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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 freebsd linux netbsd openbsd
// DNS client: see RFC 1035.
// Has to be linked into package net for Dial.
// TODO(rsc):
// Check periodically whether /etc/resolv.conf has changed.
// 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 (
"math/rand"
"sync"
"time"
)
// Send a request on the connection and hope for a reply.
// Up to cfg.attempts attempts.
func exchange(cfg *dnsConfig, c Conn, name string, qtype uint16) (*dnsMsg, error) {
if len(name) >= 256 {
return nil, &DNSError{Err: "name too long", Name: name}
}
out := new(dnsMsg)
out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
out.question = []dnsQuestion{
{name, qtype, dnsClassINET},
}
out.recursion_desired = true
msg, ok := out.Pack()
if !ok {
return nil, &DNSError{Err: "internal error - cannot pack message", Name: name}
}
for attempt := 0; attempt < cfg.attempts; attempt++ {
n, err := c.Write(msg)
if err != nil {
return nil, err
}
c.SetReadTimeout(int64(cfg.timeout) * 1e9) // nanoseconds
buf := make([]byte, 2000) // More than enough.
n, err = c.Read(buf)
if err != nil {
if e, ok := err.(Error); ok && e.Timeout() {
continue
}
return nil, err
}
buf = buf[0:n]
in := new(dnsMsg)
if !in.Unpack(buf) || in.id != out.id {
continue
}
return in, nil
}
var server string
if a := c.RemoteAddr(); a != nil {
server = a.String()
}
return nil, &DNSError{Err: "no answer from server", Name: name, Server: server, IsTimeout: true}
}
// 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) (cname string, addrs []dnsRR, err error) {
if len(cfg.servers) == 0 {
return "", nil, &DNSError{Err: "no DNS servers", Name: name}
}
for i := 0; i < len(cfg.servers); i++ {
// 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.
server := cfg.servers[i] + ":53"
c, cerr := Dial("udp", server)
if cerr != nil {
err = cerr
continue
}
msg, merr := exchange(cfg, c, name, qtype)
c.Close()
if merr != nil {
err = merr
continue
}
cname, addrs, err = answer(name, server, msg, qtype)
if err == nil || err.(*DNSError).Err == noSuchHost {
break
}
}
return
}
func convertRR_A(records []dnsRR) []IP {
addrs := make([]IP, len(records))
for i, rr := range records {
a := rr.(*dnsRR_A).A
addrs[i] = IPv4(byte(a>>24), byte(a>>16), byte(a>>8), byte(a))
}
return addrs
}
func convertRR_AAAA(records []dnsRR) []IP {
addrs := make([]IP, len(records))
for i, rr := range records {
a := make(IP, IPv6len)
copy(a, rr.(*dnsRR_AAAA).AAAA[:])
addrs[i] = a
}
return addrs
}
var cfg *dnsConfig
var dnserr error
func loadConfig() { cfg, dnserr = dnsReadConfig() }
var onceLoadConfig sync.Once
func lookup(name string, qtype uint16) (cname string, addrs []dnsRR, err error) {
if !isDomainName(name) {
return name, nil, &DNSError{Err: "invalid domain name", Name: name}
}
onceLoadConfig.Do(loadConfig)
if dnserr != nil || cfg == nil {
err = dnserr
return
}
// If name is rooted (trailing dot) or has enough dots,
// try it by itself first.
rooted := len(name) > 0 && name[len(name)-1] == '.'
if rooted || count(name, '.') >= cfg.ndots {
rname := name
if !rooted {
rname += "."
}
// Can try as ordinary name.
cname, addrs, err = tryOneName(cfg, rname, qtype)
if err == nil {
return
}
}
if rooted {
return
}
// Otherwise, try suffixes.
for i := 0; i < len(cfg.search); i++ {
rname := name + "." + cfg.search[i]
if rname[len(rname)-1] != '.' {
rname += "."
}
cname, addrs, err = tryOneName(cfg, rname, qtype)
if err == nil {
return
}
}
// Last ditch effort: try unsuffixed.
rname := name
if !rooted {
rname += "."
}
cname, addrs, err = tryOneName(cfg, rname, qtype)
if err == nil {
return
}
return
}
// 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) {
// Use entries from /etc/hosts if they match.
addrs = lookupStaticHost(name)
if len(addrs) > 0 {
return
}
onceLoadConfig.Do(loadConfig)
if dnserr != nil || cfg == nil {
err = dnserr
return
}
ips, err := goLookupIP(name)
if err != nil {
return
}
addrs = make([]string, 0, len(ips))
for _, ip := range ips {
addrs = append(addrs, ip.String())
}
return
}
// goLookupIP is the native Go implementation of LookupIP.
// Used only if cgoLookupIP refuses to handle the request
// (that is, only if cgoLookupIP 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 goLookupIP(name string) (addrs []IP, err error) {
// Use entries from /etc/hosts if possible.
haddrs := lookupStaticHost(name)
if len(haddrs) > 0 {
for _, haddr := range haddrs {
if ip := ParseIP(haddr); ip != nil {
addrs = append(addrs, ip)
}
}
if len(addrs) > 0 {
return
}
}
onceLoadConfig.Do(loadConfig)
if dnserr != nil || cfg == nil {
err = dnserr
return
}
var records []dnsRR
var cname string
cname, records, err = lookup(name, dnsTypeA)
if err != nil {
return
}
addrs = convertRR_A(records)
if cname != "" {
name = cname
}
_, records, err = lookup(name, dnsTypeAAAA)
if err != nil && len(addrs) > 0 {
// Ignore error because A lookup succeeded.
err = nil
}
if err != nil {
return
}
addrs = append(addrs, convertRR_AAAA(records)...)
return
}
// 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) {
onceLoadConfig.Do(loadConfig)
if dnserr != nil || cfg == nil {
err = dnserr
return
}
_, rr, err := lookup(name, dnsTypeCNAME)
if err != nil {
return
}
cname = rr[0].(*dnsRR_CNAME).Cname
return
}
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