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. Hypertext Transfer Protocol ( HTTP) is a Communications protocol for the transfer of information on the Internet. HTTP persistent connections also called HTTP keep-alive or HTTP connection reuse is the idea of using the same TCP connection to send and receive multiple HTTP requests responses as HTTP compression is a capability built into both Web servers and Web browsers to make better use of available bandwidth HTTP Headers form the core of an HTTP request and are very important in an HTTP response An ETag (entity tag is an HTTP response header returned by an HTTP/1 HTTP cookies, or more commonly referred to as Web cookies tracking cookies or just cookies are parcels of text sent by a server to a Web client (usually The referer, or HTTP referer, identifies from the point of view of an Internet Webpage or resource the address of the webpage (commonly the URL The following is a list of HTTP response status codes and standard associated phrases intended to give a short textual description of the status The HTTP response status code 200 OK is used to represent a successful response from the server as defined in RFC 2616 The HTTP response status code 301 Moved Permanently is used for permanent redirection The HTTP response status code 302 Found is the most common way of performing a redirection The 403 Forbidden HTTP status code indicates that the client was able to communicate with the server but the server doesn't let the user access what was requested An error message is a message displayed when an unexpected condition occurs usually on a Computer or other device Hypertext Transfer Protocol ( HTTP) is a Communications protocol for the transfer of information on the Internet. The following is a list of HTTP response status codes and standard associated phrases intended to give a short textual description of the status A web browser is a software application which enables a user to display and interact with text images videos music games and other information typically located on a 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. The term web server can mean one of two things A Computer program that is responsible for accepting HTTP requests from web clients which are

Overview

A screenshot of a 404 error in Wikipedia with Camino. Camino (Spanish for way / path / road) is a free, Open source, GUI-based Web browser based on Mozilla

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. A web browser is a software application which enables a user to display and interact with text images videos music games and other information typically located on a HTML, an initialism of HyperText Markup Language, is the predominant Markup language for Web pages It provides a means to describe the structure A web page or webpage is a resource of information that is suitable for the World Wide Web and can be accessed through a Web browser. Multipurpose Internet Mail Extensions ( MIME) is an Internet standard that extends the format of e-mail to support text in Character In the code 404, the first "4" indicates a client error, such as a mistyped URL. Uniform Resource Locator is an URI which also specifies where the identified resource is available and the protocol for retrieving it 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. The Network News Transfer Protocol or NNTP is an Internet application protocol used primarily for reading

At the HTTP level, a 404 response code is followed by a human-readable "reason phrase". The HTTP specification suggests the phrase "Not Found"^[1] and many web servers by default issue an HTML page that includes both the 404 code and the "Not Found" phrase. Webservers can typically be configured to display a more natural description, a branded page or sometimes a search form, but the protocol level phrase, which is hidden from the user, is rarely customized.

Internet Explorer (before Internet Explorer 7), however, will not display custom pages unless they are larger than 512 bytes, opting to instead display a "friendly" error page. Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated MSIE) commonly abbreviated to IE, is a series of graphical 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. A rewrite engine also known as URL rewriting, is software which modifies a web URL 's appearance 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 webserver which does not exist. These are not 404 errors, which are always returned by a webserver.

The term "404", incidentally, is also sometimes used as an obscure gibe, the implication being that nobody's home upstairs: "He's a 404. "

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 whether a link is broken.

Some proxy servers generate a 404 error when the remote host is not present, rather than returning lower level errors such as hostname lookup failing, or "connection refused". This can confuse programs that expect and act on specific responses -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 illegal by the Internet Watch Foundation. BT Group plc (formerly British Telecommunications plc) which trades as BT (ˌbiːˈtiː bee tee) (previously known as British Telecom and still Cleanfeed is a Content blocking system implemented in the UK by BT, Britain's largest Internet provider which (as of 2008) targets only child sexual The Internet Watch Foundation (IWF is a self-regulated charitable body the only such recognised organisation in the UK operating an internet ‘Hotline’ for the public 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
• Rewrite engine
• List of HTTP status codes

References

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