Index: sys/netgraph/ng_source.c
===================================================================
--- sys/netgraph/ng_source.c
+++ sys/netgraph/ng_source.c
@@ -567,8 +567,11 @@
 	}
 	KASSERT(hook == sc->input, ("%s: no hook!", __func__));
 
-	/* Enqueue packet. */
-	/* XXX should we check IF_QFULL() ? */
+	/* Enqueue packet if the queue isn't full. */
+	if (_IF_QFULL(&sc->snd_queue)) {
+		NG_FREE_M(m);
+		return (error);
+	}
 	_IF_ENQUEUE(&sc->snd_queue, m);
 	sc->queueOctets += m->m_pkthdr.len;
 	sc->last_packet = m;