Use Your MacBook’s Built-in Firewall
A firewall watches all the network communications coming into your MacBook. It automatically plays the role of security guard, blocking or denying certain network traffic that you want to prevent from reaching your laptop. It acts as another layer of security to help keep you safe from unwanted attacks.
That’s all well and good, but you must be careful to set up your firewall correctly before you turn it on: A configuration mistake could make your MacBook inaccessible from the network.
For instance, if you want to enable screen sharing on your MacBook but you also want to keep all other traffic from coming into your system, you can tell the built-in firewall to allow only screen sharing. The firewall on the MacBook will follow the rules you set up on what to block or allow.
When enabled, the firewall blocks all traffic that comes into your MacBook. By default, however, the firewall is turned off. So, your first job is to enable the firewall, which you can do by following these steps:
Click the System Preferences icon in the Dock.
Click the Security & Privacy icon.
Click the Firewall tab.
Click the Start button to turn on your firewall.
Is the Start button disabled? Don’t panic — just click the padlock icon in the lower-left corner. (If Lion prompts you for your Admin user account password, type it and then click Unlock.)
Click the Advanced button to show the settings that you see here.
Click the Automatically Allow Signed Software to Receive Incoming Connections radio button to select it.
You must enable each sharing method that you want to use — however, when you enable different sharing methods from the Sharing pane in System Preferences (such as File Sharing or Screen Sharing), you’ll notice that those types of traffic now appear in the Firewall list.
(In other words, when you turn on a sharing method, the firewall automatically allows traffic for that sharing method, which Lion calls a service. Most excellent.)
Click the up/down arrow icon to the right of any service to specify whether the firewall should allow or block connections.
Sometimes, you might want to allow other traffic through your firewall that isn’t on the firewall list of recognized services and applications. At that point, you can click the Add button (which bears a plus sign) to specify the application that your firewall should allow. Lion presents you with the familiar Add dialog, and you can choose the application that needs access.
Click OK to save your changes and return to the Security & Privacy pane, and then click the Close button to close the System Preferences window.