"The server response contains a null character" when connecting to FTP site on port 990 using SSH.NET

Question

I'm trying to use Renci SSH.NET to connect to an FTP site and download files from my C# app. I can successfully connect to the host with FileZilla and WinSCP (on Windows 10 Pro 64), and through an HTTP connection (the host is running CrushFTP), but cannot figure out how to successfully connect from my C# app, using SSH.NET. I'm getting this error:

The server response contains a null character at position 0x00000004: 00000000 15 03 03 00....A server must not send a null character before the Protocol Version Exchange is complete.

My take on the documentation from here, is that this problem appears to be an expected error. I've read the information suggested by the documentation at https://www.rfc-editor.org/rfc/rfc4253#section-4.2 and I "think" I understand that to mean that the host that I'm trying to connect to may not be correctly configured. My question is, since I don't have the ability to change how the host is configured, whether there is a way around this error with the SSH.NET library or am I just at a dead end for using SSH.NET?

Here's my code - tried to simplify to bare minimum:

using System;
using Renci.SshNet;

namespace mysftp
{
    class Program
    {
        static void Main(string[] args)
        {
            string host = "myftps.hostname.com";
            string username = "myusername";
            string password = "securepassword";
            int port = 990;

            string remoteFolder = "/";

            using (SftpClient sftp = new SftpClient(host, port, username, password)) 
            {
                try
                {
                    Console.WriteLine("Attempting to connect to " + host + " ...");
                    sftp.Connect();
                    Console.WriteLine(sftp.ConnectionInfo.ServerVersion);
                    sftp.Disconnect();
                }
                catch (Exception e)
                {
                    Console.WriteLine(e);
                    throw;
                }
            }
        }
    }
}

Here is the host information (Remote server info from WinSCP):

Remote system = UNIX Type: L8
File transfer protocol = FTP
Cryptographic protocol = TLS/SSL Implicit encryption, TLSv1.2
Encryption algorithm = TLSv1.2: ECDHE-RSA-AES256-GCM-SHA384, 2048 bit RSA, ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(256) Mac=AEAD

Compression = No
------------------------------------------------------------
Certificate fingerprint
<<fingerprint info redacted>>
------------------------------------------------------------
Can change permissions = Yes
Can change owner/group = No
Can execute arbitrary command = Protocol commands only
Can create symbolic/hard link = No/No
Can lookup user groups = No
Can duplicate remote files = No
Can check available space = No
Can calculate file checksum = Yes
Native text (ASCII) mode transfers = No
------------------------------------------------------------
Additional information
The server supports these FTP additional features:
  AUTH TLS
  AUTH SSL
  SSCN
  PBSZ
  PROT
  CCC
  CLNT
  EPSV
  EPRT
  MDTM
  MDTM YYYYMMDDHHMMSS[+-TZ];filename
  MFMT
  SIZE
  REST STREAM
  MODE Z
  LIST -Q
  SITE UTIME
  SITE MD5
  XMD5
  SITE MD5s
  SITE RANDOMACCESS
  MLST Type*;Size*;Modify*;Perm*;UNIX.owner*;UNIX.group*;
  UTF8

Martin Prikryl · Accepted Answer · 2021-01-20T07:22:24.220

Port 990 is used for FTP protocol over implicitly encrypted TLS/SSL connection (aka implicit FTPS). While SSH.NET is an SSH/SFTP library. FTPS and SFTP are completely different things. You cannot use SSH.NET for FTPS.

The implicit FTPS is obsolete thing. Not all FTP(S) libraries do support it. It's notable not supported by the built-in .NET FTP implementation, the FtpWebRequest.

For you will have to use a 3rd party library. See Does .NET FtpWebRequest Support both Implicit (FTPS) and explicit (FTPES)?

Though most FTP servers do support the standard explicit FTPS (over port 21). If your server does, use that, instead of the legacy port 990. See FTPS (FTP over SSL) in C#

If not, talk to the server administrator to enable it.

See also Difference between FTP/FTPS/SFTP - Configurable connection to any of them

A completely different problem that can lead to the same error message: Connecting with SSH.NET to OpenSSH 7.4p1 fails with "The server response contains a null character at position" but works in WinSCP.

"The server response contains a null character" when connecting to FTP site on port 990 using SSH.NET

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