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Date:	Thu, 20 Nov 2014 18:22:07 -0800
From:	Eric Dumazet <>
To:	Calvin Owens <>
Cc:	"David S. Miller" <>,
	Alexey Kuznetsov <>,
	James Morris <>,
	Eric Dumazet <>,,,
Subject: Re: [PATCH] tcp: Restore RFC5961-compliant behavior for SYN packets

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 <>

Thanks !

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