Hello
Thank you for your question and reaching out. I can understand you are having issues related to DNS.
In your case A type of split DNS with a forwarder(or Root Hints if you like) should do the trick.
InternalDNS:
- Create just a Forward zone for subsite.cmydomain.com
- Create a Blank A record "(same as parent folder)" -> 192.168.10.101
- Setup a forwarder to external Public DNS for remaining resolution (Hope you already have this in place)
- Wait for DNS to update TTL to expire or run dnscmd /clearcache on DNS servers, ipconfig /flushdns on clients.
- Now subsite.cmydomain.com should be connecting to internal where as other.contractor.com to external.
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