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// 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 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.
// Could have a small cache.
// 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"
)
// A dnsConn represents a DNS transport endpoint.
type dnsConn interface {
io.Closer
SetDeadline(time.Time) error
// dnsRoundTrip executes a single DNS transaction, returning a
// DNS response message for the provided DNS query message.
dnsRoundTrip(query *dnsMsg) (*dnsMsg, error)
}
// dnsPacketConn implements the dnsConn interface for RFC 1035's
// "UDP usage" transport mechanism. Conn is a packet-oriented connection,
// such as a *UDPConn.
type dnsPacketConn struct {
Conn
}
func (c *dnsPacketConn) dnsRoundTrip(query *dnsMsg) (*dnsMsg, error) {
b, ok := query.Pack()
if !ok {
return nil, errors.New("cannot marshal DNS message")
}
if _, err := c.Write(b); err != nil {
return nil, err
}
b = make([]byte, 512) // see RFC 1035
for {
n, err := c.Read(b)
if err != nil {
return nil, err
}
resp := &dnsMsg{}
if !resp.Unpack(b[:n]) || !resp.IsResponseTo(query) {
// Ignore invalid responses as they may be malicious
// forgery attempts. Instead continue waiting until
// timeout. See golang.org/issue/13281.
continue
}
return resp, nil
}
}
// dnsStreamConn implements the dnsConn interface for RFC 1035's
// "TCP usage" transport mechanism. Conn is a stream-oriented connection,
// such as a *TCPConn.
type dnsStreamConn struct {
Conn
}
func (c *dnsStreamConn) dnsRoundTrip(query *dnsMsg) (*dnsMsg, error) {
b, ok := query.Pack()
if !ok {
return nil, errors.New("cannot marshal DNS message")
}
l := len(b)
b = append([]byte{byte(l >> 8), byte(l)}, b...)
if _, err := c.Write(b); err != nil {
return nil, 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 nil, 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 nil, err
}
resp := &dnsMsg{}
if !resp.Unpack(b[:n]) {
return nil, errors.New("cannot unmarshal DNS message")
}
if !resp.IsResponseTo(query) {
return nil, errors.New("invalid DNS response")
}
return resp, nil
}
// exchange sends a query on the connection and hopes for a response.
func (r *Resolver) exchange(ctx context.Context, server, name string, qtype uint16, timeout time.Duration) (*dnsMsg, error) {
out := dnsMsg{
dnsMsgHdr: dnsMsgHdr{
recursion_desired: true,
},
question: []dnsQuestion{
{name, qtype, dnsClassINET},
},
}
for _, network := range []string{"udp", "tcp"} {
// TODO(mdempsky): Refactor so defers from UDP-based
// exchanges happen before TCP-based exchange.
ctx, cancel := context.WithDeadline(ctx, time.Now().Add(timeout))
defer cancel()
c, err := r.dial(ctx, network, server)
if err != nil {
return nil, err
}
defer c.Close()
if d, ok := ctx.Deadline(); ok && !d.IsZero() {
c.SetDeadline(d)
}
out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
in, err := c.dnsRoundTrip(&out)
if err != nil {
return nil, mapErr(err)
}
if in.truncated { // see RFC 5966
continue
}
return in, nil
}
return nil, errors.New("no answer from DNS server")
}
// 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 uint16) (string, []dnsRR, error) {
var lastErr error
serverOffset := cfg.serverOffset()
sLen := uint32(len(cfg.servers))
for i := 0; i < cfg.attempts; i++ {
for j := uint32(0); j < sLen; j++ {
server := cfg.servers[(serverOffset+j)%sLen]
msg, err := r.exchange(ctx, server, name, qtype, cfg.timeout)
if err != nil {
lastErr = &DNSError{
Err: err.Error(),
Name: name,
Server: server,
}
if nerr, ok := err.(Error); ok && nerr.Timeout() {
lastErr.(*DNSError).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 {
lastErr.(*DNSError).IsTemporary = true
}
continue
}
// libresolv continues to the next server when it receives
// an invalid referral response. See golang.org/issue/15434.
if msg.rcode == dnsRcodeSuccess && !msg.authoritative && !msg.recursion_available && len(msg.answer) == 0 && len(msg.extra) == 0 {
lastErr = &DNSError{Err: "lame referral", Name: name, Server: server}
continue
}
cname, rrs, err := answer(name, server, msg, qtype)
// If answer errored for rcodes dnsRcodeSuccess or dnsRcodeNameError,
// it means the response in msg was not useful and trying another
// server probably won't help. Return now in those cases.
// TODO: indicate this in a more obvious way, such as a field on DNSError?
if err == nil || msg.rcode == dnsRcodeSuccess || msg.rcode == dnsRcodeNameError {
return cname, rrs, err
}
lastErr = err
}
}
return "", nil, lastErr
}
// addrRecordList converts and returns a list of IP addresses from DNS
// address records (both A and AAAA). Other record types are ignored.
func addrRecordList(rrs []dnsRR) []IPAddr {
addrs := make([]IPAddr, 0, 4)
for _, rr := range rrs {
switch rr := rr.(type) {
case *dnsRR_A:
addrs = append(addrs, IPAddr{IP: IPv4(byte(rr.A>>24), byte(rr.A>>16), byte(rr.A>>8), byte(rr.A))})
case *dnsRR_AAAA:
ip := make(IP, IPv6len)
copy(ip, rr.AAAA[:])
addrs = append(addrs, IPAddr{IP: ip})
}
}
return addrs
}
// 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 uint16) (cname string, rrs []dnsRR, err 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 "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name}
}
resolvConf.tryUpdate("/etc/resolv.conf")
resolvConf.mu.RLock()
conf := resolvConf.dnsConfig
resolvConf.mu.RUnlock()
for _, fqdn := range conf.nameList(name) {
cname, rrs, 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, 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
}
// 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(host)
addrs, _, err = r.goLookupIPCNAMEOrder(ctx, host, order)
return
}
func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, cname string, err error) {
if order == hostLookupFilesDNS || order == hostLookupFiles {
addrs = goLookupIPFiles(name)
if len(addrs) > 0 || order == hostLookupFiles {
return addrs, name, nil
}
}
if !isDomainName(name) {
// See comment in func lookup above about use of errNoSuchHost.
return nil, "", &DNSError{Err: errNoSuchHost.Error(), Name: name}
}
resolvConf.tryUpdate("/etc/resolv.conf")
resolvConf.mu.RLock()
conf := resolvConf.dnsConfig
resolvConf.mu.RUnlock()
type racer struct {
cname string
rrs []dnsRR
error
}
lane := make(chan racer, 1)
qtypes := [...]uint16{dnsTypeA, dnsTypeAAAA}
var lastErr error
for _, fqdn := range conf.nameList(name) {
for _, qtype := range qtypes {
go func(qtype uint16) {
cname, rrs, err := r.tryOneName(ctx, conf, fqdn, qtype)
lane <- racer{cname, rrs, err}
}(qtype)
}
hitStrictError := false
for range qtypes {
racer := <-lane
if racer.error != nil {
if nerr, ok := racer.error.(Error); ok && nerr.Temporary() && r.StrictErrors {
// This error will abort the nameList loop.
hitStrictError = true
lastErr = racer.error
} else if lastErr == nil || fqdn == name+"." {
// Prefer error for original name.
lastErr = racer.error
}
continue
}
addrs = append(addrs, addrRecordList(racer.rrs)...)
if cname == "" {
cname = racer.cname
}
}
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, "", lastErr
}
}
return addrs, cname, nil
}
// goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
func (r *Resolver) goLookupCNAME(ctx context.Context, host string) (cname string, err error) {
order := systemConf().hostLookupOrder(host)
_, cname, err = r.goLookupIPCNAMEOrder(ctx, host, order)
return
}
// 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
}
_, rrs, err := r.lookup(ctx, arpa, dnsTypePTR)
if err != nil {
return nil, err
}
ptrs := make([]string, len(rrs))
for i, rr := range rrs {
ptrs[i] = rr.(*dnsRR_PTR).Ptr
}
return ptrs, nil
}