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There is 2 steps.

First of all, add to /etc/proftpd.include followed code:

TimeoutIdle                 900
TimeoutNoTransfer           900
PassivePorts                49152 65534
TimesGMT                    Off

Then test the iptables nat helper:

modprobe ip_conntrack_ftp

If there is no errors, add to directive IPTABLES_MODULES in /etc/sysconfig/iptables-config module ip_conntrack_ftp:

IPTABLES_MODULES="ip_conntrack_ftp"

If you need to load two or more helpers, separate them with spaces because the iptables script does a for-in on the IPTABLES_MODULES directive.