multicast forwarding

Greetings,

I have an interesting difficulty.  I want to be able to grab a multicast
stream from a machine (192.168.12.13) and forward/rebroadcast/rewrite
it, such that the packets seem to come from my DMZ host (192.168.12.12).

I have been trying an iptables solution, but that doesn't seem to be
working at the moment, and it was recommended to me by j_engelh on the
#iptables channel to ask at this address for further assistance.

Thank you for your time and effort on my behalf.

-dkap