How to stop/kill an autossh tunnel?

No, the proper way to kill autossh is simply to kill the process autossh, nothing else.

The reason is

# file $(which autossh)
/usr/bin/autossh: POSIX shell script, ASCII text executable

that autossh is simply a shell script, not a service. It starts a new program, in its very last line,

exec /usr/lib/autossh/autossh "$@"

again not a service. As for exec (you can double-check it in the wiki of the Bash hackers), it is a shell-builtin command which replaces the current shell with the following command (/usr/lib/autossh/autossh "$@" in this case) without starting a new process. So, the only way to stop autossh is to kill the calling script, for instance

pkill -3 autossh

(thanks to dviljoen for pointing out the importance of using the -3 flag, see below). Incidentally, killing the ssh connection will not work, because the calling command (i.e., the one above) will simply start a new connection as soon as it realizes the old one has been dropped.

Search for the process:

ps aux | grep ssh

The secon column is the PIDnumber

Kill process by PID :)

kill -9 PIDnumber

Use sudo if you dont have root privileges.

run auto ssh with:

AUTOSSH_PIDFILE=/var/run/tunnel.pid autossh

kill it with:

kill pid

BTW

pkill -9 autossh is a wrong

-9 makes sure the process not exiting gracefully, so ssh process is still there when the autossh process is killed

without -9 is still bad, if you have multiple tunnels running, pkill will kill them all

correct way is to set AUTOSSH_PIDFILE env var then kill that pid only