minor tweaks for go-micro mdns registry

* allow to receive multicast on the same host
  via multicast loopback flag
* export SendMulticast function, needed for async
  notification on registry updates

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

client.go

@@ -191,6 +191,13 @@ func newClient() (*client, error) {
 		return nil, fmt.Errorf("failed to bind to any unicast udp port")
 	}
 
+	if uconn4 == nil {
+		uconn4 = &net.UDPConn{}
+	}
+	if uconn6 == nil {
+		uconn6 = &net.UDPConn{}
+	}
+
 	mconn4, err := net.ListenUDP("udp4", mdnsWildcardAddrIPv4)
 	if err != nil {
 		log.Printf("[ERR] mdns: Failed to bind to udp4 port: %v", err)
@@ -207,6 +214,9 @@ func newClient() (*client, error) {
 	p1 := ipv4.NewPacketConn(mconn4)
 	p2 := ipv6.NewPacketConn(mconn6)
 
+	p1.SetMulticastLoopback(true)
+	p2.SetMulticastLoopback(true)
+
 	ifaces, err := net.Interfaces()
 	if err != nil {
 		return nil, err

server.go

@@ -14,7 +14,7 @@ import (
 )
 
 var (
-	mdnsGroupIPv4 = net.IPv4(224, 0, 0, 251)
+	mdnsGroupIPv4 = net.ParseIP("224.0.0.251")
 	mdnsGroupIPv6 = net.ParseIP("ff02::fb")
 
 	// mDNS wildcard addresses
@@ -73,6 +73,13 @@ func NewServer(config *Config) (*Server, error) {
 		return nil, fmt.Errorf("[ERR] mdns: Failed to bind to any udp port!")
 	}
 
+	if ipv4List == nil {
+		ipv4List = &net.UDPConn{}
+	}
+	if ipv6List == nil {
+		ipv6List = &net.UDPConn{}
+	}
+
 	// Join multicast groups to receive announcements
 	p1 := ipv4.NewPacketConn(ipv4List)
 	p2 := ipv6.NewPacketConn(ipv6List)
@@ -112,13 +119,8 @@ func NewServer(config *Config) (*Server, error) {
 		shutdownCh: make(chan struct{}),
 	}
 
-	if ipv4List != nil {
-		go s.recv(s.ipv4List)
-	}
-
-	if ipv6List != nil {
-		go s.recv(s.ipv6List)
-	}
+	go s.recv(s.ipv4List)
+	go s.recv(s.ipv6List)
 
 	s.wg.Add(1)
 	go s.probe()
@@ -180,6 +182,10 @@ func (s *Server) parsePacket(packet []byte, from net.Addr) error {
 		log.Printf("[ERR] mdns: Failed to unpack packet: %v", err)
 		return err
 	}
+	// TODO: This is a bit of a hack
+	// We decided to ignore some mDNS answers for the time being
+	// See: https://tools.ietf.org/html/rfc6762#section-7.2
+	msg.Truncated = false
 	return s.handleQuery(&msg, from)
 }
 
@@ -352,7 +358,7 @@ func (s *Server) probe() {
 	randomizer := rand.New(rand.NewSource(time.Now().UnixNano()))
 
 	for i := 0; i < 3; i++ {
-		if err := s.multicastResponse(q); err != nil {
+		if err := s.SendMulticast(q); err != nil {
 			log.Println("[ERR] mdns: failed to send probe:", err.Error())
 		}
 		time.Sleep(time.Duration(randomizer.Intn(250)) * time.Millisecond)
@@ -378,7 +384,7 @@ func (s *Server) probe() {
 	timeout := 1 * time.Second
 	timer := time.NewTimer(timeout)
 	for i := 0; i < 3; i++ {
-		if err := s.multicastResponse(resp); err != nil {
+		if err := s.SendMulticast(resp); err != nil {
 			log.Println("[ERR] mdns: failed to send announcement:", err.Error())
 		}
 		select {
@@ -393,7 +399,7 @@ func (s *Server) probe() {
 }
 
 // multicastResponse us used to send a multicast response packet
-func (s *Server) multicastResponse(msg *dns.Msg) error {
+func (s *Server) SendMulticast(msg *dns.Msg) error {
 	buf, err := msg.Pack()
 	if err != nil {
 		return err
@@ -443,5 +449,5 @@ func (s *Server) unregister() error {
 	resp.MsgHdr.Response = true
 	resp.Answer = append(resp.Answer, s.config.Zone.Records(q.Question[0])...)
 
-	return s.multicastResponse(resp)
+	return s.SendMulticast(resp)
 }