Date:   Tue, 6 Apr 2021 20:13:44 +0100
From:   Tom Cook <tom.k.cook@...il.com>
To:     Network Development <netdev@...r.kernel.org>
Subject: bind() and PACKET_MULTICAST

Can someone please suggest why the code below doesn't do as expected?
I expect it to bind an AF_PACKET socket to an interface and receive
packets with ethertype 0x5eeb that arrive at multicast MAC address
77:68:76:68:76:69 on that interface.  In practice, nothing arrives.

If I comment out the call to bind(), it receives packets with
ethertype 0x5eeb that are addressed to 77:68:76:68:76:69 and are
received on any interface on the system, not just eth0.  (There are no
packets with ethertype 0x5eeb sent to any other address, so this may
be coincidence.)

If I change either use of ether_type to be ETH_P_ALL instead (and
re-instate the bind() call), then it receives all ethernet frames
received on eth0.

Is this a bug?  Or is it as expected and I have to use some other
mechanism (BPF?) to filter the frames?

Thanks for any assistance,
Tom

Code:

#include <arpa/inet.h>
#include <linux/if_packet.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <net/ethernet.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

const unsigned short eth_type = 0x5eeb;

int main() {
    int fd = socket(AF_PACKET, SOCK_RAW, htons(eth_type));
    if (fd < 0) {
        perror("socket");
        exit(1);
    }

    struct ifreq ifr;
    const char * if_name = "eth0";
    size_t if_name_len = strlen (if_name);
    memcpy(ifr.ifr_name, if_name, if_name_len);
    ioctl(fd, SIOCGIFINDEX, &ifr);
    printf("Interface has index %d\n", ifr.ifr_ifindex);

    struct sockaddr_ll addr = {0};
    addr.sll_family = AF_PACKET;
    addr.sll_ifindex = ifr.ifr_ifindex;
    addr.sll_protocol = htons(eth_type);
    addr.sll_pkttype = PACKET_MULTICAST;
    if (bind(fd, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
        perror("bind");
        exit(1);
    }

    unsigned char mcast[ETH_ALEN] = {0x77, 0x68, 0x76, 0x68, 0x76, 0x69};
    struct packet_mreq mreq = {0};
    mreq.mr_ifindex = ifr.ifr_ifindex;
    mreq.mr_type = PACKET_MR_MULTICAST;
    memcpy(mreq.mr_address, mcast, ETH_ALEN);
    mreq.mr_alen = ETH_ALEN;
    if(setsockopt(fd, SOL_SOCKET, PACKET_ADD_MEMBERSHIP, &mreq,
sizeof(mreq)) < 0) {
        perror("setsockopt");
        exit(1);
    }

    char buf [2048];
    struct sockaddr_ll src_addr;
    socklen_t src_addr_len = sizeof(src_addr);
    ssize_t count = recvfrom(fd, buf, sizeof(buf), 0, (struct
sockaddr*)&src_addr, &src_addr_len);
    if (count == -1) {
        perror("recvfrom");
        exit(1);
    } else {
        printf("Received frame.\n");
        printf("Dest MAC: ");
        for (int ii = 0; ii < 5; ii++) {
            printf("%02hhx:", buf[ii]);
        }
        printf("%02hhx\n", buf[5]);
        printf("Src MAC: ");
        for (int ii = 6; ii < 11; ii++) {
            printf("%02hhx:", buf[ii]);
        }
        printf("%02hhx\n", buf[11]);
    }
}

And here is a short Python3 programme to generate such frames (install
pyroute2 package and run as `sudo python3 test.py eth0`):

import socket
from pyroute2 import IPDB
import sys
import struct
import binascii
import time

ip = IPDB()

SMAC=bytes.fromhex(ip.interfaces[sys.argv[1]]['address'].replace(':', ''))
DMAC=bytes.fromhex('776876687669')

s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW)
s.bind((sys.argv[1], 0x5eeb))
#s.bind((sys.argv[1], 0))

dgram = struct.pack("!6s6sHH", DMAC, SMAC, 0x5eeb, 0x7668)
print(' '.join('{:02x}'.format(x) for x in dgram))

while True:
    s.send(dgram)
    time.sleep(0.1)

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