net: tcp: Fix handling of keep-alive probes

Keep-alive probes have seq number set to SND.NXT - 1 and may or may not
contain an octet of data. The latter case was not handled properly
therefore add a special case when validating seq number to response to
keep-alive probes.

Signed-off-by: Robert Lubos <[email protected]>
(cherry picked from commit d93c5d6)

Author: rlubos

subsys/net/ip/tcp.c

@@ -2805,7 +2805,9 @@ static enum net_verdict tcp_in(struct tcp *conn, struct net_pkt *pkt)
 		/* send ACK for non-RST packet */
 		if (FL(&fl, &, RST)) {
 			net_stats_update_tcp_seg_rsterr(net_pkt_iface(pkt));
-		} else if ((len > 0) || FL(&fl, &, FIN)) {
+		} else if ((len > 0) || FL(&fl, &, FIN) ||
+			   /* Keep-alive probe */
+			   ((len == 0) && FL(&fl, &, ACK))) {
 			tcp_out(conn, ACK);
 		}
 		k_mutex_unlock(&conn->lock);