Re: Port forwarding (non-NAT)

[Jon Wilson <lists@erentil.net>]

Kristofer wrote:
> I've googled and done some searches, and the only information I can find is for port forwarding with NAT. Perhaps that's what I need to accomplish what I am trying to do. If I missed an obvious link or source with this information, I apologize and please slap me. 
>
> I currently have an SMTP server listening on port 25, and the machine has its own static IP address, no NAT is being used. 
>
> I want to use iptables to forward inbound traffic on port 587 to port 25 of that same machine, so basically making SMTP listen on both ports. I do not wish to configure the SMTP software to listen on multiple ports, since I may want to open up several more ports in the future and that would be a lot of idle daemons listening on ports they may or may not use. 
>
> So, my questions is: how can I have incoming traffic on port 587 go to port 25 of the localhost? 
>
> Thanks, 
> Kristofer 
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>   
if iptables on the same computer as the smtp server:

iptables -t nat -A PREROUTING -p tcp --dport 587 -m state --state NEW -d 
$IP_OF_MAIL_SERVER -j REDIRECT --to-ports 25

else:

iptables -t nat -A PREROUTING -p tcp --dport 587 -m state --state NEW -d 
$IP_OF_MAIL_SERVER -j DNAT --to $IP_OF_MAIL_SERVER:25

(assuming you allow established, related through)
-- 
Jon Wilson