Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -1024,7 +1024,7 @@
 #endif
 	if (!((tp->t_state == TCPS_ESTABLISHED && (thflags & TH_SYN) == 0) ||
 	      (tp->t_state == TCPS_LISTEN && (thflags & TH_SYN) &&
-	       !(tp->t_flags & TF_FASTOPEN)))) {
+	       !IS_FASTOPEN(tp->t_flags)))) {
 		if (ti_locked == TI_UNLOCKED) {
 			if (INP_INFO_TRY_RLOCK(&V_tcbinfo) == 0) {
 				in_pcbref(inp);
@@ -1502,7 +1502,9 @@
 	struct in_conninfo *inc;
 	struct mbuf *mfree;
 	struct tcpopt to;
+#ifdef TCP_RFC7413
 	int tfo_syn;
+#endif
 	
 #ifdef TCPDEBUG
 	/*
@@ -3020,8 +3022,12 @@
 	 * case PRU_RCVD).  If a FIN has already been received on this
 	 * connection then we just ignore the text.
 	 */
+#ifdef TCP_RFC7413
 	tfo_syn = ((tp->t_state == TCPS_SYN_RECEIVED) &&
 		   (tp->t_flags & TF_FASTOPEN));
+#else
+#define	tfo_syn	(false)
+#endif
 	if ((tlen || (thflags & TH_FIN) || tfo_syn) &&
 	    TCPS_HAVERCVDFIN(tp->t_state) == 0) {
 		tcp_seq save_start = th->th_seq;
@@ -3245,6 +3251,9 @@
 	if (tp != NULL)
 		INP_WUNLOCK(tp->t_inpcb);
 	m_freem(m);
+#ifndef TCP_RFC7413
+#undef	tfo_syn
+#endif
 }
 
 /*
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -349,6 +349,12 @@
 #define	ENTER_RECOVERY(t_flags) t_flags |= (TF_CONGRECOVERY | TF_FASTRECOVERY)
 #define	EXIT_RECOVERY(t_flags) t_flags &= ~(TF_CONGRECOVERY | TF_FASTRECOVERY)
 
+#if defined(_KERNEL) && !defined(TCP_RFC7413)
+#define	IS_FASTOPEN(t_flags)		(false)
+#else
+#define	IS_FASTOPEN(t_flags)		(t_flags & TF_FASTOPEN)
+#endif
+
 #define	BYTES_THIS_ACK(tp, th)	(th->th_ack - tp->snd_una)
 
 /*