Add nat packet conn

common/bufio/fallback.go

@@ -0,0 +1,37 @@
+package bufio
+
+import (
+	"net"
+
+	"github.com/sagernet/sing/common/buf"
+	M "github.com/sagernet/sing/common/metadata"
+	N "github.com/sagernet/sing/common/network"
+)
+
+var _ N.NetPacketConn = (*FallbackPacketConn)(nil)
+
+type FallbackPacketConn struct {
+	N.PacketConn
+}
+
+func (c *FallbackPacketConn) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
+	buffer := buf.With(p)
+	destination, err := c.ReadPacket(buffer)
+	if err != nil {
+		return
+	}
+	n = buffer.Len()
+	if buffer.Start() > 0 {
+		copy(p, buffer.Bytes())
+	}
+	addr = destination.UDPAddr()
+	return
+}
+
+func (c *FallbackPacketConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
+	err = c.WritePacket(buf.As(p), M.SocksaddrFromNet(addr))
+	if err == nil {
+		n = len(p)
+	}
+	return
+}

common/bufio/nat.go

@@ -0,0 +1,62 @@
+package bufio
+
+import (
+	"net"
+	"net/netip"
+
+	"github.com/sagernet/sing/common/buf"
+	M "github.com/sagernet/sing/common/metadata"
+	N "github.com/sagernet/sing/common/network"
+)
+
+type NATPacketConn struct {
+	N.NetPacketConn
+	origin      M.Socksaddr
+	destination M.Socksaddr
+}
+
+func NewNATPacketConn(conn N.NetPacketConn, origin M.Socksaddr, destination M.Socksaddr) *NATPacketConn {
+	return &NATPacketConn{
+		NetPacketConn: conn,
+		origin:        origin,
+		destination:   destination,
+	}
+}
+
+func (c *NATPacketConn) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
+	n, addr, err = c.NetPacketConn.ReadFrom(p)
+	if err == nil && M.SocksaddrFromNet(addr) == c.origin {
+		addr = c.destination.UDPAddr()
+	}
+	return
+}
+
+func (c *NATPacketConn) 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 *NATPacketConn) ReadPacket(buffer *buf.Buffer) (destination M.Socksaddr, err error) {
+	destination, err = c.NetPacketConn.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.NetPacketConn.WritePacket(buffer, destination)
+}
+
+func (c *NATPacketConn) UpdateDestination(destinationAddress netip.Addr) {
+	c.destination = M.SocksaddrFrom(destinationAddress, c.destination.Port)
+}
+
+func (c *NATPacketConn) Upstream() any {
+	return c.NetPacketConn
+}