Is it necessary to configure "round-robin" DNS in the Primary DNS or can a secondary DNS be utilized?

This can be done in any DNS server, primary or secondary. Swarm doesn't need DNS; it is for the benefit of the Swarm clients. The round-robin record shown below would allow for clients one method of distributing their initial requests across the cluster:

swarm  0 IN A 192.168.1.101
       0 IN A 192.168.1.102
       0 IN A 192.168.1.103
       0 IN A 192.168.1.104

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