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In the debug log, the connection is shown as working, but when a file is transferred or a directory is listed, the connection is closed by the server. This only happens from version 9.7.0 or later.

DEBUG [SocketController] 4 nov 2020 14:30:49.582 : FTPConnection.2 OnReceive closing (size == 0)
DEBUG [SocketController] 4 nov 2020 14:30:49.584 : FTPConnection.2 CloseConnection(e=null)
DEBUG [SocketController] 4 nov 2020 14:30:49.584 : FTPConnection.2 Shut down socket
DEBUG [SocketController] 4 nov 2020 14:30:49.585 : FTPConnection.2 Closed socket

1 Answer

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Best answer

This is a problem with Filezilla server, and it is because Filezilla server does not support the extended master secret feature in TLS. This feature makes connections more secure. 

If a server does not support the extended master secret and the client does, the RFC says the client should not resume connections. But Fiezilla server has a setting that says if a data connection (used to list directories or transfer files) does not resume, then terminate the connection. 

The Filezilla log shows it happening:

450 TLS session of data connection has not resumed or the session does not match the control connection

To fix the problem,go to FTP over TLS settings in the Filezilla server options, and uncheck "Require TLS session resumption on data connection when using PROT P".