tcp: implement RFC 5961 3.2

Implement the RFC 5691 mitigation against Blind
Reset attack using RST bit.

Idea is to validate incoming RST sequence,
to match RCV.NXT value, instead of previouly accepted
window : (RCV.NXT <= SEG.SEQ < RCV.NXT+RCV.WND)

If sequence is in window but not an exact match, send
a "challenge ACK", so that the other part can resend an
RST with the appropriate sequence.

Add a new sysctl, tcp_challenge_ack_limit, to limit
number of challenge ACK sent per second.

Add a new SNMP counter to count number of challenge acks sent.
(netstat -s | grep TCPChallengeACK)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Kiran Kumar Kella <kkiran@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 439984b9af49..85c5090bfe25 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -254,6 +254,7 @@ extern int sysctl_tcp_thin_linear_timeouts;
 extern int sysctl_tcp_thin_dupack;
 extern int sysctl_tcp_early_retrans;
 extern int sysctl_tcp_limit_output_bytes;
+extern int sysctl_tcp_challenge_ack_limit;
 
 extern atomic_long_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;