I encountered an odd issue today: using unbound as a private dns proxy for private ip address reverse lookups, like this:

dig -x 192.168.0.1

I did a quick search and found the solution, adding a stub zone like this:

local-zone: "168.192.in-addr.arpa." nodefault

stub-zone:
    name: "168.192.in-addr.arpa."
    stub-addr: private-dns-server