.Net Core Web Api Returning Binary File with No Headers Instead of Expected Response

Hey everyone,

A small issue I’ve run into while setting up a .net core web api microservice that also utilises a number of gRPC services. Symptoms were as follows:

Response Read via Fiddler
– HTTP/1.0 200 This buggy server did not return headers
– Value was binary and unable to be read

Server logs

03:59:23 DBG] Connection id “0HM01U963EUAR” accepted.
[03:59:23 DBG] Connection id “0HM01U963EUAR” started.
[03:59:23 DBG] Connection id “0HM01U963EUAR”: HTTP/2 connection error.
Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.Http2ConnectionErrorException: HTTP/2 connection error (PROTOCOL_ERROR): Invalid HTTP/2 connection preface.
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.Http2Connection.ParsePreface(ReadOnlySequence`1& buffer, SequencePosition& consumed, SequencePosition& examined)
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.Http2Connection.TryReadPrefaceAsync()
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.Http2Connection.ProcessRequestsAsync[TContext](IHttpApplication`1 application)
[03:59:23 DBG] Connection id “0HM01U963EUAR” is closed. The last processed stream ID was 0.
[03:59:23 VRB] Connection id “0HM01U963EUAR” sending GOAWAY frame for stream ID 0 with length 8 and flags 0x0
[03:59:23 DBG] Connection id “0HM01U963EUAR” stopped.
[03:59:23 DBG] Connection id “0HM01U963EUAR” sending FIN because: “The Socket transport’s send loop completed gracefully.”

The issue turned out to be that in appSettings.json the default kestrel endpoint had been set to utilise http2. Removing this immediately resolved the issue. It may also be worth double checking your launch settings, startup.cs and program.cs to ensure that nothing’s been overridden.

I expect that any similar routing issues would result in similar symptoms so hopefully this post will be able to save a few people a bit time!

Cheers,
Chris

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