Troubleshooting Multicast with Linux

I was looking for lightweight tool which would help me with troubleshooting multicast on VXLAN transport network (underlay). While both vCNS and NSX have built in tools (pings of various sizes and broadcast packets) I needed something more flexibile where I could do arbitrary IGMP joins and leaves.

I used CentOS VM with one interface directly on transport network and software SMCRoute. This link contains binary package that works on RHEL/CentOS. Some other notes:

  • if you have multiple interfaces make sure the multicast is routed through the correct one:
    route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0
  • I had to install also glibc package:

    yum -y install glibc.i686

  • Make sure the kernel supports multicast

    cat /boot/config-<kernel version> | grep CONFIG_IP_MULTICAST

  • Enable ICMP ECHO on broadcast/multicast

    sysctl net.ipv4.icmp_echo_ignore_broadcasts=0

  • Start the smcroute daemon first:
    smcroute -d

To join and leave a multicast group use -j and -l commands:
smcroute -j eth0 239.0.0.1
smcroute -l eth0 239.0.0.1

To check current memberships use:
netstat -ng

or

ip maddr

IGMP version can be changed with following command:

echo “2” > /proc/sys/net/ipv4/conf/eth0/force_igmp_version

Additional useful statistics about IGMP joins:

cat /proc/net/igmp

To see which hosts are member of particular IGMP group just ping it and see who replies:

[root@CentOS~]# ping 239.1.0.10
PING 239.1.0.10 (239.1.0.10) 56(84) bytes of data.
64 bytes from 1.1.0.1: icmp_seq=1 ttl=64 time=0.141 ms
64 bytes from 1.1.0.3: icmp_seq=1 ttl=64 time=0.256 ms (DUP!)

Hosts 1.1.0.1 and 1.1.0.3 replied to ping on 239.1.0.10 multicast group.

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2 thoughts on “Troubleshooting Multicast with Linux

  1. Another way you can check to see if multicast is working is by packet sniffing on one machine & pinging the broadcast address on another. If you received the ping from the broadcast address then multicast is working.

    Regards,

    Nicholas Lee Fagan

  2. This really helped in fixing a long pending issue
    none of my machine had this sysctl parameter but multicast totally worked fine
    But there was this special server where multicast did not work. setting this parameter in sysctl helped me

    Regards,
    Adam M

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