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// Copyright 2012 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.
package net
import "time"
// protocols contains minimal mappings between internet protocol
// names and numbers for platforms that don't have a complete list of
// protocol numbers.
// See
var protocols = map[string]int{
"icmp": 1, "ICMP": 1,
"igmp": 2, "IGMP": 2,
"tcp": 6, "TCP": 6,
"udp": 17, "UDP": 17,
"ipv6-icmp": 58, "IPV6-ICMP": 58, "IPv6-ICMP": 58,
// LookupHost looks up the given host using the local resolver.
// It returns an array of that host's addresses.
func LookupHost(host string) (addrs []string, err error) {
return lookupHost(host)
// LookupIP looks up host using the local resolver.
// It returns an array of that host's IPv4 and IPv6 addresses.
func LookupIP(host string) (addrs []IP, err error) {
return lookupIPMerge(host)
var lookupGroup singleflight
// lookupIPMerge wraps lookupIP, but makes sure that for any given
// host, only one lookup is in-flight at a time. The returned memory
// is always owned by the caller.
func lookupIPMerge(host string) (addrs []IP, err error) {
addrsi, err, shared := lookupGroup.Do(host, func() (interface{}, error) {
return lookupIP(host)
if err != nil {
return nil, err
addrs = addrsi.([]IP)
if shared {
clone := make([]IP, len(addrs))
copy(clone, addrs)
addrs = clone
return addrs, nil
func lookupIPDeadline(host string, deadline time.Time) (addrs []IP, err error) {
if deadline.IsZero() {
return lookupIPMerge(host)
// TODO(bradfitz): consider pushing the deadline down into the
// name resolution functions. But that involves fixing it for
// the native Go resolver, cgo, Windows, etc.
// In the meantime, just use a goroutine. Most users affected
// by are due to TCP connections
// to unresponsive hosts, not DNS.
timeout := deadline.Sub(time.Now())
if timeout <= 0 {
err = errTimeout
t := time.NewTimer(timeout)
defer t.Stop()
type res struct {
addrs []IP
err error
resc := make(chan res, 1)
go func() {
a, err := lookupIPMerge(host)
resc <- res{a, err}
select {
case <-t.C:
err = errTimeout
case r := <-resc:
addrs, err = r.addrs, r.err
// LookupPort looks up the port for the given network and service.
func LookupPort(network, service string) (port int, err error) {
return lookupPort(network, service)
// LookupCNAME returns the canonical DNS host for the given name.
// Callers that do not care about the canonical name can call
// LookupHost or LookupIP directly; both take care of resolving
// the canonical name as part of the lookup.
func LookupCNAME(name string) (cname string, err error) {
return lookupCNAME(name)
// LookupSRV tries to resolve an SRV query of the given service,
// protocol, and domain name. The proto is "tcp" or "udp".
// The returned records are sorted by priority and randomized
// by weight within a priority.
// LookupSRV constructs the DNS name to look up following RFC 2782.
// That is, it looks up To accommodate services
// publishing SRV records under non-standard names, if both service
// and proto are empty strings, LookupSRV looks up name directly.
func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err error) {
return lookupSRV(service, proto, name)
// LookupMX returns the DNS MX records for the given domain name sorted by preference.
func LookupMX(name string) (mx []*MX, err error) {
return lookupMX(name)
// LookupNS returns the DNS NS records for the given domain name.
func LookupNS(name string) (ns []*NS, err error) {
return lookupNS(name)
// LookupTXT returns the DNS TXT records for the given domain name.
func LookupTXT(name string) (txt []string, err error) {
return lookupTXT(name)
// LookupAddr performs a reverse lookup for the given address, returning a list
// of names mapping to that address.
func LookupAddr(addr string) (name []string, err error) {
return lookupAddr(addr)