Your problem description fits the model that BootP/DHCP client/servers use. The DHCP client "Discovers" the server via UDP broadcast. The server, if so configured, can recognize the Mac address of the client, and return client-specific/custom configuration information.

The protocol allows for customized "DHCP Options" that can be used to pass information to applications, suc as the one you are considering. This would allow the app to "discover" the server, but that discovery happens when the client gets/renews its IP address.

To research this, please google "DHCP Options".