Date:   Thu, 8 Apr 2021 17:31:51 +0100
From:   Tom Cook <tom.k.cook@...il.com>
To:     Network Development <netdev@...r.kernel.org>
Subject: Re: bind() and PACKET_MULTICAST

Never mind.  In case anyone was wondering, PACKET_ADD_MEMBERSHIP is a
SOL_PACKET option, not SOL_SOCKET.  Only took me two days to spot!

On Tue, Apr 6, 2021 at 8:13 PM Tom Cook <tom.k.cook@...il.com> wrote:
>
> 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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