Index: sys/netinet/tcp_output.c
===================================================================
--- sys/netinet/tcp_output.c
+++ sys/netinet/tcp_output.c
@@ -286,6 +286,14 @@
 	sendwin = min(tp->snd_wnd, tp->snd_cwnd);
 
 	flags = tcp_outflags[tp->t_state];
+	if (__predict_false(flags & TH_FIN) && !(tp->t_flags & TF_SENTFIN)) {
+		/* clear FIN if there is still unacknowledged data */
+		if (SEQ_LT(tp->snd_una, tp->snd_max))
+			flags &= ~TH_FIN;
+		else
+			tp->t_flags |= TF_ACKNOW;
+	    }
+	}
 	/*
 	 * Send any SACK-generated retransmissions.  If we're explicitly trying
 	 * to send out new data (when sendalot is 1), bypass this function.