Information
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 A firewall minimizes the threat of unauthorized users from gaining access to your system while connected to a network or the Internet.
Solution
Perform the following to implement the prescribed state: Open System Preferences Select Security & Privacy Select Firewall Select Turn On Firewall Alternatively: Run the following command in Terminal: defaults write /Library/Preferences/com.apple.alf globalstate - int <value> Where <value> is: 1 = on for specific services 2 = on for essential services Impact: The firewall may block legitimate traffic. Applications that are unsigned will require special handling.