A firewall is a piece of software that blocks unwanted incoming connections to a system. A firewall minimizes the threat of unauthorized users gaining access to your system while connected to a network or the Internet.
Solution
Run the following command to enable the firewall: % /usr/bin/sudo /usr/bin/defaults write /Library/Preferences/com.apple.alf globalstate -int <value> For the <value> use either 1 specific services, or 2 essential services only. Impact: The firewall may block legitimate traffic. Applications that are unsigned will require special handling.