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How Ad-Blocking Software Works

By Bill Dimm
April 23, 2001


Most ad-blocking software installs a proxy server on your computer and routes all of the data to/from your browser through the proxy server. The proxy server may then filter out the browser requests that attempt to download images etc. from ad servers. It may also modify the incoming HTML pages to strip out JavaScript code that looks like it might be for advertising purposes.

What happens when you load a web page?

When you click a link or enter a URL into your browser, it uses HyperText Transport Protocol (HTTP) to send a request to the web server that hosts the requested page. The web server responds by sending the page or an error message if it is unavailable. When the browser receives the page it parses the HTML and JavaScript to figure out what images are required and requests those images from the web server one-by-one (each such transaction is a "hit").

What does a proxy server do?

A proxy server is a piece of software (perhaps on a separate computer) that you can funnel all of your browser's data requests through before they reach the Internet. Proxy servers can be used by companies to avoid the security risk of having all their computers connect directly to the Internet. They also allow frequently requested pages to be cached or access to certain sites to be restricted (stop browsing for porn at work!).

How does ad-blocking software work?

Most ad-blocking software installs a customized proxy server on your computer and instructs you (or does it for you) to change your browser settings to route HTTP requests through this proxy server.

At the simplest level, the ad-blocking software causes each request for data to be inspected to see if it is being sent to the URL of a known ad-server (to download a banner ad, for example). Instead of passing the request along to the ad-server (as the proxy server does for normal page requests), it can throw the request away and tell the browser that the requested data could not be found, or it could send a bogus image to the browser (for example, a single transparent pixel). Thus the ad request never makes it to the ad server and the browser does not display an ad.

More sophisticated ad-blockers go beyond simply filtering out certain data requests coming from the browser. Some will actually modify the HTML and JavaScript code from a requested page as it passes through the proxy server in order to remove JavaScript code that creates pop-up windows, etc. This can take ad stripping to a higher level, but it also runs a greater risk of damaging parts of the page that are not related to advertising.

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External Resources:
Automatically Blocking Banner Ad URLs
Ad Blocking
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