Re: [tlug] [Was: iptables] Forward multicats

["Stephen J. Turnbull" <stephen@example.com>]

Bruno Raoult writes:

 > TERA -----ethernet----eth0--Linux PC-rausb0----wifi-----lan (including
 > internet router)
 > 
 > The sniffer is on the Linux PC.

Which is also the multicast router.  Quis custodiet ipsos custodes?

 > This is where I am puzzled. I don't know anything about multicast, but if
 > you are right, I should see  those "invitation" packets from client at
 > first, right?

I think so, but I am not an expert.  I don't know how multicast router
discovery is done; probably it's just a broadcast on the LAN (wifi).
See RFC 1112 (IIRC) for the host protocol for joining a group.

 > But I don't see any, and, worse, the multicast packets coming from the TERA
 > don't appear too on the rausb0 side.

But they should not, in any of three cases: (1) Linux PC doesn't know
how to route multicast packages, (2) Linux PC knows how to route but
doesn't know that it is supposed to, (3) there are no members of the
multicast group on the wifi LAN.  You say you're not seeing any
traffic of LAN trying to join the multicast froups, which sounds like
(3) to me.

 > As if my forwarding rule would not work for those

You'e jumping to conclusions.  This is the first time I've ever
thought about debugging a multicast network, and I've already come up
with at least 4 hypotheses why you're not seeing multicast traffic on
wifi.  Somebody who actually knows what they're doing can probably
come up with a dozen more (and also maybe show that some of the ones
I've proposed are irrelevant ;-).