net: append ":53" to DNS servers when reading resolv.conf

Avoids generating some redundant garbage from re-concatenating the
same string for every DNS query.

benchmark                                      old allocs     new allocs     delta
BenchmarkGoLookupIP-32                         156            154            -1.28%
BenchmarkGoLookupIPNoSuchHost-32               456            446            -2.19%
BenchmarkGoLookupIPWithBrokenNameServer-32     577            564            -2.25%

benchmark                                      old bytes     new bytes     delta
BenchmarkGoLookupIP-32                         10873         10824         -0.45%
BenchmarkGoLookupIPNoSuchHost-32               43303         43140         -0.38%
BenchmarkGoLookupIPWithBrokenNameServer-32     46824         46616         -0.44%

Update #15473.

Change-Id: I3b0173dfedf31bd08eaea1069968b416850864a1
Reviewed-on: https://go-review.googlesource.com/22556Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient_unix.go

@@ -190,7 +190,6 @@ func tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype uint16)
 	var lastErr error
 	for i := 0; i < cfg.attempts; i++ {
 		for _, server := range cfg.servers {
-			server = JoinHostPort(server, "53")
 			msg, err := exchange(ctx, server, name, qtype)
 			if err != nil {
 				lastErr = &DNSError{

src/net/dnsclient_unix_test.go

@@ -221,7 +221,7 @@ var updateResolvConfTests = []struct {
 	{
 		name:    "golang.org",
 		lines:   []string{"nameserver 8.8.8.8"},
-		servers: []string{"8.8.8.8"},
+		servers: []string{"8.8.8.8:53"},
 	},
 	{
 		name:    "",
@@ -231,7 +231,7 @@ var updateResolvConfTests = []struct {
 	{
 		name:    "www.example.com",
 		lines:   []string{"nameserver 8.8.4.4"},
-		servers: []string{"8.8.4.4"},
+		servers: []string{"8.8.4.4:53"},
 	},
 }
 

src/net/dnsconfig_unix.go

@@ -14,12 +14,12 @@ import (
 )
 
 var (
-	defaultNS   = []string{"127.0.0.1", "::1"}
+	defaultNS   = []string{"127.0.0.1:53", "[::1]:53"}
 	getHostname = os.Hostname // variable for testing
 )
 
 type dnsConfig struct {
-	servers    []string      // servers to use
+	servers    []string      // server addresses (in host:port form) to use
 	search     []string      // suffixes to append to local name
 	ndots      int           // number of dots in name to trigger absolute lookup
 	timeout    time.Duration // wait before giving up on a query, including retries
@@ -70,9 +70,9 @@ func dnsReadConfig(filename string) *dnsConfig {
 				// just an IP address. Otherwise we need DNS
 				// to look it up.
 				if parseIPv4(f[1]) != nil {
-					conf.servers = append(conf.servers, f[1])
+					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
 				} else if ip, _ := parseIPv6(f[1], true); ip != nil {
-					conf.servers = append(conf.servers, f[1])
+					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
 				}
 			}
 

src/net/dnsconfig_unix_test.go

@@ -21,7 +21,7 @@ var dnsReadConfigTests = []struct {
 	{
 		name: "testdata/resolv.conf",
 		want: &dnsConfig{
-			servers:    []string{"8.8.8.8", "2001:4860:4860::8888", "fe80::1%lo0"},
+			servers:    []string{"8.8.8.8:53", "[2001:4860:4860::8888]:53", "[fe80::1%lo0]:53"},
 			search:     []string{"localdomain"},
 			ndots:      5,
 			timeout:    10 * time.Second,
@@ -33,7 +33,7 @@ var dnsReadConfigTests = []struct {
 	{
 		name: "testdata/domain-resolv.conf",
 		want: &dnsConfig{
-			servers:  []string{"8.8.8.8"},
+			servers:  []string{"8.8.8.8:53"},
 			search:   []string{"localdomain"},
 			ndots:    1,
 			timeout:  5 * time.Second,
@@ -43,7 +43,7 @@ var dnsReadConfigTests = []struct {
 	{
 		name: "testdata/search-resolv.conf",
 		want: &dnsConfig{
-			servers:  []string{"8.8.8.8"},
+			servers:  []string{"8.8.8.8:53"},
 			search:   []string{"test", "invalid"},
 			ndots:    1,
 			timeout:  5 * time.Second,
@@ -67,7 +67,7 @@ var dnsReadConfigTests = []struct {
 			timeout:  5 * time.Second,
 			attempts: 2,
 			lookup:   []string{"file", "bind"},
-			servers:  []string{"169.254.169.254", "10.240.0.1"},
+			servers:  []string{"169.254.169.254:53", "10.240.0.1:53"},
 			search:   []string{"c.symbolic-datum-552.internal."},
 		},
 	},