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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 (
"errors"
"io"
"math/rand"
"os"
"sync"
"time"
)
// A dnsDialer provides dialing suitable for DNS queries.
type dnsDialer interface {
dialDNS(string, string) (dnsConn, error)
}
var testHookDNSDialer = func(d time.Duration) dnsDialer { return &Dialer{Timeout: d} }
// A dnsConn represents a DNS transport endpoint.
type dnsConn interface {
io.Closer
SetDeadline(time.Time) error
// readDNSResponse reads a DNS response message from the DNS
// transport endpoint and returns the received DNS response
// message.
readDNSResponse() (*dnsMsg, error)
// writeDNSQuery writes a DNS query message to the DNS
// connection endpoint.
writeDNSQuery(*dnsMsg) error
}
func (c *UDPConn) readDNSResponse() (*dnsMsg, error) {
b := make([]byte, 512) // see RFC 1035
n, err := c.Read(b)
if err != nil {
return nil, err
}
msg := &dnsMsg{}
if !msg.Unpack(b[:n]) {
return nil, errors.New("cannot unmarshal DNS message")
}
return msg, nil
}
func (c *UDPConn) writeDNSQuery(msg *dnsMsg) error {
b, ok := msg.Pack()
if !ok {
return errors.New("cannot marshal DNS message")
}
if _, err := c.Write(b); err != nil {
return err
}
return nil
}
func (c *TCPConn) readDNSResponse() (*dnsMsg, error) {
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
}
msg := &dnsMsg{}
if !msg.Unpack(b[:n]) {
return nil, errors.New("cannot unmarshal DNS message")
}
return msg, nil
}
func (c *TCPConn) writeDNSQuery(msg *dnsMsg) error {
b, ok := msg.Pack()
if !ok {
return errors.New("cannot marshal DNS message")
}
l := uint16(len(b))
b = append([]byte{byte(l >> 8), byte(l)}, b...)
if _, err := c.Write(b); err != nil {
return err
}
return nil
}
func (d *Dialer) dialDNS(network, server string) (dnsConn, error) {
switch network {
case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6":
default:
return nil, UnknownNetworkError(network)
}
// Calling Dial here is scary -- we have to be sure not to
// dial a name that will require a DNS lookup, or Dial will
// call back here to translate it. The DNS config parser has
// already checked that all the cfg.servers[i] are IP
// addresses, which Dial will use without a DNS lookup.
c, err := d.Dial(network, server)
if err != nil {
return nil, err
}
switch network {
case "tcp", "tcp4", "tcp6":
return c.(*TCPConn), nil
case "udp", "udp4", "udp6":
return c.(*UDPConn), nil
}
panic("unreachable")
}
// exchange sends a query on the connection and hopes for a response.
func exchange(server, name string, qtype uint16, timeout time.Duration) (*dnsMsg, error) {
d := testHookDNSDialer(timeout)
out := dnsMsg{
dnsMsgHdr: dnsMsgHdr{
recursion_desired: true,
},
question: []dnsQuestion{
{name, qtype, dnsClassINET},
},
}
for _, network := range []string{"udp", "tcp"} {
c, err := d.dialDNS(network, server)
if err != nil {
return nil, err
}
defer c.Close()
if timeout > 0 {
c.SetDeadline(time.Now().Add(timeout))
}
out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
if err := c.writeDNSQuery(&out); err != nil {
return nil, err
}
in, err := c.readDNSResponse()
if err != nil {
return nil, err
}
if in.id != out.id {
return nil, errors.New("DNS message ID mismatch")
}
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 tryOneName(cfg *dnsConfig, name string, qtype uint16) (string, []dnsRR, error) {
if len(cfg.servers) == 0 {
return "", nil, &DNSError{Err: "no DNS servers", Name: name}
}
timeout := time.Duration(cfg.timeout) * time.Second
var lastErr error
for i := 0; i < cfg.attempts; i++ {
for _, server := range cfg.servers {
server = JoinHostPort(server, "53")
msg, err := exchange(server, name, qtype, 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
}
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
modTime time.Time // time of resolv.conf modification
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, modTime, 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")
}
if fi, err := os.Stat("/etc/resolv.conf"); err == nil {
conf.modTime = fi.ModTime()
}
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
if fi, err := os.Stat(name); err == nil {
if fi.ModTime().Equal(conf.modTime) {
return
}
conf.modTime = fi.ModTime()
} else {
// If modTime wasn't set prior, assume nothing has changed.
if conf.modTime.IsZero() {
return
}
conf.modTime = time.Time{}
}
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 lookup(name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
if !isDomainName(name) {
return "", nil, &DNSError{Err: "invalid domain name", 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 = tryOneName(conf, fqdn, qtype)
if err == nil {
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
}
// nameList returns a list of names for sequential DNS queries.
func (conf *dnsConfig) nameList(name string) []string {
// If name is rooted (trailing dot), try only that name.
rooted := len(name) > 0 && name[len(name)-1] == '.'
if rooted {
return []string{name}
}
// Build list of search choices.
names := make([]string, 0, 1+len(conf.search))
// If name has enough dots, try unsuffixed first.
if count(name, '.') >= conf.ndots {
names = append(names, name+".")
}
// Try suffixes.
for _, suffix := range conf.search {
suffixed := name + "." + suffix
if suffixed[len(suffixed)-1] != '.' {
suffixed += "."
}
names = append(names, suffixed)
}
// Try unsuffixed, if not tried first above.
if count(name, '.') < conf.ndots {
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 goLookupHost(name string) (addrs []string, err error) {
return goLookupHostOrder(name, hostLookupFilesDNS)
}
func goLookupHostOrder(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 := goLookupIPOrder(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 goLookupIP(name string) (addrs []IPAddr, err error) {
return goLookupIPOrder(name, hostLookupFilesDNS)
}
func goLookupIPOrder(name string, order hostLookupOrder) (addrs []IPAddr, err error) {
if order == hostLookupFilesDNS || order == hostLookupFiles {
addrs = goLookupIPFiles(name)
if len(addrs) > 0 || order == hostLookupFiles {
return addrs, nil
}
}
if !isDomainName(name) {
return nil, &DNSError{Err: "invalid domain name", Name: name}
}
resolvConf.tryUpdate("/etc/resolv.conf")
resolvConf.mu.RLock()
conf := resolvConf.dnsConfig
resolvConf.mu.RUnlock()
type racer struct {
fqdn 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) {
_, rrs, err := tryOneName(conf, fqdn, qtype)
lane <- racer{fqdn, rrs, err}
}(qtype)
}
for range qtypes {
racer := <-lane
if racer.error != nil {
// Prefer error for original name.
if lastErr == nil || racer.fqdn == name+"." {
lastErr = racer.error
}
continue
}
addrs = append(addrs, addrRecordList(racer.rrs)...)
}
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, nil
}
// goLookupCNAME is the native Go implementation of LookupCNAME.
// Used only if cgoLookupCNAME refuses to handle the request
// (that is, only if cgoLookupCNAME 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 goLookupCNAME(name string) (cname string, err error) {
_, rrs, err := lookup(name, dnsTypeCNAME)
if err != nil {
return
}
cname = rrs[0].(*dnsRR_CNAME).Cname
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 goLookupPTR(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 := lookup(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
}