pkg/typify: empty messages are OtherError (#1531)

Messages with nothing in them are considered OtherError, they can not
serve any purpose for normal clients (i.e. dyn update or notifies might
have a use for them).

Also update a test in the cache plugin, so that we explicitaly test for
this case.

1 files changed, 6 insertions, 0 deletions

diff --git a/plugin/pkg/response/typify.go b/plugin/pkg/response/typify.go
index 9faa17d7b..a80b6b39f 100644
--- a/plugin/pkg/response/typify.go
+++ b/plugin/pkg/response/typify.go
@@ -55,6 +55,7 @@ func Typify(m *dns.Msg, t time.Time) (Type, *dns.OPT) {
 	if m == nil {
 		return OtherError, nil
 	}
+
 	opt := m.IsEdns0()
 	do := false
 	if opt != nil {
@@ -76,6 +77,11 @@ func Typify(m *dns.Msg, t time.Time) (Type, *dns.OPT) {
 		}
 	}
 
+	if m.Response && len(m.Answer) == 0 && len(m.Ns) == 0 {
+		// Response with nothing in it, maybe stuff in the additional section, this is not useful.
+		return OtherError, opt
+	}
+
 	// If our message contains any expired sigs and we care about that, we should return expired
 	if do {
 		if expired := typifyExpired(m, t); expired {