PROTOCOL.mux

   1 This document describes the multiplexing protocol used by ssh(1)'s
   2 ControlMaster connection-sharing.
   3
   4 Multiplexing starts with a ssh(1) configured to act as a multiplexing
   5 master. This will cause ssh(1) to listen on a Unix domain socket for
   6 requests from clients. Clients communicate over this socket using a
   7 simple packetised protocol, where each message is proceeded with
   8 a length and message type in SSH uint32 wire format:
   9
  10     uint32  packet length
  11     uint32  packet type
  12     ...     packet body
  13
  14 Most messages from the client to the server contain a "request id"
  15 field. This field is returned in replies as "client request id" to
  16 facilitate matching of responses to requests.
  17
  18 Many multiplexing (mux) client requests yield immediate responses from
  19 the mux process; requesting a forwarding, performing an alive check or
  20 requesting the master terminate itself fall in to this category.
  21
  22 The most common use of multiplexing however is to maintain multiple
  23 concurrent sessions. These are supported via two separate modes:
  24
  25 "Passenger" clients start by requesting a new session with a
  26 MUX_C_NEW_SESSION message and passing stdio file descriptors over the
  27 Unix domain control socket. The passenger client then waits until it is
  28 signaled or the mux server closes the session. This mode is so named as
  29 the client waits around while the mux server does all the driving.
  30
  31 Stdio forwarding (requested using MUX_C_NEW_STDIO_FWD) is another
  32 example of passenger mode; the client passes the stdio file descriptors
  33 and passively waits for something to happen.
  34
  35 "Proxy" clients, requested using MUX_C_PROXY, work quite differently. In
  36 this mode, the mux client/server connection socket will stop speaking
  37 the multiplexing protocol and start proxying SSH connection protocol
  38 messages between the client and server. The client therefore must
  39 speak a significant subset of the SSH protocol, but in return is able
  40 to access basically the full suite of connection protocol features.
  41 Moreover, as no file descriptor passing is required, the connection
  42 supporting a proxy client may itself be forwarded or relayed to another
  43 host if necessary.
  44
  45 1. Connection setup
  46
  47 When a multiplexing connection is made to a ssh(1) operating as a
  48 ControlMaster from a client ssh(1), the first action of each is send
  49 a hello messages to its peer:
  50
  51         uint32  MUX_MSG_HELLO
  52         uint32  protocol version
  53         string  extension name [optional]
  54         string  extension value [optional]
  55         ...
  56
  57 The current version of the mux protocol is 4. A client should refuse
  58 to connect to a master that speaks an unsupported protocol version.
  59
  60 Following the version identifier are zero or more extensions represented
  61 as a name/value pair. No extensions are currently defined.
  62
  63 2. Opening a passenger mode session
  64
  65 To open a new multiplexed session in passenger mode, a client sends the
  66 following request:
  67
  68         uint32  MUX_C_NEW_SESSION
  69         uint32  request id
  70         string  reserved
  71         bool    want tty flag
  72         bool    want X11 forwarding flag
  73         bool    want agent flag
  74         bool    subsystem flag
  75         uint32  escape char
  76         string  terminal type
  77         string  command
  78         string  environment string 0 [optional]
  79         ...
  80
  81 To disable the use of an escape character, "escape char" may be set
  82 to 0xffffffff. "terminal type" is generally set to the value of
  83 $TERM. zero or more environment strings may follow the command.
  84
  85 The client then sends its standard input, output and error file
  86 descriptors (in that order) using Unix domain socket control messages.
  87
  88 The contents of "reserved" are currently ignored.
  89
  90 If successful, the server will reply with MUX_S_SESSION_OPENED
  91
  92         uint32  MUX_S_SESSION_OPENED
  93         uint32  client request id
  94         uint32  session id
  95
  96 Otherwise it will reply with an error: MUX_S_PERMISSION_DENIED or
  97 MUX_S_FAILURE.
  98
  99 Once the server has received the fds, it will respond with MUX_S_OK
 100 indicating that the session is up. The client now waits for the
 101 session to end. When it does, the server will send an exit status
 102 message:
 103
 104         uint32  MUX_S_EXIT_MESSAGE
 105         uint32  session id
 106         uint32  exit value
 107
 108 The client should exit with this value to mimic the behaviour of a
 109 non-multiplexed ssh(1) connection. Two additional cases that the
 110 client must cope with are it receiving a signal itself and the
 111 server disconnecting without sending an exit message.
 112
 113 A master may also send a MUX_S_TTY_ALLOC_FAIL before MUX_S_EXIT_MESSAGE
 114 if remote TTY allocation was unsuccessful. The client may use this to
 115 return its local tty to "cooked" mode.
 116
 117         uint32  MUX_S_TTY_ALLOC_FAIL
 118         uint32  session id
 119
 120 3. Requesting passenger-mode stdio forwarding
 121
 122 A client may request the master to establish a stdio forwarding:
 123
 124         uint32  MUX_C_NEW_STDIO_FWD
 125         uint32  request id
 126         string  reserved
 127         string  connect host
 128         string  connect port
 129
 130 The client then sends its standard input and output file descriptors
 131 (in that order) using Unix domain socket control messages.
 132
 133 The contents of "reserved" are currently ignored.
 134
 135 A server may reply with a MUX_S_SESSION_OPENED, a MUX_S_PERMISSION_DENIED
 136 or a MUX_S_FAILURE.
 137
 138 4. Health checks
 139
 140 The client may request a health check/PID report from a server:
 141
 142         uint32  MUX_C_ALIVE_CHECK
 143         uint32  request id
 144
 145 The server replies with:
 146
 147         uint32  MUX_S_ALIVE
 148         uint32  client request id
 149         uint32  server pid
 150
 151 5. Remotely terminating a master
 152
 153 A client may request that a master terminate immediately:
 154
 155         uint32  MUX_C_TERMINATE
 156         uint32  request id
 157
 158 The server will reply with one of MUX_S_OK or MUX_S_PERMISSION_DENIED.
 159
 160 6. Requesting establishment of port forwards
 161
 162 A client may request the master to establish a port forward:
 163
 164         uint32  MUX_C_OPEN_FWD
 165         uint32  request id
 166         uint32  forwarding type
 167         string  listen host
 168         uint32  listen port
 169         string  connect host
 170         uint32  connect port
 171
 172 forwarding type may be MUX_FWD_LOCAL, MUX_FWD_REMOTE, MUX_FWD_DYNAMIC.
 173
 174 If listen port is (unsigned int) -2, then the listen host is treated as
 175 a unix socket path name.
 176
 177 If connect port is (unsigned int) -2, then the connect host is treated
 178 as a unix socket path name.
 179
 180 A server may reply with a MUX_S_OK, a MUX_S_REMOTE_PORT, a
 181 MUX_S_PERMISSION_DENIED or a MUX_S_FAILURE.
 182
 183 For dynamically allocated listen port the server replies with
 184
 185         uint32  MUX_S_REMOTE_PORT
 186         uint32  client request id
 187         uint32  allocated remote listen port
 188
 189 7. Requesting closure of port forwards
 190
 191 A client may request the master to close a port forward:
 192
 193         uint32  MUX_C_CLOSE_FWD
 194         uint32  request id
 195         uint32  forwarding type
 196         string  listen host
 197         uint32  listen port
 198         string  connect host
 199         uint32  connect port
 200
 201 A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
 202 MUX_S_FAILURE.
 203
 204 8. Requesting shutdown of mux listener
 205
 206 A client may request the master to stop accepting new multiplexing requests
 207 and remove its listener socket.
 208
 209         uint32  MUX_C_STOP_LISTENING
 210         uint32  request id
 211
 212 A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
 213 MUX_S_FAILURE.
 214
 215 9. Requesting proxy mode
 216
 217 A client may request that the control connection be placed in proxy
 218 mode:
 219
 220         uint32  MUX_C_PROXY
 221         uint32  request id
 222
 223 When a mux master receives this message, it will reply with a
 224 confirmation:
 225
 226         uint32  MUX_S_PROXY
 227         uint32  request id
 228
 229 And go into proxy mode. All subsequent data over the connection will
 230 be formatted as unencrypted, unpadded, SSH transport messages:
 231
 232         uint32  packet length
 233         byte    0 (padding length)
 234         byte    packet type
 235         byte[packet length - 2] ...
 236
 237 The mux master will accept most connection messages and global requests,
 238 and will translate channel identifiers to ensure that the proxy client has
 239 globally unique channel numbers (i.e. a proxy client need not worry about
 240 collisions with other clients).
 241
 242 10. Status messages
 243
 244 The MUX_S_OK message is empty:
 245
 246         uint32  MUX_S_OK
 247         uint32  client request id
 248
 249 The MUX_S_PERMISSION_DENIED and MUX_S_FAILURE include a reason:
 250
 251         uint32  MUX_S_PERMISSION_DENIED
 252         uint32  client request id
 253         string  reason
 254
 255         uint32  MUX_S_FAILURE
 256         uint32  client request id
 257         string  reason
 258
 259 11. Protocol numbers
 260
 261 #define MUX_MSG_HELLO           0x00000001
 262 #define MUX_C_NEW_SESSION       0x10000002
 263 #define MUX_C_ALIVE_CHECK       0x10000004
 264 #define MUX_C_TERMINATE         0x10000005
 265 #define MUX_C_OPEN_FWD          0x10000006
 266 #define MUX_C_CLOSE_FWD         0x10000007
 267 #define MUX_C_NEW_STDIO_FWD     0x10000008
 268 #define MUX_C_STOP_LISTENING    0x10000009
 269 #define MUX_S_OK                0x80000001
 270 #define MUX_S_PERMISSION_DENIED 0x80000002
 271 #define MUX_S_FAILURE           0x80000003
 272 #define MUX_S_EXIT_MESSAGE      0x80000004
 273 #define MUX_S_ALIVE             0x80000005
 274 #define MUX_S_SESSION_OPENED    0x80000006
 275 #define MUX_S_REMOTE_PORT       0x80000007
 276 #define MUX_S_TTY_ALLOC_FAIL    0x80000008
 277
 278 #define MUX_FWD_LOCAL   1
 279 #define MUX_FWD_REMOTE  2
 280 #define MUX_FWD_DYNAMIC 3
 281
 282 XXX TODO
 283 XXX extended status (e.g. report open channels / forwards)
 284 XXX lock (maybe)
 285 XXX watch in/out traffic (pre/post crypto)
 286 XXX inject packet (what about replies)
 287 XXX server->client error/warning notifications
 288 XXX send signals via mux
 289 XXX ^Z support in passengers
 290 XXX extensions for multi-agent
 291 XXX extensions for multi-X11
 292 XXX session inspection via master
 293 XXX signals via mux request
 294 XXX list active connections via mux
 295
 296 $OpenBSD: PROTOCOL.mux,v 1.14 2024/01/08 05:11:18 djm Exp $