linux-kernel - Re: [PATCH] Add IPv6 support to TCP SYN cookies

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Message-ID: <20080205205345.GA20920@basil.nowhere.org>
Date:	Tue, 5 Feb 2008 21:53:45 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Glenn Griffin <ggriffin.kernel@...il.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add IPv6 support to TCP SYN cookies

On Tue, Feb 05, 2008 at 11:39:11PM +0300, Evgeniy Polyakov wrote:
> On Tue, Feb 05, 2008 at 09:02:11PM +0100, Andi Kleen (andi@...stfloor.org) wrote:
> > On Tue, Feb 05, 2008 at 10:29:28AM -0800, Glenn Griffin wrote:
> > > > Syncookies are discouraged these days. They disable too many
> > > > valuable TCP features (window scaling, SACK) and even without them
> > > > the kernel is usually strong enough to defend against syn floods
> > > > and systems have much more memory than they used to be.
> > > >
> > > > So I don't think it makes much sense to add more code to it, sorry.
> 
> How does syncookies prevent windows from growing?

Syncookies do not allow window scaling so you can't have any windows >64k

> Most (if not all) distributions have them enabled and window growing
> works just fine. Actually I do not see any reason why connection
> establishment handshake should prevent any run-time operations at all,
> even if it was setup during handshake.

TCP only uses options negotiated during the hand shake and syncookies
is incapable to do this.

-Andi
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