How to UDP Broadcast with C in Linux?

Question

How to UDP Broadcast with C in Linux?

Answer 1

Typically using the Berkeley sockets API, to sendto() one or more datagrams to a known broadcast-class IP address.

Answer 2

I wrote udp multicast server recently for testing. To subscribe to multicast you would subscribe your client to Multicast group 225.0.0.37 port 12346 and port 12345 (2 feeds - one feeds sends "Hello, World!" the other one "Bye, Office!").

I've been using it for testing my client, both client and server run on the same box so there might be bits that may not work but give it a try first.

#include <stdlib.h>
#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>

#include <unistd.h>


#define BYE_OFFICE 12346
#define HELLO_PORT 12345
#define HELLO_GROUP "225.0.0.37"

int main(int argc, char *argv[])
{
    struct sockaddr_in addr;
    struct sockaddr_in addr2;
    int fd;
    int fd2;
    char *message = "Hello, World!";
    char *message2 = "Bye, Office!";

    if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        perror("socket");
        exit(1);
    }

    if ((fd2 = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        perror("socket");
        exit(1);
    }

    /* set up destination address */
    memset(&addr,0,sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr(HELLO_GROUP);
    addr.sin_port=htons(HELLO_PORT);

    memset(&addr2,0,sizeof(addr2));
    addr2.sin_family = AF_INET;
    addr2.sin_addr.s_addr = inet_addr(HELLO_GROUP);
    addr2.sin_port=htons(BYE_OFFICE);

    while (1)
    {
        if (sendto(fd, message, strlen(message), 0,(struct sockaddr *) &addr, sizeof(addr)) < 0)
        {
            perror("sendto");
            exit(1);
        }
        sleep(3);
        if (sendto(fd2, message2, strlen(message2), 0,(struct sockaddr *) &addr2, sizeof(addr2)) < 0)
        {
            perror("sendto2");
            exit(1);
        }
        sleep(3);
    }
}

Answer 3

Unwind has it right, except you should use 'sendto'

Here is an example, that assumes you already have a socket. It was taken from clamav

static void
broadcast(const char *mess)
{
    struct sockaddr_in s;

    if(broadcastSock < 0)
        return;

    memset(&s, '\0', sizeof(struct sockaddr_in));
    s.sin_family = AF_INET;
    s.sin_port = (in_port_t)htons(tcpSocket ? tcpSocket : 3310);
    s.sin_addr.s_addr = htonl(INADDR_BROADCAST);

    cli_dbgmsg("broadcast %s to %d\n", mess, broadcastSock);
    if(sendto(broadcastSock, mess, strlen(mess), 0, (struct sockaddr *)&s, sizeof(struct sockaddr_in)) < 0)
        perror("sendto");
}

Answer 4

In many IP stack, such as Linux, this code does not work. Your socket must have broadcast permissions. Try this:

bcast_sock = socket(AF_INET, SOCK_DGRAM, 0);
int broadcastEnable=1;
int ret=setsockopt(bcast_sock, SOL_SOCKET, SO_BROADCAST, &broadcastEnable, sizeof(broadcastEnable));

/* Add other code, sockaddr, sendto() etc. */