socket binding with INADDR_ANY and specific interface in multicast with multiple interface

Question

I'm trying to write multicast server. I've two questions:

How to send multicast announcements on multiple interfaces like ethernet/ wifi interface. Do I need to create multiple sockets for each interface and bind?

When bind socket with INADDR_ANY address, descriptor is ready to do I/O operation (using select call )but when I bind with specific interface address e.g ethernet/wifi then descriptor is not ready to perform any operation it is stuck at select api only. So what is the difference between binding a socket with default address (INADDR_ANY) or specific interface address?

int sd = socket(AF_INET, SOCK_DGRAM, 0);
if (sd < 0) {
        printf("scoket() failed");
        return sd;
}

int r = -1;

int on = 1;
if ((r = setsockopt(sd, SOL_SOCKET, SO_REUSEADDR, (char *) &on, sizeof(on))) < 0) {
        printf("recv setsockopt(SO_REUSEADDR) failed");
        return r;
}
// add membership to receiving socket
struct ip_mreq mreq;
memset(&mreq, 0, sizeof(struct ip_mreq));
mreq.imr_interface.s_addr = htonl(INADDR_ANY);
mreq.imr_multiaddr.s_addr = inet_addr(MDNS_ADDR);
if ((r = setsockopt(sd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *) &mreq, sizeof(mreq))) < 0) {
        printf("recv setsockopt(IP_ADD_MEM) failed");
        return r;
}
 // enable loopback in case someone else needs the data
if ((r = setsockopt(sd, IPPROTO_IP, IP_MULTICAST_LOOP, (char *) &on, sizeof(on))) < 0) {
        printf("recv setsockopt(IP_MULTICAST_LOOP) failed");
        return r;
}


#ifdef IP_PKTINFO
if ((r = setsockopt(sd, SOL_IP, IP_PKTINFO, (char *) &on, sizeof(on))) < 0) {
        printf("recv setsockopt(IP_PKTINFO) failed");
        return r;
}
#endif
/* bind to an address */
struct sockaddr_in serveraddr;
memset(&serveraddr, 0, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
serveraddr.sin_port = htons(MDNS_PORT);
//serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);       /* receive multicast */
serveraddr.sin_addr.s_addr = inet_addr("192.168.10.23");        /* receive multicast */
if ((r = bind(sd, (struct sockaddr *)&serveraddr, sizeof(serveraddr))) < 0) {
        printf("recv bind()");
}

In thread waiting for descriptors to ready for I/O (basically all are read file descriptors)

            FD_ZERO(&sockfd_set);
            FD_SET(svr->sockfd, &sockfd_set);
            FD_SET(svr->notify_pipe[0], &sockfd_set);
            printf("before select\n");
            select(max_fd + 1, &sockfd_set, NULL, NULL, NULL);
            printf("after select\n");

When socket id bind with INADD_ANY address select will returns and I'm able to read with file descriptor but when bind with specific interface then select never returns coz there is no file descriptors available to read.

When you bind to INADDR_ANY and then call select, you do check what select returns? And check what descriptors are set in sockfd_set? And talking about error, is any system call fails, you should really print the error (maybe with the help of strerror). — Some programmer dude
I've added socket descriptor to sockfd_set and select will returns 1 when bind with INADDR_ANY after that I'm checking socket descriptor with FD_ISSET and perform read operation on specific descriptor. In other case select is not returning at all because of i didn't mention the timeout. — Narrator
When receiving multicast, always bind to INADDR_ANY. In Linux, if you bind to a specific interface, you won't be able to receive multicast. — dbush

Stian Skjelstad Stian Skjelstad · Accepted Answer · 2016-04-14T14:30:38

Binding a socket with a specific address is a generic way to lock it down to a device. This method is a bit old-fashion, since addresses can be changed on a device after your program has started up. In general I would recommend to bind to INADDR_ANY.

You can change the output multicast device on a socket whenever you want.

Options to consider:

man 7 ip :
IP_MULTICAST_ALL - deliver to all devices (default 1)
IP_MULTICAST_IF - I'm pretty sure this only sets the output device

man 7 socket :
SO_BINDTODEVICE

Just play a little with the options and see what the result is

socket binding with INADDR_ANY and specific interface in multicast with multiple interface

2 Answers