зеркало из https://github.com/Azure/sonic-openssh.git
299 строки
8.9 KiB
Plaintext
299 строки
8.9 KiB
Plaintext
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 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 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 1.11 2018/09/26 07:30:05 djm Exp $
|