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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.
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:
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
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
To open a new multiplexed session, a client may send 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.
3. 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
4. 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.
5. 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
string listen port
string connect host
string connect port
forwarding type may be MUX_FWD_LOCAL, MUX_FWD_REMOTE, MUX_FWD_DYNAMIC.
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
6. 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
string listen host
string listen port
string connect host
string connect port
A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
MUX_S_FAILURE.
7. Requesting 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_OPEED, a MUX_S_PERMISSION_DENIED
or a MUX_S_FAILURE.
8. 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
9. 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_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_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 graceful close (delete listening socket, but keep existing sessions active)
XXX lock (maybe)
XXX watch in/out traffic (pre/post crypto)
XXX inject packet (what about replies)
XXX server->client error/warning notifications
XXX port0 rfwd (need custom response message)
XXX send signals via mux
$OpenBSD: PROTOCOL.mux,v 1.4 2011/01/31 21:42:15 djm Exp $
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