Multicast source address in recvfrom()

[Derek Tattersall]

* Robert Watson <rwatson at FreeBSD.org> [090202 17:06]:
> On Sun, 1 Feb 2009, Derek Tattersall wrote:
> 
> >In order to become familiar with multicast implementation using FreeBSD, I 
> >found via Google a pair of test programs which multicast sent a simple 
> >text message and received the text message.  I added some code to report 
> >the source address, because none of the references that I looked at 
> >specified the source IP address in the frame.
> >
> >I ran the sender on A -current system, AMD64 vintage last week.  The 
> >receiver was on a -current system I386 vintage last week.  TCPDUMP shows 
> >the source IP address in the frame as (correctly) 192.168.0.15.  The 
> >receiver reports the source IP address as 200.231.191.191.  I have also 
> >run the same test with an OpenBSD 4.4 Release I386 system as the receiver. 
> >The openBSD system reports the sender as 192.168.0.15.  A Fedora 10 system 
> >reported the source IP address as 0.0.0.0.
> >
> >Googling the RFCs and other information and referring to Comer's and 
> >Stevens' books on TCPIP I can't determine what should be reported.  Does 
> >anybody have clue for me?
> 
> Hi Derek:
> 
> It might depend on how you're querying the source address.  Could you post 
> a code excerpt?
> 
> Robert N M Watson
> Computer Laboratory
> University of Cambridge
It's a straightforward test program.

/**********************************************************************
 *
 * MulticastReceiver.c: Receive a multicast message
 *
 * Copyright (C) 2009 Derek Tattersall.
 *
 * See the COPYING file for license information.
 *
 * Author: Derek Tattersall
 *
 * Latest Revision: Sat Jan 31 07:21:04 2009
 *
 **********************************************************************/

/* Includes***********************************************************/
#include <stdio.h>      /* for printf() and fprintf() */
#include <sys/socket.h> /* for socket(), connect(), sendto(), and recvfrom() */
#include <arpa/inet.h>  /* for sockaddr_in and inet_addr() */
#include <netinet/in.h>	/* for IPPROTO_UDP */
#include <stdlib.h>     /* for atoi() and exit() */
#include <string.h>     /* for memset() */
#include <unistd.h>     /* for close() */


/* Defines ***********************************************************/
#define MAXRECVSTRING 255  /* Longest string to receive */

/* Type Definitions **************************************************/

/* Function Declarations *********************************************/
void DieWithError(char *errorMessage);  /* External error handling function */

/* Global Variables **************************************************/

/* Function Implementations ******************************************/
int main(int argc, char *argv[])
{
    int sock;                         /* Socket */
    struct sockaddr_in multicastAddr; /* Multicast Address */
    char *multicastIP;                /* IP Multicast Address */
    unsigned short multicastPort;     /* Port */
    struct sockaddr_in senderAddr;    /* Sender Address */
    int senderLen;		      /* length of sender address socket */
    char senderAddrBuf[INET_ADDRSTRLEN]; /* conversion buffer */
    char recvString[MAXRECVSTRING+1]; /* Buffer for received string */
    int recvStringLen;                /* Length of received string */
    struct ip_mreq multicastRequest;  /* Multicast address join structure */

    if (argc != 3)    /* Test for correct number of arguments */
    {
        fprintf(stderr,"Usage: %s <Multicast IP> <Multicast Port>\n", argv[0]);
        exit(1);
    }

    multicastIP = argv[1];        /* First arg: Multicast IP address (dotted quad) */
    multicastPort = atoi(argv[2]);/* Second arg: Multicast port */

    /* Create a best-effort datagram socket using UDP */
    if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
        DieWithError("socket() failed");

    /* Construct bind structure */
    memset(&multicastAddr, 0, sizeof(multicastAddr));   /* Zero out structure */
    multicastAddr.sin_family = AF_INET;                 /* Internet address family */
    multicastAddr.sin_addr.s_addr = htonl(INADDR_ANY);  /* Any incoming interface */
    multicastAddr.sin_port = htons(multicastPort);      /* Multicast port */

    /* Bind to the multicast port */
    if (bind(sock, (struct sockaddr *) &multicastAddr, sizeof(multicastAddr)) < 0)
        DieWithError("bind() failed");

    /* Specify the multicast group */
    multicastRequest.imr_multiaddr.s_addr = inet_addr(multicastIP);
    /* Accept multicast from any interface */
    multicastRequest.imr_interface.s_addr = htonl(INADDR_ANY);
    /* Join the multicast address */
    if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void *) &multicastRequest,
          sizeof(multicastRequest)) < 0)
        DieWithError("setsockopt() failed");

    /* Receive a single datagram from the server */
    if ((recvStringLen = recvfrom(sock, recvString, MAXRECVSTRING, 0, (struct sockaddr *)&senderAddr, (socklen_t *)&senderLen)) < 0)
        DieWithError("recvfrom() failed");

    if ((inet_ntop(AF_INET, &senderAddr.sin_addr.s_addr, senderAddrBuf, INET_ADDRSTRLEN)) == 0)
	    senderAddrBuf[0] = '\0';
    recvString[recvStringLen] = '\0';
    printf("Received from %s: %s\n", senderAddrBuf, recvString);    /* Print the received string */
    
    close(sock);
    exit(0);
}

/* vim: set sts=0 sw=8 ts=8: *****************************************/
Note that the sender's address is taken from the recvfrom call per the
man page and printed via inet_ntop call.  The DieWithError routine is a
simple print error message and exit with a 1.  I have used similar code
with UDP recieving apps before.
-- 
Best regards,
Derek Tattersall
dlt at mebtel.net		dlt666 at yahoo.com		 dtatters at gmail.com