A firewall is a piece of software that blocks unwanted incoming connections to a system. Apple has posted general documentation about the application firewall. http://support.apple.com/en-us/HT201642 Rationale: A firewall minimizes the threat of unauthorized users from gaining access to your system while connected to a network or the Internet. Impact: The firewall may block legitimate traffic. Applications that are unsigned will require special handling.
Solution
Perform the following to turn the firewall on: Graphical Method: Open System Preferences Select Security & Privacy Select Firewall Select Turn On Firewall Terminal Method: Run the following command to enable the firewall: $ sudo defaults write /Library/Preferences/com.apple.alf globalstate -int <value> For the <value>, use either 1, specific services, or 2, essential services only.