Is there any solution to get this working behind a NAT device ?
The problem is the NAT separates two different networks: The internet (everything outside the router, public IPs) and the server's local area network (LAN, with the internal IPs). Because of that separation, the NAT device is also called a gateway.
Now the public IPs do not work in LAN and vice versa. You see both the PORT and the PASV reply communicate the public IP. But connecting to itself is a LAN/localhost connection, and so public IPs are invalid.
You could do a test:
1. Make sure the ''Don't use external IP for local connections'' option in FZ Server's Passive settings is enabled.
2. Configure the client not to send the public IP with the port command, but 127.0.0.1 instead (the localhost IP).
3. If FZ server receives that IP it will reply with its local/localhost IP.
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