The other day I was going to test and debug some code on a Windows Server 2012RC machine when I encountered the exception ERR_HTTP2_INADEQUATE_TRANSPORT_SECURITY when running my asp.net core 3.1 razor pages website.

This site worked fine elsewhere so to try and narrow down the problem I created a brand new asp.net core website to see if it was something in my code that was the issue but I had the same error showing up in google chrome.

After some googling, I tried to reset the servers self-signed SSL certificate by using the following closing the browser in between but that had no effect:

dotnet dev-certs https --clean
dotnet dev-certs https --trust

I created a github issue and the ever-helpful @guardrex came to my rescue again and pointed me in the right direction.

It is a known bug and there is an open Github issue for it.

Workaround

So here for others and me if this happens again is my workaround:

A specific down-level machine (Windows Server 2012R2) was causing the exception and because I knew which machine, I had access to its Environment.MachineName which I use later to programmatically decide which ListenOptions.Protocols Kestrel should load.

HTTP/1 on the down-level machine but HTTP/2 elsewhere.

So, I created an appsettings.{the-environment.machinename-value-here}.json file alongside appsettings.Development.json with the following HTTP/1 settings rather than the default HTTP/2 settings that are loaded by default elsewhere.

appsettings.machinename.json

{
  "Kestrel": {
    "EndpointDefaults": {
      "Protocols": "Http1"
    }
  }
}

Then in Program.cs I modified CreateHostBuilder to read the above custom appsettings.json file.

Note the line containing: config.AddJsonFile($”appsettings.{Environment.MachineName}.json”, optional: true);

Program.cs

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

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
            .ConfigureAppConfiguration((hostingContext, config) =>
            {
                config.AddJsonFile($"appsettings.{Environment.MachineName}.json", optional: true);
                config.AddCommandLine(args);
            })
            .ConfigureWebHostDefaults(webBuilder => webBuilder.UseStartup<Startup>());
    }
}

Success 🎉