ETQW (and most id software games) had punkbuster for multiplayer protection. the serial key for etqw was only to create a official account and play online. though if for an unbelievable happening like losing the cds or the key but you had your login and password you coukd/can download the iso from a torrent site and log to your official account.

Quote Originally Posted by Kano
It is hard to create a single player protection that does not need permanent online checks that could not be hacked.
true yet useless anyway. who would hack a single player game anyway?

But as that function currently seems to use steam to identify a simple serial check for linux might not be enough. So you might end on linux only servers, no idea if you want that. Steam on Linux would fix that issue immediately...
i don't know if there are any dedicated rage servers... from what i understood, you would host the server on your pc if you create a co-op mission where your friend would connect to. the protocol should be the same for all OS clients... thought on linux with the lack on client side VAC it would be screwed for multiplayer. Valve should push an official beta to fix that but to see that happen is like winning the lotery.