~/.ssh/config

Host *
  ControlMaster auto
  ControlPersist yes
  ControlPath ~/.ssh/control/%r@%h:%p

# ssh -O ctl_cmd <server>

ctl_cmd:

• check: check that the master process is running (returns PID if running)
  • Master running (pid=3292)
  • Control socket connect(/home/user/.ssh/control/user@server:22): No such file or directory
• forward: request forwardings without command execution
• cancel: cancel forwardings
• exit: request the master to exit), and
• stop: request the master to stop accepting further multiplexing requests).

• Some infos are copied from the man page: man ssh