Robots exclusion standard

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 is different from, but can be used in conjunction with, Sitemaps, a robot inclusion standard for websites.

History

The invention of "robots.txt" is attributed to Martijn Koster, when working for WebCrawler around 1994. "robots.txt" was then popularized with the advent of AltaVista, and other popular search engines, in the following years.

About the standard

If a site owner wishes to give instructions to web robots they must place a text file called robots.txt in the root of the web site hierarchy (e.g. www.example.com/robots.txt). This text file should contain the instructions in a specific format (see examples below). Robots that choose to follow the instructions try to fetch this file and read the instructions before fetching any other file from the web site. If this file doesn't exist web robots assume that the web owner wishes to provide no specific instructions.

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 subdomains, each subdomain 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 would not apply to a.example.com.

Disadvantages

The protocol 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

Main article: Automated Content Access Protocol

ACAP, which is a possible plug-in for the Robots Exclusion Standard, was released as v1.0 on November 30, 2006.

Examples

This example allows all robots to visit all files because the wildcard "*" specifies all robots:

User-agent: *
Disallow:

This example keeps all robots out:

User-agent: *
Disallow: /

The next is an example that tells all crawlers not to enter 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 # replace the 'BadBot' with the actual user-agent of the bot
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

Nonstandard extensions

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 one disallows an entire directory but still wants some HTML documents in that directory crawled and indexed. While by standard implementation the first matching robots.txt pattern always wins, Google's implementation differs in that Allow patterns with equal or more characters in the directive path win over a matching Disallow pattern. Bing uses the Allow or Disallow directive which is the most specific.

In order to be compatible to all robots, if one wants to allow single files inside an otherwise disallowed directory, it is necessary 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.

Sitemap

Some crawlers support a Sitemap directive, allowing multiple Sitemaps in the same robots.txt in the form:

Sitemap: http://www.gstatic.com/s2/sitemaps/profiles-sitemap.xml
Sitemap: http://www.google.com/hostednews/sitemap_index.xml

The first version of the Robot Exclusion standard does not mention anything about the "*" character in the Disallow: statement. Some crawlers like Googlebot and Slurp recognize strings containing "*", while MSNbot and Teoma interpret it in different ways.

(source:wikipedia)