"550 SSL/TLS required on the data channel" using Apache Commons FTPSClient

Question

I have a probleme reading data with FTPClient on a FTP-Server (ProFTPD 1.3.3a) that requires encryption on the data channel. Everything works fine without encryption on an other server.

My code is:

FTPSClient ftpsClient = new FTPSClient("TLS", false);
log.debug("using TLS");
FTPClientConfig ftpClientConfig = new FTPClientConfig(FTPClientConfig.SYST_UNIX);
ftpClientConfig.setServerLanguageCode("de");
ftpsClient.configure(ftpClientConfig);
ftpsClient.addProtocolCommandListener(new PrintCommandListener(new PrintWriter(System.out))); // outputs all conversation to the console
ftpsClient.connect(host, 21);
ftpsClient.login(username, password);
ftpsClient.enterLocalPassiveMode();
ftpsClient.changeWorkingDirectory(pathname);
listNames = ftp.mlistDir();
ftpsClient.logout();

What I get from the output is

220 ProFTPD 1.3.3a Server (xxx) [xxx]
AUTH TLS
234 AUTH TLS successful
USER xxx
331 Password required for xxx
PASS xxx
230 User xxx logged in
CWD /www/catalog
250 CWD command successful
PASV
227 Entering Passive Mode (xxx).
MLSD
550 SSL/TLS required on the data channel
QUIT
221 Goodbye.

Any idea how to configure FTPSClient to use TLS/SSL on the data channel? Your help would be appreciated!

Answer 1

You must enable data channel encryption before executing any commands that will transfer data over the data channel (such as LIST does).

Add this to your code after connecting to the server:

// Set protection buffer size
ftpClient.execPBSZ(0);
// Set data channel protection to private
ftpClient.execPROT("P");

At least, this solved my problems (using proftpd).