.htaccess Redirect/Rewrite Tutorial

How do I redirect all links for www.domain.com to domain.com ?

Description of the problem:

By default your website can be accessed with both www.domain.com and domain.com. Since Google penalizes this due to duplicated content reasons, you should restrict the access to either www.domain.com or domain.com. Some links may be outside of your website scope and/or the search engines may have already indexed your website under both addresses.

Solution:

Create a 301 redirect forcing all http requests to use either www.domain.com or domain.com:

Example 1 - Redirect domain.com to www.domain.com:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www.domain.com$ [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [L,R=301]

Example 2 - Redirect www.domain.com to domain.com:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^domain.com$ [NC]
RewriteRule ^(.*)$ http://domain.com/$1 [L,R=301]
Explanation of this .htaccess 301 redirect:

Let's have a look at the example 1 - Redirect domain.com to www.domain.com. The first line tells apache to start the rewrite module. The next line:

RewriteCond %{HTTP_HOST} !^www.domain.com$ [NC]
specifies that the next rule only fires when the http host (that means the domain of the queried url) is not (- specified with the "!") www.domain.com.
The $ means that the host ends with www.domain.com - and the result is that all pages from www.domain.com will trigger the following rewrite rule. Combined with the inversive "!" is the result every host that is not www.domain.com will be redirected to this domain.

The [NC] specifies that the http host is case insensitive. The escapes the "." - because this is a special character (normally, the dot (.) means that one character is unspecified).

The final line describes the action that should be executed:

RewriteRule ^(.*)$ http://www.domain.com/$1 [L,R=301]
The ^(.*)$ is a little magic trick. Can you remember the meaning of the dot? If not, this can be any character(but only one). So .* means that you can have a lot of characters, not only one. This is what we need because ^(.*)$ contains the requested url, without the domain.
The next part http://www.domain.com/$1 describes the target of the rewrite rule. This is our "final" used domain name, where $1 contains the content of the (.*).

The next part is also important, since it does the 301 redirect for us automatically: [L,R=301]. L means this is the last rule in this run. After this rewrite the webserver will return a result. The R=301 means that the webserver returns a 301 moved permanently to the requesting browser or search engine.