FTP server behind firewall?

[Gilles]

Hello

We have FreeBSD server on our private LAN behind a NAT firewall on
which I'd like to add an FTP server so that customers can send us
stuff.

Problem is, since customers might have a NAT firewall on their end,
the client application must connect in passive mode... but this just
moves the problem to our end, where the FTP server will open a random
port for data... to which the client will fail connecting since our
NAT firewall is keeping them out of our LAN :-/

Is there a way to keep our server in the private LAN and still provide
a way for customers to upload data? Hard-code the socket number used
by the FTP server for data? Use a different type of server?

Thank you.