.Net core HttpClient bug? SocketException: An existing connection was forcibly closed by the remote host

Question

The following code runs without any error in a full .Net framework console program. However, it got the following error when running in .Net core 2.1.

System.AggregateException
  HResult=0x80131500
  Message=One or more errors occurred.
  Source=System.Private.CoreLib
  StackTrace:
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at ConsoleApp1.Program.requestString(String url) in C:\source\repos\ConsoleApp1\Program.cs:line 38
   at ConsoleApp1.Program.Main(String[] args) in C:\source\repos\ConsoleApp1\Program.cs:line 13

Inner Exception 1:
HttpRequestException: The SSL connection could not be established, see inner exception.

Inner Exception 2:
IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.

Inner Exception 3:
SocketException: An existing connection was forcibly closed by the remote host

Code:

class Program
{
    static void Main(string[] args)
    {
        var url = "https://google.com";
        var (statusCode, html) = requestString(url);
        Console.WriteLine("%d %s", statusCode, html);
    }

    static CookieContainer cc = new CookieContainer();

    static HttpClientHandler handler = new HttpClientHandler { AllowAutoRedirect = false, CookieContainer = cc };

    public static async Task<(int statusCode, string content)> requestStringAsync(string url)
    {
        ServicePointManager.ServerCertificateValidationCallback += (sender, cert, chain, sslPolicyErrors) => true;
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
                                                                     // | SecurityProtocolType.Ssl3;
        using (var request = new HttpRequestMessage { RequestUri = new Uri(url), Method = HttpMethod.Get })
        using (var client = new HttpClient(handler))
        {
            var response = await client.SendAsync(request); // Error (actual line)
            // response.EnsureSuccessStatusCode() |> ignore
            var statusCode = (int)response.StatusCode;
            var content = await response.Content.ReadAsStringAsync();
            return (statusCode, content);
        }
    }

    public static (int statusCode, string content) requestString(string url)
    {
        return requestStringAsync(url).Result;
    }
}

Answer 1

There is a bug for .NET Core 2.1 Preview mentioning this issue. That may be the cause. However, I also notice that your setting of TLS is incorrect. You're currently enabling it, but overwriting all other protocols that have been set. Instead of this:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

You should be using this:

ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
// ----------------------------------^

I think that's a side issue, but worth fixing all the same.

Update

The referenced GitHub issue above has a discussion which eventually links to the official announcement for .NET Core 2.1 SDK Preview 2. It has the following to say:

Sockets Performance and SocketsHttpHandler

We made major improvements to sockets in .NET Core 2.1. Sockets are the basis of both outgoing and incoming networking communication. The higher-level networking APIs in .NET Core 2.1, including HttpClient and Kestrel, are now based on .NET sockets. In earlier versions, these higher-level APIs were based on native networking implementations.

...

You can use one of the following mechanisms to configure a process to use the older HttpClientHandler:

From code, use the AppContext class:

AppContext.SetSwitch("System.Net.Http.UseSocketsHttpHandler", false);

The AppContext switch can also be set by config file.

The same can be achieved via the environment variable DOTNET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER. To opt out, set the value to either false or 0.