site off-line

Im finding sites online that run on Drupal but display a bit of inconsistancy. If you go to http://domain.com than the site works fine. However if I visit http://www.domain.com, than I get the "site offline" message. I had a similar problem with something like that using Wordpress so Im curious in what is affecting things in that adverse way.

From what I have seen, most people correct this with a band-aid by just setting a redirect on canonical names. Would this be a problem with Drupal or a problem with the domain registrar and how the DNS is set up?