lookup.go

Documentation: net

     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  	"internal/nettrace"
    10  	"internal/singleflight"
    11  	"sync"
    12  )
    13  
    14  // protocols contains minimal mappings between internet protocol
    15  // names and numbers for platforms that don't have a complete list of
    16  // protocol numbers.
    17  //
    18  // See https://www.iana.org/assignments/protocol-numbers
    19  //
    20  // On Unix, this map is augmented by readProtocols via lookupProtocol.
    21  var protocols = map[string]int{
    22  	"icmp":      1,
    23  	"igmp":      2,
    24  	"tcp":       6,
    25  	"udp":       17,
    26  	"ipv6-icmp": 58,
    27  }
    28  
    29  // services contains minimal mappings between services names and port
    30  // numbers for platforms that don't have a complete list of port numbers.
    31  //
    32  // See https://www.iana.org/assignments/service-names-port-numbers
    33  //
    34  // On Unix, this map is augmented by readServices via goLookupPort.
    35  var services = map[string]map[string]int{
    36  	"udp": {
    37  		"domain": 53,
    38  	},
    39  	"tcp": {
    40  		"ftp":    21,
    41  		"ftps":   990,
    42  		"gopher": 70, // ʕ◔ϖ◔ʔ
    43  		"http":   80,
    44  		"https":  443,
    45  		"imap2":  143,
    46  		"imap3":  220,
    47  		"imaps":  993,
    48  		"pop3":   110,
    49  		"pop3s":  995,
    50  		"smtp":   25,
    51  		"ssh":    22,
    52  		"telnet": 23,
    53  	},
    54  }
    55  
    56  // dnsWaitGroup can be used by tests to wait for all DNS goroutines to
    57  // complete. This avoids races on the test hooks.
    58  var dnsWaitGroup sync.WaitGroup
    59  
    60  const maxProtoLength = len("RSVP-E2E-IGNORE") + 10 // with room to grow
    61  
    62  func lookupProtocolMap(name string) (int, error) {
    63  	var lowerProtocol [maxProtoLength]byte
    64  	n := copy(lowerProtocol[:], name)
    65  	lowerASCIIBytes(lowerProtocol[:n])
    66  	proto, found := protocols[string(lowerProtocol[:n])]
    67  	if !found || n != len(name) {
    68  		return 0, &AddrError{Err: "unknown IP protocol specified", Addr: name}
    69  	}
    70  	return proto, nil
    71  }
    72  
    73  // maxPortBufSize is the longest reasonable name of a service
    74  // (non-numeric port).
    75  // Currently the longest known IANA-unregistered name is
    76  // "mobility-header", so we use that length, plus some slop in case
    77  // something longer is added in the future.
    78  const maxPortBufSize = len("mobility-header") + 10
    79  
    80  func lookupPortMap(network, service string) (port int, error error) {
    81  	switch network {
    82  	case "tcp4", "tcp6":
    83  		network = "tcp"
    84  	case "udp4", "udp6":
    85  		network = "udp"
    86  	}
    87  
    88  	if m, ok := services[network]; ok {
    89  		var lowerService [maxPortBufSize]byte
    90  		n := copy(lowerService[:], service)
    91  		lowerASCIIBytes(lowerService[:n])
    92  		if port, ok := m[string(lowerService[:n])]; ok && n == len(service) {
    93  			return port, nil
    94  		}
    95  	}
    96  	return 0, &AddrError{Err: "unknown port", Addr: network + "/" + service}
    97  }
    98  
    99  // ipVersion returns the provided network's IP version: '4', '6' or 0
   100  // if network does not end in a '4' or '6' byte.
   101  func ipVersion(network string) byte {
   102  	if network == "" {
   103  		return 0
   104  	}
   105  	n := network[len(network)-1]
   106  	if n != '4' && n != '6' {
   107  		n = 0
   108  	}
   109  	return n
   110  }
   111  
   112  // DefaultResolver is the resolver used by the package-level Lookup
   113  // functions and by Dialers without a specified Resolver.
   114  var DefaultResolver = &Resolver{}
   115  
   116  // A Resolver looks up names and numbers.
   117  //
   118  // A nil *Resolver is equivalent to a zero Resolver.
   119  type Resolver struct {
   120  	// PreferGo controls whether Go's built-in DNS resolver is preferred
   121  	// on platforms where it's available. It is equivalent to setting
   122  	// GODEBUG=netdns=go, but scoped to just this resolver.
   123  	PreferGo bool
   124  
   125  	// StrictErrors controls the behavior of temporary errors
   126  	// (including timeout, socket errors, and SERVFAIL) when using
   127  	// Go's built-in resolver. For a query composed of multiple
   128  	// sub-queries (such as an A+AAAA address lookup, or walking the
   129  	// DNS search list), this option causes such errors to abort the
   130  	// whole query instead of returning a partial result. This is
   131  	// not enabled by default because it may affect compatibility
   132  	// with resolvers that process AAAA queries incorrectly.
   133  	StrictErrors bool
   134  
   135  	// Dial optionally specifies an alternate dialer for use by
   136  	// Go's built-in DNS resolver to make TCP and UDP connections
   137  	// to DNS services. The host in the address parameter will
   138  	// always be a literal IP address and not a host name, and the
   139  	// port in the address parameter will be a literal port number
   140  	// and not a service name.
   141  	// If the Conn returned is also a PacketConn, sent and received DNS
   142  	// messages must adhere to RFC 1035 section 4.2.1, "UDP usage".
   143  	// Otherwise, DNS messages transmitted over Conn must adhere
   144  	// to RFC 7766 section 5, "Transport Protocol Selection".
   145  	// If nil, the default dialer is used.
   146  	Dial func(ctx context.Context, network, address string) (Conn, error)
   147  
   148  	// lookupGroup merges LookupIPAddr calls together for lookups for the same
   149  	// host. The lookupGroup key is the LookupIPAddr.host argument.
   150  	// The return values are ([]IPAddr, error).
   151  	lookupGroup singleflight.Group
   152  
   153  	// TODO(bradfitz): optional interface impl override hook
   154  	// TODO(bradfitz): Timeout time.Duration?
   155  }
   156  
   157  func (r *Resolver) preferGo() bool     { return r != nil && r.PreferGo }
   158  func (r *Resolver) strictErrors() bool { return r != nil && r.StrictErrors }
   159  
   160  func (r *Resolver) getLookupGroup() *singleflight.Group {
   161  	if r == nil {
   162  		return &DefaultResolver.lookupGroup
   163  	}
   164  	return &r.lookupGroup
   165  }
   166  
   167  // LookupHost looks up the given host using the local resolver.
   168  // It returns a slice of that host's addresses.
   169  func LookupHost(host string) (addrs []string, err error) {
   170  	return DefaultResolver.LookupHost(context.Background(), host)
   171  }
   172  
   173  // LookupHost looks up the given host using the local resolver.
   174  // It returns a slice of that host's addresses.
   175  func (r *Resolver) LookupHost(ctx context.Context, host string) (addrs []string, err error) {
   176  	// Make sure that no matter what we do later, host=="" is rejected.
   177  	// parseIP, for example, does accept empty strings.
   178  	if host == "" {
   179  		return nil, &DNSError{Err: errNoSuchHost.Error(), Name: host, IsNotFound: true}
   180  	}
   181  	if ip, _ := parseIPZone(host); ip != nil {
   182  		return []string{host}, nil
   183  	}
   184  	return r.lookupHost(ctx, host)
   185  }
   186  
   187  // LookupIP looks up host using the local resolver.
   188  // It returns a slice of that host's IPv4 and IPv6 addresses.
   189  func LookupIP(host string) ([]IP, error) {
   190  	addrs, err := DefaultResolver.LookupIPAddr(context.Background(), host)
   191  	if err != nil {
   192  		return nil, err
   193  	}
   194  	ips := make([]IP, len(addrs))
   195  	for i, ia := range addrs {
   196  		ips[i] = ia.IP
   197  	}
   198  	return ips, nil
   199  }
   200  
   201  // LookupIPAddr looks up host using the local resolver.
   202  // It returns a slice of that host's IPv4 and IPv6 addresses.
   203  func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, error) {
   204  	return r.lookupIPAddr(ctx, "ip", host)
   205  }
   206  
   207  // onlyValuesCtx is a context that uses an underlying context
   208  // for value lookup if the underlying context hasn't yet expired.
   209  type onlyValuesCtx struct {
   210  	context.Context
   211  	lookupValues context.Context
   212  }
   213  
   214  var _ context.Context = (*onlyValuesCtx)(nil)
   215  
   216  // Value performs a lookup if the original context hasn't expired.
   217  func (ovc *onlyValuesCtx) Value(key interface{}) interface{} {
   218  	select {
   219  	case <-ovc.lookupValues.Done():
   220  		return nil
   221  	default:
   222  		return ovc.lookupValues.Value(key)
   223  	}
   224  }
   225  
   226  // withUnexpiredValuesPreserved returns a context.Context that only uses lookupCtx
   227  // for its values, otherwise it is never canceled and has no deadline.
   228  // If the lookup context expires, any looked up values will return nil.
   229  // See Issue 28600.
   230  func withUnexpiredValuesPreserved(lookupCtx context.Context) context.Context {
   231  	return &onlyValuesCtx{Context: context.Background(), lookupValues: lookupCtx}
   232  }
   233  
   234  // lookupIPAddr looks up host using the local resolver and particular network.
   235  // It returns a slice of that host's IPv4 and IPv6 addresses.
   236  func (r *Resolver) lookupIPAddr(ctx context.Context, network, host string) ([]IPAddr, error) {
   237  	// Make sure that no matter what we do later, host=="" is rejected.
   238  	// parseIP, for example, does accept empty strings.
   239  	if host == "" {
   240  		return nil, &DNSError{Err: errNoSuchHost.Error(), Name: host, IsNotFound: true}
   241  	}
   242  	if ip, zone := parseIPZone(host); ip != nil {
   243  		return []IPAddr{{IP: ip, Zone: zone}}, nil
   244  	}
   245  	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
   246  	if trace != nil && trace.DNSStart != nil {
   247  		trace.DNSStart(host)
   248  	}
   249  	// The underlying resolver func is lookupIP by default but it
   250  	// can be overridden by tests. This is needed by net/http, so it
   251  	// uses a context key instead of unexported variables.
   252  	resolverFunc := r.lookupIP
   253  	if alt, _ := ctx.Value(nettrace.LookupIPAltResolverKey{}).(func(context.Context, string, string) ([]IPAddr, error)); alt != nil {
   254  		resolverFunc = alt
   255  	}
   256  
   257  	// We don't want a cancellation of ctx to affect the
   258  	// lookupGroup operation. Otherwise if our context gets
   259  	// canceled it might cause an error to be returned to a lookup
   260  	// using a completely different context. However we need to preserve
   261  	// only the values in context. See Issue 28600.
   262  	lookupGroupCtx, lookupGroupCancel := context.WithCancel(withUnexpiredValuesPreserved(ctx))
   263  
   264  	lookupKey := network + "\000" + host
   265  	dnsWaitGroup.Add(1)
   266  	ch, called := r.getLookupGroup().DoChan(lookupKey, func() (interface{}, error) {
   267  		defer dnsWaitGroup.Done()
   268  		return testHookLookupIP(lookupGroupCtx, resolverFunc, network, host)
   269  	})
   270  	if !called {
   271  		dnsWaitGroup.Done()
   272  	}
   273  
   274  	select {
   275  	case <-ctx.Done():
   276  		// Our context was canceled. If we are the only
   277  		// goroutine looking up this key, then drop the key
   278  		// from the lookupGroup and cancel the lookup.
   279  		// If there are other goroutines looking up this key,
   280  		// let the lookup continue uncanceled, and let later
   281  		// lookups with the same key share the result.
   282  		// See issues 8602, 20703, 22724.
   283  		if r.getLookupGroup().ForgetUnshared(lookupKey) {
   284  			lookupGroupCancel()
   285  		} else {
   286  			go func() {
   287  				<-ch
   288  				lookupGroupCancel()
   289  			}()
   290  		}
   291  		err := mapErr(ctx.Err())
   292  		if trace != nil && trace.DNSDone != nil {
   293  			trace.DNSDone(nil, false, err)
   294  		}
   295  		return nil, err
   296  	case r := <-ch:
   297  		lookupGroupCancel()
   298  		if trace != nil && trace.DNSDone != nil {
   299  			addrs, _ := r.Val.([]IPAddr)
   300  			trace.DNSDone(ipAddrsEface(addrs), r.Shared, r.Err)
   301  		}
   302  		return lookupIPReturn(r.Val, r.Err, r.Shared)
   303  	}
   304  }
   305  
   306  // lookupIPReturn turns the return values from singleflight.Do into
   307  // the return values from LookupIP.
   308  func lookupIPReturn(addrsi interface{}, err error, shared bool) ([]IPAddr, error) {
   309  	if err != nil {
   310  		return nil, err
   311  	}
   312  	addrs := addrsi.([]IPAddr)
   313  	if shared {
   314  		clone := make([]IPAddr, len(addrs))
   315  		copy(clone, addrs)
   316  		addrs = clone
   317  	}
   318  	return addrs, nil
   319  }
   320  
   321  // ipAddrsEface returns an empty interface slice of addrs.
   322  func ipAddrsEface(addrs []IPAddr) []interface{} {
   323  	s := make([]interface{}, len(addrs))
   324  	for i, v := range addrs {
   325  		s[i] = v
   326  	}
   327  	return s
   328  }
   329  
   330  // LookupPort looks up the port for the given network and service.
   331  func LookupPort(network, service string) (port int, err error) {
   332  	return DefaultResolver.LookupPort(context.Background(), network, service)
   333  }
   334  
   335  // LookupPort looks up the port for the given network and service.
   336  func (r *Resolver) LookupPort(ctx context.Context, network, service string) (port int, err error) {
   337  	port, needsLookup := parsePort(service)
   338  	if needsLookup {
   339  		switch network {
   340  		case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6":
   341  		case "": // a hint wildcard for Go 1.0 undocumented behavior
   342  			network = "ip"
   343  		default:
   344  			return 0, &AddrError{Err: "unknown network", Addr: network}
   345  		}
   346  		port, err = r.lookupPort(ctx, network, service)
   347  		if err != nil {
   348  			return 0, err
   349  		}
   350  	}
   351  	if 0 > port || port > 65535 {
   352  		return 0, &AddrError{Err: "invalid port", Addr: service}
   353  	}
   354  	return port, nil
   355  }
   356  
   357  // LookupCNAME returns the canonical name for the given host.
   358  // Callers that do not care about the canonical name can call
   359  // LookupHost or LookupIP directly; both take care of resolving
   360  // the canonical name as part of the lookup.
   361  //
   362  // A canonical name is the final name after following zero
   363  // or more CNAME records.
   364  // LookupCNAME does not return an error if host does not
   365  // contain DNS "CNAME" records, as long as host resolves to
   366  // address records.
   367  func LookupCNAME(host string) (cname string, err error) {
   368  	return DefaultResolver.lookupCNAME(context.Background(), host)
   369  }
   370  
   371  // LookupCNAME returns the canonical name for the given host.
   372  // Callers that do not care about the canonical name can call
   373  // LookupHost or LookupIP directly; both take care of resolving
   374  // the canonical name as part of the lookup.
   375  //
   376  // A canonical name is the final name after following zero
   377  // or more CNAME records.
   378  // LookupCNAME does not return an error if host does not
   379  // contain DNS "CNAME" records, as long as host resolves to
   380  // address records.
   381  func (r *Resolver) LookupCNAME(ctx context.Context, host string) (cname string, err error) {
   382  	return r.lookupCNAME(ctx, host)
   383  }
   384  
   385  // LookupSRV tries to resolve an SRV query of the given service,
   386  // protocol, and domain name. The proto is "tcp" or "udp".
   387  // The returned records are sorted by priority and randomized
   388  // by weight within a priority.
   389  //
   390  // LookupSRV constructs the DNS name to look up following RFC 2782.
   391  // That is, it looks up _service._proto.name. To accommodate services
   392  // publishing SRV records under non-standard names, if both service
   393  // and proto are empty strings, LookupSRV looks up name directly.
   394  func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err error) {
   395  	return DefaultResolver.lookupSRV(context.Background(), service, proto, name)
   396  }
   397  
   398  // LookupSRV tries to resolve an SRV query of the given service,
   399  // protocol, and domain name. The proto is "tcp" or "udp".
   400  // The returned records are sorted by priority and randomized
   401  // by weight within a priority.
   402  //
   403  // LookupSRV constructs the DNS name to look up following RFC 2782.
   404  // That is, it looks up _service._proto.name. To accommodate services
   405  // publishing SRV records under non-standard names, if both service
   406  // and proto are empty strings, LookupSRV looks up name directly.
   407  func (r *Resolver) LookupSRV(ctx context.Context, service, proto, name string) (cname string, addrs []*SRV, err error) {
   408  	return r.lookupSRV(ctx, service, proto, name)
   409  }
   410  
   411  // LookupMX returns the DNS MX records for the given domain name sorted by preference.
   412  func LookupMX(name string) ([]*MX, error) {
   413  	return DefaultResolver.lookupMX(context.Background(), name)
   414  }
   415  
   416  // LookupMX returns the DNS MX records for the given domain name sorted by preference.
   417  func (r *Resolver) LookupMX(ctx context.Context, name string) ([]*MX, error) {
   418  	return r.lookupMX(ctx, name)
   419  }
   420  
   421  // LookupNS returns the DNS NS records for the given domain name.
   422  func LookupNS(name string) ([]*NS, error) {
   423  	return DefaultResolver.lookupNS(context.Background(), name)
   424  }
   425  
   426  // LookupNS returns the DNS NS records for the given domain name.
   427  func (r *Resolver) LookupNS(ctx context.Context, name string) ([]*NS, error) {
   428  	return r.lookupNS(ctx, name)
   429  }
   430  
   431  // LookupTXT returns the DNS TXT records for the given domain name.
   432  func LookupTXT(name string) ([]string, error) {
   433  	return DefaultResolver.lookupTXT(context.Background(), name)
   434  }
   435  
   436  // LookupTXT returns the DNS TXT records for the given domain name.
   437  func (r *Resolver) LookupTXT(ctx context.Context, name string) ([]string, error) {
   438  	return r.lookupTXT(ctx, name)
   439  }
   440  
   441  // LookupAddr performs a reverse lookup for the given address, returning a list
   442  // of names mapping to that address.
   443  //
   444  // When using the host C library resolver, at most one result will be
   445  // returned. To bypass the host resolver, use a custom Resolver.
   446  func LookupAddr(addr string) (names []string, err error) {
   447  	return DefaultResolver.lookupAddr(context.Background(), addr)
   448  }
   449  
   450  // LookupAddr performs a reverse lookup for the given address, returning a list
   451  // of names mapping to that address.
   452  func (r *Resolver) LookupAddr(ctx context.Context, addr string) (names []string, err error) {
   453  	return r.lookupAddr(ctx, addr)
   454  }
   455
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