SSL connection could not be established

Question

I've got an issue with sending HTTP requests to as part of automation testing, I want to check URL's status code. When I send the request I have an Exception:

System.AggregateException: 'One or more errors occurred. (The SSL connection could not be established, see inner exception.)'

The inner exceptions are:

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

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

SocketException: An existing connection was forcibly closed by the remote host.

My code:

    public HttpStatusCode SendRequest(string _url)
    {
        SetUrl(_url);

        HttpClientHandler clientHandler = new HttpClientHandler();
        clientHandler.ServerCertificateCustomValidationCallback += (sender, cert, chain, sslPolicyErrors) => { return true; };
        clientHandler.SslProtocols = SslProtocols.Tls;

        HttpClient client = new HttpClient(clientHandler);

        var response = client.GetAsync(Url).Result;

        StatusCode = response.StatusCode;

        return StatusCode;
    }

Thank you in advance !

Answer 1

The remote host does (rightfully) not accept that you want to connect using TLS 1.0, which is deprecated.

Specify .Tls12 or .Tls13.

Answer 2

As @CodeCaster already mentioned: SslProtocols.Tls (TLS 1.0) is not an option nowadays. Simply use SslProtocols.None (which is the default) to let the OS choose the best option for you:

None: Allows the operating system to choose the best protocol to use, and to block protocols that are not secure. Unless your app has a specific reason not to, you should use this field.

https://docs.microsoft.com/en-us/dotnet/api/system.security.authentication.sslprotocols?view=net-5.0#fields

Furthermore skipping the server certificate validation (by letting ServerCertificateCustomValidationCallback just return true) is a very dangerous thing. You should avoid that.

Is the remote host under your control? Can you check which TLS version it's using?