Reverse SSH tunnel: how can I send my port number to the server?
- by Tom
I have two machines, Client and Server.
Client (who is behind a corporate firewall) opens a reverse SSH tunnel to Server, which has a publicly-accessible IP address, using this command:
ssh -nNT -R0:localhost:2222 [email protected]
In OpenSSH 5.3+, the 0 occurring just after the -R means "pick an available port" rather than explicitly calling for one. The reason I'm doing this is because I don't want to pick a port that's already in use. In truth, there are actually many Clients out there that need to set up similar tunnels.
The problem at this point is that the server does not know which Client is which. If we want to connect back to one of these Clients (via localhost) then how do we know which port refers to which client?
I'm aware that ssh reports the port number to the command line when used in the above manner. However, I'd also like to use autossh to keep the sessions alive. autossh runs its child process via fork/exec, presumably, so that the output of the actual ssh command is lost in the ether.
Furthermore, I can't think of any other way to get the remote port from Client. Thus, I'm wondering if there is a way to determine this port on Server.
One idea I have is to somehow use /etc/sshrc, which is supposedly a script that runs for every connection. However, I don't know how one would get the pertinent information here (perhaps the PID of the particular sshd process handling that connection?) I'd love some pointers.
Thanks!
