Editing HTTP 404 (section)

==Soft 404 errors==
{{anchor|Soft 404}}

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''.<ref>{{Cite book |author1=Ziv Bar-Yossef |author2=Andrei Z. Broder |author3=Ravi Kumar |author4=Andrew Tompkins |title=Proceedings of the 13th international conference on World Wide Web |chapter=Sic transit gloria telae |date=2004 |pages=328–337 |doi=10.1145/988672.988716 |isbn=978-1581138443 |s2cid=587547 }}</ref>

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.<ref>{{cite web|title=Why is your crawler asking for strange URLs that have never existed on my site?|url=http://help.yahoo.com/l/nl/yahoo/ysearch/slurp/slurp-10.html|url-status=dead|archive-url=https://web.archive.org/web/20140715000803/http://help.yahoo.com/l/nl/yahoo/ysearch/slurp/slurp-10.html|archive-date=15 July 2014|access-date=4 September 2013|publisher=Yahoo Ysearch Help page}}</ref> Soft 404s can occur as a result of configuration errors when using certain HTTP server software, for example with the [[Apache HTTP Server|Apache]] software, when an Error Document 404 (specified in a [[.htaccess]] file) is specified as an absolute path (e.g. <nowiki>http://example.com/error.html</nowiki>) rather than a relative path (/error.html).<ref>{{cite web|url=http://googlewebmastercentral.blogspot.com/2008/08/farewell-to-soft-404s.html|title=Farewell to soft 404s |publisher=Google Official Blog|access-date=20 September 2008}}</ref> 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 server]]s generate a 404 error when a [[List of HTTP status codes#5xx server errors|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)|Cleanfeed]] content blocking system, which returns a 404 error to any request for content identified as potentially illegal by the [[Internet Watch Foundation]].<ref>{{cite web |url=https://publicaffairs.linx.net/news/?p=154 |title=LINX Public Affairs » Cleanfeed: the facts |publisher=Publicaffairs.linx.net |date=10 September 2004 |access-date=6 March 2011 |archive-url=https://web.archive.org/web/20110513071912/https://publicaffairs.linx.net/news/?p=154 |archive-date=13 May 2011 |url-status=dead}}</ref> Other ISPs return a [[HTTP 403]] "forbidden" error in the same circumstances.<ref>{{cite web|url=https://airbrake.io/blog/http-errors/403-forbidden-error|title=DEMON – Error 403|access-date=14 June 2012}}</ref> The practice of employing fake 404 errors as a means to conceal [[censorship]] has also been reported in [[Thailand]]<ref>{{cite news|last=Sambandaraksa|first=Don|date=18 February 2009|title=The old fake '404 Not Found' routine - Dead link|newspaper=Bangkok Post|url=http://www.bangkokpost.com/tech/technews/11872/the-old-fake-404-not-found-routine|access-date=12 September 2010}}</ref> and [[Tunisia]].<ref>{{cite web |url=http://opennet.net/node/950 |title=Tunisian journalist sues government agency for blocking Facebook, claims damage for the use of 404 error message instead of 403 |first=Helmi |last=Noman |date=12 September 2008 |publisher= Open Net Initiative |access-date=21 November 2010}}</ref> In Tunisia, where censorship was severe before the [[Tunisian Revolution|2011 revolution]], people became aware of the nature of the fake 404 errors and created an imaginary character named "[[Ammar 404#Censorship during the Ben Ali regime|Ammar 404]]" who represents "the invisible censor".<ref>{{cite web|date=27 May 2010|title=Anti-censorship movement in Tunisia: creativity, courage and hope!|url=http://advocacy.globalvoicesonline.org/2010/05/27/anti-censorship-movement-in-tunisia-creativity-courage-and-hope/|access-date=28 August 2010|publisher=Global Voices Advocacy}}</ref>