Receiving UDP multicast on macOS Big Sur - c

Question

I know this might be the 10.000th question on receiving UDP multicast messages. However, I already got it working half a year ago. The following snippet I got from http://www.cs.tau.ac.il/~eddiea/samples/Multicast/multicast-listen.c.html (with minor changes: The bind-ip was changed to IPADDR_ANY) worked as expected before I upgraded to macOS Big Sur.
/**************************************************************/
/* Multicast listener (server) */
/* */
/* Activation using: {program name} {Multicast IP} {port} */
/* {program name} - This program name */
/* {Multicast IP} - The IP address to listen to (Class D) */
/* {port} - The port hnumber to listen on */
/* */
/* This is free software released under the GPL license. */
/* See the GNU GPL for details. */
/* */
/* (c) Juan-Mariano de Goyeneche. 1998, 1999. */
/**************************************************************/
#include <stdio.h> /* printf(), snprintf() */
#include <stdlib.h> /* strtol(), exit() */
#include <sys/types.h>
#include <sys/socket.h> /* socket(), setsockopt(), bind(), recvfrom(), sendto() */
#include <errno.h> /* perror() */
#include <netinet/in.h> /* IPPROTO_IP, sockaddr_in, htons(), htonl() */
#include <arpa/inet.h> /* inet_addr() */
#include <unistd.h> /* fork(), sleep() */
#include <sys/utsname.h> /* uname() */
#include <string.h> /* memset() */
#define MAXLEN 1024
int main(int argc, char* argv[])
{
u_char no = 0;
u_int yes = 1; /* Used with SO_REUSEADDR.
In Linux both u_int */
/* and u_char are valid. */
int send_s, recv_s; /* Sockets for sending and receiving. */
u_char ttl;
struct sockaddr_in mcast_group;
struct ip_mreq mreq;
struct utsname name;
int n;
socklen_t socklen;
struct sockaddr_in from;
char message [MAXLEN+1];
if (argc != 3) {
fprintf(stderr, "Usage: %s mcast_group port\n", argv[0]);
exit(1);
}
memset(&mcast_group, 0, sizeof(mcast_group));
mcast_group.sin_family = AF_INET;
mcast_group.sin_port = htons((unsigned short int)strtol(argv[2], NULL, 0));
mcast_group.sin_addr.s_addr = htonl(INADDR_ANY);
if ( (recv_s=socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
perror ("recv socket");
exit(1);
}
if (setsockopt(recv_s, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes)) < 0) {
perror("reuseaddr setsockopt");
exit(1);
}
if (bind(recv_s, (struct sockaddr*)&mcast_group, sizeof(mcast_group)) < 0) {
perror ("bind");
exit(1);
}
struct in_addr sin;
inet_pton(AF_INET, argv[1], &sin);
/* Preparatios for using Multicast */
mreq.imr_multiaddr = sin;
mreq.imr_interface.s_addr = 0;
/* Tell the kernel we want to join that multicast group. */
if (setsockopt(recv_s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)) < 0) {
perror ("add_membership setsockopt");
exit(1);
}
for (;;) {
socklen=sizeof(from);
if ( (n=recvfrom(recv_s, message, MAXLEN, 0,
(struct sockaddr*)&from, &socklen)) < 0) {
perror ("recv");
exit(1);
}
message[n] = '\0'; /* null-terminate string */
printf("%s: Received message from %s, size=%d !!\n",
name.nodename,
inet_ntoa(from.sin_addr), n);
printf("\t%s \n", message);
}
}
Now I try to get it running again and I can not find any reason why this does not work anymore.
Did some policies change in Big Sur and this is some permission error? I already run the Application as root with sudo ./multicast-listen 239.255.255.250 1900 but this did not work either. I'm trying to receive SSDP packages.

Answer 1

So the comment of #CraigEstey brought me to the conclusion it has to do something with the Firewall. Indeed it was a simple firewall issue. In Settings -> Security -> Firewall -> Firewall Options... you have to disable Block all incoming connections and disable stealth mode. Now I'm receiving all SSDP packages as intended.