Delphi Indy Connection through Proxy using NTLM: Overcoming the POST Exception

As a Delphi developer, you may have encountered an issue when trying to establish a connection through a proxy server using NTLM authentication, resulting in a frustrating POST exception. Fear not, dear reader, for we’re about to dive into the solution to this pesky problem.

What’s the Issue?

The Delphi Indy library, a popular choice for networking and internet protocols, can be finicky when it comes to proxy servers and NTLM authentication. The POST exception is a common error that arises when the proxy server requires NTLM authentication, but Indy isn’t configured correctly to handle it.

Why NTLM Authentication?

NTLM (NT LAN Manager) is a authentication protocol used by Microsoft to authenticate users and computers. In a corporate environment, NTLM is often used to authenticate users to the network, and proxy servers may require NTLM authentication to allow access to the internet. Delphi Indy needs to be configured to support NTLM authentication to successfully connect through the proxy server.

Configuring Delphi Indy for NTLM Authentication

Before we dive into the solution, ensure you have the following prerequisites:

• Delphi Indy 10 or later
• A proxy server that requires NTLM authentication
• A Delphi project with the Indy library installed

Step 1: Add the required Indy units

In your Delphi project, add the following units to your uses clause:

  uses
    IdHTTP, IdSSLOpenSSL, IdProxy, IdAuthenticationSSPI;

Step 2: Create an instance of TIdProxy

Create an instance of TIdProxy and set the proxy server’s address and port:

  var
    Proxy: TIdProxy;

  Proxy := TIdProxy.Create;
  Proxy.Host := 'your_proxy_server_address';
  Proxy.Port := 8080;

Step 3: Configure NTLM Authentication

Create an instance of TIdAuthenticationSSPI and set the authentication type to `atNTLM`:

  var
    Auth: TIdAuthenticationSSPI;

  Auth := TIdAuthenticationSSPI.Create;
  Auth.AuthenticationType := atNTLM;

Step 4: Assign the proxy and authentication objects

Assign the proxy and authentication objects to the TIdHTTP instance:

  var
    HTTP: TIdHTTP;

  HTTP := TIdHTTP.Create;
  HTTP.Proxy := Proxy;
  HTTP.Proxy.Authentication := Auth;

Step 5: Set the proxy username and password

Set the proxy username and password using the `Proxy.Username` and `Proxy.Password` properties:

  HTTP.Proxy.Username := 'your_username';
  HTTP.Proxy.Password := 'your_password';

Handling the POST Exception

Now that we’ve configured Delphi Indy for NTLM authentication, let’s handle the POST exception. The exception occurs when the proxy server returns a `407 Proxy Authentication Required` response, which Indy doesn’t handle by default.

Implementing a custom TIdHTTPOnProxyAuthEvent handler

Create a custom event handler to handle the `OnProxyAuth` event:

  procedure TForm1.HTTPProxyAuth(Sender: TObject; APayload: String);
  begin
    // Handle the 407 response by re-sending the request with NTLM authentication
    HTTP.Request.Username := HTTP.Proxy.Username;
    HTTP.Request.Password := HTTP.Proxy.Password;
    HTTP.Request.BasicAuthentication := False;
    HTTP.Request.NTLMAuthentication := True;
    HTTP.Post('https://example.com', APayload);
  end;

Assign the event handler

Assign the event handler to the `OnProxyAuth` event:

  HTTP.OnProxyAuth := HTTPProxyAuth;

Testing the Connection

Now that we’ve configured Delphi Indy for NTLM authentication and handled the POST exception, let’s test the connection. Create a button on your form and add the following code to the `OnClick` event:

  procedure TForm1.Button1Click(Sender: TObject);
  begin
    try
      HTTP.Get('https://example.com');
      ShowMessage('Connection successful!');
    except
      on E: Exception do
        ShowMessage('Error: ' + E.Message);
    end;
  end;

Run the application

Run the application and click the button. If everything is configured correctly, you should see a successful connection message. If you encounter any issues, check the proxy server’s logs for errors or adjust the configuration as needed.

Common Issues	Solutions
Proxy server returns a 401 error	Check the proxy username and password; ensure they are correct and try again.
Proxy server returns a 407 error	Check the NTLM authentication configuration; ensure the authentication type is set to `atNTLM` and the username and password are correct.
Indy throws an exception	Check the Indy logs for errors; ensure the Indy units are correctly installed and configured.

Conclusion

In this article, we’ve covered the steps to establish a Delphi Indy connection through a proxy server using NTLM authentication, overcoming the pesky POST exception. By following these instructions and configuring Delphi Indy correctly, you should be able to successfully connect to the internet through your proxy server. Remember to test your application thoroughly and troubleshoot any issues that may arise.

Happy coding, and may the connectivity be with you!

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