Nated connection behavior

Nated connection behavior

[Mr Vandeley]

Hi,
I have a question related to a NAT connection behavior:

|CLIENT|<----->[ROUTER/IPTABLES/NAT]<---->|SERVER|

Why is it that if I ping SERVER from CLIENT, I see answers comming
from SERVER internal IP instead of ROUTER external IP.

NAT rule on ROUTER is:

iptables -t nat -A POSTROUTING -s <IP SERVER> -o <external eth of
ROUTER> -j SNAT --to-source <external IP of ROUTER>

I know that it is awful too basic question for you guys, but if
someone could explain that I'll be much appreciative.

Re: Nated connection behavior

[Mr Vandeley]

On Thu, Mar 5, 2009 at 3:12 PM, James Stull <rivitir@gmail.com> wrote:
> NAT doesn't rewrite that section of a packet. Instead it "saves" the
> communication in memory this way your computer thinks it's talking directly
> to the server.
>
> You can kind of think of it like if you sent a letter to a friend:
>
> |You|<----->[Postal Service]<---->|Friend|
>
> Neither of you think about the postal service between you and how it works,
> instead you only think your talking to each other via your letters.
>
> Hope this helps.
>
> --James

Great, thanks James for your answer.

So, this "saved communication" is saved by NAT or by the conntrack module?

I ask this,  because I see that on CISCO routers, NAT doesn't behave
the same way. If I replace the Linux Router for a Cisco router,
configured nating everything that comes out on its external interface,
answers come from the router and client just ignore that answer,
right?