Initial multicast library with IGMP query response and periodic advertisements

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

listener.go

@@ -0,0 +1,124 @@
+package multicast
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	"github.com/google/gopacket/pcap"
+)
+
+type JoinLeaveHandler func(sourceIP, groupIP net.IP, join bool)
+
+type Listener struct {
+	iface       *net.Interface
+	handle      *pcap.Handle
+	joinHandler JoinLeaveHandler
+}
+
+func NewListener(iface *net.Interface, joinHandler JoinLeaveHandler) (*Listener, error) {
+	handle, err := pcap.OpenLive(iface.Name, 65536, true, 5*time.Second)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := handle.SetBPFFilter("igmp"); err != nil {
+		handle.Close()
+		return nil, err
+	}
+
+	return &Listener{
+		iface:       iface,
+		handle:      handle,
+		joinHandler: joinHandler,
+	}, nil
+}
+
+func (l *Listener) Run(ctx context.Context) {
+	defer l.handle.Close()
+
+	packetSource := gopacket.NewPacketSource(l.handle, l.handle.LinkType())
+	packets := packetSource.Packets()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case packet, ok := <-packets:
+			if !ok {
+				return
+			}
+			l.handlePacket(packet)
+		}
+	}
+}
+
+func (l *Listener) Close() {
+	l.handle.Close()
+}
+
+func (l *Listener) handlePacket(packet gopacket.Packet) {
+	ipLayer := packet.Layer(layers.LayerTypeIPv4)
+	if ipLayer == nil {
+		return
+	}
+	ip := ipLayer.(*layers.IPv4)
+	sourceIP := ip.SrcIP
+
+	igmpLayer := packet.Layer(layers.LayerTypeIGMP)
+	if igmpLayer == nil {
+		return
+	}
+
+	switch igmp := igmpLayer.(type) {
+	case *layers.IGMPv1or2:
+		l.handleIGMPv1or2(sourceIP, igmp)
+	case *layers.IGMP:
+		l.handleIGMPv3(sourceIP, igmp)
+	}
+}
+
+func (l *Listener) handleIGMPv1or2(sourceIP net.IP, igmp *layers.IGMPv1or2) {
+	switch igmp.Type {
+	case layers.IGMPMembershipReportV1, layers.IGMPMembershipReportV2:
+		groupIP := igmp.GroupAddress
+		if !groupIP.IsMulticast() || groupIP.IsLinkLocalMulticast() {
+			return
+		}
+		if l.joinHandler != nil {
+			l.joinHandler(sourceIP, groupIP, true)
+		}
+
+	case layers.IGMPLeaveGroup:
+		groupIP := igmp.GroupAddress
+		if l.joinHandler != nil {
+			l.joinHandler(sourceIP, groupIP, false)
+		}
+	}
+}
+
+func (l *Listener) handleIGMPv3(sourceIP net.IP, igmp *layers.IGMP) {
+	if igmp.Type != layers.IGMPMembershipReportV3 {
+		return
+	}
+
+	for _, record := range igmp.GroupRecords {
+		groupIP := record.MulticastAddress
+		if !groupIP.IsMulticast() || groupIP.IsLinkLocalMulticast() {
+			continue
+		}
+
+		switch record.Type {
+		case layers.IGMPIsEx, layers.IGMPToEx, layers.IGMPIsIn, layers.IGMPToIn:
+			if l.joinHandler != nil {
+				l.joinHandler(sourceIP, groupIP, true)
+			}
+		case layers.IGMPBlock:
+			if l.joinHandler != nil {
+				l.joinHandler(sourceIP, groupIP, false)
+			}
+		}
+	}
+}