Adding upstream version 1.16.10.upstream/1.16.10upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 789 insertions, 0 deletions

diff --git a/src/net/dnsclient_unix.go b/src/net/dnsclient_unix.go
new file mode 100644
index 0000000..d7db0c8
--- /dev/null
+++ b/src/net/dnsclient_unix.go
@@ -0,0 +1,789 @@
+// Copyright 2009 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.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+// DNS client: see RFC 1035.
+// Has to be linked into package net for Dial.
+
+// TODO(rsc):
+//	Could potentially handle many outstanding lookups faster.
+//	Random UDP source port (net.Dial should do that for us).
+//	Random request IDs.
+
+package net
+
+import (
+	"context"
+	"errors"
+	"io"
+	"os"
+	"sync"
+	"time"
+
+	"golang.org/x/net/dns/dnsmessage"
+)
+
+const (
+	// to be used as a useTCP parameter to exchange
+	useTCPOnly  = true
+	useUDPOrTCP = false
+)
+
+var (
+	errLameReferral              = errors.New("lame referral")
+	errCannotUnmarshalDNSMessage = errors.New("cannot unmarshal DNS message")
+	errCannotMarshalDNSMessage   = errors.New("cannot marshal DNS message")
+	errServerMisbehaving         = errors.New("server misbehaving")
+	errInvalidDNSResponse        = errors.New("invalid DNS response")
+	errNoAnswerFromDNSServer     = errors.New("no answer from DNS server")
+
+	// errServerTemporarilyMisbehaving is like errServerMisbehaving, except
+	// that when it gets translated to a DNSError, the IsTemporary field
+	// gets set to true.
+	errServerTemporarilyMisbehaving = errors.New("server misbehaving")
+)
+
+func newRequest(q dnsmessage.Question) (id uint16, udpReq, tcpReq []byte, err error) {
+	id = uint16(randInt())
+	b := dnsmessage.NewBuilder(make([]byte, 2, 514), dnsmessage.Header{ID: id, RecursionDesired: true})
+	b.EnableCompression()
+	if err := b.StartQuestions(); err != nil {
+		return 0, nil, nil, err
+	}
+	if err := b.Question(q); err != nil {
+		return 0, nil, nil, err
+	}
+	tcpReq, err = b.Finish()
+	udpReq = tcpReq[2:]
+	l := len(tcpReq) - 2
+	tcpReq[0] = byte(l >> 8)
+	tcpReq[1] = byte(l)
+	return id, udpReq, tcpReq, err
+}
+
+func checkResponse(reqID uint16, reqQues dnsmessage.Question, respHdr dnsmessage.Header, respQues dnsmessage.Question) bool {
+	if !respHdr.Response {
+		return false
+	}
+	if reqID != respHdr.ID {
+		return false
+	}
+	if reqQues.Type != respQues.Type || reqQues.Class != respQues.Class || !equalASCIIName(reqQues.Name, respQues.Name) {
+		return false
+	}
+	return true
+}
+
+func dnsPacketRoundTrip(c Conn, id uint16, query dnsmessage.Question, b []byte) (dnsmessage.Parser, dnsmessage.Header, error) {
+	if _, err := c.Write(b); err != nil {
+		return dnsmessage.Parser{}, dnsmessage.Header{}, err
+	}
+
+	b = make([]byte, 512) // see RFC 1035
+	for {
+		n, err := c.Read(b)
+		if err != nil {
+			return dnsmessage.Parser{}, dnsmessage.Header{}, err
+		}
+		var p dnsmessage.Parser
+		// Ignore invalid responses as they may be malicious
+		// forgery attempts. Instead continue waiting until
+		// timeout. See golang.org/issue/13281.
+		h, err := p.Start(b[:n])
+		if err != nil {
+			continue
+		}
+		q, err := p.Question()
+		if err != nil || !checkResponse(id, query, h, q) {
+			continue
+		}
+		return p, h, nil
+	}
+}
+
+func dnsStreamRoundTrip(c Conn, id uint16, query dnsmessage.Question, b []byte) (dnsmessage.Parser, dnsmessage.Header, error) {
+	if _, err := c.Write(b); err != nil {
+		return dnsmessage.Parser{}, dnsmessage.Header{}, err
+	}
+
+	b = make([]byte, 1280) // 1280 is a reasonable initial size for IP over Ethernet, see RFC 4035
+	if _, err := io.ReadFull(c, b[:2]); err != nil {
+		return dnsmessage.Parser{}, dnsmessage.Header{}, err
+	}
+	l := int(b[0])<<8 | int(b[1])
+	if l > len(b) {
+		b = make([]byte, l)
+	}
+	n, err := io.ReadFull(c, b[:l])
+	if err != nil {
+		return dnsmessage.Parser{}, dnsmessage.Header{}, err
+	}
+	var p dnsmessage.Parser
+	h, err := p.Start(b[:n])
+	if err != nil {
+		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotUnmarshalDNSMessage
+	}
+	q, err := p.Question()
+	if err != nil {
+		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotUnmarshalDNSMessage
+	}
+	if !checkResponse(id, query, h, q) {
+		return dnsmessage.Parser{}, dnsmessage.Header{}, errInvalidDNSResponse
+	}
+	return p, h, nil
+}
+
+// exchange sends a query on the connection and hopes for a response.
+func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Question, timeout time.Duration, useTCP bool) (dnsmessage.Parser, dnsmessage.Header, error) {
+	q.Class = dnsmessage.ClassINET
+	id, udpReq, tcpReq, err := newRequest(q)
+	if err != nil {
+		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotMarshalDNSMessage
+	}
+	var networks []string
+	if useTCP {
+		networks = []string{"tcp"}
+	} else {
+		networks = []string{"udp", "tcp"}
+	}
+	for _, network := range networks {
+		ctx, cancel := context.WithDeadline(ctx, time.Now().Add(timeout))
+		defer cancel()
+
+		c, err := r.dial(ctx, network, server)
+		if err != nil {
+			return dnsmessage.Parser{}, dnsmessage.Header{}, err
+		}
+		if d, ok := ctx.Deadline(); ok && !d.IsZero() {
+			c.SetDeadline(d)
+		}
+		var p dnsmessage.Parser
+		var h dnsmessage.Header
+		if _, ok := c.(PacketConn); ok {
+			p, h, err = dnsPacketRoundTrip(c, id, q, udpReq)
+		} else {
+			p, h, err = dnsStreamRoundTrip(c, id, q, tcpReq)
+		}
+		c.Close()
+		if err != nil {
+			return dnsmessage.Parser{}, dnsmessage.Header{}, mapErr(err)
+		}
+		if err := p.SkipQuestion(); err != dnsmessage.ErrSectionDone {
+			return dnsmessage.Parser{}, dnsmessage.Header{}, errInvalidDNSResponse
+		}
+		if h.Truncated { // see RFC 5966
+			continue
+		}
+		return p, h, nil
+	}
+	return dnsmessage.Parser{}, dnsmessage.Header{}, errNoAnswerFromDNSServer
+}
+
+// checkHeader performs basic sanity checks on the header.
+func checkHeader(p *dnsmessage.Parser, h dnsmessage.Header) error {
+	if h.RCode == dnsmessage.RCodeNameError {
+		return errNoSuchHost
+	}
+
+	_, err := p.AnswerHeader()
+	if err != nil && err != dnsmessage.ErrSectionDone {
+		return errCannotUnmarshalDNSMessage
+	}
+
+	// libresolv continues to the next server when it receives
+	// an invalid referral response. See golang.org/issue/15434.
+	if h.RCode == dnsmessage.RCodeSuccess && !h.Authoritative && !h.RecursionAvailable && err == dnsmessage.ErrSectionDone {
+		return errLameReferral
+	}
+
+	if h.RCode != dnsmessage.RCodeSuccess && h.RCode != dnsmessage.RCodeNameError {
+		// None of the error codes make sense
+		// for the query we sent. If we didn't get
+		// a name error and we didn't get success,
+		// the server is behaving incorrectly or
+		// having temporary trouble.
+		if h.RCode == dnsmessage.RCodeServerFailure {
+			return errServerTemporarilyMisbehaving
+		}
+		return errServerMisbehaving
+	}
+
+	return nil
+}
+
+func skipToAnswer(p *dnsmessage.Parser, qtype dnsmessage.Type) error {
+	for {
+		h, err := p.AnswerHeader()
+		if err == dnsmessage.ErrSectionDone {
+			return errNoSuchHost
+		}
+		if err != nil {
+			return errCannotUnmarshalDNSMessage
+		}
+		if h.Type == qtype {
+			return nil
+		}
+		if err := p.SkipAnswer(); err != nil {
+			return errCannotUnmarshalDNSMessage
+		}
+	}
+}
+
+// Do a lookup for a single name, which must be rooted
+// (otherwise answer will not find the answers).
+func (r *Resolver) tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype dnsmessage.Type) (dnsmessage.Parser, string, error) {
+	var lastErr error
+	serverOffset := cfg.serverOffset()
+	sLen := uint32(len(cfg.servers))
+
+	n, err := dnsmessage.NewName(name)
+	if err != nil {
+		return dnsmessage.Parser{}, "", errCannotMarshalDNSMessage
+	}
+	q := dnsmessage.Question{
+		Name:  n,
+		Type:  qtype,
+		Class: dnsmessage.ClassINET,
+	}
+
+	for i := 0; i < cfg.attempts; i++ {
+		for j := uint32(0); j < sLen; j++ {
+			server := cfg.servers[(serverOffset+j)%sLen]
+
+			p, h, err := r.exchange(ctx, server, q, cfg.timeout, cfg.useTCP)
+			if err != nil {
+				dnsErr := &DNSError{
+					Err:    err.Error(),
+					Name:   name,
+					Server: server,
+				}
+				if nerr, ok := err.(Error); ok && nerr.Timeout() {
+					dnsErr.IsTimeout = true
+				}
+				// Set IsTemporary for socket-level errors. Note that this flag
+				// may also be used to indicate a SERVFAIL response.
+				if _, ok := err.(*OpError); ok {
+					dnsErr.IsTemporary = true
+				}
+				lastErr = dnsErr
+				continue
+			}
+
+			if err := checkHeader(&p, h); err != nil {
+				dnsErr := &DNSError{
+					Err:    err.Error(),
+					Name:   name,
+					Server: server,
+				}
+				if err == errServerTemporarilyMisbehaving {
+					dnsErr.IsTemporary = true
+				}
+				if err == errNoSuchHost {
+					// The name does not exist, so trying
+					// another server won't help.
+
+					dnsErr.IsNotFound = true
+					return p, server, dnsErr
+				}
+				lastErr = dnsErr
+				continue
+			}
+
+			err = skipToAnswer(&p, qtype)
+			if err == nil {
+				return p, server, nil
+			}
+			lastErr = &DNSError{
+				Err:    err.Error(),
+				Name:   name,
+				Server: server,
+			}
+			if err == errNoSuchHost {
+				// The name does not exist, so trying another
+				// server won't help.
+
+				lastErr.(*DNSError).IsNotFound = true
+				return p, server, lastErr
+			}
+		}
+	}
+	return dnsmessage.Parser{}, "", lastErr
+}
+
+// A resolverConfig represents a DNS stub resolver configuration.
+type resolverConfig struct {
+	initOnce sync.Once // guards init of resolverConfig
+
+	// ch is used as a semaphore that only allows one lookup at a
+	// time to recheck resolv.conf.
+	ch          chan struct{} // guards lastChecked and modTime
+	lastChecked time.Time     // last time resolv.conf was checked
+
+	mu        sync.RWMutex // protects dnsConfig
+	dnsConfig *dnsConfig   // parsed resolv.conf structure used in lookups
+}
+
+var resolvConf resolverConfig
+
+// init initializes conf and is only called via conf.initOnce.
+func (conf *resolverConfig) init() {
+	// Set dnsConfig and lastChecked so we don't parse
+	// resolv.conf twice the first time.
+	conf.dnsConfig = systemConf().resolv
+	if conf.dnsConfig == nil {
+		conf.dnsConfig = dnsReadConfig("/etc/resolv.conf")
+	}
+	conf.lastChecked = time.Now()
+
+	// Prepare ch so that only one update of resolverConfig may
+	// run at once.
+	conf.ch = make(chan struct{}, 1)
+}
+
+// tryUpdate tries to update conf with the named resolv.conf file.
+// The name variable only exists for testing. It is otherwise always
+// "/etc/resolv.conf".
+func (conf *resolverConfig) tryUpdate(name string) {
+	conf.initOnce.Do(conf.init)
+
+	// Ensure only one update at a time checks resolv.conf.
+	if !conf.tryAcquireSema() {
+		return
+	}
+	defer conf.releaseSema()
+
+	now := time.Now()
+	if conf.lastChecked.After(now.Add(-5 * time.Second)) {
+		return
+	}
+	conf.lastChecked = now
+
+	var mtime time.Time
+	if fi, err := os.Stat(name); err == nil {
+		mtime = fi.ModTime()
+	}
+	if mtime.Equal(conf.dnsConfig.mtime) {
+		return
+	}
+
+	dnsConf := dnsReadConfig(name)
+	conf.mu.Lock()
+	conf.dnsConfig = dnsConf
+	conf.mu.Unlock()
+}
+
+func (conf *resolverConfig) tryAcquireSema() bool {
+	select {
+	case conf.ch <- struct{}{}:
+		return true
+	default:
+		return false
+	}
+}
+
+func (conf *resolverConfig) releaseSema() {
+	<-conf.ch
+}
+
+func (r *Resolver) lookup(ctx context.Context, name string, qtype dnsmessage.Type) (dnsmessage.Parser, string, error) {
+	if !isDomainName(name) {
+		// We used to use "invalid domain name" as the error,
+		// but that is a detail of the specific lookup mechanism.
+		// Other lookups might allow broader name syntax
+		// (for example Multicast DNS allows UTF-8; see RFC 6762).
+		// For consistency with libc resolvers, report no such host.
+		return dnsmessage.Parser{}, "", &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
+	}
+	resolvConf.tryUpdate("/etc/resolv.conf")
+	resolvConf.mu.RLock()
+	conf := resolvConf.dnsConfig
+	resolvConf.mu.RUnlock()
+	var (
+		p      dnsmessage.Parser
+		server string
+		err    error
+	)
+	for _, fqdn := range conf.nameList(name) {
+		p, server, err = r.tryOneName(ctx, conf, fqdn, qtype)
+		if err == nil {
+			break
+		}
+		if nerr, ok := err.(Error); ok && nerr.Temporary() && r.strictErrors() {
+			// If we hit a temporary error with StrictErrors enabled,
+			// stop immediately instead of trying more names.
+			break
+		}
+	}
+	if err == nil {
+		return p, server, nil
+	}
+	if err, ok := err.(*DNSError); ok {
+		// Show original name passed to lookup, not suffixed one.
+		// In general we might have tried many suffixes; showing
+		// just one is misleading. See also golang.org/issue/6324.
+		err.Name = name
+	}
+	return dnsmessage.Parser{}, "", err
+}
+
+// avoidDNS reports whether this is a hostname for which we should not
+// use DNS. Currently this includes only .onion, per RFC 7686. See
+// golang.org/issue/13705. Does not cover .local names (RFC 6762),
+// see golang.org/issue/16739.
+func avoidDNS(name string) bool {
+	if name == "" {
+		return true
+	}
+	if name[len(name)-1] == '.' {
+		name = name[:len(name)-1]
+	}
+	return stringsHasSuffixFold(name, ".onion")
+}
+
+// nameList returns a list of names for sequential DNS queries.
+func (conf *dnsConfig) nameList(name string) []string {
+	if avoidDNS(name) {
+		return nil
+	}
+
+	// Check name length (see isDomainName).
+	l := len(name)
+	rooted := l > 0 && name[l-1] == '.'
+	if l > 254 || l == 254 && rooted {
+		return nil
+	}
+
+	// If name is rooted (trailing dot), try only that name.
+	if rooted {
+		return []string{name}
+	}
+
+	hasNdots := count(name, '.') >= conf.ndots
+	name += "."
+	l++
+
+	// Build list of search choices.
+	names := make([]string, 0, 1+len(conf.search))
+	// If name has enough dots, try unsuffixed first.
+	if hasNdots {
+		names = append(names, name)
+	}
+	// Try suffixes that are not too long (see isDomainName).
+	for _, suffix := range conf.search {
+		if l+len(suffix) <= 254 {
+			names = append(names, name+suffix)
+		}
+	}
+	// Try unsuffixed, if not tried first above.
+	if !hasNdots {
+		names = append(names, name)
+	}
+	return names
+}
+
+// hostLookupOrder specifies the order of LookupHost lookup strategies.
+// It is basically a simplified representation of nsswitch.conf.
+// "files" means /etc/hosts.
+type hostLookupOrder int
+
+const (
+	// hostLookupCgo means defer to cgo.
+	hostLookupCgo      hostLookupOrder = iota
+	hostLookupFilesDNS                 // files first
+	hostLookupDNSFiles                 // dns first
+	hostLookupFiles                    // only files
+	hostLookupDNS                      // only DNS
+)
+
+var lookupOrderName = map[hostLookupOrder]string{
+	hostLookupCgo:      "cgo",
+	hostLookupFilesDNS: "files,dns",
+	hostLookupDNSFiles: "dns,files",
+	hostLookupFiles:    "files",
+	hostLookupDNS:      "dns",
+}
+
+func (o hostLookupOrder) String() string {
+	if s, ok := lookupOrderName[o]; ok {
+		return s
+	}
+	return "hostLookupOrder=" + itoa(int(o)) + "??"
+}
+
+// goLookupHost is the native Go implementation of LookupHost.
+// Used only if cgoLookupHost refuses to handle the request
+// (that is, only if cgoLookupHost is the stub in cgo_stub.go).
+// Normally we let cgo use the C library resolver instead of
+// depending on our lookup code, so that Go and C get the same
+// answers.
+func (r *Resolver) goLookupHost(ctx context.Context, name string) (addrs []string, err error) {
+	return r.goLookupHostOrder(ctx, name, hostLookupFilesDNS)
+}
+
+func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []string, err error) {
+	if order == hostLookupFilesDNS || order == hostLookupFiles {
+		// Use entries from /etc/hosts if they match.
+		addrs = lookupStaticHost(name)
+		if len(addrs) > 0 || order == hostLookupFiles {
+			return
+		}
+	}
+	ips, _, err := r.goLookupIPCNAMEOrder(ctx, name, order)
+	if err != nil {
+		return
+	}
+	addrs = make([]string, 0, len(ips))
+	for _, ip := range ips {
+		addrs = append(addrs, ip.String())
+	}
+	return
+}
+
+// lookup entries from /etc/hosts
+func goLookupIPFiles(name string) (addrs []IPAddr) {
+	for _, haddr := range lookupStaticHost(name) {
+		haddr, zone := splitHostZone(haddr)
+		if ip := ParseIP(haddr); ip != nil {
+			addr := IPAddr{IP: ip, Zone: zone}
+			addrs = append(addrs, addr)
+		}
+	}
+	sortByRFC6724(addrs)
+	return
+}
+
+// goLookupIP is the native Go implementation of LookupIP.
+// The libc versions are in cgo_*.go.
+func (r *Resolver) goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
+	order := systemConf().hostLookupOrder(r, host)
+	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, host, order)
+	return
+}
+
+func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, cname dnsmessage.Name, err error) {
+	if order == hostLookupFilesDNS || order == hostLookupFiles {
+		addrs = goLookupIPFiles(name)
+		if len(addrs) > 0 || order == hostLookupFiles {
+			return addrs, dnsmessage.Name{}, nil
+		}
+	}
+	if !isDomainName(name) {
+		// See comment in func lookup above about use of errNoSuchHost.
+		return nil, dnsmessage.Name{}, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
+	}
+	resolvConf.tryUpdate("/etc/resolv.conf")
+	resolvConf.mu.RLock()
+	conf := resolvConf.dnsConfig
+	resolvConf.mu.RUnlock()
+	type result struct {
+		p      dnsmessage.Parser
+		server string
+		error
+	}
+	lane := make(chan result, 1)
+	qtypes := [...]dnsmessage.Type{dnsmessage.TypeA, dnsmessage.TypeAAAA}
+	var queryFn func(fqdn string, qtype dnsmessage.Type)
+	var responseFn func(fqdn string, qtype dnsmessage.Type) result
+	if conf.singleRequest {
+		queryFn = func(fqdn string, qtype dnsmessage.Type) {}
+		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
+			dnsWaitGroup.Add(1)
+			defer dnsWaitGroup.Done()
+			p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
+			return result{p, server, err}
+		}
+	} else {
+		queryFn = func(fqdn string, qtype dnsmessage.Type) {
+			dnsWaitGroup.Add(1)
+			go func(qtype dnsmessage.Type) {
+				p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
+				lane <- result{p, server, err}
+				dnsWaitGroup.Done()
+			}(qtype)
+		}
+		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
+			return <-lane
+		}
+	}
+	var lastErr error
+	for _, fqdn := range conf.nameList(name) {
+		for _, qtype := range qtypes {
+			queryFn(fqdn, qtype)
+		}
+		hitStrictError := false
+		for _, qtype := range qtypes {
+			result := responseFn(fqdn, qtype)
+			if result.error != nil {
+				if nerr, ok := result.error.(Error); ok && nerr.Temporary() && r.strictErrors() {
+					// This error will abort the nameList loop.
+					hitStrictError = true
+					lastErr = result.error
+				} else if lastErr == nil || fqdn == name+"." {
+					// Prefer error for original name.
+					lastErr = result.error
+				}
+				continue
+			}
+
+			// Presotto says it's okay to assume that servers listed in
+			// /etc/resolv.conf are recursive resolvers.
+			//
+			// We asked for recursion, so it should have included all the
+			// answers we need in this one packet.
+			//
+			// Further, RFC 1035 section 4.3.1 says that "the recursive
+			// response to a query will be... The answer to the query,
+			// possibly preface by one or more CNAME RRs that specify
+			// aliases encountered on the way to an answer."
+			//
+			// Therefore, we should be able to assume that we can ignore
+			// CNAMEs and that the A and AAAA records we requested are
+			// for the canonical name.
+
+		loop:
+			for {
+				h, err := result.p.AnswerHeader()
+				if err != nil && err != dnsmessage.ErrSectionDone {
+					lastErr = &DNSError{
+						Err:    "cannot marshal DNS message",
+						Name:   name,
+						Server: result.server,
+					}
+				}
+				if err != nil {
+					break
+				}
+				switch h.Type {
+				case dnsmessage.TypeA:
+					a, err := result.p.AResource()
+					if err != nil {
+						lastErr = &DNSError{
+							Err:    "cannot marshal DNS message",
+							Name:   name,
+							Server: result.server,
+						}
+						break loop
+					}
+					addrs = append(addrs, IPAddr{IP: IP(a.A[:])})
+
+				case dnsmessage.TypeAAAA:
+					aaaa, err := result.p.AAAAResource()
+					if err != nil {
+						lastErr = &DNSError{
+							Err:    "cannot marshal DNS message",
+							Name:   name,
+							Server: result.server,
+						}
+						break loop
+					}
+					addrs = append(addrs, IPAddr{IP: IP(aaaa.AAAA[:])})
+
+				default:
+					if err := result.p.SkipAnswer(); err != nil {
+						lastErr = &DNSError{
+							Err:    "cannot marshal DNS message",
+							Name:   name,
+							Server: result.server,
+						}
+						break loop
+					}
+					continue
+				}
+				if cname.Length == 0 && h.Name.Length != 0 {
+					cname = h.Name
+				}
+			}
+		}
+		if hitStrictError {
+			// If either family hit an error with StrictErrors enabled,
+			// discard all addresses. This ensures that network flakiness
+			// cannot turn a dualstack hostname IPv4/IPv6-only.
+			addrs = nil
+			break
+		}
+		if len(addrs) > 0 {
+			break
+		}
+	}
+	if lastErr, ok := lastErr.(*DNSError); ok {
+		// Show original name passed to lookup, not suffixed one.
+		// In general we might have tried many suffixes; showing
+		// just one is misleading. See also golang.org/issue/6324.
+		lastErr.Name = name
+	}
+	sortByRFC6724(addrs)
+	if len(addrs) == 0 {
+		if order == hostLookupDNSFiles {
+			addrs = goLookupIPFiles(name)
+		}
+		if len(addrs) == 0 && lastErr != nil {
+			return nil, dnsmessage.Name{}, lastErr
+		}
+	}
+	return addrs, cname, nil
+}
+
+// goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
+func (r *Resolver) goLookupCNAME(ctx context.Context, host string) (string, error) {
+	order := systemConf().hostLookupOrder(r, host)
+	_, cname, err := r.goLookupIPCNAMEOrder(ctx, host, order)
+	return cname.String(), err
+}
+
+// goLookupPTR is the native Go implementation of LookupAddr.
+// Used only if cgoLookupPTR refuses to handle the request (that is,
+// only if cgoLookupPTR is the stub in cgo_stub.go).
+// Normally we let cgo use the C library resolver instead of depending
+// on our lookup code, so that Go and C get the same answers.
+func (r *Resolver) goLookupPTR(ctx context.Context, addr string) ([]string, error) {
+	names := lookupStaticAddr(addr)
+	if len(names) > 0 {
+		return names, nil
+	}
+	arpa, err := reverseaddr(addr)
+	if err != nil {
+		return nil, err
+	}
+	p, server, err := r.lookup(ctx, arpa, dnsmessage.TypePTR)
+	if err != nil {
+		return nil, err
+	}
+	var ptrs []string
+	for {
+		h, err := p.AnswerHeader()
+		if err == dnsmessage.ErrSectionDone {
+			break
+		}
+		if err != nil {
+			return nil, &DNSError{
+				Err:    "cannot marshal DNS message",
+				Name:   addr,
+				Server: server,
+			}
+		}
+		if h.Type != dnsmessage.TypePTR {
+			err := p.SkipAnswer()
+			if err != nil {
+				return nil, &DNSError{
+					Err:    "cannot marshal DNS message",
+					Name:   addr,
+					Server: server,
+				}
+			}
+			continue
+		}
+		ptr, err := p.PTRResource()
+		if err != nil {
+			return nil, &DNSError{
+				Err:    "cannot marshal DNS message",
+				Name:   addr,
+				Server: server,
+			}
+		}
+		ptrs = append(ptrs, ptr.PTR.String())
+
+	}
+	return ptrs, nil
+}