Properly supports [email protected] (#18)

* Properly supports `[email protected]`

* Check for `[email protected]` protocol version

crypto/ssh/handshake.go

@@ -699,17 +699,17 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
 	if !isClient && firstKeyExchange && contains(clientInit.KexAlgos, "ext-info-c") {
 		supportedPubKeyAuthAlgosList := strings.Join(t.publicKeyAuthAlgorithms, ",")
 		extInfo := &extInfoMsg{
-			NumExtensions: 1,
+			NumExtensions: 2,
 			Payload:       make([]byte, 0, 4+15+4+len(supportedPubKeyAuthAlgosList)+4+16+4+1),
 		}
 		extInfo.Payload = appendInt(extInfo.Payload, len("server-sig-algs"))
 		extInfo.Payload = append(extInfo.Payload, "server-sig-algs"...)
 		extInfo.Payload = appendInt(extInfo.Payload, len(supportedPubKeyAuthAlgosList))
 		extInfo.Payload = append(extInfo.Payload, supportedPubKeyAuthAlgosList...)
-		// extInfo.Payload = appendInt(extInfo.Payload, len("[email protected]"))
-		// extInfo.Payload = append(extInfo.Payload, "[email protected]"...)
-		// extInfo.Payload = appendInt(extInfo.Payload, 1)
-		// extInfo.Payload = append(extInfo.Payload, "0"...)
+		extInfo.Payload = appendInt(extInfo.Payload, len("[email protected]"))
+		extInfo.Payload = append(extInfo.Payload, "[email protected]"...)
+		extInfo.Payload = appendInt(extInfo.Payload, 1)
+		extInfo.Payload = append(extInfo.Payload, "0"...)
 		if err := t.conn.writePacket(Marshal(extInfo)); err != nil {
 			return err
 		}

crypto/ssh/pipe.go

@@ -442,12 +442,23 @@ func (s *PipeSession) Close() {
 	}
 }
 
-func pipe(dst, src packetConn) error {
+func pipe(dst, src packetConn, handlePing bool) error {
 	for {
 		msg, err := src.readPacket()
 		if err != nil {
 			return err
 		}
+		if handlePing && msg[0] == msgPing {
+			var ping pingMsg
+			if err := Unmarshal(msg, &ping); err != nil {
+				return fmt.Errorf("failed to unmarshal [email protected] message: %w", err)
+			}
+			err = src.writePacket(Marshal(pongMsg(ping)))
+			if err != nil {
+				return err
+			}
+			continue
+		}
 		err = dst.writePacket(msg)
 		if err != nil {
 			return err
@@ -459,11 +470,14 @@ func (s *PipeSession) RunPipe() error {
 	c := make(chan error)
 	go func() {
 		defer s.Downstream.transport.Close()
-		c <- pipe(s.Downstream.transport, s.Upstream.transport)
+		c <- pipe(s.Downstream.transport, s.Upstream.transport, false)
 	}()
 	go func() {
 		defer s.Upstream.transport.Close()
-		c <- pipe(s.Upstream.transport, s.Downstream.transport)
+		// If the upstream doesn't support [email protected], short-circuit with a pong response
+		upstream_ping_version := s.Upstream.extensions["[email protected]"]
+		upstream_supports_ping := len(upstream_ping_version) == 1 && upstream_ping_version[0] == byte('0')
+		c <- pipe(s.Upstream.transport, s.Downstream.transport, !upstream_supports_ping)
 	}()
 	return <-c
 }