Leopard’s Allow All Incoming Connections option is the functional equivalent of the old Stop button: it turns your firewall off. I wouldn’t recommend this setting to anyone.

The Allow Only Essential Services option will block anything except a few default networking services, such as Bonjour. It prevents file sharing, remote access, and other optional services. You should use this option only if you really want to block everything. I use this option when I’m on potentially hostile networks, such as those in hotels or public hotspots, and don’t want to bother with manually turning off all my shared services (see “Firewalls on the Road”).

The third option, Set Access For Specific Services And Applications, is the one I recommend for everyday use. It’s actually a new kind of firewall for OS X. It’s what’s known as an applicationfirewall. Previous versions of OS X used a technology known as stateful packet inspection—a fancy way of saying the firewall blocked ports that weren’t being held open for use by approved applications. An application firewall like the one in Leopard blocks traffic targeting specific applications, not specific ports.

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Posted in Leopard, Macintosh Tips & Help