tcp: add tcp_syncookies mode to allow unconditionally generation of syncookies

| If you want to test which effects syncookies have to your
| network connections you can set this knob to 2 to enable
| unconditionally generation of syncookies.

Original idea and first implementation by Eric Dumazet.

Cc: Florian Westphal <fw@strlen.de>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -440,6 +440,10 @@ tcp_syncookies - BOOLEAN
 	SYN flood warnings in logs not being really flooded, your server
 	is seriously misconfigured.
 
+	If you want to test which effects syncookies have to your
+	network connections you can set this knob to 2 to enable
+	unconditionally generation of syncookies.
+
 tcp_fastopen - INTEGER
 	Enable TCP Fast Open feature (draft-ietf-tcpm-fastopen) to send data
 	in the opening SYN packet. To use this feature, the client application

net/ipv4/tcp_ipv4.c

@@ -890,7 +890,7 @@ bool tcp_syn_flood_action(struct sock *sk,
 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
 
 	lopt = inet_csk(sk)->icsk_accept_queue.listen_opt;
-	if (!lopt->synflood_warned) {
+	if (!lopt->synflood_warned && sysctl_tcp_syncookies != 2) {
 		lopt->synflood_warned = 1;
 		pr_info("%s: Possible SYN flooding on port %d. %s.  Check SNMP counters.\n",
 			proto, ntohs(tcp_hdr(skb)->dest), msg);
@@ -1462,7 +1462,8 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 	 * limitations, they conserve resources and peer is
 	 * evidently real one.
 	 */
-	if (inet_csk_reqsk_queue_is_full(sk) && !isn) {
+	if ((sysctl_tcp_syncookies == 2 ||
+	     inet_csk_reqsk_queue_is_full(sk)) && !isn) {
 		want_cookie = tcp_syn_flood_action(sk, skb, "TCP");
 		if (!want_cookie)
 			goto drop;

net/ipv6/tcp_ipv6.c

@@ -963,7 +963,8 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 	if (!ipv6_unicast_destination(skb))
 		goto drop;
 
-	if (inet_csk_reqsk_queue_is_full(sk) && !isn) {
+	if ((sysctl_tcp_syncookies == 2 ||
+	     inet_csk_reqsk_queue_is_full(sk)) && !isn) {
 		want_cookie = tcp_syn_flood_action(sk, skb, "TCPv6");
 		if (!want_cookie)
 			goto drop;