Initial multicast library with IGMP query response and periodic advertisements

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

querier.go

@@ -0,0 +1,89 @@
+package multicast
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	"github.com/google/gopacket/pcap"
+)
+
+type Querier struct {
+	iface  *net.Interface
+	srcIP  net.IP
+	srcMAC net.HardwareAddr
+}
+
+func NewQuerier(iface *net.Interface) (*Querier, error) {
+	srcIP, err := getInterfaceIPv4(iface)
+	if err != nil {
+		return nil, err
+	}
+	if srcIP == nil {
+		return nil, nil
+	}
+
+	return &Querier{
+		iface:  iface,
+		srcIP:  srcIP,
+		srcMAC: iface.HardwareAddr,
+	}, nil
+}
+
+func (q *Querier) Run(ctx context.Context) {
+	if q == nil {
+		return
+	}
+
+	ticker := time.NewTicker(60 * time.Second)
+	defer ticker.Stop()
+
+	q.SendQuery()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			q.SendQuery()
+		}
+	}
+}
+
+func (q *Querier) SendQuery() {
+	handle, err := pcap.OpenLive(q.iface.Name, 65536, true, pcap.BlockForever)
+	if err != nil {
+		return
+	}
+	defer handle.Close()
+
+	eth := &layers.Ethernet{
+		SrcMAC:       q.srcMAC,
+		DstMAC:       net.HardwareAddr{0x01, 0x00, 0x5e, 0x00, 0x00, 0x01},
+		EthernetType: layers.EthernetTypeIPv4,
+	}
+
+	ip := &layers.IPv4{
+		Version:  4,
+		IHL:      6,
+		TTL:      1,
+		Protocol: layers.IPProtocolIGMP,
+		SrcIP:    q.srcIP,
+		DstIP:    net.IPv4(224, 0, 0, 1),
+		Options:  []layers.IPv4Option{{OptionType: 148, OptionLength: 4, OptionData: []byte{0, 0}}},
+	}
+
+	igmpPayload := buildIGMPQuery()
+
+	buf := gopacket.NewSerializeBuffer()
+	opts := gopacket.SerializeOptions{ComputeChecksums: true, FixLengths: true}
+
+	if err := gopacket.SerializeLayers(buf, opts, eth, ip, gopacket.Payload(igmpPayload)); err != nil {
+		return
+	}
+
+	handle.WritePacketData(buf.Bytes())
+}
+