How to setup primary DNS as OCI DNS and secondary DNS as EasyDNS?

Solution 1:

This is not supported per OCI DNS FAQ:

What does Oracle Cloud Infrastructure DNS support for secondary DNS today?

Oracle Cloud Infrastructure DNS zones may be configured as Secondary zones today. External name servers are not currently supported for secondary DNS.

Also, as @HåkanLindqvist mentions in the comments, the API reference for UpdateZoneDetails only has externalMasters, but there are no attributes for external secondary servers. Therefore, it seems there is no way to allow zone transfers from OCI DNS. It seems you could use EasyDNS as the primary and OCI DNS as the secondary, if you need both.

As OCI alone already provides anycast DNS, it is quite redundant and protected by itself, already. You should think carefully whether you actually need two different DNS providers. Yes, it might add some redundancy, but it also adds some costs. Also, if something changes with either of the providers, it might break the zone transfers, making part of your name servers out-of-date or even completely malfunctioning. With big DNS infrastructures it is possible that the zone transfer is invoked from another IP address than any of the actual name servers listed. If only one provider is used, they are aware of that and their access lists are updated accordingly.