Add trojan over xtls support (#334)

* Add trojan over xtls support

* Add comments

* improve PR quality

* improve PR quality

* add xtls-rprx-origin-udp443 and xtls-rprx-direct-udp443

Co-authored-by: Eken Chan <maskedeken@yahoo.com>

proxy/trojan/protocol.go

@@ -21,9 +21,17 @@ var (
 
 const (
 	maxLength = 8192
+	// XRO is constant for XTLS origin mode
+	XRO = "xtls-rprx-origin"
+	// XRD is constant for XTLS direct mode
+	XRD = "xtls-rprx-direct"
 
 	commandTCP byte = 1
 	commandUDP byte = 3
+
+	// for xtls
+	commandXRO byte = 0xf1 // XTLS origin mode
+	commandXRD byte = 0xf2 // XTLS direct mode
 )
 
 // ConnWriter is TCP Connection Writer Wrapper for trojan protocol
@@ -31,6 +39,7 @@ type ConnWriter struct {
 	io.Writer
 	Target     net.Destination
 	Account    *MemoryAccount
+	Flow       string
 	headerSent bool
 }
 
@@ -67,6 +76,10 @@ func (c *ConnWriter) writeHeader() error {
 	command := commandTCP
 	if c.Target.Network == net.Network_UDP {
 		command = commandUDP
+	} else if c.Flow == XRO {
+		command = commandXRO
+	} else if c.Flow == XRD {
+		command = commandXRD
 	}
 
 	if _, err := buffer.Write(c.Account.Key); err != nil {
@@ -160,6 +173,7 @@ func (w *PacketWriter) writePacket(payload []byte, dest net.Destination) (int, e
 type ConnReader struct {
 	io.Reader
 	Target       net.Destination
+	Flow         string
 	headerParsed bool
 }
 
@@ -183,6 +197,10 @@ func (c *ConnReader) ParseHeader() error {
 	network := net.Network_TCP
 	if command[0] == commandUDP {
 		network = net.Network_UDP
+	} else if command[0] == commandXRO {
+		c.Flow = XRO
+	} else if command[0] == commandXRD {
+		c.Flow = XRD
 	}
 
 	addr, port, err := addrParser.ReadAddressPort(nil, c.Reader)