fix: tun lookback when don't have an activated network

listener/sing_tun/server.go

@@ -173,6 +173,7 @@ func New(options LC.Tun, tunnel C.Tunnel, additions ...inbound.Addition) (l *Lis
 		closed:  false,
 		options: options,
 		handler: handler,
+		tunName: tunName,
 	}
 	defer func() {
 		if err != nil {
@@ -279,25 +280,23 @@ func New(options LC.Tun, tunnel C.Tunnel, additions ...inbound.Addition) (l *Lis
 
 func (l *Listener) FlushDefaultInterface() {
 	if l.options.AutoDetectInterface {
-		targetInterface := dialer.DefaultInterface.Load()
 		for _, destination := range []netip.Addr{netip.IPv4Unspecified(), netip.IPv6Unspecified(), netip.MustParseAddr("1.1.1.1")} {
 			autoDetectInterfaceName := l.defaultInterfaceMonitor.DefaultInterfaceName(destination)
 			if autoDetectInterfaceName == l.tunName {
 				log.Warnln("[TUN] Auto detect interface by %s get same name with tun", destination.String())
 			} else if autoDetectInterfaceName == "" || autoDetectInterfaceName == "<nil>" {
 				log.Warnln("[TUN] Auto detect interface by %s get empty name.", destination.String())
 			} else {
-				targetInterface = autoDetectInterfaceName
-				if old := dialer.DefaultInterface.Load(); old != targetInterface {
-					log.Warnln("[TUN] default interface changed by monitor, %s => %s", old, targetInterface)
-
-					dialer.DefaultInterface.Store(targetInterface)
-
+				if old := dialer.DefaultInterface.Swap(autoDetectInterfaceName); old != autoDetectInterfaceName {
+					log.Warnln("[TUN] default interface changed by monitor, %s => %s", old, autoDetectInterfaceName)
 					iface.FlushCache()
 				}
 				return
 			}
 		}
+		if dialer.DefaultInterface.CompareAndSwap("", "<invalid>") {
+			log.Warnln("[TUN] Auto detect interface failed, set '<invalid>' to DefaultInterface to avoid lookback")
+		}
 	}
 }