We are an IT service provider, and we have a client that hosts a web application from their internal network, but it is accessible from outside their network over the Internet. They have a primary and redundant Internet connection connected to a SonicWALL TZ600 firewall, which supports redundant outbound connections but not inbound. Both Internet connections have static IP addresses.

How do we use Dynu DDNS to create primary and backup DNS A records for www.hoffmancabinets.net?

Creating two A records will cause a round robin scenario which means either of those A records will be used to access the website. This will help with load balancing but not failure.

You will need a monitoring system which switches the A record's IP address in case the primary IP address is down. You can write a script which uses the API to update the A record when the IP address is not usable.

You can also update the IP address for the A record using a simple URL such as https://api.dynu.com/nic/update?hostname=somedomain.com&alias=somealias.