Mac OS X 10.5 and Mac OS X 10.6 have a multitude of ways to keep data from coming or going from a system. The traditional way is to use ipfw, although this isn’t the default way in 10.5 and above. Instead, you are meant to use the Application Layer Firewall (we’ll call it ALF for short), which is what you configure from the Security System Preference pane. You can enable the firewall simply enough by using the defaults command to augment the /Library/Preferences/com.apple.alf.plist file, setting the globalstate key to an integer of 1: defaults write /Library/Preferences/com.apple.alf globalstate -int 1 You can also configure the firewall from the command line.…