problem with redirect rules

Hi,

I really really need your help. I'm on Ubuntu Lucid and Apache2. I enabled mod_rewrite.

One thing who is really simple usually and I would like to implement, is redirect all no www request to www request (like domain.com to www.domain.com).

Usually, it's very simple. I use .htaccess to activate this rule:

RewriteEngine on

RewriteCond %{HTTP_HOST} ^domain.com$ [NC]

RewriteRule ^(.*)$ http://www.domain.com/$1 [L,R=301]

BUT it's not working. Why why why ???? :(. All others rules are working except this rule. Perhaps I forgot something.

Thanks for your help !

Julien