FireFTP: FTPS Explicit 521 error resolution WHM

We was recently trying to help a client setup his FireFox FireFTP so that it would connect to our FTP server, unfortunately though we hit some very baffling problems which took some guess work and research in order to fix. The problem seems to be that FireFTP has not implemented the FTP protocol spec properly and so our FTP server was having some problems trying to deal with the connection requests, we believe this only affects the FTPS features, we were using FTP TLS, but this may affect also FTP SSL and the others.

So our server is locked down, we dont allow normal FTP for security reasons, our server requires FTP Explicit for security, we set all the settings in FireFTP and clicked connect, it seemed like it was connecting fine but while trying to list the files it would fail with the following error message:

521 data connection cannot be opened with this prot

After some research it appears that this issue is related to the fact that FTP clients are supposed to provide a command with PROT to describe how they would like to proceed, but FireFTP seems to ignore the spec and just sends the PROT command with no parameters.

Once we realised this it was good fortune that I remembered the settings for FTP in cPanel / WHM had an option for “Broken Clients Compatibility” which I had disabled again for security reasons, so we decided to turn that on and then everything worked perfectly!


How to fix FireFTP “cannot be opened with this PROT” error in WHM

It’s very straight forward, login to your WHM and in the search box type “FTP”. Click on “FTP Server Configuration”, scroll all the way to the bottom of the page and change “Broken Clients Compatibility” to yes and click save.

That’s it FireFTP should now work correctly.

Author: Dean Williams

Professional PHP Web Developer with expertise in OpenCart Web Development, WordPress Web Development, Bespoke Systems - also a seasoned Linux Server Administrator.

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