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 can be used in conjunction with Sitemaps, a 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.
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.
On July 1, 2019 Google announced the proposal of the Robots Exclusion Protocol as an official standard under Internet Engineering Task Force. The draft will now undergo acceptance process.
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 doesn't exist, web robots assume that the website owner does not wish to place any limitations on crawling 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.
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, AOL, Baidu, DuckDuckGo, Google, Yahoo!, and Yandex. Bing is still not fully compatible with the standard as it cannot inherit settings from the wildcard (codice_1).
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."
For some years, the Internet Archive did not crawl sites with robots.txt, but in April 2017, it announced 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". This was in response to entire domains being tagged with robots.txt when the content became obsolete.
Security.
Despite the use of the terms "allow" and "disallow", the protocol is purely advisory 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, 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 codice_2 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 codice_3 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 codice_1 stands for all robots and the codice_5 directive has no value, meaning no pages are disallowed.
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:
This example tells all robots not to enter three directories:
This example tells all robots to stay away from one specific file:
All other files in the specified directory will be processed.
This example tells a specific robot to stay out of a website:
This example tells two specific robots not to enter one specific directory:
Example demonstrating how comments can be used:
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:
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 provides an interface in its search console for webmasters, to control the Googlebot's subsequent visits.
Allow directive.
Some major crawlers support an codice_6 directive, which can counteract a following codice_5 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 either the codice_6 or codice_5 directive, whichever is more specific, based on length, like Google.
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:
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 codice_10 directive, allowing multiple Sitemaps in the same <samp>robots.txt</samp> in the form codice_11:
Host.
Some crawlers (Yandex) support a codice_12 directive, allowing websites with multiple mirrors to specify their preferred domain:
This is not supported by all crawlers.
Universal "*" match.
The "Robot Exclusion Standard" does not mention the "*" character in the codice_13 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.
<meta name="robots" content="noindex" />
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.