I have written another post about accessing linux box over ssh if you are behind a firewall.
You can read it from (in Turkish) https://www.uygunteknoloji.net/drupal/?q=node/77
I have find a new way to bypass firewall which blocks ssh requests.
Also this way you don’t need putty or a linux machine to access your remote box.
shellinabox is a webserver which runs on your linux server and listens a port.
You can access it by http://yourserver.com:4200
All client-server communications are encrypted, if SSL/TLS certificates have been installed.
Its default port is 4200 but your firewall can block this port. So you can start this process
on 443 port or use apache reverse proxy.
There is no browser plugin to use it on clientside because all network traffic is on ajax.
Another interesting feature is that it supports realtime updates like tailing a log file or
editing a text file with nano. Also it supports linux shell shortcuts like tab to autocomplete
It is very easy to install it on debian. wget the deb package and dpkg -i shellinabox.deb.
Or install it from tar.gz.
For manual page and download visit : http://code.google.com/p/shellinabox/