April 20, 2001
Web site owners should communicate their position on the use of ad-blocking software on their site by providing an adblock.txt file in their top directory in much the same way that they provide a robots.txt file to indicate what parts of the site search engine spiders and other robots are permitted to access. The format and meaning of the contents of adblock.txt are the same as for robots.txt as described in the Standard for Robot Exclusion except that where robots.txt refers to "User-Agent" the relevant item for adblock.txt is the name of the ad-blocking program. If no adblock.txt file is supplied by the web site, the ad-blocking software is free to block ads on that site. Here is an example adblock.txt.
Most ad-blocking software seems to use a custom proxy server to filter HTTP requests based on the URL of the requested object. Once the software has determined that the request was for something it wants to block (and is thus an advertisement rather than an entire web page), it can determine the web page that the advertisement belongs to from the Referrer field in the HTTP header. Once this is determined the corresponding adblock.txt can be requested if it has not recently been checked.
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