pkg/response: add extra test for impossible msg (#2727)

Add another test case for impossible DNS messages which should not be
cached. This adds a check for a message that denies its own existence.

Fixes #2724.
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/pkg/response/typify_test.go

@@ -42,12 +42,24 @@ func TestTypifyRRSIG(t *testing.T) {
 	}
 
 	m = delegationMsgRRSIGFail()
-	m = addOpt(m)
+	m.Extra = append(m.Extra, test.OPT(4096, true))
 	if mt, _ := Typify(m, utc); mt != OtherError {
 		t.Errorf("Message is wrongly typified, expected OtherError, got %s", mt)
 	}
 }
 
+func TestTypifyImpossible(t *testing.T) {
+	// create impossible message that denies it's own existence
+	m := new(dns.Msg)
+	m.SetQuestion("bar.www.example.org.", dns.TypeAAAA)
+	m.Rcode = dns.RcodeNameError                                                      // name does not exist
+	m.Answer = []dns.RR{test.CNAME("bar.www.example.org. IN CNAME foo.example.org.")} // but we add a cname with the name!
+	mt, _ := Typify(m, time.Now().UTC())
+	if mt != OtherError {
+		t.Errorf("Impossible message not typified as OtherError, got %s", mt)
+	}
+}
+
 func delegationMsg() *dns.Msg {
 	return &dns.Msg{
 		Ns: []dns.RR{
@@ -77,8 +89,3 @@ func delegationMsgRRSIGFail() *dns.Msg {
 	)
 	return del
 }
-
-func addOpt(m *dns.Msg) *dns.Msg {
-	m.Extra = append(m.Extra, test.OPT(4096, true))
-	return m
-}