We have an Azure Front Door resource configured with a single backend pool and a health probe to the backend. The backend is a web API running in App Service. The App Service resource is configured with a custom domain. The Front Door health metrics show drastically different results when configuring the backend with the custom domain vs. the one provided by App Service.
We originally configured the backend with the hostname provided by App Service (xxx.azurewebsites.net). The metrics in Front Door showed a very low, very inconsistent success rate for backend health. Then, we tried configuring the backend using the custom hostname we use in the App Service resource, and all of a sudden the health metrics are at 100%.
In the following image, you can see the point where we configured the backend with the custom hostname. The average health shoots up to 100%.
I would expect the Front Door health metrics to be the same (at or near 100%) regardless of the hostname used to configure the backend.
