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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_MSG_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_FORWARD
+        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_PERMISSION_DENIED or a
+MUX_S_FAILURE.
+
+5. Requesting closure of port forwards
+
+A client may request the master to establish a port forward:
+
+        uint32  MUX_C_OPEN_FORWARD
+        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_PERMISSION_DENIED or a
+MUX_S_FAILURE.
+
+6. 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.
+
+7. 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
+
+7. 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_FORWARD      0x10000006
+#define MUX_C_CLOSE_FORWARD     0x10000007
+#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_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 1.1 2010/01/26 01:28:35 djm Exp $