How To Use SSH Tunnelling To Allow Services To Pass Through A Firewall

[this post has been partially damaged during a server failure on 11 July 2004; with the exception of the images, it was recovered on 13 October 2018]

Paul has been stuck with a problem of late – he’s now living in university accomodation, and he’s found that he can’t connect through the university firewall to his external mail server. I advised him that it’s possible to set up an ‘SSH Tunnel’ (through central.aber.ac.uk) to fix this problem, but he hasn’t met with much success (see his blog entry for more details). In any case, here’s my investigation (and solution) to the problem.

How To Use SSH Tunnelling To Allow Services To Pass Through A Firewall
In my example, I’m going to try the opposite to what Paul is trying to achieve. I’m going to try to allow my POP3 e-mail client to get access to the university e-mail server (pophost.aber.ac.uk). As things stand, this server is on the other side of the university firewall, and is inaccessible from outside. The server central.aber.ac.uk, however, is accessible from both sides of the firewall. So what I’ve got is this (yes, I know that this is a gross oversimplification):

As you can see, connecting from my home PC is futile:

C:\Documents and Settings\Dan>telnet pophost.aber.ac.uk 110
Connecting To pophost.aber.ac.uk...Could not open connection to the host, on por
t 110: Connect failed

But if I SSH-in to central.aber.ac.uk…

central:~ $ telnet pophost.aber.ac.uk 110
Trying 144.124.16.40...
Connected to pophost.aber.ac.uk.
Escape character is '^]'.
+OK mailsplit Oct 2000 ready

So, what I need to do is to tell my SSH client to connect to central.aber.ac.uk, and forward specific traffic through the firewall to the mail server. Here’s what I needed to know:

(a) A free TCP port number on my own computer from which I can virtually ‘pipe’ the connection. Most numbers over 1024 are fine. I chose ‘9110’.
(b) The name of the mail server – ‘pophost.aber.ac.uk’.
(c) The TCP port to which I wanted to connect – the standard port for a POP3 mail server is ‘110’.
(d) My user name on a server which: (1) I can connect to; (2) can connect to the server specified in (b). It happens to be ‘dlh9’.
(e) The name of the server specified in (d) (i.e. ‘central.aber.ac.uk’).
(f) My password on the server. Like I’m going to tell you that.

The syntax is:

ssh -L (a):(b):(c) (d)@(e)

I’m using the non-commercial version of SSH Secure Shell Client, so here’s what happens:

C:\Documents and Settings\Dan>"\Program Files\SSH Secure Shell\ssh2.exe" -L 9110
:pophost.aber.ac.uk:110 dlh9@central.aber.ac.uk
dlh9's password:
Authentication successful.

At this point, I’m ready to go. Look what happens when I connect to port 9110 on my own computer, now…

C:\Documents and Settings\Dan>telnet localhost 9110
+OK mailsplit Oct 2000 ready

I could simply point my e-mail program at the ‘mail server’ at localhost:9110, and I’d be able to collect my university e-mail (so long as my SSH connection remained open).

Hopefully this guide will help some folks out there who are struggling with this kind of thing, and in particular, help Paul.

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