how to detect (and kill) tunnel-only ssh connections?

[me-5yx05kfkO/aqeI1yJSURBw at public.gmane.org (Matthew Gillen)]

On 10/12/2009 12:44 PM, Dan Kressin wrote:
> Using "ssh -N" or putty's "Don't start a shell or any command at all" checkbox (Connection->SSH), it is possible to open an ssh connection to hostA for tunneling purposes even if the user's shell on hostA is set to nologin (or /bin/false, etc).  As there is no shell or command running, these connections do not appear in the output of w or who.
> 
> How might one detect these connections, assuming they come from a network with other active shell-based connections?
> 
> Platform in question is FreeBSD, but I'm interested in Linux responses also.

What I'd try is to do a full process listing to get all the 'sshd' processes.
 Then look through the full process listing for login shell processes whose
parent-PID is one of the sshd PIDs.

It would work on linux.  Can't speak for *BSD.  Of course, it's possible for
someone to 'detach' their shell from the parent-pid, but people usually don't
do that unless they're doing something nefarious (or want to launch a job
that's longer-lived than the current tty without using 'screen').

HTH,
Matt