Mac® Security Bible

17.1. Understanding How Firewalls Work

Firewalls are a bit like chess: easy to learn but difficult to master. Simply by clicking a few buttons, you can activate a firewall with basic default settings and thereby increase your Mac's network security dramatically. On the other hand, you could invest hundreds of hours learning all the complexities of network protocols and firewall configuration and still barely scratch the surface of everything your firewall has to offer. I make no attempt in this chapter to turn you into a firewall expert, but I hope to provide you with enough information so you can make informed decisions about whether to use a firewall on your Mac, what sort of firewall to use, and how to configure it to keep you reasonably safe without making your life unnecessarily complicated.

17.1.1. Firewall terminology

By way of background, you should first be familiar with some terms that are helpful to understanding firewalls:

• Port. Many kinds of data can be sent over a network, and each application or service on your Mac is designed to deal with only particular kinds. For example, your email client doesn't know about file sharing, and your web browser doesn't know about voice over IP. The way computers keep track of which sort of data needs to go where is by dividing it into ports, which you can think of as something like TV channels or radio stations. Tens of thousands of ports are available, of which some have standardized uses and others are available for individual ...