Why isn't my Linux box forwarding multicast packets?

Why isn't my Linux box forwarding multicast packets?

Post by Wei » Fri, 27 May 2005 05:09:18

My setup is as follows:

A ---------> B ---------> C
        eth0   eth1

B eth0: eth1:

A and B are both out-of-box redhat 9 machine. C is windows 2000. The
traffic flows as the arrow's direction. The B is the multicast router
and I want multicast packets coming out from A to be forwarded from
SubnetAB to Subnet BC.

I enabled packet forwarding on B (echo 1 > /proc.../ip_forward) and it
worked I can ping C from A. I also added a route for on B
(route add -net netmask dev eth0), and I am
really only interested in uni-directional multicast forwarding.

I compiled mrouted and started it with everything set as default.

./mrouted -d

Afterwards, I can see the multicast packets coming in from eth0, but
they never were forwarded to eth1.

tcpdump output on B:

B:>tcpdump -i eth1 ip multicast

15:27:03.145280 > igmp v2 report
[ttl 1]
15:27:04.422349 > dvmrp.mcast.net: igmp v2 report
dvmrp.mcast.net (DF) [ttl 1]
15:27:04.512361 > all-routers.mcast.net: igmp v2 report
all-routers.mcast.net (DF) [ttl 1]
15:27:07.040699 > dvmrp.mcast.net: igmp dvmrp Probe [tos
0xc0]  [ttl 1]
15:27:11.155794 > igmp v2 report [ttl 1]

As you can see here, C was trying to join the multicast

B:>tcpdump -i eth0 ip multicast

15:33:52.230373 > udp 188 (DF)
[ttl 1]
15:33:52.230380 > udp 188 (DF)
[ttl 1]
15:33:52.230385 > udp 188 (DF)
[ttl 1]
15:33:52.230390 > udp 132 (DF)
[ttl 1]
15:33:52.230394 > udp 8 (DF)
[ttl 1]

As you can see here, the UDP multicast packet traffic is going very
well on eth0.

The kernel is a vanilla 2.4.20-31.9 i686 kernel. The likely multicast
routing related options had all been compiled in.


What am I missing? This had been puzzling me for the past few days. Any
help would be appreciated! Thanks!



Why isn't my Linux box forwarding multicast packets?

Post by Wei » Fri, 27 May 2005 05:15:05

> My setup is as follows:

> A ---------> B ---------> C
>         eth0   eth1

> A:
> B eth0: eth1:

ugh. should have double checked the post before submitting it.

B eth1 should be




1. Why isn't Linux box "pingable" by name?

I only know enough about networking to make me dangerous...

I am using a Linksys DSL Router w/4-port switch to run a small home
network, sharing the DSL Internet connection.

I used to have 4 Windows boxes on this network, but I recently
switched one of them to being a Linux file server (SuSe Pro 8.0).

On the Linux box, I can get to the Web via a browser, and I can ping
the other nodes on the home network by their IP addresses.  But if I
try to ping them by name, I get a "not found" message.  Here's what I
find to be a kicker, tho: on the same Linux box, if I ping an
*Internet* node by name, that works, i.e. if I ping www.yahoo.com it

From any of the Windows boxes (2 XP and 1 2000 Server) I can ping the
Linux box by IP but not by name.  Another thing I find puzzling: I've
set up a Samba share on the Linux box and I can map a drive to it from
the Win2K box, using its *name*!  But I can't ping it from there using
its name!

So, I'm guessing the Linux box must be using my DSL-provider's DNS
servers to resolve hostnames like "www.yahoo.com", but what is
supposed to resolve my local nodenames for it?  Are my local nodenames
NetBIOS names?  If so, where does the translation LOCAL_NODENAME ->
LOCAL_IP_ADDR take place?  Why does this work between any two of the
existing Windows boxes but not to or from the Linux box?

I wouldn't bother with this, except for the nagging suspicion that if
I can't ping by name, there's likely some other functionality that's
broken, too.

Thanks for any help...

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