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

[Curt Sampson <cjs@example.com>]

On 2008-06-15 10:00 +0900 (Sun), Bruno Raoult wrote:

> Sorry, I was not clear enough. My network looks like:
> 
> TERA -----ethernet----eth0--Linux PC-rausb0----wifi-----lan (including
> internet router)

And is this Linux box bridging the two physical networks (Ethernet and
WiFi), or are these two separate IP networks between which it is (or
could be) routing?

> My two interfaces have the "multicast" flag.

I'm not even sure what that means, but it may not be sufficient to
ensure the packets are seen and routed.

> 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?
> 
> But I don't see any, and, worse, the multicast packets coming from the
> TERA don't appear too on the rausb0 side. As if my forwarding rule
> would not work for those (when I ftp from the lan to the TERA, I can
> see the packets in both sides, if you wonder if the sniffer works or
> not).

I'm not clear on what your "forwarding rule" is, but if it's NAT, you
probably want to disable that entirely. If you've got a firewall, you
probably want to disable that (i.e., pass through all packets) as well
unless there are folks you don't trust on one or the other networks.
(Your Internet router should do any firewalling required for that.)

If you bridge, at that point you're set; stuff will just work. (Unless
your bridge is broken, and not forwarding non-broadcast Ethernet packets
with the multicast bit set.)

If you route, your router is responsible for routing multicast as
necessary.

This means, first, that it should never forward packets or accept
subscriptions for things destined to 224.0.0.0/24, since that's reserved
for multicast on a single network only.

Second, if a host on the WiFi side wants to see multicast from the
ethernet side (or any multicast at all, really, even locally), it needs
to send an IGMP subscription message to that multicast address. Your
router should see this and start forwarding the multicast packets.

If you're still having difficulty, probably the first thing to do would
be to explain what device you have on the WiFi side that understands a
multicast protocol, which protocol it is, and how you've configured it
to subscribe.

cjs
-- 
Curt Sampson       <cjs@example.com>        +81 90 7737 2974   
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