It doesn't need to know about the changes, but it surely needs to notice them as they happen.

For example, FLOSS nginx does DNS lookups just once on startup. On config (re)loads, to be exact.

E.g. when you have "proxy_pass http://spam;" or "uwsgi_pass spam:9000;", you'll end up with permanent 502 (until a SIGHUP) after you've started a new "spam" Docker container and then removed the old one.

There are well-known workarounds like "set $backend "http://spam"; proxy_pass $backend;", but it's nicer when you don't even have to think about this and know that TTLs are properly honored. Caddy does that.