The Robot Exclusion Standard, also known as the Robots Exclusion Protocol or robots.txt
protocol, is a convention to advising cooperating web crawlers and other web robots about 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 standard was proposed by Martijn Koster,
when working for Nexor in February, 1994 on the www-talk mailing list, the main communication
channel for WWW-related activities at the time.
Charles Stross claims to have provoked Koster to suggest robots.txt, after he wrote a badly-behaved
web spider that caused an inadvertent denial of service attack on Koster's server.
It quickly became a de facto standard that present and future web crawlers were expected
to follow; most complied, including those operated by search engines such as WebCrawler,
Lycos and AltaVista.
About the standard When a site owner wishes to give instructions
to web robots they place a text file called robots.txt in the root of the web site hierarchy.
This text file contains the instructions in a specific format.
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, and crawl the entire site.
A robots.txt file on a website will function as a request that specified robots ignore
specified files or directories when crawling a site.
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.
Links to pages listed in robots.txt can still appear in search results if they are linked
to from a page that is crawled.
A robots.txt file covers one origin.
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.
In addition, each protocol and port needs its own robots.txt file; http:example.com/robots.txt
does not apply to pages under https:example.com:8080/ or https:example.com/.
Some major search engines following this standard include Ask, AOL, Baidu, Bing, Google, Yahoo!,
and Yandex.
Disadvantages Despite the use of the terms "allow" and "disallow",
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 exclusion of all web robots.
In particular, malicious web robots are unlikely to honor robots.txt; some may even use the
robots.txt as a guide and go straight to the disallowed URLs.
While it is possible to prevent directory searches by anybody including web robots by
setting up the security of the server properly, when the disallow directives are provided
in the robots.txt file, the existence of these directories is disclosed to everyone.
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.
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.
Alternatives Many robots also pass a special user-agent
to the web server when fetching content.
A web administrator could also configure the server to automatically return failure when
it detects a connection using one of the robots.
Examples This example tells all robots that they can
visit all files because the wildcard * specifies all robots:
The same result can be accomplished with an empty or missing robots.txt file.
This example tells all robots to stay out of a website:
This example tells all robots not to enter three directories:
This example tells all robots to stay away from one specific file:
Note that all other files in the specified directory will be processed.
This example tells a specific robot to stay out of a website:
This example tells two specific robots not to enter one specific directory:
Example demonstrating how comments can be used:
It is also possible to list multiple robots with their own rules.
The actual robot string is defined by the crawler.
A few sites, such as Google, support several user-agent strings that allow the operator
to deny access to a subset of their services by using specific user-agent strings.
Example demonstrating multiple user-agents:
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:
Allow directive Some major crawlers support an Allow directive
which can counteract a following Disallow directive.
This is useful when one tells robots to avoid 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 either the Allow or Disallow directive, whichever is more specific, based on length,
like Google.
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:
This example will Disallow anything in except myfile.html, since the latter will match first.
The order is only important to robots that follow the standard; in the case of the Google
or Bing bots, the order is not important.
Sitemap Some crawlers support a Sitemap directive,
allowing multiple Sitemaps in the same robots.txt in the form:
Host Some crawlers support a Host directive, allowing
websites with multiple mirrors to specify their preferred domain.
Or alternatively
Note: This is not supported by all crawlers and if used, it should be inserted at the
bottom of the robots.txt file after Crawl-delay directive.
Universal "*" match The Robot Exclusion Standard does not mention
anything about the "*" character in the Disallow: statement.
Some crawlers like Googlebot recognize strings containing "*", while MSNbot and Teoma interpret
it in different ways.
Meta tags and headers In addition to root-level robots.txt files,
robots exclusion directives can be applied at a more granular level through the use of
Robots meta tags and X-Robots-Tag HTTP headers.
A "noindex" meta tag:
A "noindex" HTTP response header:
The X-Robots-Tag is only effective after the page has been requested and the server responds,
and the robots meta tag is only effective after the page has loaded, whereas robots.txt
is effective before the page is requested.
Also, the robots meta tag only works on HTML pages, not images, text files, PDF documents,
etc.
Finally, if the pages/resources have already been excluded by a robots.txt file, then they
will not be crawled and the meta tags and headers will have no effect.
This can have the counterintuitive effect that a web address is indexed by a search
engine such as Google if it honors the site's robots.txt, stops crawling and never receives
the advice not to index the site.
See also
Automated Content Access Protocol - a failed proposal to extend robots.txt
BotSeer - now inactive search engine for robots.txt files
Distributed web crawling Focused crawler
Internet Archive Library of Congress Digital Library project
National Digital Information Infrastructure and Preservation Program
Sitemaps Nofollow and Link Spam
Spider trap Web archiving
Web crawler Meta Elements for Search Engines
References
External links w3seo robots.txt - Example,maker
www.robotstxt.org - The Web Robots Pages