upstream: Document mux proxy mode; added by Markus in openssh-7.4

Also add a little bit of information about the overall packet format

OpenBSD-Commit-ID: bdb6f6ea8580ef96792e270cae7857786ad84a95

PROTOCOL.mux

@@ -1,32 +1,69 @@
 This document describes the multiplexing protocol used by ssh(1)'s
 ControlMaster connection-sharing.
 
-Most messages from the client to the server contain a "request id" field.
-This field is returned in replies as "client request id" to facilitate
-matching of responses to requests.
+Multiplexing starts with a ssh(1) configured to act as a multiplexing
+master. This will cause ssh(1) to listen on a Unix domain socket for
+requests from clients. Clients communicate over this socket using a
+simple packetised protocol, where each message is proceeded with
+a length and message type in SSH uint32 wire format:
+
+    uint32  packet length
+    uint32  packet type
+    ...     packet body
+
+Most messages from the client to the server contain a "request id"
+field. This field is returned in replies as "client request id" to
+facilitate matching of responses to requests.
+
+Many muliplexing (mux) client requests yield immediate responses from
+the mux process; requesting a forwarding, performing an alive check or
+requesting the master terminate itself fall in to this category.
+
+The most common use of multiplexing however is to maintain multiple
+concurrent sessions. These are supported via two separate modes:
+
+"Passenger" clients start by requesting a new session with a
+MUX_C_NEW_SESSION message and passing stdio file descriptors over the
+Unix domain control socket. The passenger client then waits until it is
+signaled or the mux server closes the session. This mode is so named as
+the client waits around while the mux server does all the driving.
+
+Stdio forwarding (requested using MUX_C_NEW_STDIO_FWD) is another
+example of passenger mode; the client passes the stdio file descriptors
+and passively waits for something to happen.
+
+"Proxy" clients, requested using MUX_C_PROXY, work quite differently. In
+this mode, the mux client/server connection socket will stop speaking
+the multiplexing protocol and start proxying SSH connection protocol
+messages between the client and server. The client therefore must
+speak a significant subset of the SSH protocol, but in return is able
+to access basically the full suite of connection protocol features.
+Moreover, as no file descriptor passing is required, the connection
+supporting a proxy client may iteself be forwarded or relayed to another
+host if necessary.
 
 1. Connection setup
 
 When a multiplexing connection is made to a ssh(1) operating as a
-ControlMaster from a ssh(1) in multiplex slave mode, the first
-action of each is to exchange hello messages:
+ControlMaster from a client ssh(1), the first action of each is send
+a hello messages to its peer:
 
 	uint32	MUX_MSG_HELLO
 	uint32  protocol version
 	string  extension name [optional]
 	string  extension value [optional]
 	...
 
-The current version of the mux protocol is 4. A slave should refuse
+The current version of the mux protocol is 4. A client should refuse
 to connect to a master that speaks an unsupported protocol version.
-Following the version identifier are zero or more extensions
-represented as a name/value pair. No extensions are currently
-defined.
 
-2. Opening sessions
+Following the version identifier are zero or more extensions represented
+as a name/value pair. No extensions are currently defined.
 
-To open a new multiplexed session, a client may send the following
-request:
+2. Opening a passenger mode session
+
+To open a new multiplexed session in passenger mode, a client sends the
+following request:
 
 	uint32	MUX_C_NEW_SESSION
 	uint32  request id
@@ -80,7 +117,25 @@ return its local tty to "cooked" mode.
 	uint32	MUX_S_TTY_ALLOC_FAIL
 	uint32	session id
 
-3. Health checks
+3. Requesting passenger-mode stdio forwarding
+
+A client may request the master to establish a stdio forwarding:
+
+	uint32	MUX_C_NEW_STDIO_FWD
+	uint32	request id
+	string	reserved
+	string	connect host
+	string	connect port
+
+The client then sends its standard input and output file descriptors
+(in that order) using Unix domain socket control messages.
+
+The contents of "reserved" are currently ignored.
+
+A server may reply with a MUX_S_SESSION_OPENED, a MUX_S_PERMISSION_DENIED
+or a MUX_S_FAILURE.
+
+4. Health checks
 
 The client may request a health check/PID report from a server:
 
@@ -93,7 +148,7 @@ The server replies with:
 	uint32	client request id
 	uint32	server pid
 
-4. Remotely terminating a master
+5. Remotely terminating a master
 
 A client may request that a master terminate immediately:
 
@@ -102,7 +157,7 @@ A client may request that a master terminate immediately:
 
 The server will reply with one of MUX_S_OK or MUX_S_PERMISSION_DENIED.
 
-5. Requesting establishment of port forwards
+6. Requesting establishment of port forwards
 
 A client may request the master to establish a port forward:
 
@@ -131,7 +186,7 @@ For dynamically allocated listen port the server replies with
 	uint32	client request id
 	uint32	allocated remote listen port
 
-6. Requesting closure of port forwards
+7. Requesting closure of port forwards
 
 Note: currently unimplemented (server will always reply with MUX_S_FAILURE).
 
@@ -148,36 +203,45 @@ A client may request the master to close a port forward:
 A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
 MUX_S_FAILURE.
 
-7. Requesting stdio forwarding
+8. Requesting shutdown of mux listener
 
-A client may request the master to establish a stdio forwarding:
+A client may request the master to stop accepting new multiplexing requests
+and remove its listener socket.
 
-	uint32	MUX_C_NEW_STDIO_FWD
+	uint32	MUX_C_STOP_LISTENING
 	uint32	request id
-	string	reserved
-	string	connect host
-	string	connect port
 
-The client then sends its standard input and output file descriptors
-(in that order) using Unix domain socket control messages.
+A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
+MUX_S_FAILURE.
 
-The contents of "reserved" are currently ignored.
+9. Requesting proxy mode
 
-A server may reply with a MUX_S_SESSION_OPENED, a MUX_S_PERMISSION_DENIED
-or a MUX_S_FAILURE.
+A client may request that the the control connection be placed in proxy
+mode:
 
-8. Requesting shutdown of mux listener
+	uint32	MUX_C_PROXY
+	uint32	request id
 
-A client may request the master to stop accepting new multiplexing requests
-and remove its listener socket.
+When a mux master receives this message, it will reply with a
+confirmation:
 
-	uint32	MUX_C_STOP_LISTENING
+	uint32	MUX_S_PROXY
 	uint32	request id
 
-A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
-MUX_S_FAILURE.
+And go into proxy mode. All subsequent data over the connection will
+be formatted as unencrypted, unpadded, SSH transport messages:
+
+	uint32	packet length
+	byte	0 (padding length)
+	byte	packet type
+	byte[packet length - 2] ...
 
-9. Status messages
+The mux master will accept most connection messages and global requests,
+and will translate channel identifiers to ensure that the proxy client has
+globally unique channel numbers (i.e. a proxy client need not worry about
+collisions with other clients).
+
+10. Status messages
 
 The MUX_S_OK message is empty:
 
@@ -194,7 +258,7 @@ The MUX_S_PERMISSION_DENIED and MUX_S_FAILURE include a reason:
 	uint32	client request id
 	string	reason
 
-10. Protocol numbers
+11. Protocol numbers
 
 #define MUX_MSG_HELLO		0x00000001
 #define MUX_C_NEW_SESSION	0x10000002
@@ -224,5 +288,11 @@ XXX watch in/out traffic (pre/post crypto)
 XXX inject packet (what about replies)
 XXX server->client error/warning notifications
 XXX send signals via mux
-
-$OpenBSD: PROTOCOL.mux,v 1.10 2015/07/17 03:04:27 djm Exp $
+XXX ^Z support in passengers
+XXX extensions for multi-agent
+XXX extensions for multi-X11
+XXX session inspection via master
+XXX signals via mux request
+XXX list active connections via mux
+
+$OpenBSD: PROTOCOL.mux,v 1.11 2018/09/26 07:30:05 djm Exp $