lookup.go

     1  // Copyright 2012 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  package net
     6  
     7  import (
     8  	"context"
     9  	"errors"
    10  	"internal/nettrace"
    11  	"internal/singleflight"
    12  	"internal/stringslite"
    13  	"net/netip"
    14  	"sync"
    15  
    16  	"golang.org/x/net/dns/dnsmessage"
    17  )
    18  
    19  // protocols contains minimal mappings between internet protocol
    20  // names and numbers for platforms that don't have a complete list of
    21  // protocol numbers.
    22  //
    23  // See https://www.iana.org/assignments/protocol-numbers
    24  //
    25  // On Unix, this map is augmented by readProtocols via lookupProtocol.
    26  var protocols = map[string]int{
    27  	"icmp":      1,
    28  	"igmp":      2,
    29  	"tcp":       6,
    30  	"udp":       17,
    31  	"ipv6-icmp": 58,
    32  }
    33  
    34  // services contains minimal mappings between services names and port
    35  // numbers for platforms that don't have a complete list of port numbers.
    36  //
    37  // See https://www.iana.org/assignments/service-names-port-numbers
    38  //
    39  // On Unix, this map is augmented by readServices via goLookupPort.
    40  var services = map[string]map[string]int{
    41  	"udp": {
    42  		"domain": 53,
    43  	},
    44  	"tcp": {
    45  		"ftp":         21,
    46  		"ftps":        990,
    47  		"gopher":      70, // ʕ◔ϖ◔ʔ
    48  		"http":        80,
    49  		"https":       443,
    50  		"imap2":       143,
    51  		"imap3":       220,
    52  		"imaps":       993,
    53  		"pop3":        110,
    54  		"pop3s":       995,
    55  		"smtp":        25,
    56  		"submissions": 465,
    57  		"ssh":         22,
    58  		"telnet":      23,
    59  	},
    60  }
    61  
    62  // dnsWaitGroup can be used by tests to wait for all DNS goroutines to
    63  // complete. This avoids races on the test hooks.
    64  var dnsWaitGroup sync.WaitGroup
    65  
    66  const maxProtoLength = len("RSVP-E2E-IGNORE") + 10 // with room to grow
    67  
    68  func lookupProtocolMap(name string) (int, error) {
    69  	var lowerProtocol [maxProtoLength]byte
    70  	n := copy(lowerProtocol[:], name)
    71  	lowerASCIIBytes(lowerProtocol[:n])
    72  	proto, found := protocols[string(lowerProtocol[:n])]
    73  	if !found || n != len(name) {
    74  		return 0, &AddrError{Err: "unknown IP protocol specified", Addr: name}
    75  	}
    76  	return proto, nil
    77  }
    78  
    79  // maxPortBufSize is the longest reasonable name of a service
    80  // (non-numeric port).
    81  // Currently the longest known IANA-unregistered name is
    82  // "mobility-header", so we use that length, plus some slop in case
    83  // something longer is added in the future.
    84  const maxPortBufSize = len("mobility-header") + 10
    85  
    86  func lookupPortMap(network, service string) (port int, error error) {
    87  	switch network {
    88  	case "ip": // no hints
    89  		if p, err := lookupPortMapWithNetwork("tcp", "ip", service); err == nil {
    90  			return p, nil
    91  		}
    92  		return lookupPortMapWithNetwork("udp", "ip", service)
    93  	case "tcp", "tcp4", "tcp6":
    94  		return lookupPortMapWithNetwork("tcp", "tcp", service)
    95  	case "udp", "udp4", "udp6":
    96  		return lookupPortMapWithNetwork("udp", "udp", service)
    97  	}
    98  	return 0, &DNSError{Err: "unknown network", Name: network + "/" + service}
    99  }
   100  
   101  func lookupPortMapWithNetwork(network, errNetwork, service string) (port int, error error) {
   102  	if m, ok := services[network]; ok {
   103  		var lowerService [maxPortBufSize]byte
   104  		n := copy(lowerService[:], service)
   105  		lowerASCIIBytes(lowerService[:n])
   106  		if port, ok := m[string(lowerService[:n])]; ok && n == len(service) {
   107  			return port, nil
   108  		}
   109  		return 0, newDNSError(errUnknownPort, errNetwork+"/"+service, "")
   110  	}
   111  	return 0, &DNSError{Err: "unknown network", Name: errNetwork + "/" + service}
   112  }
   113  
   114  // ipVersion returns the provided network's IP version: '4', '6' or 0
   115  // if network does not end in a '4' or '6' byte.
   116  func ipVersion(network string) byte {
   117  	if network == "" {
   118  		return 0
   119  	}
   120  	n := network[len(network)-1]
   121  	if n != '4' && n != '6' {
   122  		n = 0
   123  	}
   124  	return n
   125  }
   126  
   127  // DefaultResolver is the resolver used by the package-level Lookup
   128  // functions and by Dialers without a specified Resolver.
   129  var DefaultResolver = &Resolver{}
   130  
   131  // A Resolver looks up names and numbers.
   132  //
   133  // A nil *Resolver is equivalent to a zero Resolver.
   134  type Resolver struct {
   135  	// PreferGo controls whether Go's built-in DNS resolver is preferred
   136  	// on platforms where it's available. It is equivalent to setting
   137  	// GODEBUG=netdns=go, but scoped to just this resolver.
   138  	PreferGo bool
   139  
   140  	// StrictErrors controls the behavior of temporary errors
   141  	// (including timeout, socket errors, and SERVFAIL) when using
   142  	// Go's built-in resolver. For a query composed of multiple
   143  	// sub-queries (such as an A+AAAA address lookup, or walking the
   144  	// DNS search list), this option causes such errors to abort the
   145  	// whole query instead of returning a partial result. This is
   146  	// not enabled by default because it may affect compatibility
   147  	// with resolvers that process AAAA queries incorrectly.
   148  	StrictErrors bool
   149  
   150  	// Dial optionally specifies an alternate dialer for use by
   151  	// Go's built-in DNS resolver to make TCP and UDP connections
   152  	// to DNS services. The host in the address parameter will
   153  	// always be a literal IP address and not a host name, and the
   154  	// port in the address parameter will be a literal port number
   155  	// and not a service name.
   156  	// If the Conn returned is also a PacketConn, sent and received DNS
   157  	// messages must adhere to RFC 1035 section 4.2.1, "UDP usage".
   158  	// Otherwise, DNS messages transmitted over Conn must adhere
   159  	// to RFC 7766 section 5, "Transport Protocol Selection".
   160  	// If nil, the default dialer is used.
   161  	Dial func(ctx context.Context, network, address string) (Conn, error)
   162  
   163  	// lookupGroup merges LookupIPAddr calls together for lookups for the same
   164  	// host. The lookupGroup key is the LookupIPAddr.host argument.
   165  	// The return values are ([]IPAddr, error).
   166  	lookupGroup singleflight.Group
   167  
   168  	// TODO(bradfitz): optional interface impl override hook
   169  	// TODO(bradfitz): Timeout time.Duration?
   170  }
   171  
   172  func (r *Resolver) preferGo() bool     { return r != nil && r.PreferGo }
   173  func (r *Resolver) strictErrors() bool { return r != nil && r.StrictErrors }
   174  
   175  func (r *Resolver) getLookupGroup() *singleflight.Group {
   176  	if r == nil {
   177  		return &DefaultResolver.lookupGroup
   178  	}
   179  	return &r.lookupGroup
   180  }
   181  
   182  // LookupHost looks up the given host using the local resolver.
   183  // It returns a slice of that host's addresses.
   184  //
   185  // LookupHost uses [context.Background] internally; to specify the context, use
   186  // [Resolver.LookupHost].
   187  func LookupHost(host string) (addrs []string, err error) {
   188  	return DefaultResolver.LookupHost(context.Background(), host)
   189  }
   190  
   191  // LookupHost looks up the given host using the local resolver.
   192  // It returns a slice of that host's addresses.
   193  func (r *Resolver) LookupHost(ctx context.Context, host string) (addrs []string, err error) {
   194  	// Make sure that no matter what we do later, host=="" is rejected.
   195  	if host == "" {
   196  		return nil, newDNSError(errNoSuchHost, host, "")
   197  	}
   198  	if _, err := netip.ParseAddr(host); err == nil {
   199  		return []string{host}, nil
   200  	}
   201  	return r.lookupHost(ctx, host)
   202  }
   203  
   204  // LookupIP looks up host using the local resolver.
   205  // It returns a slice of that host's IPv4 and IPv6 addresses.
   206  func LookupIP(host string) ([]IP, error) {
   207  	addrs, err := DefaultResolver.LookupIPAddr(context.Background(), host)
   208  	if err != nil {
   209  		return nil, err
   210  	}
   211  	ips := make([]IP, len(addrs))
   212  	for i, ia := range addrs {
   213  		ips[i] = ia.IP
   214  	}
   215  	return ips, nil
   216  }
   217  
   218  // LookupIPAddr looks up host using the local resolver.
   219  // It returns a slice of that host's IPv4 and IPv6 addresses.
   220  func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, error) {
   221  	return r.lookupIPAddr(ctx, "ip", host)
   222  }
   223  
   224  // LookupIP looks up host for the given network using the local resolver.
   225  // It returns a slice of that host's IP addresses of the type specified by
   226  // network.
   227  // network must be one of "ip", "ip4" or "ip6".
   228  func (r *Resolver) LookupIP(ctx context.Context, network, host string) ([]IP, error) {
   229  	afnet, _, err := parseNetwork(ctx, network, false)
   230  	if err != nil {
   231  		return nil, err
   232  	}
   233  	switch afnet {
   234  	case "ip", "ip4", "ip6":
   235  	default:
   236  		return nil, UnknownNetworkError(network)
   237  	}
   238  
   239  	if host == "" {
   240  		return nil, newDNSError(errNoSuchHost, host, "")
   241  	}
   242  	addrs, err := r.internetAddrList(ctx, afnet, host)
   243  	if err != nil {
   244  		return nil, err
   245  	}
   246  
   247  	ips := make([]IP, 0, len(addrs))
   248  	for _, addr := range addrs {
   249  		ips = append(ips, addr.(*IPAddr).IP)
   250  	}
   251  	return ips, nil
   252  }
   253  
   254  // LookupNetIP looks up host using the local resolver.
   255  // It returns a slice of that host's IP addresses of the type specified by
   256  // network.
   257  // The network must be one of "ip", "ip4" or "ip6".
   258  func (r *Resolver) LookupNetIP(ctx context.Context, network, host string) ([]netip.Addr, error) {
   259  	// TODO(bradfitz): make this efficient, making the internal net package
   260  	// type throughout be netip.Addr and only converting to the net.IP slice
   261  	// version at the edge. But for now (2021-10-20), this is a wrapper around
   262  	// the old way.
   263  	ips, err := r.LookupIP(ctx, network, host)
   264  	if err != nil {
   265  		return nil, err
   266  	}
   267  	ret := make([]netip.Addr, 0, len(ips))
   268  	for _, ip := range ips {
   269  		if a, ok := netip.AddrFromSlice(ip); ok {
   270  			ret = append(ret, a)
   271  		}
   272  	}
   273  	return ret, nil
   274  }
   275  
   276  // onlyValuesCtx is a context that uses an underlying context
   277  // for value lookup if the underlying context hasn't yet expired.
   278  type onlyValuesCtx struct {
   279  	context.Context
   280  	lookupValues context.Context
   281  }
   282  
   283  var _ context.Context = (*onlyValuesCtx)(nil)
   284  
   285  // Value performs a lookup if the original context hasn't expired.
   286  func (ovc *onlyValuesCtx) Value(key any) any {
   287  	select {
   288  	case <-ovc.lookupValues.Done():
   289  		return nil
   290  	default:
   291  		return ovc.lookupValues.Value(key)
   292  	}
   293  }
   294  
   295  // withUnexpiredValuesPreserved returns a context.Context that only uses lookupCtx
   296  // for its values, otherwise it is never canceled and has no deadline.
   297  // If the lookup context expires, any looked up values will return nil.
   298  // See Issue 28600.
   299  func withUnexpiredValuesPreserved(lookupCtx context.Context) context.Context {
   300  	return &onlyValuesCtx{Context: context.Background(), lookupValues: lookupCtx}
   301  }
   302  
   303  // lookupIPAddr looks up host using the local resolver and particular network.
   304  // It returns a slice of that host's IPv4 and IPv6 addresses.
   305  func (r *Resolver) lookupIPAddr(ctx context.Context, network, host string) ([]IPAddr, error) {
   306  	// Make sure that no matter what we do later, host=="" is rejected.
   307  	if host == "" {
   308  		return nil, newDNSError(errNoSuchHost, host, "")
   309  	}
   310  	if ip, err := netip.ParseAddr(host); err == nil {
   311  		return []IPAddr{{IP: IP(ip.AsSlice()).To16(), Zone: ip.Zone()}}, nil
   312  	}
   313  	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
   314  	if trace != nil && trace.DNSStart != nil {
   315  		trace.DNSStart(host)
   316  	}
   317  	// The underlying resolver func is lookupIP by default but it
   318  	// can be overridden by tests. This is needed by net/http, so it
   319  	// uses a context key instead of unexported variables.
   320  	resolverFunc := r.lookupIP
   321  	if alt, _ := ctx.Value(nettrace.LookupIPAltResolverKey{}).(func(context.Context, string, string) ([]IPAddr, error)); alt != nil {
   322  		resolverFunc = alt
   323  	}
   324  
   325  	// We don't want a cancellation of ctx to affect the
   326  	// lookupGroup operation. Otherwise if our context gets
   327  	// canceled it might cause an error to be returned to a lookup
   328  	// using a completely different context. However we need to preserve
   329  	// only the values in context. See Issue 28600.
   330  	lookupGroupCtx, lookupGroupCancel := context.WithCancel(withUnexpiredValuesPreserved(ctx))
   331  
   332  	lookupKey := network + "\000" + host
   333  	dnsWaitGroup.Add(1)
   334  	ch := r.getLookupGroup().DoChan(lookupKey, func() (any, error) {
   335  		return testHookLookupIP(lookupGroupCtx, resolverFunc, network, host)
   336  	})
   337  
   338  	dnsWaitGroupDone := func(ch <-chan singleflight.Result, cancelFn context.CancelFunc) {
   339  		<-ch
   340  		dnsWaitGroup.Done()
   341  		cancelFn()
   342  	}
   343  	select {
   344  	case <-ctx.Done():
   345  		// Our context was canceled. If we are the only
   346  		// goroutine looking up this key, then drop the key
   347  		// from the lookupGroup and cancel the lookup.
   348  		// If there are other goroutines looking up this key,
   349  		// let the lookup continue uncanceled, and let later
   350  		// lookups with the same key share the result.
   351  		// See issues 8602, 20703, 22724.
   352  		if r.getLookupGroup().ForgetUnshared(lookupKey) {
   353  			lookupGroupCancel()
   354  			go dnsWaitGroupDone(ch, func() {})
   355  		} else {
   356  			go dnsWaitGroupDone(ch, lookupGroupCancel)
   357  		}
   358  		err := newDNSError(mapErr(ctx.Err()), host, "")
   359  		if trace != nil && trace.DNSDone != nil {
   360  			trace.DNSDone(nil, false, err)
   361  		}
   362  		return nil, err
   363  	case r := <-ch:
   364  		dnsWaitGroup.Done()
   365  		lookupGroupCancel()
   366  		err := r.Err
   367  		if err != nil {
   368  			if _, ok := err.(*DNSError); !ok {
   369  				err = newDNSError(mapErr(err), host, "")
   370  			}
   371  		}
   372  		if trace != nil && trace.DNSDone != nil {
   373  			addrs, _ := r.Val.([]IPAddr)
   374  			trace.DNSDone(ipAddrsEface(addrs), r.Shared, err)
   375  		}
   376  		return lookupIPReturn(r.Val, err, r.Shared)
   377  	}
   378  }
   379  
   380  // lookupIPReturn turns the return values from singleflight.Do into
   381  // the return values from LookupIP.
   382  func lookupIPReturn(addrsi any, err error, shared bool) ([]IPAddr, error) {
   383  	if err != nil {
   384  		return nil, err
   385  	}
   386  	addrs := addrsi.([]IPAddr)
   387  	if shared {
   388  		clone := make([]IPAddr, len(addrs))
   389  		copy(clone, addrs)
   390  		addrs = clone
   391  	}
   392  	return addrs, nil
   393  }
   394  
   395  // ipAddrsEface returns an empty interface slice of addrs.
   396  func ipAddrsEface(addrs []IPAddr) []any {
   397  	s := make([]any, len(addrs))
   398  	for i, v := range addrs {
   399  		s[i] = v
   400  	}
   401  	return s
   402  }
   403  
   404  // LookupPort looks up the port for the given network and service.
   405  //
   406  // LookupPort uses [context.Background] internally; to specify the context, use
   407  // [Resolver.LookupPort].
   408  func LookupPort(network, service string) (port int, err error) {
   409  	return DefaultResolver.LookupPort(context.Background(), network, service)
   410  }
   411  
   412  // LookupPort looks up the port for the given network and service.
   413  //
   414  // The network must be one of "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6" or "ip".
   415  func (r *Resolver) LookupPort(ctx context.Context, network, service string) (port int, err error) {
   416  	port, needsLookup := parsePort(service)
   417  	if needsLookup {
   418  		switch network {
   419  		case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6", "ip":
   420  		case "": // a hint wildcard for Go 1.0 undocumented behavior
   421  			network = "ip"
   422  		default:
   423  			return 0, &AddrError{Err: "unknown network", Addr: network}
   424  		}
   425  		port, err = r.lookupPort(ctx, network, service)
   426  		if err != nil {
   427  			return 0, err
   428  		}
   429  	}
   430  	if 0 > port || port > 65535 {
   431  		return 0, &AddrError{Err: "invalid port", Addr: service}
   432  	}
   433  	return port, nil
   434  }
   435  
   436  // LookupCNAME returns the canonical name for the given host.
   437  // Callers that do not care about the canonical name can call
   438  // [LookupHost] or [LookupIP] directly; both take care of resolving
   439  // the canonical name as part of the lookup.
   440  //
   441  // A canonical name is the final name after following zero
   442  // or more CNAME records.
   443  // LookupCNAME does not return an error if host does not
   444  // contain DNS "CNAME" records, as long as host resolves to
   445  // address records.
   446  //
   447  // The returned canonical name is validated to be a properly
   448  // formatted presentation-format domain name.
   449  //
   450  // LookupCNAME uses [context.Background] internally; to specify the context, use
   451  // [Resolver.LookupCNAME].
   452  func LookupCNAME(host string) (cname string, err error) {
   453  	return DefaultResolver.LookupCNAME(context.Background(), host)
   454  }
   455  
   456  // LookupCNAME returns the canonical name for the given host.
   457  // Callers that do not care about the canonical name can call
   458  // [LookupHost] or [LookupIP] directly; both take care of resolving
   459  // the canonical name as part of the lookup.
   460  //
   461  // A canonical name is the final name after following zero
   462  // or more CNAME records.
   463  // LookupCNAME does not return an error if host does not
   464  // contain DNS "CNAME" records, as long as host resolves to
   465  // address records.
   466  //
   467  // The returned canonical name is validated to be a properly
   468  // formatted presentation-format domain name.
   469  func (r *Resolver) LookupCNAME(ctx context.Context, host string) (string, error) {
   470  	cname, err := r.lookupCNAME(ctx, host)
   471  	if err != nil {
   472  		return "", err
   473  	}
   474  	if !isDomainName(cname) {
   475  		return "", &DNSError{Err: errMalformedDNSRecordsDetail, Name: host}
   476  	}
   477  	return cname, nil
   478  }
   479  
   480  // LookupSRV tries to resolve an [SRV] query of the given service,
   481  // protocol, and domain name. The proto is "tcp" or "udp".
   482  // The returned records are sorted by priority and randomized
   483  // by weight within a priority.
   484  //
   485  // LookupSRV constructs the DNS name to look up following RFC 2782.
   486  // That is, it looks up _service._proto.name. To accommodate services
   487  // publishing SRV records under non-standard names, if both service
   488  // and proto are empty strings, LookupSRV looks up name directly.
   489  //
   490  // The returned service names are validated to be properly
   491  // formatted presentation-format domain names. If the response contains
   492  // invalid names, those records are filtered out and an error
   493  // will be returned alongside the remaining results, if any.
   494  func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err error) {
   495  	return DefaultResolver.LookupSRV(context.Background(), service, proto, name)
   496  }
   497  
   498  // LookupSRV tries to resolve an [SRV] query of the given service,
   499  // protocol, and domain name. The proto is "tcp" or "udp".
   500  // The returned records are sorted by priority and randomized
   501  // by weight within a priority.
   502  //
   503  // LookupSRV constructs the DNS name to look up following RFC 2782.
   504  // That is, it looks up _service._proto.name. To accommodate services
   505  // publishing SRV records under non-standard names, if both service
   506  // and proto are empty strings, LookupSRV looks up name directly.
   507  //
   508  // The returned service names are validated to be properly
   509  // formatted presentation-format domain names. If the response contains
   510  // invalid names, those records are filtered out and an error
   511  // will be returned alongside the remaining results, if any.
   512  func (r *Resolver) LookupSRV(ctx context.Context, service, proto, name string) (string, []*SRV, error) {
   513  	cname, addrs, err := r.lookupSRV(ctx, service, proto, name)
   514  	if err != nil {
   515  		return "", nil, err
   516  	}
   517  	if cname != "" && !isDomainName(cname) {
   518  		return "", nil, &DNSError{Err: "SRV header name is invalid", Name: name}
   519  	}
   520  	filteredAddrs := make([]*SRV, 0, len(addrs))
   521  	for _, addr := range addrs {
   522  		if addr == nil {
   523  			continue
   524  		}
   525  		if !isDomainName(addr.Target) {
   526  			continue
   527  		}
   528  		filteredAddrs = append(filteredAddrs, addr)
   529  	}
   530  	if len(addrs) != len(filteredAddrs) {
   531  		return cname, filteredAddrs, &DNSError{Err: errMalformedDNSRecordsDetail, Name: name}
   532  	}
   533  	return cname, filteredAddrs, nil
   534  }
   535  
   536  // LookupMX returns the DNS MX records for the given domain name sorted by preference.
   537  //
   538  // The returned mail server names are validated to be properly
   539  // formatted presentation-format domain names, or numeric IP addresses.
   540  // If the response contains invalid names, those records are filtered out
   541  // and an error will be returned alongside the remaining results, if any.
   542  //
   543  // LookupMX uses [context.Background] internally; to specify the context, use
   544  // [Resolver.LookupMX].
   545  func LookupMX(name string) ([]*MX, error) {
   546  	return DefaultResolver.LookupMX(context.Background(), name)
   547  }
   548  
   549  // LookupMX returns the DNS MX records for the given domain name sorted by preference.
   550  //
   551  // The returned mail server names are validated to be properly
   552  // formatted presentation-format domain names, or numeric IP addresses.
   553  // If the response contains invalid names, those records are filtered out
   554  // and an error will be returned alongside the remaining results, if any.
   555  func (r *Resolver) LookupMX(ctx context.Context, name string) ([]*MX, error) {
   556  	records, err := r.lookupMX(ctx, name)
   557  	if err != nil {
   558  		return nil, err
   559  	}
   560  	filteredMX := make([]*MX, 0, len(records))
   561  	for _, mx := range records {
   562  		if mx == nil {
   563  			continue
   564  		}
   565  		if !isDomainName(mx.Host) {
   566  			// Check for IP address. In practice we observe
   567  			// these with a trailing dot, so strip that.
   568  			ip, err := netip.ParseAddr(stringslite.TrimSuffix(mx.Host, "."))
   569  			if err != nil || ip.Zone() != "" {
   570  				continue
   571  			}
   572  		}
   573  		filteredMX = append(filteredMX, mx)
   574  	}
   575  	if len(records) != len(filteredMX) {
   576  		return filteredMX, &DNSError{Err: errMalformedDNSRecordsDetail, Name: name}
   577  	}
   578  	return filteredMX, nil
   579  }
   580  
   581  // LookupNS returns the DNS NS records for the given domain name.
   582  //
   583  // The returned name server names are validated to be properly
   584  // formatted presentation-format domain names. If the response contains
   585  // invalid names, those records are filtered out and an error
   586  // will be returned alongside the remaining results, if any.
   587  //
   588  // LookupNS uses [context.Background] internally; to specify the context, use
   589  // [Resolver.LookupNS].
   590  func LookupNS(name string) ([]*NS, error) {
   591  	return DefaultResolver.LookupNS(context.Background(), name)
   592  }
   593  
   594  // LookupNS returns the DNS NS records for the given domain name.
   595  //
   596  // The returned name server names are validated to be properly
   597  // formatted presentation-format domain names. If the response contains
   598  // invalid names, those records are filtered out and an error
   599  // will be returned alongside the remaining results, if any.
   600  func (r *Resolver) LookupNS(ctx context.Context, name string) ([]*NS, error) {
   601  	records, err := r.lookupNS(ctx, name)
   602  	if err != nil {
   603  		return nil, err
   604  	}
   605  	filteredNS := make([]*NS, 0, len(records))
   606  	for _, ns := range records {
   607  		if ns == nil {
   608  			continue
   609  		}
   610  		if !isDomainName(ns.Host) {
   611  			continue
   612  		}
   613  		filteredNS = append(filteredNS, ns)
   614  	}
   615  	if len(records) != len(filteredNS) {
   616  		return filteredNS, &DNSError{Err: errMalformedDNSRecordsDetail, Name: name}
   617  	}
   618  	return filteredNS, nil
   619  }
   620  
   621  // LookupTXT returns the DNS TXT records for the given domain name.
   622  //
   623  // If a DNS TXT record holds multiple strings, they are concatenated as a
   624  // single string.
   625  //
   626  // LookupTXT uses [context.Background] internally; to specify the context, use
   627  // [Resolver.LookupTXT].
   628  func LookupTXT(name string) ([]string, error) {
   629  	return DefaultResolver.lookupTXT(context.Background(), name)
   630  }
   631  
   632  // LookupTXT returns the DNS TXT records for the given domain name.
   633  //
   634  // If a DNS TXT record holds multiple strings, they are concatenated as a
   635  // single string.
   636  func (r *Resolver) LookupTXT(ctx context.Context, name string) ([]string, error) {
   637  	return r.lookupTXT(ctx, name)
   638  }
   639  
   640  // LookupAddr performs a reverse lookup for the given address, returning a list
   641  // of names mapping to that address.
   642  //
   643  // The returned names are validated to be properly formatted presentation-format
   644  // domain names. If the response contains invalid names, those records are filtered
   645  // out and an error will be returned alongside the remaining results, if any.
   646  //
   647  // When using the host C library resolver, at most one result will be
   648  // returned. To bypass the host resolver, use a custom [Resolver].
   649  //
   650  // LookupAddr uses [context.Background] internally; to specify the context, use
   651  // [Resolver.LookupAddr].
   652  func LookupAddr(addr string) (names []string, err error) {
   653  	return DefaultResolver.LookupAddr(context.Background(), addr)
   654  }
   655  
   656  // LookupAddr performs a reverse lookup for the given address, returning a list
   657  // of names mapping to that address.
   658  //
   659  // The returned names are validated to be properly formatted presentation-format
   660  // domain names. If the response contains invalid names, those records are filtered
   661  // out and an error will be returned alongside the remaining results, if any.
   662  func (r *Resolver) LookupAddr(ctx context.Context, addr string) ([]string, error) {
   663  	names, err := r.lookupAddr(ctx, addr)
   664  	if err != nil {
   665  		return nil, err
   666  	}
   667  	filteredNames := make([]string, 0, len(names))
   668  	for _, name := range names {
   669  		if isDomainName(name) {
   670  			filteredNames = append(filteredNames, name)
   671  		}
   672  	}
   673  	if len(names) != len(filteredNames) {
   674  		return filteredNames, &DNSError{Err: errMalformedDNSRecordsDetail, Name: addr}
   675  	}
   676  	return filteredNames, nil
   677  }
   678  
   679  // errMalformedDNSRecordsDetail is the DNSError detail which is returned when a Resolver.Lookup...
   680  // method receives DNS records which contain invalid DNS names. This may be returned alongside
   681  // results which have had the malformed records filtered out.
   682  var errMalformedDNSRecordsDetail = "DNS response contained records which contain invalid names"
   683  
   684  // dial makes a new connection to the provided server (which must be
   685  // an IP address) with the provided network type, using either r.Dial
   686  // (if both r and r.Dial are non-nil) or else Dialer.DialContext.
   687  func (r *Resolver) dial(ctx context.Context, network, server string) (Conn, error) {
   688  	// Calling Dial here is scary -- we have to be sure not to
   689  	// dial a name that will require a DNS lookup, or Dial will
   690  	// call back here to translate it. The DNS config parser has
   691  	// already checked that all the cfg.servers are IP
   692  	// addresses, which Dial will use without a DNS lookup.
   693  	var c Conn
   694  	var err error
   695  	if r != nil && r.Dial != nil {
   696  		c, err = r.Dial(ctx, network, server)
   697  	} else {
   698  		var d Dialer
   699  		c, err = d.DialContext(ctx, network, server)
   700  	}
   701  	if err != nil {
   702  		return nil, mapErr(err)
   703  	}
   704  	return c, nil
   705  }
   706  
   707  // goLookupSRV returns the SRV records for a target name, built either
   708  // from its component service ("sip"), protocol ("tcp"), and name
   709  // ("example.com."), or from name directly (if service and proto are
   710  // both empty).
   711  //
   712  // In either case, the returned target name ("_sip._tcp.example.com.")
   713  // is also returned on success.
   714  //
   715  // The records are sorted by weight.
   716  func (r *Resolver) goLookupSRV(ctx context.Context, service, proto, name string) (target string, srvs []*SRV, err error) {
   717  	if service == "" && proto == "" {
   718  		target = name
   719  	} else {
   720  		target = "_" + service + "._" + proto + "." + name
   721  	}
   722  	p, server, err := r.lookup(ctx, target, dnsmessage.TypeSRV, nil)
   723  	if err != nil {
   724  		return "", nil, err
   725  	}
   726  	var cname dnsmessage.Name
   727  	for {
   728  		h, err := p.AnswerHeader()
   729  		if err == dnsmessage.ErrSectionDone {
   730  			break
   731  		}
   732  		if err != nil {
   733  			return "", nil, &DNSError{
   734  				Err:    "cannot unmarshal DNS message",
   735  				Name:   name,
   736  				Server: server,
   737  			}
   738  		}
   739  		if h.Type != dnsmessage.TypeSRV {
   740  			if err := p.SkipAnswer(); err != nil {
   741  				return "", nil, &DNSError{
   742  					Err:    "cannot unmarshal DNS message",
   743  					Name:   name,
   744  					Server: server,
   745  				}
   746  			}
   747  			continue
   748  		}
   749  		if cname.Length == 0 && h.Name.Length != 0 {
   750  			cname = h.Name
   751  		}
   752  		srv, err := p.SRVResource()
   753  		if err != nil {
   754  			return "", nil, &DNSError{
   755  				Err:    "cannot unmarshal DNS message",
   756  				Name:   name,
   757  				Server: server,
   758  			}
   759  		}
   760  		srvs = append(srvs, &SRV{Target: srv.Target.String(), Port: srv.Port, Priority: srv.Priority, Weight: srv.Weight})
   761  	}
   762  	byPriorityWeight(srvs).sort()
   763  	return cname.String(), srvs, nil
   764  }
   765  
   766  // goLookupMX returns the MX records for name.
   767  func (r *Resolver) goLookupMX(ctx context.Context, name string) ([]*MX, error) {
   768  	p, server, err := r.lookup(ctx, name, dnsmessage.TypeMX, nil)
   769  	if err != nil {
   770  		return nil, err
   771  	}
   772  	var mxs []*MX
   773  	for {
   774  		h, err := p.AnswerHeader()
   775  		if err == dnsmessage.ErrSectionDone {
   776  			break
   777  		}
   778  		if err != nil {
   779  			return nil, &DNSError{
   780  				Err:    "cannot unmarshal DNS message",
   781  				Name:   name,
   782  				Server: server,
   783  			}
   784  		}
   785  		if h.Type != dnsmessage.TypeMX {
   786  			if err := p.SkipAnswer(); err != nil {
   787  				return nil, &DNSError{
   788  					Err:    "cannot unmarshal DNS message",
   789  					Name:   name,
   790  					Server: server,
   791  				}
   792  			}
   793  			continue
   794  		}
   795  		mx, err := p.MXResource()
   796  		if err != nil {
   797  			return nil, &DNSError{
   798  				Err:    "cannot unmarshal DNS message",
   799  				Name:   name,
   800  				Server: server,
   801  			}
   802  		}
   803  		mxs = append(mxs, &MX{Host: mx.MX.String(), Pref: mx.Pref})
   804  
   805  	}
   806  	byPref(mxs).sort()
   807  	return mxs, nil
   808  }
   809  
   810  // goLookupNS returns the NS records for name.
   811  func (r *Resolver) goLookupNS(ctx context.Context, name string) ([]*NS, error) {
   812  	p, server, err := r.lookup(ctx, name, dnsmessage.TypeNS, nil)
   813  	if err != nil {
   814  		return nil, err
   815  	}
   816  	var nss []*NS
   817  	for {
   818  		h, err := p.AnswerHeader()
   819  		if err == dnsmessage.ErrSectionDone {
   820  			break
   821  		}
   822  		if err != nil {
   823  			return nil, &DNSError{
   824  				Err:    "cannot unmarshal DNS message",
   825  				Name:   name,
   826  				Server: server,
   827  			}
   828  		}
   829  		if h.Type != dnsmessage.TypeNS {
   830  			if err := p.SkipAnswer(); err != nil {
   831  				return nil, &DNSError{
   832  					Err:    "cannot unmarshal DNS message",
   833  					Name:   name,
   834  					Server: server,
   835  				}
   836  			}
   837  			continue
   838  		}
   839  		ns, err := p.NSResource()
   840  		if err != nil {
   841  			return nil, &DNSError{
   842  				Err:    "cannot unmarshal DNS message",
   843  				Name:   name,
   844  				Server: server,
   845  			}
   846  		}
   847  		nss = append(nss, &NS{Host: ns.NS.String()})
   848  	}
   849  	return nss, nil
   850  }
   851  
   852  // goLookupTXT returns the TXT records from name.
   853  func (r *Resolver) goLookupTXT(ctx context.Context, name string) ([]string, error) {
   854  	p, server, err := r.lookup(ctx, name, dnsmessage.TypeTXT, nil)
   855  	if err != nil {
   856  		return nil, err
   857  	}
   858  	var txts []string
   859  	for {
   860  		h, err := p.AnswerHeader()
   861  		if err == dnsmessage.ErrSectionDone {
   862  			break
   863  		}
   864  		if err != nil {
   865  			return nil, &DNSError{
   866  				Err:    "cannot unmarshal DNS message",
   867  				Name:   name,
   868  				Server: server,
   869  			}
   870  		}
   871  		if h.Type != dnsmessage.TypeTXT {
   872  			if err := p.SkipAnswer(); err != nil {
   873  				return nil, &DNSError{
   874  					Err:    "cannot unmarshal DNS message",
   875  					Name:   name,
   876  					Server: server,
   877  				}
   878  			}
   879  			continue
   880  		}
   881  		txt, err := p.TXTResource()
   882  		if err != nil {
   883  			return nil, &DNSError{
   884  				Err:    "cannot unmarshal DNS message",
   885  				Name:   name,
   886  				Server: server,
   887  			}
   888  		}
   889  		// Multiple strings in one TXT record need to be
   890  		// concatenated without separator to be consistent
   891  		// with previous Go resolver.
   892  		n := 0
   893  		for _, s := range txt.TXT {
   894  			n += len(s)
   895  		}
   896  		txtJoin := make([]byte, 0, n)
   897  		for _, s := range txt.TXT {
   898  			txtJoin = append(txtJoin, s...)
   899  		}
   900  		if len(txts) == 0 {
   901  			txts = make([]string, 0, 1)
   902  		}
   903  		txts = append(txts, string(txtJoin))
   904  	}
   905  	return txts, nil
   906  }
   907  
   908  func parseCNAMEFromResources(resources []dnsmessage.Resource) (string, error) {
   909  	if len(resources) == 0 {
   910  		return "", errors.New("no CNAME record received")
   911  	}
   912  	c, ok := resources[0].Body.(*dnsmessage.CNAMEResource)
   913  	if !ok {
   914  		return "", errors.New("could not parse CNAME record")
   915  	}
   916  	return c.CNAME.String(), nil
   917  }
   918
View as plain text