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SubjectRe: [PATCH] tcp: Restore RFC5961-compliant behavior for SYN packets
From: Eric Dumazet <>
Date: Thu, 20 Nov 2014 18:22:07 -0800

> On Thu, 2014-11-20 at 17:47 -0800, Calvin Owens wrote:
>> That's actually not what led to finding this, but it's a good point. :)
>> What if the challenge-ACK counter were decremented in tcp_validate_incoming()
>> when a valid RST packet is seen? That would allow legitimate remote
>> hosts to reestablish connections without being ratelimited, and still
>> prevent a malicious host from guessing sequence numbers.
>> There would need to be a way to tell if a challenge ACK had in fact been
>> sent and only decrement in that case, since otherwise a local attacker
>> could establish and immediately reset lots of connections to keep the
>> counter below the ratelimit threshold and guess sequence numbers.
>> Simply adding a flag to struct tcp_sock would work: just set the flag
>> whenever a challenge ACK is sent, and clear it and decrement the counter
>> only if it is set when a valid RST packet is seen.
> Seems tricky, a Challenge ACK do not necessarily gives an RST.
> Anyway this certainly can wait, as we already have a sysctl to
> eventually work around the issue.
> Acked-by: Eric Dumazet <>

Applied, thanks everyone.

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