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How to Use the Firewall on Your MacBook

When you connect a network to the Internet on your MacBook, you open a door to the outside world. SO USE YOUR FIREWALL! The outside world is chock-full of malicious individuals who would dearly love to inflict damage on your data or take control of your MacBook for their own purposes. Call ’em hackers, call ’em delinquents, but don’t let them in!

Mountain Lion comes to the rescue again with the built-in firewall in OS X. When you use the firewall, you essentially build a virtual brick wall between you and the hackers out there (both on the Internet and within your local network). Follow these steps:

  1. Click the System Preferences icon in the Dock.

  2. Click the Security & Privacy icon.

  3. Click the Firewall tab.

  4. Click the Turn On Firewall button to activate your firewall.

  5. Click the Firewall Options button.

  6. Select the Enable Stealth Mode check box.

    This important feature prevents hackers from trolling for your MacBook on the Internet — or, in normal-speak, searching for an unprotected computer — so it’s much harder for them to attack you. (Stealth mode also prevents other computers from pinging you, so don’t be surprised if your MacBook suddenly clams up if you try to ping it.)

  7. Click OK.

  8. Click the Close button to exit System Preferences.

Mountain Lion even keeps track of the Internet traffic that you do want to reach your laptop, such as web page requests and file sharing. When you activate one of the network features, Mountain Lion automatically opens a tiny hole (called a port by net-types) in your firewall to allow just that type of communication to your Mac.

For example, if you decide to turn on printer sharing, Mountain Lion automatically allows incoming print jobs from other computers.

You can also add ports for applications that aren’t on the firewall’s Allow list, such as third-party instant messaging clients and multiplayer game servers. Depending on the type of connection, Mountain Lion will often automatically display a dialog prompting you for confirmation before allowing certain traffic, so most folks won’t need to do anything manually.

However, you can manually add a program to your list of allowed (or blocked) firewall ports. Follow these steps:

  1. Click the System Preferences icon in the Dock.

  2. Click the Security & Privacy icon.

  3. Click the Firewall tab, and then click the Firewall Options button.

  4. Click the Add button (which carries a plus sign).

    Mountain Lion displays a standard file-browsing sheet.

  5. Browse to the application that requires access to the outside world — or the application that you want to block from outside communication — and click it to select it.

  6. Click the Add button in the File sheet.

    The application appears in the Firewall list. By default, it’s set to Allow Incoming Connections.

  7. If you want to block any incoming communication to the application, click the Allow Incoming Connections pop-up menu and choose Block Incoming Connections instead.

  8. Click the Close button to exit System Preferences.

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