Support multiple multicast group joins per connection

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

multicast.go

@@ -2,8 +2,10 @@ package multicast
 
 import (
 	"context"
+	"fmt"
 	"math/rand"
 	"net"
+	"sync"
 	"time"
 
 	"github.com/google/gopacket"
@@ -16,12 +18,13 @@ type Conn struct {
 	*ipv4.PacketConn
 	rawConn   net.PacketConn
 	iface     *net.Interface
-	groupIP   net.IP
+	groups    []net.IP
 	srcIP     net.IP
 	srcMAC    net.HardwareAddr
 	queryChan chan struct{}
 	ctx       context.Context
 	cancel    context.CancelFunc
+	mu        sync.Mutex
 }
 
 func ListenMulticastUDP(network string, iface *net.Interface, gaddr *net.UDPAddr) (*Conn, error) {
@@ -44,13 +47,18 @@ func ListenMulticastUDP(network string, iface *net.Interface, gaddr *net.UDPAddr
 
 	ctx, cancel := context.WithCancel(context.Background())
 
+	var srcMAC net.HardwareAddr
+	if iface != nil {
+		srcMAC = iface.HardwareAddr
+	}
+
 	conn := &Conn{
 		PacketConn: p,
 		rawConn:    c,
 		iface:      iface,
-		groupIP:    gaddr.IP,
+		groups:     []net.IP{gaddr.IP},
 		srcIP:      srcIP,
-		srcMAC:     iface.HardwareAddr,
+		srcMAC:     srcMAC,
 		queryChan:  make(chan struct{}, 1),
 		ctx:        ctx,
 		cancel:     cancel,
@@ -62,9 +70,62 @@ func ListenMulticastUDP(network string, iface *net.Interface, gaddr *net.UDPAddr
 	return conn, nil
 }
 
+func ListenMulticastUDPPort(network string, iface *net.Interface, port int) (*Conn, error) {
+	srcIP, _ := getInterfaceIPv4(iface)
+
+	c, err := net.ListenPacket(network, fmt.Sprintf(":%d", port))
+	if err != nil {
+		return nil, err
+	}
+
+	p := ipv4.NewPacketConn(c)
+	if iface != nil {
+		p.SetMulticastInterface(iface)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	var srcMAC net.HardwareAddr
+	if iface != nil {
+		srcMAC = iface.HardwareAddr
+	}
+
+	conn := &Conn{
+		PacketConn: p,
+		rawConn:    c,
+		iface:      iface,
+		groups:     nil,
+		srcIP:      srcIP,
+		srcMAC:     srcMAC,
+		queryChan:  make(chan struct{}, 1),
+		ctx:        ctx,
+		cancel:     cancel,
+	}
+
+	go conn.runAdvertiser()
+	go conn.listenForQueries()
+
+	return conn, nil
+}
+
+func (c *Conn) JoinGroup(gaddr *net.UDPAddr) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if err := c.PacketConn.JoinGroup(c.iface, gaddr); err != nil {
+		return err
+	}
+	c.groups = append(c.groups, gaddr.IP)
+	return nil
+}
+
 func (c *Conn) Close() error {
 	c.cancel()
-	c.PacketConn.LeaveGroup(c.iface, &net.UDPAddr{IP: c.groupIP})
+	c.mu.Lock()
+	for _, groupIP := range c.groups {
+		c.PacketConn.LeaveGroup(c.iface, &net.UDPAddr{IP: groupIP})
+	}
+	c.mu.Unlock()
 	return c.rawConn.Close()
 }
 
@@ -76,7 +137,7 @@ func (c *Conn) runAdvertiser() {
 	ticker := time.NewTicker(60 * time.Second)
 	defer ticker.Stop()
 
-	c.sendReport()
+	c.sendReports()
 
 	for {
 		select {
@@ -85,13 +146,23 @@ func (c *Conn) runAdvertiser() {
 		case <-c.queryChan:
 			delay := time.Duration(rand.Intn(1000)) * time.Millisecond
 			time.Sleep(delay)
-			c.sendReport()
+			c.sendReports()
 		case <-ticker.C:
-			c.sendReport()
+			c.sendReports()
 		}
 	}
 }
 
+func (c *Conn) sendReports() {
+	c.mu.Lock()
+	groups := append([]net.IP{}, c.groups...)
+	c.mu.Unlock()
+
+	for _, groupIP := range groups {
+		c.sendReport(groupIP)
+	}
+}
+
 func (c *Conn) listenForQueries() {
 	handle, err := pcap.OpenLive(c.iface.Name, 65536, true, 5*time.Second)
 	if err != nil {
@@ -131,7 +202,16 @@ func (c *Conn) isQuery(packet gopacket.Packet) bool {
 	switch igmp := igmpLayer.(type) {
 	case *layers.IGMPv1or2:
 		if igmp.Type == layers.IGMPMembershipQuery {
-			return igmp.GroupAddress.IsUnspecified() || igmp.GroupAddress.Equal(c.groupIP)
+			if igmp.GroupAddress.IsUnspecified() {
+				return true
+			}
+			c.mu.Lock()
+			defer c.mu.Unlock()
+			for _, groupIP := range c.groups {
+				if igmp.GroupAddress.Equal(groupIP) {
+					return true
+				}
+			}
 		}
 	case *layers.IGMP:
 		if igmp.Type == layers.IGMPMembershipQuery {
@@ -141,8 +221,8 @@ func (c *Conn) isQuery(packet gopacket.Packet) bool {
 	return false
 }
 
-func (c *Conn) sendReport() {
+func (c *Conn) sendReport(groupIP net.IP) {
-	if c.srcIP == nil {
+	if c.srcIP == nil || c.iface == nil {
 		return
 	}
 
@@ -154,7 +234,7 @@ func (c *Conn) sendReport() {
 
 	eth := &layers.Ethernet{
 		SrcMAC:       c.srcMAC,
-		DstMAC:       multicastIPToMAC(c.groupIP),
+		DstMAC:       multicastIPToMAC(groupIP),
 		EthernetType: layers.EthernetTypeIPv4,
 	}
 
@@ -164,13 +244,13 @@ func (c *Conn) sendReport() {
 		TTL:      1,
 		Protocol: layers.IPProtocolIGMP,
 		SrcIP:    c.srcIP,
-		DstIP:    c.groupIP,
+		DstIP:    groupIP,
 		Options:  []layers.IPv4Option{{OptionType: 148, OptionLength: 4, OptionData: []byte{0, 0}}},
 	}
 
 	buf := gopacket.NewSerializeBuffer()
 	opts := gopacket.SerializeOptions{ComputeChecksums: true, FixLengths: true}
-	gopacket.SerializeLayers(buf, opts, eth, ip, gopacket.Payload(buildIGMPv2Report(c.groupIP)))
+	gopacket.SerializeLayers(buf, opts, eth, ip, gopacket.Payload(buildIGMPv2Report(groupIP)))
 	handle.WritePacketData(buf.Bytes())
 }