It is not just NATing that breaks FTP but any form of firewall in general. Iptables isn't bad because there are special kernel modules that watch ftp traffic an

It is not just NATing that breaks FTP but any form of firewall in general. Iptables isn't bad because there are special kernel modules that watch ftp traffic and auto adjust rules for you. But good luck if your firewall is not on the same box as your FTP server.