Add fakeip support

transport/fakeip/packet.go

@@ -0,0 +1,42 @@
+package fakeip
+
+import (
+	"github.com/sagernet/sing/common/buf"
+	M "github.com/sagernet/sing/common/metadata"
+	N "github.com/sagernet/sing/common/network"
+)
+
+var _ N.PacketConn = (*NATPacketConn)(nil)
+
+type NATPacketConn struct {
+	N.PacketConn
+	origin      M.Socksaddr
+	destination M.Socksaddr
+}
+
+func NewNATPacketConn(conn N.PacketConn, origin M.Socksaddr, destination M.Socksaddr) *NATPacketConn {
+	return &NATPacketConn{
+		PacketConn:  conn,
+		origin:      origin,
+		destination: destination,
+	}
+}
+
+func (c *NATPacketConn) ReadPacket(buffer *buf.Buffer) (destination M.Socksaddr, err error) {
+	destination, err = c.PacketConn.ReadPacket(buffer)
+	if destination == c.origin {
+		destination = c.destination
+	}
+	return
+}
+
+func (c *NATPacketConn) WritePacket(buffer *buf.Buffer, destination M.Socksaddr) error {
+	if destination == c.destination {
+		destination = c.origin
+	}
+	return c.PacketConn.WritePacket(buffer, destination)
+}
+
+func (c *NATPacketConn) Upstream() any {
+	return c.PacketConn
+}