OpenBSD's multicast routing is broken.

Don't bother trying just yet.

-ProFX

!! Update:

It appears multicast routing may not be 'broken' as such, just not included in the default in the monolithic 'GENERIC' OpenBSD kernel that includes just about every feature/devices/function OpenBSD has to offer.

I haven't had time to download kernel sources, edit, and compile yet so I can't confirm that it works.

If anyone wants to try, post a message here with your results.

-ProFX