How to enable ALPN in .Net 5 projects

In continuation to this topic https://github.com/grpc/grpc-dotnet/issues/1252

The problem is still being reproduced. It is not clear how it is determined whether or not there will be an ALPN section in the ClientHello message.

I created two test projects - a client project and a server project. Server: GrpcService2.zip

    public class GreeterService : Greeter.GreeterBase
    {
        private readonly ILogger<GreeterService> _logger;
        public GreeterService(ILogger<GreeterService> logger)
        {
            _logger = logger;
        }

        public override Task<HelloReply> SayHello(HelloRequest request, ServerCallContext context)
        {
            return Task.FromResult(new HelloReply
            {
                Message = "Hello " + request.Name
            });
        }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            CreateHostBuilder(args).Build().Run();
        }

        // Additional configuration is required to successfully run gRPC on macOS.
        // For instructions on how to configure Kestrel and gRPC clients on macOS, visit https://go.microsoft.com/fwlink/?linkid=2099682
        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>();
                });
    }

    public class Startup
    {
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddGrpc();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapGrpcService<GreeterService>();

                endpoints.MapGet("/", async context =>
                {
                    await context.Response.WriteAsync("Communication with gRPC endpoints must be made through a gRPC client. To learn how to create a client, visit: https://go.microsoft.com/fwlink/?linkid=2086909");
                });
            });
        }
    }

Client: ConsoleApp8.zip

        static void Main()
        {
            var url = "https://myhost:5001";                        
            var handler = new HttpClientHandler();
            handler.ServerCertificateCustomValidationCallback = HttpClientHandler.DangerousAcceptAnyServerCertificateValidator;
            var httpClient = new HttpClient(handler);
            var channel = GrpcChannel.ForAddress(url, new GrpcChannelOptions { HttpClient = httpClient });
            var client = new Greeter.GreeterClient(channel);
            var answer = client.SayHello(new HelloRequest() { Name = "111" });
            Console.WriteLine(answer.Message);
        }

When I run the client application on my Windows 10 computer, I get an error:

Error starting gRPC call. HttpRequestException: Requesting HTTP version 2.0 with version policy RequestVersionOrHigher while server offers only version fallback.

On the server side error:

Microsoft.AspNetCore.Server.Kestrel[0]. HTTP/2 over TLS was not negotiated on an HTTP/2-only endpoint.

When I run the client application on Windows Server 2016 without IIS, I get an error:

Bad gRPC response. Response protocol downgraded to HTTP/1.1.

On the Windows Server 2016 with IIS and other Windows 10 computers client application works fine.

Do I have to install or enable any Windows components or install additional libraries or frameworks in order for ALPN to be enabled?

[SLavaLL]

As it turned out, our security guards installed a DLP system to control information leakage. Apparently, the system incorrectly intercepts and replaces packets…

[JamesNK]

Sorry, I don’t know.

I suggest opening an issue at https://github.com/dotnet/runtime and providing information there. The underlying TLS connection and protocol negotiation happens in .NET library code. gRPC just happens to be using it.