Why do network firewalls prevent Active FTP from working?
Two TCP connections are formed upon the initiation of a connection to the FTP server. The connection of the second TCP is started and established by the FTP server (FTP data connection). If there is a firewall between the FTP client and server, the connection initiated from the FTP server will be blocked because it is launched from the outside. As a result, Passive FTP or modifying the firewall rule to add the FTP server as trusted can be used
