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_ robots.txt _
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Introduction
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The Robots Exclusion Protocol (often referred to by the filename
used to implement it, robots.txt) is a standard used by websites to
indicate to visiting web crawlers and other web robots which
portions of the website they are allowed to visit.
The standard, developed in 1994, relies on voluntary compliance.
Malicious bots can use the file as a directory of which pages to
visit, though standards bodies discourage countering this with
security through obscurity. Some archival sites ignore robots.txt.
The standard was used in the 1990s to mitigate server overload. In
the 2020s, websites began denying bots that collect information for
generative AI.
The "robots.txt" file can be used in conjunction with sitemaps,
another robot inclusion standard for websites.
History
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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 crawler that
inadvertently caused a denial-of-service attack on Koster's server.
The standard, initially RobotsNotWanted.txt, allowed web developers
to specify which bots should not access their website or which
pages bots should not access. The internet was small enough in 1994
to maintain a complete list of all bots; server overload was a
primary concern. By June 1994 it had become a 'de facto' standard;
most complied, including those operated by search engines such as
WebCrawler, Lycos, and AltaVista.
On July 1, 2019, Google announced the proposal of the Robots
Exclusion Protocol as an official standard under Internet
Engineering Task Force. A proposed standard was published in
September 2022 as RFC 9309.
Standard
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A site owner wishing to give instructions to web robots places a
text file called 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 does not exist, web
robots assume that the website owner does not wish to place any
limitations on crawling the entire site.
A robots.txt file contains instructions for bots indicating which
web pages they can and cannot access. Robots.txt files are
particularly important for web crawlers from search engines such as
Google.
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. Because compliant
crawlers cannot access the blocked page, search results may display
only the URL or other information available from external sources,
since the crawler cannot retrieve the page's content.
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 URI scheme and port needs
its own robots.txt file; does not apply to pages under or .
Compliance
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The robots.txt protocol is widely complied with by bot operators.
The robots.txt played a role in the 1999 legal case of 'eBay v.
Bidder's Edge', where eBay attempted to block a bot that did not
comply with robots.txt, and in May 2000 a court ordered the company
operating the bot to stop crawling eBay's servers using any
automatic means, by legal injunction on the basis of trespassing.
Bidder's Edge appealed the ruling, but agreed in March 2001 to drop
the appeal, pay an undisclosed amount to eBay, and stop accessing
eBay's auction information.
In 2007, 'Healthcare Advocates v. Harding', a company was sued for
accessing protected web pages archived via The Wayback Machine,
despite robots.txt rules that prevented those pages from being
archived. A Pennsylvania court ruled "in this situation, the
robots.txt file qualifies as a technological measure" under the
DMCA. Due to a malfunction at Internet Archive, Harding could
temporarily access these pages from the archive and thus the court
found "the Harding firm did not circumvent the protective measure".
In 2013, 'Associated Press v. Meltwater U.S. Holdings, Inc.' the
Associated Press sued Meltwater for copyright infringement and
misappropriation over copying of AP news items. Meltwater claimed
that they did not require a license and that it was fair use,
because the content was freely available and not protected by
robots.txt. The court decided in March 2013 that "Meltwater’s
copying is not protected by the fair use doctrine", mentioning
among several factors that "failure […] to employ the robots.txt
protocol did not give Meltwater […] license to copy and publish AP
content".
Search engines
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Some major search engines following this standard include Google,
Bing, Yahoo!, Yandex, DuckDuckGo, Baidu, AOL, Ask, and Kagi,
Archival sites
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Some web archiving projects ignore robots.txt. Archive Team uses
the file to discover more links, such as sitemaps. Co-founder Jason
Scott said that "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." In 2017, the
Internet Archive announced that it would stop complying with
robots.txt directives. According to 'Digital Trends', this followed
widespread use of robots.txt to remove historical sites from search
engine results, and contrasted with the nonprofit's aim to archive
"snapshots" of the internet as it previously existed.
Artificial intelligence
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Starting in the 2020s, web operators began using robots.txt to deny
access to bots collecting training data for generative AI. In 2023,
Originality.AI found that 306 of the thousand most-visited websites
blocked OpenAI's GPTBot in their robots.txt file and 85 blocked
Google's Google-Extended. Many robots.txt files named GPTBot as the
only bot explicitly disallowed on all pages. Denying access to
GPTBot was common among news websites such as the BBC and 'The New
York Times'. In 2023, blog host Medium announced it would deny
access to all artificial intelligence web crawlers as "AI companies
have leached value from writers in order to spam Internet readers".
GPTBot complies with the robots.txt standard and advises web
operators about how to disallow it, but 'The Verge's' David Pierce
said this only began after "training the underlying models that
made it so powerful". Also, some bots are used both for search
engines and artificial intelligence, and it may be impossible to
block only one of these options. '404 Media' reported that
companies like Anthropic and Perplexity.ai circumvented robots.txt
by renaming or spinning up new scrapers to replace the ones that
appeared on popular blocklists. To help identify the growing number
of web crawlers, Cloudflare maintains a public directory of known
bot user agents, organized by category and operator.
In 2025, the nonprofit RSL Collective announced the launch of the
Really Simple Licensing (RSL) open content licensing standard,
allowing web publishers to set terms for AI bots in their
robots.txt files. Participating companies at launch included
Medium, Reddit, and Yahoo.
Security
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Despite the use of the terms 'allow' and 'disallow', the protocol
is purely advisory and relies on the compliance of the web robot;
it cannot enforce any of what is stated in the file. 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, 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." In the context of robots.txt
files, security through obscurity is not recommended as a security
technique.
Alternatives
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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.
Some sites, such as Google, host a humans.txt file that displays
information meant for humans to read. Some sites such as GitHub
redirect humans.txt to an 'About' page.
Previously, Google had a joke file hosted at /killer-robots.txt
instructing the Terminator not to kill the company founders Larry
Page and Sergey Brin.
Examples
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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: *
Disallow:
This example has the same effect, allowing all files rather than
prohibiting none.
User-agent: *
Allow: /
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
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
This example tells one 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/
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.
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
The use of the wildcard * in rules
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The directive Disallow: /something/ blocks all files and
subdirectories starting with /something/.
In contrast, using a wildcard (if supported by the crawler) allows
for more complex patterns in specifying paths and files to allow or
disallow from crawling, for example Disallow: /something/*/other
blocks URLs such as:
/something/foo/other
/something/bar/other
It would not prevent the crawling of /something/foo/else, as that
would not match the pattern.
The wildcard * allows greater flexibility but may not be recognized
by all crawlers, although it is part of the Robots Exclusion
Protocol (RFC).
A wildcard at the end of a rule in effect does nothing, as that is
the standard behaviour.
Crawl-delay directive
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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. 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. Google ignores this
directive, but provides an interface in its search console for
webmasters, to control the Googlebot's subsequent visits.
User-agent: bingbot
Allow: /
Crawl-delay: 10 seconds
Sitemap
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Some crawlers support a Sitemap directive, allowing multiple
Sitemaps in the same robots.txt in the form Sitemap: 'full-url':
Sitemap: http://www.example.com/sitemap.xml
Universal "*" match
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The 'Robot Exclusion Standard' does not mention the "*" character
in the Disallow: statement.
Content-Signal
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Cloudflare introduced Content-Signal as a directive to suggest
acceptable crawler behavior by type, ai-train, ai-input, and search
with values of yes or no for each.
Content-Signal: ai-train=no, search=yes, ai-input=no
Meta tags and headers
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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.
A "noindex" HTTP response header
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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.
Maximum size of a robots.txt file
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The Robots Exclusion Protocol requires crawlers to parse at least
500 kibibytes (512000 bytes) of robots.txt files, which Google
maintains as a 500 kibibyte file size restriction for robots.txt
files.
See also
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* ads.txt, a standard for listing authorized ad sellers
* security.txt, a file to describe the process for security
researchers to follow in order to report security vulnerabilities
* trust.txt, a standard for news organizations
* 'eBay v. Bidder's Edge'
* 'hiQ Labs v. LinkedIn'
* 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
* Meta elements for search engines
* National Digital Library Program (NDLP)
* National Digital Information Infrastructure and Preservation
Program (NDIIPP)
* nofollow
* noindex
* Perma.cc
* Really Simple Licensing
* Sitemaps
* Spider trap
* Web archiving
{{No more links}}
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| DO NOT ADD EXTERNAL LINKS WITHOUT DISCUSSING THEM ON THE TALK
PAGE FIRST! |
| In particular, there are too many blogs, SEO pages and free
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| them all, and only listing a few of them unfairly gives those
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| advertising that the pages we don't list don't get.
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*
License
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All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/robots.txt
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