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 = errors.New("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  		if h.Truncated { // see RFC 5966
   198  			continue
   199  		}
   200  		return p, h, nil
   201  	}
   202  	return dnsmessage.Parser{}, dnsmessage.Header{}, errNoAnswerFromDNSServer
   203  }
   204  
   205  // checkHeader performs basic sanity checks on the header.
   206  func checkHeader(p *dnsmessage.Parser, h dnsmessage.Header) error {
   207  	rcode := extractExtendedRCode(*p, h)
   208  
   209  	if rcode == dnsmessage.RCodeNameError {
   210  		return errNoSuchHost
   211  	}
   212  
   213  	_, err := p.AnswerHeader()
   214  	if err != nil && err != dnsmessage.ErrSectionDone {
   215  		return errCannotUnmarshalDNSMessage
   216  	}
   217  
   218  	// libresolv continues to the next server when it receives
   219  	// an invalid referral response. See golang.org/issue/15434.
   220  	if rcode == dnsmessage.RCodeSuccess && !h.Authoritative && !h.RecursionAvailable && err == dnsmessage.ErrSectionDone {
   221  		return errLameReferral
   222  	}
   223  
   224  	if rcode != dnsmessage.RCodeSuccess && rcode != dnsmessage.RCodeNameError {
   225  		// None of the error codes make sense
   226  		// for the query we sent. If we didn't get
   227  		// a name error and we didn't get success,
   228  		// the server is behaving incorrectly or
   229  		// having temporary trouble.
   230  		if rcode == dnsmessage.RCodeServerFailure {
   231  			return errServerTemporarilyMisbehaving
   232  		}
   233  		return errServerMisbehaving
   234  	}
   235  
   236  	return nil
   237  }
   238  
   239  func skipToAnswer(p *dnsmessage.Parser, qtype dnsmessage.Type) error {
   240  	for {
   241  		h, err := p.AnswerHeader()
   242  		if err == dnsmessage.ErrSectionDone {
   243  			return errNoSuchHost
   244  		}
   245  		if err != nil {
   246  			return errCannotUnmarshalDNSMessage
   247  		}
   248  		if h.Type == qtype {
   249  			return nil
   250  		}
   251  		if err := p.SkipAnswer(); err != nil {
   252  			return errCannotUnmarshalDNSMessage
   253  		}
   254  	}
   255  }
   256  
   257  // extractExtendedRCode extracts the extended RCode from the OPT resource (EDNS(0))
   258  // If an OPT record is not found, the RCode from the hdr is returned.
   259  func extractExtendedRCode(p dnsmessage.Parser, hdr dnsmessage.Header) dnsmessage.RCode {
   260  	p.SkipAllAnswers()
   261  	p.SkipAllAuthorities()
   262  	for {
   263  		ahdr, err := p.AdditionalHeader()
   264  		if err != nil {
   265  			return hdr.RCode
   266  		}
   267  		if ahdr.Type == dnsmessage.TypeOPT {
   268  			return ahdr.ExtendedRCode(hdr.RCode)
   269  		}
   270  		p.SkipAdditional()
   271  	}
   272  }
   273  
   274  // Do a lookup for a single name, which must be rooted
   275  // (otherwise answer will not find the answers).
   276  func (r *Resolver) tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype dnsmessage.Type) (dnsmessage.Parser, string, error) {
   277  	var lastErr error
   278  	serverOffset := cfg.serverOffset()
   279  	sLen := uint32(len(cfg.servers))
   280  
   281  	n, err := dnsmessage.NewName(name)
   282  	if err != nil {
   283  		return dnsmessage.Parser{}, "", errCannotMarshalDNSMessage
   284  	}
   285  	q := dnsmessage.Question{
   286  		Name:  n,
   287  		Type:  qtype,
   288  		Class: dnsmessage.ClassINET,
   289  	}
   290  
   291  	for i := 0; i < cfg.attempts; i++ {
   292  		for j := uint32(0); j < sLen; j++ {
   293  			server := cfg.servers[(serverOffset+j)%sLen]
   294  
   295  			p, h, err := r.exchange(ctx, server, q, cfg.timeout, cfg.useTCP, cfg.trustAD)
   296  			if err != nil {
   297  				dnsErr := &DNSError{
   298  					Err:    err.Error(),
   299  					Name:   name,
   300  					Server: server,
   301  				}
   302  				if nerr, ok := err.(Error); ok && nerr.Timeout() {
   303  					dnsErr.IsTimeout = true
   304  				}
   305  				// Set IsTemporary for socket-level errors. Note that this flag
   306  				// may also be used to indicate a SERVFAIL response.
   307  				if _, ok := err.(*OpError); ok {
   308  					dnsErr.IsTemporary = true
   309  				}
   310  				lastErr = dnsErr
   311  				continue
   312  			}
   313  
   314  			if err := checkHeader(&p, h); err != nil {
   315  				dnsErr := &DNSError{
   316  					Err:    err.Error(),
   317  					Name:   name,
   318  					Server: server,
   319  				}
   320  				if err == errServerTemporarilyMisbehaving {
   321  					dnsErr.IsTemporary = true
   322  				}
   323  				if err == errNoSuchHost {
   324  					// The name does not exist, so trying
   325  					// another server won't help.
   326  
   327  					dnsErr.IsNotFound = true
   328  					return p, server, dnsErr
   329  				}
   330  				lastErr = dnsErr
   331  				continue
   332  			}
   333  
   334  			err = skipToAnswer(&p, qtype)
   335  			if err == nil {
   336  				return p, server, nil
   337  			}
   338  			lastErr = &DNSError{
   339  				Err:    err.Error(),
   340  				Name:   name,
   341  				Server: server,
   342  			}
   343  			if err == errNoSuchHost {
   344  				// The name does not exist, so trying another
   345  				// server won't help.
   346  
   347  				lastErr.(*DNSError).IsNotFound = true
   348  				return p, server, lastErr
   349  			}
   350  		}
   351  	}
   352  	return dnsmessage.Parser{}, "", lastErr
   353  }
   354  
   355  // A resolverConfig represents a DNS stub resolver configuration.
   356  type resolverConfig struct {
   357  	initOnce sync.Once // guards init of resolverConfig
   358  
   359  	// ch is used as a semaphore that only allows one lookup at a
   360  	// time to recheck resolv.conf.
   361  	ch          chan struct{} // guards lastChecked and modTime
   362  	lastChecked time.Time     // last time resolv.conf was checked
   363  
   364  	dnsConfig atomic.Pointer[dnsConfig] // parsed resolv.conf structure used in lookups
   365  }
   366  
   367  var resolvConf resolverConfig
   368  
   369  func getSystemDNSConfig() *dnsConfig {
   370  	resolvConf.tryUpdate("/etc/resolv.conf")
   371  	return resolvConf.dnsConfig.Load()
   372  }
   373  
   374  // init initializes conf and is only called via conf.initOnce.
   375  func (conf *resolverConfig) init() {
   376  	// Set dnsConfig and lastChecked so we don't parse
   377  	// resolv.conf twice the first time.
   378  	conf.dnsConfig.Store(dnsReadConfig("/etc/resolv.conf"))
   379  	conf.lastChecked = time.Now()
   380  
   381  	// Prepare ch so that only one update of resolverConfig may
   382  	// run at once.
   383  	conf.ch = make(chan struct{}, 1)
   384  }
   385  
   386  // tryUpdate tries to update conf with the named resolv.conf file.
   387  // The name variable only exists for testing. It is otherwise always
   388  // "/etc/resolv.conf".
   389  func (conf *resolverConfig) tryUpdate(name string) {
   390  	conf.initOnce.Do(conf.init)
   391  
   392  	if conf.dnsConfig.Load().noReload {
   393  		return
   394  	}
   395  
   396  	// Ensure only one update at a time checks resolv.conf.
   397  	if !conf.tryAcquireSema() {
   398  		return
   399  	}
   400  	defer conf.releaseSema()
   401  
   402  	now := time.Now()
   403  	if conf.lastChecked.After(now.Add(-5 * time.Second)) {
   404  		return
   405  	}
   406  	conf.lastChecked = now
   407  
   408  	switch runtime.GOOS {
   409  	case "windows":
   410  		// There's no file on disk, so don't bother checking
   411  		// and failing.
   412  		//
   413  		// The Windows implementation of dnsReadConfig (called
   414  		// below) ignores the name.
   415  	default:
   416  		var mtime time.Time
   417  		if fi, err := os.Stat(name); err == nil {
   418  			mtime = fi.ModTime()
   419  		}
   420  		if mtime.Equal(conf.dnsConfig.Load().mtime) {
   421  			return
   422  		}
   423  	}
   424  
   425  	dnsConf := dnsReadConfig(name)
   426  	conf.dnsConfig.Store(dnsConf)
   427  }
   428  
   429  func (conf *resolverConfig) tryAcquireSema() bool {
   430  	select {
   431  	case conf.ch <- struct{}{}:
   432  		return true
   433  	default:
   434  		return false
   435  	}
   436  }
   437  
   438  func (conf *resolverConfig) releaseSema() {
   439  	<-conf.ch
   440  }
   441  
   442  func (r *Resolver) lookup(ctx context.Context, name string, qtype dnsmessage.Type, conf *dnsConfig) (dnsmessage.Parser, string, error) {
   443  	if !isDomainName(name) {
   444  		// We used to use "invalid domain name" as the error,
   445  		// but that is a detail of the specific lookup mechanism.
   446  		// Other lookups might allow broader name syntax
   447  		// (for example Multicast DNS allows UTF-8; see RFC 6762).
   448  		// For consistency with libc resolvers, report no such host.
   449  		return dnsmessage.Parser{}, "", &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
   450  	}
   451  
   452  	if conf == nil {
   453  		conf = getSystemDNSConfig()
   454  	}
   455  
   456  	var (
   457  		p      dnsmessage.Parser
   458  		server string
   459  		err    error
   460  	)
   461  	for _, fqdn := range conf.nameList(name) {
   462  		p, server, err = r.tryOneName(ctx, conf, fqdn, qtype)
   463  		if err == nil {
   464  			break
   465  		}
   466  		if nerr, ok := err.(Error); ok && nerr.Temporary() && r.strictErrors() {
   467  			// If we hit a temporary error with StrictErrors enabled,
   468  			// stop immediately instead of trying more names.
   469  			break
   470  		}
   471  	}
   472  	if err == nil {
   473  		return p, server, nil
   474  	}
   475  	if err, ok := err.(*DNSError); ok {
   476  		// Show original name passed to lookup, not suffixed one.
   477  		// In general we might have tried many suffixes; showing
   478  		// just one is misleading. See also golang.org/issue/6324.
   479  		err.Name = name
   480  	}
   481  	return dnsmessage.Parser{}, "", err
   482  }
   483  
   484  // avoidDNS reports whether this is a hostname for which we should not
   485  // use DNS. Currently this includes only .onion, per RFC 7686. See
   486  // golang.org/issue/13705. Does not cover .local names (RFC 6762),
   487  // see golang.org/issue/16739.
   488  func avoidDNS(name string) bool {
   489  	if name == "" {
   490  		return true
   491  	}
   492  	if name[len(name)-1] == '.' {
   493  		name = name[:len(name)-1]
   494  	}
   495  	return stringsHasSuffixFold(name, ".onion")
   496  }
   497  
   498  // nameList returns a list of names for sequential DNS queries.
   499  func (conf *dnsConfig) nameList(name string) []string {
   500  	// Check name length (see isDomainName).
   501  	l := len(name)
   502  	rooted := l > 0 && name[l-1] == '.'
   503  	if l > 254 || l == 254 && !rooted {
   504  		return nil
   505  	}
   506  
   507  	// If name is rooted (trailing dot), try only that name.
   508  	if rooted {
   509  		if avoidDNS(name) {
   510  			return nil
   511  		}
   512  		return []string{name}
   513  	}
   514  
   515  	hasNdots := bytealg.CountString(name, '.') >= conf.ndots
   516  	name += "."
   517  	l++
   518  
   519  	// Build list of search choices.
   520  	names := make([]string, 0, 1+len(conf.search))
   521  	// If name has enough dots, try unsuffixed first.
   522  	if hasNdots && !avoidDNS(name) {
   523  		names = append(names, name)
   524  	}
   525  	// Try suffixes that are not too long (see isDomainName).
   526  	for _, suffix := range conf.search {
   527  		fqdn := name + suffix
   528  		if !avoidDNS(fqdn) && len(fqdn) <= 254 {
   529  			names = append(names, fqdn)
   530  		}
   531  	}
   532  	// Try unsuffixed, if not tried first above.
   533  	if !hasNdots && !avoidDNS(name) {
   534  		names = append(names, name)
   535  	}
   536  	return names
   537  }
   538  
   539  // hostLookupOrder specifies the order of LookupHost lookup strategies.
   540  // It is basically a simplified representation of nsswitch.conf.
   541  // "files" means /etc/hosts.
   542  type hostLookupOrder int
   543  
   544  const (
   545  	// hostLookupCgo means defer to cgo.
   546  	hostLookupCgo      hostLookupOrder = iota
   547  	hostLookupFilesDNS                 // files first
   548  	hostLookupDNSFiles                 // dns first
   549  	hostLookupFiles                    // only files
   550  	hostLookupDNS                      // only DNS
   551  )
   552  
   553  var lookupOrderName = map[hostLookupOrder]string{
   554  	hostLookupCgo:      "cgo",
   555  	hostLookupFilesDNS: "files,dns",
   556  	hostLookupDNSFiles: "dns,files",
   557  	hostLookupFiles:    "files",
   558  	hostLookupDNS:      "dns",
   559  }
   560  
   561  func (o hostLookupOrder) String() string {
   562  	if s, ok := lookupOrderName[o]; ok {
   563  		return s
   564  	}
   565  	return "hostLookupOrder=" + itoa.Itoa(int(o)) + "??"
   566  }
   567  
   568  func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder, conf *dnsConfig) (addrs []string, err error) {
   569  	if order == hostLookupFilesDNS || order == hostLookupFiles {
   570  		// Use entries from /etc/hosts if they match.
   571  		addrs, _ = lookupStaticHost(name)
   572  		if len(addrs) > 0 {
   573  			return
   574  		}
   575  
   576  		if order == hostLookupFiles {
   577  			return nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
   578  		}
   579  	}
   580  	ips, _, err := r.goLookupIPCNAMEOrder(ctx, "ip", name, order, conf)
   581  	if err != nil {
   582  		return
   583  	}
   584  	addrs = make([]string, 0, len(ips))
   585  	for _, ip := range ips {
   586  		addrs = append(addrs, ip.String())
   587  	}
   588  	return
   589  }
   590  
   591  // lookup entries from /etc/hosts
   592  func goLookupIPFiles(name string) (addrs []IPAddr, canonical string) {
   593  	addr, canonical := lookupStaticHost(name)
   594  	for _, haddr := range addr {
   595  		haddr, zone := splitHostZone(haddr)
   596  		if ip := ParseIP(haddr); ip != nil {
   597  			addr := IPAddr{IP: ip, Zone: zone}
   598  			addrs = append(addrs, addr)
   599  		}
   600  	}
   601  	sortByRFC6724(addrs)
   602  	return addrs, canonical
   603  }
   604  
   605  // goLookupIP is the native Go implementation of LookupIP.
   606  // The libc versions are in cgo_*.go.
   607  func (r *Resolver) goLookupIP(ctx context.Context, network, host string, order hostLookupOrder, conf *dnsConfig) (addrs []IPAddr, err error) {
   608  	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, network, host, order, conf)
   609  	return
   610  }
   611  
   612  func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, network, name string, order hostLookupOrder, conf *dnsConfig) (addrs []IPAddr, cname dnsmessage.Name, err error) {
   613  	if order == hostLookupFilesDNS || order == hostLookupFiles {
   614  		var canonical string
   615  		addrs, canonical = goLookupIPFiles(name)
   616  
   617  		if len(addrs) > 0 {
   618  			var err error
   619  			cname, err = dnsmessage.NewName(canonical)
   620  			if err != nil {
   621  				return nil, dnsmessage.Name{}, err
   622  			}
   623  			return addrs, cname, nil
   624  		}
   625  
   626  		if order == hostLookupFiles {
   627  			return nil, dnsmessage.Name{}, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
   628  		}
   629  	}
   630  
   631  	if !isDomainName(name) {
   632  		// See comment in func lookup above about use of errNoSuchHost.
   633  		return nil, dnsmessage.Name{}, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
   634  	}
   635  	type result struct {
   636  		p      dnsmessage.Parser
   637  		server string
   638  		error
   639  	}
   640  
   641  	if conf == nil {
   642  		conf = getSystemDNSConfig()
   643  	}
   644  
   645  	lane := make(chan result, 1)
   646  	qtypes := []dnsmessage.Type{dnsmessage.TypeA, dnsmessage.TypeAAAA}
   647  	if network == "CNAME" {
   648  		qtypes = append(qtypes, dnsmessage.TypeCNAME)
   649  	}
   650  	switch ipVersion(network) {
   651  	case '4':
   652  		qtypes = []dnsmessage.Type{dnsmessage.TypeA}
   653  	case '6':
   654  		qtypes = []dnsmessage.Type{dnsmessage.TypeAAAA}
   655  	}
   656  	var queryFn func(fqdn string, qtype dnsmessage.Type)
   657  	var responseFn func(fqdn string, qtype dnsmessage.Type) result
   658  	if conf.singleRequest {
   659  		queryFn = func(fqdn string, qtype dnsmessage.Type) {}
   660  		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
   661  			dnsWaitGroup.Add(1)
   662  			defer dnsWaitGroup.Done()
   663  			p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
   664  			return result{p, server, err}
   665  		}
   666  	} else {
   667  		queryFn = func(fqdn string, qtype dnsmessage.Type) {
   668  			dnsWaitGroup.Add(1)
   669  			go func(qtype dnsmessage.Type) {
   670  				p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
   671  				lane <- result{p, server, err}
   672  				dnsWaitGroup.Done()
   673  			}(qtype)
   674  		}
   675  		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
   676  			return <-lane
   677  		}
   678  	}
   679  	var lastErr error
   680  	for _, fqdn := range conf.nameList(name) {
   681  		for _, qtype := range qtypes {
   682  			queryFn(fqdn, qtype)
   683  		}
   684  		hitStrictError := false
   685  		for _, qtype := range qtypes {
   686  			result := responseFn(fqdn, qtype)
   687  			if result.error != nil {
   688  				if nerr, ok := result.error.(Error); ok && nerr.Temporary() && r.strictErrors() {
   689  					// This error will abort the nameList loop.
   690  					hitStrictError = true
   691  					lastErr = result.error
   692  				} else if lastErr == nil || fqdn == name+"." {
   693  					// Prefer error for original name.
   694  					lastErr = result.error
   695  				}
   696  				continue
   697  			}
   698  
   699  			// Presotto says it's okay to assume that servers listed in
   700  			// /etc/resolv.conf are recursive resolvers.
   701  			//
   702  			// We asked for recursion, so it should have included all the
   703  			// answers we need in this one packet.
   704  			//
   705  			// Further, RFC 1034 section 4.3.1 says that "the recursive
   706  			// response to a query will be... The answer to the query,
   707  			// possibly preface by one or more CNAME RRs that specify
   708  			// aliases encountered on the way to an answer."
   709  			//
   710  			// Therefore, we should be able to assume that we can ignore
   711  			// CNAMEs and that the A and AAAA records we requested are
   712  			// for the canonical name.
   713  
   714  		loop:
   715  			for {
   716  				h, err := result.p.AnswerHeader()
   717  				if err != nil && err != dnsmessage.ErrSectionDone {
   718  					lastErr = &DNSError{
   719  						Err:    errCannotUnmarshalDNSMessage.Error(),
   720  						Name:   name,
   721  						Server: result.server,
   722  					}
   723  				}
   724  				if err != nil {
   725  					break
   726  				}
   727  				switch h.Type {
   728  				case dnsmessage.TypeA:
   729  					a, err := result.p.AResource()
   730  					if err != nil {
   731  						lastErr = &DNSError{
   732  							Err:    errCannotUnmarshalDNSMessage.Error(),
   733  							Name:   name,
   734  							Server: result.server,
   735  						}
   736  						break loop
   737  					}
   738  					addrs = append(addrs, IPAddr{IP: IP(a.A[:])})
   739  					if cname.Length == 0 && h.Name.Length != 0 {
   740  						cname = h.Name
   741  					}
   742  
   743  				case dnsmessage.TypeAAAA:
   744  					aaaa, err := result.p.AAAAResource()
   745  					if err != nil {
   746  						lastErr = &DNSError{
   747  							Err:    errCannotUnmarshalDNSMessage.Error(),
   748  							Name:   name,
   749  							Server: result.server,
   750  						}
   751  						break loop
   752  					}
   753  					addrs = append(addrs, IPAddr{IP: IP(aaaa.AAAA[:])})
   754  					if cname.Length == 0 && h.Name.Length != 0 {
   755  						cname = h.Name
   756  					}
   757  
   758  				case dnsmessage.TypeCNAME:
   759  					c, err := result.p.CNAMEResource()
   760  					if err != nil {
   761  						lastErr = &DNSError{
   762  							Err:    errCannotUnmarshalDNSMessage.Error(),
   763  							Name:   name,
   764  							Server: result.server,
   765  						}
   766  						break loop
   767  					}
   768  					if cname.Length == 0 && c.CNAME.Length > 0 {
   769  						cname = c.CNAME
   770  					}
   771  
   772  				default:
   773  					if err := result.p.SkipAnswer(); err != nil {
   774  						lastErr = &DNSError{
   775  							Err:    errCannotUnmarshalDNSMessage.Error(),
   776  							Name:   name,
   777  							Server: result.server,
   778  						}
   779  						break loop
   780  					}
   781  					continue
   782  				}
   783  			}
   784  		}
   785  		if hitStrictError {
   786  			// If either family hit an error with StrictErrors enabled,
   787  			// discard all addresses. This ensures that network flakiness
   788  			// cannot turn a dualstack hostname IPv4/IPv6-only.
   789  			addrs = nil
   790  			break
   791  		}
   792  		if len(addrs) > 0 || network == "CNAME" && cname.Length > 0 {
   793  			break
   794  		}
   795  	}
   796  	if lastErr, ok := lastErr.(*DNSError); ok {
   797  		// Show original name passed to lookup, not suffixed one.
   798  		// In general we might have tried many suffixes; showing
   799  		// just one is misleading. See also golang.org/issue/6324.
   800  		lastErr.Name = name
   801  	}
   802  	sortByRFC6724(addrs)
   803  	if len(addrs) == 0 && !(network == "CNAME" && cname.Length > 0) {
   804  		if order == hostLookupDNSFiles {
   805  			var canonical string
   806  			addrs, canonical = goLookupIPFiles(name)
   807  			if len(addrs) > 0 {
   808  				var err error
   809  				cname, err = dnsmessage.NewName(canonical)
   810  				if err != nil {
   811  					return nil, dnsmessage.Name{}, err
   812  				}
   813  				return addrs, cname, nil
   814  			}
   815  		}
   816  		if lastErr != nil {
   817  			return nil, dnsmessage.Name{}, lastErr
   818  		}
   819  	}
   820  	return addrs, cname, nil
   821  }
   822  
   823  // goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
   824  func (r *Resolver) goLookupCNAME(ctx context.Context, host string, order hostLookupOrder, conf *dnsConfig) (string, error) {
   825  	_, cname, err := r.goLookupIPCNAMEOrder(ctx, "CNAME", host, order, conf)
   826  	return cname.String(), err
   827  }
   828  
   829  // goLookupPTR is the native Go implementation of LookupAddr.
   830  func (r *Resolver) goLookupPTR(ctx context.Context, addr string, order hostLookupOrder, conf *dnsConfig) ([]string, error) {
   831  	if order == hostLookupFiles || order == hostLookupFilesDNS {
   832  		names := lookupStaticAddr(addr)
   833  		if len(names) > 0 {
   834  			return names, nil
   835  		}
   836  
   837  		if order == hostLookupFiles {
   838  			return nil, &DNSError{Err: errNoSuchHost.Error(), Name: addr, IsNotFound: true}
   839  		}
   840  	}
   841  
   842  	arpa, err := reverseaddr(addr)
   843  	if err != nil {
   844  		return nil, err
   845  	}
   846  	p, server, err := r.lookup(ctx, arpa, dnsmessage.TypePTR, conf)
   847  	if err != nil {
   848  		var dnsErr *DNSError
   849  		if errors.As(err, &dnsErr) && dnsErr.IsNotFound {
   850  			if order == hostLookupDNSFiles {
   851  				names := lookupStaticAddr(addr)
   852  				if len(names) > 0 {
   853  					return names, nil
   854  				}
   855  			}
   856  		}
   857  		return nil, err
   858  	}
   859  	var ptrs []string
   860  	for {
   861  		h, err := p.AnswerHeader()
   862  		if err == dnsmessage.ErrSectionDone {
   863  			break
   864  		}
   865  		if err != nil {
   866  			return nil, &DNSError{
   867  				Err:    errCannotUnmarshalDNSMessage.Error(),
   868  				Name:   addr,
   869  				Server: server,
   870  			}
   871  		}
   872  		if h.Type != dnsmessage.TypePTR {
   873  			err := p.SkipAnswer()
   874  			if err != nil {
   875  				return nil, &DNSError{
   876  					Err:    errCannotUnmarshalDNSMessage.Error(),
   877  					Name:   addr,
   878  					Server: server,
   879  				}
   880  			}
   881  			continue
   882  		}
   883  		ptr, err := p.PTRResource()
   884  		if err != nil {
   885  			return nil, &DNSError{
   886  				Err:    errCannotUnmarshalDNSMessage.Error(),
   887  				Name:   addr,
   888  				Server: server,
   889  			}
   890  		}
   891  		ptrs = append(ptrs, ptr.PTR.String())
   892  
   893  	}
   894  
   895  	return ptrs, nil
   896  }
   897  

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