Using Mac OS X's Built-in Firewall

A firewall watches all the network communications coming into your Mac — it automatically plays the role of security guard, blocking or denying certain network traffic that you want to avoid from reaching your Mac. It acts as another layer of security to help keep you safe from unwanted attacks. That's all well and good, but you must be careful to set up your firewall correctly before you turn it on: A configuration mistake can make your Mac inaccessible from the network.

For example, if you want to enable FTP access on your Mac but you also want to keep all other traffic from coming into your Mac, you can tell the built-in firewall to allow only FTP traffic. The firewall on the Mac will block or allow only TCP/IP traffic — not AppleTalk — so AppleTalk traffic (which is always from your local network and not the Internet anyway) is always able to get in.

When enabled, the firewall blocks all traffic that comes into your Mac. By default, however, the firewall is turned off. So, your first job is to enable the firewall, following these steps:

1. Click the System Preferences icon on the Dock.

2. Click the Sharing icon.

3. Click the Firewall tab to show the settings that you see in Figure 1.

4. Click the Start button.

Figure 1: The location of user folders and the Shared folder on your hard drive.

Your firewall is now enabled, and, by default, all incoming TCP/IP traffic is blocked.

You must enable each sharing method that you want to be able to use. As you enable different sharing methods, such as Personal File Sharing or FTP Access, notice that under the Firewall tab, those types of traffic now have a check mark in the box for each type of traffic. (In other words, when you turn on a sharing method, the firewall automatically allows traffic for that sharing method. Most excellent.)

Sometimes you may want to allow other traffic through your firewall that isn't on the list. At that point, you can click the New button to create a new definition for your firewall to use. The resulting dialog box has a drop-down list with some common things that you may want to allow, such as Internet Relay Chat (IRC), I Seek You (ICQ), QuickTime, and MSN Messenger.

If you need to add ports for another application that's not in that drop-down list — for example, a multiplayer game — you need to select Other from the Port Name drop-down list. Then you can enter a port number, a port range, or a series of ports. You may need to check the documentation for a specific application to see which ports it uses.

Ports are like an extension to an IP address. For example, when you communicate with a Web server, you send a request not only to that Web server's IP address, but you send it on port 80 — the standard port for HyperText Transfer Protocol (HTTP) traffic. Different applications use different port numbers, so you need to check which ports are used when you want to adjust your firewall to allow that traffic.

Click the Advanced button whilst you're turning on your firewall — then make doggone sure that the Enable Stealth Mode check box is enabled. Hackers and other undesirables will have a much harder time locating your Mac's presence on the Internet with this feature turned on.