Egress filtering is an important concept in security. While we understand that a firewall is a hardware/software solution that prevents the bad guys from getting into your network by closing ports and allowing only ones that are are absolutely necessary. Egress filtering allows only certain traffic out of your network while the rest is blocked from leaving. This is critical in case something malicious slipped through your network and now wants to ‘phone home’ with information about your shopping habits. Or perhaps your computer has been compromised and is infected with a virus or worm and, as a result, you have a bot on your computer. This bot will attempt to establish communication with whoever is controlling it by connecting to the outside world. A firewall with egress filtering will halt thease nefarious activities.

The million dollar question is “Do I need a software firewall on my home PC to prevent outbound traffic? “. I would say that for Ingress filtering, the answer is absolutely yes. If you are running Microsoft XP Service Pack 2 you already are running a firewall or if you are behind a router you are protected from the outside. For myself, I find software firewalls to be taxing on my system. Also, many people install a firewall like ZoneAlarm or Norton Internet Security and then ,when prompted if the program should be allowed to connect to the internet, they answer yes to everything, which is essentially useless. In short, if you’re careful with your system and are not in the habit of installing software you find on the internet blindly, or clicking on links in an email, I would say you should be fine. The other alternative is to get a less susceptible OS, OS X on a MAC. I did!

Here is a a good article on the subject.