Simple multicast client and server

Multicast Server/Listener

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h> /* close */

#define SERVER_PORT 1500
#define MAX_MSG 100

int main(int argc, char *argv[]) {

  int sd, rc, n, cliLen;
  struct ip_mreq mreq;
  struct sockaddr_in cliAddr, servAddr;
  struct in_addr mcastAddr;
  struct hostent *h;
  char msg[MAX_MSG];

  if(argc!=2) {
    printf("usage : %s <mcast address>\n",argv[0]);
    exit(0);
  }

  /* get mcast address to listen to */
  h=gethostbyname(argv[1]);
  if(h==NULL) {
    printf("%s : unknown group '%s'\n",argv[0],argv[1]);
    exit(1);
  }

  memcpy(&mcastAddr, h->h_addr_list[0],h->h_length);

  /* check given address is multicast */
  if(!IN_MULTICAST(ntohl(mcastAddr.s_addr))) {
    printf("%s : given address '%s' is not multicast\n",argv[0],
           inet_ntoa(mcastAddr));
    exit(1);
  }

  /* create UDP socket */
  sd = socket(AF_INET,SOCK_DGRAM,0);
  if(sd<0) {
    printf("%s : cannot create socket\n",argv[0]);
    exit(1);
  }

  /* bind port */
  servAddr.sin_family=AF_INET;
  servAddr.sin_addr.s_addr=htonl(INADDR_ANY);
  servAddr.sin_port=htons(SERVER_PORT);
  if(bind(sd,(struct sockaddr *) &servAddr,sizeof(servAddr))<0)  
  {
    printf("%s : cannot bind port %d \n",argv[0],SERVER_PORT);
    exit(1);
  }

  /* join multicast group */
  mreq.imr_multiaddr.s_addr=mcastAddr.s_addr;
  mreq.imr_interface.s_addr=htonl(INADDR_ANY);

  rc = setsockopt(sd,IPPROTO_IP,IP_ADD_MEMBERSHIP,
                  (void *) &mreq, sizeof(mreq));
  if(rc<0) {
    printf("%s : cannot join multicast group '%s'",argv[0],
           inet_ntoa(mcastAddr));
    exit(1);
  }
  else {
    printf("%s : listening to mgroup %s:%d\n",
           argv[0],inet_ntoa(mcastAddr), SERVER_PORT);


    /* infinite server loop */
    while(1) {
      cliLen=sizeof(cliAddr);
      n = recvfrom(sd,msg,MAX_MSG,0,(struct sockaddr *) &cliAddr,&cliLen);
      if(n<n) {
        printf("%s : cannot receive data\n",argv[0]);
        continue;
      }

      printf("%s : from %s:%d on %s : %s\n",argv[0],
             inet_ntoa(cliAddr.sin_addr),ntohs(cliAddr.sin_port),
             argv[1],
             msg);
    }/* end of infinite server loop */

  }

return 0;

}

Multicast Client/Sender

#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h> /* close */

#define SERVER_PORT 1500
#define MAX_MSG 100

int main(int argc, char *argv[]) {

  int sd, rc, i;
  unsigned char ttl = 1;
  struct sockaddr_in cliAddr, servAddr;
  struct hostent *h;

  if(argc<3) {
    printf("usage %s <mgroup> <data1> <data2> ... <dataN>\n",argv[0]);
    exit(1);
  }

  h = gethostbyname(argv[1]);
  if(h==NULL) {
    printf("%s : unknown host '%s'\n",argv[0],argv[2]);
    exit(1);
  }

  servAddr.sin_family = h->h_addrtype;
  memcpy((char *) &servAddr.sin_addr.s_addr, h->h_addr_list[0],h->h_length);
  servAddr.sin_port = htons(SERVER_PORT);

  /* check if dest address is multicast */
  if(!IN_MULTICAST(ntohl(servAddr.sin_addr.s_addr))) {
    printf("%s : given address '%s' is not multicast \n",argv[0],
           inet_ntoa(servAddr.sin_addr));
    exit(1);
  }

  /* create socket */
  sd = socket(AF_INET,SOCK_DGRAM,0);
  if (sd<0) {
    printf("%s : cannot open socket\n",argv[0]);
    exit(1);
  }
  /* bind any port number */
  cliAddr.sin_family = AF_INET;
  cliAddr.sin_addr.s_addr = htonl(INADDR_ANY);
  cliAddr.sin_port = htons(0);
  if(bind(sd,(struct sockaddr *) &cliAddr,sizeof(cliAddr))<0) {
    perror("bind");
    exit(1);
  }

  if(setsockopt(sd,IPPROTO_IP,IP_MULTICAST_TTL, &ttl,sizeof(ttl))<0) {
    printf("%s : cannot set ttl = %d \n",argv[0],ttl);
    exit(1);
  }

  printf("%s : sending data on multicast group '%s' (%s)\n",argv[0],
         h->h_name,inet_ntoa(*(struct in_addr *) h->h_addr_list[0]));


  /* send data */
  for(i=2;i<argc;i++) {
    rc = sendto(sd,argv[i],strlen(argv[i])+1,0,
                (struct sockaddr *) &servAddr, sizeof(servAddr));

    if (rc<0) {
      printf("%s : cannot send data %d\n",argv[0],i-1);
      close(sd);
      exit(1);
    }

  }/* end for */

  /* close socket and exit */
  close(sd);
  exit(0);
}
 

$ ./mcastserver 224.0.0.200
./mcastserver : listening to mgroup 224.0.0.200:1500
./mcastserver : from 192.168.10.52:51654 on 224.0.0.200 : hello
./mcastserver : from 192.168.10.52:51654 on 224.0.0.200 : bye

$ ./mcastclient 224.0.0.200 hello bye
./mcastclient : sending data on multicast group '224.0.0.200' (224.0.0.200)



The switch periodically sends Query (Type 0x11) messages to broadcast address 224.0.0.1
When a server joins a new multicast group it sends a Membership report (Type 0x16) to all the hosts.
When a server leaves the group it sends a Leave Group (0x17) message to broadcast address 224.0.0.2

Also read:
http://ntrg.cs.tcd.ie/undergrad/4ba2/multicast/antony/
http://www.ibiblio.org/pub/Linux/docs/HOWTO/other-formats/html_single/Multicast-HOWTO.html