Mac® Security Bible

4.3. Firewall

In computer terms, a firewall is hardware or software that selectively blocks network traffic. Presumably, you never want to block all your Mac's network traffic; that would mean no web browsing, no email, no file sharing — no communicating with other computers at all. However, there are excellent reasons for blocking some network traffic. Many kinds of network attacks involve an outside computer attempting to send instructions to vulnerable programs on your Mac so they can install unauthorized software, use your computer to send out spam, spy on you, or simply crash your system. In some cases, hackers use robotic armies of hundreds or thousands of computers to methodically probe computers on the Internet, looking for just the right kinds of openings needed to wreak havoc.

The situation isn't as scary as it may sound though. By default, Macs run very little software that actively listens for outside network connections, and most of this software has other kinds of safeguards against misuse. Even without taking any special action, the real-world chances of becoming the victim of a random network attack are, for a Mac user, quite slim. On the other hand, as you install more third-party software, your risk increases. And hackers are constantly looking for new vulnerabilities to exploit, so a Mac that's safe today could be quite unsafe tomorrow.

In order to protect yourself against these potential threats, your first line of defense is Mac OS X's built-in firewall. In ...