How to read multicast datagrams but only from specific interface?

Getting Help
1

I’m trying to read multicast packets directed to 224.0.0.9:520, but only when received from interface eth2 (its IP address is 1.0.0.2/24).

My sample code is based on docs for golang.org/x/net/ipv4.

Roughly steps are:

  1. Find interface eth2 with: net.InterfaceByName(“eth2”)
  2. Bind socket to 1.0.0.2:520 with: net.ListenPacket(“udp”, “1.0.0.2:520”)
  3. Join multicast group 224.0.0.9: JoinGroup(ifi, 224.0.0.9)
  4. Read data with: ReadFrom(buf)

Full code is available here: http://play.golang.org/p/YOwu38OQdw

In order to test, I send UDP packets from the attached host (1.0.0.1/24):
$ echo x | nc -s 1.0.0.1 -u 224.0.0.9 520

Then I verify those packets are actually reaching the destination host interface:
$ sudo tcpdump -n -i eth2
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth2, link-type EN10MB (Ethernet), capture size 262144 bytes
12:08:34.745171 IP 1.0.0.1.51920 > 224.0.0.9.520: [|rip]

However my Go code running on host 1.0.0.2 is unable to read those packets:
$ sudo go run rip_mcast_listener.go
[sudo] password for lab:
2016/02/10 12:12:29 interfaceAdd: join error for ‘fe80::a00:27ff:fe52:9575’ on ‘eth2’: join: udp/[fe80::a00:27ff:fe52:9575]:520 listen error: listen udp [fe80::a00:27ff:fe52:9575]:520: bind: invalid argument
2016/02/10 12:12:29 main: waiting forever
2016/02/10 12:12:29 udpReader: reading from ‘1.0.0.2’ on ‘eth2’

I have also posted this question on stackoverflow: http://stackoverflow.com/questions/35300039/in-golang-how-to-receive-multicast-packets-with-socket-bound-to-specific-addres

go version is: go version go1.6rc2 linux/386

Can anyone please spot where the error is?

2

I see that you try to listen on a ipv6 address.

3

Yes, by scanning over all interface addresses, the code did hit an IPv6 address. That’s not harmful. Problem is, it seemingly succeeded in binding the IPv4 address plus UDP port (1.0.0.2:520) yet failed to read multicast datagrams. :frowning:

4

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