diff --git a/examples/coap-client.c b/examples/coap-client.c
index 3ce9f7df..73835818 100644
--- a/examples/coap-client.c
+++ b/examples/coap-client.c
@@ -318,6 +318,7 @@ event_handler(coap_session_t *session COAP_UNUSED,
   case COAP_EVENT_OSCORE_DECODE_ERROR:
   case COAP_EVENT_WS_PACKET_SIZE:
   case COAP_EVENT_WS_CLOSED:
+  case COAP_EVENT_BAD_PACKET:
     quit = 1;
     break;
   case COAP_EVENT_DTLS_CONNECTED:
@@ -331,7 +332,6 @@ event_handler(coap_session_t *session COAP_UNUSED,
   case COAP_EVENT_XMIT_BLOCK_FAIL:
   case COAP_EVENT_SERVER_SESSION_NEW:
   case COAP_EVENT_SERVER_SESSION_DEL:
-  case COAP_EVENT_BAD_PACKET:
   case COAP_EVENT_MSG_RETRANSMITTED:
   case COAP_EVENT_WS_CONNECTED:
   case COAP_EVENT_KEEPALIVE_FAILURE:
@@ -357,7 +357,6 @@ nack_handler(coap_session_t *session COAP_UNUSED,
   switch (reason) {
   case COAP_NACK_TOO_MANY_RETRIES:
   case COAP_NACK_NOT_DELIVERABLE:
-  case COAP_NACK_RST:
   case COAP_NACK_TLS_FAILED:
   case COAP_NACK_WS_FAILED:
   case COAP_NACK_TLS_LAYER_FAILED:
@@ -365,8 +364,15 @@ nack_handler(coap_session_t *session COAP_UNUSED,
     coap_log_err("cannot send CoAP pdu\n");
     quit = 1;
     break;
-  case COAP_NACK_ICMP_ISSUE:
+  case COAP_NACK_RST:
+    coap_log_info("received RST pdu response\n");
+    quit = 1;
+    break;
   case COAP_NACK_BAD_RESPONSE:
+    coap_log_info("received bad response pdu\n");
+    quit = 1;
+    break;
+  case COAP_NACK_ICMP_ISSUE:
   default:
     ;
   }
diff --git a/include/coap3/coap_net_internal.h b/include/coap3/coap_net_internal.h
index 9aafb6c0..80ce3d4b 100644
--- a/include/coap3/coap_net_internal.h
+++ b/include/coap3/coap_net_internal.h
@@ -393,6 +393,16 @@ coap_pdu_t *coap_wellknown_response(coap_context_t *context,
  */
 unsigned int coap_calc_timeout(coap_session_t *session, unsigned char r);
 
+/**
+ * Check whether the pdu contains a valid code class
+ *
+ * @param session The CoAP session.
+ * @param pdu     The PDU to check.
+ *
+ * @return        @c 1 valid, @c 0 invalid.
+ */
+int coap_check_code_class(coap_session_t *session, coap_pdu_t *pdu);
+
 /**
  * Sends a CoAP message to given peer. The memory that is
  * allocated for the pdu will be released by coap_send_internal().
diff --git a/include/coap3/coap_pdu_internal.h b/include/coap3/coap_pdu_internal.h
index df2aa6d6..767036d7 100644
--- a/include/coap3/coap_pdu_internal.h
+++ b/include/coap3/coap_pdu_internal.h
@@ -100,7 +100,8 @@
 
 #define COAP_PDU_IS_EMPTY(pdu)     ((pdu)->code == 0)
 #define COAP_PDU_IS_REQUEST(pdu)   (!COAP_PDU_IS_EMPTY(pdu) && (pdu)->code < 32)
-#define COAP_PDU_IS_RESPONSE(pdu)  ((pdu)->code >= 64 && (pdu)->code < 224)
+/* Code 1.xx (32-63) and 6.xx (192-224) currently invalid */
+#define COAP_PDU_IS_RESPONSE(pdu)  ((pdu)->code >= 64 && (pdu)->code < 192)
 #define COAP_PDU_IS_SIGNALING(pdu) ((pdu)->code >= 224)
 #define COAP_PDU_IS_PING(pdu)      ((COAP_PDU_IS_EMPTY(pdu) && \
                                      ((pdu)->type == COAP_MESSAGE_CON)) || \
diff --git a/src/coap_net.c b/src/coap_net.c
index 3f636442..1bdad675 100644
--- a/src/coap_net.c
+++ b/src/coap_net.c
@@ -1076,6 +1076,34 @@ coap_client_delay_first(coap_session_t *session) {
   return 1;
 }
 
+/*
+ * return  0 Invalid
+ *         1 Valid
+ */
+int
+coap_check_code_class(coap_session_t *session, coap_pdu_t *pdu) {
+
+  /* Check validity of sending code */
+  switch (COAP_RESPONSE_CLASS(pdu->code)) {
+  case 0: /* Empty or request */
+  case 2: /* Success */
+  case 3: /* Reserved for future use */
+  case 4: /* Client error */
+  case 5: /* Server error */
+    break;
+  case 7: /* Reliable signalling */
+    if (COAP_PROTO_RELIABLE(session->proto))
+      break;
+  /* Not valid if UDP */
+  /* Fall through */
+  case 1: /* Invalid */
+  case 6: /* Invalid */
+  default:
+    return 0;
+  }
+  return 1;
+}
+
 coap_mid_t
 coap_send(coap_session_t *session, coap_pdu_t *pdu) {
   coap_mid_t mid = COAP_INVALID_MID;
@@ -1091,6 +1119,14 @@ coap_send(coap_session_t *session, coap_pdu_t *pdu) {
   assert(pdu);
 
   coap_lock_check_locked(session->context);
+
+  /* Check validity of sending code */
+  if (!coap_check_code_class(session, pdu)) {
+    coap_log_err("coap_send: Invalid PDU code (%d.%02d)\n",
+                 COAP_RESPONSE_CLASS(pdu->code),
+                 pdu->code & 0x1f);
+    goto error;
+  }
   pdu->session = session;
 #if COAP_CLIENT_SUPPORT
   if (session->type == COAP_SESSION_TYPE_CLIENT &&
@@ -1416,11 +1452,9 @@ send_it:
 #endif /* COAP_CLIENT_SUPPORT */
   return mid;
 
-#if COAP_CLIENT_SUPPORT
 error:
   coap_delete_pdu(pdu);
   return COAP_INVALID_MID;
-#endif
 }
 
 coap_mid_t
@@ -3222,6 +3256,14 @@ handle_request(coap_context_t *context, coap_session_t *session, coap_pdu_t *pdu
   coap_lock_callback(context,
                      h(resource, session, pdu, query, response));
 
+  /* Check validity of response code */
+  if (!coap_check_code_class(session, response)) {
+    coap_log_warn("handle_request: Invalid PDU response code (%d.%02d)\n",
+                  COAP_RESPONSE_CLASS(response->code),
+                  response->code & 0x1f);
+    goto drop_it_no_debug;
+  }
+
   /* Check if lg_xmit generated and update PDU code if so */
   coap_check_code_lg_xmit(session, pdu, response, resource, query);
 
@@ -3603,6 +3645,20 @@ coap_dispatch(coap_context_t *context, coap_session_t *session,
   pdu->session = session;
   coap_show_pdu(COAP_LOG_DEBUG, pdu);
 
+  /* Check validity of received code */
+  if (!coap_check_code_class(session, pdu)) {
+    coap_log_info("coap_dispatch: Received invalid PDU code (%d.%02d)\n",
+                  COAP_RESPONSE_CLASS(pdu->code),
+                  pdu->code & 0x1f);
+    packet_is_bad = 1;
+    if (pdu->type == COAP_MESSAGE_CON) {
+      coap_send_message_type(session, pdu, COAP_MESSAGE_RST);
+    }
+    /* find message id in sendqueue to stop retransmission */
+    coap_remove_from_queue(&context->sendqueue, session, pdu->mid, &sent);
+    goto cleanup;
+  }
+
   coap_option_filter_clear(&opt_filter);
 
 #if COAP_OSCORE_SUPPORT
@@ -3917,7 +3973,8 @@ coap_dispatch(coap_context_t *context, coap_session_t *session,
               }
             }
           } else {
-            coap_send_message_type(session, pdu, COAP_MESSAGE_RST);
+            if (pdu->type == COAP_MESSAGE_CON)
+              coap_send_message_type(session, pdu, COAP_MESSAGE_RST);
           }
         }
       }
diff --git a/src/coap_resource.c b/src/coap_resource.c
index 21123656..ba03d35c 100644
--- a/src/coap_resource.c
+++ b/src/coap_resource.c
@@ -1108,6 +1108,15 @@ coap_notify_observers(coap_context_t *context, coap_resource_t *r,
         coap_lock_callback(obs->session->context,
                            h(r, obs->session, obs->pdu, query, response));
 
+        /* Check validity of response code */
+        if (!coap_check_code_class(obs->session, response)) {
+          coap_log_warn("handle_request: Invalid PDU response code (%d.%02d)\n",
+                        COAP_RESPONSE_CLASS(response->code),
+                        response->code & 0x1f);
+          coap_delete_pdu(response);
+          return;
+        }
+
         /* Check if lg_xmit generated and update PDU code if so */
         coap_check_code_lg_xmit(obs->session, obs->pdu, response, r, query);
         coap_delete_string(query);