Hello @Chris Norwood ,
Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.
I understand that you are using Azure Traffic manager with geographic routing with 2 endpoints - one app service for East Asia region and one Application gateway for all other regions and routing the www domain traffic to them but you would like to change the domain mapping of your Traffic manager from root to subdomain and would like to know if the Traffic Manager can correctly identify that it should send this traffic to the Application gateway, provided the Application gateway is already configured to receive traffic for these domains.
As explained in How Traffic Manager Works, Traffic Manager works at the DNS level. It uses DNS responses to direct clients to the appropriate service endpoint. Clients then connect to the service endpoint directly, not through Traffic Manager. Traffic Manager is not a proxy or a gateway. Traffic Manager doesn’t see HTTP traffic between client and server. When using a vanity domain with Azure Traffic Manager, you must use a CNAME to point your vanity domain name to your Traffic Manager domain name.
As long as your application is configured to accept the correct host header for the domain name you’re using, the Traffic manager will route the traffic correctly.
If you map your DNS for the subdomains to point at the Traffic Manager address and the Application gateway along with it's backend pool are configured to accept the the correct host header for those subdomains, the Traffic manager will be able to route the traffic to the Application gateway without any issues.
Kindly let us know if the above helps or you need further assistance on this issue.
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