This document describes the multiplexing protocol used by ssh(1)'s
ControlMaster connection-sharing.

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 itself 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 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 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 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
	string	reserved
	bool	want tty flag
	bool	want X11 forwarding flag
	bool	want agent flag
	bool	subsystem flag
	uint32	escape char
	string	terminal type
	string	command
	string	environment string 0 [optional]
	...

To disable the use of an escape character, "escape char" may be set
to 0xffffffff. "terminal type" is generally set to the value of
$TERM. zero or more environment strings may follow the command.

The client then sends its standard input, output and error file
descriptors (in that order) using Unix domain socket control messages.

The contents of "reserved" are currently ignored.

If successful, the server will reply with MUX_S_SESSION_OPENED

	uint32	MUX_S_SESSION_OPENED
	uint32	client request id
	uint32	session id

Otherwise it will reply with an error: MUX_S_PERMISSION_DENIED or
MUX_S_FAILURE.

Once the server has received the fds, it will respond with MUX_S_OK
indicating that the session is up. The client now waits for the
session to end. When it does, the server will send an exit status
message:

	uint32	MUX_S_EXIT_MESSAGE
	uint32	session id
	uint32	exit value

The client should exit with this value to mimic the behaviour of a
non-multiplexed ssh(1) connection. Two additional cases that the
client must cope with are it receiving a signal itself and the
server disconnecting without sending an exit message.

A master may also send a MUX_S_TTY_ALLOC_FAIL before MUX_S_EXIT_MESSAGE
if remote TTY allocation was unsuccessful. The client may use this to
return its local tty to "cooked" mode.

	uint32	MUX_S_TTY_ALLOC_FAIL
	uint32	session id

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:

	uint32	MUX_C_ALIVE_CHECK
	uint32	request id

The server replies with:

	uint32	MUX_S_ALIVE
	uint32	client request id
	uint32	server pid

5. Remotely terminating a master

A client may request that a master terminate immediately:

	uint32	MUX_C_TERMINATE
	uint32	request id

The server will reply with one of MUX_S_OK or MUX_S_PERMISSION_DENIED.

6. Requesting establishment of port forwards

A client may request the master to establish a port forward:

	uint32	MUX_C_OPEN_FWD
	uint32	request id
	uint32	forwarding type
	string	listen host
	uint32	listen port
	string	connect host
	uint32	connect port

forwarding type may be MUX_FWD_LOCAL, MUX_FWD_REMOTE, MUX_FWD_DYNAMIC.

If listen port is (unsigned int) -2, then the listen host is treated as
a unix socket path name.

If connect port is (unsigned int) -2, then the connect host is treated
as a unix socket path name.

A server may reply with a MUX_S_OK, a MUX_S_REMOTE_PORT, a
MUX_S_PERMISSION_DENIED or a MUX_S_FAILURE.

For dynamically allocated listen port the server replies with

	uint32	MUX_S_REMOTE_PORT
	uint32	client request id
	uint32	allocated remote listen port

7. Requesting closure of port forwards

Note: currently unimplemented (server will always reply with MUX_S_FAILURE).

A client may request the master to close a port forward:

	uint32	MUX_C_CLOSE_FWD
	uint32	request id
	uint32	forwarding type
	string	listen host
	uint32	listen port
	string	connect host
	uint32	connect port

A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
MUX_S_FAILURE.

8. Requesting shutdown of mux listener

A client may request the master to stop accepting new multiplexing requests
and remove its listener socket.

	uint32	MUX_C_STOP_LISTENING
	uint32	request id

A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
MUX_S_FAILURE.

9. Requesting proxy mode

A client may request that the the control connection be placed in proxy
mode:

	uint32	MUX_C_PROXY
	uint32	request id

When a mux master receives this message, it will reply with a
confirmation:

	uint32	MUX_S_PROXY
	uint32	request id

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] ...

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:

	uint32	MUX_S_OK
	uint32	client request id

The MUX_S_PERMISSION_DENIED and MUX_S_FAILURE include a reason:

	uint32	MUX_S_PERMISSION_DENIED
	uint32	client request id
	string	reason

	uint32	MUX_S_FAILURE
	uint32	client request id
	string	reason

11. Protocol numbers

#define MUX_MSG_HELLO		0x00000001
#define MUX_C_NEW_SESSION	0x10000002
#define MUX_C_ALIVE_CHECK	0x10000004
#define MUX_C_TERMINATE		0x10000005
#define MUX_C_OPEN_FWD		0x10000006
#define MUX_C_CLOSE_FWD		0x10000007
#define MUX_C_NEW_STDIO_FWD	0x10000008
#define MUX_C_STOP_LISTENING	0x10000009
#define MUX_S_OK		0x80000001
#define MUX_S_PERMISSION_DENIED	0x80000002
#define MUX_S_FAILURE		0x80000003
#define MUX_S_EXIT_MESSAGE	0x80000004
#define MUX_S_ALIVE		0x80000005
#define MUX_S_SESSION_OPENED	0x80000006
#define MUX_S_REMOTE_PORT	0x80000007
#define MUX_S_TTY_ALLOC_FAIL	0x80000008

#define MUX_FWD_LOCAL	1
#define MUX_FWD_REMOTE	2
#define MUX_FWD_DYNAMIC	3

XXX TODO
XXX extended status (e.g. report open channels / forwards)
XXX lock (maybe)
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
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 1.12 2020/03/13 03:17:07 djm Exp $