Fix behavior in presence of spurious non-CON requests

Previously, RSTs were sent, which was in violation of the protocol.

aiocoap/messagemanager.py

@@ -117,13 +117,15 @@ def dispatch_message(self, message):
                 if message.mtype is CON:
                     self._send_empty_ack(message.remote, message.mid, reason="acknowledging incoming response")
             else:
-                if message.remote.is_multicast_locally:
-                    self.log.info("Ignoring response incoming with multicast destination.")
-                else:
+                # A peer mustn't send a CON to multicast, but if a malicious
+                # peer does, we better not answer
+                if message.mtype == CON and not message.remote.is_multicast_locally:
                     self.log.info("Response not recognized - sending RST.")
                     rst = Message(mtype=RST, mid=message.mid, code=EMPTY, payload='')
                     rst.remote = message.remote.as_response_address()
                     self._send_initially(rst)
+                else:
+                    self.log.info("Ignoring unknown response (which is not a unicast CON)")
         else:
             self.log.warning("Received a message with code %s and type %s (those don't fit) from %s, ignoring it.", message.code, message.mtype, message.remote)
 

tests/test_noncoap_client.py

@@ -135,10 +135,20 @@ async def test_noresponse(self):
     @no_warnings
     @asynctest
     async def test_unknownresponse_reset(self):
-        self.mocksock.send(bytes.fromhex("4040ffff"))
+        self.mocksock.send(bytes.fromhex("4040ffff")) # CoAP CON 2.00 that the server has not sent a request for
         response = await wait_for(self.mocksock.recv(), timeout=1)
         self.assertEqual(response, bytes.fromhex("7000ffff"), "Unknown CON Response did not trigger RST")
 
+    @no_warnings
+    @asynctest
+    async def test_unknownresponse_noreset(self):
+        self.mocksock.send(bytes.fromhex("6040ffff")) # CoAP ACK 2.00 that the server has not sent a request for
+        try:
+            response = await wait_for(self.mocksock.recv(), timeout=1)
+            self.assertTrue(False, "Unknown ACK Response triggered something")
+        except TimeoutError:
+            pass
+
 # Skipping the whole class when no multicast address was given (as otherwise
 # it'd try binding :: which is bound to fail with a simplesocketserver setting)
 @_skip_unless_defaultmcif