MDNS registry fix for users on VPNs (#1759)

* filter out unsolicited responses
* send to local ip in case
* allow ip func to be passed in. add option for sending to 0.0.0.0

registry/mdns_registry.go

@@ -252,7 +252,7 @@ func (m *mdnsRegistry) Register(service *Service, opts ...RegisterOption) error
 			continue
 		}
 
-		srv, err := mdns.NewServer(&mdns.Config{Zone: s})
+		srv, err := mdns.NewServer(&mdns.Config{Zone: s, LocalhostChecking: true})
 		if err != nil {
 			gerr = err
 			continue

util/mdns/client.go

@@ -34,6 +34,7 @@ type ServiceEntry struct {
 
 // complete is used to check if we have all the info we need
 func (s *ServiceEntry) complete() bool {
+
 	return (len(s.AddrV4) > 0 || len(s.AddrV6) > 0 || len(s.Addr) > 0) && s.Port != 0 && s.hasTXT
 }
 
@@ -347,15 +348,21 @@ func (c *client) query(params *QueryParam) error {
 		select {
 		case resp := <-msgCh:
 			inp := messageToEntry(resp, inprogress)
+
 			if inp == nil {
 				continue
 			}
+			if len(resp.Question) == 0 || resp.Question[0].Name != m.Question[0].Name {
+				// discard anything which we've not asked for
+				continue
+			}
 
 			// Check if this entry is complete
 			if inp.complete() {
 				if inp.sent {
 					continue
 				}
+
 				inp.sent = true
 				select {
 				case params.Entries <- inp:

util/mdns/server.go

@@ -2,13 +2,13 @@ package mdns
 
 import (
 	"fmt"
-	"log"
 	"math/rand"
 	"net"
 	"sync"
 	"sync/atomic"
 	"time"
 
+	log "github.com/micro/go-micro/v2/logger"
 	"github.com/miekg/dns"
 	"golang.org/x/net/ipv4"
 	"golang.org/x/net/ipv6"
@@ -39,6 +39,10 @@ var (
 	}
 )
 
+// GetMachineIP is a func which returns the outbound IP of this machine.
+// Used by the server to determine whether to attempt send the response on a local address
+type GetMachineIP func() net.IP
+
 // Config is used to configure the mDNS server
 type Config struct {
 	// Zone must be provided to support responding to queries
@@ -51,9 +55,15 @@ type Config struct {
 
 	// Port If it is not 0, replace the port 5353 with this port number.
 	Port int
+
+	// GetMachineIP is a function to return the IP of the local machine
+	GetMachineIP GetMachineIP
+	// LocalhostChecking if enabled asks the server to also send responses to 0.0.0.0 if the target IP
+	// is this host (as defined by GetMachineIP). Useful in case machine is on a VPN which blocks comms on non standard ports
+	LocalhostChecking bool
 }
 
-// mDNS server is used to listen for mDNS queries and respond if we
+// Server is an mDNS server used to listen for mDNS queries and respond if we
 // have a matching local record
 type Server struct {
 	config *Config
@@ -65,6 +75,8 @@ type Server struct {
 	shutdownCh   chan struct{}
 	shutdownLock sync.Mutex
 	wg           sync.WaitGroup
+
+	outboundIP net.IP
 }
 
 // NewServer is used to create a new mDNS server from a config
@@ -118,11 +130,17 @@ func NewServer(config *Config) (*Server, error) {
 		}
 	}
 
+	ipFunc := getOutboundIP
+	if config.GetMachineIP != nil {
+		ipFunc = config.GetMachineIP
+	}
+
 	s := &Server{
 		config:     config,
 		ipv4List:   ipv4List,
 		ipv6List:   ipv6List,
 		shutdownCh: make(chan struct{}),
+		outboundIP: ipFunc(),
 	}
 
 	go s.recv(s.ipv4List)
@@ -176,7 +194,7 @@ func (s *Server) recv(c *net.UDPConn) {
 			continue
 		}
 		if err := s.parsePacket(buf[:n], from); err != nil {
-			log.Printf("[ERR] mdns: Failed to handle query: %v", err)
+			log.Errorf("[ERR] mdns: Failed to handle query: %v", err)
 		}
 	}
 }
@@ -185,7 +203,7 @@ func (s *Server) recv(c *net.UDPConn) {
 func (s *Server) parsePacket(packet []byte, from net.Addr) error {
 	var msg dns.Msg
 	if err := msg.Unpack(packet); err != nil {
-		log.Printf("[ERR] mdns: Failed to unpack packet: %v", err)
+		log.Errorf("[ERR] mdns: Failed to unpack packet: %v", err)
 		return err
 	}
 	// TODO: This is a bit of a hack
@@ -278,7 +296,7 @@ func (s *Server) handleQuery(query *dns.Msg, from net.Addr) error {
 			// caveats in the RFC), so set the Compress bit (part of the dns library
 			// API, not part of the DNS packet) to true.
 			Compress: true,
-
+			Question: query.Question,
 			Answer:   answer,
 		}
 	}
@@ -302,7 +320,6 @@ func (s *Server) handleQuery(query *dns.Msg, from net.Addr) error {
 // both.  The return values are DNS records for each transmission type.
 func (s *Server) handleQuestion(q dns.Question) (multicastRecs, unicastRecs []dns.RR) {
 	records := s.config.Zone.Records(q)
-
 	if len(records) == 0 {
 		return nil, nil
 	}
@@ -365,7 +382,7 @@ func (s *Server) probe() {
 
 	for i := 0; i < 3; i++ {
 		if err := s.SendMulticast(q); err != nil {
-			log.Println("[ERR] mdns: failed to send probe:", err.Error())
+			log.Errorf("[ERR] mdns: failed to send probe:", err.Error())
 		}
 		time.Sleep(time.Duration(randomizer.Intn(250)) * time.Millisecond)
 	}
@@ -391,7 +408,7 @@ func (s *Server) probe() {
 	timer := time.NewTimer(timeout)
 	for i := 0; i < 3; i++ {
 		if err := s.SendMulticast(resp); err != nil {
-			log.Println("[ERR] mdns: failed to send announcement:", err.Error())
+			log.Errorf("[ERR] mdns: failed to send announcement:", err.Error())
 		}
 		select {
 		case <-timer.C:
@@ -404,7 +421,7 @@ func (s *Server) probe() {
 	}
 }
 
-// multicastResponse us used to send a multicast response packet
+// SendMulticast us used to send a multicast response packet
 func (s *Server) SendMulticast(msg *dns.Msg) error {
 	buf, err := msg.Pack()
 	if err != nil {
@@ -430,13 +447,23 @@ func (s *Server) sendResponse(resp *dns.Msg, from net.Addr) error {
 
 	// Determine the socket to send from
 	addr := from.(*net.UDPAddr)
-	if addr.IP.To4() != nil {
+	conn := s.ipv4List
-		_, err = s.ipv4List.WriteToUDP(buf, addr)
+	backupTarget := net.IPv4zero
-		return err
+
-	} else {
+	if addr.IP.To4() == nil {
-		_, err = s.ipv6List.WriteToUDP(buf, addr)
+		conn = s.ipv6List
-		return err
+		backupTarget = net.IPv6zero
 	}
+	_, err = conn.WriteToUDP(buf, addr)
+	// If the address we're responding to is this machine then we can also attempt sending on 0.0.0.0
+	// This covers the case where this machine is using a VPN and certain ports are blocked so the response never gets there
+	// Sending two responses is OK
+	if s.config.LocalhostChecking && addr.IP.Equal(s.outboundIP) {
+		// ignore any errors, this is best efforts
+		conn.WriteToUDP(buf, &net.UDPAddr{IP: backupTarget, Port: addr.Port})
+	}
+	return err
+
 }
 
 func (s *Server) unregister() error {
@@ -474,3 +501,17 @@ func setCustomPort(port int) {
 		}
 	}
 }
+
+// getOutboundIP returns the IP address of this machine as seen when dialling out
+func getOutboundIP() net.IP {
+	conn, err := net.Dial("udp", "8.8.8.8:80")
+	if err != nil {
+		// no net connectivity maybe so fallback
+		return nil
+	}
+	defer conn.Close()
+
+	localAddr := conn.LocalAddr().(*net.UDPAddr)
+
+	return localAddr.IP
+}

util/mdns/server_test.go

@@ -7,7 +7,7 @@ import (
 
 func TestServer_StartStop(t *testing.T) {
 	s := makeService(t)
-	serv, err := NewServer(&Config{Zone: s})
+	serv, err := NewServer(&Config{Zone: s, LocalhostChecking: true})
 	if err != nil {
 		t.Fatalf("err: %v", err)
 	}
@@ -15,7 +15,7 @@ func TestServer_StartStop(t *testing.T) {
 }
 
 func TestServer_Lookup(t *testing.T) {
-	serv, err := NewServer(&Config{Zone: makeServiceWithServiceName(t, "_foobar._tcp")})
+	serv, err := NewServer(&Config{Zone: makeServiceWithServiceName(t, "_foobar._tcp"), LocalhostChecking: true})
 	if err != nil {
 		t.Fatalf("err: %v", err)
 	}