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1 based on http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=75ff39ccc1bd5d3c455b6822ab09e533c551f758

2 --- linux-3.2.71/net/ipv4/tcp_input.c

3 +++ linux-3.2.71/net/ipv4/tcp_input.c

4 @@ -87,7 +87,7 @@

5 EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);

6

7 /* rfc5961 challenge ack rate limiting */

8 -int sysctl_tcp_challenge_ack_limit = 100;

9 +int sysctl_tcp_challenge_ack_limit = 1000;

10

11 int sysctl_tcp_stdurg __read_mostly;

12 int sysctl_tcp_rfc1337 __read_mostly;

13 @@ -3715,13 +3715,20 @@

14 /* unprotected vars, we dont care of overwrites */

15 static u32 challenge_timestamp;

16 static unsigned int challenge_count;

17 - u32 now = jiffies / HZ;

18 + u32 count, now = jiffies / HZ;

19 +#define READ_ONCE(a) a

20 +#define WRITE_ONCE(a,b) (a = b)

21 +#define prandom_u32_max(a) (net_random() % (a))

22

23 if (now != challenge_timestamp) {

24 + u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1;

25 challenge_timestamp = now;

26 - challenge_count = 0;

27 + WRITE_ONCE(challenge_count, half +

28 + prandom_u32_max(sysctl_tcp_challenge_ack_limit));

29 }

30 - if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {

31 + count = READ_ONCE(challenge_count);

32 + if (count > 0) {

33 + WRITE_ONCE(challenge_count, count - 1);

34 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);

35 tcp_send_ack(sk);

36 }