How It Works: Personal Firewalls

Personal firewall: a technology that helps prevent intruders from accessing data on your PC via the Internet or another network, by keeping unauthorized data from entering or exiting your system.

Hackers don't just target national security organizations for cyberattacks: They want your tax returns, network passwords, or bank account numbers. And you don't want the FBI kicking in your door because someone hijacked your PC to participate in the latest denial-of-service attack on the Internet. Now that "always-on" broadband connections such as cable modems and digital subscriber line are becoming more popular, home users are at risk. Fortunately, you can protect your data. Firewalls can block malicious attacks and protect your PC from outside threats.

Here's what you need to know:

• A firewall can prevent an unauthorized user from accessing your PC, either from the Internet or from within your local network.
  
  
• It blocks some Trojan horse programs and many hostile applications that seek to take over your computer.
  
  
• New packages aimed at home users and small businesses are inexpensive and require little setup on your part.
  
  

When you're connected to the Internet, you're sending and receiving information in small units called packets. A packet contains the addresses of the sender and the recipient along with a piece of data, a request, a command, or almost anything having to do with your connection to the Internet. But just as with postal mail, not every package that arrives at your computer is one you want to open.

A firewall examines each data packet sent to or from your computer to see if it meets a set of criteria. The firewall then selectively passes or blocks the packet.

Examining Data for Cracks

The criteria a firewall uses for passing packets along depends on the kind of firewall you use. The most common type you'll find for home and small business use is called an application gateway firewall.

An application gateway, often called a proxy, acts like a customs officer for data: Anything you send or receive stops first at the firewall, which filters packets based on IP addresses and content, as well as the specific functions of an application. For instance, if you're running an FTP program, the proxy could permit file uploads while blocking other FTP functions, such as viewing or deleting files. You can also set the firewall to ignore all traffic for FTP services but allow all packets generated during Web browsing.

Other kinds of firewalls include packet filters, which examine every packet for an approved IP address; circuit-level firewalls, which allow communication only with approved computers and Internet service providers; and the newest type, stateful inspection firewalls, which note the configuration of approved packets and then pass or block traffic based on those characteristics.

Packet-filter, circuit-level, and stateful inspection firewalls are mostly found in corporate network setups. They require major upkeep, so they aren't suitable for most smaller companies and home users.