bufio: Add destination NAT packet conn

common/bufio/nat.go

@@ -30,6 +30,14 @@ func NewNATPacketConn(conn N.NetPacketConn, origin M.Socksaddr, destination M.So
 	}
 }
 
+func NewDestinationNATPacketConn(conn N.NetPacketConn, origin M.Socksaddr, destination M.Socksaddr) NATPacketConn {
+	return &destinationNATPacketConn{
+		NetPacketConn: conn,
+		origin:        origin,
+		destination:   destination,
+	}
+}
+
 type unidirectionalNATPacketConn struct {
 	N.NetPacketConn
 	origin      M.Socksaddr
@@ -144,6 +152,60 @@ func (c *bidirectionalNATPacketConn) RemoteAddr() net.Addr {
 	return c.destination.UDPAddr()
 }
 
+type destinationNATPacketConn struct {
+	N.NetPacketConn
+	origin      M.Socksaddr
+	destination M.Socksaddr
+}
+
+func (c *destinationNATPacketConn) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
+	n, addr, err = c.NetPacketConn.ReadFrom(p)
+	if err != nil {
+		return
+	}
+	if M.SocksaddrFromNet(addr) == c.origin {
+		addr = c.destination.UDPAddr()
+	}
+	return
+}
+
+func (c *destinationNATPacketConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
+	if M.SocksaddrFromNet(addr) == c.destination {
+		addr = c.origin.UDPAddr()
+	}
+	return c.NetPacketConn.WriteTo(p, addr)
+}
+
+func (c *destinationNATPacketConn) ReadPacket(buffer *buf.Buffer) (destination M.Socksaddr, err error) {
+	destination, err = c.NetPacketConn.ReadPacket(buffer)
+	if err != nil {
+		return
+	}
+	if destination == c.origin {
+		destination = c.destination
+	}
+	return
+}
+
+func (c *destinationNATPacketConn) WritePacket(buffer *buf.Buffer, destination M.Socksaddr) error {
+	if destination == c.destination {
+		destination = c.origin
+	}
+	return c.NetPacketConn.WritePacket(buffer, destination)
+}
+
+func (c *destinationNATPacketConn) UpdateDestination(destinationAddress netip.Addr) {
+	c.destination = M.SocksaddrFrom(destinationAddress, c.destination.Port)
+}
+
+func (c *destinationNATPacketConn) Upstream() any {
+	return c.NetPacketConn
+}
+
+func (c *destinationNATPacketConn) RemoteAddr() net.Addr {
+	return c.destination.UDPAddr()
+}
+
 func socksaddrWithoutPort(destination M.Socksaddr) M.Socksaddr {
 	destination.Port = 0
 	return destination