ip.7p (2010 09)

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IP(7P) IP(7P)

struct group_source_req gsr;

struct sockaddr_in *sin;

sin = (struct sockaddr_in *)&gsr.gsr_group;

bzero(sin, sizeof(*sin));

sin->sin_family = AF_INET;

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

sin = (struct sockaddr_in *)&gsr.gsr_source;

bzero(sin, sizeof(*sin));

sin->sin_family = AF_INET;

sin->sin_addr.s_addr = inet_addr("10.4.5.6");

gsr.gsr_interface = 0;

setsockopt(s, IPPROTO_IP, MCAST_JOIN_SOURCE_GROUP, &gsr, sizeof(gsr));

setsockopt(s, IPPROTO_IP, MCAST_LEAVE_SOURCE_GROUP, &gsr, sizeof(gsr));

IP_MULTICAST_IF

speciﬁes a local network interface to be used when sending multicast datagrams

through this socket. For example:

#include <arpa/inet.h>

struct in_addr addr;

addr.s_addr = inet_addr("192.1.2.3");

setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));

Normally, applications do not need to specify the interface. By default, multicast datagrams are sent

from the interface speciﬁed by the routing conﬁguration, namely the interface associated with the speciﬁc

multicast group, with the default multicast route or with the default route. If addr is set to the address

INADDR_ANY, the default interface is selected. Otherwise, addr should be the IP address of a local

interface.

IP_MULTICAST_LOOP

enables or disables loopback for multicast datagrams sent through this socket.

For example:

unsigned char loop = 1;

setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));

Note that the type of the optval parameter is unsigned char instead of int, which is common for

boolean socket options. This option is provided for compatibility only. Normally, if a multicast datagram

is sent to a group that the system has joined, a copy of the datagram is always looped back and delivered

to any applications that are bound to the destination port. See DEPENDENCIES below.

IP_MULTICAST_TTL

controls the scope a multicast by setting the time-to-live value for multicast

datagrams sent through this socket. For example:

unsigned char ttl = 64;

setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));

Note that the type of optval parameter is unsigned char instead int, which is common for socket

options. By default, the time-to-live ﬁeld (TTL) is one, which limits the multicast to the local network. If

the TTL is zero, the multicast is limited to the local system (loopback). If the TTL is two, the multicast

can be forwarded through at most one gateway; and so forth. Multicast datagrams can be forwarded to

other networks only if there are special multicast routers on the local and intermediate networks.

DEPENDENCIES

The behavior of

IP_MULTICAST_LOOP depends on the network driver and interface card. Normally,

loopback cannot be disabled, even if IP_MULTICAST_LOOP is set to zero, because it occurs in the driver

or in the network interface. However, if the outbound interface is lo0 (127.0.0.1), or if

IP_MULTICAST_TTL is set to zero, setting IP_MULTICAST_LOOP

to zero will disable loopback for

multicast datagrams sent through the socket.

ERRORS

One of the following errors may be returned if a call to

setsockopt() or getsockopt() fails.

[EADDRINUSE] The speciﬁed multicast group has been joined already on socket.

[EADDRNOTAVAIL] The speciﬁed IP address is not a local interface address; or there is no

route for the speciﬁed multicast address; or the speciﬁed multicast group

has not been joined.

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