Personal computer invisibility

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Personal computer invisibility

In a corporation there are two sources of attack on computers. The first is internal and the second is external. At home, however, even with a company machine, you really aren’t concerned about external attack other than the computer being stolen. And, in fact, you may not actually be concerned at all! If it would be anything more than an annoyance if your hard-drive were trashed, or if it would be a problem if the information on your computer appeared on a public web site or in the hands of your adversaries, then yes, you care. If not, then you probably don’t need anything other than an antivirus program.

If you decide you do care, than you do need to be somewhat concerned about outside attack when we connect to the internet. This concern is lessened if you connect via a dial-up modem, because the IP address of the computer (a unique number which identifies your computer from all the millions of other computers on the internet) changes each time you sign on, making you hard to find, and because you are on for such short periods of time that even if you are found, there is hopefully not enough time for a cracker to do anything bad to you.

This is not so if we have a permanent connection with a DSL line, a cable modem, or any other connection with which you tend to be on for long periods and with a fixed IP address. In this case we have to become more concerned about whether our computer can be located, penetrated, and used.

There are two issues here. The first is how does someone identify your IP address. Well, putting aside randomly capturing IP addresses, one way is to log on to a web site. When people load a web page, a lot of information about you, including your IP address, is captured. If you participate in online forums, some publicly display your IP address for all to see. Once your IP address is known, the bad guy can start probing your computer to look for security holes in the system.

This tells us two things: First, that you don’t want to be giving out our IP address. Second, that you want to close security gaps.

Hiding the IP address

You can hide your IP address using an anonymous proxy server. When you load a web page using an anonymous proxy server you actually send your request to the proxy server, which then requests the page and passes it back to you. The IP address captured is that of the proxy server, rather than our IP address. On our home computer we use A4Proxy, to be found at

Closing security gaps

Security gaps for the home user come in three flavors. One is the endless set of security flaws built into the operating system or software. You deal with this by checking for security updates. If you run Windows there is a Windows Update program you can click-on that will give you the latest updates. Additionally, there are programs that will notify you when there are updates to your various pieces of software. We use BigFix at Finally, many programs themselves allow you to check for updates.

A second area of danger is computer viruses. It is prudent these days to use an anti-virus program and to make sure both the program itself and the virus definitions are kept up to date. Many anti-virus programs will automatically update themselves.

Finally, cunning crackers can, if they know your IP address, hack into your system while you are online. Fortunately, you can do a lot to make your home computer invisible to the outside world. A good place to start is using the tests available online at Gibson Research Corporation at If you go to the Shields Up! Section there is a lot of good – and comprehensible – information, as well as two sets of tests: Test my Shields and Probe my Ports. These two tests will give you a reasonable assessment of how open your computer is, and suggestions on how to change settings to make it safer. Amusingly, when we re-ran these tests for this article, we came up with a lot of vulnerabilities which we thought we had dealt-with long ago. We then realized that we had run the tests through an anonymous proxy server, and what was being seen was that computer, not ours. When run with our own IP address we were relatively secure.

While closing the gaps as suggested at is helpful, for more complete control you will really need a firewall, which is software that controls what can go into and out-of your computer. Personal firewalls are not expensive.

And not particularly complicated. We use ZoneAlarm Pro from Zone Labs at While we use the snazzy $50 version, they also have a version that sells for $40, as well as a perfectly adequate version that you can download free. Which version you get depends on how sophisticated you are, but the free version is likely to do everything you need. We got the paid version because we believe in supporting good vendors who graciously offer a free version.

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