Linux Firewalls, Third Edition

Chapter 1. Preliminary Concepts Underlying Packet-Filtering Firewalls

A small site may have Internet access through a T1 line, a cable modem, DSL, ISDN, a PPP connection to a phone-line dial-up account, or wireless. The computer connected directly to the Internet is a point of focus for security issues. Whether you have one computer or a local area network (LAN) of linked computers, the initial focus for a small site will be on the machine with the direct Internet connection. This machine will be the firewall machine.

The term firewall has various meanings depending on its implementation and purpose. At this opening point in the book, firewall means the Internet-connected machine. This is where your primary security policies for Internet access ...