The robot exclusion standard, also known as the Robots Exclusion Protocol or robots.txt protocol, is a convention to prevent cooperating web spiders and other web robots from accessing all or part of a website which is otherwise publicly viewable. Robots are often used by search engines to categorize and archive web sites, or by webmasters to proofread source code. The standard complements Sitemaps, a robot inclusion standard for websites.
About the standard
A robots.txt file on a website will function as a request that specified robots ignore specified files or directories in their search. This might be, for example, out of a preference for privacy from search engine results, or the belief that the content of the selected directories might be misleading or irrelevant to the categorization of the site as a whole, or out of a desire that an application only operate on certain data.
For websites with multiple sub-domains, each sub-domain must have its own robots.txt file. If example.com had a robots.txt file but a.example.com did not, the rules that would apply for example.com will not apply to a.example.com.
Disadvantages
The protocol, however, is purely advisory. It relies on the cooperation of the web robot, so that marking an area of a site out of bounds with robots.txt does not guarantee privacy. Some web site administrators have tried to use the robots file to make private parts of a website invisible to the rest of the world, but the file is necessarily publicly available and its content is easily checked by anyone with a web browser.
There is no official standards body or RFC for the robots.txt protocol. It was created by consensus in June 1994 by members of the robots mailing list (robots-request@nexor.co.uk). The information specifying the parts that should not be accessed is specified in a file called robots.txt in the top-level directory of the website. The robots.txt patterns are matched by simple substring comparisons, so care should be taken to make sure that patterns matching directories have the final '/' character appended, otherwise all files with names starting with that substring will match, rather than just those in the directory intended.
Automated Content Access Protocol
ACAP, which is a possible plug-in for the Robots Exclusion Standard, was released as v1.0 on November 30, 2007.
robots.txt search engine
Of recent interest is the percentage of website pages that have robots.txt files and which specific robots are given privileges to crawl specific website files. To help understand these issues, a publicly available robots.txt search engine, BotSeer, has been developed to search and index robots.txt files.
Examples
This example allows all robots to visit all files because the wildcard "*" specifies all robots: User-agent:Mediapartners-Google * Disallow:
This example keeps all robots out: User-agent: * Disallow: /
The next is an example that tells all crawlers not to enter into four directories of a website: User-agent: * Disallow: /cgi-bin/ Disallow: /images/ Disallow: /tmp/ Disallow: /private/
Example that tells a specific crawler not to enter one specific directory: User-agent: BadBot Disallow: /private/
Example that tells all crawlers not to enter one specific file: User-agent: * Disallow: /directory/file.html
Note that all other files in the specified directory will be processed.
Example demonstrating how comments can be used: Comments appear after the "#" symbol at the start of a line, or after a directive User-agent: * # match all bots Disallow: / # keep them out
Compatibility
In order to prevent access to all pages by robots, do not use
Disallow: * # DO NOT USE! Use "/" instead.
as this is not a stable standard extension.
Instead: Disallow: /
should be used.
Crawl-delay directive
Several major crawlers support a Crawl-delay parameter, set to the number of seconds to wait between successive requests to the same server:
User-agent: * Crawl-delay: 10
Allow directive
Some major crawlers support an Allow directive which can counteract a following Disallow directive. This is useful when you disallow an entire directory but still want some HTML documents in that directory crawled and indexed. It should be noted that while by standard the first matching robots.txt pattern always wins, Google's implementation differs in that it first evaluates all Allow patterns and only then all Disallow patterns. Yet, in order to be compatible to all robots, if you want to allow single files inside an otherwise disallowed directory, you need to place the Allow directive(s) first, followed by the Disallow, for example:
Allow: /folder1/myfile.html Disallow: /folder1/
This example will Disallow anything in /folder1/ except /folder1/myfile.html, since the latter will match first. In case of Google, though, the order is not important.
User-agent: * Disallow: /downloads/ Request-rate: 1/5 # maximum rate is one page every 5 seconds Visit-time: 0600-0845 # only visit between 06:00 and 08:45 UTC (GMT)
The first version of the Robot Exclusion standard does not mention anything about the "*" character in the Disallow: statement. Modern crawlers like Googlebot and Slurp recognize strings containing "*", while MSNbot and Teoma interpret it in different ways.
