From the Manual linked above, I currently implemented those 2 topics.
Distribute Pi-hole as DNS server via DHCP
Hostnames instead of IP addresses in Pi-hole's web interface - Conditional forwarding
Now I want to also add
Pi-hole as upstream DNS server for your Fritz!Box to achieve seeing the requests sent by the FritzBox itself in Pi-Hole and also make use of the guest network blocking.
Can I simply do this without hassle (currently FritzBox upstream is set to 1.1.1.1) or does that generate the DNS loop warned of?
Warning
Don't set the Fitz!Box as upstream DNS server for Pi-hole if using this configuration! This will lead to a DNS loop as the Pi-hole will send the queries to the Fritz!Box which in turn will send them to Pi-hole.
While your screenshot shows that unbound is indeed your Pi-hole's sole upstream server, your FB could still be used upstream if your unbound would be configured to use your FB.
That would not be the case if you've followed our unbound guide - unbound would be running as a recursive resolver then.
This would indeed close a partial DNS loop for local hostnames.
You could trigger that by trying to resolve a non-existing hostname in your network through your FB, e.g.
nslookup bogeyman.fritz.box 192.168.178.1
(assuming a default FB network)
Any known hostnames would be answered by your FB at 192.168.178.1 straight away, while non-existing ones would be passed to Pi-hole, which in turn would conditionally forward that query back to your router, and so on, until time-out.
You should see a flood of requests for bogeyman.fritz.box in Pi-hole's Query Log.
That said, I'd consider it unlikely that you'd observe many queries that would trigger that partial loop. Nevertheless, it would still be possible that a misbehaving client would send DNS requests for non-existing local hostnames.
You could go ahead and try such a configuration and watch the Query Log closely for a while, and keep it if you don't see any signs for a loop.
