Opens a connection and goes into the background without executing a remote command so you don't need to keep a window or shell running locally just to keep that connection.

You can set it to autoask instead of auto to have ssh prompt you for whether or not to reuse an existing connection.

The configuration directive ControlPath tells ssh where it should keep its socket information. I've chosen to put these files in /tmp, however it may be best to put this into your own home directory on multi-user systems.

You will need to specify -o ControlMaster=no when using ssh to do ssh tunneling otherwise multiple tunnels to a particular host will not work.

If it doesn't work and fail silently, use -v to see whether X11 forwarding request is sent to debug this. Possible solutions:

• config solution

  add ForwardX11=yes (and ForwardX11Trusted=yes) to .ssh/config
  

• -S none option

  The last time I checked this one, it's not working, however it should according to ssh man page.

  PS: while testing this one, I crashed systemd in SLED12 beta2, which resulted in a very interesting bug.

Also, isn't the whole point of putting ControlPath in .ssh/config to make the -S argument uneccessary? And doesn't the ControlMaster auto also make the toggling -M uneccessary?