Source file src/net/dnsclient_unix.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // DNS client: see RFC 1035.
     6  // Has to be linked into package net for Dial.
     7  
     8  // TODO(rsc):
     9  //	Could potentially handle many outstanding lookups faster.
    10  //	Random UDP source port (net.Dial should do that for us).
    11  //	Random request IDs.
    12  
    13  package net
    14  
    15  import (
    16  	"context"
    17  	"errors"
    18  	"internal/bytealg"
    19  	"internal/itoa"
    20  	"io"
    21  	"os"
    22  	"runtime"
    23  	"sync"
    24  	"sync/atomic"
    25  	"time"
    26  
    27  	"golang.org/x/net/dns/dnsmessage"
    28  )
    29  
    30  const (
    31  	// to be used as a useTCP parameter to exchange
    32  	useTCPOnly  = true
    33  	useUDPOrTCP = false
    34  
    35  	// Maximum DNS packet size.
    36  	// Value taken from https://dnsflagday.net/2020/.
    37  	maxDNSPacketSize = 1232
    38  )
    39  
    40  var (
    41  	errLameReferral              = errors.New("lame referral")
    42  	errCannotUnmarshalDNSMessage = errors.New("cannot unmarshal DNS message")
    43  	errCannotMarshalDNSMessage   = errors.New("cannot marshal DNS message")
    44  	errServerMisbehaving         = errors.New("server misbehaving")
    45  	errInvalidDNSResponse        = errors.New("invalid DNS response")
    46  	errNoAnswerFromDNSServer     = errors.New("no answer from DNS server")
    47  
    48  	// errServerTemporarilyMisbehaving is like errServerMisbehaving, except
    49  	// that when it gets translated to a DNSError, the IsTemporary field
    50  	// gets set to true.
    51  	errServerTemporarilyMisbehaving = &temporaryError{"server misbehaving"}
    52  )
    53  
    54  func newRequest(q dnsmessage.Question, ad bool) (id uint16, udpReq, tcpReq []byte, err error) {
    55  	id = uint16(randInt())
    56  	b := dnsmessage.NewBuilder(make([]byte, 2, 514), dnsmessage.Header{ID: id, RecursionDesired: true, AuthenticData: ad})
    57  	if err := b.StartQuestions(); err != nil {
    58  		return 0, nil, nil, err
    59  	}
    60  	if err := b.Question(q); err != nil {
    61  		return 0, nil, nil, err
    62  	}
    63  
    64  	// Accept packets up to maxDNSPacketSize.  RFC 6891.
    65  	if err := b.StartAdditionals(); err != nil {
    66  		return 0, nil, nil, err
    67  	}
    68  	var rh dnsmessage.ResourceHeader
    69  	if err := rh.SetEDNS0(maxDNSPacketSize, dnsmessage.RCodeSuccess, false); err != nil {
    70  		return 0, nil, nil, err
    71  	}
    72  	if err := b.OPTResource(rh, dnsmessage.OPTResource{}); err != nil {
    73  		return 0, nil, nil, err
    74  	}
    75  
    76  	tcpReq, err = b.Finish()
    77  	if err != nil {
    78  		return 0, nil, nil, err
    79  	}
    80  	udpReq = tcpReq[2:]
    81  	l := len(tcpReq) - 2
    82  	tcpReq[0] = byte(l >> 8)
    83  	tcpReq[1] = byte(l)
    84  	return id, udpReq, tcpReq, nil
    85  }
    86  
    87  func checkResponse(reqID uint16, reqQues dnsmessage.Question, respHdr dnsmessage.Header, respQues dnsmessage.Question) bool {
    88  	if !respHdr.Response {
    89  		return false
    90  	}
    91  	if reqID != respHdr.ID {
    92  		return false
    93  	}
    94  	if reqQues.Type != respQues.Type || reqQues.Class != respQues.Class || !equalASCIIName(reqQues.Name, respQues.Name) {
    95  		return false
    96  	}
    97  	return true
    98  }
    99  
   100  func dnsPacketRoundTrip(c Conn, id uint16, query dnsmessage.Question, b []byte) (dnsmessage.Parser, dnsmessage.Header, error) {
   101  	if _, err := c.Write(b); err != nil {
   102  		return dnsmessage.Parser{}, dnsmessage.Header{}, err
   103  	}
   104  
   105  	b = make([]byte, maxDNSPacketSize)
   106  	for {
   107  		n, err := c.Read(b)
   108  		if err != nil {
   109  			return dnsmessage.Parser{}, dnsmessage.Header{}, err
   110  		}
   111  		var p dnsmessage.Parser
   112  		// Ignore invalid responses as they may be malicious
   113  		// forgery attempts. Instead continue waiting until
   114  		// timeout. See golang.org/issue/13281.
   115  		h, err := p.Start(b[:n])
   116  		if err != nil {
   117  			continue
   118  		}
   119  		q, err := p.Question()
   120  		if err != nil || !checkResponse(id, query, h, q) {
   121  			continue
   122  		}
   123  		return p, h, nil
   124  	}
   125  }
   126  
   127  func dnsStreamRoundTrip(c Conn, id uint16, query dnsmessage.Question, b []byte) (dnsmessage.Parser, dnsmessage.Header, error) {
   128  	if _, err := c.Write(b); err != nil {
   129  		return dnsmessage.Parser{}, dnsmessage.Header{}, err
   130  	}
   131  
   132  	b = make([]byte, 1280) // 1280 is a reasonable initial size for IP over Ethernet, see RFC 4035
   133  	if _, err := io.ReadFull(c, b[:2]); err != nil {
   134  		return dnsmessage.Parser{}, dnsmessage.Header{}, err
   135  	}
   136  	l := int(b[0])<<8 | int(b[1])
   137  	if l > len(b) {
   138  		b = make([]byte, l)
   139  	}
   140  	n, err := io.ReadFull(c, b[:l])
   141  	if err != nil {
   142  		return dnsmessage.Parser{}, dnsmessage.Header{}, err
   143  	}
   144  	var p dnsmessage.Parser
   145  	h, err := p.Start(b[:n])
   146  	if err != nil {
   147  		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotUnmarshalDNSMessage
   148  	}
   149  	q, err := p.Question()
   150  	if err != nil {
   151  		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotUnmarshalDNSMessage
   152  	}
   153  	if !checkResponse(id, query, h, q) {
   154  		return dnsmessage.Parser{}, dnsmessage.Header{}, errInvalidDNSResponse
   155  	}
   156  	return p, h, nil
   157  }
   158  
   159  // exchange sends a query on the connection and hopes for a response.
   160  func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Question, timeout time.Duration, useTCP, ad bool) (dnsmessage.Parser, dnsmessage.Header, error) {
   161  	q.Class = dnsmessage.ClassINET
   162  	id, udpReq, tcpReq, err := newRequest(q, ad)
   163  	if err != nil {
   164  		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotMarshalDNSMessage
   165  	}
   166  	var networks []string
   167  	if useTCP {
   168  		networks = []string{"tcp"}
   169  	} else {
   170  		networks = []string{"udp", "tcp"}
   171  	}
   172  	for _, network := range networks {
   173  		ctx, cancel := context.WithDeadline(ctx, time.Now().Add(timeout))
   174  		defer cancel()
   175  
   176  		c, err := r.dial(ctx, network, server)
   177  		if err != nil {
   178  			return dnsmessage.Parser{}, dnsmessage.Header{}, err
   179  		}
   180  		if d, ok := ctx.Deadline(); ok && !d.IsZero() {
   181  			c.SetDeadline(d)
   182  		}
   183  		var p dnsmessage.Parser
   184  		var h dnsmessage.Header
   185  		if _, ok := c.(PacketConn); ok {
   186  			p, h, err = dnsPacketRoundTrip(c, id, q, udpReq)
   187  		} else {
   188  			p, h, err = dnsStreamRoundTrip(c, id, q, tcpReq)
   189  		}
   190  		c.Close()
   191  		if err != nil {
   192  			return dnsmessage.Parser{}, dnsmessage.Header{}, mapErr(err)
   193  		}
   194  		if err := p.SkipQuestion(); err != dnsmessage.ErrSectionDone {
   195  			return dnsmessage.Parser{}, dnsmessage.Header{}, errInvalidDNSResponse
   196  		}
   197  		// RFC 5966 indicates that when a client receives a UDP response with
   198  		// the TC flag set, it should take the TC flag as an indication that it
   199  		// should retry over TCP instead.
   200  		// The case when the TC flag is set in a TCP response is not well specified,
   201  		// so this implements the glibc resolver behavior, returning the existing
   202  		// dns response instead of returning a "errNoAnswerFromDNSServer" error.
   203  		// See go.dev/issue/64896
   204  		if h.Truncated && network == "udp" {
   205  			continue
   206  		}
   207  		return p, h, nil
   208  	}
   209  	return dnsmessage.Parser{}, dnsmessage.Header{}, errNoAnswerFromDNSServer
   210  }
   211  
   212  // checkHeader performs basic sanity checks on the header.
   213  func checkHeader(p *dnsmessage.Parser, h dnsmessage.Header) error {
   214  	rcode, hasAdd := extractExtendedRCode(*p, h)
   215  
   216  	if rcode == dnsmessage.RCodeNameError {
   217  		return errNoSuchHost
   218  	}
   219  
   220  	_, err := p.AnswerHeader()
   221  	if err != nil && err != dnsmessage.ErrSectionDone {
   222  		return errCannotUnmarshalDNSMessage
   223  	}
   224  
   225  	// libresolv continues to the next server when it receives
   226  	// an invalid referral response. See golang.org/issue/15434.
   227  	if rcode == dnsmessage.RCodeSuccess && !h.Authoritative && !h.RecursionAvailable && err == dnsmessage.ErrSectionDone && !hasAdd {
   228  		return errLameReferral
   229  	}
   230  
   231  	if rcode != dnsmessage.RCodeSuccess && rcode != dnsmessage.RCodeNameError {
   232  		// None of the error codes make sense
   233  		// for the query we sent. If we didn't get
   234  		// a name error and we didn't get success,
   235  		// the server is behaving incorrectly or
   236  		// having temporary trouble.
   237  		if rcode == dnsmessage.RCodeServerFailure {
   238  			return errServerTemporarilyMisbehaving
   239  		}
   240  		return errServerMisbehaving
   241  	}
   242  
   243  	return nil
   244  }
   245  
   246  func skipToAnswer(p *dnsmessage.Parser, qtype dnsmessage.Type) error {
   247  	for {
   248  		h, err := p.AnswerHeader()
   249  		if err == dnsmessage.ErrSectionDone {
   250  			return errNoSuchHost
   251  		}
   252  		if err != nil {
   253  			return errCannotUnmarshalDNSMessage
   254  		}
   255  		if h.Type == qtype {
   256  			return nil
   257  		}
   258  		if err := p.SkipAnswer(); err != nil {
   259  			return errCannotUnmarshalDNSMessage
   260  		}
   261  	}
   262  }
   263  
   264  // extractExtendedRCode extracts the extended RCode from the OPT resource (EDNS(0))
   265  // If an OPT record is not found, the RCode from the hdr is returned.
   266  // Another return value indicates whether an additional resource was found.
   267  func extractExtendedRCode(p dnsmessage.Parser, hdr dnsmessage.Header) (dnsmessage.RCode, bool) {
   268  	p.SkipAllAnswers()
   269  	p.SkipAllAuthorities()
   270  	hasAdd := false
   271  	for {
   272  		ahdr, err := p.AdditionalHeader()
   273  		if err != nil {
   274  			return hdr.RCode, hasAdd
   275  		}
   276  		hasAdd = true
   277  		if ahdr.Type == dnsmessage.TypeOPT {
   278  			return ahdr.ExtendedRCode(hdr.RCode), hasAdd
   279  		}
   280  		if err := p.SkipAdditional(); err != nil {
   281  			return hdr.RCode, hasAdd
   282  		}
   283  	}
   284  }
   285  
   286  // Do a lookup for a single name, which must be rooted
   287  // (otherwise answer will not find the answers).
   288  func (r *Resolver) tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype dnsmessage.Type) (dnsmessage.Parser, string, error) {
   289  	var lastErr error
   290  	serverOffset := cfg.serverOffset()
   291  	sLen := uint32(len(cfg.servers))
   292  
   293  	n, err := dnsmessage.NewName(name)
   294  	if err != nil {
   295  		return dnsmessage.Parser{}, "", &DNSError{Err: errCannotMarshalDNSMessage.Error(), Name: name}
   296  	}
   297  	q := dnsmessage.Question{
   298  		Name:  n,
   299  		Type:  qtype,
   300  		Class: dnsmessage.ClassINET,
   301  	}
   302  
   303  	for i := 0; i < cfg.attempts; i++ {
   304  		for j := uint32(0); j < sLen; j++ {
   305  			server := cfg.servers[(serverOffset+j)%sLen]
   306  
   307  			p, h, err := r.exchange(ctx, server, q, cfg.timeout, cfg.useTCP, cfg.trustAD)
   308  			if err != nil {
   309  				dnsErr := newDNSError(err, name, server)
   310  				// Set IsTemporary for socket-level errors. Note that this flag
   311  				// may also be used to indicate a SERVFAIL response.
   312  				if _, ok := err.(*OpError); ok {
   313  					dnsErr.IsTemporary = true
   314  				}
   315  				lastErr = dnsErr
   316  				continue
   317  			}
   318  
   319  			if err := checkHeader(&p, h); err != nil {
   320  				if err == errNoSuchHost {
   321  					// The name does not exist, so trying
   322  					// another server won't help.
   323  					return p, server, newDNSError(errNoSuchHost, name, server)
   324  				}
   325  				lastErr = newDNSError(err, name, server)
   326  				continue
   327  			}
   328  
   329  			if err := skipToAnswer(&p, qtype); err != nil {
   330  				if err == errNoSuchHost {
   331  					// The name does not exist, so trying
   332  					// another server won't help.
   333  					return p, server, newDNSError(errNoSuchHost, name, server)
   334  				}
   335  				lastErr = newDNSError(err, name, server)
   336  				continue
   337  			}
   338  
   339  			return p, server, nil
   340  		}
   341  	}
   342  	return dnsmessage.Parser{}, "", lastErr
   343  }
   344  
   345  // A resolverConfig represents a DNS stub resolver configuration.
   346  type resolverConfig struct {
   347  	initOnce sync.Once // guards init of resolverConfig
   348  
   349  	// ch is used as a semaphore that only allows one lookup at a
   350  	// time to recheck resolv.conf.
   351  	ch          chan struct{} // guards lastChecked and modTime
   352  	lastChecked time.Time     // last time resolv.conf was checked
   353  
   354  	dnsConfig atomic.Pointer[dnsConfig] // parsed resolv.conf structure used in lookups
   355  }
   356  
   357  var resolvConf resolverConfig
   358  
   359  func getSystemDNSConfig() *dnsConfig {
   360  	resolvConf.tryUpdate("/etc/resolv.conf")
   361  	return resolvConf.dnsConfig.Load()
   362  }
   363  
   364  // init initializes conf and is only called via conf.initOnce.
   365  func (conf *resolverConfig) init() {
   366  	// Set dnsConfig and lastChecked so we don't parse
   367  	// resolv.conf twice the first time.
   368  	conf.dnsConfig.Store(dnsReadConfig("/etc/resolv.conf"))
   369  	conf.lastChecked = time.Now()
   370  
   371  	// Prepare ch so that only one update of resolverConfig may
   372  	// run at once.
   373  	conf.ch = make(chan struct{}, 1)
   374  }
   375  
   376  // tryUpdate tries to update conf with the named resolv.conf file.
   377  // The name variable only exists for testing. It is otherwise always
   378  // "/etc/resolv.conf".
   379  func (conf *resolverConfig) tryUpdate(name string) {
   380  	conf.initOnce.Do(conf.init)
   381  
   382  	if conf.dnsConfig.Load().noReload {
   383  		return
   384  	}
   385  
   386  	// Ensure only one update at a time checks resolv.conf.
   387  	if !conf.tryAcquireSema() {
   388  		return
   389  	}
   390  	defer conf.releaseSema()
   391  
   392  	now := time.Now()
   393  	if conf.lastChecked.After(now.Add(-5 * time.Second)) {
   394  		return
   395  	}
   396  	conf.lastChecked = now
   397  
   398  	switch runtime.GOOS {
   399  	case "windows":
   400  		// There's no file on disk, so don't bother checking
   401  		// and failing.
   402  		//
   403  		// The Windows implementation of dnsReadConfig (called
   404  		// below) ignores the name.
   405  	default:
   406  		var mtime time.Time
   407  		if fi, err := os.Stat(name); err == nil {
   408  			mtime = fi.ModTime()
   409  		}
   410  		if mtime.Equal(conf.dnsConfig.Load().mtime) {
   411  			return
   412  		}
   413  	}
   414  
   415  	dnsConf := dnsReadConfig(name)
   416  	conf.dnsConfig.Store(dnsConf)
   417  }
   418  
   419  func (conf *resolverConfig) tryAcquireSema() bool {
   420  	select {
   421  	case conf.ch <- struct{}{}:
   422  		return true
   423  	default:
   424  		return false
   425  	}
   426  }
   427  
   428  func (conf *resolverConfig) releaseSema() {
   429  	<-conf.ch
   430  }
   431  
   432  func (r *Resolver) lookup(ctx context.Context, name string, qtype dnsmessage.Type, conf *dnsConfig) (dnsmessage.Parser, string, error) {
   433  	if !isDomainName(name) {
   434  		// We used to use "invalid domain name" as the error,
   435  		// but that is a detail of the specific lookup mechanism.
   436  		// Other lookups might allow broader name syntax
   437  		// (for example Multicast DNS allows UTF-8; see RFC 6762).
   438  		// For consistency with libc resolvers, report no such host.
   439  		return dnsmessage.Parser{}, "", newDNSError(errNoSuchHost, name, "")
   440  	}
   441  
   442  	if conf == nil {
   443  		conf = getSystemDNSConfig()
   444  	}
   445  
   446  	var (
   447  		p      dnsmessage.Parser
   448  		server string
   449  		err    error
   450  	)
   451  	for _, fqdn := range conf.nameList(name) {
   452  		p, server, err = r.tryOneName(ctx, conf, fqdn, qtype)
   453  		if err == nil {
   454  			break
   455  		}
   456  		if nerr, ok := err.(Error); ok && nerr.Temporary() && r.strictErrors() {
   457  			// If we hit a temporary error with StrictErrors enabled,
   458  			// stop immediately instead of trying more names.
   459  			break
   460  		}
   461  	}
   462  	if err == nil {
   463  		return p, server, nil
   464  	}
   465  	if err, ok := err.(*DNSError); ok {
   466  		// Show original name passed to lookup, not suffixed one.
   467  		// In general we might have tried many suffixes; showing
   468  		// just one is misleading. See also golang.org/issue/6324.
   469  		err.Name = name
   470  	}
   471  	return dnsmessage.Parser{}, "", err
   472  }
   473  
   474  // avoidDNS reports whether this is a hostname for which we should not
   475  // use DNS. Currently this includes only .onion, per RFC 7686. See
   476  // golang.org/issue/13705. Does not cover .local names (RFC 6762),
   477  // see golang.org/issue/16739.
   478  func avoidDNS(name string) bool {
   479  	if name == "" {
   480  		return true
   481  	}
   482  	if name[len(name)-1] == '.' {
   483  		name = name[:len(name)-1]
   484  	}
   485  	return stringsHasSuffixFold(name, ".onion")
   486  }
   487  
   488  // nameList returns a list of names for sequential DNS queries.
   489  func (conf *dnsConfig) nameList(name string) []string {
   490  	// Check name length (see isDomainName).
   491  	l := len(name)
   492  	rooted := l > 0 && name[l-1] == '.'
   493  	if l > 254 || l == 254 && !rooted {
   494  		return nil
   495  	}
   496  
   497  	// If name is rooted (trailing dot), try only that name.
   498  	if rooted {
   499  		if avoidDNS(name) {
   500  			return nil
   501  		}
   502  		return []string{name}
   503  	}
   504  
   505  	hasNdots := bytealg.CountString(name, '.') >= conf.ndots
   506  	name += "."
   507  	l++
   508  
   509  	// Build list of search choices.
   510  	names := make([]string, 0, 1+len(conf.search))
   511  	// If name has enough dots, try unsuffixed first.
   512  	if hasNdots && !avoidDNS(name) {
   513  		names = append(names, name)
   514  	}
   515  	// Try suffixes that are not too long (see isDomainName).
   516  	for _, suffix := range conf.search {
   517  		fqdn := name + suffix
   518  		if !avoidDNS(fqdn) && len(fqdn) <= 254 {
   519  			names = append(names, fqdn)
   520  		}
   521  	}
   522  	// Try unsuffixed, if not tried first above.
   523  	if !hasNdots && !avoidDNS(name) {
   524  		names = append(names, name)
   525  	}
   526  	return names
   527  }
   528  
   529  // hostLookupOrder specifies the order of LookupHost lookup strategies.
   530  // It is basically a simplified representation of nsswitch.conf.
   531  // "files" means /etc/hosts.
   532  type hostLookupOrder int
   533  
   534  const (
   535  	// hostLookupCgo means defer to cgo.
   536  	hostLookupCgo      hostLookupOrder = iota
   537  	hostLookupFilesDNS                 // files first
   538  	hostLookupDNSFiles                 // dns first
   539  	hostLookupFiles                    // only files
   540  	hostLookupDNS                      // only DNS
   541  )
   542  
   543  var lookupOrderName = map[hostLookupOrder]string{
   544  	hostLookupCgo:      "cgo",
   545  	hostLookupFilesDNS: "files,dns",
   546  	hostLookupDNSFiles: "dns,files",
   547  	hostLookupFiles:    "files",
   548  	hostLookupDNS:      "dns",
   549  }
   550  
   551  func (o hostLookupOrder) String() string {
   552  	if s, ok := lookupOrderName[o]; ok {
   553  		return s
   554  	}
   555  	return "hostLookupOrder=" + itoa.Itoa(int(o)) + "??"
   556  }
   557  
   558  func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder, conf *dnsConfig) (addrs []string, err error) {
   559  	if order == hostLookupFilesDNS || order == hostLookupFiles {
   560  		// Use entries from /etc/hosts if they match.
   561  		addrs, _ = lookupStaticHost(name)
   562  		if len(addrs) > 0 {
   563  			return
   564  		}
   565  
   566  		if order == hostLookupFiles {
   567  			return nil, newDNSError(errNoSuchHost, name, "")
   568  		}
   569  	}
   570  	ips, _, err := r.goLookupIPCNAMEOrder(ctx, "ip", name, order, conf)
   571  	if err != nil {
   572  		return
   573  	}
   574  	addrs = make([]string, 0, len(ips))
   575  	for _, ip := range ips {
   576  		addrs = append(addrs, ip.String())
   577  	}
   578  	return
   579  }
   580  
   581  // lookup entries from /etc/hosts
   582  func goLookupIPFiles(name string) (addrs []IPAddr, canonical string) {
   583  	addr, canonical := lookupStaticHost(name)
   584  	for _, haddr := range addr {
   585  		haddr, zone := splitHostZone(haddr)
   586  		if ip := ParseIP(haddr); ip != nil {
   587  			addr := IPAddr{IP: ip, Zone: zone}
   588  			addrs = append(addrs, addr)
   589  		}
   590  	}
   591  	sortByRFC6724(addrs)
   592  	return addrs, canonical
   593  }
   594  
   595  // goLookupIP is the native Go implementation of LookupIP.
   596  // The libc versions are in cgo_*.go.
   597  func (r *Resolver) goLookupIP(ctx context.Context, network, host string, order hostLookupOrder, conf *dnsConfig) (addrs []IPAddr, err error) {
   598  	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, network, host, order, conf)
   599  	return
   600  }
   601  
   602  func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, network, name string, order hostLookupOrder, conf *dnsConfig) (addrs []IPAddr, cname dnsmessage.Name, err error) {
   603  	if order == hostLookupFilesDNS || order == hostLookupFiles {
   604  		var canonical string
   605  		addrs, canonical = goLookupIPFiles(name)
   606  
   607  		if len(addrs) > 0 {
   608  			var err error
   609  			cname, err = dnsmessage.NewName(canonical)
   610  			if err != nil {
   611  				return nil, dnsmessage.Name{}, err
   612  			}
   613  			return addrs, cname, nil
   614  		}
   615  
   616  		if order == hostLookupFiles {
   617  			return nil, dnsmessage.Name{}, newDNSError(errNoSuchHost, name, "")
   618  		}
   619  	}
   620  
   621  	if !isDomainName(name) {
   622  		// See comment in func lookup above about use of errNoSuchHost.
   623  		return nil, dnsmessage.Name{}, newDNSError(errNoSuchHost, name, "")
   624  	}
   625  	type result struct {
   626  		p      dnsmessage.Parser
   627  		server string
   628  		error
   629  	}
   630  
   631  	if conf == nil {
   632  		conf = getSystemDNSConfig()
   633  	}
   634  
   635  	lane := make(chan result, 1)
   636  	qtypes := []dnsmessage.Type{dnsmessage.TypeA, dnsmessage.TypeAAAA}
   637  	if network == "CNAME" {
   638  		qtypes = append(qtypes, dnsmessage.TypeCNAME)
   639  	}
   640  	switch ipVersion(network) {
   641  	case '4':
   642  		qtypes = []dnsmessage.Type{dnsmessage.TypeA}
   643  	case '6':
   644  		qtypes = []dnsmessage.Type{dnsmessage.TypeAAAA}
   645  	}
   646  	var queryFn func(fqdn string, qtype dnsmessage.Type)
   647  	var responseFn func(fqdn string, qtype dnsmessage.Type) result
   648  	if conf.singleRequest {
   649  		queryFn = func(fqdn string, qtype dnsmessage.Type) {}
   650  		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
   651  			dnsWaitGroup.Add(1)
   652  			defer dnsWaitGroup.Done()
   653  			p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
   654  			return result{p, server, err}
   655  		}
   656  	} else {
   657  		queryFn = func(fqdn string, qtype dnsmessage.Type) {
   658  			dnsWaitGroup.Add(1)
   659  			go func(qtype dnsmessage.Type) {
   660  				p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
   661  				lane <- result{p, server, err}
   662  				dnsWaitGroup.Done()
   663  			}(qtype)
   664  		}
   665  		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
   666  			return <-lane
   667  		}
   668  	}
   669  	var lastErr error
   670  	for _, fqdn := range conf.nameList(name) {
   671  		for _, qtype := range qtypes {
   672  			queryFn(fqdn, qtype)
   673  		}
   674  		hitStrictError := false
   675  		for _, qtype := range qtypes {
   676  			result := responseFn(fqdn, qtype)
   677  			if result.error != nil {
   678  				if nerr, ok := result.error.(Error); ok && nerr.Temporary() && r.strictErrors() {
   679  					// This error will abort the nameList loop.
   680  					hitStrictError = true
   681  					lastErr = result.error
   682  				} else if lastErr == nil || fqdn == name+"." {
   683  					// Prefer error for original name.
   684  					lastErr = result.error
   685  				}
   686  				continue
   687  			}
   688  
   689  			// Presotto says it's okay to assume that servers listed in
   690  			// /etc/resolv.conf are recursive resolvers.
   691  			//
   692  			// We asked for recursion, so it should have included all the
   693  			// answers we need in this one packet.
   694  			//
   695  			// Further, RFC 1034 section 4.3.1 says that "the recursive
   696  			// response to a query will be... The answer to the query,
   697  			// possibly preface by one or more CNAME RRs that specify
   698  			// aliases encountered on the way to an answer."
   699  			//
   700  			// Therefore, we should be able to assume that we can ignore
   701  			// CNAMEs and that the A and AAAA records we requested are
   702  			// for the canonical name.
   703  
   704  		loop:
   705  			for {
   706  				h, err := result.p.AnswerHeader()
   707  				if err != nil && err != dnsmessage.ErrSectionDone {
   708  					lastErr = &DNSError{
   709  						Err:    errCannotUnmarshalDNSMessage.Error(),
   710  						Name:   name,
   711  						Server: result.server,
   712  					}
   713  				}
   714  				if err != nil {
   715  					break
   716  				}
   717  				switch h.Type {
   718  				case dnsmessage.TypeA:
   719  					a, err := result.p.AResource()
   720  					if err != nil {
   721  						lastErr = &DNSError{
   722  							Err:    errCannotUnmarshalDNSMessage.Error(),
   723  							Name:   name,
   724  							Server: result.server,
   725  						}
   726  						break loop
   727  					}
   728  					addrs = append(addrs, IPAddr{IP: IP(a.A[:])})
   729  					if cname.Length == 0 && h.Name.Length != 0 {
   730  						cname = h.Name
   731  					}
   732  
   733  				case dnsmessage.TypeAAAA:
   734  					aaaa, err := result.p.AAAAResource()
   735  					if err != nil {
   736  						lastErr = &DNSError{
   737  							Err:    errCannotUnmarshalDNSMessage.Error(),
   738  							Name:   name,
   739  							Server: result.server,
   740  						}
   741  						break loop
   742  					}
   743  					addrs = append(addrs, IPAddr{IP: IP(aaaa.AAAA[:])})
   744  					if cname.Length == 0 && h.Name.Length != 0 {
   745  						cname = h.Name
   746  					}
   747  
   748  				case dnsmessage.TypeCNAME:
   749  					c, err := result.p.CNAMEResource()
   750  					if err != nil {
   751  						lastErr = &DNSError{
   752  							Err:    errCannotUnmarshalDNSMessage.Error(),
   753  							Name:   name,
   754  							Server: result.server,
   755  						}
   756  						break loop
   757  					}
   758  					if cname.Length == 0 && c.CNAME.Length > 0 {
   759  						cname = c.CNAME
   760  					}
   761  
   762  				default:
   763  					if err := result.p.SkipAnswer(); err != nil {
   764  						lastErr = &DNSError{
   765  							Err:    errCannotUnmarshalDNSMessage.Error(),
   766  							Name:   name,
   767  							Server: result.server,
   768  						}
   769  						break loop
   770  					}
   771  					continue
   772  				}
   773  			}
   774  		}
   775  		if hitStrictError {
   776  			// If either family hit an error with StrictErrors enabled,
   777  			// discard all addresses. This ensures that network flakiness
   778  			// cannot turn a dualstack hostname IPv4/IPv6-only.
   779  			addrs = nil
   780  			break
   781  		}
   782  		if len(addrs) > 0 || network == "CNAME" && cname.Length > 0 {
   783  			break
   784  		}
   785  	}
   786  	if lastErr, ok := lastErr.(*DNSError); ok {
   787  		// Show original name passed to lookup, not suffixed one.
   788  		// In general we might have tried many suffixes; showing
   789  		// just one is misleading. See also golang.org/issue/6324.
   790  		lastErr.Name = name
   791  	}
   792  	sortByRFC6724(addrs)
   793  	if len(addrs) == 0 && !(network == "CNAME" && cname.Length > 0) {
   794  		if order == hostLookupDNSFiles {
   795  			var canonical string
   796  			addrs, canonical = goLookupIPFiles(name)
   797  			if len(addrs) > 0 {
   798  				var err error
   799  				cname, err = dnsmessage.NewName(canonical)
   800  				if err != nil {
   801  					return nil, dnsmessage.Name{}, err
   802  				}
   803  				return addrs, cname, nil
   804  			}
   805  		}
   806  		if lastErr != nil {
   807  			return nil, dnsmessage.Name{}, lastErr
   808  		}
   809  	}
   810  	return addrs, cname, nil
   811  }
   812  
   813  // goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
   814  func (r *Resolver) goLookupCNAME(ctx context.Context, host string, order hostLookupOrder, conf *dnsConfig) (string, error) {
   815  	_, cname, err := r.goLookupIPCNAMEOrder(ctx, "CNAME", host, order, conf)
   816  	return cname.String(), err
   817  }
   818  
   819  // goLookupPTR is the native Go implementation of LookupAddr.
   820  func (r *Resolver) goLookupPTR(ctx context.Context, addr string, order hostLookupOrder, conf *dnsConfig) ([]string, error) {
   821  	if order == hostLookupFiles || order == hostLookupFilesDNS {
   822  		names := lookupStaticAddr(addr)
   823  		if len(names) > 0 {
   824  			return names, nil
   825  		}
   826  
   827  		if order == hostLookupFiles {
   828  			return nil, newDNSError(errNoSuchHost, addr, "")
   829  		}
   830  	}
   831  
   832  	arpa, err := reverseaddr(addr)
   833  	if err != nil {
   834  		return nil, err
   835  	}
   836  	p, server, err := r.lookup(ctx, arpa, dnsmessage.TypePTR, conf)
   837  	if err != nil {
   838  		var dnsErr *DNSError
   839  		if errors.As(err, &dnsErr) && dnsErr.IsNotFound {
   840  			if order == hostLookupDNSFiles {
   841  				names := lookupStaticAddr(addr)
   842  				if len(names) > 0 {
   843  					return names, nil
   844  				}
   845  			}
   846  		}
   847  		return nil, err
   848  	}
   849  	var ptrs []string
   850  	for {
   851  		h, err := p.AnswerHeader()
   852  		if err == dnsmessage.ErrSectionDone {
   853  			break
   854  		}
   855  		if err != nil {
   856  			return nil, &DNSError{
   857  				Err:    errCannotUnmarshalDNSMessage.Error(),
   858  				Name:   addr,
   859  				Server: server,
   860  			}
   861  		}
   862  		if h.Type != dnsmessage.TypePTR {
   863  			err := p.SkipAnswer()
   864  			if err != nil {
   865  				return nil, &DNSError{
   866  					Err:    errCannotUnmarshalDNSMessage.Error(),
   867  					Name:   addr,
   868  					Server: server,
   869  				}
   870  			}
   871  			continue
   872  		}
   873  		ptr, err := p.PTRResource()
   874  		if err != nil {
   875  			return nil, &DNSError{
   876  				Err:    errCannotUnmarshalDNSMessage.Error(),
   877  				Name:   addr,
   878  				Server: server,
   879  			}
   880  		}
   881  		ptrs = append(ptrs, ptr.PTR.String())
   882  
   883  	}
   884  
   885  	return ptrs, nil
   886  }
   887  

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