This does work for unencrypted DNS and DoT over port 853, but it fails for DoT over any other port and DoH, which looks just like a website visit. You can't tell whether a website is acting as a DoH server without TLS inspection, and you can't differentiate between multiple domains on the same IP address either (unless a client is stupid enough to use SNI). Some clients do actually hardcode direct IP addresses to get around entirely missing DNS servers, notably Windows.
alt Hacker News
This does work for unencrypted DNS and DoT over port 853, but it fails for DoT over any other port and DoH, which looks just like a website visit. You can't tell whether a website is acting as a DoH server without TLS inspection, and you can't differentiate between multiple domains on the same IP address either (unless a client is stupid enough to use SNI). Some clients do actually hardcode direct IP addresses to get around entirely missing DNS servers, notably Windows.