All things related to multicast

[Ethan Sommer <>]

As we've (Dan Oachs and I) been working on multicast routing, etc, I've 
often wanted to see if multicast was working to a particular host to 
which I only had ssh access (and sometimes only user level ssh access.)

After looking around, there didn't seem to be an easy way to test that.

Finally I got fed up and wrote a little program which joins the SAP 
announcement multicast group and outputs all the SAP announcements it 
sees. It can be run as a normal user (although you obviously won't be 
able open the firewall.)

Anyway, I thought some other people might find the program useful. I've 
included it below my signature.

Ethan

--
Ethan Sommer
Associate Director of Core Services
Gustavus Adolphus College
507-933-7042


/*
* listener.c -- joins a multicast group and echoes all data it receives from
*               the group to its stdout...
*
* Antony Courtney,      25/11/94
* Modified by: Frédéric Bastien (25/03/04)
* to compile without warning and work correctly
*
* Modified by: Ethan Sommer to look for SAP Announcements (25/09/07)
* 
*/

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
#include <string.h>
#include <stdio.h>


#define SAP_PORT 9875
#define SAP_GROUP "224.2.127.254"
#define MSGBUFSIZE 100000

main(int argc, char *argv[])
{
    struct sockaddr_in addr;
    int fd, nbytes,addrlen;
    struct ip_mreq mreq;
    char msgbuf[MSGBUFSIZE];
    int x;

    u_int yes=1;            /*** MODIFICATION TO ORIGINAL */

    /* create what looks like an ordinary UDP socket */
    if ((fd=socket(AF_INET,SOCK_DGRAM,0)) < 0) {
          perror("socket");
          exit(1);
    }


/**** MODIFICATION TO ORIGINAL */
   /* allow multiple sockets to use the same PORT number */
   if (setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(yes)) < 0) {
      perror("Reusing ADDR failed");
      exit(1);
      }
/*** END OF MODIFICATION TO ORIGINAL */

    /* set up destination address */
    memset(&addr,0,sizeof(addr));
    addr.sin_family=AF_INET;
    addr.sin_addr.s_addr=htonl(INADDR_ANY); /* N.B.: differs from sender */
    addr.sin_port=htons(SAP_PORT);
    
    /* bind to receive address */
    if (bind(fd,(struct sockaddr *) &addr,sizeof(addr)) < 0) {
          perror("bind");
          exit(1);
    }
    
    /* use setsockopt() to request that the kernel join a multicast group */
    mreq.imr_multiaddr.s_addr=inet_addr(SAP_GROUP);
    mreq.imr_interface.s_addr=htonl(INADDR_ANY);
    if (setsockopt(fd,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq)) < 0) {
          perror("setsockopt");
          exit(1);
    }

    /* now just enter a read-print loop */
    while (1) {
          addrlen=sizeof(addr);
          if ((nbytes=recvfrom(fd,msgbuf,MSGBUFSIZE,0,
                               (struct sockaddr *) &addr,&addrlen)) < 0) {
               perror("recvfrom");
               exit(1);
          }
          for (x=8;x<nbytes; x++) {
	    if (isprint(msgbuf[x])) 
	      putchar(msgbuf[x]);
	    else if (msgbuf[x]=='\r' || msgbuf[x]=='\n') 
	      putchar(msgbuf[x]);
	    else 
	      putchar('.');

          }
          printf("\n---\n");
    }
}