Receiving UDP multicast on macOS Big Sur

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.