"425 Unable to build data connection" when attempting to download files from a FTPS server using .Net with TLS 1.3 [duplicate] - .net-4.8

Question

I am trying to open an FTP connection over SSL in my code. I'm able to connect and list a directory using FileZilla of WinSCP. But when listing the directory through .NET code using FtpWebClient, I get the error
(425) Can't open data connection
Since I'm able to connect using FileZilla from the same computer, I'm not sure how to go about troubleshooting this.
Here's my code
public void FtpStuff()
{
string url = "ftp://my.server.com";
FtpWebRequest request = (FtpWebRequest)WebRequest.Create(url);
request.Credentials = new NetworkCredential("myname", "password");
request.EnableSsl = true;
request.Method = WebRequestMethods.Ftp.ListDirectory;
FtpWebResponse response = (FtpWebResponse)request.GetResponse();
StreamReader streamReader = new StreamReader(response.GetResponseStream());
// This is the line that throws the exception
string line = streamReader.ReadLine();
}
I also tried FluentFTP. Here's my code for that. I get the exception
Unable to build data connection: Operation not permitted.
public void FtpStuff()
{
FtpClient client = new FtpClient();
client.Host = "my.server.com";
client.Credentials = new NetworkCredential("myname", "password");
client.EncryptionMode = FtpEncryptionMode.Explicit;
client.Connect();
// This line gives me an exception.
var files = client.GetListing();
}
Here is the logging information from FluentFTP. I changed the real user name and IP, but the rest of the data (including the port) is the real data. My FTP service provider specifies that I have to connect on port 21. The problem seems to happen towards the end after the EPSV command is issued and a connection on a new port is established.
# Connect()
The thread 0x5514 has exited with code 0 (0x0).
The thread 0xc80 has exited with code 0 (0x0).
The thread 0x89d4 has exited with code 0 (0x0).
Status: Connecting to 123.123.123.123:21
Response: 220 FTP Server Ready
Command: AUTH TLS
Response: 234 AUTH TLS successful
Status: FTPS Authentication Successful
Status: Time to activate encryption: 0h 0m 0s. Total Seconds: 0.1339995.
Command: USER me#mysite.com
The thread 0x6ddc has exited with code 0 (0x0).
Response: 331 Password required for me#mysite.com
Status: Testing connectivity using Socket.Poll()...
Command: PASS ***
Response: 230-***************************************************************************
Response: NOTICE TO USERS
Response: This computer system is private property. It is for authorized use only.
Response: Users (authorized or unauthorized) have no explicit or implicit
Response: expectation of privacy.
Response:
Response: Any or all uses of this system and all files on this system may be
Response: intercepted, monitored, recorded, copied, audited and inspected by
Response: using this system, the user consents to such interception, monitoring,
Response: recording, copying, auditing, inspection, and disclosure at the
Response: discretion of such personnel or officials. Unauthorized or improper use
Response: of this system may result in civil and criminal penalties and
Response: administrative or disciplinary action, as appropriate. By continuing to
Response: use this system you indicate your awareness of and consent to these terms
Response: and conditions of use. LOG OFF IMMEDIATELY if you do not agree to the
Response: conditions stated in this warning.
Response: ****************************************************************************
Response: 230 User me#mysite.com logged in
Command: PBSZ 0
Response: 200 PBSZ 0 successful
Command: PROT P
Response: 200 Protection set to Private
Command: FEAT
Response: 211-Features:
Response: AUTH TLS
Response: CCC
Response: CLNT
Response: EPRT
Response: EPSV
Response: HOST
Response: MDTM
Response: MFF modify;UNIX.group;UNIX.mode;
Response: MFMT
Response: MLST modify*;perm*;size*;type*;unique*;UNIX.group*;UNIX.groupname*;UNIX.mode*;UNIX.owner*;UNIX.ownername*;
Response: PBSZ
Response: PROT
Response: REST STREAM
Response: SIZE
Response: SSCN
Response: TVFS
Response: 211 End
Status: Text encoding: System.Text.ASCIIEncoding
Command: SYST
Response: 215 UNIX Type: L8
# GetListing(null, Auto)
# GetWorkingDirectory()
Command: PWD
Response: 257 "/" is the current directory
Command: TYPE I
Response: 200 Type set to I
# OpenPassiveDataStream(AutoPassive, "MLSD /", 0)
Command: EPSV
Response: 229 Entering Extended Passive Mode (|||50304|)
Status: Connecting to 123.123.123.123:50304
Command: MLSD /
Response: 150 Opening BINARY mode data connection for MLSD
Status: FTPS Authentication Successful
Status: Time to activate encryption: 0h 0m 0s. Total Seconds: 0.1210002.
+---------------------------------------+
-----------------------------------------
Status: Disposing FtpSocketStream...
# CloseDataStream()
Response: 425 Unable to build data connection: Operation not permitted
Status: Disposing FtpSocketStream...
Exception thrown: 'FluentFTP.FtpCommandException' in FluentFTP.dll
Here are my FileZilla logs.
Status: Resolving address of mysite.com
Status: Connecting to 123.123.123.123:21...
Status: Connection established, waiting for welcome message...
Response: 220 FTP Server Ready
Command: AUTH TLS
Response: 234 AUTH TLS successful
Status: Initializing TLS...
Status: Verifying certificate...
Status: TLS connection established.
Command: USER me#mysite.com
Response: 331 Password required for me#mysite.com
Command: PASS ************
Response: 230-***************************************************************************
Response: NOTICE TO USERS
Response: This computer system is private property. It is for authorized use only.
Response: Users (authorized or unauthorized) have no explicit or implicit
Response: expectation of privacy.
Response:
Response: Any or all uses of this system and all files on this system may be
Response: intercepted, monitored, recorded, copied, audited and inspected by
Response: using this system, the user consents to such interception, monitoring,
Response: recording, copying, auditing, inspection, and disclosure at the
Response: discretion of such personnel or officials. Unauthorized or improper use
Response: of this system may result in civil and criminal penalties and
Response: administrative or disciplinary action, as appropriate. By continuing to
Response: use this system you indicate your awareness of and consent to these terms
Response: and conditions of use. LOG OFF IMMEDIATELY if you do not agree to the
Response: conditions stated in this warning.
Response: ****************************************************************************
Response: 230 User me#mysite.com logged in
Command: SYST
Response: 215 UNIX Type: L8
Command: FEAT
Response: 211-Features:
Response: AUTH TLS
Response: CCC
Response: CLNT
Response: EPRT
Response: EPSV
Response: HOST
Response: MDTM
Response: MFF modify;UNIX.group;UNIX.mode;
Response: MFMT
Response: MLST modify*;perm*;size*;type*;unique*;UNIX.group*;UNIX.groupname*;UNIX.mode*;UNIX.owner*;UNIX.ownername*;
Response: PBSZ
Response: PROT
Response: REST STREAM
Response: SIZE
Response: SSCN
Response: TVFS
Response: 211 End
Status: Server does not support non-ASCII characters.
Command: PBSZ 0
Response: 200 PBSZ 0 successful
Command: PROT P
Response: 200 Protection set to Private
Status: Logged in
Status: Retrieving directory listing...
Command: PWD
Response: 257 "/" is the current directory
Command: TYPE I
Response: 200 Type set to I
Command: PASV
Response: 227 Entering Passive Mode (123,123,123,123,197,68).
Command: MLSD
Response: 150 Opening BINARY mode data connection for MLSD
Response: 226 Transfer complete
Status: Directory listing of "/" successful
I can also connect using WinSCP. As suggested in comments, I did check if TLS/SSL session ID is reused when opening the data connection. It seems that it is.
227 Entering Passive Mode (???)
MLSD
Connecting to ??? ...
Connection pending
Data connection opened
Trying reuse main TLS session ID
Session ID reused
150 Opening data channel for directory listing of "/"

Answer 1

.NET framework does not support TLS/SSL session reuse. If your server requires it (what it looks it does and what is quite common nowadays and what is good thing for security), you cannot use FtpWebRequest nor FluentFTP. Both use the .NET implementation of TLS/SSL.
You will have to use FTP library that uses own TLS/SSL implementation.
You can use my WinSCP .NET assembly. Though contrary to FluentFTP, it's not a native .NET library, it has dependencies on an external binary. But that's what makes it working.
Some references:
https://github.com/robinrodricks/FluentFTP/issues/347
https://github.com/dotnet/runtime/issues/27916
"Authentication failed because the remote party has closed the transport stream" when transferring to/from FTP server over TLS/SSL using FluentFTP
Upload file to implicit FTPS server in C# with TLS session reuse
Suddenly getting "150 Opening Data channel for file download from server" after the FTP downloads was working for years – According to this post and other references elsewhere, the TLS/SSL session reuse was supported earlier with .NET Framework, but some update broke it. In .NET Core it was never working (see also the dotnet GitHub link above).