More than one remote port fwd request for the same local port

[Rob Landley]

On Thursday 30 October 2008 20:17:04 Ming-Ching Tiew wrote:
> Ming-Ching Tiew wrote:
> > Matt Johnston wrote:
> >> On Thu, Oct 30, 2008 at 02:37:44PM +0100, Michael Wiedmann wrote:
> >>> Hi,
> >>>
> >>> how deals dropbear with different clients which are requesting each
> >>> a remote port forwarding to the same local port (on the server
> >>> side), e.g.
> >>>
> >>> system-1> dbclient -l user1 -N -R 7777:client-ip-1:80 server-ip
> >>> ...
> >>> system-2> dbclient -l user2 -N -R 7777:client-ip-2:80 server-ip
> >>>
> >>> Doing a quick test it looks like dropbear accepts the client
> >>> requests but the port forwarding does not work (actually it cannot
> >>> because there is more than one 'target').
> >>
> >> Unix sockets inherently only allow a single process (so a
> >> single user) to listen on a port. What behaviour would
> >> you expect?
> >
> > I do face the same issue. In my usage, more than one system
> > execute the same command ( ie client-ip is actually one only )
> > and I don't run any remote shell, the sole purpose of the
> > dbclient connection is to establish remote port forward, I
> > would prefer the last command succeed and it drops the previous
> > connection. However, I do realise  such a behaviour is rather
> >  "unfair".
>
> And I did try implement a remote shell method to solve my
> problem, but I was stuck at a stage where I could not identify
> the correct dropbear server process  to kill ( ie to kill the
> previous instance of dropbear which started up the
> same port forward ).

lsof -i

(Note that you may have to run it as root to list the sockets belonging to 
processes other than your current user.)

Of course it may be easier to grovel around in /proc yourself than try to beat 
lsof into _not_ displaying port 80 as "www".  (You can always 
delete /etc/services...)

Rob