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Many people use their home's Internet connection to access computers located at a university, a library, or at work. The reverse is also true: an Internet connection can be used to connect to your computer and, if you have DSL, you may be more vulnerable to having the wrong person connect to your system.

The reason that DSL offers a greater invitation to crackers (hackers of malicious intent) is that, unlike a dialup connection, DSL is always connected. The greater the time your computer is visible to the Internet, the greater the likelihood that it may be spotted by some nefarious individual.

Of course big companies and glamorous networks like those at NASA and the Pentagon are still the favored targets of crackers, but the allure of an individual's computer is not necessarily the contents of the hard drive. Crackers like to launch attacks from computers belonging to ordinary citizens as means of disguising their identities (and ruining yours).

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DSL SECURITY: STEPS YOU CAN TAKE TO PROTECT YOURSELF   Disable file sharing on your system   Install the latest software patches available for your operating system (certain bugs may make your computer more vulnerable to attack)   Turn your computer off when you're not using it (admittedly, this ruins some of the fun of having an "always on" connection, but you'll minimize the time that your computer is visible to the world)   Be sure to password-protect your router, if you're using one   Keep up to date on security issues by checking out sites with security-oriented content like icsa.net, The International Computer Security Association and cert.org, The Computer Emergency Response Team   Consider installing 'firewall' software (see below)

Firewall Software

Firewall software is designed to prevent users beyond your network from accessing files and programs residing on your computer(s). Up until recently, most firewall software was designed for use by administrators of medium to large networks and the process of installing and configuring the software was fairly complicated. Now you can obtain firewall software intended for the small office/home office (SOHO) and individual user environments.

Noted below are a few such firewall programs designed with the individual user in mind. The features are outlined in detail at the product Web sites, but each lets you specify the particular applications (such as your email client) that will be able to come through the firewall. For example, in the default settings, systems on your local network or individual computer can access resources such as servers and printers, but systems on the Internet cannot. You also have the ability to allow specific users to access your system from the Internet side and all ports not in use are blocked.

Windows XP has a built-in firewall. To activate it, go from the "Start" button to "Control Panel". Switch to "Classic View", if not already in Classic View, and double-click on "Network Connections". Double-click "Local Area Network connection" and then click the "Properties" button. Select the "Advanced" tab. Put a check in the checkbox that says "Protect my computer and network by limiting or preventing access to this computer from the Internet".

A firewall program for Windows 95, 98, 2000, or NT users called ZoneAlarm is free for personal use and costs only $19.95 per year for corporate users (after a 60-day free evaluation).

Mac OS 10.2 has a built-in firewall that can be activated by launching System Preferences, selecting the Sharing icon, clicking the Firewall tab, and clicking the "Start" button.

Mac OS 10.1 includes the command line program ipfw. You may want to see a tutorial for help configuring ipfw. If you are unfamiliar with command-line tasks in the Terminal, you may want to use a third-party program such as BrickHouse. BrickHouse is shareware and costs $25 per machine.

Unfortunately, the firewall software designed for use with Mac OS 9 or earlier is not free, but the file sharing security issues really aren't pressing, unless you're specifically running OS9 - OS9.2.2. Unlike previous versions of the Mac OS, OS9 can share files over TCP/IP, which is the protocol that computers on the Internet use to talk to each other. If you are operating a LAN, which provides services like FTP and Web access, you may be interested in obtaining firewall software for your Mac, even if you're running a version of the OS that is earlier than OS9. In this case, check out Netbarrier and Doorstop which retail for $74.95 and 59.95 respectively. Netbarrier requires Mac OS 7.5.5 or later, PowerPC, Open Transport, 16MB, 5MB of free disk space, Internet Config., and a screen resolution of 800x600 or higher. Doorstop firewall software requires Mac OS 8.1 and PowerPC.