The FTP module supports Passive Mode transfers. In this mode, the FTP module opens a separate listener port/socket, sends the IP address and port number of that socket to the client, and the client opens a TCP connection to the specified address and port.
When the CommuniGate Pro Server is located behind a NAT/Firewall, external (WAN) clients using the Passive Mode connect to an external WAN address, rather than the Server own IP address. If the NAT/Firewall cannot fix this problem, use the Send WAN Address option.
The FTP Module uses the TCP Media Proxy ports for Passive Mode transfers.