@dionedomingo

When it comes to setting up a DNS cluster, PowerDNS would probably be the easiest to work with as it has a built-in API that you can use to create a module for WHMCS (if there’s not already one).

Ideally, you’d setup two, three, or more Droplets and install PowerDNS on each one. This would be a standard master/slave setup (replication), though you could setup something more robust, it all really depends on what you need and how many domains you’re hosting or plan to host.

There’s an existing guide here at DigitalOcean that details how to go about setting up PowerDNS, so I’d take a look at it to get familiar. It was written for Ubuntu 14.04, though it should work the same with Ubuntu 16.04 or 16.10.

https://www.digitalocean.com/community/tutorials/how-to-configure-dns-replication-on-a-slave-powerdns-server-on-ubuntu-14-04

The PowerDNS API can be found here:

https://doc.powerdns.com/md/httpapi/README/