Should You Use a Firewall with OS X Mountain Lion?
Using a firewall protects your computer running OS X Mountain Lion from malicious users on other networks or the Internet and keeps them from gaining access to your Mac.
According to the OS X built-in Oxford American Dictionary, a firewall is
Part of a computer system or network that is designed to block unauthorized access while permitting outward communication.
Unlike older versions of Windows, OS X is quite difficult to crack. There have been few (if any) reports of outsiders gaining access to Macintosh computers running OS X.
One reason might be that OS X has a built-in firewall. That’s the good news. The bad news is that said firewall is disabled by default. You’ll need to activate it if you want to be protected against unauthorized access to your computer.
If you use a router with its own firewall (and the router’s firewall is enabled), do not activate the Mountain Lion firewall. Running multiple firewalls can cause issues.
To activate your firewall, follow these steps:
Open the System Preferences application (from the Applications folder, Apple menu, Launchpad, or Dock).
Click the Security & Privacy icon and click the Firewall tab.
The default setting is Allow All Incoming Connections, which is the least secure option.
Click the Turn On Firewall button to turn the firewall on, if it’s not already running.
(Optional) If the lock in the bottom-left corner of the Security & Privacy pane is locked, click it, and provide your administrator password.
Click the Firewall Options button to configure your firewall’s settings.
For the highest level of protection, select the Block All Incoming Connections check box.
Alas, you probably won’t want to keep this setting for long, because you won’t be able to use awesome OS X features such as Messages and file, screen, printer, and music sharing, to name a few. If (or when) it becomes desirable to allow certain incoming connections from outside computers, enable them in the Sharing System Preference pane.
The only other issue you’re likely to face is when a particular application needs you to allow outside connections to it in order to function. How would you know?
Check the user manual, Read Me file, or application Help. Or you might see an error message that the program can’t connect to the Internet. Don’t worry — if a program requires you to open your firewall, you can almost certainly find some information in one (or more) of these places.
The solution is to click the little + button on the left near the bottom of the Firewall Options window. A standard Open File sheet drops down over the window; select the appropriate program and click the Add button. Your firewall will then allow incoming connections to that particular application evermore.