So here is most probably the dumbest question on earth... You install your brand spanking new Ubuntu 11.04 Linux on your screaming fast $1200 machine, only to discover whenever you put a DVD in the DVD-ROM, it does not play. So you install VLC, MPlayer etc... All do the same - they see the name of the DVD as it gets mounted properly, but it refuses to play it. No error, nothing. It just flickers when you click Play, then reverts back to stopped.
So you think to yourself... This is the year 2011. Playing a DVD should not be hard... Especially since everybody thinks Ubuntu with its new Unity desktop is going to revolutionise the world. Fortunately for me I know a thing or three about Linux and DVDs... But if I did not? then what? So here is what you need to do.
Open up a terminal, and run the following command:
apt-get install libdvdread4
This is the crucial step. It actually fetches the stuff necessary to decrypt the DVD CSS encryption. Ubuntu and most vendors cannot preload this due to legal issues, so you have to perform this step manually.
Once done you should be able to play back DVDs just fine.