Handle TUN loopback in direct outbound

protocol/direct/outbound.go

@@ -20,6 +20,7 @@ import (
 	"github.com/sagernet/sing/common/logger"
 	M "github.com/sagernet/sing/common/metadata"
 	N "github.com/sagernet/sing/common/network"
+	"github.com/sagernet/sing/service"
 )
 
 func RegisterOutbound(registry *outbound.Registry) {
@@ -37,10 +38,12 @@ type Outbound struct {
 	outbound.Adapter
 	ctx            context.Context
 	logger         logger.ContextLogger
+	network        adapter.NetworkManager
 	dialer         dialer.ParallelInterfaceDialer
 	domainStrategy C.DomainStrategy
 	fallbackDelay  time.Duration
 	isEmpty        bool
+	myAddresses    common.TypedValue[[]netip.Prefix]
 }
 
 func NewOutbound(ctx context.Context, router adapter.Router, logger log.ContextLogger, tag string, options option.DirectOutboundOptions) (adapter.Outbound, error) {
@@ -61,6 +64,7 @@ func NewOutbound(ctx context.Context, router adapter.Router, logger log.ContextL
 		Adapter: outbound.NewAdapterWithDialerOptions(C.TypeDirect, tag, []string{N.NetworkTCP, N.NetworkUDP, N.NetworkICMP}, options.DialerOptions),
 		ctx:     ctx,
 		logger:  logger,
+		network: service.FromContext[adapter.NetworkManager](ctx),
 		//nolint:staticcheck
 		domainStrategy: C.DomainStrategy(options.DomainStrategy),
 		fallbackDelay:  time.Duration(options.FallbackDelay),
@@ -74,7 +78,48 @@ func NewOutbound(ctx context.Context, router adapter.Router, logger log.ContextL
 	return outbound, nil
 }
 
+func (h *Outbound) Start(stage adapter.StartStage) error {
+	switch stage {
+	case adapter.StartStatePostStart, adapter.StartStateStarted:
+		h.fetchMyAddresses()
+	}
+	return nil
+}
+
+func (h *Outbound) fetchMyAddresses() {
+	if len(h.myAddresses.Load()) > 0 {
+		return
+	}
+	myInterfaceNames := h.network.InterfaceMonitor().MyInterfaces()
+	if len(myInterfaceNames) == 0 {
+		return
+	}
+	var myAddresses []netip.Prefix
+	for _, myInterfaceName := range myInterfaceNames {
+		myInterface, err := h.network.InterfaceFinder().ByName(myInterfaceName)
+		if err != nil {
+			continue
+		}
+		myAddresses = append(myAddresses, myInterface.Addresses...)
+	}
+	h.myAddresses.Store(myAddresses)
+}
+
+func (h *Outbound) isMyLoopbackAddress(addresses ...netip.Addr) bool {
+	for _, prefix := range h.myAddresses.Load() {
+		for _, address := range addresses {
+			if prefix.Addr() != address && prefix.Contains(address) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 func (h *Outbound) DialContext(ctx context.Context, network string, destination M.Socksaddr) (net.Conn, error) {
+	if h.isMyLoopbackAddress(destination.Addr) {
+		return nil, E.New("loopback connection to TUN range")
+	}
 	ctx, metadata := adapter.ExtendContext(ctx)
 	metadata.Outbound = h.Tag()
 	metadata.Destination = destination
@@ -89,6 +134,9 @@ func (h *Outbound) DialContext(ctx context.Context, network string, destination
 }
 
 func (h *Outbound) ListenPacket(ctx context.Context, destination M.Socksaddr) (net.PacketConn, error) {
+	if h.isMyLoopbackAddress(destination.Addr) {
+		return nil, E.New("loopback connection to TUN range")
+	}
 	ctx, metadata := adapter.ExtendContext(ctx)
 	metadata.Outbound = h.Tag()
 	metadata.Destination = destination
@@ -111,6 +159,9 @@ func (h *Outbound) NewDirectRouteConnection(metadata adapter.InboundContext, rou
 }
 
 func (h *Outbound) DialParallel(ctx context.Context, network string, destination M.Socksaddr, destinationAddresses []netip.Addr) (net.Conn, error) {
+	if h.isMyLoopbackAddress(destinationAddresses...) {
+		return nil, E.New("loopback connection to TUN range")
+	}
 	ctx, metadata := adapter.ExtendContext(ctx)
 	metadata.Outbound = h.Tag()
 	metadata.Destination = destination
@@ -125,6 +176,9 @@ func (h *Outbound) DialParallel(ctx context.Context, network string, destination
 }
 
 func (h *Outbound) DialParallelNetwork(ctx context.Context, network string, destination M.Socksaddr, destinationAddresses []netip.Addr, networkStrategy *C.NetworkStrategy, networkType []C.InterfaceType, fallbackNetworkType []C.InterfaceType, fallbackDelay time.Duration) (net.Conn, error) {
+	if h.isMyLoopbackAddress(destinationAddresses...) {
+		return nil, E.New("loopback connection to TUN range")
+	}
 	ctx, metadata := adapter.ExtendContext(ctx)
 	metadata.Outbound = h.Tag()
 	metadata.Destination = destination
@@ -139,6 +193,9 @@ func (h *Outbound) DialParallelNetwork(ctx context.Context, network string, dest
 }
 
 func (h *Outbound) ListenSerialNetworkPacket(ctx context.Context, destination M.Socksaddr, destinationAddresses []netip.Addr, networkStrategy *C.NetworkStrategy, networkType []C.InterfaceType, fallbackNetworkType []C.InterfaceType, fallbackDelay time.Duration) (net.PacketConn, netip.Addr, error) {
+	if h.isMyLoopbackAddress(destinationAddresses...) {
+		return nil, netip.Addr{}, E.New("loopback connection to TUN range")
+	}
 	ctx, metadata := adapter.ExtendContext(ctx)
 	metadata.Outbound = h.Tag()
 	metadata.Destination = destination