URL Redirection(转)

http://en.wikipedia.org/wiki/URL_redirection

URL redirection, also called URL forwarding and the very similar technique domain redirection also called domain forwarding, are techniques on the World Wide Web for making a web page available under many URLs.

[edit] Purposes

There are several reasons to use URL redirection:

[edit] Similar domain names

A user might mis-type a URL—for example, "example.com" and "exmaple.com". Organizations often register these "mis-spelled" domains and re-direct them to the "correct" location: example.com. The addresses example.com and example.net could both redirect to a single domain, or web page, such as example.org. This technique is often used to "reserve" other top-level domains (TLD) with the same name, or make it easier for a true ".edu" or ".net" to redirect to a more recognizable ".com" domain.

[edit] Moving a site to a new domain

A web page may be redirected for several reasons:

• a web site might need to change its domain name;
• an author might move his or her pages to a new domain;
• two web sites might merge.

With URL redirects, incoming links to an outdated URL can be sent to the correct location.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 They often have the older/outdated domain names and links in their database and will send search users to these old URLs.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 up at the correct page. Also, in the next search engine pass, the search engine should detect and use the newer URL.

[edit] Logging outgoing links

The access logs of most web servers keep detailed information about where visitors came from and how they browsed the hosted site. They do not, however, log which links visitors left by. This is because the visitor's browser has no need to communicate with the original server when the visitor clicks on an outgoing link.

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 technique bears the downside of the delay caused by the additional request to the original website's server. As this added request will leave a trace in the server log, revealing exactly which link was followed, it can also be a privacy issue.^[1]

The same technique is also used by some corporate websites to implement a statement that the subsequent content is at another site, and therefore not necessarily affiliated with the corporation. In such scenarios, displaying the warning causes an additional delay.

[edit] Short aliases for long URLs

Main article: URL shortening

Web applications often include lengthy descriptive attributes in their URLs which represent data hierarchies, command structures, transaction paths and session information. This practice results in a URL that is aesthetically unpleasant and difficult to remember, and which may not fit within the size limitations of microblogging sites. URL shortening services provide a solution to this problem by redirecting a user from a longer URL to a shorter one.

[edit] Meaningful, persistent aliases for long or changing URLs

Sometimes the URL of a page changes even though the content stays the same. Therefore URL redirection can help users who have bookmarks. This is routinely done on Wikipedia whenever a page is renamed.

[edit] Manipulating search engines

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."^[2][3]

Search engine providers noticed the problem and took appropriate actions^{[citation needed]}. 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.

[edit] Satire and criticism

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. The website shadyurl.com offers a satirical service that will create an apparently "suspicious and frightening" redirection URL for even benign webpages. For example, an input of en.wikipedia.org generates 5z8.info/hookers_e4u5_inject_worm.

[edit] Manipulating visitors

URL redirection is sometimes used as a part of phishing attacks that confuse visitors about which web site they are visiting^{[citation needed]}. Because modern browsers always show the real URL in the address bar, the threat is lessened. However, redirects can also take you to sites that will otherwise attempt to attack in other ways. For example, a redirect might take a user to a site that would attempt to trick them into downloading antivirus software and ironically installing a trojan of some sort instead.

[edit] Techniques

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.^[4]

[edit] Manual redirect

The simplest technique is to ask the visitor to follow a link to the new page, usually using an HTML anchor as such:

Please follow this <a href="http://www.example.com/">link</a>.

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.

[edit] HTTP status codes 3xx

In the HTTP protocol used by the World Wide Web, a redirect is a response with a status code beginning with 3 that induces a browser to go to another location, with annotation describing the reason, which allows for the correct subsequent action (such as changing links in the case of code 301, a permanent change of address)

The HTTP standard defines several status codes for redirection:

• 300 multiple choices (e.g. offer different languages)
• 301 moved permanently
• 302 found (originally temporary redirect, but now commonly used to specify redirection for unspecified reason)
• 303 see other (e.g. for results of cgi-scripts)
• 307 temporary redirect

All of these status codes require that the URL of the redirect target be 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):

• 304 not modified
• 305 use proxy

This is a sample of an HTTP response that uses the 301 "moved permanently" redirect:

HTTP/1.1 301 Moved Permanently
Location: http://www.example.org/
Content-Type: text/html
Content-Length: 174
 
<html>
<head>
<title>Moved</title>
</head>
<body>
<h1>Moved</h1>
<p>This page has moved to <a href="http://www.example.org/">http://www.example.org/</a>.</p>
</body>
</html>

[edit] Using server-side scripting for redirection

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 the header function as follows:

header('HTTP/1.1 301 Moved Permanently');
header('Location: http://www.example.com');
exit();

According to the HTTP protocol, the Location header must contain an absolute URI.^[5] 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.

The above format used to work until somewhere around version 2.2.14. In the Apache HTTP server version 2.2.14 it has been found^{[by whom?]} (on three separate servers^{[citation needed]}) that placing a line of the above format in an .htaccess file causes an Internal Server Error for the entire site. Instead you should use the format dictated by the Apache Foundation ^[6] such as:

Redirect permanent /oldpage.html http://www.example.com/newpage.html
Redirect 301 /oldpage.html http://www.example.com/newpage.html

To change domain names using example.com/.htaccess or within a section in an Apache config file:

RewriteEngine on
RewriteCond %{HTTP_HOST} ^([^.:]+\.)*oldsite\.example\.com\.?(:[0-9]*)?$ [NC]
RewriteRule ^(.*)$ http://newsite.example.net/$1 [R=301,L,QSA]

Use of .htaccess for this purpose usually does not require administrative permissions. However, .htaccess can be disabled by your host, and so may not work (or continue to work) if they do so.

In addition, some server configurations may require the addition of the line:

Options +FollowSymLinks

ahead of the "RewriteEngine on" directive, in order to enable the mod_rewrite module.

When you have access to the main Apache config files (such as httpd.conf), it is best to avoid the use of .htaccess files.

If the code is placed into an Apache config file and not within any container, then the RewriteRule pattern must be changed to include a leading slash:

RewriteEngine on
 
RewriteCond %{HTTP_HOST} ^([^.:]+\.)*oldwebsite\.com\.?(:[0-9]*)?$ [NC]
RewriteRule ^/(.*)$ http://www.preferredwebsite.net/$1 [R=301,L]

[edit] Refresh Meta tag and HTTP refresh header

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:

• HTML tag
• An exploration of dynamic documents
• Meta refresh

A timeout of 0 seconds means an immediate redirect. Meta Refresh with a timeout of 0 seconds is accepted as a 301 permanent redirect by Google, allowing to transfer PageRank from static html files.^[7]

This is an example of a simple HTML document that uses this technique:

<html>
<head>
<meta http-equiv="Refresh" content="0; url=http://www.example.com/" />
</head>
<body>
<p>Please follow this <a href="http://www.example.com/">link</a>.</p>
</body>
</html>

• This technique is usable by all web authors because the meta tag is contained inside the document itself.
• The meta tag must be placed in the "head" section of the HTML file.
• The number "0" in this example may be replaced by another number to achieve a delay of that many seconds.
• This is a proprietary extension to HTML introduced by Netscape but supported by most web browsers. The manual link in the "body" section is for users whose browsers do not support this feature.

This is an example of achieving the same effect by issuing an HTTP refresh header:

HTTP/1.1 200 ok
Refresh: 0; url=http://www.example.com/
Content-type: text/html
Content-length: 78
 
Please follow <a href="http://www.example.com/">link</a>!

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:

#!/usr/bin/perl
print "Refresh: 0; url=http://www.example.com/\r\n";
print "Content-type: text/html\r\n";
print "\r\n";
print "Please follow \"http://www.example.com/\">link!"

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!

---

This example works best for a refresh, or in simple terms - a redirect for webpages, as follows, however, for a refresh under 4 seconds, your webpage will not be given priority listing on search engines. For some users, this is preferred not to be listed. Inline, you will find the time as in seconds: CONTENT="2 this number can be adjusted to suit your needs.

Place in your head:

---

[edit] JavaScript redirects

JavaScript offers several ways to display a different page in the current browser window. Quite frequently, they are used for a redirect. However, there are several reasons to prefer HTTP header or the refresh meta tag (whenever it is possible) over JavaScript redirects:

• Security considerations
• Some browsers don't support JavaScript
• many web crawlers don't execute JavaScript.

[edit] Frame redirects

A slightly different effect can be achieved by creating a single HTML frame that contains the target page:

<frameset rows="100%">
  <frame src="http://www.example.com/">
</frameset>
<noframes>
  <body>Please follow <a href="http://www.example.com/">link</a>!</body>
</noframes>

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.

This technique is commonly called cloaking. This may be used so that the reader sees a more memorable URL or, with fraudulent intentions, to conceal a phishing site as part of website spoofing.^[8]

[edit] Redirect loops

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, and 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.

[edit] Services

There exist services that can perform URL redirection on demand, with no need for technical work or access to the webserver your site is hosted on.

[edit] URL redirection services

A redirect service is an information management system, which provides an internet link that redirects users to the desired content. The typical benefit to the user is the use of a memorable domain name, and a reduction in the length of the URL or web address. A redirecting link can also be used as a permanent address for content that frequently changes hosts, similarly to the Domain Name System.

Hyperlinks involving URL redirection services are frequently used in spam messages directed at blogs and wikis. Thus, one way to reduce spam is to reject all edits and comments containing hyperlinks to known URL redirection services; however, this will also remove legitimate edits and comments and may not be an effective method to reduce spam.

Recently, URL redirection services have taken to using AJAX as an efficient, user friendly method for creating shortened URLs.

A major drawback of some URL redirection services is the use of delay pages, or frame based advertising, to generate revenue.

[edit] History

The first redirect services took advantage of top-level domains (TLD) such as ".to" (Tonga), ".at" (Austria) and ".is" (Iceland). Their goal was to make memorable URLs. The first mainstream redirect service was V3.com that boasted 4 million users at its peak in 2000. V3.com success was attributed to having a wide variety of short memorable domains including "r.im", "go.to", "i.am", "come.to" and "start.at". V3.com was acquired by FortuneCity.com, a large free web hosting company, in early 1999. In 2001 emerged .tk (Tokelau) as a TLD used for memorable names.^[9] As the sales price of top level domains started falling from $70.00 per year to less than $10.00, the demand for memorable redirection services eroded.^{[citation needed]}

With the launch of TinyURL in 2002 a new kind of redirecting service was born, namely URL shortening. Their goal was to make long URLs short, to be able to post them on internet forums. Since 2006, with the 140 character limit on the extremely popular Twitter service, these short URL services have seen a resurgence.

[edit] Referrer Masking

Redirection services can hide the referrer by placing an intermediate page between the the page the link is on and its destination. Although these are conceptually similar to other URL redirection services, they serve a different purpose, and they rarely attempt to shorten or obfuscate the destination URL (as their only intended side-effect is to hide referrer information and provide a clear gateway between other websites.)

This type of redirection is often used to prevent potentially-malicious links from gaining information using the referrer, for example a session ID in the query string. Many large community websites use link redirection on external links to lessen the chance of an exploit that could be used to steal account information, as well as make it clear when a user is leaving a service, to lessen the chance of effective phishing.

Here is a simplistic example of such a service, written in PHP.

$url = htmlspecialchars($_GET['url']);
header( 'Location: http://'.$url.'' );
?>


 
  
   echo $url; ?>">
 
 
 Attempting to redirect to  echo $url; ?>">http:// echo $url; ?>.
 

转载于:https://www.cnblogs.com/limei/archive/2011/04/21/2023189.html