HTTP 404 Totally Explained

Everything about Http 404 totally explained

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 couldn't find what was requested, or it was configured not to fulfill the request and not reveal the reason why. 404 errors shouldn't be confused with "server not found" or similar errors, in which a connection to the destination server can't be made at all.

Overview

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. In the code 404, the first "4" indicates a client error, such as a mistyped 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. 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, won't display custom pages unless they're larger than 512 bytes, opting to instead display a "friendly" error page. 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. Because these two options require special server configuration, most websites don't make use of them.
   404 errors shouldn't be confused with DNS errors, which appear when the given URL refers to a webserver which doesn't 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 isn't 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's 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. Governments that censor the Internet also often return a fake 404 error when a user tries to access a blocked website.

Further Information

Get more info on 'Http 404'.

External Link Exchanges

Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:

<a href="http://http_404.totallyexplained.com">HTTP 404 Totally Explained</a>

Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned.