There are times you need to access a service or website that runs on a port that is blocked for public access. For example you have a test site on your company computer running on 8080 and this is not accessible from the public internet. If you have ssh access to the machine or any machine on that network you can easily setup a tunnel to access that port

This will explain how to setup a tunnel and will works for Mac OS X, Linux, FreeBSD, cywgin, or any other OS with ssh shell program.

Lets say you want to access a website on port 8080 on a machine that is behind a firewall but you have ssh access to this machine. In this case you can do the following to access the website on port 8080
> ssh -L 8080/localhost/8080 user@mycompany.com
log in when your are prompted for your password and you will appear to have a typical ssh shell, but in addition you are running a tunnel. Now you can access the website by going to http://localhost:8080 this will really be going to http://mycompany.com:8080

Lets say the website is running on a different machine than you have access to but the one you have access to can access 8080. In this case you can do the following to access the website on port 8080
> ssh -L 8080/10.2.2.2/8080 user@mycompany.com
This will forward all traffic to 8080 on your local machine to 8080 on 10.2.2.2 so going to http://localhost:8080 would really be the sam e as going to http://10.2.2.2:8080

In most cases the ssh session will timeout in a certain time of inactivity. To prevent this I usually use a script that executes vmstat -20 on the remote machine. This gives the output of vmstat every 20 seconds and keeps the connection open. Here is a sample script I use to tunnel ports

#!/bin/sh

ssh  
-L 993/imap.gmail.com/993 
-L 465/smtp.gmail.com/465 
-L 587/smtp.gmail.com/587 
-L 23/localhost/22 
-L 3128/localhost/3128 
chad@linux.server.com vmstat 20

Another place this comes in handy is when you are at a place that blocks or limits web access. In this case you can setup a squid proxy server on your machine at home than just open an ssh tunnel to your home machine and forward port 3128. Now just setup your browser to use a proxy on localhost 3128 and you will be surfing the web over ssh via your proxy. Nobody can monitor your web usage because it all goes thru ssh.

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