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Date:	Mon, 11 Aug 2014 18:32:50 -0700
From:	Eric Dumazet <>
To:	Hannes Frederic Sowa <>
Cc:, Florian Westphal <>
Subject: Re: [PATCH net] tcp: don't allow syn packets without timestamps to
 pass tcp_tw_recycle logic

On Tue, 2014-08-12 at 02:21 +0200, Hannes Frederic Sowa wrote:
> If tw_recycle is enabled, non-timestamped SYN packets could get past
> the tw_recycle check and create a new connection. This is dangerous
> as we cannot verify that segments from an old connection won't be
> accepted by the new one in tcp_validate_incoming because of the missing
> timestamps. Note that Windows seems to have timestamps disabled by
> default. Thus this broken situation could easily arise by a Linux and
> Windows box sharing one IP address and talking to a tcp_tw_recycle
> enabled server.
> We don't change the behavior regarding how many SYNs we queue up from
> non timestamping hosts (the second tcp_peer_is_proven check), because the
> second call to tcp_peer_is_proven does not use the new boolean timestamp
> argument at all because PAWS check is disabled.
> Cc: Florian Westphal <>
> Signed-off-by: Hannes Frederic Sowa <>
> ---

Not sure what you try to achieve here.

tw_recycle can only be used in very controlled environment, no NAT, and
all hosts using timestamps.

If using NAT, then tw_recycle can not be used, even if all hosts are
linux boxes with timestamps enabled.

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