Why can't I play my DVD on Linux VLC?

[clark]

I'm running Linux BackTrack 4 from an external hard drive and I'm trying to play a DVD through VLC. The error message I get is "VLC is unable to open the MRL 'dvd:///dev/hda'." I've tried this on several media players so I'm guessing its not the media players. I know BackTrack is mainly for network penetration but it seems like playing DVDs shouldn't really be a big deal. I appreciate any help I can get.