Make mDNS server response more compatible with RFC 6762 spec.
- dns.Msg construction is now explicit, with comments pointing to the RFC about the values of each field. - dns.Msg responses no longer contain a Question section, which is not allowed according to the spec. - handleQuestion's interface returns both unicast and multicast answer records. (Note: These are not yet treated differently by the sendResponse method.) - removed unused error return value of handleQuestion
This commit is contained in:
Red Daly
parent
6c44326b32
commit
68622dea49
147
server.go
147
server.go
@ -2,16 +2,19 @@ package mdns
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import (
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"fmt"
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"github.com/miekg/dns"
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"log"
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"net"
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"strings"
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"sync"
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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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@ -127,41 +130,145 @@ func (s *Server) parsePacket(packet []byte, from net.Addr) error {
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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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var resp dns.Msg
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resp.SetReply(query)
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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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if len(query.Question) > 0 {
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for i, _ := range query.Question {
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if err := s.handleQuestion(query.Question[i], &resp); err != nil {
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log.Printf("[ERR] mdns: failed to handle question %v: %v",
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query.Question[i], err)
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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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// Check if there is an answer
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if len(resp.Answer) > 0 {
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return s.sendResponse(&resp, from)
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if 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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func (s *Server) handleQuestion(q dns.Question, resp *dns.Msg) error {
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// Bail if we have no zone
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if s.config.Zone == nil {
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return nil
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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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// Add all the query answers
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records := s.config.Zone.Records(q)
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resp.Answer = append(resp.Answer, records...)
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return nil
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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) error {
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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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