HTTP 404
Encyclopedia
The 404 or Not Found error message
is a HTTP
standard response code indicating that the client
was able to communicate with the server, but the server could not find what was requested. 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.
'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.
Webservers can typically be configured to display a customised error page, including a more natural description, the parent site's branding or sometimes a search form. 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 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. Another problem is that if the page does not provide a favicon
, and a separate custom 404 page exists, extra traffic and longer loading times will be generated.
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".
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.
Error message
An error message is information displayed when an unexpected condition occurs, usually on a computer or other device. On modern operating systems with graphical user interfaces, error messages are often displayed using dialog boxes...
is a HTTP
Hypertext Transfer Protocol
The Hypertext Transfer Protocol is a networking protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web....
standard response code indicating that the client
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
was able to communicate with the server, but the server could not find what was requested. A 404 error should not be confused with "server
Web server
Web server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....
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.
Overview
When communicating via HTTP, a server is required to respond to a request, such as a web browserWeb browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
's request for a web page
Web page
A web page or webpage is a document or information resource that is suitable for the World Wide Web and can be accessed through a web browser and displayed on a monitor or mobile device. This information is usually in HTML or XHTML format, and may provide navigation to other web pages via hypertext...
, 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
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
(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
File Transfer Protocol
File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...
and NNTP
Network News Transfer Protocol
The Network News Transfer Protocol is an Internet application protocol used for transporting Usenet news articles between news servers and for reading and posting articles by end user client applications...
.
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
Rewrite engine
A rewrite engine is software that modifies a web URL's appearance . Rewritten URLs are used to provide shorter and more relevant-looking links to web pages...
; 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
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
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.
Custom error pages
Internet Explorer
Internet Explorer
Windows Internet Explorer is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 that year...
(before Internet Explorer 7
Internet Explorer 7
Windows Internet Explorer 7 is a web browser released by Microsoft in October 2006. Internet Explorer 7 is part of a long line of versions of Internet Explorer and was the first major update to the browser in more than 5 years...
), however, will not display custom pages unless they are larger than 512 bytes, opting instead to display a "friendly" error page. Google Chrome
Google Chrome
Google Chrome is a web browser developed by Google that uses the WebKit layout engine. It was first released as a beta version for Microsoft Windows on September 2, 2008, and the public stable release was on December 11, 2008. The name is derived from the graphical user interface frame, or...
includes similar functionality, where the 404 is replaced with alternative suggestions generated by Google algorithms, if the page is under 512 bytes in size. Another problem is that if the page does not provide a favicon
Favicon
A favicon , also known as a shortcut icon, Web site icon, URL icon, or bookmark icon, is a file containing one small icons, most commonly 16×16 pixels, associated with a particular Web site or Web page...
, and a separate custom 404 page exists, extra traffic and longer loading times will be generated.
Soft 404
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 ApacheApache HTTP Server
The Apache HTTP Server, commonly referred to as Apache , is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million website milestone...
software, when an Error Document 404 (specified in a .htaccess
.htaccess
A .htaccess file is a directory-level configuration file supported by several web servers, that allows for decentralized management of web server configuration....
file) is specified as an absolute path (e.g.
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
BT Group
BT Group plc is a global telecommunications services company headquartered in London, United Kingdom. It is one of the largest telecommunications services companies in the world and has operations in more than 170 countries. Through its BT Global Services division it is a major supplier of...
deployed the Cleanfeed
Cleanfeed (content blocking system)
Cleanfeed is the name given to privately administered ISP level content filtering systems operating in the United Kingdom and Canada. It is also the name of a proposed mandatory Australian ISP level content filtering system which is undergoing testing...
content blocking system, which returns a 404 error to any request for content identified as potentially illegal by the Internet Watch Foundation
Internet Watch Foundation
The Internet Watch Foundation is a non-governmental charitable body based in the United Kingdom. It states that its remit is "to minimise the availability of 'potentially criminal' Internet content, specifically images of child sexual abuse hosted anywhere, and criminally obscene adult content in...
. Other ISPs return a HTTP 403
HTTP 403
In the HTTP protocol used on the World Wide Web, 403 Forbidden is an HTTP status code returned by a web server when a user requests a web page or media that the server does not allow them to. In other words, the server can be reached, but the server declined to allow access to the page. This...
"forbidden" error in the same circumstances. The practice of employing fake 404 errors as a means to conceal censorship
Censorship
thumb|[[Book burning]] following the [[1973 Chilean coup d'état|1973 coup]] that installed the [[Military government of Chile |Pinochet regime]] in Chile...
has also been reported in Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
and Tunisia
Tunisia
Tunisia , officially the Tunisian RepublicThe long name of Tunisia in other languages used in the country is: , is the northernmost country in Africa. It is a Maghreb country and is bordered by Algeria to the west, Libya to the southeast, and the Mediterranean Sea to the north and east. Its area...
. 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".
Slang usage
In 2008, a study carried out by the telecommunications arm of the Post OfficeRoyal Mail
Royal Mail is the government-owned postal service in the United Kingdom. Royal Mail Holdings plc owns Royal Mail Group Limited, which in turn operates the brands Royal Mail and Parcelforce Worldwide...
found that "404" had become a slang synonym for "clueless" in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
. 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.
404 page widgets
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—there are also much more advanced extensions available as widgets that endeavor to find the correct web page the user wanted.In popular culture
- During the May 2011 Greek protests, one of the most popular slogans was "Error 404: Democracy not found".
- In TunisiaTunisiaTunisia , officially the Tunisian RepublicThe long name of Tunisia in other languages used in the country is: , is the northernmost country in Africa. It is a Maghreb country and is bordered by Algeria to the west, Libya to the southeast, and the Mediterranean Sea to the north and east. Its area...
political censorship led to 404 becoming so ubiquitous that Tunisian bloggers invented a character called Ammar they held responsible for its deployment.
External links
- ErrorDocument Directive – instructions on custom error pages for the Apache 2.0 web serverApache HTTP ServerThe Apache HTTP Server, commonly referred to as Apache , is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million website milestone...
- Help for HTTP 404 'Not Found' errors
- HTTP Error Resource
- Wikipedia 404 error page