Robots exclusion standard
||This article contains instructions, advice, or how-to content. (November 2009)|
The Robot Exclusion Standard, also known as the Robots Exclusion Protocol or robots.txt protocol, is a convention to prevent cooperating web crawlers 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.
The invention of "robots.txt" is attributed to Martijn Koster, when working for Nexor then WebCrawler in 1994. "robots.txt" was then popularized with the advent of AltaVista, and other established 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. https://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, 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/.
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 will even jump straight into the disallowed sections.
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 (firstname.lastname@example.org). 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.
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 (or pass alternative content) when it detects a connection using one of the robots.
The following examples are from robotstxt.org
This example tells all robots to visit all files because the wildcard
* specifies all robots:
User-agent: * Disallow:
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:
User-agent: * Disallow: /
This example tells all robots not to enter three directories:
User-agent: * Disallow: /cgi-bin/ Disallow: /tmp/ Disallow: /junk/
This example tells all robots to stay away from one specific file:
User-agent: * Disallow: /directory/file.html
Note that all other files in the specified directory will be processed.
This example tells a specific robot to stay out of a website::
User-agent: BadBot # replace 'BadBot' with the actual user-agent of the bot Disallow: /
This example tells a specific robot not to enter one specific directory:
User-agent: BadBot # replace 'BadBot' with the actual user-agent of the bot Disallow: /private/
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
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 you to turn off a subset of their services by using specific user-agent strings.
Example demonstrating multiple user-agents:
User-agent: googlebot # all services Disallow: /private/ # disallow this directory User-agent: googlebot-news # only the news service Disallow: / # on everything User-agent: * # all robots Disallow: /something/ # on this folder
Nonstandard extensions 
Crawl-delay directive 
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 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 the
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. 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: http://www.gstatic.com/s2/sitemaps/profiles-sitemap.xml Sitemap: http://www.google.com/hostednews/sitemap_index.xml
Some crawlers (Yandex,Google) support a
Host directive, allowing websites with multiple mirrors to specify their preferred domain.
Note: This is not supported by all crawlers and if used, it should be inserted at the bottom of the host 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.
See also 
- Automated Content Access Protocol - a failed proposal to extend robots.txt
- BotSeer - search engine for robots.txt files
- Distributed web crawling
- Focused crawler
- Humans.txt - a file intended for humans to read as opposed to robots
- Internet Archive
- Library of Congress Digital Library project
- National Digital Information Infrastructure and Preservation Program
- Nofollow and Link Spam
- Spider trap
- Web archiving
- Web crawler
- Meta Elements for Search Engines
- Martijn, Koster. "Martijn Koster".
- "About Ask.com: Webmasters". Retrieved 16 February 2013.
- "About AOL Search". Retrieved 16 February 2013.
- "Baiduspider". Retrieved 16 February 2013.
- "Robots Exclusion Protocol - joining together to provide better documentation". Retrieved 16 February 2013.
- "Google Developers - Robots.txt Specifications". Retrieved 16 February 2013.
- "Submitting your website to Yahoo! Search". Retrieved 16 February 2013.
- "Using robots.txt". Retrieved 16 February 2013.
- Rick DeJarnette (10 August 2009). "Crawl delay and the Bing crawler, MSNBot". Retrieved 16 February 2013.
- "Webmaster Help Center - How do I block Googlebot?". Retrieved 2007-11-20.
- "How do I prevent my site or certain subdirectories from being crawled? - Yahoo Search Help". Retrieved 2007-11-20.
- "Google's Hidden Interpretation of Robots.txt". Retrieved 2010-11-15.
- "Yahoo! Search Blog - Webmasters can now auto-discover with Sitemaps". Retrieved 2009-03-23.
- "Yandex - Using robots.txt". Retrieved 2013-05-13.
- "Search engines and dynamic content issues". MSNbot issues with robots.txt. Retrieved 2007-04-01.
- www.robotstxt.org - The Web Robots Pages
- History of robots.txt - (how Charles Stross prompted its invention; original comment on Slashdot)
- Block or remove pages using a robots.txt file - Google Webmaster Tools Help = Using the robots.txt analysis tool
- About Robots.txt at the Mediawiki website
- List of Bad Bots - rogue robots and spiders which ignore these guidelines
- Wikipedia's Robots.txt - an example
- Robots.txt Generator + Tutorial
- Robots.txt Generator Tool