Here's a thing:

 

http://www.vachu.com/duke3d/

 

I've tried the single player version & it works wonderfully. I'll have a play with multiplayer and get back to you. Looks straightforward enough (or at least as straightforward as you'd expect getting a 12 year old DOS game to run on XP over the internet).

Is the multiplayer a part of one of the ready-to-plays? or do you need the multiplayer launcher that says "its not done yet, and its not very near being done.

http://jonof.edgenetwork.org/index.php?p=jfduke3d