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

internal/cli/export_test.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package cli
 
 import "github.com/spf13/cobra"

internal/cli/root.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 // Package cli wires command-line flags to the network monitor and runs it.
 package cli
 
@@ -7,12 +5,14 @@ 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.
@@ -59,8 +59,8 @@ func newRootCmd() *cobra.Command {
 		Short: "Real-time network monitoring dashboard",
 		Long: `rtnetmon is a dual-interface network monitoring dashboard that provides
 real-time visibility into network health, packet loss, and latency.`,
-		RunE: func(_ *cobra.Command, _ []string) error {
-			return runMonitor(cfg)
+		RunE: func(cmd *cobra.Command, _ []string) error {
+			return runMonitor(cmd, cfg)
 		},
 	}
 	registerFlags(cmd, cfg)
@@ -86,14 +86,17 @@ func registerFlags(cmd *cobra.Command, cfg *config) {
 	}
 }
 
-// runMonitor constructs and runs the monitor from cfg.
-func runMonitor(cfg *config) error {
+// runMonitor detects the interfaces to monitor, then constructs and runs the
+// monitor from cfg.
+func runMonitor(cmd *cobra.Command, cfg *config) error {
 	monitor.Logf(cfg.LogFile, "Starting rtnetmon")
-	monitor.Logf(cfg.LogFile, "Monitoring interfaces %s and %s",
-		cfg.IfaceA, cfg.IfaceB)
 
-	mon := monitor.NewMonitor(cfg.IfaceA, cfg.LabelA, cfg.IfaceB, cfg.LabelB,
-		cfg.LogFile)
+	specs, err := detectInterfaces(cmd, cfg)
+	if err != nil {
+		return err
+	}
+
+	mon := monitor.NewMonitor(specs, cfg.LogFile)
 
 	for _, host := range cfg.Hosts {
 		mon.AddReachabilityHost(host)
@@ -122,6 +125,45 @@ func runMonitor(cfg *config) 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, cfg *config,
+) ([]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 builds the root command and runs it.
 func Execute() error {
 	return newRootCmd().Execute()

internal/cli/root_test.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package cli_test
 
 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/export_test.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package monitor
 
 // Test-only accessors exposing unexported state for white-box assertions.
@@ -13,8 +11,13 @@ func (m *Monitor) PacketLossHosts() []string { return m.packetLossHosts }
 // TCPHosts returns the configured TCP hosts.
 func (m *Monitor) TCPHosts() []string { return m.tcpHosts }
 
-// InterfaceA returns the first monitored interface.
-func (m *Monitor) InterfaceA() *InterfaceStatus { return m.interfaceA }
+// Interfaces returns the monitored interfaces.
+func (m *Monitor) Interfaces() []*InterfaceStatus { return m.interfaces }
 
-// InterfaceB returns the second monitored interface.
-func (m *Monitor) InterfaceB() *InterfaceStatus { return m.interfaceB }
+// PingArgs exposes pingArgs for external tests.
+func PingArgs(goos, iface, host string) []string { return pingArgs(goos, iface, host) }
+
+// LossArgs exposes lossArgs for external tests.
+func LossArgs(goos, iface, host string, count int) []string {
+	return lossArgs(goos, iface, host, count)
+}

internal/monitor/loops.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package monitor
 
 import (

internal/monitor/monitor.go

@@ -1,8 +1,6 @@
-//go:build linux
-
-// Package monitor implements the dual-interface real-time network
-// monitoring dashboard: ICMP reachability, packet loss, TCP latency, and
-// the terminal UI that renders them.
+// Package monitor implements the real-time network monitoring dashboard:
+// ICMP reachability, packet loss, TCP latency, and the terminal UI that
+// renders them. It monitors one or two interfaces.
 package monitor
 
 import (
@@ -14,6 +12,7 @@ import (
 	"net"
 	"os"
 	"os/exec"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -88,9 +87,8 @@ type Monitor struct {
 	packetLossHosts   []string
 	tcpHosts          []string
 
-	// Interfaces
-	interfaceA *InterfaceStatus
-	interfaceB *InterfaceStatus
+	// Interfaces to monitor (one or two)
+	interfaces []*InterfaceStatus
 
 	// Logging
 	logFile string
@@ -104,9 +102,16 @@ type Monitor struct {
 	mu sync.RWMutex
 }
 
-// NewMonitor creates a new Monitor instance with default settings.
-func NewMonitor(ifaceA, labelA, ifaceB, labelB, logFile string) *Monitor {
-	return &Monitor{
+// IfaceSpec names one interface to monitor and its display label.
+type IfaceSpec struct {
+	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{
 		ICMPTimeout:      defaultICMPTimeout,
 		TCPTimeout:       defaultTCPTimeout,
 		PacketLossPings:  defaultPacketLossPings,
@@ -127,13 +132,16 @@ func NewMonitor(ifaceA, labelA, ifaceB, labelB, logFile string) *Monitor {
 		packetLossHosts:   []string{},
 		tcpHosts:          []string{},
 
-		interfaceA: NewInterfaceStatus(ifaceA, labelA),
-		interfaceB: NewInterfaceStatus(ifaceB, labelB),
-
 		logFile:   logFile,
 		startTime: time.Now(),
 		uiUpdate:  make(chan struct{}, uiUpdateBuffer),
 	}
+
+	for _, spec := range ifaces {
+		m.interfaces = append(m.interfaces, NewInterfaceStatus(spec.Name, spec.Label))
+	}
+
+	return m
 }
 
 // AddReachabilityHost adds a host for reachability monitoring.
@@ -194,12 +202,11 @@ func (m *Monitor) Run(ctx context.Context) error {
 	tcpHosts := append([]string{}, m.tcpHosts...)
 	m.mu.RUnlock()
 
-	go m.reachLoop(ctx, m.interfaceA, reachHosts)
-	go m.reachLoop(ctx, m.interfaceB, reachHosts)
-	go m.lossLoop(ctx, m.interfaceA, lossHosts)
-	go m.lossLoop(ctx, m.interfaceB, lossHosts)
-	go m.tcpLoop(ctx, m.interfaceA, tcpHosts)
-	go m.tcpLoop(ctx, m.interfaceB, tcpHosts)
+	for _, st := range m.interfaces {
+		go m.reachLoop(ctx, st, reachHosts)
+		go m.lossLoop(ctx, st, lossHosts)
+		go m.tcpLoop(ctx, st, tcpHosts)
+	}
 
 	m.logf("Starting UI loop")
 	m.uiLoop(ctx)
@@ -306,13 +313,36 @@ 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 over the named interface.
 func (m *Monitor) pingOnce(ctx context.Context, iface, host string) bool {
 	ctx, cancel := context.WithTimeout(ctx, m.ICMPTimeout)
 	defer cancel()
 
+	args := pingArgs(runtime.GOOS, iface, host)
+
 	cmd := exec.CommandContext( //nolint:gosec // G204: fixed argv, operator CLI input
-		ctx, "ping", "-I", iface, "-c1", "-W1", host)
+		ctx, "ping", args...)
 
 	return cmd.Run() == nil
 }
@@ -322,9 +352,10 @@ func (m *Monitor) lossPercent(ctx context.Context, iface, host string) float64 {
 	ctx, cancel := context.WithTimeout(ctx, lossQueryTimeout)
 	defer cancel()
 
+	args := lossArgs(runtime.GOOS, iface, host, m.PacketLossPings)
+
 	cmd := exec.CommandContext( //nolint:gosec // G204: fixed argv, operator CLI input
-		ctx, "ping", "-q", "-i", "0.05",
-		"-c", strconv.Itoa(m.PacketLossPings), "-W1", "-I", iface, host)
+		ctx, "ping", args...)
 
 	out, err := cmd.CombinedOutput()
 	if err != nil {
@@ -375,7 +406,7 @@ func (m *Monitor) tcpDuration(iface, hp string) time.Duration {
 		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)

internal/monitor/monitor_test.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package monitor_test
 
 import (
@@ -8,26 +6,33 @@ import (
 	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
 )
 
+// Interface names and hosts reused across the monitor package tests.
+const (
+	ifaceTest0 = "test0"
+	ifaceTest1 = "test1"
+	ifaceEth0  = "eth0"
+	host8888   = "8.8.8.8"
+)
+
 func TestNewMonitor(t *testing.T) {
 	t.Parallel()
 
-	mon := monitor.NewMonitor("test0", "Test Interface A", "test1",
-		"Test Interface B", "/tmp/test.log")
+	mon := monitor.NewMonitor([]monitor.IfaceSpec{
+		{Name: ifaceTest0, Label: "Test Interface A"},
+		{Name: ifaceTest1, Label: "Test Interface B"},
+	}, "/tmp/test.log")
 
-	if mon.InterfaceA() == nil {
-		t.Fatal("interfaceA is nil")
+	ifaces := mon.Interfaces()
+	if len(ifaces) != 2 {
+		t.Fatalf("interfaces = %d, want 2", len(ifaces))
 	}
 
-	if mon.InterfaceB() == nil {
-		t.Fatal("interfaceB is nil")
+	if ifaces[0].Name != ifaceTest0 {
+		t.Errorf("interfaces[0].Name = %q, want %q", ifaces[0].Name, ifaceTest0)
 	}
 
-	if mon.InterfaceA().Name != "test0" {
-		t.Errorf("interfaceA.Name = %q, want %q", mon.InterfaceA().Name, "test0")
-	}
-
-	if mon.InterfaceB().Name != "test1" {
-		t.Errorf("interfaceB.Name = %q, want %q", mon.InterfaceB().Name, "test1")
+	if ifaces[1].Name != ifaceTest1 {
+		t.Errorf("interfaces[1].Name = %q, want %q", ifaces[1].Name, ifaceTest1)
 	}
 
 	if mon.ICMPTimeout.Milliseconds() != 500 {
@@ -51,12 +56,31 @@ func TestNewMonitor(t *testing.T) {
 	}
 }
 
+func TestNewMonitorSingle(t *testing.T) {
+	t.Parallel()
+
+	mon := monitor.NewMonitor(
+		[]monitor.IfaceSpec{{Name: ifaceEth0, Label: "default route"}}, "")
+
+	ifaces := mon.Interfaces()
+	if len(ifaces) != 1 {
+		t.Fatalf("interfaces = %d, want 1", len(ifaces))
+	}
+
+	if ifaces[0].Name != ifaceEth0 {
+		t.Errorf("interfaces[0].Name = %q, want %q", ifaces[0].Name, ifaceEth0)
+	}
+}
+
 func TestAddHosts(t *testing.T) {
 	t.Parallel()
 
-	mon := monitor.NewMonitor("test0", "Test A", "test1", "Test B", "")
+	mon := monitor.NewMonitor([]monitor.IfaceSpec{
+		{Name: ifaceTest0, Label: "Test A"},
+		{Name: ifaceTest1, Label: "Test B"},
+	}, "")
 
-	mon.AddReachabilityHost("8.8.8.8")
+	mon.AddReachabilityHost(host8888)
 	mon.AddReachabilityHost("google.com")
 
 	if len(mon.ReachabilityHosts()) != 2 {

internal/monitor/ping_test.go

@@ -0,0 +1,58 @@
+package monitor_test
+
+import (
+	"reflect"
+	"testing"
+
+	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
+)
+
+func TestPingArgs(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		goos string
+		want []string
+	}{
+		{"linux", []string{"-I", ifaceEth0, "-c1", "-W1", host8888}},
+		{"darwin", []string{"-b", ifaceEth0, "-c1", "-W1000", host8888}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.goos, func(t *testing.T) {
+			t.Parallel()
+
+			got := monitor.PingArgs(tt.goos, ifaceEth0, host8888)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("PingArgs(%q) = %v, want %v", tt.goos, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestLossArgs(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		goos string
+		want []string
+	}{
+		{"linux", []string{
+			"-q", "-i", "0.05", "-c", "20", "-W1", "-I", ifaceEth0, host8888,
+		}},
+		{"darwin", []string{
+			"-q", "-i", "0.05", "-c", "20", "-W1000", "-b", ifaceEth0, host8888,
+		}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.goos, func(t *testing.T) {
+			t.Parallel()
+
+			got := monitor.LossArgs(tt.goos, ifaceEth0, host8888, 20)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("LossArgs(%q) = %v, want %v", tt.goos, got, tt.want)
+			}
+		})
+	}
+}

internal/monitor/styles.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package monitor
 
 import tcell "github.com/gdamore/tcell/v2"

internal/monitor/styles_test.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package monitor_test
 
 import (

internal/monitor/ui.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package monitor
 
 import (
@@ -383,9 +381,11 @@ func (m *Monitor) uiLoop(ctx context.Context) {
 		runtime := time.Since(m.startTime).Round(time.Second).String()
 		Put(m.screen, colLeft, rowRuntime, "Runtime: "+runtime, tcell.StyleDefault)
 
+		// Draw one pane per interface; a single interface draws one pane.
 		y := rowFirstIface
-		y = m.DrawInterface(m.screen, y, w, m.interfaceA)
-		_ = m.DrawInterface(m.screen, y, w, m.interfaceB)
+		for _, st := range m.interfaces {
+			y = m.DrawInterface(m.screen, y, w, st)
+		}
 
 		m.screen.Show()
 	}

internal/netdetect/export_test.go

@@ -0,0 +1,12 @@
+package netdetect
+
+// Test-only accessors exposing the unexported route parsers.
+
+// ParseIPRoute exposes parseIPRoute for external tests.
+func ParseIPRoute(out string) []Route { return parseIPRoute(out) }
+
+// ParseProcNetRoute exposes parseProcNetRoute for external tests.
+func ParseProcNetRoute(out string) []Route { return parseProcNetRoute(out) }
+
+// ParseNetstat exposes parseNetstat for external tests.
+func ParseNetstat(out string) []Route { return parseNetstat(out) }

internal/netdetect/netdetect.go

@@ -0,0 +1,375 @@
+// 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 (
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"net"
+	"sort"
+	"strings"
+)
+
+// Selection failures. Those needing the offending interface names are
+// wrapped with %w at the call site.
+var (
+	errUnsupportedOS = errors.New("unsupported operating system")
+	errOneOfPair     = errors.New(
+		"found only one of the configured interfaces; " +
+			"rtnetmon needs both, or neither (default route only)")
+	errNoDefaultRoute = errors.New(
+		"no default route found; rtnetmon needs one internet interface")
+	errManyDefaultRoutes = errors.New(
+		"multiple default-route interfaces found; rtnetmon supports only one")
+	errNoPhysRoute = errors.New(
+		"no physical default-route interface found; " +
+			"rtnetmon needs one internet interface")
+	errManyPhysRoutes = errors.New(
+		"multiple physical default-route interfaces found; " +
+			"rtnetmon supports only one")
+)
+
+// 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("%w: %q", errUnsupportedOS, 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("%w (%q, %q)", errOneOfPair, 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 "", errNoDefaultRoute
+	default:
+		return "", fmt.Errorf("%w (%s)",
+			errManyDefaultRoutes, 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 "", errNoPhysRoute
+	default:
+		return "", fmt.Errorf("%w (%s)",
+			errManyPhysRoutes, 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 := range len(fields) - 1 {
+			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 {
+	b, err := hex.DecodeString(h)
+	if err != nil || len(b) != net.IPv4len {
+		return ""
+	}
+
+	// /proc/net/route stores the address little-endian.
+	return net.IPv4(b[3], b[2], b[1], b[0]).String()
+}

internal/netdetect/netdetect_test.go

@@ -0,0 +1,310 @@
+package netdetect_test
+
+import (
+	"reflect"
+	"testing"
+
+	"git.eeqj.de/sneak/rtnetmon/internal/netdetect"
+)
+
+// Values reused across the selection tests.
+const (
+	osLinux  = "linux"
+	osDarwin = "darwin"
+
+	ifaceGu0      = "gu0"
+	ifaceBackhaul = "backhaul0"
+	ifaceEth0     = "eth0"
+	ifaceWlan0    = "wlan0"
+	ifaceEn0      = "en0"
+	ifaceUtun4    = "utun4"
+
+	labelGu      = "gu LAN - VPN outbound"
+	labelCox     = "Cox cable direct"
+	labelDefault = "default route"
+
+	addrEn0 = "192.168.1.20"
+)
+
+// linuxFlags describes the Linux bridge interfaces rtnetmon was built for.
+func linuxFlags() netdetect.Flags {
+	return netdetect.Flags{
+		IfaceA: ifaceGu0, LabelA: labelGu,
+		IfaceB: ifaceBackhaul, LabelB: labelCox,
+	}
+}
+
+// selectCase is one Select scenario with fake interfaces and routes.
+type selectCase struct {
+	name    string
+	goos    string
+	ifaces  []netdetect.Interface
+	routes  []netdetect.Route
+	flags   netdetect.Flags
+	want    []netdetect.Pane
+	wantErr bool
+}
+
+// runSelectCases runs each case as a parallel subtest.
+func runSelectCases(t *testing.T, cases []selectCase) {
+	t.Helper()
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := netdetect.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 TestSelectLinuxPanes(t *testing.T) {
+	t.Parallel()
+
+	runSelectCases(t, []selectCase{
+		{
+			name: "both bridge interfaces present",
+			goos: osLinux,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceGu0, Up: true},
+				{Name: ifaceBackhaul, Up: true},
+				{Name: ifaceEth0, Up: true},
+			},
+			routes: []netdetect.Route{{Iface: ifaceEth0, Default: true}},
+			flags:  linuxFlags(),
+			want: []netdetect.Pane{
+				{Name: ifaceGu0, Label: labelGu},
+				{Name: ifaceBackhaul, Label: labelCox},
+			},
+		},
+		{
+			name: "no bridge interfaces, single default route",
+			goos: osLinux,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceEth0, Up: true},
+				{Name: "lo", Up: true},
+			},
+			routes: []netdetect.Route{{Iface: ifaceEth0, Default: true}},
+			flags:  linuxFlags(),
+			want:   []netdetect.Pane{{Name: ifaceEth0, Label: labelDefault}},
+		},
+		{
+			name:   "single default route keeps explicit label",
+			goos:   osLinux,
+			ifaces: []netdetect.Interface{{Name: ifaceWlan0, Up: true}},
+			routes: []netdetect.Route{{Iface: ifaceWlan0, Default: true}},
+			flags: netdetect.Flags{
+				IfaceA: ifaceGu0, LabelA: "Home WiFi", LabelASet: true,
+				IfaceB: ifaceBackhaul, LabelB: labelCox,
+			},
+			want: []netdetect.Pane{{Name: ifaceWlan0, Label: "Home WiFi"}},
+		},
+	})
+}
+
+func TestSelectLinuxErrors(t *testing.T) {
+	t.Parallel()
+
+	runSelectCases(t, []selectCase{
+		{
+			name: "only one bridge interface present",
+			goos: osLinux,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceGu0, Up: true},
+				{Name: ifaceEth0, Up: true},
+			},
+			routes:  []netdetect.Route{{Iface: ifaceEth0, Default: true}},
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name:    "no bridge, no default route",
+			goos:    osLinux,
+			ifaces:  []netdetect.Interface{{Name: ifaceEth0, Up: true}},
+			routes:  nil,
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name: "no bridge, multiple default routes",
+			goos: osLinux,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceEth0, Up: true},
+				{Name: "eth1", Up: true},
+			},
+			routes: []netdetect.Route{
+				{Iface: ifaceEth0, Default: true},
+				{Iface: "eth1", Default: true},
+			},
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+	})
+}
+
+func TestSelectDarwinPanes(t *testing.T) {
+	t.Parallel()
+
+	runSelectCases(t, []selectCase{
+		{
+			name: "vpn tunnel plus physical default route",
+			goos: osDarwin,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceEn0, Up: true, IPv4: []string{addrEn0}},
+				{Name: ifaceUtun4, Up: true, IPv4: []string{"10.64.0.2"}},
+				{Name: "utun0", Up: true, IPv4: nil},
+			},
+			routes: []netdetect.Route{
+				{Iface: ifaceUtun4, Default: true},
+				{Iface: ifaceEn0, Default: true},
+			},
+			flags: linuxFlags(),
+			want: []netdetect.Pane{
+				{Name: ifaceUtun4, Label: "VPN"},
+				{Name: ifaceEn0, Label: labelDefault},
+			},
+		},
+		{
+			name: "vpn detected by routable address without its own route",
+			goos: osDarwin,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceEn0, Up: true, IPv4: []string{addrEn0}},
+				{Name: "utun6", Up: true, IPv4: []string{"10.2.0.2"}},
+			},
+			routes: []netdetect.Route{{Iface: ifaceEn0, Default: true}},
+			flags:  linuxFlags(),
+			want: []netdetect.Pane{
+				{Name: "utun6", Label: "VPN"},
+				{Name: ifaceEn0, Label: labelDefault},
+			},
+		},
+		{
+			name: "vpn pane honors explicit labels",
+			goos: osDarwin,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceEn0, Up: true, IPv4: []string{addrEn0}},
+				{Name: ifaceUtun4, Up: true, IPv4: []string{"10.64.0.2"}},
+			},
+			routes: []netdetect.Route{
+				{Iface: ifaceUtun4, Default: true},
+				{Iface: ifaceEn0, Default: true},
+			},
+			flags: netdetect.Flags{
+				LabelA: "Mullvad", LabelASet: true,
+				LabelB: "Fiber", LabelBSet: true,
+			},
+			want: []netdetect.Pane{
+				{Name: ifaceUtun4, Label: "Mullvad"},
+				{Name: ifaceEn0, Label: "Fiber"},
+			},
+		},
+	})
+}
+
+func TestSelectDarwinSingleAndErrors(t *testing.T) {
+	t.Parallel()
+
+	runSelectCases(t, []selectCase{
+		{
+			name: "no vpn, single physical default route",
+			goos: osDarwin,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceEn0, Up: true, IPv4: []string{addrEn0}},
+				{Name: "utun0", Up: true, IPv4: nil},
+				{Name: "utun1", Up: true, IPv4: []string{"169.254.1.1"}},
+			},
+			routes: []netdetect.Route{{Iface: ifaceEn0, Default: true}},
+			flags:  linuxFlags(),
+			want:   []netdetect.Pane{{Name: ifaceEn0, Label: labelDefault}},
+		},
+		{
+			name: "no default route",
+			goos: osDarwin,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceEn0, Up: true, IPv4: []string{addrEn0}},
+			},
+			routes:  nil,
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name: "two physical default routes",
+			goos: osDarwin,
+			ifaces: []netdetect.Interface{
+				{Name: ifaceEn0, Up: true, IPv4: []string{addrEn0}},
+				{Name: "en1", Up: true, IPv4: []string{"192.168.2.20"}},
+			},
+			routes: []netdetect.Route{
+				{Iface: ifaceEn0, Default: true},
+				{Iface: "en1", Default: true},
+			},
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name:    "unsupported operating system",
+			goos:    "windows",
+			wantErr: true,
+		},
+	})
+}
+
+func TestParseIPRoute(t *testing.T) {
+	t.Parallel()
+
+	out := "default via 192.168.1.1 dev eth0 proto dhcp metric 100\n"
+	got := netdetect.ParseIPRoute(out)
+	want := []netdetect.Route{{Iface: ifaceEth0, 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) {
+	t.Parallel()
+
+	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 := netdetect.ParseProcNetRoute(out)
+	want := []netdetect.Route{{Iface: ifaceEth0, 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) {
+	t.Parallel()
+
+	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 := netdetect.ParseNetstat(out)
+	want := []netdetect.Route{
+		{Iface: ifaceEn0, Gateway: "10.0.0.1", Default: true},
+		{Iface: ifaceUtun4, 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 +1,27 @@
+//go:build darwin
+
+package netdetect
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+	"time"
+)
+
+// routeQueryTimeout bounds the external route-table query.
+const routeQueryTimeout = 2 * time.Second
+
+// DefaultRoutes returns the host's IPv4 default routes, read from the macOS
+// routing table.
+func DefaultRoutes() ([]Route, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), routeQueryTimeout)
+	defer cancel()
+
+	out, err := exec.CommandContext(ctx, "netstat", "-rn", "-f", "inet").Output()
+	if err != nil {
+		return nil, fmt.Errorf("running netstat: %w", err)
+	}
+
+	return parseNetstat(string(out)), nil
+}

internal/netdetect/routes_linux.go

@@ -0,0 +1,36 @@
+//go:build linux
+
+package netdetect
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"time"
+)
+
+// routeQueryTimeout bounds the external route-table query.
+const routeQueryTimeout = 2 * time.Second
+
+// 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) {
+	ctx, cancel := context.WithTimeout(context.Background(), routeQueryTimeout)
+	defer cancel()
+
+	out, err := exec.CommandContext(ctx,
+		"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, fmt.Errorf("reading /proc/net/route: %w", err)
+	}
+
+	return parseProcNetRoute(string(data)), nil
+}