NAT Enough?

By Larry Seltzer  |  Posted 2004-09-07 Print this article Print

A home router is not a complete security solution.

You may have seen inexpensive home routers described as "NAT firewalls." For example, Linksys says of its BEFSR11 EtherFast Cable/DSL Router: "[T]he built-in NAT technology acts as a firewall protecting your internal network." Its an interesting—if disingenuous—claim, but it raises a legitimate question: How much security do you get with a typical SOHO router?

The answer is that you do get protection, and its not negligible. Consider that several of the most important network attacks of the last year or two—Blaster, Sasser and most of the other protocol-level attacks on Windows systems—could not reach systems on a network behind a NAT router. (NAT, or Network Address Translation, is the ability to show one IP address to the world while concealing the IP addresses of the computers on the network.)

This is because these worms typically scan the Internet, perpetrating attacks that work against unprotected Windows systems. If you are using a router, the IP address that outsiders see is that of the router itself, which doesnt run Windows. But if one system behind the router were to become infected, the others would be doomed, because the router doesnt protect traffic within the network.

Real firewalls do what is called "stateful inspection"; they examine the headers and potentially the content of each network packet in the context of its connection to check for validity. For example, a good firewall will recognize a smurf attack or the similar Fraggle attack, which uses a form of address spoofing (the forging of a sender IP address so that communications appear to come from a trusted source) to cause your network to flood a third party with unwanted traffic. Such firewalls also block off access to TCP ports unless they have been opened for an approved reason.

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Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.

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