I need to establish a websocket connection to a ASP.NET Core 6 backend over a 3rd party API Gateway. For some reason, the API Gateway is adding an additional User-Agent header to the request which results in a bad request (400).
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN""http://www.w3.org/TR/html4/strict.dtd">
<HTML><HEAD><TITLE>Bad Request</TITLE>
<META HTTP-EQUIV="Content-Type" Content="text/html; charset=us-ascii"></HEAD>
<BODY><h2>Bad Request - Invalid Header</h2>
<hr><p>HTTP Error 400. The request has an invalid header name.</p>
</BODY></HTML>
Ok, sending two User-Agents is at least questionable and needs to be fixed by the Gateway product.
What boggles me is that the same request works when it is sent to a "public" Azure App Service, but not when sent to an App Service hosted in our "Azure App Service Environment V2" (internal URL). Deployed is the exact same sample code and the app service is configured using the same settings. I was not able to log the "Bad Request" in the backend (not using Application Insights and not using App Service Logs / Kudu).
I'm starting to doubt that the error is produced by the App Service itself. Any idea what could cause the different behavior or how I can allow these duplicated headers until they can be avoided?
Update:
It looks like this behavior comes from IIS. We also tested it on an endpoint on ASEv3 and there the duplicated headers doesn't seem to be problem.