Update

vendor/tailscale.com/net/netmon/defaultroute_darwin.go

@@ -6,6 +6,8 @@
 package netmon
 
 import (
+	"errors"
+	"fmt"
 	"log"
 	"net"
 
@@ -16,14 +18,26 @@ var (
 	lastKnownDefaultRouteIfName syncs.AtomicValue[string]
 )
 
-// UpdateLastKnownDefaultRouteInterface is called by ipn-go-bridge in the iOS app when
+// UpdateLastKnownDefaultRouteInterface is called by ipn-go-bridge from apple network extensions when
 // our NWPathMonitor instance detects a network path transition.
 func UpdateLastKnownDefaultRouteInterface(ifName string) {
 	if ifName == "" {
 		return
 	}
 	if old := lastKnownDefaultRouteIfName.Swap(ifName); old != ifName {
-		log.Printf("defaultroute_darwin: update from Swift, ifName = %s (was %s)", ifName, old)
+		interfaces, err := netInterfaces()
+		if err != nil {
+			log.Printf("defaultroute_darwin: UpdateLastKnownDefaultRouteInterface could not get interfaces: %v", err)
+			return
+		}
+
+		netif, err := getInterfaceByName(ifName, interfaces)
+		if err != nil {
+			log.Printf("defaultroute_darwin: UpdateLastKnownDefaultRouteInterface could not find interface index for %s: %v", ifName, err)
+			return
+		}
+
+		log.Printf("defaultroute_darwin: updated last known default if from OS, ifName = %s index: %d (was %s)", ifName, netif.Index, old)
 	}
 }
 
@@ -40,45 +54,12 @@ func defaultRoute() (d DefaultRouteDetails, err error) {
 	//
 	// If for any reason the Swift machinery didn't work and we don't get any updates, we will
 	// fallback to the BSD logic.
-
-	// Start by getting all available interfaces.
-	interfaces, err := netInterfaces()
-	if err != nil {
-		log.Printf("defaultroute_darwin: could not get interfaces: %v", err)
-		return d, ErrNoGatewayIndexFound
-	}
-
-	getInterfaceByName := func(name string) *Interface {
-		for _, ifc := range interfaces {
-			if ifc.Name != name {
-				continue
-			}
-
-			if !ifc.IsUp() {
-				log.Printf("defaultroute_darwin: %s is down", name)
-				return nil
-			}
-
-			addrs, _ := ifc.Addrs()
-			if len(addrs) == 0 {
-				log.Printf("defaultroute_darwin: %s has no addresses", name)
-				return nil
-			}
-			return &ifc
-		}
-		return nil
-	}
-
-	// Did Swift set lastKnownDefaultRouteInterface? If so, we should use it and don't bother
-	// with anything else. However, for sanity, do check whether Swift gave us with an interface
-	// that exists, is up, and has an address.
-	if swiftIfName := lastKnownDefaultRouteIfName.Load(); swiftIfName != "" {
-		ifc := getInterfaceByName(swiftIfName)
-		if ifc != nil {
-			d.InterfaceName = ifc.Name
-			d.InterfaceIndex = ifc.Index
-			return d, nil
-		}
+	osRoute, osRouteErr := OSDefaultRoute()
+	if osRouteErr == nil {
+		// If we got a valid interface from the OS, use it.
+		d.InterfaceName = osRoute.InterfaceName
+		d.InterfaceIndex = osRoute.InterfaceIndex
+		return d, nil
 	}
 
 	// Fallback to the BSD logic
@@ -94,3 +75,48 @@ func defaultRoute() (d DefaultRouteDetails, err error) {
 	d.InterfaceIndex = idx
 	return d, nil
 }
+
+// OSDefaultRoute returns the DefaultRouteDetails for the default interface as provided by the OS
+// via UpdateLastKnownDefaultRouteInterface.  If UpdateLastKnownDefaultRouteInterface has not been called,
+// the interface name is not valid, or we cannot find its index, an error is returned.
+func OSDefaultRoute() (d DefaultRouteDetails, err error) {
+
+	// Did Swift set lastKnownDefaultRouteInterface? If so, we should use it and don't bother
+	// with anything else. However, for sanity, do check whether Swift gave us with an interface
+	// that exists, is up, and has an address and is not the tunnel itself.
+	if swiftIfName := lastKnownDefaultRouteIfName.Load(); swiftIfName != "" {
+		// Start by getting all available interfaces.
+		interfaces, err := netInterfaces()
+		if err != nil {
+			log.Printf("defaultroute_darwin: could not get interfaces: %v", err)
+			return d, err
+		}
+
+		if ifc, err := getInterfaceByName(swiftIfName, interfaces); err == nil {
+			d.InterfaceName = ifc.Name
+			d.InterfaceIndex = ifc.Index
+			return d, nil
+		}
+	}
+	err = errors.New("no os provided default route interface found")
+	return d, err
+}
+
+func getInterfaceByName(name string, interfaces []Interface) (*Interface, error) {
+	for _, ifc := range interfaces {
+		if ifc.Name != name {
+			continue
+		}
+
+		if !ifc.IsUp() {
+			return nil, fmt.Errorf("defaultroute_darwin: %s is down", name)
+		}
+
+		addrs, _ := ifc.Addrs()
+		if len(addrs) == 0 {
+			return nil, fmt.Errorf("defaultroute_darwin: %s has no addresses", name)
+		}
+		return &ifc, nil
+	}
+	return nil, errors.New("no interfaces found")
+}