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net: detect changes to /etc/resolv.conf.

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Implement the changes as suggested by rsc.
Fixes #6670.

LGTM=josharian, iant
R=golang-codereviews, iant, josharian, mikioh.mikioh, alex, gobot
CC=golang-codereviews, rsc
https://golang.org/cl/83690045
This commit is contained in:
Guillaume J. Charmes 2014-05-14 17:11:00 -07:00 committed by Ian Lance Taylor
parent 79fb16d32c
commit bf1d400d1c
2 changed files with 203 additions and 29 deletions
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100
src/pkg/net/dnsclient_unix.go
View File

@ -8,7 +8,6 @@
// 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).
@ -19,6 +18,7 @@ package net
import (
"io"
"math/rand"
"os"
"sync"
"time"
)
@ -156,33 +156,90 @@ func convertRR_AAAA(records []dnsRR) []IP {
return addrs
}
var cfg *dnsConfig
var dnserr error
var cfg struct {
ch chan struct{}
mu sync.RWMutex // protects dnsConfig and dnserr
dnsConfig *dnsConfig
dnserr error
}
var onceLoadConfig sync.Once
// Assume dns config file is /etc/resolv.conf here
func loadConfig() { cfg, dnserr = dnsReadConfig("/etc/resolv.conf") }
func loadDefaultConfig() {
loadConfig("/etc/resolv.conf", 5*time.Second, nil)
}
var onceLoadConfig sync.Once
func loadConfig(resolvConfPath string, reloadTime time.Duration, quit <-chan chan struct{}) {
var mtime time.Time
cfg.ch = make(chan struct{}, 1)
if fi, err := os.Stat(resolvConfPath); err != nil {
cfg.dnserr = err
} else {
mtime = fi.ModTime()
cfg.dnsConfig, cfg.dnserr = dnsReadConfig(resolvConfPath)
}
go func() {
for {
time.Sleep(reloadTime)
select {
case qresp := <-quit:
qresp <- struct{}{}
return
case <-cfg.ch:
}
// In case of error, we keep the previous config
fi, err := os.Stat(resolvConfPath)
if err != nil {
continue
}
// If the resolv.conf mtime didn't change, do not reload
m := fi.ModTime()
if m.Equal(mtime) {
continue
}
mtime = m
// In case of error, we keep the previous config
ncfg, err := dnsReadConfig(resolvConfPath)
if err != nil || len(ncfg.servers) == 0 {
continue
}
cfg.mu.Lock()
cfg.dnsConfig = ncfg
cfg.dnserr = nil
cfg.mu.Unlock()
}
}()
}
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
onceLoadConfig.Do(loadDefaultConfig)
select {
case cfg.ch <- struct{}{}:
default:
}
cfg.mu.RLock()
defer cfg.mu.RUnlock()
if cfg.dnserr != nil || cfg.dnsConfig == nil {
err = cfg.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 {
if rooted || count(name, '.') >= cfg.dnsConfig.ndots {
rname := name
if !rooted {
rname += "."
}
// Can try as ordinary name.
cname, addrs, err = tryOneName(cfg, rname, qtype)
cname, addrs, err = tryOneName(cfg.dnsConfig, rname, qtype)
if err == nil {
return
}
@ -192,12 +249,12 @@ func lookup(name string, qtype uint16) (cname string, addrs []dnsRR, err error)
}
// Otherwise, try suffixes.
for i := 0; i < len(cfg.search); i++ {
rname := name + "." + cfg.search[i]
for i := 0; i < len(cfg.dnsConfig.search); i++ {
rname := name + "." + cfg.dnsConfig.search[i]
if rname[len(rname)-1] != '.' {
rname += "."
}
cname, addrs, err = tryOneName(cfg, rname, qtype)
cname, addrs, err = tryOneName(cfg.dnsConfig, rname, qtype)
if err == nil {
return
}
@ -208,7 +265,7 @@ func lookup(name string, qtype uint16) (cname string, addrs []dnsRR, err error)
if !rooted {
rname += "."
}
cname, addrs, err = tryOneName(cfg, rname, qtype)
cname, addrs, err = tryOneName(cfg.dnsConfig, rname, qtype)
if err == nil {
return
}
@ -233,11 +290,6 @@ func goLookupHost(name string) (addrs []string, err error) {
if len(addrs) > 0 {
return
}
onceLoadConfig.Do(loadConfig)
if dnserr != nil || cfg == nil {
err = dnserr
return
}
ips, err := goLookupIP(name)
if err != nil {
return
@ -268,11 +320,6 @@ func goLookupIP(name string) (addrs []IP, err error) {
return
}
}
onceLoadConfig.Do(loadConfig)
if dnserr != nil || cfg == nil {
err = dnserr
return
}
var records []dnsRR
var cname string
var err4, err6 error
@ -308,11 +355,6 @@ func goLookupIP(name string) (addrs []IP, err error) {
// 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

132
src/pkg/net/dnsclient_unix_test.go
View File

@ -7,7 +7,13 @@
package net
import (
"io"
"io/ioutil"
"os"
"path"
"reflect"
"testing"
"time"
)
func TestTCPLookup(t *testing.T) {
@ -25,3 +31,129 @@ func TestTCPLookup(t *testing.T) {
t.Fatalf("exchange failed: %v", err)
}
}
type resolvConfTest struct {
*testing.T
dir string
path string
started bool
quitc chan chan struct{}
}
func newResolvConfTest(t *testing.T) *resolvConfTest {
dir, err := ioutil.TempDir("", "resolvConfTest")
if err != nil {
t.Fatalf("could not create temp dir: %v", err)
}
// Disable the default loadConfig
onceLoadConfig.Do(func() {})
r := &resolvConfTest{
T: t,
dir: dir,
path: path.Join(dir, "resolv.conf"),
quitc: make(chan chan struct{}),
}
return r
}
func (r *resolvConfTest) Start() {
loadConfig(r.path, 100*time.Millisecond, r.quitc)
r.started = true
}
func (r *resolvConfTest) SetConf(s string) {
// Make sure the file mtime will be different once we're done here,
// even on systems with coarse (1s) mtime resolution.
time.Sleep(time.Second)
f, err := os.OpenFile(r.path, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0600)
if err != nil {
r.Fatalf("failed to create temp file %s: %v", r.path, err)
}
if _, err := io.WriteString(f, s); err != nil {
f.Close()
r.Fatalf("failed to write temp file: %v", err)
}
f.Close()
if r.started {
cfg.ch <- struct{}{} // fill buffer
cfg.ch <- struct{}{} // wait for reload to begin
cfg.ch <- struct{}{} // wait for reload to complete
}
}
func (r *resolvConfTest) WantServers(want []string) {
cfg.mu.RLock()
defer cfg.mu.RUnlock()
if got := cfg.dnsConfig.servers; !reflect.DeepEqual(got, want) {
r.Fatalf("Unexpected dns server loaded, got %v want %v", got, want)
}
}
func (r *resolvConfTest) Close() {
resp := make(chan struct{})
r.quitc <- resp
<-resp
if err := os.RemoveAll(r.dir); err != nil {
r.Logf("failed to remove temp dir %s: %v", r.dir, err)
}
}
func TestReloadResolvConfFail(t *testing.T) {
if testing.Short() || !*testExternal {
t.Skip("skipping test to avoid external network")
}
r := newResolvConfTest(t)
defer r.Close()
// resolv.conf.tmp does not exist yet
r.Start()
if _, err := goLookupIP("golang.org"); err == nil {
t.Fatal("goLookupIP(missing) succeeded")
}
r.SetConf("nameserver 8.8.8.8")
if _, err := goLookupIP("golang.org"); err != nil {
t.Fatalf("goLookupIP(missing; good) failed: %v", err)
}
// Using a bad resolv.conf while we had a good
// one before should not update the config
r.SetConf("")
if _, err := goLookupIP("golang.org"); err != nil {
t.Fatalf("goLookupIP(missing; good; bad) failed: %v", err)
}
}
func TestReloadResolvConfChange(t *testing.T) {
if testing.Short() || !*testExternal {
t.Skip("skipping test to avoid external network")
}
r := newResolvConfTest(t)
defer r.Close()
r.SetConf("nameserver 8.8.8.8")
r.Start()
if _, err := goLookupIP("golang.org"); err != nil {
t.Fatalf("goLookupIP(good) failed: %v", err)
}
r.WantServers([]string{"[8.8.8.8]"})
// Using a bad resolv.conf when we had a good one
// before should not update the config
r.SetConf("")
if _, err := goLookupIP("golang.org"); err != nil {
t.Fatalf("goLookupIP(good; bad) failed: %v", err)
}
// A new good config should get picked up
r.SetConf("nameserver 8.8.4.4")
r.WantServers([]string{"[8.8.4.4]"})
}