Writting iptables rules to accept inbound connections to kvm guests

Hi all,

I have installed a CentOS 5.3 x86_64 server with kvm and libvirt to do
some tests for future virtualized deployments.

My environment:

eth0 --> 172.25.50.1/24 ("public" host ip)
virbr0 ---> 192.168.122.1/24 (natted interface installed by libvirt)
virbr1 ---> 172.26.50.0/24 (isolated internal virtualized network, whithout physical interface binded)

Between 192.168.122.0/24 network and 172.26.50.0/24 network I have
installed a virtual firewall. So to reach 172.26.50.0/24 network, all connections needs to go to 192.168.122.0/24 network.

I need to insert some iptables rules to allow access to some services
installed on 172.26.50.0/24 network like smtp,http,ftp, etc.

How can I configure libvirt to forward and nat these services coming
from 172.25.50.0/24 network to a specific IP under 192.168.122.0/24 network?? Or do I need to use /etc/sysconfig/iptables configuration and disable libvirt's iptables rules ?? Some examples, please??.

many thanks.

CL Martinez
carlopmart {at} gmail {d0t} com