netdev - Re: [PATCH] xt_TCPMSS: SYN packets are allowed to contain data

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Date:	Wed, 20 Jan 2010 12:59:16 -0000
From:	"Simon Arlott" <simon@...e.lp0.eu>
To:	"William Allen Simpson" <william.allen.simpson@...il.com>,
	"Patrick McHardy" <kaber@...sh.net>
Cc:	"netdev" <netdev@...r.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH] xt_TCPMSS: SYN packets are allowed to contain data

On Tue, January 19, 2010 15:44, William Allen Simpson wrote:
> Simon Arlott wrote:
>> On Tue, January 19, 2010 09:17, William Allen Simpson wrote:
>> I could change the comment too, but the same logic applies when
>> there is data and no MSS option - the packet can't be increased
>> in size if it would then exceed 576 bytes and/or the destination
>> MTU.
>>
> Please change the comment.

I've made a new version of the patch which I'll be able to test
tonight.

> If there is no MSS option, it should *not* be added, under *ANY*
> circumstances.  That violates the end-to-end arguments (some call
> them principles).

Agreed. The added MSS is likely to be larger than 536 too... I've
removed this code.

> MSS isn't about the _destination_ MTU, it's about the *source*.
> If you cannot guarantee you know the source MTU, there's no basis
> for deciding the MSS.

I was referring to the SYN packet itself. It wouldn't always be
possible to add an option without exceeding the MTU of that packet's
destination if it had data.


On 19/01/10 12:53, Patrick McHardy wrote:
> Simon Arlott wrote:
>> If this is 20 (IPv4 Header) + 20 (TCP Header) = 40 bytes, then
>> there is no data and the header offset is wrong so it hasn't been
>> checked.
>
> That's odd. If the packet is really only 40 bytes large, then there
> are no TCP options, so your patch shouldn't have any effect.

Except to remove the printk which fills up my serial console (because
the header offset is wrong).

-- 
Simon Arlott
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