micro/util/mdns/server.go

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2020-04-11 15:02:53 +03:00
package mdns
import (
"fmt"
"log"
"math/rand"
"net"
"sync"
"sync/atomic"
"time"
"github.com/miekg/dns"
"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,
}
)
// 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
}
// 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 bool
shutdownCh chan struct{}
shutdownLock sync.Mutex
wg sync.WaitGroup
}
// 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!")
}
}
s := &Server{
config: config,
ipv4List: ipv4List,
ipv6List: ipv6List,
shutdownCh: make(chan struct{}),
}
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)
s.unregister()
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 {
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
}
// 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,
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 {
log.Println("[ERR] mdns: failed to send probe:", err.Error())
}
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 {
log.Println("[ERR] mdns: failed to send announcement:", err.Error())
}
select {
case <-timer.C:
timeout *= 2
timer.Reset(timeout)
case <-s.shutdownCh:
timer.Stop()
return
}
}
}
// multicastResponse 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)
if addr.IP.To4() != nil {
_, err = s.ipv4List.WriteToUDP(buf, addr)
return err
} else {
_, err = s.ipv6List.WriteToUDP(buf, addr)
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
}
}
}