Using the Built-in Firewall in Mac OS X Panther

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 could make your Mac inaccessible from the network.

For instance, 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 only allow FTP traffic. (The firewall on the Mac will only block or allow TCP/IP traffic — not AppleTalk — so AppleTalk traffic is always able to get in.)

When enabled, the firewall, by default, will block 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. Open System Preferences from the Apple menu or the Dock.

2. Click the Sharing icon.

3. Click the Firewall tab, as shown in Figure 1.

4. Click the Start button.

Figure 1: Enabling the firewall in Mac OS X.

This will enable the firewall — and, by default, all incoming TCP/IP traffic will be 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, you might 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 might 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 has a drop-down list with some common things that you might want to allow, such as America Online (AOL) Instant Messenger, ICQ, and MSN Messenger.

If you need to add ports for another application that's not in that drop-down list — for instance, a multiplayer game — you need to select Other from the Port Name drop-down list (as shown in Figure 2). Then you can enter a port number, a port range, or a series of ports. You might need to check the documentation for a specific application to see which ports it uses.

Figure 2: Configure a "hole" in the firewall for a specific application.

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.