ssh -O stop server1
ssh -O cancel -L 8080:localhost:80 -S ~/.ssh/controlmaster/fred@server.example.org:22 server.example.org
https://en.wikibooks.org/wiki/OpenSS...k/Multiplexing
There are however caveats:
http://www.anchor.com.au/blog/2010/0...-bad-the-ugly/
[Edit: if there were only one connection per socket then I can't see there would be any issues.]