An anonymous request was received in between authentication handshake requests

I was receiving this error while hosting a webapi project on .net 5, using kestrel.

fail: Microsoft.AspNetCore.Authentication.Negotiate.NegotiateHandler[5]
      An exception occurred while processing the authentication request.
      System.InvalidOperationException: An anonymous request was received in between authentication handshake requests.
         at Microsoft.AspNetCore.Authentication.Negotiate.NegotiateHandler.HandleRequestAsync()
fail: Microsoft.AspNetCore.Server.Kestrel[13]
      Connection id "0HM6A34N67LO5", Request id "0HM6A34N67LO5:00000005": An unhandled exception was thrown by the application.
      System.InvalidOperationException: An anonymous request was received in between authentication handshake requests.
         at Microsoft.AspNetCore.Authentication.Negotiate.NegotiateHandler.HandleRequestAsync()
         at Microsoft.AspNetCore.Authentication.Negotiate.NegotiateHandler.HandleRequestAsync()
         at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
         at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
         at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
         at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)

Symptoms

Windows authentication works just fine when debugging locally, however, fails when deployed to a server, or accessing remotely via dns, with this error: System.InvalidOperationException: An anonymous request was received in between authentication handshake requests.

How I corrected it

My kestrel endpoints were configured to https://0.0.0.0:1234, with a certificate from myservice.mydomain.com.

I resolve the issue by setting my endpoint to https://myservice.mydomain.com:1234

As well, ensure you have a SPN configured using setspn -S HTTP/myserver.mydomain.com myserviceaccount

Why am I blogging about this?

Because all of the advice I found while googling was incredibly unhelpful.

• Github Issue #1 Closed by Microsoft bot. No help.
• Github Issue #2 Closed, states does not work by design.
• Stackoverflow #1. No help at all.
• Stackoverflow #2. No Help.
• Stackoverflow #3 Links back to a github article stating windows auth is not supported.
• Microsoft Documentation Says to use http.sys instead of kestrel, and/or update SPN.

So, If I helped a single person, by taking the 5 minutes to write this article, it has served its purpose.

If you do stumble upon this, and it helps you, leave a comment down below.