How to redirect www.domain.com to domain.com (or reverse)

How to redirect www.domain.com to domain.com (or reverse)

Most sites can be reached by two different URLs. For example http://b2evolution.net/about/features.html and http://www.b2evolution.net/about/features.html .

This is good for the users who can choose whether or not to type in the "www." part. However, it is not good for search engines which will tend to see a lot of duplicate contents.

If you are on an Apache webserver, you can use mod_rewrite in order to redirect all traffic that goes to the www.domain to the "non www" domain. (If you want it the other way round, see below)

Try adding this to a file named ".htaccess" at the root of your site (create that file if it doesn't exist):

Code:
    RewriteEngine on
    RewriteCond %{HTTP_HOST}   !^$
    RewriteCond %{HTTP_HOST}   !^your-domain\.com [NC]
    RewriteRule ^/(.*)         http://your-domain\.com/$1 [L,R=301]

Note: More advanced users will prefer to add this to the httpd.conf / apache2.conf instead in order to gain a little efficiency and also to not interfere with tests on development/staging servers.

Explanations

RewriteEngine on activates mod_rewrite.

The first RewriteCond makes sure we have a non(!) empty(^$) hostname to work with.

The second RewriteCond detects that the host name is not(!) the one we want (e-g: it has an extra www. part in it).

The RewriteRule sends out a permanent(301) redirection to the right domain name followed by the currently requested path/page ($1).

Handling multiple domains at once

Below is more elegant (yet complex to read) solution that can handle multiple domains at once (if you have "ServerAlias"es) and that doesn't require to type-in your domain (just copy/paste):

Code:
    RewriteEngine on
    RewriteCond %{HTTP_HOST}   !^$
    RewriteCond %{HTTP_HOST}   ^www\.(.+)$ [NC]
    RewriteRule ^/(.*)         http://%1/$1 [L,R=301]

The trick here is to use %1 which matches the canonical part of the domain name in the second RewriteCond (.+) .

Reverse redirection

By popular demand, here is how to redirect from somedomain.com to www.somedomain.com

Code:
    RewriteEngine on
    RewriteCond %{HTTP_HOST}   !^$
    RewriteCond %{HTTP_HOST}   !^www\. [NC]
    RewriteRule ^/(.*)         http://www.%{HTTP_HOST}/$1 [L,R=301]

【转自：http://fplanque.com/dev/http-ssl/www-domain-canonical-urls-mod_rewrite】