Duke Nukem 3D Multiplayer Setup by VatVit 2005

Features

Automatically detect IP addresses and interfaces [Interface:].
Support for broadcast networking mode, where Duke3D sends broadcast packets for others on the LAN to pick up and reply to.
Support for specifying IP addresses that are allowed to connect to the Duke3D client, useful for playing over the internet.
Screenname [Nick:] used in Duke 3D can be set.
Support for peer-2-peer mode.
Port [Port:] used for UDP networking can be set.
Number of players can be selected if you're using broadcast mode.
Duke3D can be launched with the right command line options, and Duke3D's output is showed in a command line window so you can see the connection progress.
Your settings are stored in the same directory where the program is located so each time you play your settings will be restored.
Configurable Duke3D-directory and filename (Windows version only).

Download Duke3D Multiplayer Setup (Or in linux you can also compile it from source with Free Pascal Compiler).
Place the file in the same directory with duke3d_w32.exe(win32)/duke3d(linux).
Execute it.
Place your name/nick to the [nick:] input.
Place your own ip to the [Interface:] input.
Select the port that you want to use.
Add your opponents IP to the Allowed Ip list. Or select broadcast on lan and enter the Number Of Players.
Press "Save & Play" and have fun.