Article Aggregator:

adblock.txt Standard

By Bill Dimm
April 20, 2001


Most content sites have "Terms of Use" statements that prohibit "modifying" or "creating derivative works" from the site content. Much like Napster, ad-blocking software acts as a tool allowing people to violate content owners' rights and deprive them of income. Ad-blocking software should face restrictions similar to those imposed upon Napster in the courts. Ad-blocking software should not block ads displayed on sites which have communicated that blocking is prohibited. This proposal specifies an automated mechanism for sites to communicate that ad blocking is prohibited.


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.

Use of this format allows adblock.txt a great deal of flexibility in specifying permitted ad-blocking on a site. This proposed standard is, in fact, probably far more flexible (and complicated) than is really necessary. It would probably be quite sufficient (and a huge improvement over the current situation) if web site owners could simply specify that all ad-blocking is either permitted or prohibited for an entire site. There are two reasons for proposing this more complicated standard for adblock.txt. First, the format is one that most webmasters are already familiar with from robots.txt. Second, some site owners may be generally willing to permit ad-blocking on their site while wanting to prohibit certain very abusive (e.g. software that replaces ads with something else) or aggressive (e.g. software that modifies and can break JavaScript code) programs. Ad-blocking software authors who do not want to implement a parser for such a complicated adblock.txt can follow the much simpler and more conservative procedure of turning-off ad-blocking if an adblock.txt file is present on the site (regardless of its contents).


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.

Comment or vote on this proposal.

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