FTP behind NAT on a non-standard port

[mikhalich123 <mikhalich123@gmail.com>]

Hello

Gateway to Debian 7.11 (3.2.0-4-686-pae), iptables v1.4.14.

external interface ip: 1.1.1.1

internal interface ip: 192.168.1.1

An ftp server works inside the local network, for which you need to organize access from the outside.

ip ftp server on the internal network: 192.168.1.55

Port of ftp server on internal network: 51

Lsmod output | grep ftp

nf_nat_ftp 12 420 0
nf_conntrack_ftp 12533 ​​2 nf_nat_ftp
nf_nat 17913 2 iptable_nat, nf_nat_ftp
nf_conntrack 43121 9 nf_conntrack_ipv4, nf_nat, iptable_nat, xt_conntrack, xt_state, nf_conntrack_ftp, nf_nat_ftp, xt_CT, nf_conntrack_netlink

It doesn't work like this:

iptables -t raw -A PREROUTING --dst 1.1.1.1 -p tcp --dport 55555 -j CT --helper ftp
iptables -t nat -A PREROUTING -i ext --dst 1.1.1.1 -p tcp --dport 55555 -j DNAT --to-destination 192.168.1.55:51

The control connection opens, but there is no data flow. conntrack -E expect shows nothing.

If we change so that the ftp server port is standard (ftp server settings and iptables settings), then everything works:

iptables -t nat -A PREROUTING -i ext --dst 1.1.1.1 -p tcp --dport 55555 -j DNAT --to-destination 192.168.1.55:21

Please tell me what settings are needed to make available an ftp server running on a non-standard port?