I'm going to talk a little bit about what firewalls are, what they do, and what they protect us against. In terms of firewalls, I'll focus on Linux ipchains packet-filtering firewalls.
Throughout all of this, it's important to remember that security requires a hierarchical approach, security in depth. A firewall is not a panacea, and it's not any one thing. There is no single magic bullet.

A static firewall can do nothing for a poorly administered system that’s connected to the Internet.