Index: head/sys/netinet6/ip6_output.c
===================================================================
--- head/sys/netinet6/ip6_output.c
+++ head/sys/netinet6/ip6_output.c
@@ -1393,6 +1393,15 @@
 	int retval;
 #endif
 
+/*
+ * Don't use more than a quarter of mbuf clusters.  N.B.:
+ * nmbclusters is an int, but nmbclusters * MCLBYTES may overflow
+ * on LP64 architectures, so cast to u_long to avoid undefined
+ * behavior.  ILP32 architectures cannot have nmbclusters
+ * large enough to overflow for other reasons.
+ */
+#define IPV6_PKTOPTIONS_MBUF_LIMIT	((u_long)nmbclusters * MCLBYTES / 4)
+
 	level = sopt->sopt_level;
 	op = sopt->sopt_dir;
 	optname = sopt->sopt_name;
@@ -1448,6 +1457,12 @@
 			{
 				struct mbuf *m;
 
+				if (optlen > IPV6_PKTOPTIONS_MBUF_LIMIT) {
+					printf("ip6_ctloutput: mbuf limit hit\n");
+					error = ENOBUFS;
+					break;
+				}
+
 				error = soopt_getm(sopt, &m); /* XXX */
 				if (error != 0)
 					break;