From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Richard Pickett" <Richard.Pickett@CSRTechnologies.com>
Subject: RE: Forward specific port to another ip
Date: Tue, 27 Dec 2005 13:11:12 -0600
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References: <a46d47540512271036v71e355epf1f87cbb564b2996@mail.gmail.com>
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To: 'netfilter' <netfilter@lists.netfilter.org>

Here's something to keep in mind on NAT, it only works if the NAT box is
in-between both systems or you can double-NAT.

Take this example:


    NAT BOX
       |
-----------------
|               |
SYS A          SYS B


A connects to NAT A's connection looks like this:

From: A:Portx
To:   NAT:Porty

NAT NATs the packet to send it to B, the packet now looks like this:

From: A:Portx
To:   B:Porty

When B replies to the connection his packet looks like this:

From: B:Porty
To:   A:Portx

When this packet arrives at SYS A it doesn't recognize the connection,
it's expecting all return packets to look like this:

From: NAT:Porty
To:   A:Portx

To make it work you have to double-NAT the packets. You have to NAT the
PREROUTING to change the "TO", and the POSTROUTING to change the "FROM".