UDP broadcasting on same machine? - linux

Question

I wrote a broadcaster and a listener which recvfrom a particular port on the system. I used REUSEADDR option for the socket in listener, to make multiple instances of listener monitor the same port on same system.
When I run the listener on different machines, and send packet from another machine, all the listeners receives the packet. but when I run multiple instances of listener on same machine and if I try sending udp packets, only the first instance of listener gets the packet not all. I want to broadcast UDP packets on same machine and want all the listeners to receive the packet. I am on linux.
I followed the Beej's Guide.
Edit 01
listener code
/*
** listener.c -- a datagram sockets "server" demo
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#define MYPORT "4950" // the port users will be connecting to
#define MAXBUFLEN 100
// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET) {
return &(((struct sockaddr_in*)sa)->sin_addr);
}
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
int main(int argc, char *argv[])
{
int sockfd;
struct addrinfo hints, *servinfo, *p;
int rv;
int numbytes;
struct sockaddr_storage their_addr;
char buf[MAXBUFLEN];
socklen_t addr_len;
char s[INET6_ADDRSTRLEN];
int reuse_addr = 1;
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_PASSIVE; // use my IP
if ((rv = getaddrinfo(NULL, MYPORT, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
return 1;
}
// loop through all the results and bind to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror("listener: socket");
continue;
}
if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
if(errno == EADDRINUSE)
{
if(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR,
&reuse_addr, sizeof reuse_addr) < 0)
perror("setsockopt(): REUSEADDR\n"),exit(1);
}
else
{
close(sockfd);
perror("listener: bind");
continue;
}
}
break;
}
if (p == NULL) {
fprintf(stderr, "listener: failed to bind socket\n");
return 2;
}
freeaddrinfo(servinfo);
printf("listener: waiting to recvfrom...\n");
addr_len = sizeof their_addr;
if ((numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1 , 0,
(struct sockaddr *)&their_addr, &addr_len)) == -1) {
perror("recvfrom");
exit(1);
}
printf("listener: got packet from %s\n",
inet_ntop(their_addr.ss_family,
get_in_addr((struct sockaddr *)&their_addr),
s, sizeof s));
printf("listener: packet is %d bytes long\n", numbytes);
buf[numbytes] = '\0';
printf("listener: packet contains \"%s\"\n", buf);
close(sockfd);
return 0;
}

Answer 1

Set socket option to SO_REUSEPORT in linstener.
SO_REUSEPORT - socket option allows multiple sockets on the same host to bind to the same port

Answer 2

Make accept_local as 1 using echo in SYSFS.
/proc/sys/net/ipv4/conf/all # cat accept_local
1
This will allow local communication.