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 web sites. 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.

History

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 caused an inadvertent 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. https://www.example.com/robots.txt). 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 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.^[7]

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,^[8] AOL,^[9] Baidu,^[10] Bing,^[11] Google,^[12] Yahoo!,^[13] and Yandex.^[14]

Security

Despite the use of the terms "allow" and "disallow", the protocol is purely advisory.^[15] 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,^[16] 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."^[17] In the context of robots.txt files, security through obscurity is not recommended as a security technique.^[18]

Alternatives

Many robots also pass a special user-agent to the web server when fetching content.^[19] 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.^[20][21]

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:

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 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.^[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. 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.^[22]

User-agent: *
Crawl-delay: 10

Allow directive

Some major crawlers support an Allow directive, which can counteract a following Disallow directive.^{[23] [24]} 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.^[25] Bing uses either the Allow or Disallow directive, whichever is more specific, based on length, like Google.^[11]

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.

Sitemap

Some crawlers support a Sitemap directive, allowing multiple Sitemaps in the same robots.txt in the form:^[26]

Sitemap: http://www.gstatic.com/s2/sitemaps/profiles-sitemap.xml

Sitemap: http://www.google.com/hostednews/sitemap_index.xml

Host

Some crawlers (Yandex) support a Host directive, allowing websites with multiple mirrors to specify their preferred domain:^[27]

Host: example.com

Or alternatively:

Host: www.example.com

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.^[28]

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.^[28] Even if a robot honors robots.txt, it is still possible for the robot to find and index a disallowed URL from other places on the web. This can be prevented by using robots.txt directives in combination with robots meta tags or X-Robots-Tag headers.^[29]

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
• National Digital Library Program (NDLP)
• National Digital Information Infrastructure and Preservation Program (NDIIPP)
• Nofollow
• Meta elements for search engines
• Sitemaps
• Spider trap
• Web archiving
• Web crawler

External resources

• Robots Database (list of bot names)

References

External links

• Official website