Unix path too long for domain socket

I wanted to SSH into a machine, and I got a very weird error message: unix_listener: path "/Users/me/.ssh/sockets/myservername_ansible@000-servername001.subdomain.main-domain-in-use.de-22.gFthlK8q5qNkrXW4" too long for Unix domain socket.

This seems to be the culprit:

• I am “persisting” SSH connections in my SSH config folder, precisely here: $HOME/.ssh/sockets
• The configuration for this is:

  Host ...
    ControlMaster auto
    ControlPath ~/.ssh/sockets/%r@%h-%p
    ControlPersist 600

• Before opening a connection, SSH looks for an existing socket in $HOME/.ssh/sockets, and if found re-uses it
• There is a 100-something length limit for “path to a socket” under Mac OS
• This path length limit was overshot with that 106-character long path

Solution: Shorten the path by using %C instead of %r@%h-%p:

Host ...
  ControlMaster auto
  ControlPath ~/.ssh/sockets/%C

That is it.

Sources:

• gist
• tarmac