OS X,General Tips
FireWall in OSX
Mac OS X has a built-in firewall, though it is slightly misunderstood. In this case, the firewall is a piece of software that can restrict network access to your computer based on the sharing protocol to which the traffic belongs. There are various ways in which an OS X machine could be accessed over the network, and they all require positive action on the part of the user in order to enable these methods. These sharing mechanisms are controlled with the "Sharing" pane of System Preferences. It is important to understand that with these sharing mechanisms turned off (the default state), there is no way that your computer can be remotely accessed, regardless of the firewall state (viri notwithstanding, though there are no known viri that create remotely accessible back doors in OS X). Bottom line: Mac OS X is secure without a firewall due to conservative default settings. No network services are enabled unless a user turns them on, in which case the firewall would need to allow network traffic for that service in order for it to be useful.