net: allow single dot in resolv.conf search statement

A single dot in a search statement doesn't affect DNS lookup, so just ignore it and a syntax error.

Fixes #54124

Change-Id: Idd43bd34c5c16af50cba51f0b6e24f992eec6e57
Reviewed-on: https://go-review.googlesource.com/c/go/+/423875
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/net/dnsconfig_unix.go

@@ -64,9 +64,13 @@ func dnsReadConfig(filename string) *dnsConfig {
 			}
 
 		case "search": // set search path to given servers
-			conf.search = make([]string, len(f)-1)
-			for i := 0; i < len(conf.search); i++ {
-				conf.search[i] = ensureRooted(f[i+1])
+			conf.search = make([]string, 0, len(f)-1)
+			for i := 1; i < len(f); i++ {
+				name := ensureRooted(f[i])
+				if name == "." {
+					continue
+				}
+				conf.search = append(conf.search, name)
 			}
 
 		case "options": // magic options

src/net/dnsconfig_unix_test.go

@@ -52,6 +52,16 @@ var dnsReadConfigTests = []struct {
 			attempts: 2,
 		},
 	},
+	{
+		name: "testdata/search-single-dot-resolv.conf",
+		want: &dnsConfig{
+			servers:  []string{"8.8.8.8:53"},
+			search:   []string{},
+			ndots:    1,
+			timeout:  5 * time.Second,
+			attempts: 2,
+		},
+	},
 	{
 		name: "testdata/empty-resolv.conf",
 		want: &dnsConfig{
@@ -166,6 +176,9 @@ func TestDNSReadConfig(t *testing.T) {
 	getHostname = func() (string, error) { return "host.domain.local", nil }
 
 	for _, tt := range dnsReadConfigTests {
+		if len(tt.want.search) == 0 {
+			tt.want.search = append(tt.want.search, dnsDefaultSearch()...)
+		}
 		conf := dnsReadConfig(tt.name)
 		if conf.err != nil {
 			t.Fatal(conf.err)

src/net/testdata/search-single-dot-resolv.conf

@@ -0,0 +1,5 @@
+# /etc/resolv.conf
+
+domain localdomain
+search .
+nameserver 8.8.8.8