Index: sys/netinet/in.c
===================================================================
--- sys/netinet/in.c
+++ sys/netinet/in.c
@@ -87,6 +87,12 @@
 	&VNET_NAME(nosameprefix), 0,
 	"Refuse to create same prefixes on different interfaces");
 
+VNET_DEFINE_STATIC(bool, broadcast_zero);
+#define	V_broadcast_zero		VNET(broadcast_zero)
+SYSCTL_BOOL(_net_inet_ip, OID_AUTO, broadcast_zero, CTLFLAG_VNET | CTLFLAG_RW,
+	&VNET_NAME(broadcast_zero), 0,
+	"Treat host address 0 on a subnet as broadcast");
+
 VNET_DECLARE(struct inpcbinfo, ripcbinfo);
 #define	V_ripcbinfo			VNET(ripcbinfo)
 
@@ -1135,10 +1141,10 @@
 
 	return ((in.s_addr == ia->ia_broadaddr.sin_addr.s_addr ||
 	     /*
-	      * Check for old-style (host 0) broadcast, but
+	      * Optionally check for old-style (host 0) broadcast, but
 	      * taking into account that RFC 3021 obsoletes it.
 	      */
-	    (ia->ia_subnetmask != IN_RFC3021_MASK &&
+	    (V_broadcast_zero && ia->ia_subnetmask != IN_RFC3021_MASK &&
 	    ntohl(in.s_addr) == ia->ia_subnet)) &&
 	     /*
 	      * Check for an all one subnetmask. These