Special Edition Using Linux®, Sixth Edition

Introduction to Firewalls

The term firewall comes from the firewall that is used in cars (and other motorized vehicles), which protects the occupants in the cabin in case of a fire in the engine compartment. A firewall on a network protects both users and the data that is behind it on the local network from the savagery of the Internet (or extranet). It can also be used to prevent users on the local network from connecting to prohibited sites, and to compartmentalize the internal network.

Some people learn too late that a firewall does not protect against insiders. Internal firewalls can be used to isolate departments so that damage is not widespread; for example, firewalls can be used to protect accounting from engineering, and engineering ...