SSL error on downlevel windows versions

The other day I was going to test and debug some code on a Windows Server 2012RC machine.

When running my asp.net core 3.1 razor pages website I encountered the exception:

ERR_HTTP2_INADEQUATE_TRANSPORT_SECURITY

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

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

Success 🎉