IP multicasting is supported only on
AF_INET sockets of type
SOCK_DGRAM and
SOCK_RAW, and only on networks where the interface driver supports multicasting.
The
IP_MULTICAST_TTL option changes the time-to-live (TTL) for outgoing multicast datagrams in order to control the scope of the multicasts:
u_char ttl; /* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
Datagrams with a TTL of 1 are not forwarded beyond the local network. Multicast datagrams with a TTL of 0 will not be transmitted on any network, but may be delivered locally if the sending host belongs to the destination group and if multicast loopback has not been disabled on the sending socket (see below). Multicast datagrams with TTL greater than 1 may be forwarded to other networks if a multicast router is attached to the local network.
For hosts with multiple interfaces, each multicast transmission is sent from the primary network interface. The
IP_MULTICAST_IF option overrides the default for subsequent transmissions from a given socket:
struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
where "addr" is the local IP address of the desired interface or
INADDR_ANY to specify the default interface. An interface's local IP address and multicast capability can be obtained via the
SIOCGIFCONF and
SIOCGIFFLAGS ioctls. An application may also specify an alternative to the default network interface by index:
struct uint32_t idx = htonl(ifindex);
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &idx, sizeof(idx));
where "ifindex" is an interface index as returned by
if_nametoindex(3).
Normal applications should not need to use
IP_MULTICAST_IF.
If a multicast datagram is sent to a group to which the sending host itself belongs (on the outgoing interface), a copy of the datagram is, by default, looped back by the IP layer for local delivery. The
IP_MULTICAST_LOOP option gives the sender explicit control over whether or not subsequent datagrams are looped back:
u_char loop; /* 0 = disable, 1 = enable (default) */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
This option improves performance for applications that may have no more than one instance on a single host (such as a router demon), by eliminating the overhead of receiving their own transmissions. It should generally not be used by applications for which there may be more than one instance on a single host (such as a conferencing program) or for which the sender does not belong to the destination group (such as a time querying program).
A multicast datagram sent with an initial TTL greater than 1 may be delivered to the sending host on a different interface from that on which it was sent, if the host belongs to the destination group on that other interface. The loopback control option has no effect on such delivery.
A host must become a member of a multicast group before it can receive datagrams sent to the group. To join a multicast group, use the
IP_ADD_MEMBERSHIP option:
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
where
mreq is the following structure:
struct ip_mreq {
struct in_addr imr_multiaddr; /* multicast group to join */
struct in_addr imr_interface; /* interface to join on */
}
imr_interface should be
INADDR_ANY to choose the default multicast interface, or the IP address of a particular multicast-capable interface if the host is multihomed. Membership is associated with a single interface; programs running on multihomed hosts may need to join the same group on more than one interface. Up to
IP_MAX_MEMBERSHIPS (currently 20) memberships may be added on a single socket.
To drop a membership, use:
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
where
mreq contains the same values as used to add the membership. Memberships are dropped when the socket is closed or the process exits.