You ever frequently connect to the same SSH server to perform subsequent commands? Maybe you rsync files to the server, then you just want to tell it to restart a service, for example.
Regularly, every time you call ssh, it establishes a new connection. It does so by sending a command to an SSH server to setup a new connection, and once the command is carried out or the shell is exit, sends another command to disconnect.
But, you can also tell SSH to not disconnect right away, and instead keep and re-use the same connection for any subsequent commands you want to run. That’s called “connection multiplexing,” and it’s easy to set up. Saves up some time from always handshaking and authenticating every time you want to do something on the same server via SSH.
By the way, since it’s for SSH, this also works with RSync via SSH!
Do yourself a favour and add the configuration lines below in ~/.ssh/multiplexing.inc:
# ~/.ssh/multiplexing.inc
ControlMaster auto
ControlPath ~/.ssh/cm-%r@%h:%p
ControlPersist 10m
Then, wherever you want to use connection multiplexing, maybe only your favourite machines or all of them, include the file above:
# ~/.ssh/config
# Example host only
Host example.com
Include ~/.ssh/multiplexing.inc
(You can also include the lines for ~/.ssh/multiplexing.inc directly in ~/.ssh/config, if you want. The above is just a suggestion to help organise the SSH config.)
The next time you connect to an SSH server with multiplexing set up, a socket file will be created in ~/.ssh. This will keep the connection open for up to 10 minutes after the last command. (Feel free to change the ControlPersist 10m line to how long you want connections to remain.) In the case a connection expires, or the socket file is missing, only then ssh will establish a new connection.
You can also close the connection immediately, if you want:
ssh -O exit example.com
Enjoy the re-usability of the SSH connections!
This article expands on the original tip I posted on X back on .