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03/31/2004 Archived Entry: "What your web browser reveals"

I see that Sir Bill of the Blue Screen is forecasting that computer hardware will be almost free in 10 years. Funny how he neglects to mention how much his software will cost then -- $1000 per PC? -- for the poor souls who are still locked into it.

On that note, I'm delighted to see that Mandrakesoft (publishers of Mandrake Linux) have emerged from bankruptcy protection. Their timing couldn't be better, since they're widely regarded as one of the better desktop Linux systems. It's time for me to give their product a test drive.

A recent posting on the ifeminists bulletin board caused a friend to ask, what does web browsing reveal about you?

Of particular concern was this web page, which, when you visit it, tells you your IP address, ISP, operating system, and browser. How do they do that? How much more do they know?

For starters, when you visit a web page, you have to send your IP address. Remember, this is how the web server knows how to send the information back to you. That's unavoidable.

IP addresses are assigned in blocks to Internet Service Providers. So, from your IP address, it's easy to determine what ISP you're using. Here's a web page with lots of Internet information services; try typing an IP address like into the Reverse DNS lookup or IPWHOIS lookup. Depending on your ISP, your IP address might even indicate what city you're dialing into.

The only way to block this information is to use an "anonymizer" proxy server. This is a computer which will accept your web page request, and relay it without revealing your IP address. Then it accepts the reply and relays that reply back to you. The only computer that knows your real IP address is the anonymizer proxy; the rest of the world just sees the proxy's IP address.

(Two such proxy servers are anonymizer.com or The Cloak. You can find more by searching "anonymous web surfing" on Google. These provide other privacy services too, like managing cookies...more on that in a future blog.)

So where does the rest of the information come from? Your web browser sends it, whenever it asks for a web page. Here's a typical request as logged at our server:

"GET /mac/isil.htm HTTP/1.1" 200 11256
"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)"

From this we learn the following:
1. Someone wants to read the web page /mac/isil.htm on our web site.
2. The "referring page" is google.com. This means that this person followed a link from google to our site. Whenever you follow a link from one web page to another, the "referring page" is identified.
3. Furthermore, we can see that this person searched for "women and pornography" on Google. This is because Google keeps the search terms in their web page URL.
4. Finally we see that this person is using a Mozilla/4.0 compatible browser, specifically MS Internet Explorer 6.0, and using the Windows NT 5.1 (I presume that's Windows XP) operating system.

Generally this is out of your control (although Opera will let you identify yourself as using Netscape or MSIE). This goes back to the early days, when the people building the World Wide Web thought this would be useful information to have. It does allow, for example, a web site to adjust its presentation to the specific browser you are using... although some dim-witted web sites will simply refuse to accept certain browsers.

Note that this does not reveal your name, email address, or other personal information, or leak any files from your computer.

Personally, I don't worry about revealing this information. I don't care if the world knows I'm running Mozilla on Linux (and for the stupid sites that insist on MSIE, I can use Opera to lie to them). If it's an issue for you, look for an anonymizer proxy that removes this information (not all of them do). Test them by feeding web page http://www.idzap.com/userdata.php into their free trial.


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