Source: dnsclient_unix.go in package net

+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"
)

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
		}

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
	}

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
				}

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,
			}

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.

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) {

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 we hit a temporary error with StrictErrors enabled, stop immediately instead of trying more names.

			break
		}
	}
	if err == nil {
		return p, server, nil
	}

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.

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

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) {

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
		}
	}

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 {

This error will abort the nameList loop.

					hitStrictError = true
					lastErr = result.error

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 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
		}
	}

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