Error "You must issue the PBSZ command prior to PROT" when connecting to an FTPS server

Symptom

When executing aprocess which connects to an FTPS server, the following error occurs on the FTP action:

javax.net.ssl.SSLException: 503 You must issue the PBSZ command prior to PROT
    at org.apache.commons.net.ftp.FTPSClient.execPROT(FTPSClient.java:426)
    at com.indy.engine.actionCodes.d.b.connect(SourceFile:285)
    at com.indy.engine.actionCodes.FtpPutActionCodeI.executeSimpleCode(SourceFile:277)
    at com.indy.engine.action.common.ActionCodeTypeI.executeCode(SourceFile:1608)
    at com.indy.engine.action.common.ActionCodeTypeI.run(SourceFile:1767)
    at java.lang.Thread.run(Unknown Source)

 

Solution

This error message is raised by the FTPS server.

See RFC2228 for more information on the PROTECTION BUFFER SIZE (PBSZ) command:

      This command allows the FTP client and server to negotiate a
      maximum protected buffer size for the connection.  There is no
      default size; the client must issue a PBSZ command before it can
      issue the first PROT command.

The solution is to enable this parameter on the FTP action:

 

 Please contact the FTPS server's admin to define the value.