn pane honors explicit labels",
+			goos: "darwin",
+			ifaces: []Interface{
+				{Name: "en0", Up: true, IPv4: []string{"192.168.1.20"}},
+				{Name: "utun4", Up: true, IPv4: []string{"10.64.0.2"}},
+			},
+			routes: []Route{
+				{Iface: "utun4", Default: true},
+				{Iface: "en0", Default: true},
+			},
+			flags: Flags{
+				LabelA: "Mullvad", LabelASet: true,
+				LabelB: "Fiber", LabelBSet: true,
+			},
+			want: []Pane{
+				{Name: "utun4", Label: "Mullvad"},
+				{Name: "en0", Label: "Fiber"},
+			},
+		},
+		{
+			name:    "macos no default route is an error",
+			goos:    "darwin",
+			ifaces:  []Interface{{Name: "en0", Up: true, IPv4: []string{"192.168.1.20"}}},
+			routes:  nil,
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name: "macos two physical default routes is an error",
+			goos: "darwin",
+			ifaces: []Interface{
+				{Name: "en0", Up: true, IPv4: []string{"192.168.1.20"}},
+				{Name: "en1", Up: true, IPv4: []string{"192.168.2.20"}},
+			},
+			routes: []Route{
+				{Iface: "en0", Default: true},
+				{Iface: "en1", Default: true},
+			},
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name:    "unsupported operating system is an error",
+			goos:    "windows",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := Select(tt.goos, tt.ifaces, tt.routes, tt.flags)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("Select() expected error, got panes %v", got)
+				}
+
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("Select() unexpected error: %v", err)
+			}
+
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("Select() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestParseIPRoute(t *testing.T) {
+	out := "default via 192.168.1.1 dev eth0 proto dhcp metric 100\n"
+	got := parseIPRoute(out)
+	want := []Route{{Iface: "eth0", Gateway: "192.168.1.1", Default: true}}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("parseIPRoute() = %v, want %v", got, want)
+	}
+}
+
+func TestParseProcNetRoute(t *testing.T) {
+	out := "Iface\tDestination\tGateway\tFlags\tRefCnt\tUse\tMetric\tMask\n" +
+		"eth0\t00000000\t0102A8C0\t0003\t0\t0\t100\t00000000\n" +
+		"eth0\t0002A8C0\t00000000\t0001\t0\t0\t0\t00FFFFFF\n"
+	got := parseProcNetRoute(out)
+	want := []Route{{Iface: "eth0", Gateway: "192.168.2.1", Default: true}}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("parseProcNetRoute() = %v, want %v", got, want)
+	}
+}
+
+func TestParseNetstat(t *testing.T) {
+	out := "Routing tables\n\nInternet:\n" +
+		"Destination        Gateway            Flags        Netif Expire\n" +
+		"default            10.0.0.1           UGScg          en0\n" +
+		"default            link#15            UCSg         utun4\n" +
+		"127.0.0.1          127.0.0.1          UH             lo0\n"
+	got := parseNetstat(out)
+	want := []Route{
+		{Iface: "en0", Gateway: "10.0.0.1", Default: true},
+		{Iface: "utun4", Gateway: "link#15", Default: true},
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("parseNetstat() = %v, want %v", got, want)
+	}
+}

internal/netdetect/routes_darwin.go

@@ -0,0 +1,16 @@
+//go:build darwin
+
+package netdetect
+
+import "os/exec"
+
+// DefaultRoutes returns the host's IPv4 default routes, read from the macOS
+// routing table.
+func DefaultRoutes() ([]Route, error) {
+	out, err := exec.Command("netstat", "-rn", "-f", "inet").Output()
+	if err != nil {
+		return nil, err
+	}
+
+	return parseNetstat(string(out)), nil
+}

internal/netdetect/routes_linux.go

@@ -0,0 +1,26 @@
+//go:build linux
+
+package netdetect
+
+import (
+	"os"
+	"os/exec"
+)
+
+// DefaultRoutes returns the host's IPv4 default routes. It prefers the `ip`
+// command and falls back to reading /proc/net/route.
+func DefaultRoutes() ([]Route, error) {
+	out, err := exec.Command("ip", "-4", "route", "show", "default").Output()
+	if err == nil {
+		if routes := parseIPRoute(string(out)); len(routes) > 0 {
+			return routes, nil
+		}
+	}
+
+	data, err := os.ReadFile("/proc/net/route")
+	if err != nil {
+		return nil, err
+	}
+
+	return parseProcNetRoute(string(data)), nil
+}