diff --git a/examples/coap-client.c b/examples/coap-client.c
index 82320dea..2c4be330 100644
--- a/examples/coap-client.c
+++ b/examples/coap-client.c
@@ -360,6 +360,7 @@ 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:
   default:
     break;
   }
@@ -381,6 +382,7 @@ nack_handler(coap_session_t *session COAP_UNUSED,
     quit = 1;
     break;
   case COAP_NACK_ICMP_ISSUE:
+  case COAP_NACK_BAD_RESPONSE:
   default:
     ;
   }
diff --git a/examples/coap-server.c b/examples/coap-server.c
index 76e8941b..c13f7958 100644
--- a/examples/coap-server.c
+++ b/examples/coap-server.c
@@ -1645,6 +1645,7 @@ proxy_event_handler(coap_session_t *session,
   case COAP_EVENT_XMIT_BLOCK_FAIL:
   case COAP_EVENT_SERVER_SESSION_NEW:
   case COAP_EVENT_SERVER_SESSION_DEL:
+  case COAP_EVENT_BAD_PACKET:
   default:
     break;
   }
@@ -1788,6 +1789,7 @@ proxy_nack_handler(coap_session_t *session,
     remove_proxy_association(session, 1);
     break;
   case COAP_NACK_ICMP_ISSUE:
+  case COAP_NACK_BAD_RESPONSE:
   default:
     break;
   }
diff --git a/include/coap3/coap_event.h b/include/coap3/coap_event.h
index 2b5cb014..bda21ef5 100644
--- a/include/coap3/coap_event.h
+++ b/include/coap3/coap_event.h
@@ -91,7 +91,13 @@ typedef enum {
  * The session will still contain valid data when the event handler is
  * called.
  */
-  COAP_EVENT_SERVER_SESSION_DEL = 0x4002
+  COAP_EVENT_SERVER_SESSION_DEL = 0x4002,
+
+/*
+ * Received packet formatting errors
+ */
+  /** Triggered when badly formatted packet received */
+  COAP_EVENT_BAD_PACKET         = 0x5001
 
 } coap_event_t;
 
diff --git a/include/coap3/coap_io.h b/include/coap3/coap_io.h
index ef2e72d8..e4f3f0e0 100644
--- a/include/coap3/coap_io.h
+++ b/include/coap3/coap_io.h
@@ -71,7 +71,8 @@ typedef enum {
   COAP_NACK_NOT_DELIVERABLE,
   COAP_NACK_RST,
   COAP_NACK_TLS_FAILED,
-  COAP_NACK_ICMP_ISSUE
+  COAP_NACK_ICMP_ISSUE,
+  COAP_NACK_BAD_RESPONSE
 } coap_nack_reason_t;
 
 #endif /* COAP_IO_H_ */
diff --git a/man/coap_handler.txt.in b/man/coap_handler.txt.in
index 9f453b45..200842ff 100644
--- a/man/coap_handler.txt.in
+++ b/man/coap_handler.txt.in
@@ -144,6 +144,7 @@ COAP_NACK_NOT_DELIVERABLE
 COAP_NACK_RST
 COAP_NACK_TLS_FAILED
 COAP_NACK_ICMP_ISSUE
+COAP_NACK_BAD_RESPONSE
 ----
 
 _sent_ can be NULL.  _mid_ can be used for determing which is the transmitting
@@ -217,6 +218,10 @@ COAP_EVENT_XMIT_BLOCK_FAIL    0x3002
  */
 COAP_EVENT_SERVER_SESSION_NEW 0x4001
 COAP_EVENT_SERVER_SESSION_DEL 0x4002
+/**
+ * Unrecognizable received packet events
+ */
+COAP_EVENT_BAD_PACKET         0x5001
 ----
 
 EXAMPLES
diff --git a/src/net.c b/src/net.c
index 98859443..f3c7c3a9 100644
--- a/src/net.c
+++ b/src/net.c
@@ -2054,6 +2054,7 @@ coap_handle_dgram(coap_context_t *ctx, coap_session_t *session,
     goto error;
 
   if (!coap_pdu_parse(session->proto, msg, msg_len, pdu)) {
+    coap_handle_event(session->context, COAP_EVENT_BAD_PACKET, session);
     coap_log(LOG_WARNING, "discard malformed PDU\n");
     goto error;
   }
@@ -3163,6 +3164,7 @@ coap_dispatch(coap_context_t *context, coap_session_t *session,
   coap_pdu_t *response;
   coap_opt_filter_t opt_filter;
   int is_ping_rst;
+  int packet_is_bad = 0;
 
   if (LOG_DEBUG <= coap_get_log_level()) {
     /* FIXME: get debug to work again **
@@ -3189,8 +3191,10 @@ coap_dispatch(coap_context_t *context, coap_session_t *session,
           /* Flush out any entries on session->delayqueue */
           coap_session_connected(session);
       }
-      if (coap_option_check_critical(session, pdu, &opt_filter) == 0)
+      if (coap_option_check_critical(session, pdu, &opt_filter) == 0) {
+        packet_is_bad = 1;
         goto cleanup;
+      }
 
 #if COAP_SERVER_SUPPORT
       /* if sent code was >= 64 the message might have been a
@@ -3271,6 +3275,7 @@ coap_dispatch(coap_context_t *context, coap_session_t *session,
       coap_remove_from_queue(&context->sendqueue, session, pdu->mid, &sent);
       /* check for unknown critical options */
       if (coap_option_check_critical(session, pdu, &opt_filter) == 0) {
+        packet_is_bad = 1;
         coap_send_rst(session, pdu);
         goto cleanup;
       }
@@ -3278,7 +3283,7 @@ coap_dispatch(coap_context_t *context, coap_session_t *session,
 
     case COAP_MESSAGE_CON:        /* check for unknown critical options */
       if (coap_option_check_critical(session, pdu, &opt_filter) == 0) {
-
+        packet_is_bad = 1;
         if (COAP_PDU_IS_REQUEST(pdu)) {
           response =
             coap_new_error_response(pdu, COAP_RESPONSE_CODE(402), &opt_filter);
@@ -3322,6 +3327,8 @@ coap_dispatch(coap_context_t *context, coap_session_t *session,
       if (context->ping_handler) {
         context->ping_handler(session, pdu, pdu->mid);
       }
+    } else {
+      packet_is_bad = 1;
     }
     coap_log(LOG_DEBUG, "dropped message with invalid code (%d.%02d)\n",
              COAP_RESPONSE_CLASS(pdu->code),
@@ -3345,6 +3352,16 @@ coap_dispatch(coap_context_t *context, coap_session_t *session,
   }
 
 cleanup:
+  if (packet_is_bad) {
+    if (sent) {
+      if (context->nack_handler) {
+        context->nack_handler(session, sent->pdu, COAP_NACK_BAD_RESPONSE, sent->id);
+      }
+    }
+    else {
+      coap_handle_event(context, COAP_EVENT_BAD_PACKET, session);
+    }
+  }
   coap_delete_node(sent);
 }