Just wasted a couple of hours of my life helping a colleague at work troubleshooting why we got a 502 response when trying to access our application in Azure.

Regular ASP.NET Core 10 api, nothing fancy.

Using Kudu, I could see that the dotnet process worked correctly, and I could curl it from inside the "container".

But when accessing it via a browser, a 502 Bad Gateway response was returned. Weird.

First I thought that it was some kind of network policy or similar.

Just for the fun of it, I tried curling from my own computer as well.

It worked, 200 OK. Wtf.

Tried the browser again -> 502 Bad Gateway.

So curl worked, but not the browser.

Turns out, curl sends HTTP/1 requests if you don't specify that it should use HTTP/2.

When I instructed curl to send HTTP/2 requests, I got the same 502 Bad Gateway error.

Solution

You see that little HTTP 2.0 Proxy dropdown? For some reason, this was set to On, which means that Azure tries to talk HTTP/2 all the way to your application. Our application only runs on http, meaning that HTTP/2 is not supported.

Changing this to Off and restarting the app service -> Problem solved.