FluentFTP: Authentication failed because the remote party has closed the transport stream

Question

I am using a C# console app to repro the issue. It's a .NET Framework 4.7 app. The FluentFTP version is 24.0.0, installed as a Nuget package.

This is my code:

using FluentFTP;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;

namespace FTP_Test
{
    class Program
    {
        private static string host = "<my host>";
        private static string username = "<my username>";
        private static string pass = "<my pass>";
        private static int port = 990;

        static void Main(string[] args)
        {
            FtpClient ftpClient = new FtpClient(host, port, username, pass);

            ftpClient.DataConnectionType = FtpDataConnectionType.EPSV;
            ftpClient.EncryptionMode = FtpEncryptionMode.Implicit;

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

                        ftpClient.DataConnectionEncryption = true;
            ftpClient.ValidateCertificate += new FtpSslValidation(OnValidateCertificate);
            var cer = new System.Security.Cryptography.X509Certificates.X509Certificate2();
            ftpClient.ClientCertificates.Add(cer);
            ServicePointManager.ServerCertificateValidationCallback = ServerCertificateValidationCallback;

            ftpClient.Connect();
        }

        private static void OnValidateCertificate(FtpClient control, FtpSslValidationEventArgs e)
        {
            // add logic to test if certificate is valid here
            e.Accept = true;
        }
        private static bool ServerCertificateValidationCallback(object sender,
                                                System.Security.Cryptography.X509Certificates.X509Certificate certificate,
                                                System.Security.Cryptography.X509Certificates.X509Chain chain,
                                                System.Net.Security.SslPolicyErrors sslPolicyErrors)
        {
            return true;
        }
    }
}

The error (and the stacktrace) I'm getting in the console:

Unhandled Exception: System.IO.IOException: Authentication failed because the remote party has closed the transport stream.
   at System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)
   at System.Net.Security.SslStream.AuthenticateAsClient(String targetHost, X509CertificateCollection clientCertificates, SslProtocols enabledSslProtocols, Boolean checkCertificateRevocation)
   at FluentFTP.FtpSocketStream.ActivateEncryption(String targethost, X509CertificateCollection clientCerts, SslProtocols sslProtocols)
   at FluentFTP.FtpClient.Connect()
   at FTP_Test.Program.Main(String[] args) in C:\Users\Nemanja\source\repos\FTP Test\Program.cs:line 35

What am I missing here?

I am able to connect through Filezilla, using the same set of host/user/pass. In Filezilla, I have to set the following:

• Ecryption: Require implicit FTP over TLS (in the "General" tab)
• Transfer mode: Passive (in the "Transfer Settings" tab)

When I run the app in debug mode and set breakpoints inside functions OnValidateCertificate and ServerCertificateValidationCallback, those breakpoints are not hit.

Answer 1

Turns out FluentFTP doesn't support Implicit SSL, so I had to switch to Chilkat.

example-code.com has good examples for this library and here is the link to the example for Implicit SSL: https://www.example-code.com/csharp/ftp_implicitSSL.asp

Copying the example (from the link above) here in case the link stops working.

// This example requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

Chilkat.Ftp2 ftp = new Chilkat.Ftp2();

// If this example does not work, try using passive mode
// by setting this to true.
ftp.Passive = false;
ftp.Hostname = "ftp.something.com";
ftp.Username = "test";
ftp.Password = "test";
ftp.Port = 990;

// We don't want AUTH SSL:
ftp.AuthTls = false;

// We want Implicit SSL:
ftp.Ssl = true;

// Connect and login to the FTP server.
bool success = ftp.Connect();
if (success != true) {
    Debug.WriteLine(ftp.LastErrorText);
    return;
}
else {
    // LastErrorText contains information even when
    // successful. This allows you to visually verify
    // that the secure connection actually occurred.
    Debug.WriteLine(ftp.LastErrorText);
}

Debug.WriteLine("FTPS Channel Established!");

// Do whatever you're doing to do ...
// upload files, download files, etc...

success = ftp.Disconnect();