Network Interface Guide

Receiving IPv4 Multicast Datagrams

Before a host can receive IP multicast datagrams, it must become a member of one, or more, IP multicast group. A process can ask the host to join a multicast group by using the following socket option:

    struct ip_mreq mreq;
    setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)) 
where mreq is the structure
    struct ip_mreq {
        struct in_addr imr_multiaddr;   /* multicast group to join */
        struct in_addr imr_interface;   /* interface to join on */
Each membership is associated with a single interface, and it is possible to join the same group on more than one interface. Specify imr_interface to be in6addr_any to choose the default multicast interface, or one of the host's local addresses to choose a particular (multicast-capable) interface.

To drop a membership, use

    struct ip_mreq mreq;
    setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq))  
where mreq contains the same values used to add the membership. The memberships associated with a socket are also dropped when the socket is closed or the process holding the socket is killed. More than one socket can claim a membership in a particular group, and the host remains a member of that group until the last claim is dropped.

Incoming multicast packets are accepted by the kernel IP layer if any socket has claimed a membership in the destination group of the datagram. Delivery of a multicast datagram to a particular socket is based on the destination port and the memberships associated with the socket (or protocol type, for raw sockets), just as with unicast datagrams. To receive multicast datagrams sent to a particular port, bind to the local port, leaving the local address unspecified (such as, INADDR_ANY).

More than one process can bind to the same SOCK_DGRAM UDP port if the bind(3SOCKET) is preceded by:

    int one = 1;
    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one))  
In this case, every incoming multicast or broadcast UDP datagram destined to the shared port is delivered to all sockets bound to the port. For backwards compatibility reasons, this does not apply to incoming unicast datagrams. Unicast datagrams are never delivered to more than one socket, regardless of how many sockets are bound to the datagram's destination port. SOCK_RAW sockets do not require the SO_REUSEADDR option to share a single IP protocol type.

The definitions required for the new, multicast-related socket options are found in <netinet/in.h>. All IP addresses are passed in network byte-order.