With URL redirects, incoming links to an outdated URL can be sent to the correct location. These links might be from other sites that have not realized that there is a change or from bookmarks/favorites that users have saved in their browsers.
The same applies to search engines. They often have the older/outdated domain names and links in their database and will send search users to these old URLs. By using a "moved permanently" redirect to the new URL, visitors will still end at the correct page. Also, in the next search engine pass, the search engine should detect and use the newer URL.
This information can be captured in several ways. One way involves URL redirection. Instead of sending the visitor straight to the other site, links on the site can direct to a URL on the original website's domain that automatically redirects to the real target. This added request will leave a trace in the server logs saying exactly which link was followed. This technique is also used by some corporate websites to have a "warning" page that the content is off-site and not necessarily affiliated with the corporation. This technique does bear the downside in the delay of an additional request to the original website's server. For websites that wish to display a "warning" page before automatically forwarding, the length of time the warning is displayed is an additional delay.
Some years ago, redirect techniques were used to fool search engines. For example, one page could show popular search terms to search engines but redirect the visitors to a different target page. There are also cases where redirects have been used to "steal" the page rank of one popular page and use it for a different page, usually involving the 302 HTTP status code of "moved temporarily."
Search engine providers noticed the problem and took appropriate actions. Usually, sites that employ such techniques to manipulate search engines are punished automatically by reducing their ranking or by excluding them from the search index.
As a result, today, such manipulations usually result in less rather than more site exposure.
In the same way that a Google bomb can be used for satire and political criticism, a domain name that conveys one meaning can be redirected to any other web page, sometimes with malicious intent.
URL redirection is sometimes used as a part of phishing attacks that confuse visitors about which web site they are visiting.
There are several techniques to implement a redirect. In many cases, Refresh meta tag is the simplest one. However, there exist several strong opinions discouraging this method.
The simplest technique is to ask the visitor to follow a link to the new page, usually using an HTML anchor as such: This method is often used as a fall-back for automatic methods — if the visitor's browser does not support the automatic redirect method, the visitor can still reach the target document by following the link.
All of these status codes require that the URL of the redirect target is given in the Location: header of the HTTP response. The 300 multiple choices will usually list all choices in the body of the message and show the default choice in the Location: header.
Within the 3xx range, there are also some status codes that are quite different from the above redirects (they are not discussed here with their details):
This is a sample of a HTTP response that uses the 301 "moved permanently" redirect:
Often, web authors don't have sufficient permissions to produce these status codes: The HTTP header is generated by the web server program and not read from the file for that URL. Even for CGI scripts, the web server usually generates the status code automatically and allows custom headers to be added by the script. To produce HTTP status codes with cgi-scripts, one needs to enable non-parsed-headers.
Sometimes, it is sufficient to print the "Location: 'url'" header line from a normal CGI script. Many web servers choose one of the 3xx status codes for such replies.
Frameworks for server-side content generation typically require that HTTP headers be generated before response data. As a result, the web programmer who is using such a scripting language to redirect the user's browser to another page must ensure that the redirect is the first or only part of the response. In the ASP scripting language, this can also be accomplished using the methods response.buffer=true and response.redirect "http://www.example.com". Using PHP, one can use header("Location: http://www.example.com");.
According to the HTTP protocol, the Location header must contain an absolute URI. When redirecting from one page to another within the same site, it is a common mistake to use a relative URI. As a result most browsers tolerate relative URIs in the Location header, but some browsers display a warning to the end user.
To change domain names:
Use of .htaccess for this purpose usually does not require administrative permissions, though it can be disabled. When you have access to apache main config file (such as httpd.conf), it is best to avoid the use of .htaccess files.
Netscape introduced a feature to refresh the displayed page after a certain amount of time. This method is often called meta refresh. It is possible to specify the URL of the new page, thus replacing one page after some time by another page:
A timeout of 0 seconds means an immediate redirect.
This is an example of a simple HTML document that uses this technique:
This is an example of achieving the same effect by issuing a HTTP refresh header:
This response is easier to generate by CGI programs because one does not need to change the default status code. Here is a simple CGI program that effects this redirect:
Note: Usually, the HTTP server adds the status line and the Content-length header automatically.
This method is considered by the W3C to be a poor method of redirection, since it does not communicate any information about either the original or new resource, to the browser (or search engine). The W3C's Web Content Accessibility Guidelines (7.4) discourage the creation of auto-refreshing pages, since most web browsers do not allow the user to disable or control the refresh rate. Some articles that they have written on the issue include W3C Web Content Accessibility Guidelines (1.0): Ensure user control of time-sensitive content changes and Use standard redirects: don't break the back button!
A slightly different effect can be achieved by creating a single HTML frame that contains the target page:
One main difference to the above redirect methods is that for a frame redirect, the browser displays the URL of the frame document and not the URL of the target page in the URL bar.
It is quite possible that one redirect leads to another redirect. For example, the URL http://www.wikipedia.com/wiki/URL_redirection (note the differences in the domain name) is first redirected to http://www.wikipedia.org/wiki/URL_redirection and again redirected to the correct URL: http://en.wikipedia.org/wiki/URL_redirection. This is appropriate: the first redirection corrects the wrong domain name. The second redirection selects the correct language section. Finally, the browser displays the correct page.
Sometimes, however, a mistake can cause the redirection to point back to the first page, leading to an infinite loop of redirects. Browsers usually break that loop after a few steps and display an error message instead.
The HTTP standard states:
A client SHOULD detect infinite redirection loops, since such loops generate network traffic for each redirection.
Previous versions of this specification recommended a maximum of five redirections; some clients may exist that implement such a fixed limitation.
Recently, URL redirection services have taken to using AJAX as an efficient, user friendly method for creating shortened URLs.
This is very easy to do with PHP, such as in this example. This code can then be accessed by example,
Further, the above example code may not work correctly with URLs containing variables, unless the input is first encoded, or code is added that loops across the $_GETs and pieces together the final URL.