serpus2k2

You have to run it on a separate box or on a vps. You can't run a server and play the game on the same computer since steam will throw a hissy fit. Take a look at the first post in this thread for more clarification. http://forums.bistudio.com/showthread.php?147537-Tutorial-How-to-run-ArmA3-server-on-a-dedicated-server