HTTP 404

A 404 error is presented to the user.

The 404 or Not Found error message is an HTTP standard response code indicating that the client was able to communicate with the server, but either the server could not find what was requested, or it was configured not to fulfill the request and not reveal the reason why. 404 errors should not be confused with "server not found" or similar errors, in which a connection to the destination server cannot be made at all.

Overview

When communicating via HTTP, a server is required to respond to a request, such as a web browser's request for an HTML document (web page), with a numeric response code and an email-like MIME message. In the code 404, the first "4" indicates a client error, such as a mistyped 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.

A common myth holds that the error code was named after a room 404 in the CERN laboratories, where the World Wide Web was invented. There is, however, no room 404 in CERN.

Each response code has an associated string of English text that must also be present; response code 404's associated string is "Not Found". When sending a 404 response, web servers usually include in the response message a short HTML document that mentions both the numeric code and this string. These messages can be customized on a large number of such servers to display a page that could be of more help than a default. For example, this can be achieved in Apache by placing a .htaccess file on the Web server or editing httpd.conf.

Internet Explorer, however, will not display custom pages unless they are larger than 512 bytes, opting to instead display a "friendly" error page. This default behaviour can be changed under Tools | Internet Options by clicking on the Advanced tab and un-checking the 'Show friendly HTTP error messages' check box.

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.

The popularity of the World Wide Web has led to the use of "404" as a neologism denoting missing a thing or person. Creating humorous 404 pages has become popular, and websites have been created for the sole purpose of linking to numerous amusing 404 error pages.

False 404 errors

Some websites report a "not found" error by returning a standard web page with a "200 OK" response code; this is called a soft 404. Soft 404s are problematic for automated methods of discovering if a link is broken or not. A heuristic for identifying soft 404s was given by Bar-Yossef, et al.

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 illegal by the Internet Watch Foundation. Governments that censor the Internet also often return a fake 404 error when a user tries to access a blocked website.

See also

• Dead link
• .htaccess
• Link rot
• List of HTTP status codes
• Rewrite engine

External links

• ErrorDocument Directive - instructions on custom error pages for the Apache 2.0 web server
• The Perfect 404 - Guide to building a custom 404 page
• 404 Research Lab - Various 404 error pages
• How To Troubleshoot 404 Not Found Messages - Some easy things to check when you get the 404 Not Found message