Troubleshooting Leopard

From SAI Wiki

New Firewall Configuration (Mac OS X 10.5)

In the new Mac OS X Leopard, one of the new features is a revamped network firewall. In the first versions of 10.5, the firewall was disabled by default! This should never be the case. To ensure that your firewall is enabled, open up your System Preferences (Apple Menu -> System Preferences) ans select Security. From the Security panel, choose the Firewall tab.
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Make sure "Allow all incoming connections" IS NOT CHECKED! Most users should simply set the "Allow only essential services" option. If you have some special needs or have a specific application that needs access through the firewall, you can select the "Set access for specific services and applications" option and then add your application(s) to the access list with the "+" button.

The firewall is an essential defense for your Mac on the Internet. For more information on Mac OS's firewall, refer to Apple's Website.