diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index 40ed7ee41141..8a55d28e38c5 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -83,7 +83,7 @@
 extern struct mbuf *m_copypack();
 #endif
 
-static int path_mtu_discovery = 1;
+int path_mtu_discovery = 1;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, path_mtu_discovery, CTLFLAG_RW,
 	&path_mtu_discovery, 1, "Enable Path MTU Discovery");
 
diff --git a/sys/netinet/tcp_syncache.c b/sys/netinet/tcp_syncache.c
index 40defbe8b4e2..e4c467e68b99 100644
--- a/sys/netinet/tcp_syncache.c
+++ b/sys/netinet/tcp_syncache.c
@@ -1128,6 +1128,24 @@ syncache_respond(sc, m)
 
 		th = (struct tcphdr *)(ip + 1);
 	}
+
+	ip->ip_ttl = sc->sc_tp->t_inpcb->inp_ip_ttl;   /* XXX */
+	ip->ip_tos = sc->sc_tp->t_inpcb->inp_ip_tos;   /* XXX */
+
+	/*
+	 * See if we should do MTU discovery.  We do it only if the following
+	 * are true:
+	 *      1) we have a valid route to the destination
+	 *      2) the MTU is not locked (if it is, then discovery has been  
+	 *         disabled)
+	 */
+	if (path_mtu_discovery
+	    && (rt != NULL)
+	    && rt->rt_flags & RTF_UP
+	    && !(rt->rt_rmx.rmx_locks & RTV_MTU)) {
+	       ip->ip_off |= IP_DF;
+	}
+
 	th->th_sport = sc->sc_inc.inc_lport;
 	th->th_dport = sc->sc_inc.inc_fport;
 
diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index e8eb361506e9..bce5d29637d8 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -432,6 +432,7 @@ extern	struct tcpstat tcpstat;	/* tcp statistics */
 extern	int tcp_mssdflt;	/* XXX */
 extern	int tcp_delack_enabled;
 extern	int tcp_do_newreno;
+extern	int path_mtu_discovery;
 extern	int ss_fltsz;
 extern	int ss_fltsz_local;