OS X (both Leopard and Tiger) comes with a built-in firewall that’s disabled by default. The Leopard firewall is a little bit different than Tiger’s, so I’m focusing on that. The underlying firewall is ipfw – the same as on FreeBSD, so if you know what you’re doing, you can edit it to your heart’s content. More details on controlling the firewall from the command line are available in this O’Reilly article. This article is going to talk about dealing with the firewall through the available GUI interface. First, you have to access the firewall. System Preferences -> Security then the Firewall tab. By default, it’ll have “Allow all incoming connections”. If you’re not quite sure what you’re doing, “only[…]