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When setting TLSRequired to on instead of off in my proftp tls.conf, I always get a

550 SSL/TLS required on the data channel

error when trying to connect. I tried it out with both filezilla and winscp and proftpd versions 1.3.3a and 1.3.4a

This is my tls.conf:

<IfModule mod_tls.c>
TLSEngine                               on
TLSProtocol                             SSLv23 TLSv1
TLSRSACertificateFile                   /etc/ssl/private/cert.crt
TLSRSACertificateKeyFile                /etc/ssl/private/cert.key
TLSVerifyClient                         off
TLSRequired                             on

The error occurs after executing the FTP-LISTcommand. This is a short excerpt from the debug log in filezilla:

Command:    PASV
Trace:  CFtpControlSocket::OnReceive()
Response:   227 Entering Passive Mode (78,46,187,75,208,71).
Trace:  CFtpControlSocket::TransferParseResponse()
Trace:  CFtpControlSocket::SendNextCommand()
Trace:  CFtpControlSocket::TransferSend()
Command:    LIST
Trace:  CFtpControlSocket::OnReceive()
Response:   550 SSL/TLS required on the data channel
Trace:  CFtpControlSocket::TransferParseResponse()
Trace:  CFtpControlSocket::ResetOperation(2)
Trace:  CControlSocket::ResetOperation(2)
Trace:  CFtpControlSocket::ParseSubcommandResult(2)
Trace:  CFtpControlSocket::ListSubcommandResult()
Trace:  CFtpControlSocket::ResetOperation(2)
Trace:  CControlSocket::ResetOperation(2)
Error:  Directory listing could not be retrieved
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How was configured your FTP client? In FileZilla you can use a short form 'ftpes://' – ALex_hha Feb 27 at 20:44
SSLv2 and v3 are considered deprecated and should not be used. – Jesse Keilson Apr 28 at 19:05

I recently had a similar problem but not with filezilla. When I use "Filezilla" as my ftp client all was good. But for some reason phpstorm's internal ftp client had the above issue, it could connect but cannot list the directory.

In proftpd.conf I used the following config and now phpstorm's ftp client can connect to the ftp server. However, data channel is not required to be encrypted with the below option:

TLSRequired                    ctrl

Another possible solution could be flipping the ftp connection to "passive" since this is a directory listing issue.

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