Hi ,
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When you run "dcdiag /test:dns" tests forwarders by communicating through RPC protocol and DCOM.
You might see events like:
DCOM errors 10009 in the event logs (DCOM was unable to communicate with the computer X.X.X.X using any of the configured protocols).
If the Forwarded server is responding fine to nslookup, this error can be ignored. You can disregard this warning if the DNS server is a BIND or other non-Microsoft DNS server.
For your reference:
Best Regards,
Candy