When the 404 Strikes and What it Means

Posted: 15/12/2011 in Class Room, Geek Stuff, How to...., Tutorial
Tags: , , ,

The Error 404 “Page not found” is the error page displayed whenever someone asks for a page that’s simply not available on your site. The reason for this is that there may be a link on your site that was wrong or the page might have been recently removed from the site. As there is no web page to display, the web server sends a page that simply says “404 Page not found”.

The 404 error message is an HTTP (Hypertext Transfer Protocol) standard status code. This “Not Found” response code indicates that although the client could communicate to the server, the server could not find what was requested or it was configured not to fulfil the request.

The 404 “Not Found” error is not the same as the “Server Not Found” error which you see whenever a connection to the destination server could not be established at all.

The default 404 error page as shown on Internet Explorer is given below.

HTTP Status Code

Whenever you visit a web page, your computer will request data from a server through HTTP. Even before the requested page is displayed in your browser, the web server will send the HTTP header that has the status code. The status code provides information about the status of the request. A normal web page gets the status code as 200. But we do not see this as the server proceeds to send the contents of the page. It’s only when there is an error, we see the status code 404 Not Found.

Origin of Status Codes

As a part of the HTTP 0.9 specifications, the World Wide Web Consortium (W3C) established HTTP status codes in 1992. Tim Berners-Lee, who invented the web and the first web browser in 1990, defined the status codes.

List of Status Codes

A brief overview of HTTP status codes is given below.

HTTP Status Code

Whenever you visit a web page, your computer will request data from a server through HTTP. Even before the requested page is displayed in your browser, the web server will send the HTTP header that has the status code. The status code provides information about the status of the request. A normal web page gets the status code as 200. But we do not see this as the server proceeds to send the contents of the page. It’s only when there is an error, we see the status code 404 Not Found.

Origin of Status Codes

As a part of the HTTP 0.9 specifications, the World Wide Web Consortium (W3C) established HTTP status codes in 1992. Tim Berners-Lee, who invented the web and the first web browser in 1990, defined the status codes.

List of Status Codes

A brief overview of HTTP status codes is given below.

Code Meaning Description
100 Continue Confirms the client about the arrival of the first part of the request and informs to continue with the rest of the request or ignore if the request has been fulfilled
101 Switching Protocols Informs the client about the server switching the protocols to that specified in the Upgrade message header field during the current connection.
200 OK Standard response for successful requests
201 Created Request fulfilled and new resource created
202 Accepted Request accepted, but not yet processed
203 Non-Authoritative Information Returned meta information was not the definitive set from the origin server.
204 No Content Request succeeded without requiring the return of an entity-body
205 Reset Content Request succeeded but require resetting of the document view that caused the request
206 Partial Content Partial GET request was successful
300 Multiple Choices Requested resource has multiple choices at different locations.
301 Moved Permanently Resource permanently moved to a different URL.
302 Found Requested resource was found under a different URL but the client should continue to use the original URL.
303 See Other Requested response is at a different URL and can be accessed only through a GET command.
304 Not Modified Resource not modified since the last request.
305 Use Proxy Requested resource should be accessed through the proxy specified in the location field.
306 No Longer Used Reserved for future use
307 Temporary Redirect Resource has been moved temporarily to a different URL.
400 Bad Request Syntax of the request not understood by the server.
401 Not Authorized Request requires user authentication
402 Payment Required Reserved for future use.
403 Forbidden Server refuses to fulfill the request.
404 Not Found Document or file requested by the client was not found.
405 Method Not Allowed Method specified in the Request-Line was not allowed for the specified resource.
406 Not Acceptable Resource requested generates response entities that has content characteristics not specified in the accept headers.
407 Proxy Authentication Required Request requires the authentication with the proxy.
408 Request Timeout Client fails to send a request in the time allowed by the server.
409 Conflict Request was unsuccessful due to a conflict in the state of the resource.
410 Gone Resource requested is no longer available with no forwarding address
411 Length Required Server doesn’t accept the request without a valid Content-Length header field.
412 Precondition Failed Precondition specified in the Request-Header field returns false.
413 Request Entity Too Large Request unsuccessful as the request entity is larger than that allowed by the server
414 Request URL Too Long Request unsuccessful as the URL specified is longer than the one, the server is willing to process.
415 Unsupported Media Type Request unsuccessful as the entity of the request is in a format not supported by the requested resource
416 Requested Range Not Satisfiable Request included a Range request-header field without any range-specifier value
417 Expectation Failed Expectation given in the Expect request-header was not fulfilled by the server.
422 Unprocessable Entity Request well-formed but unable to process because of semantic errors
423 Locked Resource accessed was locked
424 Failed Dependency Request failed because of the failure of a previous request
426 Upgrade Required Client should switch to Transport Layer Security
500 Internal Server Error Request unsuccessful because of an unexpected condition encountered by the server.
501 Not Implemented Request unsuccessful as the server could not support the functionality needed to fulfill the request.
502 Bad Gateway Server received an invalid response from the upstream server while trying to fulfill the request.
503 Service Unavailable Request unsuccessful to the server being down or overloaded.
504 Gateway Timeout Upstream server failed to send a request in the time allowed by the server.
505 HTTP Version Not Supported Server does not support the HTTP version specified in the request.

Meaning of 404

When we expand the code 404, the first digit “4” represents a client error. The server indicates that you did a mistake like misspelling the URL or requesting for a page that is no longer available.

The middle digit, 0 represents a general syntax error and could indicate a spelling mistake.

The last digit, 4 refers to a specific error in the group of 40x.

The World Wide Web Consortium (W3C) states that 404 Not Found should be used in cases where the server fails to find the requested location and is unsure of its status. Whenever a page has been permanently removed, the status code used must be 410. But hardly have we seen a 410 page. Instead, 404 Not Found page has become popular and the most commonly used error page.

Content of a 404 Error Page

A 404 response code is always followed by a human readable reason phrase as per the HTTP specification. Generally, a web server issues an HTML page that has the 404 code and the “Not Found” phrase by default. You can configure a web server to display a branded page with a better description and a search form. But the protocol level phrase requires no customization as it is hidden from the user.

Soft 404s

Soft 404 errors are actually “Not Found” errors returned by a web server as a standard web page with a 200 Ok response code. In an automated process of discovering a broken link, the soft 404 errors are problematic.

The BT Group of UK has a clean feed content blocking system that returns a 404 error to the requests for content identified as illegal by the Internet Watch Foundation. Even when the user tries to access the Government censored websites, a fake 404 error will be returned.

404 Error Percentages

A sample web trends’ summary report by ARCHIVI shows the client error details for 404 Page.

Client Errors

Error

Hits

% of Failed Hits

000 Incomplete / Undefined

29,164

69.62%

404 Page or File Not Found

12,651

30.2%

400 Bad Request

57

0.13%

18745 Incomplete / Undefined

5

0.01%

18747 Incomplete / Undefined

4

0%

401 Unauthorized Access

4

0%

Total

41,885

100%

Although the web statistics generally vary from month to month, based on the strategy used to eliminate 404 errors, and how active the website is, the percentage of 404 errors varies. Most active websites that have frequently changed or added content generally experience a higher number of Page Not Found errors. But there are many large and busy sites that achieve zero percent 404 errors over a period. On an average, around 7% of visits to any given web site will result in a 404 error page.

Tracking and Preventing 404 Errors

  • Log Files – Web Server log files help in tracking the 404 errors. These standard log files are just ASCII text files that have each HTTP protocol transaction, whether completed or not, recorded in them. Most of the HTTP errors are recorded in the transfer log and the error log files. If you have access to the log files of your website, you can observe the HTTP status code field. This field gives you an idea about the occurrence of 404 errors, their frequencies, consistencies, and also the referred document that led to the errors. Also find out the existence of any broken link on your site and the misspelled URL that led to the error. When you know all these information, you can easily correct the link and prevent 404 errors on your website.
  • Redirects – If you find a page that is consistently getting a 404 error, you can create a redirect page using the .htaccess file that automatically takes the users from an older page to its newer replacement. You can use Permanent and Temporary Redirects to “catch” old referrals from other sites and send the visitors to their intended information.
  • Robots File – If you have a section of your site with pages that are frequently changed, you can block the search engines from indexing them in their databases using robots.txt file so that you can prevent 404 errors.

Using Log Files

A sample line from a common transfer log file is given below.

Address or DNS revacsystems.com
RFC931
AuthUser
TimeStamp [18/June/2008:12:13:03 -0700]
Access Request GET /download/windows/happiness.zip HTTP/1.0
Status Code 200
Transfer Volume 9887
Referer URL http://www.payoneer.com/
User Agent Mozilla/4.7 [en]C-SYMPA (Win95; U)
  •  Address or DNS – This field refers to the address of the computer making the HTTP request.
  • RFC931 – This field identifies the requestor. If no information is available, you’ll find the symbol – for this column in the log file.
  • AuthUser – This field has the authenticated user sent via clear text.
  • TimeStamp – This field has the date, time, and offset from Greenwich Mean Time (GMT x 100) recorded for each hit. You can even compare the time stamps between entries so that you’ll know how long a visitor stayed on a given page.
  • Access Request – This field has one of the three types of HTTP requests. Get request is for a document or program. POST is to tell the server that the data is following. HEAD is for use by the link checking programs.
  • Status Code – This field has the status code of 200 meaning that the transaction was successful. If the requested URL doesn’t exist, then you would have noticed the 404 code in the log.
  • Transfer Volume – This field shows the number of bytes transferred.
  • Referer URL – This field refers to the page where the visitor was located when making the next request.
  • User Agent – This field refers to the information such as the browser, version, and operating system of the reader.

Using Redirects

Redirects are very useful when used in conjunction with a 404 error page. To redirect a page, simply follow the steps given below.

1. Create a file “notfound404.htm” with a message such as:

Using Redirects

Redirects are very useful when used in conjunction with a 404 error page. To redirect a page, simply follow the steps given below.

1. Create a file “notfound404.htm” with a message such as:

“Sorry, this page was not found. In a few seconds, you will be redirected to the Home page.”

2. Allow 5 seconds for reading the message and then redirect.

3. A sample redirect code is:

Note: The value for CONTENT specifies the number of seconds you allow the user to read the message before redirecting.

 Using robots.txt File

Robots.txt file is useful when there are frequently changing sections on your webpage. To use a robots.txt file, simply follow the steps given below.

1. Create a file “robots.txt” in the root directory.

2. A sample robots.txt code is:

Note: User-agent: * will apply to all search engines. Disallow command helps you to block complete directories or only the individual files that change.

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