Robots.txt. robots.txt is the filename used for implementing the Robots Exclusion Protocol, 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 artificial intelligence.

The "robots.txt" file can be used in conjunction with sitemaps, another 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 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
When a site owner wishes to give instructions to web robots they place 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. Additionally, optimizing the robots.txt file can help websites prioritize valuable pages and avoid search engines wasting their crawl budget on irrelevant or duplicate content, which improves overall SEO performance.

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.

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 .

Compliance
The robots.txt protocol is commonly followed by major search engines and web crawlers. It gained legal recognition during the case of eBay v. Bidder's Edge, where eBay successfully argued that unauthorized web crawling could constitute trespassing. The court issued an injunction ordering the company to stop automated crawling of eBay’s servers. This decision reinforced the significance of respecting robots.txt directives among legitimate operators.

Search engines
Some major search engines following this standard include Ask, AOL, Baidu, Bing,  DuckDuckGo, Kagi, Google, Yahoo!, and Yandex.

Archival sites
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
In the 2020s, website operators increasingly used robots.txt files to block bots that collected data for generative AI models. By 2023, Originality.AI reported that 306 of the thousand most-visited websites had blocked OpenAI's GPTBot, while 85 had blocked Google's Google-Extended bot. Many websites explicitly listed GPTBot as the only bot disallowed on all pages. News organizations, such as the BBC and The New York Times, commonly restricted GPTBot’s access.

In 2023, the blogging platform Medium announced it would block all AI web crawlers, stating that "AI companies have extracted value from writers to flood readers with spam-like content."

GPTBot adheres to the robots.txt protocol and offers instructions to website operators on how to block it. However, The Verges David Pierce noted that OpenAI began providing these guidelines only after completing the training of its powerful models. Additionally, some bots serve dual purposes, functioning both as search engine crawlers and AI data collectors, making it difficult for operators to block only one function.

According to 404 Media, companies like Anthropic and Perplexity.ai have bypassed robots.txt restrictions by launching new scrapers or renaming existing ones to evade popular blocklists.

Security
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
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
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.

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.

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

Difference Between Disallow: /something/ and Disallow: /something/* 
The directive Disallow: /something/ blocks all files and subdirectories starting with /something/. In contrast, Disallow: /something/* (if supported by the crawler) blocks URLs with more complex patterns, such as /something-folder/page. The wildcard * allows greater flexibility but may not be recognized by all crawlers.

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. 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

Sitemap
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
The Robot Exclusion Standard does not mention the "*" character in the Disallow: statement.

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.

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.

Maximum size of a robots.txt file
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

 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
 eBay v. Bidder's Edge
 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
 Sitemaps
 Spider trap
 Web archiving
 Web crawler

References

Further reading

External links

 

Search engine optimization
Websites
Web scraping
Text files