What's a firewall, and how do I set one up?

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Summary: Firewalls are an important part of keeping your computer safe when connected to the internet. We'll look at what a firewall is and your choices.

Viruses like the recent Sasser worm can be prevented simply by using a good firewall. What's a firewall? Well, in your car it's the "wall" of metal behind the dashboard between you and the engine that prevents engine fires from roasting you and your passengers.

A firewall for your computer is much the same - it's to keep you from getting burned.

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A firewall's purpose is very simple: to block or filter certain types of network traffic from reaching your computer. What do I mean by "certain types"? There are things you want to get like the pages of web sites you visit or the software you might download. And then there are other things you might not want like people accessing your computer remotely or viruses and worms infecting your machine.

A firewall knows the difference.

Firewalls are also usually configurable; they can allow you to say "this kind of connection from the outside is OK". A good example is Remote Desktop. A firewall will by default prevent it from working. But you can also configure the firewall to allow that type of connection to come through. That way you would be able to access your computer from another, across the room, or across the internet. But other types of traffic like viruses are still blocked.

Some firewalls will also monitor outgoing traffic for suspicious behavior. One characteristic of many viruses is that once you're infected they attempt to establish connections to other computer to spread. Many software firewalls will detect and either warn you or simply prevent it.

And that leads to a very important distinction in firewalls - there are two types: hardware and software.

A hardware firewall is just that - a box that sits between you and the internet that performs the filtering function. Traffic that is filtered out never reaches your computer. Broadband routers perform the function of a firewall quite nicely and are typically what I recommend. The downside for a hardware devices is that most will not filter outgoing traffic.

A software firewall is a program that runs on your computer, and at the very lowest level monitors your network traffic. The firewall prevents filtered traffic from getting through to the operating system. All network traffic reaches your computer but the firewall prevents your system from actually doing anything with it.

The good news is that if you're running Windows XP, you already have a firewall built-in. It's a simple matter of turning it on to get the protection you're looking for.

Video Tip: Enabling the Internet Firewall. A short walk-through of the steps you can take to enable your Windows XP firewall and help prevent viruses like Sasser from spreading.

Related Links:

• Ask Leo! - What are "LSASS.EXE" and "Sasser" and how do I know if I'm infected? What do I do if I am?

• Ask Leo! - What's the difference between a Hub, a Switch and a Router?

• Ask Leo! - How do I keep my computer safe on the internet?

• How Stuff Works - How firewalls work. A more detailed look at the workings of firewalls.

• Lockergnome - Video Tip of the Week: Enabling the Internet Firewall

• Webopedia - firewall - a definition.

Article C1941 - May 14, 2004