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I have both FTP client and server behind different routers (doing NAT). The router in the server side has opened the proper port to work. My FTP client is configured to use active mode and it sends the following command when I try to upload a file to the server:

PORT 192,168,1,42,184,89

It is ok, my client with internal IP 192.168.1.42 is trying to setup a data connection with the server in the port specified by the PORT command. The problem is that the server is capable of responding me. As I am behind a router, how it is possible? from where the TCP server gets the public IP to reach my router through the Internet?

Thanks.

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The NAT probably translates the IP address in PORT command. That's possible if the traffic is not encrypted. Try enabling TLS/SSL encryption. The data connection should fail then.

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