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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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