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CONTENTS

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 * (Top)
 * 1Overview
 * 2History
 * 3Soft 404 errors
   Toggle Soft 404 errors subsection
   * 3.1Proxy servers
   * 3.2Intentional 404s
 * 4Microsoft Internet Server 404 substatus error codes
   Toggle Microsoft Internet Server 404 substatus error codes subsection
   * 4.1Substatus codes
 * 5Custom error pages
 * 6Tracking 404 errors
 * 7Causes
 * 8Solutions
 * 9See also
 * 10References
 * 11External links

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HTTP 404

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From Wikipedia, the free encyclopedia

Internet error message
"404 Not Found" redirects here. For the TV episode, see 404 Not Found (Mr.
Robot).



HTTP
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Response status codes
 * 301 Moved Permanently
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 * 404 Not Found
 * 451 Unavailable for Legal Reasons

Security access control methods
 * Basic access authentication
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Security vulnerabilities
 * HTTP header injection
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 * HTTP parameter pollution

 * v
 * t
 * e

In computer network communications, the HTTP 404, 404 not found, 404, 404 error,
page not found or file not found error message is a hypertext transfer protocol
(HTTP) standard response code, to indicate that the browser was able to
communicate with a given server, but the server could not find what was
requested. The error may also be used when a server does not wish to disclose
whether it has the requested information.[1]

The website 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 encountered on the World Wide Web.

English Wikipedia's 404 Page


OVERVIEW

When communicating via HTTP, a server is required to respond to a request, such
as a web browser request for a web page, with a numeric response code and an
optional, mandatory, or disallowed (based upon the status code) message. In 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"[1] 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, it is better to employ URL mapping or URL redirection by returning 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.


HISTORY

The origin of the 404 error code dates back to the early days of the World Wide
Web. In 1992, Tim Berners-Lee, the inventor of the Web, and his team at CERN,
the European Organization for Nuclear Research, created the first web server
software, called CERN httpd.[2] This software used a simple file system to store
and retrieve web pages, and it assigned a three-digit number to each type of
request and response. The number 404 was chosen to indicate that the requested
file was not found on the server.[3]

The term "404 Not Found" was coined by Berners-Lee himself, who explained in a
1998 interview that he wanted to make the error message "slightly
apologetic".[4][3] He also said that he considered using "400 Bad Request"
instead, but decided that it was too vague and technical.[4][3]

The first documented case of a 404 error appearing on a web page was in 1993,
when a user tried to access a page about the Mosaic web browser on the NCSA
website. The page had been moved to a different location, but the link had not
been updated.[2] The user reported the error to the NCSA team, who fixed the
link and added a humorous message to their 404 page: "We're sorry, but the
document you requested is not here. Maybe you should try someplace else."[3]

Since then, 404 errors have become one of the most common and recognizable
errors on the Web. Many websites have customized their 404 pages with creative
designs, messages, or features to entertain or assist their visitors. For
example, Google's 404 page features a broken robot and a link to its
homepage,[5] while GitHub's 404 page shows a random image of a parallax star
field and a link to its status page.[6] Some websites have also used their 404
pages to showcase their brand personality, humor, or social causes. For
instance, Lego's 404 page shows an image of a Lego character searching for a
missing brick,[7] while Amazon's 404 page displays the image of a dog with a
message about conservation.[8]


SOFT 404 ERRORS



Some websites report a "not found" error by returning a standard web page with a
"200 OK" response code, falsely reporting that the page loaded properly; this is
known as a soft 404. The term "soft 404" was introduced in 2004 by Ziv
Bar-Yossef et al.[9]

Soft 404s are problematic for automated methods of discovering whether a link is
broken. Some search engines, like Yahoo and Google, use automated processes to
detect soft 404s.[10] 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).[11] This can also be done on purpose to force some browsers (like
Internet Explorer) to display a customized 404 error message rather than
replacing what is served with a browser-specific "friendly" error message (in
Internet Explorer, this behavior is triggered when a 404 is served and the
received HTML is shorter than a certain length, and can be manually disabled by
the user).

There are also "soft 3XX" errors where content is returned with a status 200 but
comes from a redirected page, such as when missing pages are redirected to the
domain root/home page.


PROXY SERVERS

Some proxy servers generate a 404 error when a 500-range error code would be
more correct. If the proxy server is unable to satisfy a request for a page
because of a problem with the remote host (such as hostname resolution failures
or refused TCP connections), this should be described as a 5xx Internal Server
Error, but might deliver a 404 instead. 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.


INTENTIONAL 404S

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.[12] Other ISPs return a
HTTP 403 "forbidden" error in the same circumstances.[13] The practice of
employing fake 404 errors as a means to conceal censorship has also been
reported in Thailand[14] and Tunisia.[15] In Tunisia, where censorship was
severe before the 2011 revolution, people became aware of the nature of the fake
404 errors and created an imaginary character named "Ammar 404" who represents
"the invisible censor".[16]


MICROSOFT INTERNET SERVER 404 SUBSTATUS ERROR CODES

The webserver software developed by Microsoft, Microsoft's Internet Information
Services (IIS), returns a set of substatus codes with its 404 responses. The
substatus codes take the form of decimal numbers appended to the 404 status
code. The substatus codes are not officially recognized by IANA and are not
returned by non-Microsoft servers.


SUBSTATUS CODES

Microsoft's IIS 7.0, IIS 7.5, and IIS 8.0 servers define the following HTTP
substatus codes to indicate a more specific cause of a 404 error:

 * 404.0 – Not found.
 * 404.1 – Site Not Found.
 * 404.2 – ISAPI or CGI restriction.
 * 404.3 – MIME type restriction.
 * 404.4 – No handler configured.
 * 404.5 – Denied by request filtering configuration.
 * 404.6 – Verb denied.
 * 404.7 – File extension denied.
 * 404.8 – Hidden namespace.
 * 404.9 – File attribute hidden.
 * 404.10 – Request header too long.
 * 404.11 – Request contains double escape sequence.
 * 404.12 – Request contains high-bit characters.
 * 404.13 – Content length too large.
 * 404.14 – Request URL too long.
 * 404.15 – Query string too long.
 * 404.16 – DAV request sent to the static file handler.
 * 404.17 – Dynamic content mapped to the static file handler via a wildcard
   MIME mapping.
 * 404.18 – Query string sequence denied.
 * 404.19 – Denied by filtering rule.
 * 404.20 – Too Many URL Segments.


CUSTOM ERROR PAGES

The Wikimedia 404 message

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.[17] Google Chrome included similar
functionality, where the 404 is replaced with alternative suggestions generated
by Google algorithms, if the page is under 512 bytes in size.[18] 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 on
every page view.[19][20]

Many organizations use 404 error pages as an opportunity to inject humor into
what may otherwise be a serious website. For example, Metro UK shows a polar
bear on a skateboard, and the web development agency Left Logic has a simple
drawing program.[21] During the 2015 UK general election campaign the main
political parties all used their 404 pages to either take aim at political
opponents or show relevant policies to potential supporters.[22] In Europe, the
NotFound project, created by multiple European organizations including Missing
Children Europe and Child Focus, encourages site operators to add a snippet of
code to serve customized 404 error pages[23] which provide data about missing
children.[24]

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 content
management systems (CMSs) to do this.[25]


TRACKING 404 ERRORS

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.[26]
One way around this is to find 404 errors by analyzing external links.[27]

One of the most effective ways to discover 404 errors is by using Google Search
Console, Google Analytics or crawling software.

Another common method is tracking traffic to 404 pages using log file
analysis.[28] This can be useful to understand more about what 404s users
reached on the site. Another method of tracking traffic to 404 pages is using
JavaScript-based traffic tracking tools.[29]


CAUSES

There are many possible causes for a page not to exist. Some of the common ones
are:[30][31][32]

 * The page was deleted by the owner or administrator of the website.
 * The page was moved to a different location or renamed without updating the
   links that point to it.
 * The page was never created in the first place or is still under construction.
 * The page is temporarily unavailable due to maintenance or technical issues.
 * The page is blocked by the user's network or firewall settings.
 * The page is restricted by the website's privacy or security policies.
 * The page contains illegal or harmful content that was removed by the
   authorities or the website itself.


SOLUTIONS

If a user encounters a page that doesn't exist, there are some steps they can
take to try to find the information they are looking for or to report the
problem.[30][31][32]

 * Check the URL of the page. Sometimes, a simple typo or spelling mistake can
   cause a page not to load. Make sure the correct address is entered and try
   again.
 * Refresh the page. Sometimes, a temporary glitch or network issue can prevent
   a page from loading. Try reloading the page by pressing F5 or clicking on the
   refresh button on the browser.
 * Go back to the previous page. Sometimes, a link might be broken or outdated.
   Try going back to the page where the link was found and see if there is an
   updated or alternative link to the same information.
 * Use a search engine. Sometimes, a page might be indexed by a search engine
   even if it doesn't exist anymore. Try searching for keywords related to the
   topic of the page and see if other sources of information can be found.
 * Contact the website owner or administrator. Sometimes, a page might be
   removed or moved without notice. Try contacting the person or organization
   responsible for the website and ask them about the status of the page. Their
   contact information can usually be found on their homepage or in their footer
   section.
 * Report the error. Sometimes, a page might not exist due to an error on the
   website's part. Try reporting the error to the website owner or administrator
   so they can fix it as soon as possible. Their feedback form or email address
   can usually be found on their homepage or in their footer section.


SEE ALSO

 * Internet portal
 * Computer programming portal

 * Blue screen of death
 * Funky caching
 * Link rot
 * List of HTTP status codes


REFERENCES

 1.  ^ Jump up to: a b Fielding, R.; Reschke, J. (June 2014). Fielding, R;
     Reschke, J (eds.). "RFC 7231, HTTP/1.1 Semantics and Content, Section 6.5.4
     404 Not Found". ietf.org. doi:10.17487/RFC7231. S2CID 14399078. Retrieved
     13 December 2018. {{cite journal}}: Cite journal requires |journal= (help)
 2.  ^ Jump up to: a b "404 page design: best practices and awesome examples".
     www.justinmind.com. Retrieved 19 May 2023. {{cite web}}: Cite uses generic
     title (help)
 3.  ^ Jump up to: a b c d "What is a 404 error and what should I do if I get
     one? » Internet » Windows » Tech Ease". Retrieved 19 May 2023.
 4.  ^ Jump up to: a b What is the world wide web? - Twila Camp, retrieved 19
     May 2023
 5.  ^ "Google 404 Error Page". Google. {{cite web}}: Check |archive-url= value
     (help)CS1 maint: url-status (link)
 6.  ^ "Github 404 Error Page". Github. {{cite web}}: Check |archive-url= value
     (help)CS1 maint: url-status (link)
 7.  ^ "LEGO 404 Error Page". Lego. {{cite web}}: Check |archive-url= value
     (help)CS1 maint: url-status (link)
 8.  ^ "Amazon's 404 error page". Amazon. {{cite web}}: Check |archive-url=
     value (help)CS1 maint: url-status (link)
 9.  ^ Ziv Bar-Yossef; Andrei Z. Broder; Ravi Kumar; Andrew Tompkins (2004). Sic
     Transit Gloria Telae: Towards an Understanding of the Web's Decay.
     Proceedings of the 13th International Conference on World Wide Web (WWW).
     pp. 328–337. doi:10.1145/988672.988716. ISBN 978-1581138443. S2CID 587547.
 10. ^ "Why is your crawler asking for strange URLs that have never existed on
     my site?". Yahoo Ysearch Help page. Archived from the original on 15 July
     2014. Retrieved 4 September 2013.
 11. ^ "Farewell to soft 404s". Google Official Blog. Retrieved 20 September
     2008.
 12. ^ "LINX Public Affairs » Cleanfeed: the facts". Publicaffairs.linx.net. 10
     September 2004. Archived from the original on 13 May 2011. Retrieved 6
     March 2011.
 13. ^ "DEMON – Error 403". Retrieved 14 June 2012.
 14. ^ Sambandaraksa, Don (18 February 2009). "The old fake '404 Not Found'
     routine - Dead link". Bangkok Post. Retrieved 12 September 2010.
 15. ^ Noman, Helmi (12 September 2008). "Tunisian journalist sues government
     agency for blocking Facebook, claims damage for the use of 404 error
     message instead of 403". Open Net Initiative. Retrieved 21 November 2010.
 16. ^ "Anti-censorship movement in Tunisia: creativity, courage and hope!".
     Global Voices Advocacy. 27 May 2010. Retrieved 28 August 2010.
 17. ^ "Friendly HTTP Error Pages". msdn.com. 18 August 2010. Archived from the
     original on 2 December 2010. Retrieved 14 June 2012.
 18. ^ "Issue 1695: Chrome needs option to turn off "Friendly 404" displays".
     bugs.chromium.org. Retrieved 25 December 2021.
 19. ^ Heng, Christopher (7 September 2008). "What is Favicon.ico and How to
     Create a Favicon Icon for Your Website". The Site Wizard. Retrieved 23
     February 2011.
 20. ^ "The Dastardly "favicon.ico not found" Error". Internet Folks. 3 August
     1999.
 21. ^ "From skateboarding bears to missing children: The power of the 404 Not
     Found error page". Metro. 6 June 2011. Retrieved 16 April 2013.
 22. ^ "The political Page 404 war". BBC Newsbeat. 27 April 2015. Retrieved 18
     May 2018.
 23. ^ "Notfound.org". notfound. notfound. Archived from the original on 2
     September 2014.
 24. ^ "Missing children messages go on 404 error pages". BBC News. 27 September
     2012. Retrieved 20 September 2014.
 25. ^ Swenson, Sahala (19 August 2008). "Make your 404 pages more useful".
     Official Google Webmaster Central Blog. Google, Inc. Retrieved 28 August
     2009.
 26. ^ "Sources Leading To 404s". SpringTrax. Retrieved 11 February 2013.
 27. ^ Cushing, Anne (2 April 2013). "A Data-Centric Approach To Identifying 404
     Pages Worth Saving". Search Engine Land. Retrieved 7 June 2013.
 28. ^ "Tracking and Preventing 404 Errors". 404errorpages.com. Retrieved 7 June
     2013.
 29. ^ "Understand 404 Errors". SpringTrax.com. Retrieved 7 June 2013.
 30. ^ Jump up to: a b Edgar, Matthew (11 April 2023). "How To Fix 404 Errors On
     Your Website". Matthew Edgar. Retrieved 19 May 2023.
 31. ^ Jump up to: a b Frickey, Dean (18 November 2008). "A More Useful 404". A
     List Apart. Retrieved 19 May 2023.
 32. ^ Jump up to: a b "What 'Error 404' means and how to fix it". IONOS Digital
     Guide. 31 January 2023. Retrieved 19 May 2023.


EXTERNAL LINKS

 * A More Useful 404
 * 404 Not Found of the Hypertext Transfer Protocol (HTTP/1.1): Semantics and
   Content specification, at the Internet Engineering Task Force
 * ErrorDocument Directive – instructions on custom error pages for the Apache
   2.0 web server
 * 404: Not Found – an award-winning song about the error code



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