C++ receiving multicast on particular interface

Question

Docs on IP_ADD_MEMBERSHIP says:

IP_ADD_MEMBERSHIP (since Linux 1.2) Join a multicast group. Argument is an ip_mreqn structure.

 struct ip_mreqn { struct in_addr imr_multiaddr; /* IP multicast group address */ struct in_addr imr_address; /* IP address of local interface */ int imr_ifindex; /* interface index */ }; imr_multiaddr contains the address of the multicast group the appli‐ cation wants to join or leave. It must be a valid multicast address (or setsockopt(2) fails with the error EINVAL). imr_address is the address of the local interface with which the system should join the multicast group; if it is equal to INADDR_ANY, an appropriate inter‐ face is chosen by the system. imr_ifindex is the interface index of the interface that should join/leave the imr_multiaddr group, or 0 to indicate any interface.

So I have an interface "eth0" with ip 192.168.1.5. I want to join this interface to multicast group 225.1.1.1. I'm a little bit confused on how to setup ip_mreqn structure properly? I found 2 possible ways:

1.

ip_mreqn group;
group.imr_multiaddr.s_addr = inet_addr("225.1.1.1");
group.imr_address.s_addr = inet_addr("192.168.1.5");
group.imr_ifindex = 0;

2.

ip_mreqn group;
group.imr_multiaddr.s_addr = inet_addr("225.1.1.1");
group.imr_address.s_addr = htonl(INADDR_ANY);
group.imr_ifindex = if_nametoindex("eth0");

And 3rd way is to use SO_BINDTODEVICE socket option.

My questions are.

1) What is correct way to join particular interface to multicast group?

2) What is functional difference between imr_address and imr_ifindex?

3) How option SO_BINDTODEVICE could be useful?

EDIT: I did some research.

Suppose that I have two network interfaces: eth0 with ip 192.168.1.5 and eth1 with ip 192.168.1.255 and I receive multicast on eth0 with ip 192.168.1.5.

These ways work properly (I get multicast messages on eth0):

group.imr_address.s_addr = inet_addr("192.168.1.5");
group.imr_ifindex = 0;

or

group.imr_address.s_addr = htonl(INADDR_ANY);
group.imr_ifindex = if_nametoindex("eth0");

or obviously

group.imr_address.s_addr = inet_addr("192.168.1.5");
group.imr_ifindex = if_nametoindex("eth0");

and even

group.imr_address.s_addr = inet_addr("192.168.1.255");
group.imr_ifindex = if_nametoindex("eth0");

And these ways do not (I don't get multicast messages on eth0):

group.imr_address.s_addr = inet_addr("192.168.1.5");
group.imr_ifindex = if_nametoindex("eth1");

and

group.imr_address.s_addr = inet_addr("192.168.1.255");
group.imr_ifindex = if_nametoindex("eth1");

Answer 1

I've always used the older struct ip_mreq instead of struct ip_mreqn , as both are supported. This struct doesn't have the index field, so it's less ambiguous as to what you need to set.

struct ip_mreq
  {
    /* IP multicast address of group.  */
    struct in_addr imr_multiaddr;

    /* Local IP address of interface.  */
    struct in_addr imr_interface;
  };

You would then set it like this:

struct ip_mreq group;
group.imr_multiaddr.s_addr = inet_addr("225.1.1.1");
group.imr_interface.s_addr = inet_addr("192.168.1.5");

Answer 2

ip_mreqn is used to find network interface devices in the kernel source code at linux/net/ipv4/igmp.c (linux 5.13), as follow.

static struct in_device *ip_mc_find_dev(struct net *net, struct ip_mreqn *imr)
{
    struct net_device *dev = NULL;
    struct in_device *idev = NULL;

    if (imr->imr_ifindex) {
        idev = inetdev_by_index(net, imr->imr_ifindex);
        return idev;
    }
    if (imr->imr_address.s_addr) {
        dev = __ip_dev_find(net, imr->imr_address.s_addr, false);
        if (!dev)
            return NULL;
    }

    if (!dev) {
        struct rtable *rt = ip_route_output(net,
                            imr->imr_multiaddr.s_addr,
                            0, 0, 0);
        if (!IS_ERR(rt)) {
            dev = rt->dst.dev;
            ip_rt_put(rt);
        }
    }
    if (dev) {
        imr->imr_ifindex = dev->ifindex;
        idev = __in_dev_get_rtnl(dev);
    }
    return idev;
}

imr->imr_ifindex is first used to search for the device. If NOT found, imr->imr_address is used.