netdev - RE: [PATCH 3/5] netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of MSS option

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Date:	Tue, 11 Jun 2013 17:09:11 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Phil Oester" <kernel@...uxace.com>
Cc:	"Pablo Neira Ayuso" <pablo@...filter.org>,
	<netfilter-devel@...r.kernel.org>, <davem@...emloft.net>,
	<netdev@...r.kernel.org>
Subject: RE: [PATCH 3/5] netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of MSS option

> On Tue, Jun 11, 2013 at 04:00:06PM +0100, David Laight wrote:
> > To quote that bug:
> >
> > I stumbled upon this problem in debian bug #541658[1] ("[iceweasel] cannot open
> > research.microsoft.com" - only worth reading for entertainment purposes) and,
> > after that bug was closed, analysed it in my blog[2] until a friend of mine
> > found out why the page loads when clamping mss to pmtu is disabled or
> > restricted to a range (like with "iptables -A FORWARD -p tcp --tcp-flags
> > SYN,RST SYN -m tcpmss --mss 1400:1536 -j TCPMSS --clamp-mss-to-pmtu") but
> > doesn't load with "simple" clamping. His really great and detailed analysation
> > of the problem may be seen at [3].
> >
> > If I read/understand that correctly, clamping to 1400 worked - there was
> > no need to clamp all the way down to 536.
> 
> You are not understanding the issue correctly.  The reason the command worked with
> "-m tcpmss --mss 1400:1536" is because that implies an MSS option was provided.
> The issue occurs only when NO MSS option is sent.  In these cases, we cannot
> ASSUME that it is ok to use some arbitrarily high value (1400 as you propose).
> The RFC is clear on this point.

My problem is that I don't want TCP connections to drop the mss to
536 when talking to minimal/old implementations that don't add any
options to SYN packets.

	David



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