Before starting a data transfer (e.g. creating a new data connection outside the control connection to transfer files or listing) you have to specify the protection level using the PROT
command. The main protection levels are P
for protected (e.g. SSL encryption) or C
for clear (no encryption). The server complains, because you don't specify a protection level and so it does not know how you want to get your data.
The PROT
command must be implemented and used by all ftps clients.