Advertisers always need to know how effective their current marketing campaign may be. Since the Internet spans the world, it's nearly impossible to tell how many people looked at a particular ad and who they might be. To solve these two problems, advertisers created web bugs.
When you visit a website, your browser asks the website server to send your computer all the text and graphic images that make up the web page. Thus, every webserver needs to know the IP address of your computer so it knows where to send the text and graphics.
When your browser receives information about a web page, that information appears in the form of HTML (Hypertext Markup Language) code, which tells your browser exactly how to display and position text and graphics. The specific HTML code that your browser receives from a web page defines the name of the graphic file, its size, and the name of the server it came from. In the following HTML example, the graphic file is called dotclear.gif, its width and height are both one pixel, and the server it came from is http://ad.doubleclick.net:
<IMG SRC=http://ad.doubleclick.net/dotclear.gif width=1 height=1>
Web bugs hide on ordinary web pages as invisible, one pixel by one pixel size images so you won't notice when you're being tracked. When the server sends the web bug to your browser, the server can immediately identify the following:
The IP address of the computer that fetched the web bug
The specific web page that contains the web bug (useful for seeing which web pages someone might have visited)
The time and date the web bug was retrieved
The type of browser that fetched the web bug
At the simplest level, web bugs help advertisers determine how many people have visited a particular website and viewed a particular web page. On a more insidious level, web bugs can work with cookies to track which websites each person visits so they can display advertisements specific to that individual.
Web bugs can sometimes appear in spam too (see Chapter 16), buried inside email so an advertiser can see how many times people read (or at least open) a particular message. If someone doesn't bother to view a web bug in an email, this tells the advertiser that the email address may not be valid or that this particular person didn't bother to read it. In either case, the advertiser will likely remove that person's email address to avoid wasting time sending advertisements that no one will read.
Some companies accused of planting web bugs in email marketing messages include Experian (http://www.experian.com), Digital Impact (http://www.digitalimpact.com), and Responsys (http://www.responsys.com). By browsing their websites, you can get a better idea of how email marketing firms work, and how they might target you sometime in the future.
Besides slipping web bugs in target email messages, it's possible to embed a web bug into a newsgroup message, too. Not only could this tell an advertiser how many times someone looked at the ad, but it can also track down the specific IP address of each person who downloaded the web bug.
The extremely paranoid believe that web bugs can identify people who subscribe to politically incorrect newsgroups, while less-conspiracy-minded people believe that governments might use web bugs to track down anyone trading child pornography or illegal MP3 files. Since web bugs are invisible, and you aren't likely to even notice their presence, it's possible that a web bug has already given away your IP address and browsing habits to a faceless corporation without you even knowing it.
Since web bugs often work with cookies to track your browsing habits, your first line of defense is to make sure your browser refuses all cookies. Since this won't always be practical, especially when you visit online shopping sites, visit Bugnosis.org (http://www.bugnosis.org) and download their free Bugnosis tool (see Figure 17-1).
As you browse through different websites, Bugnosis scans each web page, gives an audible warning, and highlights suspicious web bugs on a web page. By using Bugnosis with Internet Explorer, you can see how prevalent web bugs may actually be, especially if Bugnosis finds suspicious GIF images on your favorite websites, such as the DM News site (http://www.dmnews.com), the Detroit News (http://www.detnews.com), or the New York Times (http://www.nytimes.com).