net/netmon: use Monitor's tsIfName if set by SetTailscaleInterfaceName

Currently nobody calls SetTailscaleInterfaceName yet, so this is a
no-op. I checked oss, android, and the macOS/iOS client. Nobody calls
this, or ever did.

But I want to in the future.

Updates #15408
Updates #9040

Change-Id: I05dfabe505174f9067b929e91c6e0d8bc42628d7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -4968,7 +4968,7 @@ func (b *LocalBackend) authReconfig() {
 		return
 	}
 
-	oneCGNATRoute := shouldUseOneCGNATRoute(b.logf, b.sys.ControlKnobs(), version.OS())
+	oneCGNATRoute := shouldUseOneCGNATRoute(b.logf, b.sys.NetMon.Get(), b.sys.ControlKnobs(), version.OS())
 	rcfg := b.routerConfig(cfg, prefs, oneCGNATRoute)
 
 	err = b.e.Reconfig(cfg, rcfg, dcfg)
@@ -4992,7 +4992,7 @@ func (b *LocalBackend) authReconfig() {
 //
 // The versionOS is a Tailscale-style version ("iOS", "macOS") and not
 // a runtime.GOOS.
-func shouldUseOneCGNATRoute(logf logger.Logf, controlKnobs *controlknobs.Knobs, versionOS string) bool {
+func shouldUseOneCGNATRoute(logf logger.Logf, mon *netmon.Monitor, controlKnobs *controlknobs.Knobs, versionOS string) bool {
 	if controlKnobs != nil {
 		// Explicit enabling or disabling always take precedence.
 		if v, ok := controlKnobs.OneCGNAT.Load().Get(); ok {
@@ -5007,7 +5007,7 @@ func shouldUseOneCGNATRoute(logf logger.Logf, controlKnobs *controlknobs.Knobs,
 	// use fine-grained routes if another interfaces is also using the CGNAT
 	// IP range.
 	if versionOS == "macOS" {
-		hasCGNATInterface, err := netmon.HasCGNATInterface()
+		hasCGNATInterface, err := mon.HasCGNATInterface()
 		if err != nil {
 			logf("shouldUseOneCGNATRoute: Could not determine if any interfaces use CGNAT: %v", err)
 			return false

ipn/ipnlocal/peerapi.go

@@ -481,7 +481,7 @@ func (h *peerAPIHandler) handleServeInterfaces(w http.ResponseWriter, r *http.Re
 		fmt.Fprintf(w, "<h3>Could not get the default route: %s</h3>\n", html.EscapeString(err.Error()))
 	}
 
-	if hasCGNATInterface, err := netmon.HasCGNATInterface(); hasCGNATInterface {
+	if hasCGNATInterface, err := h.ps.b.sys.NetMon.Get().HasCGNATInterface(); hasCGNATInterface {
 		fmt.Fprintln(w, "<p>There is another interface using the CGNAT range.</p>")
 	} else if err != nil {
 		fmt.Fprintf(w, "<p>Could not check for CGNAT interfaces: %s</p>\n", html.EscapeString(err.Error()))

net/netmon/interfaces_test.go

@@ -13,7 +13,7 @@ import (
 )
 
 func TestGetState(t *testing.T) {
-	st, err := getState()
+	st, err := getState("")
 	if err != nil {
 		t.Fatal(err)
 	}

net/netmon/netmon.go

@@ -161,7 +161,7 @@ func (m *Monitor) InterfaceState() *State {
 }
 
 func (m *Monitor) interfaceStateUncached() (*State, error) {
-	return getState()
+	return getState(m.tsIfName)
 }
 
 // SetTailscaleInterfaceName sets the name of the Tailscale interface. For

net/netmon/state.go

@@ -461,21 +461,22 @@ func isTailscaleInterface(name string, ips []netip.Prefix) bool {
 // getPAC, if non-nil, returns the current PAC file URL.
 var getPAC func() string
 
-// GetState returns the state of all the current machine's network interfaces.
+// getState returns the state of all the current machine's network interfaces.
 //
 // It does not set the returned State.IsExpensive. The caller can populate that.
 //
-// Deprecated: use netmon.Monitor.InterfaceState instead.
+// optTSInterfaceName is the name of the Tailscale interface, if known.
-func getState() (*State, error) {
+func getState(optTSInterfaceName string) (*State, error) {
 	s := &State{
 		InterfaceIPs: make(map[string][]netip.Prefix),
 		Interface:    make(map[string]Interface),
 	}
 	if err := ForeachInterface(func(ni Interface, pfxs []netip.Prefix) {
+		isTSInterfaceName := optTSInterfaceName != "" && ni.Name == optTSInterfaceName
 		ifUp := ni.IsUp()
 		s.Interface[ni.Name] = ni
 		s.InterfaceIPs[ni.Name] = append(s.InterfaceIPs[ni.Name], pfxs...)
-		if !ifUp || isTailscaleInterface(ni.Name, pfxs) {
+		if !ifUp || isTSInterfaceName || isTailscaleInterface(ni.Name, pfxs) {
 			return
 		}
 		for _, pfx := range pfxs {
@@ -755,11 +756,12 @@ func DefaultRoute() (DefaultRouteDetails, error) {
 
 // HasCGNATInterface reports whether there are any non-Tailscale interfaces that
 // use a CGNAT IP range.
-func HasCGNATInterface() (bool, error) {
+func (m *Monitor) HasCGNATInterface() (bool, error) {
 	hasCGNATInterface := false
 	cgnatRange := tsaddr.CGNATRange()
 	err := ForeachInterface(func(i Interface, pfxs []netip.Prefix) {
-		if hasCGNATInterface || !i.IsUp() || isTailscaleInterface(i.Name, pfxs) {
+		isTSInterfaceName := m.tsIfName != "" && i.Name == m.tsIfName
+		if hasCGNATInterface || !i.IsUp() || isTSInterfaceName || isTailscaleInterface(i.Name, pfxs) {
 			return
 		}
 		for _, pfx := range pfxs {