How to check SSH multiplexing session status

Multiplexed SSH sessions can leave later shells, file copies, and tunnels dependent on one local control master. Checking that master before reusing a saved host alias helps separate a missing control socket from a server login problem or a stalled automation run.

OpenSSH records the local listener for a multiplexing master at the ControlPath selected by the client configuration. The effective value from ssh -G shows the exact path for a host alias after Host and Match rules have been applied, including hashed %C socket names.

The socket listing and ssh -O check test different parts of the same state. A socket entry shows that the local file exists, while a Master running response proves that the ssh client can speak to the control master through that socket; a Control socket connect error means no master is listening at the resolved path.

1. Print the resolved client configuration for the multiplexed host alias.

   $ ssh -G host-mux
   host host-mux
   user user
   hostname host.example.net
   port 22
   ##### snipped #####
   controlmaster auto
   controlpath /home/user/.ssh/muxmasters/25bf3bd04004b80c0f2062755b4bd08f468420a1
   controlpersist 600

   Use the same alias, user, port, and command-line options that the real session uses. If controlmaster is no or controlpath is none, that target is not using a reusable control master.

   Related: How to show SSH client configuration

2. List the directory shown by ControlPath.

   $ ls -l ~/.ssh/muxmasters
   total 0
   srw------- 1 user user 0 Jun 13 11:23 25bf3bd04004b80c0f2062755b4bd08f468420a1

   The leading s in srw——- marks a Unix-domain socket. An empty directory usually means the master expired, exited, or was created under a different ControlPath.

3. Query the control master through the configured socket.

   $ ssh -O check host-mux
   Master running (pid=26)

   ssh -O check uses the target's resolved ControlPath and returns the process ID of the reachable master.

4. Match the reported process ID to the local ssh process when ownership or cleanup needs confirmation.

   $ ps -fC ssh
   UID          PID    PPID  C STIME TTY          TIME CMD
   user          26       1  0 11:23 ?        00:00:00 ssh -fN host-mux

   The process is local to the client machine, not the remote server. Different platforms may show the command as a master, mux, or background ssh process.

5. Interpret a missing control socket as an inactive master for that host alias.

   $ ssh -O check host-mux
   Control socket connect(/home/user/.ssh/muxmasters/25bf3bd04004b80c0f2062755b4bd08f468420a1): No such file or directory

   Do not use ssh -O exit as a status check because it closes multiplexed sessions that still depend on the master. Use the dedicated stop workflow only when ending the master is intentional.

   Related: How to stop SSH multiplexing