All things related to multicast

[Marshall Eubanks <>]

Of course, since SAP frequently goes encapsulated in TCP MSDP  
announcements, the presence of
SAP does not prove that multicast routing works.

(The absence of SAP, though, may prove that it doesn't.)

Regards
Marshall

On Sep 25, 2007, at 11:34 AM, Ethan Sommer wrote:

> 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");
>     }
> }