Manuel Bouyer:
> I'm not sure passive ftp would work though NAT without the proxy.
> At last the source address, and possibly the source port, need to be
> translasted in the PORT command.
>

(actually I did not follow the entire discussion...)
but according to me passive ftp should work with only ipnat:

 client -----> NAT box ------> FTP server

All the connection originate from the client (using the passive mode) and
this is sufficient to establish the FTP-CTRL and FTP-DATA connection: in
the passive mode is the server that publicize its IP and port number not
the client behind the NAT.

Tell me if I'm forgot something.

Regards,
Roberto