src/net/dnsclient_unix.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // +build darwin dragonfly freebsd linux netbsd openbsd solaris

 // DNS client: see RFC 1035.
 // Has to be linked into package net for Dial.

 // TODO(rsc):
 //	Could potentially handle many outstanding lookups faster.
 //	Could have a small cache.
 //	Random UDP source port (net.Dial should do that for us).
 //	Random request IDs.

 package net

 import (
 	"context"
 	"errors"
 	"io"
 	"math/rand"
 	"os"
 	"sync"
 	"time"
 )

 // A dnsDialer provides dialing suitable for DNS queries.
 type dnsDialer interface {
 	dialDNS(ctx context.Context, network, addr string) (dnsConn, error)
 }

 var testHookDNSDialer = func() dnsDialer { return &Dialer{} }

 // A dnsConn represents a DNS transport endpoint.
 type dnsConn interface {
 	io.Closer

 	SetDeadline(time.Time) error

 	// dnsRoundTrip executes a single DNS transaction, returning a
 	// DNS response message for the provided DNS query message.
 	dnsRoundTrip(query *dnsMsg) (*dnsMsg, error)
 }

 func (c *UDPConn) dnsRoundTrip(query *dnsMsg) (*dnsMsg, error) {
 	return dnsRoundTripUDP(c, query)
 }

 // dnsRoundTripUDP implements the dnsRoundTrip interface for RFC 1035's
 // "UDP usage" transport mechanism. c should be a packet-oriented connection,
 // such as a *UDPConn.
 func dnsRoundTripUDP(c io.ReadWriter, query *dnsMsg) (*dnsMsg, error) {
 	b, ok := query.Pack()
 	if !ok {
 		return nil, errors.New("cannot marshal DNS message")
 	}
 	if _, err := c.Write(b); err != nil {
 		return nil, err
 	}

 	b = make([]byte, 512) // see RFC 1035
 	for {
 		n, err := c.Read(b)
 		if err != nil {
 			return nil, err
 		}
 		resp := &dnsMsg{}
 		if !resp.Unpack(b[:n]) || !resp.IsResponseTo(query) {
 			// Ignore invalid responses as they may be malicious
 			// forgery attempts. Instead continue waiting until
 			// timeout. See golang.org/issue/13281.
 			continue
 		}
 		return resp, nil
 	}
 }

 func (c *TCPConn) dnsRoundTrip(out *dnsMsg) (*dnsMsg, error) {
 	return dnsRoundTripTCP(c, out)
 }

 // dnsRoundTripTCP implements the dnsRoundTrip interface for RFC 1035's
 // "TCP usage" transport mechanism. c should be a stream-oriented connection,
 // such as a *TCPConn.
 func dnsRoundTripTCP(c io.ReadWriter, query *dnsMsg) (*dnsMsg, error) {
 	b, ok := query.Pack()
 	if !ok {
 		return nil, errors.New("cannot marshal DNS message")
 	}
 	l := len(b)
 	b = append([]byte{byte(l >> 8), byte(l)}, b...)
 	if _, err := c.Write(b); err != nil {
 		return nil, err
 	}

 	b = make([]byte, 1280) // 1280 is a reasonable initial size for IP over Ethernet, see RFC 4035
 	if _, err := io.ReadFull(c, b[:2]); err != nil {
 		return nil, err
 	}
 	l = int(b[0])<<8 | int(b[1])
 	if l > len(b) {
 		b = make([]byte, l)
 	}
 	n, err := io.ReadFull(c, b[:l])
 	if err != nil {
 		return nil, err
 	}
 	resp := &dnsMsg{}
 	if !resp.Unpack(b[:n]) {
 		return nil, errors.New("cannot unmarshal DNS message")
 	}
 	if !resp.IsResponseTo(query) {
 		return nil, errors.New("invalid DNS response")
 	}
 	return resp, nil
 }

 func (d *Dialer) dialDNS(ctx context.Context, network, server string) (dnsConn, error) {
 	switch network {
 	case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6":
 	default:
 		return nil, UnknownNetworkError(network)
 	}
 	// Calling Dial here is scary -- we have to be sure not to
 	// dial a name that will require a DNS lookup, or Dial will
 	// call back here to translate it. The DNS config parser has
 	// already checked that all the cfg.servers are IP
 	// addresses, which Dial will use without a DNS lookup.
 	c, err := d.DialContext(ctx, network, server)
 	if err != nil {
 		return nil, mapErr(err)
 	}
 	switch network {
 	case "tcp", "tcp4", "tcp6":
 		return c.(*TCPConn), nil
 	case "udp", "udp4", "udp6":
 		return c.(*UDPConn), nil
 	}
 	panic("unreachable")
 }

 // exchange sends a query on the connection and hopes for a response.
 func exchange(ctx context.Context, server, name string, qtype uint16, timeout time.Duration) (*dnsMsg, error) {
 	d := testHookDNSDialer()
 	out := dnsMsg{
 		dnsMsgHdr: dnsMsgHdr{
 			recursion_desired: true,
 		},
 		question: []dnsQuestion{
 			{name, qtype, dnsClassINET},
 		},
 	}
 	for _, network := range []string{"udp", "tcp"} {
 		// TODO(mdempsky): Refactor so defers from UDP-based
 		// exchanges happen before TCP-based exchange.

 		ctx, cancel := context.WithDeadline(ctx, time.Now().Add(timeout))
 		defer cancel()

 		c, err := d.dialDNS(ctx, network, server)
 		if err != nil {
 			return nil, err
 		}
 		defer c.Close()
 		if d, ok := ctx.Deadline(); ok && !d.IsZero() {
 			c.SetDeadline(d)
 		}
 		out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
 		in, err := c.dnsRoundTrip(&out)
 		if err != nil {
 			return nil, mapErr(err)
 		}
 		if in.truncated { // see RFC 5966
 			continue
 		}
 		return in, nil
 	}
 	return nil, errors.New("no answer from DNS server")
 }

 // Do a lookup for a single name, which must be rooted
 // (otherwise answer will not find the answers).
 func tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype uint16) (string, []dnsRR, error) {
 	var lastErr error
 	serverOffset := cfg.serverOffset()
 	sLen := uint32(len(cfg.servers))

 	for i := 0; i < cfg.attempts; i++ {
 		for j := uint32(0); j < sLen; j++ {
 			server := cfg.servers[(serverOffset+j)%sLen]

 			msg, err := exchange(ctx, server, name, qtype, cfg.timeout)
 			if err != nil {
 				lastErr = &DNSError{
 					Err:    err.Error(),
 					Name:   name,
 					Server: server,
 				}
 				if nerr, ok := err.(Error); ok && nerr.Timeout() {
 					lastErr.(*DNSError).IsTimeout = true
 				}
 				continue
 			}
 			// libresolv continues to the next server when it receives
 			// an invalid referral response. See golang.org/issue/15434.
 			if msg.rcode == dnsRcodeSuccess && !msg.authoritative && !msg.recursion_available && len(msg.answer) == 0 && len(msg.extra) == 0 {
 				lastErr = &DNSError{Err: "lame referral", Name: name, Server: server}
 				continue
 			}
 			cname, rrs, err := answer(name, server, msg, qtype)
 			// If answer errored for rcodes dnsRcodeSuccess or dnsRcodeNameError,
 			// it means the response in msg was not useful and trying another
 			// server probably won't help. Return now in those cases.
 			// TODO: indicate this in a more obvious way, such as a field on DNSError?
 			if err == nil || msg.rcode == dnsRcodeSuccess || msg.rcode == dnsRcodeNameError {
 				return cname, rrs, err
 			}
 			lastErr = err
 		}
 	}
 	return "", nil, lastErr
 }

 // addrRecordList converts and returns a list of IP addresses from DNS
 // address records (both A and AAAA). Other record types are ignored.
 func addrRecordList(rrs []dnsRR) []IPAddr {
 	addrs := make([]IPAddr, 0, 4)
 	for _, rr := range rrs {
 		switch rr := rr.(type) {
 		case *dnsRR_A:
 			addrs = append(addrs, IPAddr{IP: IPv4(byte(rr.A>>24), byte(rr.A>>16), byte(rr.A>>8), byte(rr.A))})
 		case *dnsRR_AAAA:
 			ip := make(IP, IPv6len)
 			copy(ip, rr.AAAA[:])
 			addrs = append(addrs, IPAddr{IP: ip})
 		}
 	}
 	return addrs
 }

 // A resolverConfig represents a DNS stub resolver configuration.
 type resolverConfig struct {
 	initOnce sync.Once // guards init of resolverConfig

 	// ch is used as a semaphore that only allows one lookup at a
 	// time to recheck resolv.conf.
 	ch          chan struct{} // guards lastChecked and modTime
 	lastChecked time.Time     // last time resolv.conf was checked

 	mu        sync.RWMutex // protects dnsConfig
 	dnsConfig *dnsConfig   // parsed resolv.conf structure used in lookups
 }

 var resolvConf resolverConfig

 // init initializes conf and is only called via conf.initOnce.
 func (conf *resolverConfig) init() {
 	// Set dnsConfig and lastChecked so we don't parse
 	// resolv.conf twice the first time.
 	conf.dnsConfig = systemConf().resolv
 	if conf.dnsConfig == nil {
 		conf.dnsConfig = dnsReadConfig("/etc/resolv.conf")
 	}
 	conf.lastChecked = time.Now()

 	// Prepare ch so that only one update of resolverConfig may
 	// run at once.
 	conf.ch = make(chan struct{}, 1)
 }

 // tryUpdate tries to update conf with the named resolv.conf file.
 // The name variable only exists for testing. It is otherwise always
 // "/etc/resolv.conf".
 func (conf *resolverConfig) tryUpdate(name string) {
 	conf.initOnce.Do(conf.init)

 	// Ensure only one update at a time checks resolv.conf.
 	if !conf.tryAcquireSema() {
 		return
 	}
 	defer conf.releaseSema()

 	now := time.Now()
 	if conf.lastChecked.After(now.Add(-5 * time.Second)) {
 		return
 	}
 	conf.lastChecked = now

 	var mtime time.Time
 	if fi, err := os.Stat(name); err == nil {
 		mtime = fi.ModTime()
 	}
 	if mtime.Equal(conf.dnsConfig.mtime) {
 		return
 	}

 	dnsConf := dnsReadConfig(name)
 	conf.mu.Lock()
 	conf.dnsConfig = dnsConf
 	conf.mu.Unlock()
 }

 func (conf *resolverConfig) tryAcquireSema() bool {
 	select {
 	case conf.ch <- struct{}{}:
 		return true
 	default:
 		return false
 	}
 }

 func (conf *resolverConfig) releaseSema() {
 	<-conf.ch
 }

 func lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
 	if !isDomainName(name) {
 		// We used to use "invalid domain name" as the error,
 		// but that is a detail of the specific lookup mechanism.
 		// Other lookups might allow broader name syntax
 		// (for example Multicast DNS allows UTF-8; see RFC 6762).
 		// For consistency with libc resolvers, report no such host.
 		return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name}
 	}
 	resolvConf.tryUpdate("/etc/resolv.conf")
 	resolvConf.mu.RLock()
 	conf := resolvConf.dnsConfig
 	resolvConf.mu.RUnlock()
 	for _, fqdn := range conf.nameList(name) {
 		cname, rrs, err = tryOneName(ctx, conf, fqdn, qtype)
 		if err == nil {
 			break
 		}
 	}
 	if err, ok := err.(*DNSError); ok {
 		// Show original name passed to lookup, not suffixed one.
 		// In general we might have tried many suffixes; showing
 		// just one is misleading. See also golang.org/issue/6324.
 		err.Name = name
 	}
 	return
 }

 // avoidDNS reports whether this is a hostname for which we should not
 // use DNS. Currently this includes only .onion, per RFC 7686. See
 // golang.org/issue/13705. Does not cover .local names (RFC 6762),
 // see golang.org/issue/16739.
 func avoidDNS(name string) bool {
 	if name == "" {
 		return true
 	}
 	if name[len(name)-1] == '.' {
 		name = name[:len(name)-1]
 	}
 	return stringsHasSuffixFold(name, ".onion")
 }

 // nameList returns a list of names for sequential DNS queries.
 func (conf *dnsConfig) nameList(name string) []string {
 	if avoidDNS(name) {
 		return nil
 	}

 	// If name is rooted (trailing dot), try only that name.
 	rooted := len(name) > 0 && name[len(name)-1] == '.'
 	if rooted {
 		return []string{name}
 	}

 	hasNdots := count(name, '.') >= conf.ndots
 	name += "."

 	// Build list of search choices.
 	names := make([]string, 0, 1+len(conf.search))
 	// If name has enough dots, try unsuffixed first.
 	if hasNdots {
 		names = append(names, name)
 	}
 	// Try suffixes.
 	for _, suffix := range conf.search {
 		names = append(names, name+suffix)
 	}
 	// Try unsuffixed, if not tried first above.
 	if !hasNdots {
 		names = append(names, name)
 	}
 	return names
 }

 // hostLookupOrder specifies the order of LookupHost lookup strategies.
 // It is basically a simplified representation of nsswitch.conf.
 // "files" means /etc/hosts.
 type hostLookupOrder int

 const (
 	// hostLookupCgo means defer to cgo.
 	hostLookupCgo      hostLookupOrder = iota
 	hostLookupFilesDNS                 // files first
 	hostLookupDNSFiles                 // dns first
 	hostLookupFiles                    // only files
 	hostLookupDNS                      // only DNS
 )

 var lookupOrderName = map[hostLookupOrder]string{
 	hostLookupCgo:      "cgo",
 	hostLookupFilesDNS: "files,dns",
 	hostLookupDNSFiles: "dns,files",
 	hostLookupFiles:    "files",
 	hostLookupDNS:      "dns",
 }

 func (o hostLookupOrder) String() string {
 	if s, ok := lookupOrderName[o]; ok {
 		return s
 	}
 	return "hostLookupOrder=" + itoa(int(o)) + "??"
 }

 // goLookupHost is the native Go implementation of LookupHost.
 // Used only if cgoLookupHost refuses to handle the request
 // (that is, only if cgoLookupHost is the stub in cgo_stub.go).
 // Normally we let cgo use the C library resolver instead of
 // depending on our lookup code, so that Go and C get the same
 // answers.
 func goLookupHost(ctx context.Context, name string) (addrs []string, err error) {
 	return goLookupHostOrder(ctx, name, hostLookupFilesDNS)
 }

 func goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []string, err error) {
 	if order == hostLookupFilesDNS || order == hostLookupFiles {
 		// Use entries from /etc/hosts if they match.
 		addrs = lookupStaticHost(name)
 		if len(addrs) > 0 || order == hostLookupFiles {
 			return
 		}
 	}
 	ips, err := goLookupIPOrder(ctx, name, order)
 	if err != nil {
 		return
 	}
 	addrs = make([]string, 0, len(ips))
 	for _, ip := range ips {
 		addrs = append(addrs, ip.String())
 	}
 	return
 }

 // lookup entries from /etc/hosts
 func goLookupIPFiles(name string) (addrs []IPAddr) {
 	for _, haddr := range lookupStaticHost(name) {
 		haddr, zone := splitHostZone(haddr)
 		if ip := ParseIP(haddr); ip != nil {
 			addr := IPAddr{IP: ip, Zone: zone}
 			addrs = append(addrs, addr)
 		}
 	}
 	sortByRFC6724(addrs)
 	return
 }

 // goLookupIP is the native Go implementation of LookupIP.
 // The libc versions are in cgo_*.go.
 func goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
 	order := systemConf().hostLookupOrder(host)
 	return goLookupIPOrder(ctx, host, order)
 }

 func goLookupIPOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, err error) {
 	if order == hostLookupFilesDNS || order == hostLookupFiles {
 		addrs = goLookupIPFiles(name)
 		if len(addrs) > 0 || order == hostLookupFiles {
 			return addrs, nil
 		}
 	}
 	if !isDomainName(name) {
 		// See comment in func lookup above about use of errNoSuchHost.
 		return nil, &DNSError{Err: errNoSuchHost.Error(), Name: name}
 	}
 	resolvConf.tryUpdate("/etc/resolv.conf")
 	resolvConf.mu.RLock()
 	conf := resolvConf.dnsConfig
 	resolvConf.mu.RUnlock()
 	type racer struct {
 		fqdn string
 		rrs  []dnsRR
 		error
 	}
 	lane := make(chan racer, 1)
 	qtypes := [...]uint16{dnsTypeA, dnsTypeAAAA}
 	var lastErr error
 	for _, fqdn := range conf.nameList(name) {
 		for _, qtype := range qtypes {
 			go func(qtype uint16) {
 				_, rrs, err := tryOneName(ctx, conf, fqdn, qtype)
 				lane <- racer{fqdn, rrs, err}
 			}(qtype)
 		}
 		for range qtypes {
 			racer := <-lane
 			if racer.error != nil {
 				// Prefer error for original name.
 				if lastErr == nil || racer.fqdn == name+"." {
 					lastErr = racer.error
 				}
 				continue
 			}
 			addrs = append(addrs, addrRecordList(racer.rrs)...)
 		}
 		if len(addrs) > 0 {
 			break
 		}
 	}
 	if lastErr, ok := lastErr.(*DNSError); ok {
 		// Show original name passed to lookup, not suffixed one.
 		// In general we might have tried many suffixes; showing
 		// just one is misleading. See also golang.org/issue/6324.
 		lastErr.Name = name
 	}
 	sortByRFC6724(addrs)
 	if len(addrs) == 0 {
 		if order == hostLookupDNSFiles {
 			addrs = goLookupIPFiles(name)
 		}
 		if len(addrs) == 0 && lastErr != nil {
 			return nil, lastErr
 		}
 	}
 	return addrs, nil
 }

 // goLookupCNAME is the native Go implementation of LookupCNAME.
 // Used only if cgoLookupCNAME refuses to handle the request
 // (that is, only if cgoLookupCNAME is the stub in cgo_stub.go).
 // Normally we let cgo use the C library resolver instead of
 // depending on our lookup code, so that Go and C get the same
 // answers.
 func goLookupCNAME(ctx context.Context, name string) (cname string, err error) {
 	_, rrs, err := lookup(ctx, name, dnsTypeCNAME)
 	if err != nil {
 		return
 	}
 	cname = rrs[0].(*dnsRR_CNAME).Cname
 	return
 }

 // goLookupPTR is the native Go implementation of LookupAddr.
 // Used only if cgoLookupPTR refuses to handle the request (that is,
 // only if cgoLookupPTR is the stub in cgo_stub.go).
 // Normally we let cgo use the C library resolver instead of depending
 // on our lookup code, so that Go and C get the same answers.
 func goLookupPTR(ctx context.Context, addr string) ([]string, error) {
 	names := lookupStaticAddr(addr)
 	if len(names) > 0 {
 		return names, nil
 	}
 	arpa, err := reverseaddr(addr)
 	if err != nil {
 		return nil, err
 	}
 	_, rrs, err := lookup(ctx, arpa, dnsTypePTR)
 	if err != nil {
 		return nil, err
 	}
 	ptrs := make([]string, len(rrs))
 	for i, rr := range rrs {
 		ptrs[i] = rr.(*dnsRR_PTR).Ptr
 	}
 	return ptrs, nil
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// +build darwin dragonfly freebsd linux netbsd openbsd solaris

	// DNS client: see RFC 1035.
	// Has to be linked into package net for Dial.

	// TODO(rsc):
	// Could potentially handle many outstanding lookups faster.
	// Could have a small cache.
	// Random UDP source port (net.Dial should do that for us).
	// Random request IDs.

	package net

	import (
	"context"
	"errors"
	"io"
	"math/rand"
	"os"
	"sync"
	"time"
	)

	// A dnsDialer provides dialing suitable for DNS queries.
	type dnsDialer interface {
	dialDNS(ctx context.Context, network, addr string) (dnsConn, error)
	}

	var testHookDNSDialer = func() dnsDialer { return &Dialer{} }

	// A dnsConn represents a DNS transport endpoint.
	type dnsConn interface {
	io.Closer

	SetDeadline(time.Time) error

	// dnsRoundTrip executes a single DNS transaction, returning a
	// DNS response message for the provided DNS query message.
	dnsRoundTrip(query dnsMsg) (dnsMsg, error)
	}

	func (c UDPConn) dnsRoundTrip(query dnsMsg) (*dnsMsg, error) {
	return dnsRoundTripUDP(c, query)
	}

	// dnsRoundTripUDP implements the dnsRoundTrip interface for RFC 1035's
	// "UDP usage" transport mechanism. c should be a packet-oriented connection,
	// such as a *UDPConn.
	func dnsRoundTripUDP(c io.ReadWriter, query dnsMsg) (dnsMsg, error) {
	b, ok := query.Pack()
	if !ok {
	return nil, errors.New("cannot marshal DNS message")
	}
	if _, err := c.Write(b); err != nil {
	return nil, err
	}

	b = make([]byte, 512) // see RFC 1035
	for {
	n, err := c.Read(b)
	if err != nil {
	return nil, err
	}
	resp := &dnsMsg{}
	if !resp.Unpack(b[:n]) \|\| !resp.IsResponseTo(query) {
	// Ignore invalid responses as they may be malicious
	// forgery attempts. Instead continue waiting until
	// timeout. See golang.org/issue/13281.
	continue
	}
	return resp, nil
	}
	}

	func (c TCPConn) dnsRoundTrip(out dnsMsg) (*dnsMsg, error) {
	return dnsRoundTripTCP(c, out)
	}

	// dnsRoundTripTCP implements the dnsRoundTrip interface for RFC 1035's
	// "TCP usage" transport mechanism. c should be a stream-oriented connection,
	// such as a *TCPConn.
	func dnsRoundTripTCP(c io.ReadWriter, query dnsMsg) (dnsMsg, error) {
	b, ok := query.Pack()
	if !ok {
	return nil, errors.New("cannot marshal DNS message")
	}
	l := len(b)
	b = append([]byte{byte(l >> 8), byte(l)}, b...)
	if _, err := c.Write(b); err != nil {
	return nil, err
	}

	b = make([]byte, 1280) // 1280 is a reasonable initial size for IP over Ethernet, see RFC 4035
	if _, err := io.ReadFull(c, b[:2]); err != nil {
	return nil, err
	}
	l = int(b[0])<<8 \| int(b[1])
	if l > len(b) {
	b = make([]byte, l)
	}
	n, err := io.ReadFull(c, b[:l])
	if err != nil {
	return nil, err
	}
	resp := &dnsMsg{}
	if !resp.Unpack(b[:n]) {
	return nil, errors.New("cannot unmarshal DNS message")
	}
	if !resp.IsResponseTo(query) {
	return nil, errors.New("invalid DNS response")
	}
	return resp, nil
	}

	func (d *Dialer) dialDNS(ctx context.Context, network, server string) (dnsConn, error) {
	switch network {
	case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6":
	default:
	return nil, UnknownNetworkError(network)
	}
	// Calling Dial here is scary -- we have to be sure not to
	// dial a name that will require a DNS lookup, or Dial will
	// call back here to translate it. The DNS config parser has
	// already checked that all the cfg.servers are IP
	// addresses, which Dial will use without a DNS lookup.
	c, err := d.DialContext(ctx, network, server)
	if err != nil {
	return nil, mapErr(err)
	}
	switch network {
	case "tcp", "tcp4", "tcp6":
	return c.(*TCPConn), nil
	case "udp", "udp4", "udp6":
	return c.(*UDPConn), nil
	}
	panic("unreachable")
	}

	// exchange sends a query on the connection and hopes for a response.
	func exchange(ctx context.Context, server, name string, qtype uint16, timeout time.Duration) (*dnsMsg, error) {
	d := testHookDNSDialer()
	out := dnsMsg{
	dnsMsgHdr: dnsMsgHdr{
	recursion_desired: true,
	},
	question: []dnsQuestion{
	{name, qtype, dnsClassINET},
	},
	}
	for _, network := range []string{"udp", "tcp"} {
	// TODO(mdempsky): Refactor so defers from UDP-based
	// exchanges happen before TCP-based exchange.

	ctx, cancel := context.WithDeadline(ctx, time.Now().Add(timeout))
	defer cancel()

	c, err := d.dialDNS(ctx, network, server)
	if err != nil {
	return nil, err
	}
	defer c.Close()
	if d, ok := ctx.Deadline(); ok && !d.IsZero() {
	c.SetDeadline(d)
	}
	out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
	in, err := c.dnsRoundTrip(&out)
	if err != nil {
	return nil, mapErr(err)
	}
	if in.truncated { // see RFC 5966
	continue
	}
	return in, nil
	}
	return nil, errors.New("no answer from DNS server")
	}

	// Do a lookup for a single name, which must be rooted
	// (otherwise answer will not find the answers).
	func tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype uint16) (string, []dnsRR, error) {
	var lastErr error
	serverOffset := cfg.serverOffset()
	sLen := uint32(len(cfg.servers))

	for i := 0; i < cfg.attempts; i++ {
	for j := uint32(0); j < sLen; j++ {
	server := cfg.servers[(serverOffset+j)%sLen]

	msg, err := exchange(ctx, server, name, qtype, cfg.timeout)
	if err != nil {
	lastErr = &DNSError{
	Err: err.Error(),
	Name: name,
	Server: server,
	}
	if nerr, ok := err.(Error); ok && nerr.Timeout() {
	lastErr.(*DNSError).IsTimeout = true
	}
	continue
	}
	// libresolv continues to the next server when it receives
	// an invalid referral response. See golang.org/issue/15434.
	if msg.rcode == dnsRcodeSuccess && !msg.authoritative && !msg.recursion_available && len(msg.answer) == 0 && len(msg.extra) == 0 {
	lastErr = &DNSError{Err: "lame referral", Name: name, Server: server}
	continue
	}
	cname, rrs, err := answer(name, server, msg, qtype)
	// If answer errored for rcodes dnsRcodeSuccess or dnsRcodeNameError,
	// it means the response in msg was not useful and trying another
	// server probably won't help. Return now in those cases.
	// TODO: indicate this in a more obvious way, such as a field on DNSError?
	if err == nil \|\| msg.rcode == dnsRcodeSuccess \|\| msg.rcode == dnsRcodeNameError {
	return cname, rrs, err
	}
	lastErr = err
	}
	}
	return "", nil, lastErr
	}

	// addrRecordList converts and returns a list of IP addresses from DNS
	// address records (both A and AAAA). Other record types are ignored.
	func addrRecordList(rrs []dnsRR) []IPAddr {
	addrs := make([]IPAddr, 0, 4)
	for _, rr := range rrs {
	switch rr := rr.(type) {
	case *dnsRR_A:
	addrs = append(addrs, IPAddr{IP: IPv4(byte(rr.A>>24), byte(rr.A>>16), byte(rr.A>>8), byte(rr.A))})
	case *dnsRR_AAAA:
	ip := make(IP, IPv6len)
	copy(ip, rr.AAAA[:])
	addrs = append(addrs, IPAddr{IP: ip})
	}
	}
	return addrs
	}

	// A resolverConfig represents a DNS stub resolver configuration.
	type resolverConfig struct {
	initOnce sync.Once // guards init of resolverConfig

	// ch is used as a semaphore that only allows one lookup at a
	// time to recheck resolv.conf.
	ch chan struct{} // guards lastChecked and modTime
	lastChecked time.Time // last time resolv.conf was checked

	mu sync.RWMutex // protects dnsConfig
	dnsConfig *dnsConfig // parsed resolv.conf structure used in lookups
	}

	var resolvConf resolverConfig

	// init initializes conf and is only called via conf.initOnce.
	func (conf *resolverConfig) init() {
	// Set dnsConfig and lastChecked so we don't parse
	// resolv.conf twice the first time.
	conf.dnsConfig = systemConf().resolv
	if conf.dnsConfig == nil {
	conf.dnsConfig = dnsReadConfig("/etc/resolv.conf")
	}
	conf.lastChecked = time.Now()

	// Prepare ch so that only one update of resolverConfig may
	// run at once.
	conf.ch = make(chan struct{}, 1)
	}

	// tryUpdate tries to update conf with the named resolv.conf file.
	// The name variable only exists for testing. It is otherwise always
	// "/etc/resolv.conf".
	func (conf *resolverConfig) tryUpdate(name string) {
	conf.initOnce.Do(conf.init)

	// Ensure only one update at a time checks resolv.conf.
	if !conf.tryAcquireSema() {
	return
	}
	defer conf.releaseSema()

	now := time.Now()
	if conf.lastChecked.After(now.Add(-5 * time.Second)) {
	return
	}
	conf.lastChecked = now

	var mtime time.Time
	if fi, err := os.Stat(name); err == nil {
	mtime = fi.ModTime()
	}
	if mtime.Equal(conf.dnsConfig.mtime) {
	return
	}

	dnsConf := dnsReadConfig(name)
	conf.mu.Lock()
	conf.dnsConfig = dnsConf
	conf.mu.Unlock()
	}

	func (conf *resolverConfig) tryAcquireSema() bool {
	select {
	case conf.ch <- struct{}{}:
	return true
	default:
	return false
	}
	}

	func (conf *resolverConfig) releaseSema() {
	<-conf.ch
	}

	func lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
	if !isDomainName(name) {
	// We used to use "invalid domain name" as the error,
	// but that is a detail of the specific lookup mechanism.
	// Other lookups might allow broader name syntax
	// (for example Multicast DNS allows UTF-8; see RFC 6762).
	// For consistency with libc resolvers, report no such host.
	return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name}
	}
	resolvConf.tryUpdate("/etc/resolv.conf")
	resolvConf.mu.RLock()
	conf := resolvConf.dnsConfig
	resolvConf.mu.RUnlock()
	for _, fqdn := range conf.nameList(name) {
	cname, rrs, err = tryOneName(ctx, conf, fqdn, qtype)
	if err == nil {
	break
	}
	}
	if err, ok := err.(*DNSError); ok {
	// Show original name passed to lookup, not suffixed one.
	// In general we might have tried many suffixes; showing
	// just one is misleading. See also golang.org/issue/6324.
	err.Name = name
	}
	return
	}

	// avoidDNS reports whether this is a hostname for which we should not
	// use DNS. Currently this includes only .onion, per RFC 7686. See
	// golang.org/issue/13705. Does not cover .local names (RFC 6762),
	// see golang.org/issue/16739.
	func avoidDNS(name string) bool {
	if name == "" {
	return true
	}
	if name[len(name)-1] == '.' {
	name = name[:len(name)-1]
	}
	return stringsHasSuffixFold(name, ".onion")
	}

	// nameList returns a list of names for sequential DNS queries.
	func (conf *dnsConfig) nameList(name string) []string {
	if avoidDNS(name) {
	return nil
	}

	// If name is rooted (trailing dot), try only that name.
	rooted := len(name) > 0 && name[len(name)-1] == '.'
	if rooted {
	return []string{name}
	}

	hasNdots := count(name, '.') >= conf.ndots
	name += "."

	// Build list of search choices.
	names := make([]string, 0, 1+len(conf.search))
	// If name has enough dots, try unsuffixed first.
	if hasNdots {
	names = append(names, name)
	}
	// Try suffixes.
	for _, suffix := range conf.search {
	names = append(names, name+suffix)
	}
	// Try unsuffixed, if not tried first above.
	if !hasNdots {
	names = append(names, name)
	}
	return names
	}

	// hostLookupOrder specifies the order of LookupHost lookup strategies.
	// It is basically a simplified representation of nsswitch.conf.
	// "files" means /etc/hosts.
	type hostLookupOrder int

	const (
	// hostLookupCgo means defer to cgo.
	hostLookupCgo hostLookupOrder = iota
	hostLookupFilesDNS // files first
	hostLookupDNSFiles // dns first
	hostLookupFiles // only files
	hostLookupDNS // only DNS
	)

	var lookupOrderName = map[hostLookupOrder]string{
	hostLookupCgo: "cgo",
	hostLookupFilesDNS: "files,dns",
	hostLookupDNSFiles: "dns,files",
	hostLookupFiles: "files",
	hostLookupDNS: "dns",
	}

	func (o hostLookupOrder) String() string {
	if s, ok := lookupOrderName[o]; ok {
	return s
	}
	return "hostLookupOrder=" + itoa(int(o)) + "??"
	}

	// goLookupHost is the native Go implementation of LookupHost.
	// Used only if cgoLookupHost refuses to handle the request
	// (that is, only if cgoLookupHost is the stub in cgo_stub.go).
	// Normally we let cgo use the C library resolver instead of
	// depending on our lookup code, so that Go and C get the same
	// answers.
	func goLookupHost(ctx context.Context, name string) (addrs []string, err error) {
	return goLookupHostOrder(ctx, name, hostLookupFilesDNS)
	}

	func goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []string, err error) {
	if order == hostLookupFilesDNS \|\| order == hostLookupFiles {
	// Use entries from /etc/hosts if they match.
	addrs = lookupStaticHost(name)
	if len(addrs) > 0 \|\| order == hostLookupFiles {
	return
	}
	}
	ips, err := goLookupIPOrder(ctx, name, order)
	if err != nil {
	return
	}
	addrs = make([]string, 0, len(ips))
	for _, ip := range ips {
	addrs = append(addrs, ip.String())
	}
	return
	}

	// lookup entries from /etc/hosts
	func goLookupIPFiles(name string) (addrs []IPAddr) {
	for _, haddr := range lookupStaticHost(name) {
	haddr, zone := splitHostZone(haddr)
	if ip := ParseIP(haddr); ip != nil {
	addr := IPAddr{IP: ip, Zone: zone}
	addrs = append(addrs, addr)
	}
	}
	sortByRFC6724(addrs)
	return
	}

	// goLookupIP is the native Go implementation of LookupIP.
	// The libc versions are in cgo_*.go.
	func goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
	order := systemConf().hostLookupOrder(host)
	return goLookupIPOrder(ctx, host, order)
	}

	func goLookupIPOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, err error) {
	if order == hostLookupFilesDNS \|\| order == hostLookupFiles {
	addrs = goLookupIPFiles(name)
	if len(addrs) > 0 \|\| order == hostLookupFiles {
	return addrs, nil
	}
	}
	if !isDomainName(name) {
	// See comment in func lookup above about use of errNoSuchHost.
	return nil, &DNSError{Err: errNoSuchHost.Error(), Name: name}
	}
	resolvConf.tryUpdate("/etc/resolv.conf")
	resolvConf.mu.RLock()
	conf := resolvConf.dnsConfig
	resolvConf.mu.RUnlock()
	type racer struct {
	fqdn string
	rrs []dnsRR
	error
	}
	lane := make(chan racer, 1)
	qtypes := [...]uint16{dnsTypeA, dnsTypeAAAA}
	var lastErr error
	for _, fqdn := range conf.nameList(name) {
	for _, qtype := range qtypes {
	go func(qtype uint16) {
	_, rrs, err := tryOneName(ctx, conf, fqdn, qtype)
	lane <- racer{fqdn, rrs, err}
	}(qtype)
	}
	for range qtypes {
	racer := <-lane
	if racer.error != nil {
	// Prefer error for original name.
	if lastErr == nil \|\| racer.fqdn == name+"." {
	lastErr = racer.error
	}
	continue
	}
	addrs = append(addrs, addrRecordList(racer.rrs)...)
	}
	if len(addrs) > 0 {
	break
	}
	}
	if lastErr, ok := lastErr.(*DNSError); ok {
	// Show original name passed to lookup, not suffixed one.
	// In general we might have tried many suffixes; showing
	// just one is misleading. See also golang.org/issue/6324.
	lastErr.Name = name
	}
	sortByRFC6724(addrs)
	if len(addrs) == 0 {
	if order == hostLookupDNSFiles {
	addrs = goLookupIPFiles(name)
	}
	if len(addrs) == 0 && lastErr != nil {
	return nil, lastErr
	}
	}
	return addrs, nil
	}

	// goLookupCNAME is the native Go implementation of LookupCNAME.
	// Used only if cgoLookupCNAME refuses to handle the request
	// (that is, only if cgoLookupCNAME is the stub in cgo_stub.go).
	// Normally we let cgo use the C library resolver instead of
	// depending on our lookup code, so that Go and C get the same
	// answers.
	func goLookupCNAME(ctx context.Context, name string) (cname string, err error) {
	_, rrs, err := lookup(ctx, name, dnsTypeCNAME)
	if err != nil {
	return
	}
	cname = rrs[0].(*dnsRR_CNAME).Cname
	return
	}

	// goLookupPTR is the native Go implementation of LookupAddr.
	// Used only if cgoLookupPTR refuses to handle the request (that is,
	// only if cgoLookupPTR is the stub in cgo_stub.go).
	// Normally we let cgo use the C library resolver instead of depending
	// on our lookup code, so that Go and C get the same answers.
	func goLookupPTR(ctx context.Context, addr string) ([]string, error) {
	names := lookupStaticAddr(addr)
	if len(names) > 0 {
	return names, nil
	}
	arpa, err := reverseaddr(addr)
	if err != nil {
	return nil, err
	}
	_, rrs, err := lookup(ctx, arpa, dnsTypePTR)
	if err != nil {
	return nil, err
	}
	ptrs := make([]string, len(rrs))
	for i, rr := range rrs {
	ptrs[i] = rr.(*dnsRR_PTR).Ptr
	}
	return ptrs, nil
	}