Multisite non-www domain redirects to sign-up

I have Multisite enabled and can only view my site with the www. When I try without the www it sends me to the sign-up page and says

Registration has been disabled.

www.domain.com/portfolio  <-- load correctly
domain.com/portfolio      <-- redirects to sign-up page

I’ve tried NOBLOGREDIRECT, but nothing changes.

define('NOBLOGREDIRECT', 'http://www.domain.com');

In .htaccess, I’ve also tried the following with no success.

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

How do I get my site viewable with and without the “www.” in the domain?

The answer is here:
Domain redirect in WordPress multisite.

You need to use the domain mapping plugin.

http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/

A must read http://ottopress.com/2010/wordpress-3-0-multisite-domain-mapping-tutorial/