As someone browsing the Internet you're bound to come across an error message now and again. The usual message is either that the page you're searching for can't be found, or that there's a problem locating the Domain Name Server. There are however many other error messages and this article not only provides the low down on these, but also shows how good websites provide their own customised messages like the screenshot of AbleStable's 404 error message on the left. [click to enlarge]

Defining the W3C

The World Wide Web Consortium (W3C) was founded in 1994 by Tim Berners-Lee, the original architect of the World Wide Web.

The organization's purpose is to develop open standards so that the Web evolves in a single direction rather than being splintered among competing factions. The W3C is the chief standards body for HTTP, HTML, and includes responsibilities for defining Internet error messages.

Web page Elements

It's important to know a little about how a web page is structured when discussing browser error messages. Web pages have two distinct sections: the 'head' and 'body'. The 'head' carries the title of the page, information that is unseen by the general user, and may contain important instructions to the browser about how to display certain elements of the page, how the user can interact with page elements, and 'Meta' tags which search engines and directories can harvest so the web page may be effectively indexed. The 'body' section contains all the page contents that are 'viewable' by the user.

It Happens To Us All

A browser error message occurs because there's a misconfiguration or failure on the server side, a break-down in the data stream between the server and your computer, or a refusal to serve you the page without authorisation. Browser error messages do not signify there's anything wrong with your computer or necessarily wrong with the website serving the page. Error messages attempt to provide information about the nature of the problem so that you can decide on the best course of action to take.



The rare screenshot above shows how a search query to Yahoo! resulted in an error in the way the site processed the request. Yahoo! like all good sites, delivers custom error pages so that if an error occurs there's more help than a standard error page which in general are not particularly informative or helpful.

Common Error Pages

	The most common error page by far is the '404 page not found' which denotes that the server has not located anything matching the requested web page. In this instance the requested page has either been moved, deleted, the user has entered the wrong URL, or the referring page has provided an incorrect URL. No indication in the error message is given of whether the condition is temporary or permanent.
Rather than serving a 404 error page, a 410 status code should be used if the server knows that an old resource is permanently unavailable and has no forwarding address, however this is not generally implemented. The 410 status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable. The second most frequent error page occurs when the browser cannot find the Domain Name Server (DNS).
	If all goes well you request a Web page > the DNS tells the request where to look for the web page by providing the website IP Address > the Web page is served from that IP Address and resolves in your browser. A 500 Internal Server Error page denotes that the server encountered an unexpected condition which prevented it from fulfilling the request. This is most likely caused by a problem in a CGI script called by the server, or possibly by a server or DNS misconfiguration. The DNS is often on a different server than the website files and hosted by an entirely different hosting company. It is possible that although the website itself is fully functional, the DNS may not be.

Less Common Error Pages

There are many additional error messages that you may also encounter in your Internet browsing. The following section details these messages in number order:

52 Runtime Error

This is a 'bad file name or number' error in a VBScript or JavaScript. These scripting languages are used to embed small programs such as pop-up windows into the HTML code of a Web page.

301 Moved Permanently

The requested resource has been assigned a new permanent URL and any future references to this resource should use one of the returned URLs. Clients with link editing capabilities ought to automatically re-link references to the Request URL to one or more of the new references returned by the server.

306 (Unused)

The 306 status code was advised in a previous version of the HTML W3C specification but is no longer used.

307 Temporary Redirect

The requested resource resides temporarily under a different URL.

400 Bad Request

The request could not be understood by the server due to malformed syntax. This message usually means the syntax used in the URL is incorrect (e.g. an uppercase letter should be a lowercase letter; wrong punctuation marks etc.).

401 Unauthorized

The request requires user authentication: the server is looking for some encryption key from the client and is not getting it, or access control has been misconfigured on the server.

402 Payment Required

This code is reserved for future use.

403 Forbidden

The server understood the request, but is refusing to fulfil it. Authorization will not help, and the request should not be repeated. If the server wishes to make public why the request has not been fulfilled, it may describe the reason for the refusal. If the server does not wish to make this information available to the client, the status code 404 (Not Found) may be used instead.

405 Method Not Allowed

The server is refusing to service your request because the method specified in the request-line is not allowed for the resource identified by the request-URL.

406 Not Acceptable

The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.

407 Proxy Authentication Required

This code is similar to 401 (Unauthorized), but indicates that the client must first authenticate itself with the proxy.

408 Request Timeout

The client did not produce a request within the time that the server was prepared to wait.

409 Conflict

The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request.

410 Gone

The requested resource is no longer available at the server and no forwarding address is known. This condition is expected to be considered permanent.

The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as 'gone' or to keep the mark for any length of time - that is left to the discretion of the server owner.

411 Length Required

The server refuses to accept the request without a defined Content Length.

412 Precondition Failed

The precondition given in one or more of the request-header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource meta information and thus prevent the requested method from being applied to a resource other than the one intended.

413 Request Entity Too Large

The server is refusing to process a request because the request entity is larger than the server is willing or able to process.

414 Request-URI Too Long

The server is refusing to service the request because the Request-URI is longer than the server is willing to interpret.

415 Unsupported Media Type

The server is refusing to service the request because the entity of the request is in a format not supported by the requested resource for the requested method.

501 Not Implemented

The server does not support the functionality required to fulfil the request.

502 Bad Gateway

The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfil the request: server congestion; too many connections; high traffic etc.

503 Service Unavailable

The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The server may be busy, site may have moved ,or the user may have lost their dial-up Internet connection. The implication is that this is a temporary condition which will be alleviated after some delay. Some servers however may simply refuse the connection.

504 Gateway Timeout

The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server specified by the URL (e.g. HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed to access in attempting to complete the request.

Note: Note to implementers: some deployed proxies are known to return 400 or 500 when DNS lookups time out.

505 HTTP Version Not Supported

The server does not support, or refuses to support, the HTTP protocol version that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client.

Web page Contains No Data

The page is there but is not showing anything. Error occurs in the document attributable to bad table formatting, or stripped header information.

Unable to Locate Host

Host server is down, Internet connection is lost, or the URL is typed incorrectly.

Could Do Better Error

Here's a surprise. After entering the URL http://www.google/test the utterly inadequate error message on the left was delivered. Someone somewhere isn't doing their job so well...

Error messages can guide the Internet user to their destination in a painless and effective manner, however too many websites continue to deliver inadequate error provision. In the case of Google above, it would have taken very little effort to provide a additional information about the error, and a search field so the user could continue browsing. The only options the user is left with is to either use their back button, or enter a new URL in the browser address field.

Conclusion

The result of serving a default 404 page is almost always that the visitor leaves the site never to return. Most hosting providers now enable customisable error pages which, if developed well, encourage the user to stay rather than giving the impression that the site as 'failing' in some way. The irony is that much of the time errors occur as a result of incorrect user input or external linking, rather than a fault in the site's server or web page. Sites that don't implement good error handling not only fail from a usability perspective, but also loose a significant percentage of their potential custom.

If you run a site, spend time testing and delivering effective error pages. If you don't, you now know a site that provides great error pages cares about detail, and that professional attitude will likely carry through to other areas of its business and organisation...