RE: Forward specific port to another ip

["Richard Pickett" <Richard.Pickett@CSRTechnologies.com>]

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".