Linux has a powerful firewall (netfilter) that can be configured with a command line tool (iptables). But this is not very intuitive.
Ubuntu 8.10 (Intrepid Ibex) is supplied with a simplified configuration of the firewall, gufw (Graphic uncomplicated firewall).
Type in a terminal: sudo apt-get install gufw
or go through Synaptic to install the gufw package.
Start the firewall
Go to System> Administration> Firewall configuration.
Enter the administrator password if requested.
And check "Firewall enabled".
That's it! You're protected.
Nobody can connect to your computer remotely
Note that you're protected even if the guwf window is not displayed.
(You do need to run gufw that to change the configuration of the firewall.)
The firewall will remain active even after restarting the computer.
Opening a port
If you need to open a port, follow the procedure:
Enter the port number to be opened.
Indicate whether it is TCP, UDP or both.
For example, for BitTorrent, we open the port 6881 for both TCP and UDP:
Opening of ports is effective immediately and this configuration will be retained even after you restart the computer.