doc/linux-syn-cookies.txt

SYN cookie analysis on 3.10

include/net/request_sock.h:

static inline int reqsk_queue_is_full(const struct request_sock_queue *queue)
{
	return queue->listen_opt->qlen >> queue->listen_opt->max_qlen_log;
}

include/net/inet_connection_sock.h:

static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk)
{
	return reqsk_queue_is_full(&inet_csk(sk)->icsk_accept_queue);
}

max_qlen_log is computed to equal log2(min(min(listen_backlog,somaxconn), sysctl_max_syn_backlog),
and this is done this way following this path :

 socket.c:listen(fd, backlog) :

   backlog = min(backlog, somaxconn)
   => af_inet.c:inet_listen(sock, backlog)

     => inet_connection_sock.c:inet_csk_listen_start(sk, backlog)

       sk_max_ack_backlog = backlog
       => request_sock.c:reqsk_queue_alloc(sk, backlog (=nr_table_entries))

         nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog); 
         nr_table_entries = max_t(u32, nr_table_entries, 8);
         nr_table_entries = roundup_pow_of_two(nr_table_entries + 1);
         for (lopt->max_qlen_log = 3;
              (1 << lopt->max_qlen_log) < nr_table_entries;
              lopt->max_qlen_log++);


tcp_ipv4.c:tcp_v4_conn_request()
   - inet_csk_reqsk_queue_is_full() returns true when the listening socket's
     qlen is larger than 1 << max_qlen_log, so basically qlen >= min(backlog,max_backlog)

   - tcp_syn_flood_action() returns true when sysctl_tcp_syncookies is set. It
     also emits a warning once per listening socket when activating the feature.

	if (inet_csk_reqsk_queue_is_full(sk) && !isn) {
		want_cookie = tcp_syn_flood_action(sk, skb, "TCP");
		if (!want_cookie)
			goto drop;
	}

   => when the socket's current backlog is >= min(backlog,max_backlog),
      either tcp_syn_cookies is set so we set want_cookie to 1, or we drop.


	/* Accept backlog is full. If we have already queued enough
	 * of warm entries in syn queue, drop request. It is better than
	 * clogging syn queue with openreqs with exponentially increasing
	 * timeout.
	 */

sock.h:sk_acceptq_is_full() = sk_ack_backlog > sk_max_ack_backlog
                            = sk_ack_backlog > min(somaxconn, listen_backlog)

	if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
		goto drop;
	}

====> the following algorithm is applied in the reverse order but with these
      priorities :

      1) IF socket's accept queue >= min(somaxconn, listen_backlog) THEN drop

      2) IF socket's SYN backlog < min(somaxconn, listen_backlog, tcp_max_syn_backlog) THEN accept

      3) IF tcp_syn_cookies THEN send_syn_cookie

      4) otherwise drop

====> the problem is the accept queue being filled, but it's supposed to be
      filled only with validated client requests (step 1).



	req = inet_reqsk_alloc(&tcp_request_sock_ops);
	if (!req)
		goto drop;

	...
		if (!sysctl_tcp_syncookies &&
			 (sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
			  (sysctl_max_syn_backlog >> 2)) &&
			 !tcp_peer_is_proven(req, dst, false)) {
			/* Without syncookies last quarter of
			 * backlog is filled with destinations,
			 * proven to be alive.
			 * It means that we continue to communicate
			 * to destinations, already remembered
			 * to the moment of synflood.
			 */
			LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("drop open request from %pI4/%u\n"),
				       &saddr, ntohs(tcp_hdr(skb)->source));
			goto drop_and_release;
		}