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.
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 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 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 http://example.com:8080/ or https://example.com/.
Some major search engines following this standard include Ask, AOL, Baidu, DuckDuckGo, Google, Yahoo!, and Yandex. Bing still is not fully compatible with the standard 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.”
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.
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.
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.
User-agent: BadBot # replace 'BadBot' with the actual user-agent of the bot
This example tells a specific robot to stay out of a website:
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
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
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 : /
Some major crawlers support an Allow directive, which can counteract a following Disallow 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 Allow or Disallow 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.
Some crawlers support a Sitemap directive, allowing multiple Sitemaps in the same robots.txt in the form Sitemap: full-url:
1 Sitemap: http://www.example.com/sitemap.xml
Some crawlers (Yandex) support a Host directive, allowing websites with multiple mirrors to specify their preferred domain:
1 Host: hosting.example.com
This is not supported by all crawlers.
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
1 <meta name="robots" content="noindex" />
A “noindex” HTTP response header
1 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.