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 16:00:06 +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 09:43:07AM +0100, David Laight wrote:
> > Is setting the mss to 536 actually ever sensible?
> > RFC 879 might say that it is the default (and the minimum
> > that must be supported), but in practise the actual mss
> > is very likely to be only slightly shorter than the standard
> > ethernet mss.
> > Although strict conformance with RFC 879 might require the mss
> > be clamped to 536, pragmatically a value much nearer 1400 would
> > make sense - systems with very low mtu/mss are probably likely
> > to advertise it.
> 
> Read the associated bugzilla - there was at least one real world
> example where setting a higher MSS was causing breakage.
> 
> Phil
> 
> https://bugzilla.netfilter.org/show_bug.cgi?id=662

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.

	David

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