Impossible to connect to data port using implicit FTPS about proftpd

[Castaglia]

See also #759 (comment)

[Castaglia]

ProFTPD requires that the client send a data transfer command (such as LIST, RETR, STOR, etc) before it will open a passive data socket. Why? If it opened a socket on commands such as PASV or EPSV, it means a malicious (or badly-written) client could spew such commands and open many sockets, exhausting the number of possible sockets (which are also used by other processes on a host), without ever actually performing a data transfer.

Thus ProFTPD requires an actual FTP data transfer command before it will open a data transfer socket, to avoid such denial-of-service situations.