MS365 local internet egress



I'm trying to figure out the correct configuration for DNS when deploying M365. In a local office, the set up is: private network -> on-premise DNS server -> firewall -> ISP router -> Internet


When resolving M365 endpoints for geographic locations, should the on-premise DNS server be configured as a resolving DNS server or should it be configured as a forwarding DNS server pointing to an ISP provided DNS server?


I'm guessing that if it is configured as a resolving DNS server then the source IP address in any DNS query will be the public IP address of the ISP router which should be ideal from a Geo-DNS lookup viewpoint.  But I'm not sure - any advice is welcome.




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