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Robots exclusion standard

Robots exclusion standardThe 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 harvestersspambotsmalware, 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.

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 WebCrawlerLycos and AltaVista

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] DuckDuckGo, [12] Google,[13] Yahoo!,[14] and Yandex.[15]

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."[16] 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".[17] This was in response to entire domains being tagged with robots.txt when the content became obsolete.

Despite the use of the terms "allow" and "disallow", the protocol is purely advisory.[18] 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,[19] 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."[20] In the context of robots.txt files, security through obscurity is not recommended as a security technique.

Many robots also pass a special user-agent to the web server when fetching content.[22] 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.

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