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"Can I configure reverse DNS for #IPv6 PublicIpAddress resources?

No. #Azure DNS does not currently support reverse DNS (PTR records) for Azure-owned public IPv6 addresses."

https://learn.microsoft.com/en-us/azure/dns/dns-reverse-dns-for-azure-services#can-i-configure-reverse-dns-for-ipv6-publicipaddress-resources

Reverse DNS for Azure services - Azure DNS

With this learning path, get started configuring reverse DNS lookups for services hosted in Azure.