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The web site hosting server will typically generate a "404 Not Found" web page when a user attempts to follow a broken or dead link; hence the 404 error is one of the most recognizable errors users can find on the web.
A 404 error should not be confused with "server not found" or similar errors, in which a connection to the destination server could not be made at all. A 404 error indicates that the requested resource may be available again in the future; however, the fact does not guarantee the same content.
When communicating via HTTP, a server is required to respond to a request, such as a web browser's request for a web page, with a numeric response code and an optional, mandatory, or disallowed (based upon the status code) message. In the code 404, the first digit indicates a client error, such as a mistyped Uniform Resource Locator (URL). The following two digits indicate the specific error encountered. HTTP's use of three-digit codes is similar to the use of such codes in earlier protocols such as FTP and NNTP.
At the HTTP level, a 404 response code is followed by a human-readable "reason phrase". The HTTP specification suggests the phrase "Not Found" and many web servers by default issue an HTML page that includes both the 404 code and the "Not Found" phrase.
A 404 error is often returned when pages have been moved or deleted. In the first case, a better response is to return a 301 Moved Permanently response, which can be configured in most server configuration files, or through URL rewriting; in the second case, a 410 Gone should be returned. Because these two options require special server configuration, most websites do not make use of them.
404 errors should not be confused with DNS errors, which appear when the given URL refers to a server name that does not exist. A 404 error indicates that the server itself was found, but that the server was not able to retrieve the requested page.
Web servers can typically be configured to display a customised 404 error page, including a more natural description, the parent site's branding, and sometimes a site map, a search form or 404 page widget. The protocol level phrase, which is hidden from the user, is rarely customized.
Internet Explorer, however, will not display custom pages unless they are larger than 512 bytes, opting instead to display a "friendly" error page. Google Chrome includes similar functionality, where the 404 is replaced with alternative suggestions generated by Google algorithms, if the page is under 512 bytes in size. This goes against the recommendations of RFC 7231, which states that "User agents SHOULD display any included representation to the user".
A number of tools exist that crawl through a website to find pages that return 404 status codes. These tools can be helpful in finding links that exist within a particular website. The limitation of these tools is that they only find links within one particular website, and ignore 404s resulting from links on other websites. As a result, these tools miss out on 83% of the 404s on websites. One way around this is to find 404 errors by analyzing external links.
While many websites send additional information in a 404 error message—such as a link to the homepage of a website or a search box—some also endeavor to find the correct web page the user wanted. Extensions are available for some popular CMS to do this. In Europe, the NotFound Project encourages site operators to add a snippet of code to serve customised 404 error pages which provide data about missing children.
Some websites report a "not found" error by returning a standard web page with a "200 OK" response code; this is known as a soft 404. Soft 404s are problematic for automated methods of discovering whether a link is broken. Some search engines, like Yahoo, use automated processes to detect soft 404s. Soft 404s can occur as a result of configuration errors when using certain HTTP server software, for example with the Apache software, when an Error Document 404 (specified in a .htaccess file) is specified as an absolute path (e.g. http://example.com/error.html) rather than a relative path (/error.html).
Some proxy servers generate a 404 error when the remote host is not present, rather than returning the correct 500-range code when errors such as hostname resolution failures or refused TCP connections prevent the proxy server from satisfying the request. This can confuse programs that expect and act on specific responses, as they can no longer easily distinguish between an absent web server and a missing web page on a web server that is present.
In July 2004, the UK telecom provider BT Group deployed the Cleanfeed content blocking system, which returns a 404 error to any request for content identified as potentially illegal by the Internet Watch Foundation. Other ISPs return a HTTP 403 "forbidden" error in the same circumstances. The practice of employing fake 404 errors as a means to conceal censorship has also been reported in Thailand and Tunisia. In Tunisia, where censorship is reportedly severe, people have become aware of the nature of the fake 404 errors and have created an imaginary character named "Ammar 404" who represents "the invisible censor".
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In 2008, a study carried out by the telecommunications arm of the Post Office found that "404" had become a slang synonym for "clueless" in the United Kingdom. Slang lexicographer Jonathon Green said that "404" as a slang term had been driven by the "influence of technology" and young people, but at the time, such usage was relatively confined to London and other urban areas.
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