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Force the www.
 
RewriteEngine On
RewriteCond %{HTTP_HOST} ^your-site.com [NC]
RewriteRule ^(.*)$ http://www.your-site.com/$1 [L,R=301]
 
Remove the www.
 
RewriteEngine On
RewriteCond %{HTTP_HOST} !^your-site.com$ [NC]
RewriteRule ^(.*)$ http://your-site.com/$1 [L,R=301]