// 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"
"math/rand"
"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")
// errServerTemporarlyMisbehaving is like errServerMisbehaving, except
// that when it gets translated to a DNSError, the IsTemporary field
// gets set to true.
errServerTemporarlyMisbehaving = errors.New("server misbehaving")
)
func newRequest(q dnsmessage.Question) (id uint16, udpReq, tcpReq []byte, err error) {
id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
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 errServerTemporarlyMisbehaving
}
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 == errServerTemporarlyMisbehaving {
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
}
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