tcp: Change possible SYN flooding messages

"Possible SYN flooding on port xxxx " messages can fill logs on servers.

Change logic to log the message only once per listener, and add two new
SNMP counters to track :

TCPReqQFullDoCookies : number of times a SYNCOOKIE was replied to client

TCPReqQFullDrop : number of times a SYN request was dropped because
syncookies were not enabled.

Based on a prior patch from Tom Herbert, and suggestions from David.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 28 insertions, 21 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 1c12b8ec849d..c34f01513945 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -808,20 +808,38 @@ static void tcp_v4_reqsk_destructor(struct request_sock *req)
         kfree(inet_rsk(req)->opt);
 }
 
-static void syn_flood_warning(const struct sk_buff *skb)
+/*
+ * Return 1 if a syncookie should be sent
+ */
+int tcp_syn_flood_action(struct sock *sk,
+                         const struct sk_buff *skb,
+                         const char *proto)
 {
-        const char *msg;
+        const char *msg = "Dropping request";
+        int want_cookie = 0;
+        struct listen_sock *lopt;
+
+
 
 #ifdef CONFIG_SYN_COOKIES
-        if (sysctl_tcp_syncookies)
+        if (sysctl_tcp_syncookies) {
                 msg = "Sending cookies";
-        else
+                want_cookie = 1;
+                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPREQQFULLDOCOOKIES);
+        } else
 #endif
-                msg = "Dropping request";
+                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
 
-        pr_info("TCP: Possible SYN flooding on port %d. %s.\n",
-                                ntohs(tcp_hdr(skb)->dest), msg);
+        lopt = inet_csk(sk)->icsk_accept_queue.listen_opt;
+        if (!lopt->synflood_warned) {
+                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);
+        }
+        return want_cookie;
 }
+EXPORT_SYMBOL(tcp_syn_flood_action);
 
 /*
  * Save and compile IPv4 options into the request_sock if needed.
@@ -1235,11 +1253,7 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
         __be32 saddr = ip_hdr(skb)->saddr;
         __be32 daddr = ip_hdr(skb)->daddr;
         __u32 isn = TCP_SKB_CB(skb)->when;
-#ifdef CONFIG_SYN_COOKIES
         int want_cookie = 0;
-#else
-#define want_cookie 0 /* Argh, why doesn't gcc optimize this :( */
-#endif
 
         /* Never answer to SYNs send to broadcast or multicast */
         if (skb_rtable(skb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
@@ -1250,14 +1264,9 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
          * evidently real one.
          */
         if (inet_csk_reqsk_queue_is_full(sk) && !isn) {
-                if (net_ratelimit())
-                        syn_flood_warning(skb);
-#ifdef CONFIG_SYN_COOKIES
-                if (sysctl_tcp_syncookies) {
-                        want_cookie = 1;
-                } else
-#endif
-                goto drop;
+                want_cookie = tcp_syn_flood_action(sk, skb, "TCP");
+                if (!want_cookie)
+                        goto drop;
         }
 
         /* Accept backlog is full. If we have already queued enough
@@ -1303,9 +1312,7 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
                 while (l-- > 0)
                         *c++ ^= *hash_location++;
 
-#ifdef CONFIG_SYN_COOKIES
                 want_cookie = 0;        /* not our kind of cookie */
-#endif
                 tmp_ext.cookie_out_never = 0; /* false */
                 tmp_ext.cookie_plus = tmp_opt.cookie_plus;
         } else if (!tp->rx_opt.cookie_in_always) {