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Masquerade address when connecting from the internal LAN

File transfer protocol.
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Masquerade address when connecting from the internal LAN


Post by pozz »

I configured FTP service for external connections. NAS4Free is behind a NAT ADSL router, so I configured Masquerade Address (dyndns hostname) and the passive range ports. All works ok from external.

Now if I connect from inside the LAN (client on using the internal NAS server (, it returns the external IP address during passive data transfers. I thought it wasn't a real problem, because I already checked external transfers, but it really doesn't work. I see on Wireshark that the client tries to make a connection to the correct combination <external ip address>:<passive port>, I see also some answers from the server (SYN, ACK), but data transfers don't occur.

If I setup the FTP client on the internal LAN to use the external NAS ip address (dyndns hostname), all works ok.

I know I have found a solution, but I'm wondering why this strange behaviour.

The only explanation that I could think of, is the following. The server (actually proFtpd) starts listening on a port in the passive range after "Entering passive mode", but it accepts incoming connections only from the ip address that started the initial ftp connection. If starts the connection to, the server knows that the client is After entering passive mode, it listens for connections only from
But the client makes a connection to the external IP address (the answer of PASV command), so it seems the request comes from the external IP address and doesn't allow the data transfer.

Is it correct?

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Re: Masquerade address when connecting from the internal LAN


Post by doktornotor »

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