Built in. On the forums it says a start option in the game.
From this post:
https://forums.eagle.ru/showthread.php?t=2334861. What is needed to run a dedicated server?
Dedicated server is not a standalone software, it's one of DCS start option. To run a server, a (separate) DCS base install is needed with all terrains (you wish to host) purchased and installed. We are considering possibilities for running terrains without purchase them in --norender mode.
2. How to run a dedicated server?
Currently, the dedicated server can only be started manually from the command line appending the following parameters to file
--server
runs DCS in