Error 502 “Active FTP not Allowed” when trying to list files in a FTP session behind Forefront TMG 2010

Consider a scenario where you have a client workstation behind Forefront TMG 2010 and you are trying to download files from a FTP Server. You are successfully able to logon on the FTP but after type the command “dir” you get the error message below: The message is pretty clear about what is going on,…