package mdns import ( "context" "fmt" "math/rand" "net" "sync" "sync/atomic" "time" "github.com/miekg/dns" "github.com/unistack-org/micro/v3/logger" "golang.org/x/net/ipv4" "golang.org/x/net/ipv6" ) var ( mdnsGroupIPv4 = net.ParseIP("224.0.0.251") mdnsGroupIPv6 = net.ParseIP("ff02::fb") // mDNS wildcard addresses mdnsWildcardAddrIPv4 = &net.UDPAddr{ IP: net.ParseIP("224.0.0.0"), Port: 5353, } mdnsWildcardAddrIPv6 = &net.UDPAddr{ IP: net.ParseIP("ff02::"), Port: 5353, } // mDNS endpoint addresses ipv4Addr = &net.UDPAddr{ IP: mdnsGroupIPv4, Port: 5353, } ipv6Addr = &net.UDPAddr{ IP: mdnsGroupIPv6, Port: 5353, } ) // 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 Zone Zone // Iface if provided binds the multicast listener to the given // interface. If not provided, the system default multicase interface // is used. Iface *net.Interface // 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 Context context.Context } // 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 ipv4List *net.UDPConn ipv6List *net.UDPConn shutdown bool shutdownCh chan struct{} shutdownLock sync.Mutex wg sync.WaitGroup outboundIP net.IP } // NewServer is used to create a new mDNS server from a config func NewServer(config *Config) (*Server, error) { setCustomPort(config.Port) // Create the listeners // Create wildcard connections (because :5353 can be already taken by other apps) ipv4List, _ := net.ListenUDP("udp4", mdnsWildcardAddrIPv4) ipv6List, _ := net.ListenUDP("udp6", mdnsWildcardAddrIPv6) if ipv4List == nil && ipv6List == nil { 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) p1.SetMulticastLoopback(true) p2.SetMulticastLoopback(true) if config.Iface != nil { if err := p1.JoinGroup(config.Iface, &net.UDPAddr{IP: mdnsGroupIPv4}); err != nil { return nil, err } if err := p2.JoinGroup(config.Iface, &net.UDPAddr{IP: mdnsGroupIPv6}); err != nil { return nil, err } } else { ifaces, err := net.Interfaces() if err != nil { return nil, err } errCount1, errCount2 := 0, 0 for _, iface := range ifaces { if err := p1.JoinGroup(&iface, &net.UDPAddr{IP: mdnsGroupIPv4}); err != nil { errCount1++ } if err := p2.JoinGroup(&iface, &net.UDPAddr{IP: mdnsGroupIPv6}); err != nil { errCount2++ } } if len(ifaces) == errCount1 && len(ifaces) == errCount2 { return nil, fmt.Errorf("Failed to join multicast group on all interfaces!") } } ipFunc := getOutboundIP if config.GetMachineIP != nil { ipFunc = config.GetMachineIP } s := &Server{ config: config, ipv4List: ipv4List, ipv6List: ipv6List, shutdownCh: make(chan struct{}), outboundIP: ipFunc(), } if s.config.Context == nil { s.config.Context = context.Background() } go s.recv(s.ipv4List) go s.recv(s.ipv6List) s.wg.Add(1) go s.probe() return s, nil } // Shutdown is used to shutdown the listener func (s *Server) Shutdown() error { s.shutdownLock.Lock() defer s.shutdownLock.Unlock() if s.shutdown { return nil } s.shutdown = true close(s.shutdownCh) if err := s.unregister(); err != nil { return err } if s.ipv4List != nil { s.ipv4List.Close() } if s.ipv6List != nil { s.ipv6List.Close() } s.wg.Wait() return nil } // recv is a long running routine to receive packets from an interface func (s *Server) recv(c *net.UDPConn) { if c == nil { return } buf := make([]byte, 65536) for { s.shutdownLock.Lock() if s.shutdown { s.shutdownLock.Unlock() return } s.shutdownLock.Unlock() n, from, err := c.ReadFrom(buf) if err != nil { continue } if err := s.parsePacket(buf[:n], from); err != nil { logger.Errorf(s.config.Context, "[ERR] mdns: Failed to handle query: %v", err) } } } // parsePacket is used to parse an incoming packet func (s *Server) parsePacket(packet []byte, from net.Addr) error { var msg dns.Msg if err := msg.Unpack(packet); err != nil { logger.Errorf(s.config.Context, "[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) } // handleQuery is used to handle an incoming query func (s *Server) handleQuery(query *dns.Msg, from net.Addr) error { if query.Opcode != dns.OpcodeQuery { // "In both multicast query and multicast response messages, the OPCODE MUST // be zero on transmission (only standard queries are currently supported // over multicast). Multicast DNS messages received with an OPCODE other // than zero MUST be silently ignored." Note: OpcodeQuery == 0 return fmt.Errorf("mdns: received query with non-zero Opcode %v: %v", query.Opcode, *query) } if query.Rcode != 0 { // "In both multicast query and multicast response messages, the Response // Code MUST be zero on transmission. Multicast DNS messages received with // non-zero Response Codes MUST be silently ignored." return fmt.Errorf("mdns: received query with non-zero Rcode %v: %v", query.Rcode, *query) } // TODO(reddaly): Handle "TC (Truncated) Bit": // In query messages, if the TC bit is set, it means that additional // Known-Answer records may be following shortly. A responder SHOULD // record this fact, and wait for those additional Known-Answer records, // before deciding whether to respond. If the TC bit is clear, it means // that the querying host has no additional Known Answers. if query.Truncated { return fmt.Errorf("[ERR] mdns: support for DNS requests with high truncated bit not implemented: %v", *query) } var unicastAnswer, multicastAnswer []dns.RR // Handle each question for _, q := range query.Question { mrecs, urecs := s.handleQuestion(q) multicastAnswer = append(multicastAnswer, mrecs...) unicastAnswer = append(unicastAnswer, urecs...) } // See section 18 of RFC 6762 for rules about DNS headers. resp := func(unicast bool) *dns.Msg { // 18.1: ID (Query Identifier) // 0 for multicast response, query.Id for unicast response id := uint16(0) if unicast { id = query.Id } var answer []dns.RR if unicast { answer = unicastAnswer } else { answer = multicastAnswer } if len(answer) == 0 { return nil } return &dns.Msg{ MsgHdr: dns.MsgHdr{ Id: id, // 18.2: QR (Query/Response) Bit - must be set to 1 in response. Response: true, // 18.3: OPCODE - must be zero in response (OpcodeQuery == 0) Opcode: dns.OpcodeQuery, // 18.4: AA (Authoritative Answer) Bit - must be set to 1 Authoritative: true, // The following fields must all be set to 0: // 18.5: TC (TRUNCATED) Bit // 18.6: RD (Recursion Desired) Bit // 18.7: RA (Recursion Available) Bit // 18.8: Z (Zero) Bit // 18.9: AD (Authentic Data) Bit // 18.10: CD (Checking Disabled) Bit // 18.11: RCODE (Response Code) }, // 18.12 pertains to questions (handled by handleQuestion) // 18.13 pertains to resource records (handled by handleQuestion) // 18.14: Name Compression - responses should be compressed (though see // 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, } } if mresp := resp(false); mresp != nil { if err := s.sendResponse(mresp, from); err != nil { return fmt.Errorf("mdns: error sending multicast response: %v", err) } } if uresp := resp(true); uresp != nil { if err := s.sendResponse(uresp, from); err != nil { return fmt.Errorf("mdns: error sending unicast response: %v", err) } } return nil } // handleQuestion is used to handle an incoming question // // The response to a question may be transmitted over multicast, unicast, or // 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 } // Handle unicast and multicast responses. // TODO(reddaly): The decision about sending over unicast vs. multicast is not // yet fully compliant with RFC 6762. For example, the unicast bit should be // ignored if the records in question are close to TTL expiration. For now, // we just use the unicast bit to make the decision, as per the spec: // RFC 6762, section 18.12. Repurposing of Top Bit of qclass in Question // Section // // In the Question Section of a Multicast DNS query, the top bit of the // qclass field is used to indicate that unicast responses are preferred // for this particular question. (See Section 5.4.) if q.Qclass&(1<<15) != 0 { return nil, records } return records, nil } func (s *Server) probe() { defer s.wg.Done() sd, ok := s.config.Zone.(*MDNSService) if !ok { return } name := fmt.Sprintf("%s.%s.%s.", sd.Instance, trimDot(sd.Service), trimDot(sd.Domain)) q := new(dns.Msg) q.SetQuestion(name, dns.TypePTR) q.RecursionDesired = false srv := &dns.SRV{ Hdr: dns.RR_Header{ Name: name, Rrtype: dns.TypeSRV, Class: dns.ClassINET, Ttl: defaultTTL, }, Priority: 0, Weight: 0, Port: uint16(sd.Port), Target: sd.HostName, } txt := &dns.TXT{ Hdr: dns.RR_Header{ Name: name, Rrtype: dns.TypeTXT, Class: dns.ClassINET, Ttl: defaultTTL, }, Txt: sd.TXT, } q.Ns = []dns.RR{srv, txt} randomizer := rand.New(rand.NewSource(time.Now().UnixNano())) for i := 0; i < 3; i++ { if err := s.SendMulticast(q); err != nil { logger.Errorf(s.config.Context, "[ERR] mdns: failed to send probe: %v", err) } time.Sleep(time.Duration(randomizer.Intn(250)) * time.Millisecond) } resp := new(dns.Msg) resp.MsgHdr.Response = true // set for query q.SetQuestion(name, dns.TypeANY) resp.Answer = append(resp.Answer, s.config.Zone.Records(q.Question[0])...) // reset q.SetQuestion(name, dns.TypePTR) // From RFC6762 // The Multicast DNS responder MUST send at least two unsolicited // responses, one second apart. To provide increased robustness against // packet loss, a responder MAY send up to eight unsolicited responses, // provided that the interval between unsolicited responses increases by // at least a factor of two with every response sent. timeout := 1 * time.Second timer := time.NewTimer(timeout) for i := 0; i < 3; i++ { if err := s.SendMulticast(resp); err != nil { logger.Errorf(s.config.Context, "[ERR] mdns: failed to send announcement: %v", err) } select { case <-timer.C: timeout *= 2 timer.Reset(timeout) case <-s.shutdownCh: timer.Stop() return } } } // SendMulticast us used to send a multicast response packet func (s *Server) SendMulticast(msg *dns.Msg) error { buf, err := msg.Pack() if err != nil { return err } if s.ipv4List != nil { s.ipv4List.WriteToUDP(buf, ipv4Addr) } if s.ipv6List != nil { s.ipv6List.WriteToUDP(buf, ipv6Addr) } return nil } // sendResponse is used to send a response packet func (s *Server) sendResponse(resp *dns.Msg, from net.Addr) error { // TODO(reddaly): Respect the unicast argument, and allow sending responses // over multicast. buf, err := resp.Pack() if err != nil { return err } // Determine the socket to send from addr := from.(*net.UDPAddr) conn := s.ipv4List backupTarget := net.IPv4zero if addr.IP.To4() == nil { conn = s.ipv6List 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 { sd, ok := s.config.Zone.(*MDNSService) if !ok { return nil } atomic.StoreUint32(&sd.TTL, 0) name := fmt.Sprintf("%s.%s.%s.", sd.Instance, trimDot(sd.Service), trimDot(sd.Domain)) q := new(dns.Msg) q.SetQuestion(name, dns.TypeANY) resp := new(dns.Msg) resp.MsgHdr.Response = true resp.Answer = append(resp.Answer, s.config.Zone.Records(q.Question[0])...) return s.SendMulticast(resp) } func setCustomPort(port int) { if port != 0 { if mdnsWildcardAddrIPv4.Port != port { mdnsWildcardAddrIPv4.Port = port } if mdnsWildcardAddrIPv6.Port != port { mdnsWildcardAddrIPv6.Port = port } if ipv4Addr.Port != port { ipv4Addr.Port = port } if ipv6Addr.Port != port { ipv6Addr.Port = port } } } // 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 }