Knowledge Base

/**
* Demonstration of how to set TTL on a multicast socket
**/
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>

struct in_addr localInterface;
struct sockaddr_in groupSock;
int sd;
int datalen;
char databuf[1024];

int main( int argc, char *argv[] )
{
// Open the datagram socket for the control stream
sd = socket( AF_INET, SOCK_DGRAM, 0 );
if( sd < 0 ) {
perror( "opening datagram socket" );
exit( 1 );
}

// Create a multicast socket
memset( ( char * ) &groupSock, 0, sizeof( groupSock ) );
groupSock.sin_family = AF_INET;
groupSock.sin_addr.s_addr = inet_addr( "225.1.1.1" );
groupSock.sin_port = htons( 5555 );
{
char loopch = 0;
if( setsockopt
( sd, IPPROTO_IP, IP_MULTICAST_LOOP, ( char * ) &loopch,
sizeof( loopch ) ) < 0 ) {
perror( "setting IP_MULTICAST_LOOP:" );
close( sd );
exit( 1 );
}
}

// Get the local interface and mark it as multicast interface
localInterface.s_addr = inet_addr( "192.168.12.240" );
if( setsockopt
( sd, IPPROTO_IP, IP_MULTICAST_IF, ( char * ) &localInterface,
sizeof( localInterface ) ) < 0 ) {
perror( "setting local interface multicast" );
exit( 1 );
}

// Now set the TTL
// In this case to 31
{
unsigned char ttl;
ttl = 31;
// Write TTL for IP Multicasrt packets into the control socket
if( setsockopt
( sd, IPPROTO_IP, IP_MULTICAST_TTL, ( void * ) &ttl,
sizeof( ttl ) ) < 0 ) {
perror( "setting TTL on local interface" );
exit( 1 );
}
}

// Do a testsend on the socket so one can validate the packets
datalen = 10;
if( sendto
( sd, databuf, datalen, 0, ( struct sockaddr * ) &groupSock,
sizeof( groupSock ) ) < 0 ) {
perror( "sending datagram message" );
}