open-consul/vendor/github.com/hashicorp/mdns/server.go

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package mdns
import (
"fmt"
"log"
"net"
"strings"
"sync/atomic"
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"github.com/miekg/dns"
)
const (
ipv4mdns = "224.0.0.251"
ipv6mdns = "ff02::fb"
mdnsPort = 5353
forceUnicastResponses = false
)
var (
ipv4Addr = &net.UDPAddr{
IP: net.ParseIP(ipv4mdns),
Port: mdnsPort,
}
ipv6Addr = &net.UDPAddr{
IP: net.ParseIP(ipv6mdns),
Port: mdnsPort,
}
)
// 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
// LogEmptyResponses indicates the server should print an informative message
// when there is an mDNS query for which the server has no response.
LogEmptyResponses bool
}
// mDNS server is 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 int32
shutdownCh chan struct{}
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}
// NewServer is used to create a new mDNS server from a config
func NewServer(config *Config) (*Server, error) {
// Create the listeners
ipv4List, _ := net.ListenMulticastUDP("udp4", config.Iface, ipv4Addr)
ipv6List, _ := net.ListenMulticastUDP("udp6", config.Iface, ipv6Addr)
// Check if we have any listener
if ipv4List == nil && ipv6List == nil {
return nil, fmt.Errorf("No multicast listeners could be started")
}
s := &Server{
config: config,
ipv4List: ipv4List,
ipv6List: ipv6List,
shutdownCh: make(chan struct{}),
}
if ipv4List != nil {
go s.recv(s.ipv4List)
}
if ipv6List != nil {
go s.recv(s.ipv6List)
}
return s, nil
}
// Shutdown is used to shutdown the listener
func (s *Server) Shutdown() error {
if !atomic.CompareAndSwapInt32(&s.shutdown, 0, 1) {
// something else already closed us
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return nil
}
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close(s.shutdownCh)
if s.ipv4List != nil {
s.ipv4List.Close()
}
if s.ipv6List != nil {
s.ipv6List.Close()
}
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 atomic.LoadInt32(&s.shutdown) == 0 {
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n, from, err := c.ReadFrom(buf)
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if err != nil {
continue
}
if err := s.parsePacket(buf[:n], from); err != nil {
log.Printf("[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 {
log.Printf("[ERR] mdns: Failed to unpack packet: %v", err)
return err
}
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,
Answer: answer,
}
}
if s.config.LogEmptyResponses && len(multicastAnswer) == 0 && len(unicastAnswer) == 0 {
questions := make([]string, len(query.Question))
for i, q := range query.Question {
questions[i] = q.Name
}
log.Printf("no responses for query with questions: %s", strings.Join(questions, ", "))
}
if mresp := resp(false); mresp != nil {
if err := s.sendResponse(mresp, from, false); err != nil {
return fmt.Errorf("mdns: error sending multicast response: %v", err)
}
}
if uresp := resp(true); uresp != nil {
if err := s.sendResponse(uresp, from, true); 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 || forceUnicastResponses {
return nil, records
}
return records, nil
}
// sendResponse is used to send a response packet
func (s *Server) sendResponse(resp *dns.Msg, from net.Addr, unicast bool) 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)
if addr.IP.To4() != nil {
_, err = s.ipv4List.WriteToUDP(buf, addr)
return err
} else {
_, err = s.ipv6List.WriteToUDP(buf, addr)
return err
}
}