linux-kernel - Re: [PATCH] tcp: sysctl to disable TCP simultaneous connect

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Message-ID: <20130207103950.662698ea@nehalam.linuxnetplumber.net>
Date:	Thu, 7 Feb 2013 10:39:50 -0800
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org, Rob Landley <rob@...dley.net>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Neil Horman <nhorman@...driver.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Shan Wei <davidshan@...cent.com>, linux-doc@...r.kernel.org,
	netdev@...r.kernel.org, Willy Tarreau <w@....eu>
Subject: Re: [PATCH] tcp: sysctl to disable TCP simultaneous connect

On Thu, 7 Feb 2013 09:52:40 -0800
Kees Cook <keescook@...omium.org> wrote:

> This is based on Willy Tarreau's patch from 2008[1]. The goal is to
> close a corner-case of TCP that isn't used and poses a small DoS risk.
> For systems that do not want to take any risk at all, this is a desirable
> configuration knob.
> 
> It is possible for two clients to connect with crossed SYNs without
> checking sequence numbers. As such, it might be possible to guess a source
> port number to block a system from making connections to well-known
> ports and IP addresses (e.g. auto-update checks) without requiring a
> MiTM position.
>

This patch probably also breaks TCP STUNT that is used by some applications for NAT
traversal.
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