Box2d on authoritative server for Unity client

I’m creating a multiplayer platform and shooting game in unity, using unity2d’s own physics system (box2d).

But the truth is that I am quite lost in the creation of an authoritative server in c ++.
I have quite clear and experience in the use of socket for communication between client and server, I do not know how to approach the issue of simulating the physics of my game on the server so that it decides the positions/movements of the clients.

Can anyone guide me to use box2d on the server, as I simulate the game on the server? Is this a good option or would it be better to implement my own game physics than to use the non-deterministic engine?

Thank you very much and sorry for my level of English