nat and multicast over ipv4

hi,
after trying some configuration i didn't come with a solution for the following problem:

a Linux box with two eths is acting as a gateway that nat a private lan to go to the internet.
The ISP use multicast, and all the isp routers are capable of forwardin multicast traffic.
i would like to make it possible to join to multicast group from the inside lan.
So the linux box has to forward/nat (i read igmp nat is in the TODO of netfilter) igmp announcement, and after the join
it has to forward udp traffic that comes from 224/4.
What are the steps to do that with iptables ?
Does the linux box has to have Multicast Routing options in the kernel enable ? (looking to the help from the kernel it doesn't look it has to be enable) .

Multicast-HOWTO is rather old, and it doesn't have any hint, also mrouted is needed only when you want to tunnel the multicast traffic from a lan to an another that is multicast capable, but that's not the examples 'cause the isp after the linux box can "understand" multicast.

Any hint, suggestion, resource for this problem ?

Thanks
Samuele

When all the network has eyes, even if we were to send out minds turned into
light or electrons...  
It is a time when "one" is not able to make a "solid", a complex, into 
data yet...