Add L3 routing support

transport/wireguard/device_nat_gvisor.go

@@ -0,0 +1,27 @@
+//go:build with_gvisor
+
+package wireguard
+
+import (
+	"github.com/sagernet/sing/common"
+	"github.com/sagernet/sing/common/buf"
+
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+func (d *natDestinationWrapper) WritePacketBuffer(buffer *stack.PacketBuffer) error {
+	defer buffer.DecRef()
+	if d.device.writer != nil {
+		d.device.writer.RewritePacketBuffer(buffer)
+	}
+	var packetLen int
+	for _, slice := range buffer.AsSlices() {
+		packetLen += len(slice)
+	}
+	packet := buf.NewSize(packetLen)
+	for _, slice := range buffer.AsSlices() {
+		common.Must1(packet.Write(slice))
+	}
+	d.device.outbound <- packet
+	return nil
+}