Restrict access to the server using vpn?

Question

linux

Good time, I ran into a problem that I can not solve using the methods known to me. There is a server, several services and vpn are running on the server, the task is to restrict access to some of the services, namely, so that they are available only through vpn, while maintaining the ability to access them by domain name.
How to solve this problem?
PS I understand that there is little information, but there will be many letters to describe everything, but I am ready to answer constructive questions

Answer 1

Deny connection to all IPs except the VPN network.