How can I transfer only A records from one DNS zone to another without using scripts, full zone transfers?

I have two DNS servers for different zones: subdomain.example.com (subdomain) example.com (main domain) The goal is to dynamically transfer A records from subdomain.example.com to example.com without using scripts, full zone transfers. Here are the constraints and requirements: The two servers have different SOA records and configurations. I cannot use scripts, zone transfers (AXFR) for synchronization. The A records need to be added automatically to the example.com zone without overwriting existing records. Can anyone help with the configuration steps and advice on how to set this up? Ideally, I need to ensure: Secure and authenticated updates between the servers. Only the A records should be transferred. No existing data in the example.com zone should be overwritten.