[Linux-cluster] people having trouble with the default multicast address 239.192.x.x

[Nikolas Lam]

On Fri, 2007-08-24 at 09:17 -0700, Steven Dake wrote:
> If you are having problems with the default multicast address
> 239.192.x.x it is probably because your switch is not sending those
> packets because of a configuration issue.
> 
> If you have this problem please contact me.  I would like to make a
> cookbook of switch manufacturer/models and possible solutions.
> 
> Using 224.0.0.x is not recommended as these multicast packets are
> broadcoast over the network.
> 
> Regards
> -steve

Hi Steve,

I'm not sure if the problems I'm having are caused by switch
configuration or compatibility - would the following be a good way to
determine if the multicast communications are working?

1) Check what multicast group (IP) the nodes have chosen using netstat
-gn ?

e.g. for host0, I'm guessing that cman is using 239.192.205.2 (as I
think 225.0.0.12 is for the fence_xvmd Xen virtualised host group and I
can't see any activity on 224.0.0.251).

[root at host0 ~]# netstat -g
IPv6/IPv4 Group Memberships
Interface       RefCnt Group
--------------- ------ ---------------------
lo              1      all-systems.mcast.net
eth0            1      224.0.0.251
eth0            1      225.0.0.12
eth0            1      239.192.205.2
eth0            1      all-systems.mcast.net
[root at host0 ~]# 



2) Use tcpdump to check that this node is receiving transmissions from
*other* nodes to the multicast group.

e.g. 

[root at host0 ~]# tcpdump -i any -nn host 239.192.205.2 and ip multicast
tcpdump: WARNING: Promiscuous mode not supported on the "any" device
tcpdump: verbose output suppressed, use -v or -vv for full protocol
decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 96
bytes
13:42:03.674676 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
118
13:42:03.767820 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
74
13:42:03.962382 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
74
13:42:04.162652 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
118
13:42:04.267768 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
74
13:42:04.450195 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
74
13:42:04.650646 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
118
13:42:04.766404 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
74
13:42:04.940367 IP 172.31.99.50.5149 > 239.192.205.2.5405: UDP, length
74

9 packets captured
19 packets received by filter
0 packets dropped by kernel
[root at host0 ~]# 

In the above example I can only see transmissions to 239.192.205.2 from
172.31.99.50 (hosts0's IP address). If things were working properly, I'd
be able to see transmissions from other source addresses wouldn't I?

E.g. I should have at least some entries like this

13:42:04.940367 IP 172.31.99.100.5149 > 239.192.205.2.5405: UDP, length
74

where the source address is a different node, right?


Regards,

Nik Lam