About c : Sending-and-receiving-multicast-on-the-same-Linux-machine-from-different-interfaces

I have a network with several computers when each computer has several network interfaces with a single destination. I’m developing an application that actively uses multicast. In general everything works as expected. Except for the moment that I cannot receive a multicast on the same machine from which I’m sending via a second network interface. Other computers on the network can receive multicast through any network interface. Is it possible to send a multicast through one interface and receive it through another within the same network? If so, where did I make a mistake: in the client code, in the recipient code, or in the system settings?

A typical machine configuration:

$ ifconfig 
enp0s31f6: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
    inet 192.168.88.230  netmask 255.255.255.0  broadcast 192.168.88.255

enp10s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
    inet 192.168.88.229  netmask 255.255.255.0  broadcast 192.168.88.255

wlp6s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
    inet 192.168.88.48  netmask 255.255.255.0  broadcast 192.168.88.255

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
    inet 127.0.0.1  netmask 255.0.0.0

Routing table:

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.88.1    0.0.0.0         UG    100    0        0 enp0s31f6
0.0.0.0         192.168.88.1    0.0.0.0         UG    101    0        0 enp10s0
0.0.0.0         192.168.88.1    0.0.0.0         UG    600    0        0 wlp6s0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 enp0s31f6
192.168.88.0    0.0.0.0         255.255.255.0   U     100    0        0 enp0s31f6
192.168.88.0    0.0.0.0         255.255.255.0   U     101    0        0 enp10s0
192.168.88.0    0.0.0.0         255.255.255.0   U     600    0        0 wlp6s0

These combinations work as expected

./sender 239.255.255.251 27335 192.168.88.48
./listener 239.255.255.251 27335 192.168.88.48

or

./sender 239.255.255.251 27335 192.168.88.229
./listener 239.255.255.251 27335 192.168.88.229

or

./sender 239.255.255.251 27335 192.168.88.230
./listener 239.255.255.251 27335 192.168.88.230

But these don’t work:

./sender 239.255.255.251 27335 192.168.88.48
./listener 239.255.255.251 27335 192.168.88.229

or

./sender 239.255.255.251 27335 192.168.88.48
./listener 239.255.255.251 27335 192.168.88.230

or

./sender 239.255.255.251 27335 192.168.88.229
./listener 239.255.255.251 27335 192.168.88.230

For my task, I have adapted the code from several examples from https://tldp.org/HOWTO/Multicast-HOWTO-6.html

sender.c

//
// Simple sender.c program for UDP
//

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if (argc < 3) {
        printf("Command line args should be multicast group and port\n");
        printf("(e.g. for SSDP, `sender 239.255.255.250 1900 [interface_ip]`)\n");
        return 1;
    }

    const char* group = argv[1]; // e.g. 239.255.255.250 for SSDP
    const int port = atoi(argv[2]); // 0 if error, which is an invalid port
    const char* source_iface = (argc == 4 ? argv[3] : NULL);
   
    //
    // create what looks like an ordinary UDP socket
    //
    const int fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) {
        perror("socket");
        return 1;
    }

    struct ip_mreq mreq;
    memset(&mreq, 0, sizeof(mreq));
    mreq.imr_interface.s_addr = source_iface ? inet_addr(source_iface) : htonl(INADDR_ANY);

    if (
        setsockopt(
            fd, IPPROTO_IP, IP_MULTICAST_IF, (char*) &mreq, sizeof(mreq)
        ) < 0
    ){
        perror("setsockopt");
        return 1;
    }

    //
    // set up destination address
    //
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr(group);
    addr.sin_port = htons(port);

    //
    // now just sendto() our destination
    //
    for (unsigned i = 0; ; i++) {
        char buffer[64];
        memset(buffer, '\0', sizeof(buffer));
        snprintf(buffer, sizeof(buffer), "Hello, World! Sequence: %u", i & 0xFF);

        const int nbytes = sendto(
            fd,
            buffer,
            sizeof(buffer),
            0,
            (struct sockaddr*) &addr,
            sizeof(addr)
        );
        if (nbytes < 0) {
            perror("sendto");
            return 1;
        }

        const int delay_secs = 1;
        sleep(delay_secs);
    }

    return 0;
}

listener.c

//
// Simple listener.c program for UDP multicast
//

#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 <stdlib.h>

#define MSGBUFSIZE 4096

int main(int argc, char *argv[])
{
    if (argc < 3) {
       printf("Command line args should be multicast group and port and [interface] optional\n");
       printf("(e.g. for SSDP, `listener 239.255.255.250 1900 [192.168.1.1]`)\n");
       return 1;
    }

    const char* group = argv[1]; // e.g. 239.255.255.250 for SSDP
    const int port = atoi(argv[2]); // 0 if error, which is an invalid port
    const char* source_iface = (argc == 4) ? argv[3] : NULL;

    //
    // create what looks like an ordinary UDP socket
    //
    const int fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) {
        perror("socket");
        return 1;
    }

    //
    // allow multiple sockets to use the same PORT number
    //
    const u_int yes = 1;
    if (
        setsockopt(
            fd, SOL_SOCKET, SO_REUSEADDR, (char*) &yes, sizeof(yes)
        ) < 0
    ){
       perror("Reusing ADDR failed");
       return 1;
    }

    //
    // set up destination address
    //
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr(group);
    addr.sin_port = htons(port);

    //
    // bind to receive address
    //
    if (bind(fd, (struct sockaddr*) &addr, sizeof(addr)) < 0) {
        perror("bind");
        return 1;
    }

    //
    // use setsockopt() to request that the kernel join a multicast group
    //
    struct ip_mreq mreq;
    memset(&mreq, 0, sizeof(mreq));
    mreq.imr_multiaddr.s_addr = inet_addr(group);
    mreq.imr_interface.s_addr = source_iface ? inet_addr(source_iface) : htonl(INADDR_ANY);
    if (
        setsockopt(
            fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*) &mreq, sizeof(mreq)
        ) < 0
    ){
        perror("setsockopt");
        return 1;
    }

    //
    // now just enter a read-print loop
    //
    while (1) {
        char msgbuf[MSGBUFSIZE];
        unsigned addrlen = sizeof(addr);
        int const nbytes = recvfrom(
            fd,
            msgbuf,
            MSGBUFSIZE,
            0,
            (struct sockaddr *) &addr,
            &addrlen
        );
        if (nbytes < 0) {
            perror("recvfrom");
            return 1;
        }
        msgbuf[nbytes] = '\0';
        printf("from: %s message: %s\n", inet_ntoa(addr.sin_addr), msgbuf);
     }

    return 0;
}

tcpdump looks good for all interfaces:

$ sudo tcpdump -i wlp6s0 -s0 -vv host 239.255.255.251
tcpdump: listening on wlp6s0, link-type EN10MB (Ethernet), capture size 262144 bytes
18:03:40.547226 IP (tos 0x0, ttl 1, id 55512, offset 0, flags [DF], proto UDP (17), 
length 92)
    i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
18:03:41.547602 IP (tos 0x0, ttl 1, id 55691, offset 0, flags [DF], proto UDP (17), 
length 92)
    i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64

$ sudo tcpdump -i enp0s31f6 -s0 -vv host 239.255.255.251
tcpdump: listening on enp0s31f6, link-type EN10MB (Ethernet), capture size 262144 bytes
18:07:42.639153 IP (tos 0x0, ttl 1, id 20849, offset 0, flags [DF], proto UDP (17), 
length 92)
    i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
18:07:43.639911 IP (tos 0x0, ttl 1, id 20997, offset 0, flags [DF], proto UDP (17), 
length 92)
    i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64

$ sudo tcpdump -i enp10s0 -s0 -vv host 239.255.255.251
tcpdump: listening on enp10s0, link-type EN10MB (Ethernet), capture size 262144 bytes
18:08:57.666159 IP (tos 0x0, ttl 1, id 30039, offset 0, flags [DF], proto UDP (17), 
length 92)
    i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
18:08:58.666518 IP (tos 0x0, ttl 1, id 30171, offset 0, flags [DF], proto UDP (17), 
length 92)
    ci7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64

You can work around this by having your listening socket join the multicast group on multiple interfaces.

First, rather than binding to the multicast address, bind to INADDR_ANY:

struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = INADDR_ANY;
addr.sin_port = htons(port);

This will prevent any filtering based on the IP. Then you can accept a comma separated list of interface IPs and join on each one.

char *ptr = strtok(source_iface, ",");
while (ptr) {
    struct ip_mreq mreq;
    memset(&mreq, 0, sizeof(mreq));
    mreq.imr_multiaddr.s_addr = inet_addr(group);
    mreq.imr_interface.s_addr = inet_addr(ptr);

    if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)) < 0) {
        perror("setsockopt");
        return 1;
    }

    ptr=strtok(NULL, ",");
}