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Affordable IT: Desktop Security: Page 2 of 5

The Always-On Alternative

Applications or drivers that run on the end user's PC--software firewalls--usually act as a kernel shim. The software intercepts the data being passed between the kernel and network card drivers, inspecting all network traffic passed through it.

There are two major types of software firewalls: port blockers and application blockers. Port blockers, which include the built-in Windows 2000/XP firewall and the IPtables on Linux, work just like gateway or Internet firewalls and can block communications only to or from specific TCP/UDP ports.

Regrettably, port blockers are useless on the desktop. For one thing, you'd have to open a wide range of ports for a user to take advantage of his or her most common applications. What's more, these firewalls can't distinguish between Internet Explorer and a hostile program sending traffic over Port 80.