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299 lines
8.9 KiB
Plaintext
299 lines
8.9 KiB
Plaintext
This document describes the multiplexing protocol used by ssh(1)'s
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ControlMaster connection-sharing.
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Multiplexing starts with a ssh(1) configured to act as a multiplexing
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master. This will cause ssh(1) to listen on a Unix domain socket for
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requests from clients. Clients communicate over this socket using a
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simple packetised protocol, where each message is proceeded with
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a length and message type in SSH uint32 wire format:
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uint32 packet length
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uint32 packet type
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... packet body
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Most messages from the client to the server contain a "request id"
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field. This field is returned in replies as "client request id" to
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facilitate matching of responses to requests.
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Many multiplexing (mux) client requests yield immediate responses from
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the mux process; requesting a forwarding, performing an alive check or
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requesting the master terminate itself fall in to this category.
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The most common use of multiplexing however is to maintain multiple
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concurrent sessions. These are supported via two separate modes:
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"Passenger" clients start by requesting a new session with a
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MUX_C_NEW_SESSION message and passing stdio file descriptors over the
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Unix domain control socket. The passenger client then waits until it is
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signaled or the mux server closes the session. This mode is so named as
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the client waits around while the mux server does all the driving.
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Stdio forwarding (requested using MUX_C_NEW_STDIO_FWD) is another
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example of passenger mode; the client passes the stdio file descriptors
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and passively waits for something to happen.
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"Proxy" clients, requested using MUX_C_PROXY, work quite differently. In
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this mode, the mux client/server connection socket will stop speaking
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the multiplexing protocol and start proxying SSH connection protocol
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messages between the client and server. The client therefore must
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speak a significant subset of the SSH protocol, but in return is able
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to access basically the full suite of connection protocol features.
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Moreover, as no file descriptor passing is required, the connection
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supporting a proxy client may itself be forwarded or relayed to another
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host if necessary.
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1. Connection setup
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When a multiplexing connection is made to a ssh(1) operating as a
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ControlMaster from a client ssh(1), the first action of each is send
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a hello messages to its peer:
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uint32 MUX_MSG_HELLO
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uint32 protocol version
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string extension name [optional]
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string extension value [optional]
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...
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The current version of the mux protocol is 4. A client should refuse
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to connect to a master that speaks an unsupported protocol version.
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Following the version identifier are zero or more extensions represented
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as a name/value pair. No extensions are currently defined.
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2. Opening a passenger mode session
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To open a new multiplexed session in passenger mode, a client sends the
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following request:
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uint32 MUX_C_NEW_SESSION
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uint32 request id
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string reserved
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bool want tty flag
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bool want X11 forwarding flag
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bool want agent flag
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bool subsystem flag
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uint32 escape char
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string terminal type
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string command
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string environment string 0 [optional]
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...
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To disable the use of an escape character, "escape char" may be set
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to 0xffffffff. "terminal type" is generally set to the value of
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$TERM. zero or more environment strings may follow the command.
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The client then sends its standard input, output and error file
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descriptors (in that order) using Unix domain socket control messages.
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The contents of "reserved" are currently ignored.
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If successful, the server will reply with MUX_S_SESSION_OPENED
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uint32 MUX_S_SESSION_OPENED
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uint32 client request id
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uint32 session id
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Otherwise it will reply with an error: MUX_S_PERMISSION_DENIED or
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MUX_S_FAILURE.
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Once the server has received the fds, it will respond with MUX_S_OK
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indicating that the session is up. The client now waits for the
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session to end. When it does, the server will send an exit status
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message:
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uint32 MUX_S_EXIT_MESSAGE
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uint32 session id
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uint32 exit value
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The client should exit with this value to mimic the behaviour of a
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non-multiplexed ssh(1) connection. Two additional cases that the
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client must cope with are it receiving a signal itself and the
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server disconnecting without sending an exit message.
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A master may also send a MUX_S_TTY_ALLOC_FAIL before MUX_S_EXIT_MESSAGE
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if remote TTY allocation was unsuccessful. The client may use this to
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return its local tty to "cooked" mode.
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uint32 MUX_S_TTY_ALLOC_FAIL
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uint32 session id
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3. Requesting passenger-mode stdio forwarding
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A client may request the master to establish a stdio forwarding:
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uint32 MUX_C_NEW_STDIO_FWD
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uint32 request id
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string reserved
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string connect host
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string connect port
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The client then sends its standard input and output file descriptors
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(in that order) using Unix domain socket control messages.
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The contents of "reserved" are currently ignored.
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A server may reply with a MUX_S_SESSION_OPENED, a MUX_S_PERMISSION_DENIED
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or a MUX_S_FAILURE.
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4. Health checks
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The client may request a health check/PID report from a server:
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uint32 MUX_C_ALIVE_CHECK
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uint32 request id
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The server replies with:
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uint32 MUX_S_ALIVE
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uint32 client request id
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uint32 server pid
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5. Remotely terminating a master
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A client may request that a master terminate immediately:
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uint32 MUX_C_TERMINATE
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uint32 request id
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The server will reply with one of MUX_S_OK or MUX_S_PERMISSION_DENIED.
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6. Requesting establishment of port forwards
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A client may request the master to establish a port forward:
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uint32 MUX_C_OPEN_FWD
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uint32 request id
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uint32 forwarding type
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string listen host
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uint32 listen port
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string connect host
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uint32 connect port
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forwarding type may be MUX_FWD_LOCAL, MUX_FWD_REMOTE, MUX_FWD_DYNAMIC.
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If listen port is (unsigned int) -2, then the listen host is treated as
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a unix socket path name.
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If connect port is (unsigned int) -2, then the connect host is treated
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as a unix socket path name.
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A server may reply with a MUX_S_OK, a MUX_S_REMOTE_PORT, a
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MUX_S_PERMISSION_DENIED or a MUX_S_FAILURE.
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For dynamically allocated listen port the server replies with
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uint32 MUX_S_REMOTE_PORT
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uint32 client request id
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uint32 allocated remote listen port
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7. Requesting closure of port forwards
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Note: currently unimplemented (server will always reply with MUX_S_FAILURE).
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A client may request the master to close a port forward:
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uint32 MUX_C_CLOSE_FWD
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uint32 request id
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uint32 forwarding type
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string listen host
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uint32 listen port
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string connect host
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uint32 connect port
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A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
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MUX_S_FAILURE.
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8. Requesting shutdown of mux listener
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A client may request the master to stop accepting new multiplexing requests
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and remove its listener socket.
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uint32 MUX_C_STOP_LISTENING
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uint32 request id
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A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
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MUX_S_FAILURE.
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9. Requesting proxy mode
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A client may request that the control connection be placed in proxy
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mode:
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uint32 MUX_C_PROXY
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uint32 request id
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When a mux master receives this message, it will reply with a
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confirmation:
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uint32 MUX_S_PROXY
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uint32 request id
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And go into proxy mode. All subsequent data over the connection will
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be formatted as unencrypted, unpadded, SSH transport messages:
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uint32 packet length
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byte 0 (padding length)
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byte packet type
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byte[packet length - 2] ...
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The mux master will accept most connection messages and global requests,
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and will translate channel identifiers to ensure that the proxy client has
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globally unique channel numbers (i.e. a proxy client need not worry about
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collisions with other clients).
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10. Status messages
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The MUX_S_OK message is empty:
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uint32 MUX_S_OK
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uint32 client request id
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The MUX_S_PERMISSION_DENIED and MUX_S_FAILURE include a reason:
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uint32 MUX_S_PERMISSION_DENIED
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uint32 client request id
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string reason
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uint32 MUX_S_FAILURE
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uint32 client request id
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string reason
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11. Protocol numbers
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#define MUX_MSG_HELLO 0x00000001
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#define MUX_C_NEW_SESSION 0x10000002
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#define MUX_C_ALIVE_CHECK 0x10000004
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#define MUX_C_TERMINATE 0x10000005
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#define MUX_C_OPEN_FWD 0x10000006
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#define MUX_C_CLOSE_FWD 0x10000007
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#define MUX_C_NEW_STDIO_FWD 0x10000008
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#define MUX_C_STOP_LISTENING 0x10000009
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#define MUX_S_OK 0x80000001
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#define MUX_S_PERMISSION_DENIED 0x80000002
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#define MUX_S_FAILURE 0x80000003
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#define MUX_S_EXIT_MESSAGE 0x80000004
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#define MUX_S_ALIVE 0x80000005
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#define MUX_S_SESSION_OPENED 0x80000006
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#define MUX_S_REMOTE_PORT 0x80000007
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#define MUX_S_TTY_ALLOC_FAIL 0x80000008
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#define MUX_FWD_LOCAL 1
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#define MUX_FWD_REMOTE 2
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#define MUX_FWD_DYNAMIC 3
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XXX TODO
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XXX extended status (e.g. report open channels / forwards)
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XXX lock (maybe)
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XXX watch in/out traffic (pre/post crypto)
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XXX inject packet (what about replies)
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XXX server->client error/warning notifications
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XXX send signals via mux
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XXX ^Z support in passengers
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XXX extensions for multi-agent
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XXX extensions for multi-X11
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XXX session inspection via master
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XXX signals via mux request
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XXX list active connections via mux
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$OpenBSD: PROTOCOL.mux,v 1.13 2022/01/01 01:55:30 jsg Exp $
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