289 lines
8 KiB
Go
289 lines
8 KiB
Go
package mdns
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import (
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"fmt"
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"log"
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"net"
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"strings"
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"sync/atomic"
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"github.com/miekg/dns"
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)
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const (
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ipv4mdns = "224.0.0.251"
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ipv6mdns = "ff02::fb"
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mdnsPort = 5353
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forceUnicastResponses = false
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)
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var (
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ipv4Addr = &net.UDPAddr{
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IP: net.ParseIP(ipv4mdns),
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Port: mdnsPort,
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}
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ipv6Addr = &net.UDPAddr{
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IP: net.ParseIP(ipv6mdns),
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Port: mdnsPort,
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}
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)
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// Config is used to configure the mDNS server
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type Config struct {
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// Zone must be provided to support responding to queries
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Zone Zone
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// Iface if provided binds the multicast listener to the given
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// interface. If not provided, the system default multicase interface
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// is used.
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Iface *net.Interface
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// LogEmptyResponses indicates the server should print an informative message
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// when there is an mDNS query for which the server has no response.
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LogEmptyResponses bool
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}
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// mDNS server is used to listen for mDNS queries and respond if we
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// have a matching local record
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type Server struct {
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config *Config
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ipv4List *net.UDPConn
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ipv6List *net.UDPConn
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shutdown int32
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shutdownCh chan struct{}
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}
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// NewServer is used to create a new mDNS server from a config
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func NewServer(config *Config) (*Server, error) {
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// Create the listeners
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ipv4List, _ := net.ListenMulticastUDP("udp4", config.Iface, ipv4Addr)
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ipv6List, _ := net.ListenMulticastUDP("udp6", config.Iface, ipv6Addr)
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// Check if we have any listener
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if ipv4List == nil && ipv6List == nil {
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return nil, fmt.Errorf("No multicast listeners could be started")
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}
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s := &Server{
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config: config,
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ipv4List: ipv4List,
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ipv6List: ipv6List,
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shutdownCh: make(chan struct{}),
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}
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if ipv4List != nil {
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go s.recv(s.ipv4List)
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}
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if ipv6List != nil {
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go s.recv(s.ipv6List)
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}
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return s, nil
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}
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// Shutdown is used to shutdown the listener
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func (s *Server) Shutdown() error {
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if !atomic.CompareAndSwapInt32(&s.shutdown, 0, 1) {
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// something else already closed us
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return nil
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}
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close(s.shutdownCh)
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if s.ipv4List != nil {
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s.ipv4List.Close()
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}
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if s.ipv6List != nil {
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s.ipv6List.Close()
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}
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return nil
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}
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// recv is a long running routine to receive packets from an interface
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func (s *Server) recv(c *net.UDPConn) {
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if c == nil {
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return
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}
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buf := make([]byte, 65536)
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for atomic.LoadInt32(&s.shutdown) == 0 {
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n, from, err := c.ReadFrom(buf)
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if err != nil {
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continue
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}
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if err := s.parsePacket(buf[:n], from); err != nil {
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log.Printf("[ERR] mdns: Failed to handle query: %v", err)
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}
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}
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}
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// parsePacket is used to parse an incoming packet
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func (s *Server) parsePacket(packet []byte, from net.Addr) error {
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var msg dns.Msg
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if err := msg.Unpack(packet); err != nil {
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log.Printf("[ERR] mdns: Failed to unpack packet: %v", err)
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return err
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}
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return s.handleQuery(&msg, from)
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}
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// handleQuery is used to handle an incoming query
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func (s *Server) handleQuery(query *dns.Msg, from net.Addr) error {
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if query.Opcode != dns.OpcodeQuery {
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// "In both multicast query and multicast response messages, the OPCODE MUST
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// be zero on transmission (only standard queries are currently supported
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// over multicast). Multicast DNS messages received with an OPCODE other
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// than zero MUST be silently ignored." Note: OpcodeQuery == 0
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return fmt.Errorf("mdns: received query with non-zero Opcode %v: %v", query.Opcode, *query)
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}
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if query.Rcode != 0 {
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// "In both multicast query and multicast response messages, the Response
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// Code MUST be zero on transmission. Multicast DNS messages received with
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// non-zero Response Codes MUST be silently ignored."
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return fmt.Errorf("mdns: received query with non-zero Rcode %v: %v", query.Rcode, *query)
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}
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// TODO(reddaly): Handle "TC (Truncated) Bit":
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// In query messages, if the TC bit is set, it means that additional
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// Known-Answer records may be following shortly. A responder SHOULD
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// record this fact, and wait for those additional Known-Answer records,
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// before deciding whether to respond. If the TC bit is clear, it means
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// that the querying host has no additional Known Answers.
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if query.Truncated {
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return fmt.Errorf("[ERR] mdns: support for DNS requests with high truncated bit not implemented: %v", *query)
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}
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var unicastAnswer, multicastAnswer []dns.RR
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// Handle each question
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for _, q := range query.Question {
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mrecs, urecs := s.handleQuestion(q)
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multicastAnswer = append(multicastAnswer, mrecs...)
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unicastAnswer = append(unicastAnswer, urecs...)
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}
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// See section 18 of RFC 6762 for rules about DNS headers.
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resp := func(unicast bool) *dns.Msg {
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// 18.1: ID (Query Identifier)
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// 0 for multicast response, query.Id for unicast response
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id := uint16(0)
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if unicast {
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id = query.Id
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}
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var answer []dns.RR
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if unicast {
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answer = unicastAnswer
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} else {
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answer = multicastAnswer
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}
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if len(answer) == 0 {
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return nil
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}
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return &dns.Msg{
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MsgHdr: dns.MsgHdr{
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Id: id,
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// 18.2: QR (Query/Response) Bit - must be set to 1 in response.
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Response: true,
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// 18.3: OPCODE - must be zero in response (OpcodeQuery == 0)
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Opcode: dns.OpcodeQuery,
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// 18.4: AA (Authoritative Answer) Bit - must be set to 1
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Authoritative: true,
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// The following fields must all be set to 0:
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// 18.5: TC (TRUNCATED) Bit
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// 18.6: RD (Recursion Desired) Bit
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// 18.7: RA (Recursion Available) Bit
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// 18.8: Z (Zero) Bit
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// 18.9: AD (Authentic Data) Bit
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// 18.10: CD (Checking Disabled) Bit
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// 18.11: RCODE (Response Code)
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},
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// 18.12 pertains to questions (handled by handleQuestion)
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// 18.13 pertains to resource records (handled by handleQuestion)
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// 18.14: Name Compression - responses should be compressed (though see
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// caveats in the RFC), so set the Compress bit (part of the dns library
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// API, not part of the DNS packet) to true.
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Compress: true,
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Answer: answer,
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}
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}
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if s.config.LogEmptyResponses && len(multicastAnswer) == 0 && len(unicastAnswer) == 0 {
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questions := make([]string, len(query.Question))
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for i, q := range query.Question {
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questions[i] = q.Name
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}
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log.Printf("no responses for query with questions: %s", strings.Join(questions, ", "))
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}
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if mresp := resp(false); mresp != nil {
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if err := s.sendResponse(mresp, from, false); err != nil {
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return fmt.Errorf("mdns: error sending multicast response: %v", err)
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}
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}
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if uresp := resp(true); uresp != nil {
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if err := s.sendResponse(uresp, from, true); err != nil {
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return fmt.Errorf("mdns: error sending unicast response: %v", err)
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}
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}
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return nil
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}
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// handleQuestion is used to handle an incoming question
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//
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// The response to a question may be transmitted over multicast, unicast, or
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// both. The return values are DNS records for each transmission type.
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func (s *Server) handleQuestion(q dns.Question) (multicastRecs, unicastRecs []dns.RR) {
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records := s.config.Zone.Records(q)
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if len(records) == 0 {
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return nil, nil
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}
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// Handle unicast and multicast responses.
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// TODO(reddaly): The decision about sending over unicast vs. multicast is not
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// yet fully compliant with RFC 6762. For example, the unicast bit should be
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// ignored if the records in question are close to TTL expiration. For now,
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// we just use the unicast bit to make the decision, as per the spec:
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// RFC 6762, section 18.12. Repurposing of Top Bit of qclass in Question
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// Section
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//
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// In the Question Section of a Multicast DNS query, the top bit of the
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// qclass field is used to indicate that unicast responses are preferred
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// for this particular question. (See Section 5.4.)
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if q.Qclass&(1<<15) != 0 || forceUnicastResponses {
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return nil, records
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}
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return records, nil
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}
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// sendResponse is used to send a response packet
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func (s *Server) sendResponse(resp *dns.Msg, from net.Addr, unicast bool) error {
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// TODO(reddaly): Respect the unicast argument, and allow sending responses
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// over multicast.
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buf, err := resp.Pack()
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if err != nil {
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return err
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}
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// Determine the socket to send from
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addr := from.(*net.UDPAddr)
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if addr.IP.To4() != nil {
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_, err = s.ipv4List.WriteToUDP(buf, addr)
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return err
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} else {
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_, err = s.ipv6List.WriteToUDP(buf, addr)
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return err
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}
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}
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