Forward specific port to another ip

[Edmundo Carmona <eantoranz@gmail.com>]

Remember I said "for starters". You have to consider more things in
order to make it work... depending on your networking environment. :-)

On 12/27/05, Richard Pickett <Richard.Pickett@csrtechnologies.com> wrote:
> 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".
>
>
>