Domain Name Canonicalization and the .htaccess File

Lee Zoumas - December 17, 2009

The .htaccess file is the main configuration file for URL Rewriting software, such as Apache’s mod_rewrite and Helicon’s ISAPI_Rewrite. An .htaccess file can be used to perform many different SEO-related tasks. Whether or not your web host allows the use of the .htaccess file can mean all the difference in the world when planning an SEO strategy for your website. In all of our client projects, we use the .htaccess file to perform some SEO-critical functions. One of the most important functions that the .htaccess file can perform is domain name canonicalization.

Domain Name Canonicalization

If a domain name is not canonicalized, it means that the same site will be presented to the browser when different combinations of a domain are requested. For example, consider the following urls:

http://www.domain.com
http://domain.com

While both of the examples above look the same, they are in fact quite different. Search engines may regard them as different URLs altogether. As a result, some pages may get indexed under the www version, while others may get indexed under the non-www version. One way to ensure that search engines will only index one version is by adding the following rule into your .htaccess file:

RewriteCond   %{HTTP:Host} ^domain.com$
RewriteRule   (.*)     http://www.domain.com/$1 [QSA]

With that rule in place, when the non-www version of the site is requested, the user will be redirected to the canonicalized www version. It should be noted that this rule will not just work for the homepage, but all pages within that domain. For example:

domain.com/page1.htm

… will redirect to this…

http://www.domain.com/page1.htm

As you can see, that rule is pretty powerful. In a future post, I will demonstrate how the .htaccess file can be used for page level redirects.