HTTP 404
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]
"404 Not Found" redirects here. For the TV episode, see 404 Not Found (Mr. Robot).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.
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 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".[2] He also said that he considered using "400 Bad Request" instead, but decided that it was too vague and technical.[2]
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.[3] 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."[2]
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,[4] while GitHub's 404 page shows a random image of a parallax star field and a link to its status page.[5] Some websites have also used their 404 pages to showcase their brand personality, humor, or social causes. For instance, Lego's 404 page shows the Lego minifigure Emmet along with a humorous message,[6] Amazon's displays the image of a dog,[7] Peugeot's shows a picture of its 404 model, [8] and RTÉ's shows an image of Bosco.[9][10]
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.[28] One way around this is to find 404 errors by analyzing external links.[29]
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.[30] 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.[31]