The majority of backend web apps only talk to a limited few services; db’s, internal queues, external apis. My main app talks to two internal db’s, and maybe 4

The majority of backend web apps only talk to a limited few services; db’s, internal queues, external apis. My main app talks to two internal db’s, and maybe 4 external apis. It should be possibles to specify the host names of these services and block everything else, certainly on a PAAS such a Heroku (they even know what internal services you are using), but that functionality isn’t there… I simply don’t understand why not. It should be possible to build this sort of firewall based on hostnames not IP addresses? Maybe with a custom DNS server? And to be honest I would take the configuration complexity if it made my apps more secure. There will be apps that talk to more, and as you said use UDP etc. but these will be the minority.