App Service not getting requests from its external IP

Question

I currently have an asp.net core middleware app currently running on a server and I want to move it to an Azure app service. This app is used to take requests from various domains that we own and redirect them to the appropriate URL. So the domains we own have A records in DNS to send them all to the IP of the app. and the app sees the request come in as whatever domain, then looks up where that should domain go, and redirects it as needed. This works on the server currently, but I moved it to an app service and it does not work. My app doesn't see the requests from the domain. If you go to one of those domains it says timed out and nothing loads. If I setup the domain as a custom domain, then it does work. But I can't do that for the 800+ domains we have. I did setup one custom domain with a self signed cert so i can make sure the app has a static IP, but this doesn't change anything.

What I believe is that the app service needs to be able to allow requests to come into the public IP from various domains without me having to add it as a custom domain. Is there a way I can do this without creating a VM instead in Azure? Maybe through some networking configuration?

Thanks for any help that can be provided!

Answer 1

App Services by default are routed by the host header in the request. If the custom domain hasn't been mapped/bound to the app service you'll see a 404. This is the expected behavior of app services since its a multi-tenant deployment. There is a limit on the number of custom domains that can be added to a we bapp so that could be a pontial blocker as well.

https://azure.github.io/AppService/2017/08/08/FAQ-App-Service-Domain-and-Custom-Domains.html

Answer 2

Update: I didn't end up finding an acceptable way to do this via App Service, and had to go the VM route here.