"500 Syntax error, command unrecognized" with FtpWebRequest in active mode (UsePassive = False) on Azure VM

Question

I have a test C# console app that uses FtpWebRequest to transfer a file to a public FTP server.

The app works from my Dev PC with UsePassive set to False. However, when I copy the app to an Azure VM it raises the following exception:

The remote server returned an error: (500) Syntax error, command unrecognized.

So in summary, these are the results when running the app locally and on Azure:

UsePassive  Local PC   Azure VM
----------  --------   --------
True        ok         ok
False       ok         ERROR!

Enabling tracing for System.Net shows the following (extract):

System.Net Information: 0 : [5716] FtpControlStream#60068066 - Created connection from 10.0.0.4:49706 to 90.130.70.73:21.
System.Net Information: 0 : [5716] Associating FtpWebRequest#45004109 with FtpControlStream#60068066
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Received response [220 (vsFTPd 2.3.5)]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Sending command [USER anonymous]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Received response [331 Please specify the password.]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Sending command [PASS ********]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Received response [230 Login successful.]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Sending command [OPTS utf8 on]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Received response [200 Always in UTF8 mode.]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Sending command [PWD]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Received response [257 "/"]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Sending command [TYPE I]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Received response [200 Switching to Binary mode.]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Sending command [PORT 10,0,0,4,194,43]
System.Net Information: 0 : [5716] FtpControlStream#60068066 - Received response [500 Illegal PORT command.]
System.Net Information: 0 : [5716] FtpWebRequest#45004109::(Releasing FTP connection#60068066.)
System.Net Error: 0 : [5716] Exception in FtpWebRequest#45004109::GetRequestStream - The remote server returned an error: (500) Syntax error, command unrecognized..
   at System.Net.FtpWebRequest.SyncRequestCallback(Object obj)

Here is the source code to my app:

var fileContents = Encoding.UTF8.GetBytes("this is a test");

Console.Write("UsePassive? (y/n)");
var passive = Console.ReadKey();

var usePassive = passive.Key == ConsoleKey.Y;

var request = (FtpWebRequest)WebRequest.Create("ftp://speedtest.tele2.net/upload/mytest.txt");
request.Credentials = new NetworkCredential("anonymous", "anonymous");
request.Method = WebRequestMethods.Ftp.UploadFile;
request.UsePassive = usePassive;
request.ContentLength = fileContents.Length;

using (var requestStream = request.GetRequestStream())
{
    requestStream.WriteAsync(fileContents, 0, fileContents.Length);
}
using (FtpWebResponse response = (FtpWebResponse)request.GetResponse())
{
    Console.WriteLine($"Upload File Complete, status {response.StatusDescription}");
}

The Azure VM is not part of a network security group. I've tried disabling the firewall on the VM but the app still raises the exception.

How can I resolve this? It's a requirement that my app has UsePassive set to False.

Answer 1

PORT 10,0,0,4,194,43
500 Illegal PORT command.

10.0.0.4 is a private IP address. You cannot send a private IP address to FTP server, as it cannot connect back to it (hence 500 error). You have to use your public IP address. I do not know how to override a local IP address for FtpWebRequest. Checking .NET source code, it actually seems to me, that it's not even possible.

As I have already wrote you: Do not even try to use the active mode. It is complicated to setup from a network perspective. See my article on Network configuration for FTP active and passive modes.

So again, use passive mode. I'm aware that you also have problems with the passive mode. But they are more likely solvable than the problems with the active mode.

---

If you insist on the active mode, use FTP client library that supports specifying a local IP address.

For example with WinSCP .NET assembly:

// Set up session options
SessionOptions sessionOptions = new SessionOptions
{
    Protocol = Protocol.Ftp,
    HostName = "10.0.0.4",
    FtpMode = FtpMode.Active,
};

using (Session session = new Session())
{
    session.AddRawConfiguration(@"Interface\ExternalIpAddress", "your public IP address");

    // Connect
    session.Open(sessionOptions);

    // Upload
    session.PutFiles(@"C:\local\path\file.txt", "/upload/mytest.txt").Check();
}

(I'm the author of WinSCP)

You still have to open the VM firewall for all data connection ports.

---

While I didn't test it, it looks like that if local machine (VM) has only IPv6 IP address, FtpWebRequest would use EPRT command without an IP address, instead of PORT command with IP address. What would also solve your problem. You still have to open the VM firewall for all data connection ports.