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Testing your firewall for open ports

For several years now I’ve used a neat tool at Gibson Research to test a clients firewall quick and easy from the web browser. They have a tool called Shields Up that does a limited port scan to determine of network ports are open, closed or “stealth”.


One of the things I usually have to explain is that even though you only have one connection to the outside world (the internet), on that connection a computer can listen for incoming requests on as many as 65,535 different “ports”, think of them as different “tv channels.” To a computer on the outside an open port means that the request was accepted by the listening computer. A closed port means the listening computer answered back that it’s not letting anyone in (kind of like saying “nobody’s here right now”.) The stealth port means that the computer doing the probe sent a request but there was no response. This is usually the ideal because 1) it takes longer for the “attacker” to get an idea of who if anyone is at a given IP address and 2) it makes it look as though there’s no machine listening at that port.

The most ambitious of his scans does the first 1056 ports (remember there are over 65,000 total). You can do custom probes specifying a list of specific ports as well.

Gibson Research is home to lot’s of other neat utilities. Spinrite is the brainchild of the sites owner Steve Gibson. One of the things that has impressed me about him is his dedication to efficient programming (assembly language guru….) This gives utilities which are small, fast and not cluttered with the cruft you usually get these days from “utilities”. He also has dedicated a lot of time to computer security. It’s a site worth reading.

I should also mention another utility before I let this one go….

Leaktest is a utility he has to test personal (software) firewall software to see if it is fooled by a program claiming to be another application. You see one of the benefits of a personal firewall is that it can prevent data from leaving your computer without your knowledge (backdoor keylogger software for instance.) Unfortunately some firewalls can be tricked say if you’ve allowed internet explorer to connect and send data out and another program decides to tell the system “I’m internet explorer”, the firewall can be fooled. Leaktest basically tests your personal firewalls abilities….

All in all www.grc.com is a VERY useful site. It’s run and maintained by a VERY sharp fellow, Steve Gibson and I think you’re just about guaranteed to LEARN something reading it. For that reason I’m adding it to the links area as well.

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Posted by on August 12, 2005.

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Categories: Computers, Networking, Security

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