ssh: add server side support for ping@openssh.com protocol extension

Fixes golang/go#62390

Change-Id: Ie4dc577fb55b45a0c26a9e2dc5903af2bd382e00
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/524775
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

ssh/doc.go

@@ -13,6 +13,7 @@ others.
 
 References:
 
+	[PROTOCOL]: https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL?rev=HEAD
 	[PROTOCOL.certkeys]: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?rev=HEAD
 	[SSH-PARAMETERS]:    http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xml#ssh-parameters-1
 

ssh/handshake.go

@@ -647,16 +647,20 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
 
 	// On the server side, after the first SSH_MSG_NEWKEYS, send a SSH_MSG_EXT_INFO
 	// message with the server-sig-algs extension if the client supports it. See
-	// RFC 8308, Sections 2.4 and 3.1.
+	// RFC 8308, Sections 2.4 and 3.1, and [PROTOCOL], Section 1.9.
 	if !isClient && firstKeyExchange && contains(clientInit.KexAlgos, "ext-info-c") {
 		extInfo := &extInfoMsg{
-			NumExtensions: 1,
-			Payload:       make([]byte, 0, 4+15+4+len(supportedPubKeyAuthAlgosList)),
+			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("ping@openssh.com"))
+		extInfo.Payload = append(extInfo.Payload, "ping@openssh.com"...)
+		extInfo.Payload = appendInt(extInfo.Payload, 1)
+		extInfo.Payload = append(extInfo.Payload, "0"...)
 		if err := t.conn.writePacket(Marshal(extInfo)); err != nil {
 			return err
 		}

ssh/messages.go

@@ -349,6 +349,20 @@ type userAuthGSSAPIError struct {
 	LanguageTag string
 }
 
+// Transport layer OpenSSH extension. See [PROTOCOL], section 1.9
+const msgPing = 192
+
+type pingMsg struct {
+	Data string `sshtype:"192"`
+}
+
+// Transport layer OpenSSH extension. See [PROTOCOL], section 1.9
+const msgPong = 193
+
+type pongMsg struct {
+	Data string `sshtype:"193"`
+}
+
 // typeTags returns the possible type bytes for the given reflect.Type, which
 // should be a struct. The possible values are separated by a '|' character.
 func typeTags(structType reflect.Type) (tags []byte) {

ssh/mux.go

@@ -231,6 +231,12 @@ func (m *mux) onePacket() error {
 		return m.handleChannelOpen(packet)
 	case msgGlobalRequest, msgRequestSuccess, msgRequestFailure:
 		return m.handleGlobalPacket(packet)
+	case msgPing:
+		var msg pingMsg
+		if err := Unmarshal(packet, &msg); err != nil {
+			return fmt.Errorf("failed to unmarshal ping@openssh.com message: %w", err)
+		}
+		return m.sendMessage(pongMsg(msg))
 	}
 
 	// assume a channel packet.