net: search domain from hostname if no search directives

Fixes #14897

Change-Id: Iffe7462983a5623a37aa0dc6f74c8c70e10c3244
Reviewed-on: https://go-review.googlesource.com/21464
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/net/dnsconfig_unix.go

@@ -8,9 +8,15 @@
 
 package net
 
-import "time"
+import (
+	"os"
+	"time"
+)
 
-var defaultNS = []string{"127.0.0.1", "::1"}
+var (
+	defaultNS   = []string{"127.0.0.1", "::1"}
+	getHostname = os.Hostname // variable for testing
+)
 
 type dnsConfig struct {
 	servers    []string  // servers to use
@@ -26,8 +32,6 @@ type dnsConfig struct {
 }
 
 // See resolv.conf(5) on a Linux machine.
-// TODO(rsc): Supposed to call uname() and chop the beginning
-// of the host name to get the default search domain.
 func dnsReadConfig(filename string) *dnsConfig {
 	conf := &dnsConfig{
 		ndots:    1,
@@ -37,6 +41,7 @@ func dnsReadConfig(filename string) *dnsConfig {
 	file, err := open(filename)
 	if err != nil {
 		conf.servers = defaultNS
+		conf.search = dnsDefaultSearch()
 		conf.err = err
 		return conf
 	}
@@ -45,6 +50,7 @@ func dnsReadConfig(filename string) *dnsConfig {
 		conf.mtime = fi.ModTime()
 	} else {
 		conf.servers = defaultNS
+		conf.search = dnsDefaultSearch()
 		conf.err = err
 		return conf
 	}
@@ -122,9 +128,24 @@ func dnsReadConfig(filename string) *dnsConfig {
 	if len(conf.servers) == 0 {
 		conf.servers = defaultNS
 	}
+	if len(conf.search) == 0 {
+		conf.search = dnsDefaultSearch()
+	}
 	return conf
 }
 
+func dnsDefaultSearch() []string {
+	hn, err := getHostname()
+	if err != nil {
+		// best effort
+		return nil
+	}
+	if i := byteIndex(hn, '.'); i >= 0 && i < len(hn)-1 {
+		return []string{hn[i+1:]}
+	}
+	return nil
+}
+
 func hasPrefix(s, prefix string) bool {
 	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
 }

src/net/dnsconfig_unix_test.go

@@ -7,6 +7,7 @@
 package net
 
 import (
+	"errors"
 	"os"
 	"reflect"
 	"testing"
@@ -56,6 +57,7 @@ var dnsReadConfigTests = []struct {
 			ndots:    1,
 			timeout:  5,
 			attempts: 2,
+			search:   []string{"domain.local"},
 		},
 	},
 	{
@@ -72,6 +74,10 @@ var dnsReadConfigTests = []struct {
 }
 
 func TestDNSReadConfig(t *testing.T) {
+	origGetHostname := getHostname
+	defer func() { getHostname = origGetHostname }()
+	getHostname = func() (string, error) { return "host.domain.local", nil }
+
 	for _, tt := range dnsReadConfigTests {
 		conf := dnsReadConfig(tt.name)
 		if conf.err != nil {
@@ -85,6 +91,10 @@ func TestDNSReadConfig(t *testing.T) {
 }
 
 func TestDNSReadMissingFile(t *testing.T) {
+	origGetHostname := getHostname
+	defer func() { getHostname = origGetHostname }()
+	getHostname = func() (string, error) { return "host.domain.local", nil }
+
 	conf := dnsReadConfig("a-nonexistent-file")
 	if !os.IsNotExist(conf.err) {
 		t.Errorf("missing resolv.conf:\ngot: %v\nwant: %v", conf.err, os.ErrNotExist)
@@ -95,8 +105,52 @@ func TestDNSReadMissingFile(t *testing.T) {
 		ndots:    1,
 		timeout:  5,
 		attempts: 2,
+		search:   []string{"domain.local"},
 	}
 	if !reflect.DeepEqual(conf, want) {
 		t.Errorf("missing resolv.conf:\ngot: %+v\nwant: %+v", conf, want)
 	}
 }
+
+var dnsDefaultSearchTests = []struct {
+	name string
+	err  error
+	want []string
+}{
+	{
+		name: "host.long.domain.local",
+		want: []string{"long.domain.local"},
+	},
+	{
+		name: "host.local",
+		want: []string{"local"},
+	},
+	{
+		name: "host",
+		want: nil,
+	},
+	{
+		name: "host.domain.local",
+		err:  errors.New("errored"),
+		want: nil,
+	},
+	{
+		// ensures we don't return []string{""}
+		// which causes duplicate lookups
+		name: "foo.",
+		want: nil,
+	},
+}
+
+func TestDNSDefaultSearch(t *testing.T) {
+	origGetHostname := getHostname
+	defer func() { getHostname = origGetHostname }()
+
+	for _, tt := range dnsDefaultSearchTests {
+		getHostname = func() (string, error) { return tt.name, tt.err }
+		got := dnsDefaultSearch()
+		if !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("dnsDefaultSearch with hostname %q and error %+v = %q, wanted %q", tt.name, tt.err, got, tt.want)
+		}
+	}
+}