From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thorsten Scherf <tscherf@web.de>
Subject: Re: Nat with a dynamic IP
Date: Fri, 5 Sep 2003 16:05:36 +0200
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To: Peter Marshall <peter.marshall@caris.com>
Cc: netfilter@lists.netfilter.org

>Hi.  I was wondering if someone could tell me what a good way is to set up
>DNAT and SNAT on a firewall if the external IP is not static ....

Do not use SNAT, use Maquerading. You don't have to know your external IP 
here:

iptables -t nat -A POSTROUTING -s $INT_NET -o $EXT_DEV -j MASQ

DNAT is also very simple, don't specify your external IP in DNAT Rule, just 
the Destination Port:

iptables -t nat -A PREROUTING -p tcp --dport 25 -i $EXT_DEV -j DNAT 
--to-destination $SMTP_SERVER


Greetings,

Thorsten Scherf,
RHCE, RHCX