ip.7p (2010 09)

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IP(7P) IP(7P)

IP_MULTICAST_IF

(struct in_addr) Speciﬁes a network interface other than the default

to be used when sending multicast datagrams through this socket.

Default: multicast datagrams are sent from the interface associated with

the speciﬁc multicast group, with the default multicast route or with the

default route.

IP_MULTICAST_LOOP

(unsigned char; boolean) Enables or disables loopback in the IP layer

for multicast datagrams sent through this socket. The value of the vari-

able pointed to by optval is zero (disable) or non-zero (enable). This

option is provided for compatibility only. Normally, multicast datagrams

are always looped back if the system has joined the group. See DEPEN-

DENCIES below. Default: enabled.

IP_MULTICAST_TTL

(unsigned char) Speciﬁes the time-to-live value for multicast

datagrams sent through this socket. The value of the variable pointed to

by optval can be zero through 255. Default: one.

An application joins a multicast group on a network interface in order to receive multicast datagrams

sent on the network to which that interface connects. An application can join up to

IP_MAX_MEMBERSHIPS

multicast groups on each socket. IP_MAX_MEMBERSHIPS

is deﬁned in

. However, each network interface may impose a smaller system-wide limit because

of interface resource limitations and because the system uses some link-layer multicast addresses.

The application must also bind to the destination port number in order to receive datagrams that are sent

to that port number. If the application binds to the address

INADDR_ANY, it may receive all datagrams

that are sent to the port number. If the application binds to a multicast group address, it may receive

only datagrams sent to that group and port number. It is not necessary to join a multicast group in order

to send datagrams to it.

For each multicast group that an application joins on a given socket and network interface, there is an

associated ﬁlter mode and source list. The ﬁlter mode can be "exclude" mode or "include" mode. The

source list is a set of IP addresses which will be compared to the source addresses of received multicast

datagrams.

An application uses "exclude" mode when it wants to block reception of multicast datagrams from a

speciﬁc set of sources, while allowing multicast datagrams from all other sources. For groups in "exclude"

mode, the source list may be empty, thus allowing reception of multicast datagrams from all sources.

An application uses "include" mode when it wants to allow reception of multicast datagrams from a

speciﬁc set of sources, while blocking multicast datagrams from all other sources. For groups in "include"

mode, the source list needs to contain at least one member in order for the application to stay joined to

the group; removing the last address from an "include" mode source list will cause the application to leave

the group.

IP_ADD_MEMBERSHIP

and MCAST_JOIN_GROUP request that the system join a multicast group on the

speciﬁed interface. The group is joined in "exclude" mode, with an empty source list. The

imr_interface ﬁeld holds the IPv4 address of a local interface, or INADDR_ANY. The

gr_interface ﬁeld holds the interface index of a local interface, or zero. If the interface address is

INADDR_ANY, or if the interface index is zero, the system joins the speciﬁed group on the interface from

which datagrams for that group would be sent, based on the routing conﬁguration. For example:

struct ip_mreq mreq;

mreq.imr_multiaddr.s_addr = inet_addr("224.1.2.3");

mreq.imr_interface.s_addr = INADDR_ANY;

setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));

struct group_req gr;

struct sockaddr_in *sin;

sin = (struct sockaddr_in *)&gr.gr_group;

bzero(sin, sizeof(*sin));

sin->sin_family = AF_INET;

sin->sin_addr.s_addr = inet_addr("224.1.2.3");

gr.gr_interface = 0;

setsockopt(s, IPPROTO_IP, MCAST_JOIN_GROUP, &gr, sizeof(gr));

IP_DROP_MEMBERSHIP and MCAST_LEAVE_GROUP allow the system to leave a multicast group. The

imr_interface ﬁeld holds the IPv4 address of a local interface, or INADDR_ANY. The

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