You have the following or similar Azure setup in a single tenant/subscription:
- Azure Front Door with routing rules mapping Azure Front Door Frontends to backend App services (Web apps)
- App Services (Web Apps) running on .NET/MVC which internally utilize web sockets and Signal R.
- Azure SQL as the backend datastore.
During testing of the system you encounter Web socket and SignalR errors in Azure Front Door when you call the Azure Front Door URLs from the Internet. When you call the App service URLs directly (*.azurewebsites.net) the issue does not exist.
Regarding Web socket and SignalR errors in Azure Front Door, Microsoft community forum threads exist on this matter, as follows:
If Azure Front Door does not officially support SignalR, the equivalent design comprises Azure Application Gateway with Web Application Firewall and Azure CDN. Also in this case, you will have to use your our own SSL certificates to secure your custom DNS domains. With Front Door, you can utilize the free Front Door-managed certificate service by Digicert. You can refer to this blog article for a comparison between Azure’s load balancing and L7 application gateway appliances (Azure Front Door, Application Gateway, Traffic Manager and Load Balancer).
Update (Dec 2021):
Microsoft Technical Support team’s official response is that as of end of 2021, SignalR and Web socket technologies are not yet supported in Azure Front Door.
Update (Oct 2022):
As per below article, possible ETA for Azure Front Door Web Socket support preview is March 2023.
Issue is still being tracked in Microsoft Azure Feedback portal: