From mboxrd@z Thu Jan  1 00:00:00 1970
From: Daniel Lopes <lopsch@lopsch.com>
Subject: Re: UDP nat question
Date: Tue, 19 Apr 2005 15:31:10 +0200
Message-ID: <4265081E.7@lopsch.com>
References: <4263C3A2.70006@inescporto.pt>
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Filipe Abrantes schrieb:
> Hi all,
> 
> I have a basic question about iptables UDP NAT.
> 
> Imagine that you have 2 UDP sessions in your private LAN using the same 
> port. How does iptables nat these connections? One of the sessions will 
> get one external port of the NAT machine and the other session will get 
> another one? Does the usual MASQUERADE command suffice to achieve this?
> 
> iptables MASQUERADE command:
> 
> $IPTABLES -t nat -A POSTROUTING -o $OUTBOUND_IFACE -j MASQUERADE
> 
> Hope I have made myself clear and,
> 
> Thanks for your attention
> 
> Filipe Abrantes
> 

For example:
machine 1 running an application on port 1465
machine 2 running an application also on 1465
both make a DNS request to Port 53 UDP
the srcip of the second request reaching the router will be mapped for 
that time to a free port on the router. AFAIK