Firewall and Proxy Server HOWTO This document is designed to describe the basics of firewall systems and give you some detail on setting up both a filtering and proxy firewall on a Linux based system. An HTML version of this document is available at http://www.grennan.com/Firewall-HOWTO.html.
Understanding FirewallsA firewall is a structure intended to keep a fire from spreading. Building have firewalls made of brick walls completely dividing sections of the building. In a car a firewall is the metal wall separating the engine and passenger compartments. Internet firewalls are intended to keep the flames of Internet hell out of your private LAN. Or, to keep the members of your LAN pure and chaste by denying them access the all the evil Internet temptations. The first computer firewall was a non-routing Unix host with connections to two different networks. One network card connected to the Internet and the other to the private LAN. To reach the Internet from the private network, you had to logon to the firewall (Unix) server. You then used the resources of the system to access the Internet. For example, you could use X-windows to run Netscapes browser on the firewall system and have the display on your work station. With the browser running on the firewall it has access to both networks. This sort of dual homed system (a system with two network connections) is great if you can TRUST ALL of your users. You can simple setup a Linux system and give an account accounts on it to everyone needing Internet access. With this setup, the only computer on your private network that knows anything about the outside world is the firewall. No one can download to their personal workstations. They must first download a file to the firewall and then download the file from the firewall to their workstation. BIG NOTE: 99% of all break-ins start with gaining account level access on the system being attacked. Because of this I dont recommend this type of firewall. It is also very limiting.
Loopback This is a network on your computer, allowing programs to communicate with it like a remote machine. It's required for running the X Window System.
