I have set OpenDNS as primary DNS and Google (184.108.40.206.) as the secondary one in PiHole. I always thought that the secondary DNS is a fall-back solution; only used in case the primary DNS fails. However, looking at the admin page it seems that the Pi is simply using both at the same time. The primary DNS gets about 60% of the queries and the other 40% go to the secondary one. Why is this?
Now I'm wondering if my Router, which has a secondary DNS set in case my Pi goes down, will also send a portion of the queries directly to OpenDNS in stead of the Pi. This wouldn't be good since I'd get ads and tracking but I also don't want to cripple my home network whenever the Pi is offline.