How to check SSH multiplexing session status

Monitoring the status of multiplexed SSH connections avoids surprises when many shells, tunnels, or automation tasks reuse the same TCP connection. When a single master connection silently dies or stalls, new clients can fail in confusing ways and long‑running jobs may lose their control channel unexpectedly.

In OpenSSH multiplexing, a master connection created with ControlMaster and ControlPath listens on a Unix domain socket, and subsequent sessions attach to that socket instead of opening new TCP connections. The socket path, often under /tmp or /home in a dedicated directory, is the main indicator that a multiplexing master is alive and accepting new clients.

Status checks focus on three pieces of information: whether the control socket exists and is open, whether the associated ssh process is still running, and whether ssh -O check can talk to the master. The examples assume a POSIX shell with OpenSSH on Linux or a similar Unix‑like system; care is required when using ssh -O exit or ssh -O stop, because those control commands can disconnect other sessions that rely on the same master connection.

Steps to check SSH multiplexing session status:

Open a terminal on the client system that establishes multiplexed connections.
```
$ whoami
user
```
Inspect the effective configuration for the target host to confirm multiplexing settings.
```
$ ssh -G host-mux | grep -iE 'control(master|path|persist)'
controlmaster auto
controlpath /home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129
controlpersist 600
```
The generated ssh -G output shows the final ControlMaster, ControlPath, and ControlPersist values after all configuration files are merged.
Identify the concrete control socket path that corresponds to the expanded ControlPath pattern.
```
$ ssh -G host-mux | awk '$1 == "controlpath" {print $2}'
/home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129
```
The %C token in ControlPath expands to a hash of the connection parameters, so reading the resolved path from ssh -G is the simplest way to find the socket.
List the control socket on disk to verify that a multiplexing master is currently listening.
```
$ ls -l /home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129
srw------- 1 user user 0 Jan 10 13:55 /home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129
```
A srw——- entry indicates a Unix stream socket, which is the usual type for the OpenSSH control master.
Examine the ssh process that owns the control socket to confirm the master is still running.
```
$ ps aux | grep ssh | grep -E 'master|mux'
user       14473  0.0  0.1  18252  4920 ?        Ss   13:55   0:00 ssh: /home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129 [mux]
```
Command lines containing \[mux] or similar labels identify multiplexing master processes serving one or more control sockets.

List open Unix sockets that reference the control path to see how many file descriptors reference the master.

$ lsof -U | grep /home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129
ssh       14473 user    4u  unix 0x0000000000000000      0t0 71787 /home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129.cx3xzBqSgI15kQ9u type=STREAM (LISTEN)

Multiple file descriptors to the same control socket usually indicate active or recently active multiplexed sessions.

Query the multiplexing master directly using the ssh -O check control command.
```
$ ssh -O check host-mux
Master running (pid=14473)
```
A Master running response confirms that the master process is reachable via the control socket and is accepting multiplexed connections.

Increase verbosity when additional diagnostics are needed about the control connection.

$ ssh -vvv -O check host-mux
OpenSSH_9.6p1 Ubuntu-3ubuntu13.14, OpenSSL 3.0.13 30 Jan 2024
##### snipped #####
debug1: auto-mux: Trying existing master at '/home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129'
debug2: mux_client_hello_exchange: master version 4
debug3: mux_client_request_alive: done pid = 14473
Master running (pid=14473)

Verbose output with -vvv helps trace configuration loading, control socket negotiation, and potential permission or path issues.

Optionally stop an unwanted or stale multiplexing master once status has been confirmed.
```
$ ssh -O stop host-mux
Stop listening request sent.
```
Stopping the master with ssh -O stop prevents new multiplexed sessions from attaching but allows existing sessions to finish; using ssh -O exit would immediately terminate all sessions that share the same master.

Related: How to stop multiplexing sessions in SSH
Verify the final status by running another check against the same host.
```
$ ssh -O check host-mux
Control socket connect(/home/user/.ssh/muxmasters/79b54c680bc3a687c54a4b2e86e6605d44c58129): No such file or directory
```
Failure to connect to the control socket after stop indicates that the master has removed its listener and multiplexing is no longer active for that host.