Using multicast addresses and groups - sockets

Question

For a week or so I have been researching methods to send datagrams to multiple clients from a single source, and found that the IP supports multicasting over the range of 224.0.0.0 and 234.255.255.255. I have read a few documents that describe the technology, but am finding nothing that has helped me understand how to use them.
From what I have gathered, it seems that a program asks the OS kernel to join a non-associated multicast group, then starts sending data over a socket via UDP to the multicast address. Everything else is handled by WAN without needing anything something physically at the address being sent to.
This line of thinking has been vaguely supported by what I've read, but has not been confirmed at all. Is this thinking correct? What is mistaken if there are issues with it?

Answer 1

This is essentially correct.
On the receiver side, you first open up a socket listening on a particular port. Then you join a multicast address on a given network interface. Doing so causes the OS to send out IGMP messages to the network informing routers that is joining a group.
On the sender side, you open a socket, then configure the outgoing network interface for multicast. Once you do that, you can send packets to any multicast address.
Here's an example sender in C:
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/in.h>
#include <strings.h>
#include <arpa/inet.h>
int main(int argc, char *argv[])
{
struct sockaddr_in sin;
struct in_addr out_addr;
char mes[255], ttl;
int sock;
printf("Enter message: ");
fgets(mes, sizeof(mes), stdin);
bzero(&sin,sizeof(sin));
sin.sin_family=AF_INET;
sin.sin_addr.s_addr=inet_addr(argv[1]);
sin.sin_port=htons(atoi(argv[2]));
if ((sock=socket(AF_INET,SOCK_DGRAM,0))==-1) {
perror("Error creating socket");
exit(1);
}
out_addr.s_addr=inet_addr(argv[3]);
if (setsockopt(sock,IPPROTO_IP,IP_MULTICAST_IF,(char *)&out_addr,sizeof(out_addr))== -1) {
perror("Error setting outgoing interface");
close(sock);
exit(1);
}
if (sendto(sock,mes,strlen(mes),0,(struct sockaddr *)&sin,sizeof(sin))==-1) {
close(sock);
perror("Send error");
}
close(sock);
}
And here's a sample receiver in C:
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int main(int argc, char *argv[])
{
int addr_len;
struct sockaddr_in sin,sin_recv;
struct ip_mreq multi;
char mes[255];
int sock;
bzero(&sin,sizeof(sin));
multi.imr_multiaddr.s_addr=inet_addr(argv[1]);
multi.imr_interface.s_addr=inet_addr(argv[2]);
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(atoi(argv[3]));
if ((sock=socket(AF_INET,SOCK_DGRAM,0))==-1) {
perror("Error creating socket");
exit(1);
}
if (setsockopt(sock,IPPROTO_IP,IP_ADD_MEMBERSHIP,(char *)&multi,sizeof(multi))== -1) {
perror("Error joining multicast group");
close(sock);
exit(1);
}
if (bind(sock,(struct sockaddr *)&sin,sizeof(sin))==-1) {
perror("Error binding socket");
close(sock);
exit(1);
}
while (1) {
bzero(mes,sizeof(mes));
bzero(&sin_recv,sizeof(sin_recv));
addr_len=sizeof(sin_recv);
printf("Waiting for packet...\n");
if (recvfrom(sock,mes,sizeof(mes),0,(struct sockaddr *)&sin_recv,&addr_len)==-1) {
perror("error recving socket");
close(sock);
exit(1);
}
printf("Got packet\n");
printf("Origin: %s port %d\n",inet_ntoa(sin_recv.sin_addr),sin_recv.sin_port);
printf("Message: %s\n",mes);
}
}