Redirecting flows among one machine's interfaces

[Kostas Pelechrinis <kpele_ntua@yahoo.com>]

Hi all,

I am not very familiar with iptables, but what I am trying to achieve is the following:

I have a machine with two interfaces (let's say if1 -- with ip address a.b.c.d -- and if2 -- with ip address x.y.z.w) both connected to the internet. One of the two interfaces is the default interface (e.g., if1).  Whatever flow is initiated towards a destination in the Internet is going out if1.  I want to use iptables in order to redirect some of the flows to if2.  Is there a way to do this using iptables?  

I have tried some rules (e.g., iptables -A FORWARD -d k.l.m.n -o if2 -j ACCEPT // where k.l.m.n is the destination ip address of the flow I want to redirect) but with no luck.  From what I could figure out the rule FORWARD is mainly for packets that go through the machine and not that much for packets originating from the machine.  Is there a way to perform what I want with iptables ?  Given that this is possible, another question is weather iptables changing the source ip address from a.b.c.d to x.y.z.w or not.

Thanks a lot and hopefully someone can seed light on this.

Best,
Kostas