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= 404.php =
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Introduction
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In HTTP, the 404 HTTP status code indicates that a web client (i.e.
browser) was able to communicate with a server, but the server could
not provide the requested resource. The server may not have the
resource or it may not wish to disclose whether it has the resource.
The code is often associated with response reason 'Not Found' and is
often referred to as 'page not found' or 'file not found'.
Often, the server generates a web page for the condition and the
client displays it, and often the content indicates the error code.
Since the condition is relatively commonly-encountered (i.e. due to
broken or dead links), this code is one of the most recognizable on
the World Wide Web.
Overview
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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" 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.
Soft 404 errors
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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'.
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. 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). 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. 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 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".
Microsoft Internet Server 404 substatus error codes
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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
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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. 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. 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.
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. 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. 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 which provide data about missing
children.
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.
Tracking 404 errors
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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. One way around this is
to find 404 errors by analyzing external links.
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. 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.
See also
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*
*
*
* List of HTTP status codes
External links
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* [http://alistapart.com/article/amoreuseful404 A More Useful 404]
*
* [http://httpd.apache.org/docs/2.4/mod/core.html#errordocument
ErrorDocument Directive] - instructions on custom error pages for the
Apache 2.0 web server
* [http://www.ovff.org/pegasus/songs/404-not-found.html 404: Not
Found] - an award-winning filk song about the error code
License
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All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/404.php
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