Fix socks5 udp associate address

protocol/socks/handshake.go

@@ -9,6 +9,8 @@ import (
 
 	"github.com/sagernet/sing/common"
 	"github.com/sagernet/sing/common/auth"
+	"github.com/sagernet/sing/common/buf"
+	"github.com/sagernet/sing/common/bufio"
 	E "github.com/sagernet/sing/common/exceptions"
 	M "github.com/sagernet/sing/common/metadata"
 	N "github.com/sagernet/sing/common/network"
@@ -76,6 +78,9 @@ func ClientHandshake5(conn io.ReadWriter, command byte, destination M.Socksaddr,
 	} else if authResponse.Method != socks5.AuthTypeNotRequired {
 		return socks5.Response{}, E.New("socks5: unsupported auth method: ", authResponse.Method)
 	}
+	if command == socks5.CommandUDPAssociate {
+		destination = M.SocksaddrFrom(netip.IPv4Unspecified(), 0)
+	}
 	err = socks5.WriteRequest(conn, socks5.Request{
 		Command:     command,
 		Destination: destination,
@@ -215,13 +220,23 @@ func HandleConnection0(ctx context.Context, conn net.Conn, version byte, authent
 			if err != nil {
 				return err
 			}
-			metadata.Protocol = "socks5"
-			metadata.Destination = request.Destination
 			var innerError error
 			done := make(chan struct{})
 			associatePacketConn := NewAssociatePacketConn(udpConn, request.Destination, conn)
+			var cachedConn N.PacketConn
+			buffer := buf.NewPacket()
+			defer buffer.Release()
+			destination, err := associatePacketConn.ReadPacket(buffer)
+			request.Destination = destination
+			if !buffer.IsEmpty() {
+				cachedConn = bufio.NewCachedPacketConn(associatePacketConn, buffer, destination)
+			} else {
+				return err
+			}
+			metadata.Protocol = "socks5"
+			metadata.Destination = request.Destination
 			go func() {
-				innerError = handler.NewPacketConnection(ctx, associatePacketConn, metadata)
+				innerError = handler.NewPacketConnection(ctx, cachedConn, metadata)
 				close(done)
 			}()
 			err = common.Error(io.Copy(io.Discard, conn))