Robots exclusion standard

The robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to communicate with web crawlers and other web robots. The standard specifies how to inform the web robot about which areas of the website should not be processed or scanned. Robots are often used by search engines to categorize websites. Not all robots cooperate with the standard; email harvesters, spambots, malware and robots that scan for security vulnerabilities may even start with the portions of the website where they have been told to stay out. The standard is different from but can be used in conjunction with, Sitemaps, a robot inclusion standard for websites.


The standard was proposed by Martijn Koster,[1][2] when working for Nexor[3] in February 1994[4] 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 crawler that inadvertently caused a denial-of-service attack on Koster's server.[5]

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.[6]

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 (e.g. This text file contains 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 website. 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 operates 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.[7]

A robots.txt file covers one origin. For websites with multiple subdomains, each subdomain must have its own robots.txt file. If had a robots.txt file but did not, the rules that would apply for would not apply to In addition, each protocol and port needs its own robots.txt file; does not apply to pages under or

Some major search engines following this standard include Ask,[8] AOL,[9] Baidu,[10] DuckDuckGo,[11] Google,[12] Yahoo!,[13] and Yandex.[14] Bing[15] still is not fully[16] compatible with the standard[3] as it cannot inherit settings from the wildcard (*).

The volunteering group Archive Team explicitly ignores robots.txt for the most part, viewing it as an obsolete standard that hinders web archival efforts. According to project leader Jason Scott, "unchecked, and left alone, the robots.txt file ensures no mirroring or reference for items that may have general use and meaning beyond the website's context."[17]

For some years, the Internet Archive did not crawl sites with robots.txt, but in April 2017, it announced[18] that it would no longer honour directives in the robots.txt files. “Over time we have observed that the robots.txt files that are geared toward search engine crawlers do not necessarily serve our archival purposes".[19] This was in response to entire domains being tagged with robots.txt when the content became obsolete.[19]


Despite the use of the terms "allow" and "disallow", the protocol is purely advisory[20] and relies on the compliance of the web robot. Malicious web robots are unlikely to honor robots.txt; some may even use the robots.txt as a guide to find disallowed links and go straight to them. While this is sometimes claimed to be a security risk,[21] this sort of security through obscurity is discouraged by standards bodies. The National Institute of Standards and Technology (NIST) in the United States specifically recommends against this practice: "System security should not depend on the secrecy of the implementation or its components."[22] In the context of robots.txt files, security through obscurity is not recommended as a security technique.[23]


Many robots also pass a special user-agent to the web server when fetching content.[24] 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.[25][26]

Some sites, notably Google, host a humans.txt file that displays site contributor information.[27] Some sites such as GitHub redirect to an about page.[28] Previously Google also had a joke file hosted at /killer-robots.txt.[29]


This example tells all robots that they can visit all files because the wildcard * stands for all robots and the Disallow directive has no value, meaning no pages are disallowed.

User-agent: *

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 two specific robots not to enter one specific directory:

User-agent: BadBot # replace 'BadBot' with the actual user-agent of the bot
User-agent: Googlebot
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 robot operators, 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.[12]

Example demonstrating multiple user-agents:

User-agent: googlebot        # all Google services
Disallow: /private/          # disallow this directory

User-agent: googlebot-news   # only the news service
Disallow: /                  # disallow everything

User-agent: *                # any robot
Disallow: /something/        # disallow this directory

Nonstandard extensions

Crawl-delay directive

The crawl-delay value is supported by some crawlers to throttle their visits to the host. Since this value is not part of the standard, its interpretation is dependent on the crawler reading it. It is used when the multiple burst of visits from bots is slowing down the host. Yandex interprets the value as the number of seconds to wait between subsequent visits.[14] Bing defines crawl-delay as the size of a time window (from 1 to 30 seconds) during which BingBot will access a web site only once.[30] Google provides an interface in its search console for webmasters, to control the GoogleBot's subsequent visits.[31]

User-agent: *
Crawl-delay: 10

Allow directive

Some major crawlers support an Allow directive, which can counteract a following Disallow directive.[32][33] 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.[34] Bing uses either the Allow or Disallow directive, whichever is more specific, based on length, like Google.[15]

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: /directory1/myfile.html
Disallow: /directory1/

This example will Disallow anything in /directory1/ except /directory1/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.


Some crawlers support a Sitemap directive, allowing multiple Sitemaps in the same robots.txt in the form:[35]




Some crawlers (Yandex) support a Host directive, allowing websites with multiple mirrors to specify their preferred domain:[36]


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. The robots meta tag cannot be used for non-HTML files such as images, text files, or PDF documents. On the other hand, the X-Robots-Tag can be added to non-HTML files by using .htaccess and httpd.conf files.[37]

A "noindex" meta tag
<meta name="robots" content="noindex" />
A "noindex" HTTP response header
X-Robots-Tag: noindex

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. Thus if a page is excluded by a robots.txt file, any robots meta tags or X-Robots-Tag headers are effectively ignored because the robot will not see them in the first place.[37]

See also


  1. ^ "Historical". Archived from the original on 2017-04-03. Retrieved 2017-03-03.
  2. ^ Fielding, Roy (1994). "Maintaining Distributed Hypertext Infostructures: Welcome to MOMspider's Web" (PostScript). First International Conference on the World Wide Web. Geneva. Archived from the original on 2013-09-27. Retrieved September 25, 2013.
  3. ^ a b "The Web Robots Pages". 1994-06-30. Archived from the original on 2014-01-12. Retrieved 2013-12-29.
  4. ^ Koster, Martijn (25 February 1994). "Important: Spiders, Robots and Web Wanderers". www-talk mailing list. Archived from the original (Hypermail archived message) on October 29, 2013.
  5. ^ "How I got here in the end, part five: "things can only get better!"". Charlie's Diary. 19 June 2006. Archived from the original on 2013-11-25. Retrieved 19 April 2014.
  6. ^ Barry Schwartz (30 June 2014). "Robots.txt Celebrates 20 Years Of Blocking Search Engines". Search Engine Land. Archived from the original on 2015-09-07. Retrieved 2015-11-19.
  7. ^ "Uncrawled URLs in search results". YouTube. Oct 5, 2009. Archived from the original on 2014-01-06. Retrieved 2013-12-29.
  8. ^ "About Webmasters". Retrieved 16 February 2013.
  9. ^ "About AOL Search". Retrieved 16 February 2013.
  10. ^ "Baiduspider". Retrieved 16 February 2013.
  11. ^ "DuckDuckGo Bot". Retrieved 25 April 2017.
  12. ^ a b "Webmasters: Robots.txt Specifications". Google Developers. Archived from the original on 2013-01-15. Retrieved 16 February 2013.
  13. ^ "Submitting your website to Yahoo! Search". Archived from the original on 2013-01-21. Retrieved 16 February 2013.
  14. ^ a b "Using robots.txt". Archived from the original on 2013-01-25. Retrieved 16 February 2013.
  15. ^ a b "Robots Exclusion Protocol: joining together to provide better documentation". Archived from the original on 2014-08-18. Retrieved 16 February 2013.
  16. ^ "How to Create a Robots.txt File - Bing Webmaster Tools". Retrieved 2019-02-06.
  17. ^ Jason Scott. "Robots.txt is a suicide note". Archive Team. Archived from the original on 2017-02-18. Retrieved 18 February 2017.
  18. ^ "Robots.txt meant for search engines don't work well for web archives | Internet Archive Blogs". Archived from the original on 2018-12-04. Retrieved 2018-12-01.
  19. ^ a b Jones, Brad (24 April 2017). "The Internet Archive Will Ignore Robots.txt Files to Maintain Accuracy". Digital Trends. Archived from the original on 2017-05-16. Retrieved 8 May 2017.
  20. ^ "Block URLs with robots.txt: Learn about robots.txt files". Google. Archived from the original on 2015-08-14. Retrieved 2015-08-10.
  21. ^ "Robots.txt tells hackers the places you don't want them to look". The Register. Archived from the original on 2015-08-21. Retrieved August 12, 2015.
  22. ^ "Guide to General Server Security" (PDF). National Institute of Standards and Technology. July 2008. Archived (PDF) from the original on 2011-10-08. Retrieved August 12, 2015.
  23. ^ Sverre H. Huseby (2004). Innocent Code: A Security Wake-Up Call for Web Programmers. John Wiley & Sons. pp. 91–92. ISBN 9780470857472. Archived from the original on 2016-04-01. Retrieved 2015-08-12.
  24. ^ "List of User-Agents (Spiders, Robots, Browser)". Archived from the original on 2014-01-07. Retrieved 2013-12-29.
  25. ^ "Access Control - Apache HTTP Server". Archived from the original on 2013-12-29. Retrieved 2013-12-29.
  26. ^ "Deny Strings for Filtering Rules : The Official Microsoft IIS Site". 2013-11-06. Archived from the original on 2014-01-01. Retrieved 2013-12-29.
  27. ^ "Archived copy". Archived from the original on 2017-01-24. Retrieved 2017-01-24.CS1 maint: Archived copy as title (link)
  28. ^ "Archived copy". Archived from the original on 2016-05-30. Retrieved 2017-01-24.CS1 maint: Archived copy as title (link)
  29. ^ "Wayback Machine". 2018-01-10. Retrieved 2018-05-25.
  30. ^ "To crawl or not to crawl, that is BingBot's question". 3 May 2012. Archived from the original on 2016-02-03. Retrieved 9 February 2016.
  31. ^ "Change Googlebot crawl rate - Search Console Help". Archived from the original on 2018-11-18. Retrieved 22 October 2018.
  32. ^ "Webmaster Help Center - How do I block Googlebot?". Archived from the original on 2010-08-01. Retrieved 2007-11-20.
  33. ^ "How do I prevent my site or certain subdirectories from being crawled? - Yahoo Search Help". Archived from the original on 2007-10-13. Retrieved 2007-11-20.
  34. ^ "Google's Hidden Interpretation of Robots.txt". Archived from the original on 2010-11-20. Retrieved 2010-11-15.
  35. ^ "Yahoo! Search Blog - Webmasters can now auto-discover with Sitemaps". Archived from the original on 2009-03-05. Retrieved 2009-03-23.
  36. ^ "Yandex - Using robots.txt". Archived from the original on 2013-05-09. Retrieved 2013-05-13.
  37. ^ a b "Robots meta tag and X-Robots-Tag HTTP header specifications - Webmasters — Google Developers". Archived from the original on 2013-08-08. Retrieved 2013-08-17.

External links


ALIWEB (Archie Like Indexing for the WEB) is considered the first Web search engine, as its predecessors were either built with different purposes (the Wanderer, Gopher) or were literally just indexers (Archie, Veronica and Jughead).

First announced in November 1993 by developer Martijn Koster while working at Nexor, and presented in May 1994 at the First International Conference on the World Wide Web at CERN in Geneva, ALIWEB preceded WebCrawler by several months.ALIWEB allowed users to submit the locations of index files on their sites which enabled the search engine to include webpages and add user-written page descriptions and keywords. This empowered webmasters to define the terms that would lead users to their pages, and also avoided setting bots (e.g. the Wanderer, JumpStation) which used up bandwidth. As relatively few people submitted their sites, ALIWEB was not very widely used.

Martijn Koster, who was also instrumental in the creation of the Robots Exclusion Standard, detailed the background and objectives of ALIWEB with an overview of its functions and framework in the paper he presented at CERN.Koster is not associated with a commercial website which uses the aliweb name.


Allow may refer to:

River Allow, river in Ireland

Allow, a low-carbon aluminium brand by Rusal

"Allow", a directive in the website robots exclusion standard

"Allow", a song on the 2016 album Bad Hair Extensions

Automated Content Access Protocol

Automated Content Access Protocol ("ACAP") was proposed in 2006 as a method of providing machine-readable permissions information for content, in the hope that it would have allowed automated processes (such as search-engine web crawling) to be compliant with publishers' policies without the need for human interpretation of legal terms. ACAP was developed by organisations that claimed to represent sections of the publishing industry (World Association of Newspapers, European Publishers Council, International Publishers Association). It was intended to provide support for more sophisticated online publishing business models, but was criticised for being biased towards the fears of publishers who see search and aggregation as a threat rather than as a source of traffic and new readers.


Bingbot is a web-crawling robot (type of internet bot), deployed by Microsoft October 2010 to supply Bing. It collects documents from the web to build a searchable index for the Bing (search engine). It performs the same function as Google's Googlebot.

A typical user agent string for Bingbot is "Mozilla/5.0 (compatible; bingbot/2.0; +". This appears in the web server logs to tell the webmaster who is requesting a file. Each webmaster is able to use the included agent identifier, "bingbot", to disallow or allow access to their site (by default access is allowed). If they don't want to grant access they can use the Robots Exclusion Standard to block it (relying on the assumed good behaviour of bingbot), or use other server specific means (relying on the web server to do the blocking).


BotSeer was a Web-based information system and search tool used for research on Web robots and trends in Robot Exclusion Protocol deployment and adherence. It was created and designed by Yang Sun, Isaac G. Councill, Ziming Zhuang and C. Lee Giles. BotSeer is now inactive; the original URL was

Deep linking

In the context of the World Wide Web, deep linking is the use of a hyperlink that links to a specific, generally searchable or indexed, piece of web content on a website (e.g. ""), rather than the website's home page (e.g., ""). The URL contains all the information needed to point to a particular item, in this case the "Example" section of the English Wikipedia article entitled "Deep linking", as opposed to only the information needed to point to the highest-level home page of Wikipedia at

Deep web

The deep web, invisible web, or hidden web are parts of the World Wide Web whose contents are not indexed by standard web search engines. The opposite term to the deep web is the surface web, which is accessible to anyone using the Internet. Computer scientist Michael K. Bergman is credited with coining the term deep web in 2001 as a search indexing term.The content of the deep web is hidden behind HTTP forms and includes many very common uses such as web mail, online banking, and services that users must pay for, and which are protected by paywalls, such as video on demand and some online magazines and newspapers.

The content of the deep web can be located and accessed by a direct URL or IP address, and may require a password or other security access past the public website page.

Employment website

An employment website is a website that deals specifically with employment or careers. Many employment websites are designed to allow employers to post job requirements for a position to be filled and are commonly known as job boards. Other employment sites offer employer reviews, career and job-search advice, and describe different job descriptions or employers. Through a job website a prospective employee can locate and fill out a job application or submit resumes over the Internet for the advertised position.


HTTrack is a free and open-source Web crawler and offline browser, developed by Xavier Roche and licensed under the GNU General Public License Version 3.

HTTrack allows users to download World Wide Web sites from the Internet to a local computer. By default, HTTrack arranges the downloaded site by the original site's relative link-structure. The downloaded (or "mirrored") website can be browsed by opening a page of the site in a browser.

HTTrack can also update an existing mirrored site and resume interrupted downloads. HTTrack is configurable by options and by filters (include/exclude), and has an integrated help system. There is a basic command line version and two GUI versions (WinHTTrack and WebHTTrack); the former can be part of scripts and cron jobs.

HTTrack uses a Web crawler to download a website. Some parts of the website may not be downloaded by default due to the robots exclusion protocol unless disabled during the program. HTTrack can follow links that are generated with basic JavaScript and inside Applets or Flash, but not complex links (generated using functions or expressions) or server-side image maps.

Martijn Koster

Martijn Koster (born ca 1970) is a Dutch software engineer noted for his pioneering work on Internet searching.

Koster created Aliweb, the Internet's first Search Engine, which was announced in November 1993 while working at Nexor and presented in May 1994 at the First International Conference on the World Wide Web. Koster also developed Achiplex, a search engine for FTP sites that pre-dates the Web, and CUSI, a simple tool that allowed you to search different search engines in quick succession, useful in the early days of search when services provided varying results.

Koster also created the Robots Exclusion Standard.


Nexor Limited is a privately held company based in Nottingham, providing product and services to safeguard government, defence and critical national infrastructure computer systems. It was originally known as X-Tel Services Limited.


nofollow is a value that can be assigned to the rel attribute of an HTML a element to instruct some search engines that the hyperlink should not influence the ranking of the link's target in the search engine's index. It is intended to reduce the effectiveness of certain types of internet advertising because their search algorithm depends heavily on the number of links to a website when determining which websites should be listed in what order in their search results for any given term.


The noindex value of an HTML robots meta tag requests that automated Internet bots avoid indexing a web page. Reasons why one might want to use this meta tag include advising robots not to index a very large database, web pages that are very transitory, web pages that are under development, web pages that one wishes to keep slightly more private, or the printer and mobile-friendly versions of pages. Since the burden of honoring a website's noindex tag lies with the author of the search robot, sometimes these tags are ignored. Also the interpretation of the noindex tag is sometimes slightly different from one search engine company to the next.

Processing Instruction

A Processing Instruction (PI) is an SGML and XML node type, which may occur anywhere in the document, intended to carry instructions to the application.Processing instructions are exposed in the Document Object Model as Node.PROCESSING_INSTRUCTION_NODE, and they can be used in XPath and XQuery with the 'processing-instruction()' command.

Spider trap

A spider trap (or crawler trap) is a set of web pages that may intentionally or unintentionally be used to cause a web crawler or search bot to make an infinite number of requests or cause a poorly constructed crawler to crash. Web crawlers are also called web spiders, from which the name is derived. Spider traps may be created to "catch" spambots or other crawlers that waste a website's bandwidth. They may also be created unintentionally by calendars that use dynamic pages with links that continually point to the next day or year.

Common techniques used are:

creation of indefinitely deep directory structures like

Dynamic pages that produce an unbounded number of documents for a web crawler to follow. Examples include calendars and algorithmically generated language poetry.

documents filled with a large number of characters, crashing the lexical analyzer parsing the document.

documents with session-id's based on required cookies.There is no algorithm to detect all spider traps. Some classes of traps can be detected automatically, but new, unrecognized traps arise quickly.

User agent

In computing, a user agent is software (a software agent) that is acting on behalf of a user. One common use of the term refers to a web browser that "retrieves, renders and facilitates end user interaction with Web content".There are other uses of the term "user agent". For example, an email reader is a mail user agent. In many cases, a user agent acts as a client in a network protocol used in communications within a client–server distributed computing system. In particular, the Hypertext Transfer Protocol (HTTP) identifies the client software originating the request, using a user-agent header, even when the client is not operated by a user. The Session Initiation Protocol (SIP) protocol (based on HTTP) followed this usage. In the SIP, the term user agent refers to both end points of a communications session.

Wayback Machine

The Wayback Machine is a digital archive of the World Wide Web and other information on the Internet. It was launched in 2001 by the Internet Archive, a nonprofit organization based in San Francisco, California, United States.


GNU Wget (or just Wget, formerly Geturl, also written as its package name, wget) is a computer program that retrieves content from web servers. It is part of the GNU Project. Its name derives from World Wide Web and get. It supports downloading via HTTP, HTTPS, and FTP.

Its features include recursive download, conversion of links for offline viewing of local HTML, and support for proxies. It appeared in 1996, coinciding with the boom of popularity of the Web, causing its wide use among Unix users and distribution with most major Linux distributions. Written in portable C, Wget can be easily installed on any Unix-like system. Wget has been ported to Microsoft Windows, Mac OS X, OpenVMS, HP-UX, MorphOS and AmigaOS. Since version 1.14 Wget has been able to save its output in the web archiving standard WARC format.It has been used as the basis for graphical programs such as GWget for the GNOME Desktop.

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