android: bind to active network

net/netns/netns_android.go

@@ -10,6 +10,7 @@
 	"sync"
 	"syscall"
 
+	"tailscale.com/envknob"
 	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
 )
@@ -17,8 +18,14 @@
 var (
 	androidProtectFuncMu sync.Mutex
 	androidProtectFunc   func(fd int) error
+
+	androidBindToNetworkFuncMu sync.Mutex
+	androidBindToNetworkFunc   func(fd int) error
 )
 
+// If enabled, we skip the bind-to-network hook and let the routing table decide.
+var bindToInterfaceByRouteEnv = envknob.RegisterBool("TS_BIND_TO_INTERFACE_BY_ROUTE")
+
 // UseSocketMark reports whether SO_MARK is in use. Android does not use SO_MARK.
 func UseSocketMark() bool {
 	return false
@@ -50,6 +57,16 @@ func SetAndroidProtectFunc(f func(fd int) error) {
 	androidProtectFunc = f
 }
 
+// SetAndroidBindToNetworkFunc registers a func that Android provides that binds
+// the socket FD to the currently selected underlying etwork
+//
+// A nil func disables the hook.
+func SetAndroidBindToNetworkFunc(f func(fd int) error) {
+	androidBindToNetworkFuncMu.Lock()
+	defer androidBindToNetworkFuncMu.Unlock()
+	androidBindToNetworkFunc = f
+}
+
 func control(logger.Logf, *netmon.Monitor) func(network, address string, c syscall.RawConn) error {
 	return controlC
 }
@@ -60,14 +77,42 @@ func control(logger.Logf, *netmon.Monitor) func(network, address string, c sysca
 // and net.ListenConfig.Control.
 func controlC(network, address string, c syscall.RawConn) error {
 	var sockErr error
+
+	// If route-based binding is enabled, we preserve the historical behavior:
+	// protect from VPN loops, but do NOT force-bind to a particular Network.
+	// This lets the OS routing table make per-destination decisions.
+	useRoute := bindToInterfaceByRoute.Load() || bindToInterfaceByRouteEnv()
+
 	err := c.Control(func(fd uintptr) {
+		fdInt := int(fd)
+
+		// Protect from VPN loops
 		androidProtectFuncMu.Lock()
-		f := androidProtectFunc
+		pf := androidProtectFunc
 		androidProtectFuncMu.Unlock()
-		if f != nil {
-			sockErr = f(int(fd))
+		if pf != nil {
+			if err := pf(fdInt); err != nil && sockErr == nil {
+				sockErr = err
+				return
+			}
+		}
+
+		// Maybe bbind to currently active network
+		if useRoute {
+			return
+		}
+
+		androidBindToNetworkFuncMu.Lock()
+		bf := androidBindToNetworkFunc
+		androidBindToNetworkFuncMu.Unlock()
+		if bf != nil {
+			if err := bf(fdInt); err != nil && sockErr == nil {
+				sockErr = err
+				return
+			}
 		}
 	})
+
 	if err != nil {
 		return fmt.Errorf("RawConn.Control on %T: %w", c, err)
 	}