macOS: ignore on-link default routes when picking the physical uplink

A VM with NAT networking makes vmnet install a default route scoped to
its bridge (bridge100) with no gateway. The detector counted it as a
second physical default route and refused to start:

    multiple physical default-route interfaces found (bridge100, en0)

Scoped routes cannot simply be dropped: while a VPN holds the default
route, the physical interface keeps only a default route scoped to
itself. That route still names its gateway (flag G). The on-link one
does not, so a physical default route now requires a next-hop gateway.

parseNetstat records a gateway only when the G flag is set; a link#N
gateway column is the interface itself, not a next hop.

README.md

@@ -55,7 +55,12 @@ primary pane alongside the physical interface. A tunnel that is up without the
 default route is ignored, whatever its address: Tailscale without an exit node,
 or a tunnel a disconnected client left behind. A default route scoped to the
 tunnel alone (flag `I`) does not count either. With no VPN connected, only the
-physical interface is monitored. Interface names are detected on macOS; the
+physical interface is monitored. The physical interface is the one non-tunnel
+`default` row with a next-hop gateway (flag `G`); it keeps that gateway even
+while a VPN holds the default route and its own default is scoped. An on-link
+`default` row with no gateway, such as the one vmnet or Internet Sharing
+installs on `bridge100` for a VM's subnet, is ignored. Interface names are
+detected on macOS; the
 `--ifaceA`/`--ifaceB` flags are not used there, but `--labelA`/`--labelB` still
 set the pane labels.
 

internal/netdetect/netdetect.go

@@ -41,9 +41,11 @@ type Interface struct {
 	IPv4 []string
 }
 
-// Route is one routing-table entry reduced to what detection needs. Scoped
-// marks a macOS interface-scoped route (flag I), which only traffic bound to
-// that interface uses.
+// Route is one routing-table entry reduced to what detection needs. Gateway
+// is the next-hop address, empty for an on-link route that has none: a
+// tunnel, or the pseudo-default that vmnet and Internet Sharing install on a
+// bridge so the hosted subnet works. Scoped marks a macOS interface-scoped
+// route (flag I), which only traffic bound to that interface uses.
 type Route struct {
 	Iface   string
 	Gateway string
@@ -188,7 +190,9 @@ func findVPN(ifaces []Interface, routes []Route) string {
 // 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, "")
+	names := defaultRouteIfaces(routes, func(r Route) bool {
+		return !isTunnel(r.Iface)
+	})
 
 	switch len(names) {
 	case 1:
@@ -202,9 +206,16 @@ func onlyDefaultRoute(routes []Route) (string, error) {
 }
 
 // onlyPhysicalDefaultRoute returns the single non-tunnel default-route
-// interface (ignoring the VPN), or an error if there is not exactly one.
+// interface (ignoring the VPN), or an error if there is not exactly one. Only
+// a default route with a next-hop gateway counts: scoped routes do, since
+// while a VPN holds the default route the physical interface keeps only a
+// default route scoped to itself, but that route still names the gateway. An
+// on-link default without one (vmnet's on bridge100) reaches nothing beyond
+// its own subnet and is not an uplink.
 func onlyPhysicalDefaultRoute(routes []Route, vpn string) (string, error) {
-	names := defaultRouteIfaces(routes, vpn)
+	names := defaultRouteIfaces(routes, func(r Route) bool {
+		return r.Iface != vpn && !isTunnel(r.Iface) && r.Gateway != ""
+	})
 
 	switch len(names) {
 	case 1:
@@ -218,16 +229,14 @@ func onlyPhysicalDefaultRoute(routes []Route, vpn string) (string, error) {
 }
 
 // defaultRouteIfaces returns the sorted, unique interface names that carry a
-// default route, excluding the named VPN interface and any other tunnel.
-// Scoped routes count: while a VPN holds the default route, the physical
-// interface keeps only a default route scoped to itself.
-func defaultRouteIfaces(routes []Route, vpn string) []string {
+// default route accepted by keep.
+func defaultRouteIfaces(routes []Route, keep func(Route) bool) []string {
 	seen := map[string]bool{}
 
 	var names []string
 
 	for _, r := range routes {
-		if !r.Default || r.Iface == vpn || isTunnel(r.Iface) || seen[r.Iface] {
+		if !r.Default || !keep(r) || seen[r.Iface] {
 			continue
 		}
 
@@ -327,11 +336,20 @@ func parseProcNetRoute(out string) []Route {
 	return routes
 }
 
+// netstat route flags (see `man netstat`).
+const (
+	netstatFlagGateway = "G" // destination is reached through a next-hop gateway
+	netstatFlagScoped  = "I" // route is scoped to its interface
+)
+
 // parseNetstat reads `netstat -rn -f inet` output (macOS); the Netif column
 // (field 4) names the interface. A row whose destination is "default" is a
-// default route, scoped when its flags include I. A "0/1" row and a "128.0/1"
-// row on one interface together also make a default route there: VPN clients
-// that leave the physical default in place send all traffic through them.
+// default route, scoped when its flags include I. Its Gateway column is the
+// next hop only when the flags include G; otherwise it names the link itself
+// (link#N) and the route is on-link, with no gateway. A "0/1" row and a
+// "128.0/1" row on one interface together also make a default route there:
+// VPN clients that leave the physical default in place send all traffic
+// through them.
 func parseNetstat(out string) []Route {
 	const columns = 4 // Destination, Gateway, Flags, Netif; Expire is optional
 
@@ -351,11 +369,15 @@ func parseNetstat(out string) []Route {
 
 		switch dest {
 		case "default":
+			if !strings.Contains(flags, netstatFlagGateway) {
+				gateway = ""
+			}
+
 			routes = append(routes, Route{
 				Iface:   iface,
 				Gateway: gateway,
 				Default: true,
-				Scoped:  strings.Contains(flags, "I"),
+				Scoped:  strings.Contains(flags, netstatFlagScoped),
 			})
 		case "0/1":
 			lowHalf = append(lowHalf, iface)

internal/netdetect/netdetect_test.go

@@ -17,6 +17,7 @@ const (
 	ifaceEth0     = "eth0"
 	ifaceWlan0    = "wlan0"
 	ifaceEn0      = "en0"
+	ifaceBridge   = "bridge100"
 	ifaceUtun0    = "utun0"
 	ifaceUtun3    = "utun3"
 	ifaceUtun4    = "utun4"
@@ -27,6 +28,8 @@ const (
 	labelVPN     = "VPN"
 
 	addrEn0       = "192.168.1.20"
+	gatewayEn0    = "192.168.1.1"
+	addrBridge    = "192.168.64.1"
 	addrVPN       = "10.64.0.2"
 	addrTailscale = "100.101.102.103"
 )
@@ -106,6 +109,32 @@ default            192.168.1.1        UGScg          en0
 100.101.102.103/32 link#22            UCS          utun3
 127.0.0.1          127.0.0.1          UH             lo0
 128.0/1            utun4              USc          utun4
+`
+
+	// A VM with NAT networking: vmnet installs an on-link default route,
+	// scoped to its bridge and with no gateway, next to the real one.
+	netstatVMBridge = netstatHeader + `
+default            192.168.1.1        UGScg          en0
+default            link#45            UCSIg    bridge100      !
+127.0.0.1          127.0.0.1          UH             lo0
+192.168.64         link#45            UC       bridge100      !
+`
+
+	// The same VM bridge while a VPN holds the default route.
+	netstatVMBridgeAndVPN = netstatHeader + `
+default            link#15            UCSg         utun4
+default            192.168.1.1        UGScIg         en0
+default            link#45            UCSIg    bridge100      !
+10.64.0.2          10.64.0.2          UH           utun4
+127.0.0.1          127.0.0.1          UH             lo0
+192.168.64         link#45            UC       bridge100      !
+`
+
+	// Only the VM bridge's on-link default route, no uplink at all.
+	netstatVMBridgeOnly = netstatHeader + `
+default            link#45            UCSIg    bridge100      !
+127.0.0.1          127.0.0.1          UH             lo0
+192.168.64         link#45            UC       bridge100      !
 `
 )
 
@@ -265,7 +294,7 @@ func TestSelectDarwinPanes(t *testing.T) {
 			},
 			routes: []netdetect.Route{
 				{Iface: ifaceUtun4, Default: true},
-				{Iface: ifaceEn0, Default: true},
+				{Iface: ifaceEn0, Gateway: gatewayEn0, Default: true, Scoped: true},
 			},
 			flags: linuxFlags(),
 			want: []netdetect.Pane{
@@ -282,7 +311,7 @@ func TestSelectDarwinPanes(t *testing.T) {
 			},
 			routes: []netdetect.Route{
 				{Iface: ifaceUtun4, Default: true},
-				{Iface: ifaceEn0, Default: true},
+				{Iface: ifaceEn0, Gateway: gatewayEn0, Default: true, Scoped: true},
 			},
 			flags: netdetect.Flags{
 				LabelA: "Mullvad", LabelASet: true,
@@ -308,7 +337,7 @@ func TestSelectDarwinSingleAndErrors(t *testing.T) {
 				{Name: ifaceUtun0, Up: true, IPv4: nil},
 				{Name: "utun1", Up: true, IPv4: []string{"169.254.1.1"}},
 			},
-			routes: []netdetect.Route{{Iface: ifaceEn0, Default: true}},
+			routes: []netdetect.Route{{Iface: ifaceEn0, Gateway: gatewayEn0, Default: true}},
 			flags:  linuxFlags(),
 			want:   []netdetect.Pane{{Name: ifaceEn0, Label: labelDefault}},
 		},
@@ -319,7 +348,7 @@ func TestSelectDarwinSingleAndErrors(t *testing.T) {
 				{Name: ifaceEn0, Up: true, IPv4: []string{addrEn0}},
 				{Name: "utun6", Up: true, IPv4: []string{"10.2.0.2"}},
 			},
-			routes: []netdetect.Route{{Iface: ifaceEn0, Default: true}},
+			routes: []netdetect.Route{{Iface: ifaceEn0, Gateway: gatewayEn0, Default: true}},
 			flags:  linuxFlags(),
 			want:   []netdetect.Pane{{Name: ifaceEn0, Label: labelDefault}},
 		},
@@ -341,8 +370,8 @@ func TestSelectDarwinSingleAndErrors(t *testing.T) {
 				{Name: "en1", Up: true, IPv4: []string{"192.168.2.20"}},
 			},
 			routes: []netdetect.Route{
-				{Iface: ifaceEn0, Default: true},
-				{Iface: "en1", Default: true},
+				{Iface: ifaceEn0, Gateway: gatewayEn0, Default: true},
+				{Iface: "en1", Gateway: "192.168.2.1", Default: true},
 			},
 			flags:   linuxFlags(),
 			wantErr: true,
@@ -430,6 +459,63 @@ func TestSelectDarwinFromNetstat(t *testing.T) {
 	})
 }
 
+// TestSelectDarwinVMBridge covers a VM's NAT bridge: vmnet installs a default
+// route scoped to the bridge with no gateway, which is not an uplink.
+func TestSelectDarwinVMBridge(t *testing.T) {
+	t.Parallel()
+
+	vpn := netdetect.Interface{Name: ifaceUtun4, Up: true, IPv4: []string{addrVPN}}
+	bridge := netdetect.Interface{Name: ifaceBridge, Up: true, IPv4: []string{addrBridge}}
+	physicalOnly := []netdetect.Pane{{Name: ifaceEn0, Label: labelDefault}}
+	vpnAndPhysical := []netdetect.Pane{
+		{Name: ifaceUtun4, Label: labelVPN},
+		{Name: ifaceEn0, Label: labelDefault},
+	}
+
+	runSelectCases(t, []selectCase{
+		{
+			name:   "on-link default route is not an uplink",
+			goos:   osDarwin,
+			ifaces: macIfaces(bridge),
+			routes: []netdetect.Route{
+				{Iface: ifaceEn0, Gateway: gatewayEn0, Default: true},
+				{Iface: ifaceBridge, Default: true, Scoped: true},
+			},
+			flags: linuxFlags(),
+			want:  physicalOnly,
+		},
+		{
+			name:    "only an on-link default route",
+			goos:    osDarwin,
+			ifaces:  macIfaces(bridge),
+			routes:  []netdetect.Route{{Iface: ifaceBridge, Default: true, Scoped: true}},
+			flags:   linuxFlags(),
+			wantErr: true,
+		},
+		{
+			name:   "netstat: vm bridge next to the physical default",
+			goos:   osDarwin,
+			ifaces: macIfaces(bridge),
+			routes: netdetect.ParseNetstat(netstatVMBridge),
+			want:   physicalOnly,
+		},
+		{
+			name:   "netstat: vm bridge next to a connected vpn",
+			goos:   osDarwin,
+			ifaces: macIfaces(bridge, vpn),
+			routes: netdetect.ParseNetstat(netstatVMBridgeAndVPN),
+			want:   vpnAndPhysical,
+		},
+		{
+			name:    "netstat: vm bridge with no uplink",
+			goos:    osDarwin,
+			ifaces:  macIfaces(bridge),
+			routes:  netdetect.ParseNetstat(netstatVMBridgeOnly),
+			wantErr: true,
+		},
+	})
+}
+
 func TestParseIPRoute(t *testing.T) {
 	t.Parallel()
 
@@ -463,11 +549,13 @@ func TestParseNetstat(t *testing.T) {
 		"Destination        Gateway            Flags        Netif Expire\n" +
 		"default            10.0.0.1           UGScg          en0\n" +
 		"default            link#15            UCSg         utun4\n" +
+		"default            link#45            UCSIg    bridge100      !\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},
+		{Iface: ifaceUtun4, Default: true},
+		{Iface: ifaceBridge, Default: true, Scoped: true},
 	}
 
 	if !reflect.DeepEqual(got, want) {