Why does the WHM interface for the Nameserver selection report the nameserver as not available for selection?
If your resolver configuration references a local IP address that is bound to the server, PowerDNS will not be compatible with said configuration and you will need to remove the local IP address reference.
To determine if your server has a configuration which PowerDNS is not compatible with, you may use the following commands to determine this:
List the IP addresses bound to your server:
# ip a
[root@the ~]$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP>
[Truncated for visibility]
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP>
link/ether 82:a5:1f:af:fb:1b brd ff:ff:ff:ff:ff:ff
inet 126.96.36.199/20 brd 188.8.131.52 scope global eth0
Then dump the contents of your resolver configuration:
# cat /etc/resolv.conf
[root@the ~]$ cat /etc/resolv.conf
; Created by cloud-init on instance boot automatically, do not edit.
If you find that an IP that is bound to your server is also listed in the resolvers' configuration, then this will cause PowerDNS to display as unavailable until this configuration is updated to use external resolvers.
For more information on how to determine which resolvers are best for you, please reference the following article.