IPv6 multicast on Windows (fixes #1817)

lib/beacon/beacon.go

@@ -18,7 +18,11 @@ type Interface interface {
 	Recv() ([]byte, net.Addr)
 }
 
-func genericReader(conn *net.UDPConn, outbox chan<- recv) {
+type readerFrom interface {
+	ReadFrom([]byte) (int, net.Addr, error)
+}
+
+func genericReader(conn readerFrom, outbox chan<- recv) {
 	bs := make([]byte, 65536)
 	for {
 		n, addr, err := conn.ReadFrom(bs)

lib/beacon/multicast.go

@@ -6,38 +6,65 @@
 
 package beacon
 
-import "net"
+import (
+	"errors"
+	"fmt"
+	"net"
+
+	"golang.org/x/net/ipv6"
+)
 
 type Multicast struct {
-	conn   *net.UDPConn
+	conn   *ipv6.PacketConn
 	addr   *net.UDPAddr
-	intf   *net.Interface
 	inbox  chan []byte
 	outbox chan recv
+	intfs  []net.Interface
 }
 
-func NewMulticast(addr, ifname string) (*Multicast, error) {
+func NewMulticast(addr string) (*Multicast, error) {
 	gaddr, err := net.ResolveUDPAddr("udp6", addr)
 	if err != nil {
 		return nil, err
 	}
-	intf, err := net.InterfaceByName(ifname)
+
+	conn, err := net.ListenPacket("udp6", fmt.Sprintf("[::]:%d", gaddr.Port))
 	if err != nil {
 		return nil, err
 	}
-	conn, err := net.ListenMulticastUDP("udp6", intf, gaddr)
-	if err != nil {
-		return nil, err
-	}
-	b := &Multicast{
-		conn:   conn,
-		addr:   gaddr,
-		intf:   intf,
-		inbox:  make(chan []byte),
-		outbox: make(chan recv, 16),
-	}
 
-	go genericReader(b.conn, b.outbox)
+	intfs, err := net.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+
+	p := ipv6.NewPacketConn(conn)
+	joined := 0
+	for _, intf := range intfs {
+		err := p.JoinGroup(&intf, &net.UDPAddr{IP: gaddr.IP})
+		if debug {
+			if err != nil {
+				l.Debugln("IPv6 join", intf.Name, "failed:", err)
+			} else {
+				l.Debugln("IPv6 join", intf.Name, "success")
+			}
+		}
+		joined++
+	}
+
+	if joined == 0 {
+		return nil, errors.New("no multicast interfaces available")
+	}
+
+	b := &Multicast{
+		conn:   p,
+		addr:   gaddr,
+		inbox:  make(chan []byte),
+		outbox: make(chan recv, 16),
+		intfs:  intfs,
+	}
+
+	go genericReader(ipv6ReaderAdapter{b.conn}, b.outbox)
 	go b.writer()
 
 	return b, nil
@@ -53,14 +80,30 @@ func (b *Multicast) Recv() ([]byte, net.Addr) {
 }
 
 func (b *Multicast) writer() {
-	addr := *b.addr
-	addr.Zone = b.intf.Name
+	wcm := &ipv6.ControlMessage{
+		HopLimit: 1,
+	}
+
 	for bs := range b.inbox {
-		_, err := b.conn.WriteTo(bs, &addr)
-		if err != nil && debug {
-			l.Debugln(err, "on write to", addr)
-		} else if debug {
-			l.Debugf("sent %d bytes to %s", len(bs), addr.String())
+		for _, intf := range b.intfs {
+			wcm.IfIndex = intf.Index
+			_, err := b.conn.WriteTo(bs, wcm, b.addr)
+			if err != nil && debug {
+				l.Debugln(err, "on write to", b.addr)
+			} else if debug {
+				l.Debugf("sent %d bytes to %v on %s", len(bs), b.addr, intf.Name)
+			}
 		}
 	}
 }
+
+// This makes ReadFrom on an *ipv6.PacketConn behave like ReadFrom on a
+// net.PacketConn.
+type ipv6ReaderAdapter struct {
+	c *ipv6.PacketConn
+}
+
+func (i ipv6ReaderAdapter) ReadFrom(bs []byte) (int, net.Addr, error) {
+	n, _, src, err := i.c.ReadFrom(bs)
+	return n, src, err
+}

lib/discover/discover.go

@@ -14,7 +14,6 @@ import (
 	"io"
 	"net"
 	"net/url"
-	"runtime"
 	"sort"
 	"time"
 
@@ -101,38 +100,13 @@ func (d *Discoverer) startLocalIPv4Broadcasts(localPort int) {
 }
 
 func (d *Discoverer) startLocalIPv6Multicasts(localMCAddr string) {
-	intfs, err := net.Interfaces()
+	mb, err := beacon.NewMulticast(localMCAddr)
 	if err != nil {
-		if debug {
-			l.Debugln("discover: interfaces:", err)
-		}
 		l.Infoln("Local discovery over IPv6 unavailable")
 		return
 	}
-
-	v6Intfs := 0
-	for _, intf := range intfs {
-		// Interface flags seem to always be 0 on Windows
-		if runtime.GOOS != "windows" && (intf.Flags&net.FlagUp == 0 || intf.Flags&net.FlagMulticast == 0) {
-			continue
-		}
-
-		mb, err := beacon.NewMulticast(localMCAddr, intf.Name)
-		if err != nil {
-			if debug {
-				l.Debugln("discover: Start local v6:", err)
-			}
-			continue
-		}
-
-		d.beacons = append(d.beacons, mb)
-		go d.recvAnnouncements(mb)
-		v6Intfs++
-	}
-
-	if v6Intfs == 0 {
-		l.Infoln("Local discovery over IPv6 unavailable")
-	}
+	d.beacons = append(d.beacons, mb)
+	go d.recvAnnouncements(mb)
 }
 
 func (d *Discoverer) StartGlobal(servers []string, extPort uint16) {