NAT FROM public ip TO public ip

["Mario Ohnewald" <mario.Ohnewald@gmx.de>]

Hello List!

This is a real easy question, but i will explain it in detail so that you
know what i am on about ;)

I have two internet boxes, both have only one nic.
Box A is my root box.
Box B is a ftp server (where i am not admin, but i have an ftp account).

Both, Box A and B have a public ip address.
You can NOT access the ftp server from the outside.
ONLY Box A can connect to that ftp server (Box B).

So i though i could simply nat a connection through my Box A, to Box B.
If i connect from the real bad world, to my Box A, port 2121. Box A could
forward/NAT that to the internal ftp box.

I have tried it with this rule:
$IPT -t nat -A PREROUTING -i eth0 -p tcp -d 123.123.123.123 -m helper
--helper ftp-2121 -j DNAT --to-destination 123.123.123.111

and

modprobe ip_conntrack_ftp ports=21,2121


But it looks like its not working.
I did a tcpdump -i eth0 -n host 123.123.123.111


And no packages where transfered to the ftp box (123.123.123.111).
Any idea why? Am i going the right way at all?

Thanks, Mario

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