net: accept "." as a valid domain name

Fixes #45715

Change-Id: Ibdaa91c97d34473061b377325ebe9a3bf5696c8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/360314
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/dnsclient.go

@@ -76,6 +76,11 @@ func equalASCIIName(x, y dnsmessage.Name) bool {
 // (currently restricted to hostname-compatible "preferred name" LDH labels and
 // SRV-like "underscore labels"; see golang.org/issue/12421).
 func isDomainName(s string) bool {
+	// The root domain name is valid. See golang.org/issue/45715.
+	if s == "." {
+		return true
+	}
+
 	// See RFC 1035, RFC 3696.
 	// Presentation format has dots before every label except the first, and the
 	// terminal empty label is optional here because we assume fully-qualified

src/net/dnsclient_unix_test.go

@@ -2120,3 +2120,44 @@ func TestNullMX(t *testing.T) {
 		t.Errorf("records = [%v]; want [%v]", strings.Join(records, " "), want[0])
 	}
 }
+
+func TestRootNS(t *testing.T) {
+	// See https://golang.org/issue/45715.
+	fake := fakeDNSServer{
+		rh: func(n, _ string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {
+			r := dnsmessage.Message{
+				Header: dnsmessage.Header{
+					ID:       q.Header.ID,
+					Response: true,
+					RCode:    dnsmessage.RCodeSuccess,
+				},
+				Questions: q.Questions,
+				Answers: []dnsmessage.Resource{
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:  q.Questions[0].Name,
+							Type:  dnsmessage.TypeNS,
+							Class: dnsmessage.ClassINET,
+						},
+						Body: &dnsmessage.NSResource{
+							NS: dnsmessage.MustNewName("i.root-servers.net."),
+						},
+					},
+				},
+			}
+			return r, nil
+		},
+	}
+	r := Resolver{PreferGo: true, Dial: fake.DialContext}
+	rrset, err := r.LookupNS(context.Background(), ".")
+	if err != nil {
+		t.Fatalf("LookupNS: %v", err)
+	}
+	if want := []*NS{&NS{Host: "i.root-servers.net."}}; !reflect.DeepEqual(rrset, want) {
+		records := []string{}
+		for _, rr := range rrset {
+			records = append(records, fmt.Sprintf("%v", rr))
+		}
+		t.Errorf("records = [%v]; want [%v]", strings.Join(records, " "), want[0])
+	}
+}

src/net/lookup.go

@@ -558,9 +558,7 @@ func (r *Resolver) LookupMX(ctx context.Context, name string) ([]*MX, error) {
 		if mx == nil {
 			continue
 		}
-		// Bypass the hostname validity check for targets which contain only a dot,
-		// as this is used to represent a 'Null' MX record.
-		if mx.Host != "." && !isDomainName(mx.Host) {
+		if !isDomainName(mx.Host) {
 			continue
 		}
 		filteredMX = append(filteredMX, mx)