multicast data from specific ip address - c

Question

For example I am having two different segment IP of same server 172.17.8.90 172.19.8.100. Now, I want my program to send multicast from only 172.19.8.100 this ip. Currently my program is sending multicast from 172.17.8.90 which is not allowed to send multicast because of this, i want multicast data from specific IP
#include <stdio.h> /* for fprintf() */
#include <sys/socket.h> /* for socket(), connect(), send(), and recv() */
#include <arpa/inet.h> /* for sockaddr_in and inet_addr() */
#include <stdlib.h> /* for atoi() and exit() */
#include <string.h> /* for memset() */
#include <unistd.h> /* for sleep() */
void DieWithError(char *errorMessage) /* External error handling function */
{
fprintf(stderr,"Dieing baby\n");
}
int main(int argc, char *argv[])
{
int sock; /* Socket */
struct sockaddr_in multicastAddr; /* Multicast address */
char *multicastIP; /* IP Multicast address */
unsigned short multicastPort; /* Server port */
char *sendString; /* String to multicast */
unsigned char multicastTTL; /* TTL of multicast packets */
unsigned int sendStringLen; /* Length of string to multicast */
if ((argc < 4) || (argc > 5)) /* Test for correct number of parameters */
{
fprintf(stderr,"Usage: %s <Multicast Address> <Port> <Send String> [<TTL>]\n",
argv[0]);
exit(1);
}
multicastIP = argv[1]; /* First arg: multicast IP address */
multicastPort = atoi(argv[2]); /* Second arg: multicast port */
sendString = argv[3]; /* Third arg: String to multicast */
if (argc == 5) /* Is TTL specified on command-line? */
multicastTTL = atoi(argv[4]); /* Command-line specified TTL */
else
multicastTTL = 1; /* Default TTL = 1 */
/* Create socket for sending/receiving datagrams */
if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
DieWithError("socket() failed");
/* Set TTL of multicast packet */
if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, (void *) &multicastTTL,
sizeof(multicastTTL)) < 0)
DieWithError("setsockopt() failed");
/* Construct local address structure */
memset(&multicastAddr, 0, sizeof(multicastAddr)); /* Zero out structure */
multicastAddr.sin_family = AF_INET; /* Internet address family */
multicastAddr.sin_addr.s_addr = inet_addr(multicastIP);/* Multicast IP address */
multicastAddr.sin_port = htons(multicastPort); /* Multicast port */
sendStringLen = strlen(sendString); /* Find length of sendString */
for (;;) /* Run forever */
{
/* Multicast sendString in datagram to clients every 3 seconds */
if (sendto(sock, sendString, sendStringLen, 0, (struct sockaddr *)
&multicastAddr, sizeof(multicastAddr)) != sendStringLen)
DieWithError("sendto() sent a different number of bytes than expected");
sleep(3);
}
}

Answer 1

When sending out multicast packets, the OS will by default chose one particular interface to send from. To change the outgoing interface, you need to set the IP_MULTICAST_IF option:
struct in_addr multi_out;
multi_out.s_addr = inet_addr("172.19.8.100");
if (setsockopt(sock,IPPROTO_IP,IP_MULTICAST_IF, (char *)&multi_out, sizeof(multi_out)) == -1) {
perror("error setting IP_MULTICAST_IF option");
exit(1);
}
Also be sure to use the perror function as above for error checking to see why a particular library call failed.