src/net/lookup.go - go - Git at Google

 // Copyright 2012 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package net

 import (
 	"context"
 	"internal/nettrace"
 	"internal/singleflight"
 	"sync"
 )

 // protocols contains minimal mappings between internet protocol
 // names and numbers for platforms that don't have a complete list of
 // protocol numbers.
 //
 // See https://www.iana.org/assignments/protocol-numbers
 //
 // On Unix, this map is augmented by readProtocols via lookupProtocol.
 var protocols = map[string]int{
 	"icmp":      1,
 	"igmp":      2,
 	"tcp":       6,
 	"udp":       17,
 	"ipv6-icmp": 58,
 }

 // services contains minimal mappings between services names and port
 // numbers for platforms that don't have a complete list of port numbers.
 //
 // See https://www.iana.org/assignments/service-names-port-numbers
 //
 // On Unix, this map is augmented by readServices via goLookupPort.
 var services = map[string]map[string]int{
 	"udp": {
 		"domain": 53,
 	},
 	"tcp": {
 		"ftp":    21,
 		"ftps":   990,
 		"gopher": 70, // ʕ◔ϖ◔ʔ
 		"http":   80,
 		"https":  443,
 		"imap2":  143,
 		"imap3":  220,
 		"imaps":  993,
 		"pop3":   110,
 		"pop3s":  995,
 		"smtp":   25,
 		"ssh":    22,
 		"telnet": 23,
 	},
 }

 // dnsWaitGroup can be used by tests to wait for all DNS goroutines to
 // complete. This avoids races on the test hooks.
 var dnsWaitGroup sync.WaitGroup

 const maxProtoLength = len("RSVP-E2E-IGNORE") + 10 // with room to grow

 func lookupProtocolMap(name string) (int, error) {
 	var lowerProtocol [maxProtoLength]byte
 	n := copy(lowerProtocol[:], name)
 	lowerASCIIBytes(lowerProtocol[:n])
 	proto, found := protocols[string(lowerProtocol[:n])]
 	if !found || n != len(name) {
 		return 0, &AddrError{Err: "unknown IP protocol specified", Addr: name}
 	}
 	return proto, nil
 }

 // maxPortBufSize is the longest reasonable name of a service
 // (non-numeric port).
 // Currently the longest known IANA-unregistered name is
 // "mobility-header", so we use that length, plus some slop in case
 // something longer is added in the future.
 const maxPortBufSize = len("mobility-header") + 10

 func lookupPortMap(network, service string) (port int, error error) {
 	switch network {
 	case "tcp4", "tcp6":
 		network = "tcp"
 	case "udp4", "udp6":
 		network = "udp"
 	}

 	if m, ok := services[network]; ok {
 		var lowerService [maxPortBufSize]byte
 		n := copy(lowerService[:], service)
 		lowerASCIIBytes(lowerService[:n])
 		if port, ok := m[string(lowerService[:n])]; ok && n == len(service) {
 			return port, nil
 		}
 	}
 	return 0, &AddrError{Err: "unknown port", Addr: network + "/" + service}
 }

 // ipVersion returns the provided network's IP version: '4', '6' or 0
 // if network does not end in a '4' or '6' byte.
 func ipVersion(network string) byte {
 	if network == "" {
 		return 0
 	}
 	n := network[len(network)-1]
 	if n != '4' && n != '6' {
 		n = 0
 	}
 	return n
 }

 // DefaultResolver is the resolver used by the package-level Lookup
 // functions and by Dialers without a specified Resolver.
 var DefaultResolver = &Resolver{}

 // A Resolver looks up names and numbers.
 //
 // A nil *Resolver is equivalent to a zero Resolver.
 type Resolver struct {
 	// PreferGo controls whether Go's built-in DNS resolver is preferred
 	// on platforms where it's available. It is equivalent to setting
 	// GODEBUG=netdns=go, but scoped to just this resolver.
 	PreferGo bool

 	// StrictErrors controls the behavior of temporary errors
 	// (including timeout, socket errors, and SERVFAIL) when using
 	// Go's built-in resolver. For a query composed of multiple
 	// sub-queries (such as an A+AAAA address lookup, or walking the
 	// DNS search list), this option causes such errors to abort the
 	// whole query instead of returning a partial result. This is
 	// not enabled by default because it may affect compatibility
 	// with resolvers that process AAAA queries incorrectly.
 	StrictErrors bool

 	// Dial optionally specifies an alternate dialer for use by
 	// Go's built-in DNS resolver to make TCP and UDP connections
 	// to DNS services. The host in the address parameter will
 	// always be a literal IP address and not a host name, and the
 	// port in the address parameter will be a literal port number
 	// and not a service name.
 	// If the Conn returned is also a PacketConn, sent and received DNS
 	// messages must adhere to RFC 1035 section 4.2.1, "UDP usage".
 	// Otherwise, DNS messages transmitted over Conn must adhere
 	// to RFC 7766 section 5, "Transport Protocol Selection".
 	// If nil, the default dialer is used.
 	Dial func(ctx context.Context, network, address string) (Conn, error)

 	// lookupGroup merges LookupIPAddr calls together for lookups for the same
 	// host. The lookupGroup key is the LookupIPAddr.host argument.
 	// The return values are ([]IPAddr, error).
 	lookupGroup singleflight.Group

 	// TODO(bradfitz): optional interface impl override hook
 	// TODO(bradfitz): Timeout time.Duration?
 }

 func (r *Resolver) preferGo() bool     { return r != nil && r.PreferGo }
 func (r *Resolver) strictErrors() bool { return r != nil && r.StrictErrors }

 func (r *Resolver) getLookupGroup() *singleflight.Group {
 	if r == nil {
 		return &DefaultResolver.lookupGroup
 	}
 	return &r.lookupGroup
 }

 // LookupHost looks up the given host using the local resolver.
 // It returns a slice of that host's addresses.
 //
 // LookupHost uses context.Background internally; to specify the context, use
 // Resolver.LookupHost.
 func LookupHost(host string) (addrs []string, err error) {
 	return DefaultResolver.LookupHost(context.Background(), host)
 }

 // LookupHost looks up the given host using the local resolver.
 // It returns a slice of that host's addresses.
 func (r *Resolver) LookupHost(ctx context.Context, host string) (addrs []string, err error) {
 	// Make sure that no matter what we do later, host=="" is rejected.
 	// parseIP, for example, does accept empty strings.
 	if host == "" {
 		return nil, &DNSError{Err: errNoSuchHost.Error(), Name: host, IsNotFound: true}
 	}
 	if ip, _ := parseIPZone(host); ip != nil {
 		return []string{host}, nil
 	}
 	return r.lookupHost(ctx, host)
 }

 // LookupIP looks up host using the local resolver.
 // It returns a slice of that host's IPv4 and IPv6 addresses.
 func LookupIP(host string) ([]IP, error) {
 	addrs, err := DefaultResolver.LookupIPAddr(context.Background(), host)
 	if err != nil {
 		return nil, err
 	}
 	ips := make([]IP, len(addrs))
 	for i, ia := range addrs {
 		ips[i] = ia.IP
 	}
 	return ips, nil
 }

 // LookupIPAddr looks up host using the local resolver.
 // It returns a slice of that host's IPv4 and IPv6 addresses.
 func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, error) {
 	return r.lookupIPAddr(ctx, "ip", host)
 }

 // LookupIP looks up host for the given network using the local resolver.
 // It returns a slice of that host's IP addresses of the type specified by
 // network.
 // network must be one of "ip", "ip4" or "ip6".
 func (r *Resolver) LookupIP(ctx context.Context, network, host string) ([]IP, error) {
 	afnet, _, err := parseNetwork(ctx, network, false)
 	if err != nil {
 		return nil, err
 	}
 	switch afnet {
 	case "ip", "ip4", "ip6":
 	default:
 		return nil, UnknownNetworkError(network)
 	}
 	addrs, err := r.internetAddrList(ctx, afnet, host)
 	if err != nil {
 		return nil, err
 	}
 	ips := make([]IP, 0, len(addrs))
 	for _, addr := range addrs {
 		ips = append(ips, addr.(*IPAddr).IP)
 	}
 	return ips, nil
 }

 // onlyValuesCtx is a context that uses an underlying context
 // for value lookup if the underlying context hasn't yet expired.
 type onlyValuesCtx struct {
 	context.Context
 	lookupValues context.Context
 }

 var _ context.Context = (*onlyValuesCtx)(nil)

 // Value performs a lookup if the original context hasn't expired.
 func (ovc *onlyValuesCtx) Value(key interface{}) interface{} {
 	select {
 	case <-ovc.lookupValues.Done():
 		return nil
 	default:
 		return ovc.lookupValues.Value(key)
 	}
 }

 // withUnexpiredValuesPreserved returns a context.Context that only uses lookupCtx
 // for its values, otherwise it is never canceled and has no deadline.
 // If the lookup context expires, any looked up values will return nil.
 // See Issue 28600.
 func withUnexpiredValuesPreserved(lookupCtx context.Context) context.Context {
 	return &onlyValuesCtx{Context: context.Background(), lookupValues: lookupCtx}
 }

 // lookupIPAddr looks up host using the local resolver and particular network.
 // It returns a slice of that host's IPv4 and IPv6 addresses.
 func (r *Resolver) lookupIPAddr(ctx context.Context, network, host string) ([]IPAddr, error) {
 	// Make sure that no matter what we do later, host=="" is rejected.
 	// parseIP, for example, does accept empty strings.
 	if host == "" {
 		return nil, &DNSError{Err: errNoSuchHost.Error(), Name: host, IsNotFound: true}
 	}
 	if ip, zone := parseIPZone(host); ip != nil {
 		return []IPAddr{{IP: ip, Zone: zone}}, nil
 	}
 	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
 	if trace != nil && trace.DNSStart != nil {
 		trace.DNSStart(host)
 	}
 	// The underlying resolver func is lookupIP by default but it
 	// can be overridden by tests. This is needed by net/http, so it
 	// uses a context key instead of unexported variables.
 	resolverFunc := r.lookupIP
 	if alt, _ := ctx.Value(nettrace.LookupIPAltResolverKey{}).(func(context.Context, string, string) ([]IPAddr, error)); alt != nil {
 		resolverFunc = alt
 	}

 	// We don't want a cancellation of ctx to affect the
 	// lookupGroup operation. Otherwise if our context gets
 	// canceled it might cause an error to be returned to a lookup
 	// using a completely different context. However we need to preserve
 	// only the values in context. See Issue 28600.
 	lookupGroupCtx, lookupGroupCancel := context.WithCancel(withUnexpiredValuesPreserved(ctx))

 	lookupKey := network + "\000" + host
 	dnsWaitGroup.Add(1)
 	ch, called := r.getLookupGroup().DoChan(lookupKey, func() (interface{}, error) {
 		defer dnsWaitGroup.Done()
 		return testHookLookupIP(lookupGroupCtx, resolverFunc, network, host)
 	})
 	if !called {
 		dnsWaitGroup.Done()
 	}

 	select {
 	case <-ctx.Done():
 		// Our context was canceled. If we are the only
 		// goroutine looking up this key, then drop the key
 		// from the lookupGroup and cancel the lookup.
 		// If there are other goroutines looking up this key,
 		// let the lookup continue uncanceled, and let later
 		// lookups with the same key share the result.
 		// See issues 8602, 20703, 22724.
 		if r.getLookupGroup().ForgetUnshared(lookupKey) {
 			lookupGroupCancel()
 		} else {
 			go func() {
 				<-ch
 				lookupGroupCancel()
 			}()
 		}
 		err := mapErr(ctx.Err())
 		if trace != nil && trace.DNSDone != nil {
 			trace.DNSDone(nil, false, err)
 		}
 		return nil, err
 	case r := <-ch:
 		lookupGroupCancel()
 		if trace != nil && trace.DNSDone != nil {
 			addrs, _ := r.Val.([]IPAddr)
 			trace.DNSDone(ipAddrsEface(addrs), r.Shared, r.Err)
 		}
 		return lookupIPReturn(r.Val, r.Err, r.Shared)
 	}
 }

 // lookupIPReturn turns the return values from singleflight.Do into
 // the return values from LookupIP.
 func lookupIPReturn(addrsi interface{}, err error, shared bool) ([]IPAddr, error) {
 	if err != nil {
 		return nil, err
 	}
 	addrs := addrsi.([]IPAddr)
 	if shared {
 		clone := make([]IPAddr, len(addrs))
 		copy(clone, addrs)
 		addrs = clone
 	}
 	return addrs, nil
 }

 // ipAddrsEface returns an empty interface slice of addrs.
 func ipAddrsEface(addrs []IPAddr) []interface{} {
 	s := make([]interface{}, len(addrs))
 	for i, v := range addrs {
 		s[i] = v
 	}
 	return s
 }

 // LookupPort looks up the port for the given network and service.
 //
 // LookupPort uses context.Background internally; to specify the context, use
 // Resolver.LookupPort.
 func LookupPort(network, service string) (port int, err error) {
 	return DefaultResolver.LookupPort(context.Background(), network, service)
 }

 // LookupPort looks up the port for the given network and service.
 func (r *Resolver) LookupPort(ctx context.Context, network, service string) (port int, err error) {
 	port, needsLookup := parsePort(service)
 	if needsLookup {
 		switch network {
 		case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6":
 		case "": // a hint wildcard for Go 1.0 undocumented behavior
 			network = "ip"
 		default:
 			return 0, &AddrError{Err: "unknown network", Addr: network}
 		}
 		port, err = r.lookupPort(ctx, network, service)
 		if err != nil {
 			return 0, err
 		}
 	}
 	if 0 > port || port > 65535 {
 		return 0, &AddrError{Err: "invalid port", Addr: service}
 	}
 	return port, nil
 }

 // LookupCNAME returns the canonical name for the given host.
 // Callers that do not care about the canonical name can call
 // LookupHost or LookupIP directly; both take care of resolving
 // the canonical name as part of the lookup.
 //
 // A canonical name is the final name after following zero
 // or more CNAME records.
 // LookupCNAME does not return an error if host does not
 // contain DNS "CNAME" records, as long as host resolves to
 // address records.
 //
 // The returned canonical name is validated to be a properly
 // formatted presentation-format domain name.
 //
 // LookupCNAME uses context.Background internally; to specify the context, use
 // Resolver.LookupCNAME.
 func LookupCNAME(host string) (cname string, err error) {
 	return DefaultResolver.LookupCNAME(context.Background(), host)
 }

 // LookupCNAME returns the canonical name for the given host.
 // Callers that do not care about the canonical name can call
 // LookupHost or LookupIP directly; both take care of resolving
 // the canonical name as part of the lookup.
 //
 // A canonical name is the final name after following zero
 // or more CNAME records.
 // LookupCNAME does not return an error if host does not
 // contain DNS "CNAME" records, as long as host resolves to
 // address records.
 //
 // The returned canonical name is validated to be a properly
 // formatted presentation-format domain name.
 func (r *Resolver) LookupCNAME(ctx context.Context, host string) (string, error) {
 	cname, err := r.lookupCNAME(ctx, host)
 	if err != nil {
 		return "", err
 	}
 	if !isDomainName(cname) {
 		return "", &DNSError{Err: "CNAME target is invalid", Name: host}
 	}
 	return cname, nil
 }

 // LookupSRV tries to resolve an SRV query of the given service,
 // protocol, and domain name. The proto is "tcp" or "udp".
 // The returned records are sorted by priority and randomized
 // by weight within a priority.
 //
 // LookupSRV constructs the DNS name to look up following RFC 2782.
 // That is, it looks up _service._proto.name. To accommodate services
 // publishing SRV records under non-standard names, if both service
 // and proto are empty strings, LookupSRV looks up name directly.
 //
 // The returned service names are validated to be properly
 // formatted presentation-format domain names.
 func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err error) {
 	return DefaultResolver.LookupSRV(context.Background(), service, proto, name)
 }

 // LookupSRV tries to resolve an SRV query of the given service,
 // protocol, and domain name. The proto is "tcp" or "udp".
 // The returned records are sorted by priority and randomized
 // by weight within a priority.
 //
 // LookupSRV constructs the DNS name to look up following RFC 2782.
 // That is, it looks up _service._proto.name. To accommodate services
 // publishing SRV records under non-standard names, if both service
 // and proto are empty strings, LookupSRV looks up name directly.
 //
 // The returned service names are validated to be properly
 // formatted presentation-format domain names.
 func (r *Resolver) LookupSRV(ctx context.Context, service, proto, name string) (string, []*SRV, error) {
 	cname, addrs, err := r.lookupSRV(ctx, service, proto, name)
 	if err != nil {
 		return "", nil, err
 	}
 	if cname != "" && !isDomainName(cname) {
 		return "", nil, &DNSError{Err: "SRV header name is invalid", Name: name}
 	}
 	for _, addr := range addrs {
 		if addr == nil {
 			continue
 		}
 		if !isDomainName(addr.Target) {
 			return "", nil, &DNSError{Err: "SRV target is invalid", Name: name}
 		}
 	}
 	return cname, addrs, nil
 }

 // LookupMX returns the DNS MX records for the given domain name sorted by preference.
 //
 // The returned mail server names are validated to be properly
 // formatted presentation-format domain names.
 //
 // LookupMX uses context.Background internally; to specify the context, use
 // Resolver.LookupMX.
 func LookupMX(name string) ([]*MX, error) {
 	return DefaultResolver.LookupMX(context.Background(), name)
 }

 // LookupMX returns the DNS MX records for the given domain name sorted by preference.
 //
 // The returned mail server names are validated to be properly
 // formatted presentation-format domain names.
 func (r *Resolver) LookupMX(ctx context.Context, name string) ([]*MX, error) {
 	records, err := r.lookupMX(ctx, name)
 	if err != nil {
 		return nil, err
 	}
 	for _, mx := range records {
 		if mx == nil {
 			continue
 		}
 		if !isDomainName(mx.Host) {
 			return nil, &DNSError{Err: "MX target is invalid", Name: name}
 		}
 	}
 	return records, nil
 }

 // LookupNS returns the DNS NS records for the given domain name.
 //
 // The returned name server names are validated to be properly
 // formatted presentation-format domain names.
 //
 // LookupNS uses context.Background internally; to specify the context, use
 // Resolver.LookupNS.
 func LookupNS(name string) ([]*NS, error) {
 	return DefaultResolver.LookupNS(context.Background(), name)
 }

 // LookupNS returns the DNS NS records for the given domain name.
 //
 // The returned name server names are validated to be properly
 // formatted presentation-format domain names.
 func (r *Resolver) LookupNS(ctx context.Context, name string) ([]*NS, error) {
 	records, err := r.lookupNS(ctx, name)
 	if err != nil {
 		return nil, err
 	}
 	for _, ns := range records {
 		if ns == nil {
 			continue
 		}
 		if !isDomainName(ns.Host) {
 			return nil, &DNSError{Err: "NS target is invalid", Name: name}
 		}
 	}
 	return records, nil
 }

 // LookupTXT returns the DNS TXT records for the given domain name.
 //
 // LookupTXT uses context.Background internally; to specify the context, use
 // Resolver.LookupTXT.
 func LookupTXT(name string) ([]string, error) {
 	return DefaultResolver.lookupTXT(context.Background(), name)
 }

 // LookupTXT returns the DNS TXT records for the given domain name.
 func (r *Resolver) LookupTXT(ctx context.Context, name string) ([]string, error) {
 	return r.lookupTXT(ctx, name)
 }

 // LookupAddr performs a reverse lookup for the given address, returning a list
 // of names mapping to that address.
 //
 // The returned names are validated to be properly formatted presentation-format
 // domain names.
 //
 // When using the host C library resolver, at most one result will be
 // returned. To bypass the host resolver, use a custom Resolver.
 //
 // LookupAddr uses context.Background internally; to specify the context, use
 // Resolver.LookupAddr.
 func LookupAddr(addr string) (names []string, err error) {
 	return DefaultResolver.LookupAddr(context.Background(), addr)
 }

 // LookupAddr performs a reverse lookup for the given address, returning a list
 // of names mapping to that address.
 //
 // The returned names are validated to be properly formatted presentation-format
 // domain names.
 func (r *Resolver) LookupAddr(ctx context.Context, addr string) ([]string, error) {
 	names, err := r.lookupAddr(ctx, addr)
 	if err != nil {
 		return nil, err
 	}
 	for _, name := range names {
 		if !isDomainName(name) {
 			return nil, &DNSError{Err: "PTR target is invalid", Name: addr}
 		}
 	}
 	return names, nil
 }
	// Copyright 2012 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package net

	import (
	"context"
	"internal/nettrace"
	"internal/singleflight"
	"sync"
	)

	// protocols contains minimal mappings between internet protocol
	// names and numbers for platforms that don't have a complete list of
	// protocol numbers.
	//
	// See https://www.iana.org/assignments/protocol-numbers
	//
	// On Unix, this map is augmented by readProtocols via lookupProtocol.
	var protocols = map[string]int{
	"icmp": 1,
	"igmp": 2,
	"tcp": 6,
	"udp": 17,
	"ipv6-icmp": 58,
	}

	// services contains minimal mappings between services names and port
	// numbers for platforms that don't have a complete list of port numbers.
	//
	// See https://www.iana.org/assignments/service-names-port-numbers
	//
	// On Unix, this map is augmented by readServices via goLookupPort.
	var services = map[string]map[string]int{
	"udp": {
	"domain": 53,
	},
	"tcp": {
	"ftp": 21,
	"ftps": 990,
	"gopher": 70, // ʕ◔ϖ◔ʔ
	"http": 80,
	"https": 443,
	"imap2": 143,
	"imap3": 220,
	"imaps": 993,
	"pop3": 110,
	"pop3s": 995,
	"smtp": 25,
	"ssh": 22,
	"telnet": 23,
	},
	}

	// dnsWaitGroup can be used by tests to wait for all DNS goroutines to
	// complete. This avoids races on the test hooks.
	var dnsWaitGroup sync.WaitGroup

	const maxProtoLength = len("RSVP-E2E-IGNORE") + 10 // with room to grow

	func lookupProtocolMap(name string) (int, error) {
	var lowerProtocol [maxProtoLength]byte
	n := copy(lowerProtocol[:], name)
	lowerASCIIBytes(lowerProtocol[:n])
	proto, found := protocols[string(lowerProtocol[:n])]
	if !found \|\| n != len(name) {
	return 0, &AddrError{Err: "unknown IP protocol specified", Addr: name}
	}
	return proto, nil
	}

	// maxPortBufSize is the longest reasonable name of a service
	// (non-numeric port).
	// Currently the longest known IANA-unregistered name is
	// "mobility-header", so we use that length, plus some slop in case
	// something longer is added in the future.
	const maxPortBufSize = len("mobility-header") + 10

	func lookupPortMap(network, service string) (port int, error error) {
	switch network {
	case "tcp4", "tcp6":
	network = "tcp"
	case "udp4", "udp6":
	network = "udp"
	}

	if m, ok := services[network]; ok {
	var lowerService [maxPortBufSize]byte
	n := copy(lowerService[:], service)
	lowerASCIIBytes(lowerService[:n])
	if port, ok := m[string(lowerService[:n])]; ok && n == len(service) {
	return port, nil
	}
	}
	return 0, &AddrError{Err: "unknown port", Addr: network + "/" + service}
	}

	// ipVersion returns the provided network's IP version: '4', '6' or 0
	// if network does not end in a '4' or '6' byte.
	func ipVersion(network string) byte {
	if network == "" {
	return 0
	}
	n := network[len(network)-1]
	if n != '4' && n != '6' {
	n = 0
	}
	return n
	}

	// DefaultResolver is the resolver used by the package-level Lookup
	// functions and by Dialers without a specified Resolver.
	var DefaultResolver = &Resolver{}

	// A Resolver looks up names and numbers.
	//
	// A nil *Resolver is equivalent to a zero Resolver.
	type Resolver struct {
	// PreferGo controls whether Go's built-in DNS resolver is preferred
	// on platforms where it's available. It is equivalent to setting
	// GODEBUG=netdns=go, but scoped to just this resolver.
	PreferGo bool

	// StrictErrors controls the behavior of temporary errors
	// (including timeout, socket errors, and SERVFAIL) when using
	// Go's built-in resolver. For a query composed of multiple
	// sub-queries (such as an A+AAAA address lookup, or walking the
	// DNS search list), this option causes such errors to abort the
	// whole query instead of returning a partial result. This is
	// not enabled by default because it may affect compatibility
	// with resolvers that process AAAA queries incorrectly.
	StrictErrors bool

	// Dial optionally specifies an alternate dialer for use by
	// Go's built-in DNS resolver to make TCP and UDP connections
	// to DNS services. The host in the address parameter will
	// always be a literal IP address and not a host name, and the
	// port in the address parameter will be a literal port number
	// and not a service name.
	// If the Conn returned is also a PacketConn, sent and received DNS
	// messages must adhere to RFC 1035 section 4.2.1, "UDP usage".
	// Otherwise, DNS messages transmitted over Conn must adhere
	// to RFC 7766 section 5, "Transport Protocol Selection".
	// If nil, the default dialer is used.
	Dial func(ctx context.Context, network, address string) (Conn, error)

	// lookupGroup merges LookupIPAddr calls together for lookups for the same
	// host. The lookupGroup key is the LookupIPAddr.host argument.
	// The return values are ([]IPAddr, error).
	lookupGroup singleflight.Group

	// TODO(bradfitz): optional interface impl override hook
	// TODO(bradfitz): Timeout time.Duration?
	}

	func (r *Resolver) preferGo() bool { return r != nil && r.PreferGo }
	func (r *Resolver) strictErrors() bool { return r != nil && r.StrictErrors }

	func (r Resolver) getLookupGroup() singleflight.Group {
	if r == nil {
	return &DefaultResolver.lookupGroup
	}
	return &r.lookupGroup
	}

	// LookupHost looks up the given host using the local resolver.
	// It returns a slice of that host's addresses.
	//
	// LookupHost uses context.Background internally; to specify the context, use
	// Resolver.LookupHost.
	func LookupHost(host string) (addrs []string, err error) {
	return DefaultResolver.LookupHost(context.Background(), host)
	}

	// LookupHost looks up the given host using the local resolver.
	// It returns a slice of that host's addresses.
	func (r *Resolver) LookupHost(ctx context.Context, host string) (addrs []string, err error) {
	// Make sure that no matter what we do later, host=="" is rejected.
	// parseIP, for example, does accept empty strings.
	if host == "" {
	return nil, &DNSError{Err: errNoSuchHost.Error(), Name: host, IsNotFound: true}
	}
	if ip, _ := parseIPZone(host); ip != nil {
	return []string{host}, nil
	}
	return r.lookupHost(ctx, host)
	}

	// LookupIP looks up host using the local resolver.
	// It returns a slice of that host's IPv4 and IPv6 addresses.
	func LookupIP(host string) ([]IP, error) {
	addrs, err := DefaultResolver.LookupIPAddr(context.Background(), host)
	if err != nil {
	return nil, err
	}
	ips := make([]IP, len(addrs))
	for i, ia := range addrs {
	ips[i] = ia.IP
	}
	return ips, nil
	}

	// LookupIPAddr looks up host using the local resolver.
	// It returns a slice of that host's IPv4 and IPv6 addresses.
	func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, error) {
	return r.lookupIPAddr(ctx, "ip", host)
	}

	// LookupIP looks up host for the given network using the local resolver.
	// It returns a slice of that host's IP addresses of the type specified by
	// network.
	// network must be one of "ip", "ip4" or "ip6".
	func (r *Resolver) LookupIP(ctx context.Context, network, host string) ([]IP, error) {
	afnet, _, err := parseNetwork(ctx, network, false)
	if err != nil {
	return nil, err
	}
	switch afnet {
	case "ip", "ip4", "ip6":
	default:
	return nil, UnknownNetworkError(network)
	}
	addrs, err := r.internetAddrList(ctx, afnet, host)
	if err != nil {
	return nil, err
	}
	ips := make([]IP, 0, len(addrs))
	for _, addr := range addrs {
	ips = append(ips, addr.(*IPAddr).IP)
	}
	return ips, nil
	}

	// onlyValuesCtx is a context that uses an underlying context
	// for value lookup if the underlying context hasn't yet expired.
	type onlyValuesCtx struct {
	context.Context
	lookupValues context.Context
	}

	var _ context.Context = (*onlyValuesCtx)(nil)

	// Value performs a lookup if the original context hasn't expired.
	func (ovc *onlyValuesCtx) Value(key interface{}) interface{} {
	select {
	case <-ovc.lookupValues.Done():
	return nil
	default:
	return ovc.lookupValues.Value(key)
	}
	}

	// withUnexpiredValuesPreserved returns a context.Context that only uses lookupCtx
	// for its values, otherwise it is never canceled and has no deadline.
	// If the lookup context expires, any looked up values will return nil.
	// See Issue 28600.
	func withUnexpiredValuesPreserved(lookupCtx context.Context) context.Context {
	return &onlyValuesCtx{Context: context.Background(), lookupValues: lookupCtx}
	}

	// lookupIPAddr looks up host using the local resolver and particular network.
	// It returns a slice of that host's IPv4 and IPv6 addresses.
	func (r *Resolver) lookupIPAddr(ctx context.Context, network, host string) ([]IPAddr, error) {
	// Make sure that no matter what we do later, host=="" is rejected.
	// parseIP, for example, does accept empty strings.
	if host == "" {
	return nil, &DNSError{Err: errNoSuchHost.Error(), Name: host, IsNotFound: true}
	}
	if ip, zone := parseIPZone(host); ip != nil {
	return []IPAddr{{IP: ip, Zone: zone}}, nil
	}
	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
	if trace != nil && trace.DNSStart != nil {
	trace.DNSStart(host)
	}
	// The underlying resolver func is lookupIP by default but it
	// can be overridden by tests. This is needed by net/http, so it
	// uses a context key instead of unexported variables.
	resolverFunc := r.lookupIP
	if alt, _ := ctx.Value(nettrace.LookupIPAltResolverKey{}).(func(context.Context, string, string) ([]IPAddr, error)); alt != nil {
	resolverFunc = alt
	}

	// We don't want a cancellation of ctx to affect the
	// lookupGroup operation. Otherwise if our context gets
	// canceled it might cause an error to be returned to a lookup
	// using a completely different context. However we need to preserve
	// only the values in context. See Issue 28600.
	lookupGroupCtx, lookupGroupCancel := context.WithCancel(withUnexpiredValuesPreserved(ctx))

	lookupKey := network + "\000" + host
	dnsWaitGroup.Add(1)
	ch, called := r.getLookupGroup().DoChan(lookupKey, func() (interface{}, error) {
	defer dnsWaitGroup.Done()
	return testHookLookupIP(lookupGroupCtx, resolverFunc, network, host)
	})
	if !called {
	dnsWaitGroup.Done()
	}

	select {
	case <-ctx.Done():
	// Our context was canceled. If we are the only
	// goroutine looking up this key, then drop the key
	// from the lookupGroup and cancel the lookup.
	// If there are other goroutines looking up this key,
	// let the lookup continue uncanceled, and let later
	// lookups with the same key share the result.
	// See issues 8602, 20703, 22724.
	if r.getLookupGroup().ForgetUnshared(lookupKey) {
	lookupGroupCancel()
	} else {
	go func() {
	<-ch
	lookupGroupCancel()
	}()
	}
	err := mapErr(ctx.Err())
	if trace != nil && trace.DNSDone != nil {
	trace.DNSDone(nil, false, err)
	}
	return nil, err
	case r := <-ch:
	lookupGroupCancel()
	if trace != nil && trace.DNSDone != nil {
	addrs, _ := r.Val.([]IPAddr)
	trace.DNSDone(ipAddrsEface(addrs), r.Shared, r.Err)
	}
	return lookupIPReturn(r.Val, r.Err, r.Shared)
	}
	}

	// lookupIPReturn turns the return values from singleflight.Do into
	// the return values from LookupIP.
	func lookupIPReturn(addrsi interface{}, err error, shared bool) ([]IPAddr, error) {
	if err != nil {
	return nil, err
	}
	addrs := addrsi.([]IPAddr)
	if shared {
	clone := make([]IPAddr, len(addrs))
	copy(clone, addrs)
	addrs = clone
	}
	return addrs, nil
	}

	// ipAddrsEface returns an empty interface slice of addrs.
	func ipAddrsEface(addrs []IPAddr) []interface{} {
	s := make([]interface{}, len(addrs))
	for i, v := range addrs {
	s[i] = v
	}
	return s
	}

	// LookupPort looks up the port for the given network and service.
	//
	// LookupPort uses context.Background internally; to specify the context, use
	// Resolver.LookupPort.
	func LookupPort(network, service string) (port int, err error) {
	return DefaultResolver.LookupPort(context.Background(), network, service)
	}

	// LookupPort looks up the port for the given network and service.
	func (r *Resolver) LookupPort(ctx context.Context, network, service string) (port int, err error) {
	port, needsLookup := parsePort(service)
	if needsLookup {
	switch network {
	case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6":
	case "": // a hint wildcard for Go 1.0 undocumented behavior
	network = "ip"
	default:
	return 0, &AddrError{Err: "unknown network", Addr: network}
	}
	port, err = r.lookupPort(ctx, network, service)
	if err != nil {
	return 0, err
	}
	}
	if 0 > port \|\| port > 65535 {
	return 0, &AddrError{Err: "invalid port", Addr: service}
	}
	return port, nil
	}

	// LookupCNAME returns the canonical name for the given host.
	// Callers that do not care about the canonical name can call
	// LookupHost or LookupIP directly; both take care of resolving
	// the canonical name as part of the lookup.
	//
	// A canonical name is the final name after following zero
	// or more CNAME records.
	// LookupCNAME does not return an error if host does not
	// contain DNS "CNAME" records, as long as host resolves to
	// address records.
	//
	// The returned canonical name is validated to be a properly
	// formatted presentation-format domain name.
	//
	// LookupCNAME uses context.Background internally; to specify the context, use
	// Resolver.LookupCNAME.
	func LookupCNAME(host string) (cname string, err error) {
	return DefaultResolver.LookupCNAME(context.Background(), host)
	}

	// LookupCNAME returns the canonical name for the given host.
	// Callers that do not care about the canonical name can call
	// LookupHost or LookupIP directly; both take care of resolving
	// the canonical name as part of the lookup.
	//
	// A canonical name is the final name after following zero
	// or more CNAME records.
	// LookupCNAME does not return an error if host does not
	// contain DNS "CNAME" records, as long as host resolves to
	// address records.
	//
	// The returned canonical name is validated to be a properly
	// formatted presentation-format domain name.
	func (r *Resolver) LookupCNAME(ctx context.Context, host string) (string, error) {
	cname, err := r.lookupCNAME(ctx, host)
	if err != nil {
	return "", err
	}
	if !isDomainName(cname) {
	return "", &DNSError{Err: "CNAME target is invalid", Name: host}
	}
	return cname, nil
	}

	// LookupSRV tries to resolve an SRV query of the given service,
	// protocol, and domain name. The proto is "tcp" or "udp".
	// The returned records are sorted by priority and randomized
	// by weight within a priority.
	//
	// LookupSRV constructs the DNS name to look up following RFC 2782.
	// That is, it looks up _service._proto.name. To accommodate services
	// publishing SRV records under non-standard names, if both service
	// and proto are empty strings, LookupSRV looks up name directly.
	//
	// The returned service names are validated to be properly
	// formatted presentation-format domain names.
	func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err error) {
	return DefaultResolver.LookupSRV(context.Background(), service, proto, name)
	}

	// LookupSRV tries to resolve an SRV query of the given service,
	// protocol, and domain name. The proto is "tcp" or "udp".
	// The returned records are sorted by priority and randomized
	// by weight within a priority.
	//
	// LookupSRV constructs the DNS name to look up following RFC 2782.
	// That is, it looks up _service._proto.name. To accommodate services
	// publishing SRV records under non-standard names, if both service
	// and proto are empty strings, LookupSRV looks up name directly.
	//
	// The returned service names are validated to be properly
	// formatted presentation-format domain names.
	func (r Resolver) LookupSRV(ctx context.Context, service, proto, name string) (string, []SRV, error) {
	cname, addrs, err := r.lookupSRV(ctx, service, proto, name)
	if err != nil {
	return "", nil, err
	}
	if cname != "" && !isDomainName(cname) {
	return "", nil, &DNSError{Err: "SRV header name is invalid", Name: name}
	}
	for _, addr := range addrs {
	if addr == nil {
	continue
	}
	if !isDomainName(addr.Target) {
	return "", nil, &DNSError{Err: "SRV target is invalid", Name: name}
	}
	}
	return cname, addrs, nil
	}

	// LookupMX returns the DNS MX records for the given domain name sorted by preference.
	//
	// The returned mail server names are validated to be properly
	// formatted presentation-format domain names.
	//
	// LookupMX uses context.Background internally; to specify the context, use
	// Resolver.LookupMX.
	func LookupMX(name string) ([]*MX, error) {
	return DefaultResolver.LookupMX(context.Background(), name)
	}

	// LookupMX returns the DNS MX records for the given domain name sorted by preference.
	//
	// The returned mail server names are validated to be properly
	// formatted presentation-format domain names.
	func (r Resolver) LookupMX(ctx context.Context, name string) ([]MX, error) {
	records, err := r.lookupMX(ctx, name)
	if err != nil {
	return nil, err
	}
	for _, mx := range records {
	if mx == nil {
	continue
	}
	if !isDomainName(mx.Host) {
	return nil, &DNSError{Err: "MX target is invalid", Name: name}
	}
	}
	return records, nil
	}

	// LookupNS returns the DNS NS records for the given domain name.
	//
	// The returned name server names are validated to be properly
	// formatted presentation-format domain names.
	//
	// LookupNS uses context.Background internally; to specify the context, use
	// Resolver.LookupNS.
	func LookupNS(name string) ([]*NS, error) {
	return DefaultResolver.LookupNS(context.Background(), name)
	}

	// LookupNS returns the DNS NS records for the given domain name.
	//
	// The returned name server names are validated to be properly
	// formatted presentation-format domain names.
	func (r Resolver) LookupNS(ctx context.Context, name string) ([]NS, error) {
	records, err := r.lookupNS(ctx, name)
	if err != nil {
	return nil, err
	}
	for _, ns := range records {
	if ns == nil {
	continue
	}
	if !isDomainName(ns.Host) {
	return nil, &DNSError{Err: "NS target is invalid", Name: name}
	}
	}
	return records, nil
	}

	// LookupTXT returns the DNS TXT records for the given domain name.
	//
	// LookupTXT uses context.Background internally; to specify the context, use
	// Resolver.LookupTXT.
	func LookupTXT(name string) ([]string, error) {
	return DefaultResolver.lookupTXT(context.Background(), name)
	}

	// LookupTXT returns the DNS TXT records for the given domain name.
	func (r *Resolver) LookupTXT(ctx context.Context, name string) ([]string, error) {
	return r.lookupTXT(ctx, name)
	}

	// LookupAddr performs a reverse lookup for the given address, returning a list
	// of names mapping to that address.
	//
	// The returned names are validated to be properly formatted presentation-format
	// domain names.
	//
	// When using the host C library resolver, at most one result will be
	// returned. To bypass the host resolver, use a custom Resolver.
	//
	// LookupAddr uses context.Background internally; to specify the context, use
	// Resolver.LookupAddr.
	func LookupAddr(addr string) (names []string, err error) {
	return DefaultResolver.LookupAddr(context.Background(), addr)
	}

	// LookupAddr performs a reverse lookup for the given address, returning a list
	// of names mapping to that address.
	//
	// The returned names are validated to be properly formatted presentation-format
	// domain names.
	func (r *Resolver) LookupAddr(ctx context.Context, addr string) ([]string, error) {
	names, err := r.lookupAddr(ctx, addr)
	if err != nil {
	return nil, err
	}
	for _, name := range names {
	if !isDomainName(name) {
	return nil, &DNSError{Err: "PTR target is invalid", Name: addr}
	}
	}
	return names, nil
	}