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resolver.go
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resolver.go
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package libnetwork
import (
"context"
"fmt"
"math/rand"
"net"
"strings"
"sync"
"time"
"github.com/docker/docker/libnetwork/types"
"github.com/miekg/dns"
"github.com/sirupsen/logrus"
"golang.org/x/sync/semaphore"
"golang.org/x/time/rate"
)
// DNSBackend represents a backend DNS resolver used for DNS name
// resolution. All the queries to the resolver are forwarded to the
// backend resolver.
type DNSBackend interface {
// ResolveName resolves a service name to an IPv4 or IPv6 address by searching
// the networks the sandbox is connected to. For IPv6 queries, second return
// value will be true if the name exists in docker domain but doesn't have an
// IPv6 address. Such queries shouldn't be forwarded to external nameservers.
ResolveName(name string, iplen int) ([]net.IP, bool)
// ResolveIP returns the service name for the passed in IP. IP is in reverse dotted
// notation; the format used for DNS PTR records
ResolveIP(name string) string
// ResolveService returns all the backend details about the containers or hosts
// backing a service. Its purpose is to satisfy an SRV query
ResolveService(name string) ([]*net.SRV, []net.IP)
// ExecFunc allows a function to be executed in the context of the backend
// on behalf of the resolver.
ExecFunc(f func()) error
// NdotsSet queries the backends ndots dns option settings
NdotsSet() bool
// HandleQueryResp passes the name & IP from a response to the backend. backend
// can use it to maintain any required state about the resolution
HandleQueryResp(name string, ip net.IP)
}
const (
dnsPort = "53"
ptrIPv4domain = ".in-addr.arpa."
ptrIPv6domain = ".ip6.arpa."
respTTL = 600
maxExtDNS = 3 // max number of external servers to try
extIOTimeout = 4 * time.Second
maxConcurrent = 1024
logInterval = 2 * time.Second
)
type extDNSEntry struct {
IPStr string
port uint16 // for testing
HostLoopback bool
}
// Resolver is the embedded DNS server in Docker. It operates by listening on
// the container's loopback interface for DNS queries.
type Resolver struct {
backend DNSBackend
extDNSList [maxExtDNS]extDNSEntry
server *dns.Server
conn *net.UDPConn
tcpServer *dns.Server
tcpListen *net.TCPListener
err error
listenAddress string
proxyDNS bool
startCh chan struct{}
logger *logrus.Logger
fwdSem *semaphore.Weighted // Limit the number of concurrent external DNS requests in-flight
logInverval rate.Sometimes // Rate-limit logging about hitting the fwdSem limit
}
// NewResolver creates a new instance of the Resolver
func NewResolver(address string, proxyDNS bool, backend DNSBackend) *Resolver {
return &Resolver{
backend: backend,
proxyDNS: proxyDNS,
listenAddress: address,
err: fmt.Errorf("setup not done yet"),
startCh: make(chan struct{}, 1),
fwdSem: semaphore.NewWeighted(maxConcurrent),
logInverval: rate.Sometimes{Interval: logInterval},
}
}
func (r *Resolver) log() *logrus.Logger {
if r.logger == nil {
return logrus.StandardLogger()
}
return r.logger
}
// SetupFunc returns the setup function that should be run in the container's
// network namespace.
func (r *Resolver) SetupFunc(port int) func() {
return func() {
var err error
// DNS operates primarily on UDP
addr := &net.UDPAddr{
IP: net.ParseIP(r.listenAddress),
Port: port,
}
r.conn, err = net.ListenUDP("udp", addr)
if err != nil {
r.err = fmt.Errorf("error in opening name server socket %v", err)
return
}
// Listen on a TCP as well
tcpaddr := &net.TCPAddr{
IP: net.ParseIP(r.listenAddress),
Port: port,
}
r.tcpListen, err = net.ListenTCP("tcp", tcpaddr)
if err != nil {
r.err = fmt.Errorf("error in opening name TCP server socket %v", err)
return
}
r.err = nil
}
}
// Start starts the name server for the container.
func (r *Resolver) Start() error {
r.startCh <- struct{}{}
defer func() { <-r.startCh }()
// make sure the resolver has been setup before starting
if r.err != nil {
return r.err
}
if err := r.setupIPTable(); err != nil {
return fmt.Errorf("setting up IP table rules failed: %v", err)
}
s := &dns.Server{Handler: dns.HandlerFunc(r.serveDNS), PacketConn: r.conn}
r.server = s
go func() {
if err := s.ActivateAndServe(); err != nil {
r.log().WithError(err).Error("[resolver] failed to start PacketConn DNS server")
}
}()
tcpServer := &dns.Server{Handler: dns.HandlerFunc(r.serveDNS), Listener: r.tcpListen}
r.tcpServer = tcpServer
go func() {
if err := tcpServer.ActivateAndServe(); err != nil {
r.log().WithError(err).Error("[resolver] failed to start TCP DNS server")
}
}()
return nil
}
// Stop stops the name server for the container. A stopped resolver can be
// reused after running the SetupFunc again.
func (r *Resolver) Stop() {
r.startCh <- struct{}{}
defer func() { <-r.startCh }()
if r.server != nil {
r.server.Shutdown() //nolint:errcheck
}
if r.tcpServer != nil {
r.tcpServer.Shutdown() //nolint:errcheck
}
r.conn = nil
r.tcpServer = nil
r.err = fmt.Errorf("setup not done yet")
r.fwdSem = semaphore.NewWeighted(maxConcurrent)
}
// SetExtServers configures the external nameservers the resolver should use
// when forwarding queries.
func (r *Resolver) SetExtServers(extDNS []extDNSEntry) {
l := len(extDNS)
if l > maxExtDNS {
l = maxExtDNS
}
for i := 0; i < l; i++ {
r.extDNSList[i] = extDNS[i]
}
}
// NameServer returns the IP of the DNS resolver for the containers.
func (r *Resolver) NameServer() string {
return r.listenAddress
}
// ResolverOptions returns resolv.conf options that should be set.
func (r *Resolver) ResolverOptions() []string {
return []string{"ndots:0"}
}
func setCommonFlags(msg *dns.Msg) {
msg.RecursionAvailable = true
}
//nolint:gosec // The RNG is not used in a security-sensitive context.
var (
shuffleRNG = rand.New(rand.NewSource(time.Now().Unix()))
shuffleRNGMu sync.Mutex
)
func shuffleAddr(addr []net.IP) []net.IP {
shuffleRNGMu.Lock()
defer shuffleRNGMu.Unlock()
for i := len(addr) - 1; i > 0; i-- {
r := shuffleRNG.Intn(i + 1) //nolint:gosec // gosec complains about the use of rand here. It should be fine.
addr[i], addr[r] = addr[r], addr[i]
}
return addr
}
func createRespMsg(query *dns.Msg) *dns.Msg {
resp := new(dns.Msg)
resp.SetReply(query)
setCommonFlags(resp)
return resp
}
func (r *Resolver) handleMXQuery(query *dns.Msg) (*dns.Msg, error) {
name := query.Question[0].Name
addrv4, _ := r.backend.ResolveName(name, types.IPv4)
addrv6, _ := r.backend.ResolveName(name, types.IPv6)
if addrv4 == nil && addrv6 == nil {
return nil, nil
}
// We were able to resolve the name. Respond with an empty list with
// RcodeSuccess/NOERROR so that email clients can treat it as "implicit MX"
// [RFC 5321 Section-5.1] and issue a Type A/AAAA query for the name.
resp := createRespMsg(query)
return resp, nil
}
func (r *Resolver) handleIPQuery(query *dns.Msg, ipType int) (*dns.Msg, error) {
var (
addr []net.IP
ipv6Miss bool
name = query.Question[0].Name
)
addr, ipv6Miss = r.backend.ResolveName(name, ipType)
if addr == nil && ipv6Miss {
// Send a reply without any Answer sections
r.log().Debugf("[resolver] lookup name %s present without IPv6 address", name)
resp := createRespMsg(query)
return resp, nil
}
if addr == nil {
return nil, nil
}
r.log().Debugf("[resolver] lookup for %s: IP %v", name, addr)
resp := createRespMsg(query)
if len(addr) > 1 {
addr = shuffleAddr(addr)
}
if ipType == types.IPv4 {
for _, ip := range addr {
rr := new(dns.A)
rr.Hdr = dns.RR_Header{Name: name, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: respTTL}
rr.A = ip
resp.Answer = append(resp.Answer, rr)
}
} else {
for _, ip := range addr {
rr := new(dns.AAAA)
rr.Hdr = dns.RR_Header{Name: name, Rrtype: dns.TypeAAAA, Class: dns.ClassINET, Ttl: respTTL}
rr.AAAA = ip
resp.Answer = append(resp.Answer, rr)
}
}
return resp, nil
}
func (r *Resolver) handlePTRQuery(query *dns.Msg) (*dns.Msg, error) {
ptr := query.Question[0].Name
name, after, found := strings.Cut(ptr, ptrIPv4domain)
if !found || after != "" {
name, after, found = strings.Cut(ptr, ptrIPv6domain)
}
if !found || after != "" {
// Not a known IPv4 or IPv6 PTR domain.
// Maybe the external DNS servers know what to do with the query?
return nil, nil
}
host := r.backend.ResolveIP(name)
if host == "" {
return nil, nil
}
r.log().Debugf("[resolver] lookup for IP %s: name %s", name, host)
fqdn := dns.Fqdn(host)
resp := new(dns.Msg)
resp.SetReply(query)
setCommonFlags(resp)
rr := new(dns.PTR)
rr.Hdr = dns.RR_Header{Name: ptr, Rrtype: dns.TypePTR, Class: dns.ClassINET, Ttl: respTTL}
rr.Ptr = fqdn
resp.Answer = append(resp.Answer, rr)
return resp, nil
}
func (r *Resolver) handleSRVQuery(query *dns.Msg) (*dns.Msg, error) {
svc := query.Question[0].Name
srv, ip := r.backend.ResolveService(svc)
if len(srv) == 0 {
return nil, nil
}
if len(srv) != len(ip) {
return nil, fmt.Errorf("invalid reply for SRV query %s", svc)
}
resp := createRespMsg(query)
for i, r := range srv {
rr := new(dns.SRV)
rr.Hdr = dns.RR_Header{Name: svc, Rrtype: dns.TypePTR, Class: dns.ClassINET, Ttl: respTTL}
rr.Port = r.Port
rr.Target = r.Target
resp.Answer = append(resp.Answer, rr)
rr1 := new(dns.A)
rr1.Hdr = dns.RR_Header{Name: r.Target, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: respTTL}
rr1.A = ip[i]
resp.Extra = append(resp.Extra, rr1)
}
return resp, nil
}
func (r *Resolver) serveDNS(w dns.ResponseWriter, query *dns.Msg) {
var (
resp *dns.Msg
err error
)
if query == nil || len(query.Question) == 0 {
return
}
queryName := query.Question[0].Name
queryType := query.Question[0].Qtype
switch queryType {
case dns.TypeA:
resp, err = r.handleIPQuery(query, types.IPv4)
case dns.TypeAAAA:
resp, err = r.handleIPQuery(query, types.IPv6)
case dns.TypeMX:
resp, err = r.handleMXQuery(query)
case dns.TypePTR:
resp, err = r.handlePTRQuery(query)
case dns.TypeSRV:
resp, err = r.handleSRVQuery(query)
default:
r.log().Debugf("[resolver] query type %s is not supported by the embedded DNS and will be forwarded to external DNS", dns.TypeToString[queryType])
}
reply := func(msg *dns.Msg) {
if err = w.WriteMsg(msg); err != nil {
r.log().WithError(err).Errorf("[resolver] failed to write response")
}
}
if err != nil {
r.log().WithError(err).Errorf("[resolver] failed to handle query: %s (%s)", queryName, dns.TypeToString[queryType])
reply(new(dns.Msg).SetRcode(query, dns.RcodeServerFailure))
return
}
if resp != nil {
// We are the authoritative DNS server for this request so it's
// on us to truncate the response message to the size limit
// negotiated by the client.
maxSize := dns.MinMsgSize
if w.LocalAddr().Network() == "tcp" {
maxSize = dns.MaxMsgSize
} else {
if optRR := query.IsEdns0(); optRR != nil {
if udpsize := int(optRR.UDPSize()); udpsize > maxSize {
maxSize = udpsize
}
}
}
resp.Truncate(maxSize)
reply(resp)
return
}
if r.proxyDNS {
// If the user sets ndots > 0 explicitly and the query is
// in the root domain don't forward it out. We will return
// failure and let the client retry with the search domain
// attached.
if (queryType == dns.TypeA || queryType == dns.TypeAAAA) && r.backend.NdotsSet() &&
!strings.Contains(strings.TrimSuffix(queryName, "."), ".") {
resp = createRespMsg(query)
} else {
resp = r.forwardExtDNS(w.LocalAddr().Network(), query)
}
}
if resp == nil {
// We were unable to get an answer from any of the upstream DNS
// servers or the backend doesn't support proxying DNS requests.
resp = new(dns.Msg).SetRcode(query, dns.RcodeServerFailure)
}
reply(resp)
}
func (r *Resolver) dialExtDNS(proto string, server extDNSEntry) (net.Conn, error) {
var (
extConn net.Conn
dialErr error
)
extConnect := func() {
if server.port == 0 {
server.port = 53
}
addr := fmt.Sprintf("%s:%d", server.IPStr, server.port)
extConn, dialErr = net.DialTimeout(proto, addr, extIOTimeout)
}
if server.HostLoopback {
extConnect()
} else {
execErr := r.backend.ExecFunc(extConnect)
if execErr != nil {
return nil, execErr
}
}
if dialErr != nil {
return nil, dialErr
}
return extConn, nil
}
func (r *Resolver) forwardExtDNS(proto string, query *dns.Msg) *dns.Msg {
for _, extDNS := range r.extDNSList {
if extDNS.IPStr == "" {
break
}
// limits the number of outstanding concurrent queries.
ctx, cancel := context.WithTimeout(context.Background(), extIOTimeout)
err := r.fwdSem.Acquire(ctx, 1)
cancel()
if err != nil {
r.logInverval.Do(func() {
r.log().Errorf("[resolver] more than %v concurrent queries", maxConcurrent)
})
return new(dns.Msg).SetRcode(query, dns.RcodeRefused)
}
resp := func() *dns.Msg {
defer r.fwdSem.Release(1)
return r.exchange(proto, extDNS, query)
}()
if resp == nil {
continue
}
switch resp.Rcode {
case dns.RcodeServerFailure, dns.RcodeRefused:
// Server returned FAILURE: continue with the next external DNS server
// Server returned REFUSED: this can be a transitional status, so continue with the next external DNS server
r.log().Debugf("[resolver] external DNS %s:%s returned failure:\n%s", proto, extDNS.IPStr, resp)
continue
}
answers := 0
for _, rr := range resp.Answer {
h := rr.Header()
switch h.Rrtype {
case dns.TypeA:
answers++
ip := rr.(*dns.A).A
r.log().Debugf("[resolver] received A record %q for %q from %s:%s", ip, h.Name, proto, extDNS.IPStr)
r.backend.HandleQueryResp(h.Name, ip)
case dns.TypeAAAA:
answers++
ip := rr.(*dns.AAAA).AAAA
r.log().Debugf("[resolver] received AAAA record %q for %q from %s:%s", ip, h.Name, proto, extDNS.IPStr)
r.backend.HandleQueryResp(h.Name, ip)
}
}
if len(resp.Answer) == 0 {
r.log().Debugf("[resolver] external DNS %s:%s returned response with no answers:\n%s", proto, extDNS.IPStr, resp)
}
resp.Compress = true
return resp
}
return nil
}
func (r *Resolver) exchange(proto string, extDNS extDNSEntry, query *dns.Msg) *dns.Msg {
extConn, err := r.dialExtDNS(proto, extDNS)
if err != nil {
r.log().WithError(err).Warn("[resolver] connect failed")
return nil
}
defer extConn.Close()
log := r.log().WithFields(logrus.Fields{
"dns-server": extConn.RemoteAddr().Network() + ":" + extConn.RemoteAddr().String(),
"client-addr": extConn.LocalAddr().Network() + ":" + extConn.LocalAddr().String(),
"question": query.Question[0].String(),
})
log.Debug("[resolver] forwarding query")
resp, _, err := (&dns.Client{
Timeout: extIOTimeout,
// Following the robustness principle, make a best-effort
// attempt to receive oversized response messages without
// truncating them on our end to forward verbatim to the client.
// Some DNS servers (e.g. Mikrotik RouterOS) don't support
// EDNS(0) and may send replies over UDP longer than 512 bytes
// regardless of what size limit, if any, was advertized in the
// query message. Note that ExchangeWithConn will override this
// value if it detects an EDNS OPT record in query so only
// oversized replies to non-EDNS queries will benefit.
UDPSize: dns.MaxMsgSize,
}).ExchangeWithConn(query, &dns.Conn{Conn: extConn})
if err != nil {
r.log().WithError(err).Errorf("[resolver] failed to query DNS server: %s, query: %s", extConn.RemoteAddr().String(), query.Question[0].String())
return nil
}
if resp == nil {
// Should be impossible, so make noise if it happens anyway.
log.Error("[resolver] external DNS returned empty response")
}
return resp
}