285 lines
7.0 KiB
Go
285 lines
7.0 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build darwin freebsd linux netbsd openbsd
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// DNS client: see RFC 1035.
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// Has to be linked into package net for Dial.
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// TODO(rsc):
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// Check periodically whether /etc/resolv.conf has changed.
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// Could potentially handle many outstanding lookups faster.
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// Could have a small cache.
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// Random UDP source port (net.Dial should do that for us).
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// Random request IDs.
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package net
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import (
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"math/rand"
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"sync"
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"time"
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)
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// Send a request on the connection and hope for a reply.
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// Up to cfg.attempts attempts.
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func exchange(cfg *dnsConfig, c Conn, name string, qtype uint16) (*dnsMsg, error) {
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if len(name) >= 256 {
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return nil, &DNSError{Err: "name too long", Name: name}
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}
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out := new(dnsMsg)
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out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
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out.question = []dnsQuestion{
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{name, qtype, dnsClassINET},
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}
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out.recursion_desired = true
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msg, ok := out.Pack()
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if !ok {
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return nil, &DNSError{Err: "internal error - cannot pack message", Name: name}
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}
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for attempt := 0; attempt < cfg.attempts; attempt++ {
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n, err := c.Write(msg)
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if err != nil {
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return nil, err
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}
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if cfg.timeout == 0 {
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c.SetReadDeadline(time.Time{})
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} else {
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c.SetReadDeadline(time.Now().Add(time.Duration(cfg.timeout) * time.Second))
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}
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buf := make([]byte, 2000) // More than enough.
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n, err = c.Read(buf)
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if err != nil {
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if e, ok := err.(Error); ok && e.Timeout() {
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continue
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}
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return nil, err
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}
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buf = buf[0:n]
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in := new(dnsMsg)
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if !in.Unpack(buf) || in.id != out.id {
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continue
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}
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return in, nil
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}
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var server string
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if a := c.RemoteAddr(); a != nil {
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server = a.String()
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}
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return nil, &DNSError{Err: "no answer from server", Name: name, Server: server, IsTimeout: true}
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}
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// Do a lookup for a single name, which must be rooted
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// (otherwise answer will not find the answers).
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func tryOneName(cfg *dnsConfig, name string, qtype uint16) (cname string, addrs []dnsRR, err error) {
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if len(cfg.servers) == 0 {
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return "", nil, &DNSError{Err: "no DNS servers", Name: name}
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}
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for i := 0; i < len(cfg.servers); i++ {
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// Calling Dial here is scary -- we have to be sure
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// not to dial a name that will require a DNS lookup,
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// or Dial will call back here to translate it.
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// The DNS config parser has already checked that
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// all the cfg.servers[i] are IP addresses, which
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// Dial will use without a DNS lookup.
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server := cfg.servers[i] + ":53"
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c, cerr := Dial("udp", server)
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if cerr != nil {
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err = cerr
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continue
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}
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msg, merr := exchange(cfg, c, name, qtype)
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c.Close()
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if merr != nil {
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err = merr
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continue
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}
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cname, addrs, err = answer(name, server, msg, qtype)
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if err == nil || err.(*DNSError).Err == noSuchHost {
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break
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}
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}
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return
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}
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func convertRR_A(records []dnsRR) []IP {
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addrs := make([]IP, len(records))
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for i, rr := range records {
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a := rr.(*dnsRR_A).A
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addrs[i] = IPv4(byte(a>>24), byte(a>>16), byte(a>>8), byte(a))
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}
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return addrs
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}
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func convertRR_AAAA(records []dnsRR) []IP {
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addrs := make([]IP, len(records))
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for i, rr := range records {
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a := make(IP, IPv6len)
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copy(a, rr.(*dnsRR_AAAA).AAAA[:])
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addrs[i] = a
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}
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return addrs
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}
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var cfg *dnsConfig
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var dnserr error
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func loadConfig() { cfg, dnserr = dnsReadConfig() }
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var onceLoadConfig sync.Once
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func lookup(name string, qtype uint16) (cname string, addrs []dnsRR, err error) {
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if !isDomainName(name) {
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return name, nil, &DNSError{Err: "invalid domain name", Name: name}
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}
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onceLoadConfig.Do(loadConfig)
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if dnserr != nil || cfg == nil {
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err = dnserr
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return
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}
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// If name is rooted (trailing dot) or has enough dots,
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// try it by itself first.
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rooted := len(name) > 0 && name[len(name)-1] == '.'
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if rooted || count(name, '.') >= cfg.ndots {
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rname := name
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if !rooted {
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rname += "."
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}
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// Can try as ordinary name.
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cname, addrs, err = tryOneName(cfg, rname, qtype)
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if err == nil {
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return
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}
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}
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if rooted {
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return
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}
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// Otherwise, try suffixes.
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for i := 0; i < len(cfg.search); i++ {
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rname := name + "." + cfg.search[i]
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if rname[len(rname)-1] != '.' {
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rname += "."
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}
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cname, addrs, err = tryOneName(cfg, rname, qtype)
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if err == nil {
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return
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}
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}
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// Last ditch effort: try unsuffixed.
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rname := name
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if !rooted {
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rname += "."
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}
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cname, addrs, err = tryOneName(cfg, rname, qtype)
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if err == nil {
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return
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}
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return
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}
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// goLookupHost is the native Go implementation of LookupHost.
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// Used only if cgoLookupHost refuses to handle the request
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// (that is, only if cgoLookupHost is the stub in cgo_stub.go).
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// Normally we let cgo use the C library resolver instead of
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// depending on our lookup code, so that Go and C get the same
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// answers.
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func goLookupHost(name string) (addrs []string, err error) {
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// Use entries from /etc/hosts if they match.
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addrs = lookupStaticHost(name)
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if len(addrs) > 0 {
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return
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}
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onceLoadConfig.Do(loadConfig)
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if dnserr != nil || cfg == nil {
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err = dnserr
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return
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}
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ips, err := goLookupIP(name)
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if err != nil {
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return
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}
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addrs = make([]string, 0, len(ips))
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for _, ip := range ips {
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addrs = append(addrs, ip.String())
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}
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return
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}
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// goLookupIP is the native Go implementation of LookupIP.
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// Used only if cgoLookupIP refuses to handle the request
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// (that is, only if cgoLookupIP is the stub in cgo_stub.go).
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// Normally we let cgo use the C library resolver instead of
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// depending on our lookup code, so that Go and C get the same
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// answers.
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func goLookupIP(name string) (addrs []IP, err error) {
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// Use entries from /etc/hosts if possible.
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haddrs := lookupStaticHost(name)
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if len(haddrs) > 0 {
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for _, haddr := range haddrs {
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if ip := ParseIP(haddr); ip != nil {
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addrs = append(addrs, ip)
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}
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}
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if len(addrs) > 0 {
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return
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}
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}
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onceLoadConfig.Do(loadConfig)
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if dnserr != nil || cfg == nil {
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err = dnserr
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return
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}
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var records []dnsRR
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var cname string
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var err4, err6 error
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cname, records, err4 = lookup(name, dnsTypeA)
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addrs = convertRR_A(records)
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if cname != "" {
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name = cname
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}
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_, records, err6 = lookup(name, dnsTypeAAAA)
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if err4 != nil && err6 == nil {
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// Ignore A error because AAAA lookup succeeded.
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err4 = nil
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}
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if err6 != nil && len(addrs) > 0 {
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// Ignore AAAA error because A lookup succeeded.
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err6 = nil
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}
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if err4 != nil {
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return nil, err4
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}
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if err6 != nil {
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return nil, err6
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}
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addrs = append(addrs, convertRR_AAAA(records)...)
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return addrs, nil
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}
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// goLookupCNAME is the native Go implementation of LookupCNAME.
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// Used only if cgoLookupCNAME refuses to handle the request
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// (that is, only if cgoLookupCNAME is the stub in cgo_stub.go).
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// Normally we let cgo use the C library resolver instead of
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// depending on our lookup code, so that Go and C get the same
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// answers.
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func goLookupCNAME(name string) (cname string, err error) {
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onceLoadConfig.Do(loadConfig)
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if dnserr != nil || cfg == nil {
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err = dnserr
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return
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}
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_, rr, err := lookup(name, dnsTypeCNAME)
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if err != nil {
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return
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}
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cname = rr[0].(*dnsRR_CNAME).Cname
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return
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}
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