"The underlying connection was closed: An unexpected error occurred on a send.". Postman goes OK with same headers

Question

Scenario

• Win10 x64
• VS2013

I'm trying to make a WebRequest, but I'm getting the following error:

The underlying connection was closed: An unexpected error occurred on a send.

Digging into the inner exception, I got:

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

The code which does the request is the following:

private static Hashtable exec (String method, String uri, Object data, String contentType) {
    Hashtable response;

    HttpWebRequest request = (HttpWebRequest)WebRequest.Create (API_BASE_URL + uri);

    request.UserAgent = "MercadoPago .NET SDK v"+MP.version; //version resolves to 0.3.4
    request.Accept = MIME_JSON; // application/json
    request.Method = method; //GET
    request.ContentType = contentType; //application/json
    setData (request, data, contentType); //setData in this case does nothing.

    String responseBody = null;
    try {
      HttpWebResponse apiResult = (HttpWebResponse)request.GetResponse (); //Error throws here
      responseBody = new StreamReader (apiResult.GetResponseStream ()).ReadToEnd ();

      response = new Hashtable();
      response["status"] = (int) apiResult.StatusCode;
      response["response"] = JSON.JsonDecode(responseBody);
    } catch (WebException e) {
      Console.WriteLine (e.Message);
    }
}

What i've already done:

• Made the request via Console Application and MVC Application controller. Both throws the same exception
• Called the API via Postman with the exact same headers, which brings me the content correctly.

Those requests were working okay via c# about 4 days ago and I suddenly started having issues, but considering the fact that it responds okay for Postman, I can't figure out where's the problem.

Here's Postman's response

EDIT: Did both requests with Fiddler listening. The result for Postman shows a direct request to the API with HTTPS. When trying with my ConsoleApplication, it shows a HTTP request, which makes a tunnel to the API endpoint, port 443.

The TextView from Fiddler for the tunnel request says the following:

I noticed the "time" field which refers to a very old date, but i don't know what does it mean.

Answer 1

It is kind of bad practice to enable Tls12 like this-

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

In future, if you'd need to use higher version of TLS, you'll have to update your code.

If you are using an older version of .NET, you can simply switch it higher version in which Tls12 is enabled by default.

For example, this simple change in your web.config will enable Tls12 automatically-

<httpRuntime targetFramework="4.6.1"/>