Source file src/net/dnsclient_unix.go

Documentation: net

     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  		if err := p.SkipAdditional(); err != nil {
   271  			return hdr.RCode
   272  		}
   273  	}
   274  }
   275  
   276  // Do a lookup for a single name, which must be rooted
   277  // (otherwise answer will not find the answers).
   278  func (r *Resolver) tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype dnsmessage.Type) (dnsmessage.Parser, string, error) {
   279  	var lastErr error
   280  	serverOffset := cfg.serverOffset()
   281  	sLen := uint32(len(cfg.servers))
   282  
   283  	n, err := dnsmessage.NewName(name)
   284  	if err != nil {
   285  		return dnsmessage.Parser{}, "", errCannotMarshalDNSMessage
   286  	}
   287  	q := dnsmessage.Question{
   288  		Name:  n,
   289  		Type:  qtype,
   290  		Class: dnsmessage.ClassINET,
   291  	}
   292  
   293  	for i := 0; i < cfg.attempts; i++ {
   294  		for j := uint32(0); j < sLen; j++ {
   295  			server := cfg.servers[(serverOffset+j)%sLen]
   296  
   297  			p, h, err := r.exchange(ctx, server, q, cfg.timeout, cfg.useTCP, cfg.trustAD)
   298  			if err != nil {
   299  				dnsErr := &DNSError{
   300  					Err:    err.Error(),
   301  					Name:   name,
   302  					Server: server,
   303  				}
   304  				if nerr, ok := err.(Error); ok && nerr.Timeout() {
   305  					dnsErr.IsTimeout = true
   306  				}
   307  				// Set IsTemporary for socket-level errors. Note that this flag
   308  				// may also be used to indicate a SERVFAIL response.
   309  				if _, ok := err.(*OpError); ok {
   310  					dnsErr.IsTemporary = true
   311  				}
   312  				lastErr = dnsErr
   313  				continue
   314  			}
   315  
   316  			if err := checkHeader(&p, h); err != nil {
   317  				dnsErr := &DNSError{
   318  					Err:    err.Error(),
   319  					Name:   name,
   320  					Server: server,
   321  				}
   322  				if err == errServerTemporarilyMisbehaving {
   323  					dnsErr.IsTemporary = true
   324  				}
   325  				if err == errNoSuchHost {
   326  					// The name does not exist, so trying
   327  					// another server won't help.
   328  
   329  					dnsErr.IsNotFound = true
   330  					return p, server, dnsErr
   331  				}
   332  				lastErr = dnsErr
   333  				continue
   334  			}
   335  
   336  			err = skipToAnswer(&p, qtype)
   337  			if err == nil {
   338  				return p, server, nil
   339  			}
   340  			lastErr = &DNSError{
   341  				Err:    err.Error(),
   342  				Name:   name,
   343  				Server: server,
   344  			}
   345  			if err == errNoSuchHost {
   346  				// The name does not exist, so trying another
   347  				// server won't help.
   348  
   349  				lastErr.(*DNSError).IsNotFound = true
   350  				return p, server, lastErr
   351  			}
   352  		}
   353  	}
   354  	return dnsmessage.Parser{}, "", lastErr
   355  }
   356  
   357  // A resolverConfig represents a DNS stub resolver configuration.
   358  type resolverConfig struct {
   359  	initOnce sync.Once // guards init of resolverConfig
   360  
   361  	// ch is used as a semaphore that only allows one lookup at a
   362  	// time to recheck resolv.conf.
   363  	ch          chan struct{} // guards lastChecked and modTime
   364  	lastChecked time.Time     // last time resolv.conf was checked
   365  
   366  	dnsConfig atomic.Pointer[dnsConfig] // parsed resolv.conf structure used in lookups
   367  }
   368  
   369  var resolvConf resolverConfig
   370  
   371  func getSystemDNSConfig() *dnsConfig {
   372  	resolvConf.tryUpdate("/etc/resolv.conf")
   373  	return resolvConf.dnsConfig.Load()
   374  }
   375  
   376  // init initializes conf and is only called via conf.initOnce.
   377  func (conf *resolverConfig) init() {
   378  	// Set dnsConfig and lastChecked so we don't parse
   379  	// resolv.conf twice the first time.
   380  	conf.dnsConfig.Store(dnsReadConfig("/etc/resolv.conf"))
   381  	conf.lastChecked = time.Now()
   382  
   383  	// Prepare ch so that only one update of resolverConfig may
   384  	// run at once.
   385  	conf.ch = make(chan struct{}, 1)
   386  }
   387  
   388  // tryUpdate tries to update conf with the named resolv.conf file.
   389  // The name variable only exists for testing. It is otherwise always
   390  // "/etc/resolv.conf".
   391  func (conf *resolverConfig) tryUpdate(name string) {
   392  	conf.initOnce.Do(conf.init)
   393  
   394  	if conf.dnsConfig.Load().noReload {
   395  		return
   396  	}
   397  
   398  	// Ensure only one update at a time checks resolv.conf.
   399  	if !conf.tryAcquireSema() {
   400  		return
   401  	}
   402  	defer conf.releaseSema()
   403  
   404  	now := time.Now()
   405  	if conf.lastChecked.After(now.Add(-5 * time.Second)) {
   406  		return
   407  	}
   408  	conf.lastChecked = now
   409  
   410  	switch runtime.GOOS {
   411  	case "windows":
   412  		// There's no file on disk, so don't bother checking
   413  		// and failing.
   414  		//
   415  		// The Windows implementation of dnsReadConfig (called
   416  		// below) ignores the name.
   417  	default:
   418  		var mtime time.Time
   419  		if fi, err := os.Stat(name); err == nil {
   420  			mtime = fi.ModTime()
   421  		}
   422  		if mtime.Equal(conf.dnsConfig.Load().mtime) {
   423  			return
   424  		}
   425  	}
   426  
   427  	dnsConf := dnsReadConfig(name)
   428  	conf.dnsConfig.Store(dnsConf)
   429  }
   430  
   431  func (conf *resolverConfig) tryAcquireSema() bool {
   432  	select {
   433  	case conf.ch <- struct{}{}:
   434  		return true
   435  	default:
   436  		return false
   437  	}
   438  }
   439  
   440  func (conf *resolverConfig) releaseSema() {
   441  	<-conf.ch
   442  }
   443  
   444  func (r *Resolver) lookup(ctx context.Context, name string, qtype dnsmessage.Type, conf *dnsConfig) (dnsmessage.Parser, string, error) {
   445  	if !isDomainName(name) {
   446  		// We used to use "invalid domain name" as the error,
   447  		// but that is a detail of the specific lookup mechanism.
   448  		// Other lookups might allow broader name syntax
   449  		// (for example Multicast DNS allows UTF-8; see RFC 6762).
   450  		// For consistency with libc resolvers, report no such host.
   451  		return dnsmessage.Parser{}, "", &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
   452  	}
   453  
   454  	if conf == nil {
   455  		conf = getSystemDNSConfig()
   456  	}
   457  
   458  	var (
   459  		p      dnsmessage.Parser
   460  		server string
   461  		err    error
   462  	)
   463  	for _, fqdn := range conf.nameList(name) {
   464  		p, server, err = r.tryOneName(ctx, conf, fqdn, qtype)
   465  		if err == nil {
   466  			break
   467  		}
   468  		if nerr, ok := err.(Error); ok && nerr.Temporary() && r.strictErrors() {
   469  			// If we hit a temporary error with StrictErrors enabled,
   470  			// stop immediately instead of trying more names.
   471  			break
   472  		}
   473  	}
   474  	if err == nil {
   475  		return p, server, nil
   476  	}
   477  	if err, ok := err.(*DNSError); ok {
   478  		// Show original name passed to lookup, not suffixed one.
   479  		// In general we might have tried many suffixes; showing
   480  		// just one is misleading. See also golang.org/issue/6324.
   481  		err.Name = name
   482  	}
   483  	return dnsmessage.Parser{}, "", err
   484  }
   485  
   486  // avoidDNS reports whether this is a hostname for which we should not
   487  // use DNS. Currently this includes only .onion, per RFC 7686. See
   488  // golang.org/issue/13705. Does not cover .local names (RFC 6762),
   489  // see golang.org/issue/16739.
   490  func avoidDNS(name string) bool {
   491  	if name == "" {
   492  		return true
   493  	}
   494  	if name[len(name)-1] == '.' {
   495  		name = name[:len(name)-1]
   496  	}
   497  	return stringsHasSuffixFold(name, ".onion")
   498  }
   499  
   500  // nameList returns a list of names for sequential DNS queries.
   501  func (conf *dnsConfig) nameList(name string) []string {
   502  	// Check name length (see isDomainName).
   503  	l := len(name)
   504  	rooted := l > 0 && name[l-1] == '.'
   505  	if l > 254 || l == 254 && !rooted {
   506  		return nil
   507  	}
   508  
   509  	// If name is rooted (trailing dot), try only that name.
   510  	if rooted {
   511  		if avoidDNS(name) {
   512  			return nil
   513  		}
   514  		return []string{name}
   515  	}
   516  
   517  	hasNdots := bytealg.CountString(name, '.') >= conf.ndots
   518  	name += "."
   519  	l++
   520  
   521  	// Build list of search choices.
   522  	names := make([]string, 0, 1+len(conf.search))
   523  	// If name has enough dots, try unsuffixed first.
   524  	if hasNdots && !avoidDNS(name) {
   525  		names = append(names, name)
   526  	}
   527  	// Try suffixes that are not too long (see isDomainName).
   528  	for _, suffix := range conf.search {
   529  		fqdn := name + suffix
   530  		if !avoidDNS(fqdn) && len(fqdn) <= 254 {
   531  			names = append(names, fqdn)
   532  		}
   533  	}
   534  	// Try unsuffixed, if not tried first above.
   535  	if !hasNdots && !avoidDNS(name) {
   536  		names = append(names, name)
   537  	}
   538  	return names
   539  }
   540  
   541  // hostLookupOrder specifies the order of LookupHost lookup strategies.
   542  // It is basically a simplified representation of nsswitch.conf.
   543  // "files" means /etc/hosts.
   544  type hostLookupOrder int
   545  
   546  const (
   547  	// hostLookupCgo means defer to cgo.
   548  	hostLookupCgo      hostLookupOrder = iota
   549  	hostLookupFilesDNS                 // files first
   550  	hostLookupDNSFiles                 // dns first
   551  	hostLookupFiles                    // only files
   552  	hostLookupDNS                      // only DNS
   553  )
   554  
   555  var lookupOrderName = map[hostLookupOrder]string{
   556  	hostLookupCgo:      "cgo",
   557  	hostLookupFilesDNS: "files,dns",
   558  	hostLookupDNSFiles: "dns,files",
   559  	hostLookupFiles:    "files",
   560  	hostLookupDNS:      "dns",
   561  }
   562  
   563  func (o hostLookupOrder) String() string {
   564  	if s, ok := lookupOrderName[o]; ok {
   565  		return s
   566  	}
   567  	return "hostLookupOrder=" + itoa.Itoa(int(o)) + "??"
   568  }
   569  
   570  func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder, conf *dnsConfig) (addrs []string, err error) {
   571  	if order == hostLookupFilesDNS || order == hostLookupFiles {
   572  		// Use entries from /etc/hosts if they match.
   573  		addrs, _ = lookupStaticHost(name)
   574  		if len(addrs) > 0 {
   575  			return
   576  		}
   577  
   578  		if order == hostLookupFiles {
   579  			return nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
   580  		}
   581  	}
   582  	ips, _, err := r.goLookupIPCNAMEOrder(ctx, "ip", name, order, conf)
   583  	if err != nil {
   584  		return
   585  	}
   586  	addrs = make([]string, 0, len(ips))
   587  	for _, ip := range ips {
   588  		addrs = append(addrs, ip.String())
   589  	}
   590  	return
   591  }
   592  
   593  // lookup entries from /etc/hosts
   594  func goLookupIPFiles(name string) (addrs []IPAddr, canonical string) {
   595  	addr, canonical := lookupStaticHost(name)
   596  	for _, haddr := range addr {
   597  		haddr, zone := splitHostZone(haddr)
   598  		if ip := ParseIP(haddr); ip != nil {
   599  			addr := IPAddr{IP: ip, Zone: zone}
   600  			addrs = append(addrs, addr)
   601  		}
   602  	}
   603  	sortByRFC6724(addrs)
   604  	return addrs, canonical
   605  }
   606  
   607  // goLookupIP is the native Go implementation of LookupIP.
   608  // The libc versions are in cgo_*.go.
   609  func (r *Resolver) goLookupIP(ctx context.Context, network, host string, order hostLookupOrder, conf *dnsConfig) (addrs []IPAddr, err error) {
   610  	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, network, host, order, conf)
   611  	return
   612  }
   613  
   614  func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, network, name string, order hostLookupOrder, conf *dnsConfig) (addrs []IPAddr, cname dnsmessage.Name, err error) {
   615  	if order == hostLookupFilesDNS || order == hostLookupFiles {
   616  		var canonical string
   617  		addrs, canonical = goLookupIPFiles(name)
   618  
   619  		if len(addrs) > 0 {
   620  			var err error
   621  			cname, err = dnsmessage.NewName(canonical)
   622  			if err != nil {
   623  				return nil, dnsmessage.Name{}, err
   624  			}
   625  			return addrs, cname, nil
   626  		}
   627  
   628  		if order == hostLookupFiles {
   629  			return nil, dnsmessage.Name{}, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
   630  		}
   631  	}
   632  
   633  	if !isDomainName(name) {
   634  		// See comment in func lookup above about use of errNoSuchHost.
   635  		return nil, dnsmessage.Name{}, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
   636  	}
   637  	type result struct {
   638  		p      dnsmessage.Parser
   639  		server string
   640  		error
   641  	}
   642  
   643  	if conf == nil {
   644  		conf = getSystemDNSConfig()
   645  	}
   646  
   647  	lane := make(chan result, 1)
   648  	qtypes := []dnsmessage.Type{dnsmessage.TypeA, dnsmessage.TypeAAAA}
   649  	if network == "CNAME" {
   650  		qtypes = append(qtypes, dnsmessage.TypeCNAME)
   651  	}
   652  	switch ipVersion(network) {
   653  	case '4':
   654  		qtypes = []dnsmessage.Type{dnsmessage.TypeA}
   655  	case '6':
   656  		qtypes = []dnsmessage.Type{dnsmessage.TypeAAAA}
   657  	}
   658  	var queryFn func(fqdn string, qtype dnsmessage.Type)
   659  	var responseFn func(fqdn string, qtype dnsmessage.Type) result
   660  	if conf.singleRequest {
   661  		queryFn = func(fqdn string, qtype dnsmessage.Type) {}
   662  		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
   663  			dnsWaitGroup.Add(1)
   664  			defer dnsWaitGroup.Done()
   665  			p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
   666  			return result{p, server, err}
   667  		}
   668  	} else {
   669  		queryFn = func(fqdn string, qtype dnsmessage.Type) {
   670  			dnsWaitGroup.Add(1)
   671  			go func(qtype dnsmessage.Type) {
   672  				p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
   673  				lane <- result{p, server, err}
   674  				dnsWaitGroup.Done()
   675  			}(qtype)
   676  		}
   677  		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
   678  			return <-lane
   679  		}
   680  	}
   681  	var lastErr error
   682  	for _, fqdn := range conf.nameList(name) {
   683  		for _, qtype := range qtypes {
   684  			queryFn(fqdn, qtype)
   685  		}
   686  		hitStrictError := false
   687  		for _, qtype := range qtypes {
   688  			result := responseFn(fqdn, qtype)
   689  			if result.error != nil {
   690  				if nerr, ok := result.error.(Error); ok && nerr.Temporary() && r.strictErrors() {
   691  					// This error will abort the nameList loop.
   692  					hitStrictError = true
   693  					lastErr = result.error
   694  				} else if lastErr == nil || fqdn == name+"." {
   695  					// Prefer error for original name.
   696  					lastErr = result.error
   697  				}
   698  				continue
   699  			}
   700  
   701  			// Presotto says it's okay to assume that servers listed in
   702  			// /etc/resolv.conf are recursive resolvers.
   703  			//
   704  			// We asked for recursion, so it should have included all the
   705  			// answers we need in this one packet.
   706  			//
   707  			// Further, RFC 1034 section 4.3.1 says that "the recursive
   708  			// response to a query will be... The answer to the query,
   709  			// possibly preface by one or more CNAME RRs that specify
   710  			// aliases encountered on the way to an answer."
   711  			//
   712  			// Therefore, we should be able to assume that we can ignore
   713  			// CNAMEs and that the A and AAAA records we requested are
   714  			// for the canonical name.
   715  
   716  		loop:
   717  			for {
   718  				h, err := result.p.AnswerHeader()
   719  				if err != nil && err != dnsmessage.ErrSectionDone {
   720  					lastErr = &DNSError{
   721  						Err:    errCannotUnmarshalDNSMessage.Error(),
   722  						Name:   name,
   723  						Server: result.server,
   724  					}
   725  				}
   726  				if err != nil {
   727  					break
   728  				}
   729  				switch h.Type {
   730  				case dnsmessage.TypeA:
   731  					a, err := result.p.AResource()
   732  					if err != nil {
   733  						lastErr = &DNSError{
   734  							Err:    errCannotUnmarshalDNSMessage.Error(),
   735  							Name:   name,
   736  							Server: result.server,
   737  						}
   738  						break loop
   739  					}
   740  					addrs = append(addrs, IPAddr{IP: IP(a.A[:])})
   741  					if cname.Length == 0 && h.Name.Length != 0 {
   742  						cname = h.Name
   743  					}
   744  
   745  				case dnsmessage.TypeAAAA:
   746  					aaaa, err := result.p.AAAAResource()
   747  					if err != nil {
   748  						lastErr = &DNSError{
   749  							Err:    errCannotUnmarshalDNSMessage.Error(),
   750  							Name:   name,
   751  							Server: result.server,
   752  						}
   753  						break loop
   754  					}
   755  					addrs = append(addrs, IPAddr{IP: IP(aaaa.AAAA[:])})
   756  					if cname.Length == 0 && h.Name.Length != 0 {
   757  						cname = h.Name
   758  					}
   759  
   760  				case dnsmessage.TypeCNAME:
   761  					c, err := result.p.CNAMEResource()
   762  					if err != nil {
   763  						lastErr = &DNSError{
   764  							Err:    errCannotUnmarshalDNSMessage.Error(),
   765  							Name:   name,
   766  							Server: result.server,
   767  						}
   768  						break loop
   769  					}
   770  					if cname.Length == 0 && c.CNAME.Length > 0 {
   771  						cname = c.CNAME
   772  					}
   773  
   774  				default:
   775  					if err := result.p.SkipAnswer(); err != nil {
   776  						lastErr = &DNSError{
   777  							Err:    errCannotUnmarshalDNSMessage.Error(),
   778  							Name:   name,
   779  							Server: result.server,
   780  						}
   781  						break loop
   782  					}
   783  					continue
   784  				}
   785  			}
   786  		}
   787  		if hitStrictError {
   788  			// If either family hit an error with StrictErrors enabled,
   789  			// discard all addresses. This ensures that network flakiness
   790  			// cannot turn a dualstack hostname IPv4/IPv6-only.
   791  			addrs = nil
   792  			break
   793  		}
   794  		if len(addrs) > 0 || network == "CNAME" && cname.Length > 0 {
   795  			break
   796  		}
   797  	}
   798  	if lastErr, ok := lastErr.(*DNSError); ok {
   799  		// Show original name passed to lookup, not suffixed one.
   800  		// In general we might have tried many suffixes; showing
   801  		// just one is misleading. See also golang.org/issue/6324.
   802  		lastErr.Name = name
   803  	}
   804  	sortByRFC6724(addrs)
   805  	if len(addrs) == 0 && !(network == "CNAME" && cname.Length > 0) {
   806  		if order == hostLookupDNSFiles {
   807  			var canonical string
   808  			addrs, canonical = goLookupIPFiles(name)
   809  			if len(addrs) > 0 {
   810  				var err error
   811  				cname, err = dnsmessage.NewName(canonical)
   812  				if err != nil {
   813  					return nil, dnsmessage.Name{}, err
   814  				}
   815  				return addrs, cname, nil
   816  			}
   817  		}
   818  		if lastErr != nil {
   819  			return nil, dnsmessage.Name{}, lastErr
   820  		}
   821  	}
   822  	return addrs, cname, nil
   823  }
   824  
   825  // goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
   826  func (r *Resolver) goLookupCNAME(ctx context.Context, host string, order hostLookupOrder, conf *dnsConfig) (string, error) {
   827  	_, cname, err := r.goLookupIPCNAMEOrder(ctx, "CNAME", host, order, conf)
   828  	return cname.String(), err
   829  }
   830  
   831  // goLookupPTR is the native Go implementation of LookupAddr.
   832  func (r *Resolver) goLookupPTR(ctx context.Context, addr string, order hostLookupOrder, conf *dnsConfig) ([]string, error) {
   833  	if order == hostLookupFiles || order == hostLookupFilesDNS {
   834  		names := lookupStaticAddr(addr)
   835  		if len(names) > 0 {
   836  			return names, nil
   837  		}
   838  
   839  		if order == hostLookupFiles {
   840  			return nil, &DNSError{Err: errNoSuchHost.Error(), Name: addr, IsNotFound: true}
   841  		}
   842  	}
   843  
   844  	arpa, err := reverseaddr(addr)
   845  	if err != nil {
   846  		return nil, err
   847  	}
   848  	p, server, err := r.lookup(ctx, arpa, dnsmessage.TypePTR, conf)
   849  	if err != nil {
   850  		var dnsErr *DNSError
   851  		if errors.As(err, &dnsErr) && dnsErr.IsNotFound {
   852  			if order == hostLookupDNSFiles {
   853  				names := lookupStaticAddr(addr)
   854  				if len(names) > 0 {
   855  					return names, nil
   856  				}
   857  			}
   858  		}
   859  		return nil, err
   860  	}
   861  	var ptrs []string
   862  	for {
   863  		h, err := p.AnswerHeader()
   864  		if err == dnsmessage.ErrSectionDone {
   865  			break
   866  		}
   867  		if err != nil {
   868  			return nil, &DNSError{
   869  				Err:    errCannotUnmarshalDNSMessage.Error(),
   870  				Name:   addr,
   871  				Server: server,
   872  			}
   873  		}
   874  		if h.Type != dnsmessage.TypePTR {
   875  			err := p.SkipAnswer()
   876  			if err != nil {
   877  				return nil, &DNSError{
   878  					Err:    errCannotUnmarshalDNSMessage.Error(),
   879  					Name:   addr,
   880  					Server: server,
   881  				}
   882  			}
   883  			continue
   884  		}
   885  		ptr, err := p.PTRResource()
   886  		if err != nil {
   887  			return nil, &DNSError{
   888  				Err:    errCannotUnmarshalDNSMessage.Error(),
   889  				Name:   addr,
   890  				Server: server,
   891  			}
   892  		}
   893  		ptrs = append(ptrs, ptr.PTR.String())
   894  
   895  	}
   896  
   897  	return ptrs, nil
   898  }
   899  

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