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, 5 insertions, 0 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index e20c17a7d34..e1e021594cf 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -565,6 +565,11 @@ tcp_limit_output_bytes - INTEGER
         reduce the size of individual GSO packet (64KB being the max)
         Default: 131072
 
+tcp_challenge_ack_limit - INTEGER
+        Limits number of Challenge ACK sent per second, as recommended
+        in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
+        Default: 100
+
 UDP variables:
 
 udp_mem - vector of 3 INTEGERs: min, pressure, max