From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jiri Bohac Subject: Re: Quick Blind TCP Connection Spoofing with SYN Cookies Date: Fri, 16 Aug 2013 01:57:43 +0200 Message-ID: <20130815235743.GA25665@midget.suse.cz> References: <520A3B4A.1050704@jakoblell.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev@vger.kernel.org, davem@davemloft.net To: Jakob Lell Return-path: Received: from cantor2.suse.de ([195.135.220.15]:46139 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752540Ab3HOX5s (ORCPT ); Thu, 15 Aug 2013 19:57:48 -0400 Content-Disposition: inline In-Reply-To: <520A3B4A.1050704@jakoblell.com> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, Aug 13, 2013 at 03:57:30PM +0200, Jakob Lell wrote: > VIII. Possible mitigation options > > The simplification of TCP Connection Spoofing described here is an > inherent problem of TCP SYN Cookies and so there won't be a simple > patch which just solves the issue and makes the Spoofing Attack as > hard as it is without SYN Cookies. It is only possible to gradually > increase the required effort for successfully spoofing a connection > e.g. by only accepting the last two instead of four counter values > (which will lead to a 60-120s If the counter is slowed down 4 times, accepting only two values should result in similar behaviour as we have today. Can anyone think of a reason this should not be done? Additionally, I believe we should reduce the number of possible MSS values. I think 3 values should be enough - not supporting jumbo frames and wasting a few bytes on sub-optimal MSS around 1400 bytes should be acceptable when a system is under a DoS attack. I have 3 patches doing just that: 1 - slow down the timer and improve the situation by a factor of 2 2 - since not everyone will be happy with exactly 3 MSS values, let's make this configurable via sysctl 3 - decrease the default number of MSS values to 3 and improve the situation by a factor of 8/3 These patches combined make the attack 5.3 times harder. By using the sysctl to only set two possible MSSs, one can make the attack 8 times harder - and only 4 times easier than previously believed. The patches are compile-tested only. Thoughts? -- Jiri Bohac SUSE Labs, SUSE CZ