net: case insensitivity of DNS labels in built-in stub resolver

This change adds support for case insensitivity of DNS labels to
built-in DNS stub resolver as described in RFC 4343.

Fixes #9215.

Change-Id: Ia752fe71866a3bfa3ea08371985b799d419ddea3
Reviewed-on: https://go-review.googlesource.com/3685Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/dnsclient.go

@@ -94,7 +94,7 @@ Cname:
 				continue
 			}
 			h := rr.Header()
-			if h.Class == dnsClassINET && h.Name == name {
+			if h.Class == dnsClassINET && equalASCIILabel(h.Name, name) {
 				switch h.Rrtype {
 				case qtype:
 					addrs = append(addrs, rr)
@@ -114,6 +114,26 @@ Cname:
 	return "", nil, &DNSError{Err: "too many redirects", Name: name, Server: server}
 }
 
+func equalASCIILabel(x, y string) bool {
+	if len(x) != len(y) {
+		return false
+	}
+	for i := 0; i < len(x); i++ {
+		a := x[i]
+		b := y[i]
+		if 'A' <= a && a <= 'Z' {
+			a += 0x20
+		}
+		if 'A' <= b && b <= 'Z' {
+			b += 0x20
+		}
+		if a != b {
+			return false
+		}
+	}
+	return true
+}
+
 func isDomainName(s string) bool {
 	// See RFC 1035, RFC 3696.
 	if len(s) == 0 {

src/net/dnsmsg_test.go

@@ -18,7 +18,7 @@ func TestDNSParseSRVReply(t *testing.T) {
 	msg := new(dnsMsg)
 	ok := msg.Unpack(data)
 	if !ok {
-		t.Fatalf("unpacking packet failed")
+		t.Fatal("unpacking packet failed")
 	}
 	msg.String() // exercise this code path
 	if g, e := len(msg.answer), 5; g != e {
@@ -32,23 +32,29 @@ func TestDNSParseSRVReply(t *testing.T) {
 			t.Errorf("answer[%d] = %T; want *dnsRR_SRV", idx, rr)
 		}
 	}
-	_, addrs, err := answer("_xmpp-server._tcp.google.com.", "foo:53", msg, uint16(dnsTypeSRV))
+	for _, name := range [...]string{
+		"_xmpp-server._tcp.google.com.",
+		"_XMPP-Server._TCP.Google.COM.",
+		"_XMPP-SERVER._TCP.GOOGLE.COM.",
+	} {
+		_, addrs, err := answer(name, "foo:53", msg, uint16(dnsTypeSRV))
 		if err != nil {
-		t.Fatalf("answer: %v", err)
+			t.Error(err)
 		}
 		if g, e := len(addrs), 5; g != e {
 			t.Errorf("len(addrs) = %d; want %d", g, e)
 			t.Logf("addrs = %#v", addrs)
 		}
+	}
 	// repack and unpack.
 	data2, ok := msg.Pack()
 	msg2 := new(dnsMsg)
 	msg2.Unpack(data2)
 	switch {
 	case !ok:
-		t.Errorf("failed to repack message")
+		t.Error("failed to repack message")
 	case !reflect.DeepEqual(msg, msg2):
-		t.Errorf("repacked message differs from original")
+		t.Error("repacked message differs from original")
 	}
 }