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

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Message-ID: <51140084.3080902@zytor.com>
Date:	Thu, 07 Feb 2013 11:29:08 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <keescook@...omium.org>
CC:	Stephen Hemminger <stephen@...workplumber.org>,
	LKML <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" <linux-doc@...r.kernel.org>,
	netdev@...r.kernel.org, Willy Tarreau <w@....eu>
Subject: Re: [PATCH] tcp: sysctl to disable TCP simultaneous connect

On 02/07/2013 10:44 AM, Kees Cook wrote:
>>
>> This patch probably also breaks TCP STUNT that is used by some applications for NAT
>> traversal.
> 
> The patch would not break it -- it defaults the sysctl to staying enabled.
> 
> If you mean the documentation should be updated, sure, that's easy to do.
> 
> David: I know you aren't a fan of this patch, but I'd like to try to
> convince you. :) This leaves the feature enabled and add a toggle for
> systems (like Chrome OS) that don't want to risk this DoS at all.
> There are so very many other toggle, I don't see why this one would be
> a problem to add.
> 

It is not just STUNT, but in NAT-less configurations behind stateful
firewalls (which is expected to be the norm for IPv6), TCP rendezvous
via crossed SYN is a very effective way to establish peer-to-peer
connections.

	-hpa

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