Support for listening to multicast group

[sgtsquiggs]

Feature Request

I believe this feature could be added easily to the exisitng socket_listener input plugin. I have it working locally but I'm not sure how to cleanly integrate. For my requirements I only need the very simple multicast implementation (via net.ListenMulticastUDP) so there isn't much change.

Proposal:

Adding multicast:// multicast4:// multicast6:// network type handling to service_address config option:

case "udp", "udp4", "udp6", "ip", "ip4", "ip6", "unixgram", "multicast", "multicast4", "multicast6":
	var (
		err error
		pc net.PacketConn
	)

	switch spl[0] {
	case "multicast", "multicast4", "multicast6":
		var addr *net.UDPAddr
		spl[0] = strings.Replace(spl[0], "multicast", "udp", 1)
		if addr, err = net.ResolveUDPAddr(spl[0], spl[1]); err == nil {
			pc, err = net.ListenMulticastUDP(spl[0], nil, addr)
		}
	default:
		pc, err = net.ListenPacket(spl[0], spl[1])
	}
	if err != nil {
		return err
	}

	if sl.ReadBufferSize.Size > 0 {
		if srb, ok := pc.(setReadBufferer); ok {
			srb.SetReadBuffer(int(sl.ReadBufferSize.Size))
		} else {
			log.Printf("W! Unable to set read buffer on a %s socket", spl[0])
		}
	}

	psl := &packetSocketListener{
		PacketConn:     pc,
		SocketListener: sl,
	}

	sl.Closer = psl
	go psl.listen()

Current behavior:

Does not support multicast

Desired behavior:

Support multicast

Use case: [Why is this important (helps with prioritizing requests)]

I'm monitoring alerts sent from one application to other related applications via multicast group

[danielnelson]

I'm new to working with multicast, but it seems that a more advanced configuration would allow listening to a specific address on an interface and potentially with multiple multicast groups? We could potentially expose these as settings, or it might just be overkill.

@phemmer Any thoughts on this one?

[phemmer]

In principal I don't think there's anything wrong with the idea.
However in regards to the exposed interface, I don't think I'd do this by adding multicast:// as a protocol. The multicast address space is well defined. If we added it as a separate protocol, then udp://224.0.0.0 would be invalid configuration, just as multicast://1.0.0.1. I think it'd be better to support udp://224.0.0.1, recognize this as multicast, and do the appropriate setup.

In regards to listening on a specific interface, we can add support for udp://1.2.3.4%ifname. This would be usable on both unicast and multicast addresses. The syntax %ifname is stolen from the official IPv6 address specification format, so would be broadly compatible.

[sgtsquiggs]

With @phemmer's input:

	case "udp", "udp4", "udp6", "ip", "ip4", "ip6", "unixgram":
		pc, err := udpListen(spl[0], spl[1])
		if err != nil {
			return err
		}

		if sl.ReadBufferSize.Size > 0 {
			if srb, ok := pc.(setReadBufferer); ok {
				srb.SetReadBuffer(int(sl.ReadBufferSize.Size))
			} else {
				log.Printf("W! Unable to set read buffer on a %s socket", spl[0])
			}
		}

		psl := &packetSocketListener{
			PacketConn:     pc,
			SocketListener: sl,
		}

		sl.Closer = psl
		go psl.listen()

and

func udpListen(network string, address string) (net.PacketConn, error) {
	switch network {
	case "udp", "udp4", "udp6":
		var addr *net.UDPAddr
		var err error
		var ifi *net.Interface
		if spl := strings.SplitN(address, "%", 2); len(spl) == 2 {
			address = spl[0]
			ifi, err = net.InterfaceByName(spl[1])
			if err != nil {
				return nil, err
			}
		}
		addr, err = net.ResolveUDPAddr(network, address)
		if err == nil {
			if addr.IP.IsMulticast() {
				return net.ListenMulticastUDP(network, ifi, addr)
			}
		}
	}
	return net.ListenPacket(network, address)
}

[danielnelson]

Looks good, can you create a pull request?