Multicast Join on Linux and IGMPv3

Question

We've run into a thorny problem. We are writing a c++ program that receives multicast UDP traffic. We're in the process of moving our applications to a different network environment and our operations team has requested that we support IGMPv3 membership announcements from our applications. Initial investigations indicate that Linux 2.6 kernels do support IGMPv3. Therefore, I'm puzzled that when we run tcpdump we see the following output traces:

[rtv@myhost]$ sudo /usr/sbin/tcpdump -i eth1.22 igmp
tcpdump: listening on eth1.22
00:20:09.007094 switch-a.stage > ALL-SYSTEMS.MCAST.NET: igmp query v2 [max resp time 20] [ttl 1]
00:20:09.241946 10.129.22.236 > 232.0.1.10: igmp v2 report 232.0.1.10 (DF) [tos 0xc0]  [ttl 1]
00:20:10.472159 10.129.22.236 > 236.0.1.101: igmp v2 report 236.0.1.101 (DF) [tos 0xc0]  [ttl 1]

44 packets received by filter

My understanding is that one could force the kernel to use a lower version of IGMP by specifying a non-zero value in the file /proc/sys/net/ipv4/conf/eth1.22/force_igmp_version; however, I've confirmed that the file has a zero value configuration.

Our application is using the following code to join the multicast group:

... joinMulticast(in_addr mcast_addr, in_addr interface_addr)
{
  struct ip_mreq  mcast_req;

  mcast_req.imr_multiaddr.s_addr = mcast_addr;
  mcast_req.imr_interface.s_addr = interface_addr;
  int err = setsockopt(fFileDesc, IPPROTO_IP, IP_ADD_MEMBERSHIP,
    (char*)&theMulti, sizeof(theMulti));
  // handle errors etc.
  // ...
}

Is there something extra that we need to include in the source program to force IGMPv3?

Answer 1

Couple of things to be aware of.

The first is that (as I understand it) setting the /proc/sys/net/ipv4/conf/eth1.22/force_igmp_version to 0 doesn't mean "use v3", but actually sets it to "auto". I beleive that you can set it to 3 to force it use igmp v3.

However, the other thing to be aware of is that the behavior of the igmp stack is determined by the environment that it finds itself in. If your linux box is receiving igmp v2 membership queries from an upstream igmp router, then I believe that the default linux behavior (as mandated by the igmp v3 rfc) is to use only igmp v2 for reports.

As I understand it when you set /proc/sys/net/ipv4/conf/eth1.22/force_igmp_version to 0 it uses this behavior.