Desktop firewall products have been popular for a year or two now and are often employed for the obvious reason...
that millions of people are connecting their PCs to "always on" cable and DSL modems and they need protection from the evil hackers on the Internet. The focus has clearly been on preventing unwanted traffic from entering your PC, but most desktop firewall software is equally adept at blocking unwanted traffic from leaving your PC as well.
There are a lot of reasons you should consider using a desktop firewall package, even if you're considering it for a number of desktops that sit behind a full-featured firewall. The first reason is employing a diverse and deep defense to prevent attacks. Given the nominal expense of these packages (some of which are free, of course), two firewalls are better than one, because, if properly configured, an attacker would have to breach two systems, probably by researching two different exploits, rather than one. Besides, external firewalls, which are always placed on a network perimeter, can't help you if the attacker is inside the network.
An even better reason to consider installing desktop firewall software on your workstations is that while external firewalls like Cisco's PIX or Checkpoint's Firewall-1 products are capable of blocking some types of outbound traffic, it would be practically impossible to manage application granularity, such as blocking cookies or HTML referrers, using these external solutions. Desktop firewall software handles these issues with ease, and as a bonus it can tell you which process on the PC is responsible for the offending packets. By comparison, if an external firewall can even recognize the outbound traffic as bad, it will most likely silently drop the packets and you won't even be alerted that one of your PCs is sending the data.
For example, if some "ad-ware" has managed to install itself on one of your PCs and is sending data to some marketing company, the majority of external firewall products will be configured to permit this traffic, but a properly configured desktop firewall would alert you before sending the packets and tell you the offending application. This is also an excellent backup defense for detecting any Trojan horses missed by a virus scanner, since they will often attempt to phone home.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.