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 the server either could not find the file that was requested, or it was configured not to fulfill the request and not reveal the reason why.
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, sometimes followed by an email-like MIME message. 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. Many browsers opt to show this message to the user, while some browsers instead display a "friendlier" error message.
HTTP response codes have a specific interpretation. In 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.
These errors 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. Internet Explorer will not display these pages, however, unless they are larger than 512 bytes. Creating humorous 404 pages has become popular and websites (see external links below) have been created for the sole purpose of linking to numerous amusing 404 error pages.
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 is also a neologism denoting when a thing or person is not found.
Soft 404 Edit
A soft 404 is the reporting of a "not found" page via a 200 response code. 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 implemented the Cleanfeed content blocking system that returns a 404 error to any request for content identified as illegal by the Internet Watch Foundation.
- ↑ Sic Transit Gloria Telae: Towards an Understanding of the Web’s Decay, §3, submitted to the 13th World Wide Web Conference in New York City, 17–22 May 2004 (WWW 2004).