Fix: drop requests with *cough* qr set.

Err, yeah.

defaults.go

@@ -169,7 +169,7 @@ func IsMsg(buf []byte) error {
 		return errors.New("dns: bad message header")
 	}
 	// Header: Opcode
-
+	// TODO(miek): more checks here, e.g. check all header bits.
 	return nil
 }
 

server.go

@@ -77,8 +77,7 @@ func (f HandlerFunc) ServeDNS(w ResponseWriter, r *Msg) {
 	f(w, r)
 }
 
-// FailedHandler returns a HandlerFunc
-// returns SERVFAIL for every request it gets.
+// FailedHandler returns a HandlerFunc that returns SERVFAIL for every request it gets.
 func HandleFailed(w ResponseWriter, r *Msg) {
 	m := new(Msg)
 	m.SetRcode(r, RcodeServerFailure)
@@ -437,6 +436,7 @@ func (srv *Server) serve(a net.Addr, h Handler, m []byte, u *net.UDPConn, s *ses
 		}
 	}()
 Redo:
+	// Ideally we want use isMsg here before we allocate memory to actually parse the packet.
 	req := new(Msg)
 	err := req.Unpack(m)
 	if err != nil { // Send a FormatError back
@@ -445,6 +445,9 @@ Redo:
 		w.WriteMsg(x)
 		goto Exit
 	}
+	if req.Response {
+		goto Exit
+	}
 
 	w.tsigStatus = nil
 	if w.tsigSecret != nil {

server_test.go

@@ -302,6 +302,31 @@ func TestServingLargeResponses(t *testing.T) {
 	}
 }
 
+func TestServingResponse(t *testing.T) {
+	HandleFunc("miek.nl.", HelloServer)
+	s, addrstr, err := RunLocalUDPServer("127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("Unable to run test server: %s", err)
+	}
+	defer s.Shutdown()
+
+	c := new(Client)
+	m := new(Msg)
+	m.SetQuestion("miek.nl.", TypeTXT)
+	m.Response = false
+	_, _, err = c.Exchange(m, addrstr)
+	if err != nil {
+		t.Log("failed to exchange", err)
+		t.Fatal()
+	}
+	m.Response = true
+	_, _, err = c.Exchange(m, addrstr)
+	if err == nil {
+		t.Log("exchanged response message", err)
+		t.Fatal()
+	}
+}
+
 func TestShutdownTCP(t *testing.T) {
 	s, _, err := RunLocalTCPServer("127.0.0.1:0")
 	if err != nil {