No that was not really what I was referring to. I was talking about domains in a Web context, not ad context. I give you a concrete example:
In my home network I have a few devices that provide webinterfaces. There is for example my nas, but also a raspberry pi. I have a router running openwrt and on that router I have nginx running. All traffic on port 80 and 443 from the outside world terminates on that nginx instance. If there is a request for nas.my-domain.com the request get proxied to my nas, if there is a request for pi.my-domain.com it get proxied to the pi. That functionality is referred to as a reverse proxy.
Ps: instead of trying to achieve this with squid on the Univention host, I’d rather recommend to either solve this on your firewall (if possible) or on the system that currently receives your http traffic. Good tools for reverse proxies are nginx, or if you want something dead simple (which also takes care of automatic https) caddy.