From the multicast howto:

IP_ADD_MEMBERSHIP.

Recall that you need to tell the kernel which multicast groups you are interested in. If no process is interested in a group, packets destined to it that arrive to the host are discarded.

If you don't do that, you won't see those packets with tcpdump.

• Is it possible to subscribe to all multicast traffic so I can do a tcpdump for all existing traffic? I would think IGMP doesn't allow this, so probably not.. but maybe you can configure a switch to still send all multicast traffic. Is that possible?
• Is it possible to do subscription (for a specific IP) with a command line tool? (note: I know how to do this in C.. but would prefer to use an existing tool and not compile a separate program for this)