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

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

Curt Sampson writes:

 > On 2008-06-14 21:49 +0900 (Sat), Bruno Raoult wrote:
 > 
 > > To: 228.5.6.7 (port 9442, UDP)
 > > To: 239.255.255.0 (port 1900, UDP)
 > > 
 > > I would like to my second computer to see these messages.
 > > As I did put a masquerade rule...
 > 
 > However, the above addresses are multicast addresses, and thus all
 > computers see them (assuming that they're running software that's
 > listening).

I don't much like that way of expressing it.  I would say on an
Ethernet all computers see all messages (that's why it's called an
"ether").  The question is what are they being sent and what are they
ignoring or looking at.

You said that Tera is sending these messages on Wifi according to
Wireshark.  Are you sure?  That is did you check this by running
Wireshark on the other computer, not on Tera?  If so, the question
becomes why aren't they looking at what they see?  If not, maybe Tera
needs to be configured as a multicast router?  (I've heard of these,
but I'm no expert on how it works or if it is normally enabled.  See
STD0005, RFC1112.)

First, they can be filtered at the "hardware" layer based on Ethernet
(ie, MAC) addresses.  I'm not really up on IEEE 802, but I would
suppose that there Ethernet broadcast (and/or multicast) addresses
such that the MAC looks for MY_ADDRESS and the broadcast and multicast
address(es).  ifconfig should report something like:

eth0      Link encap:Ethernet  HWaddr 00:E0:81:54:F6:70  
          inet addr:130.158.99.156  Bcast:130.158.99.0  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          [...]

Note the "MULTICAST" in the third line.

Second, AFAIK you need to run an application that registers to receive
packets at the multicast address or a multicast router won't send it
(that's why it's multi-, not broad-, cast).

RFC5135 (BCP for multicast over NAT) may be relevant, or contain
relevant references.

HTH