netdev - Re: [PATCH net-next v3 1/3] ipv4: introduce ip_dst_mtu_secure and protect forwarding path against pmtu spoofing

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Message-Id: <20140108.010213.884915830982848289.davem@davemloft.net>
Date:	Wed, 08 Jan 2014 01:02:13 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	netdev@...r.kernel.org, eric.dumazet@...il.com,
	johnwheffner@...il.com, steffen.klassert@...unet.com,
	fweimer@...hat.com
Subject: Re: [PATCH net-next v3 1/3] ipv4: introduce ip_dst_mtu_secure and
 protect forwarding path against pmtu spoofing

From: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Wed, 8 Jan 2014 06:02:25 +0100

> Hi David!
> 
> On Tue, Jan 07, 2014 at 07:13:14PM -0500, David Miller wrote:
>> From: Hannes Frederic Sowa <hannes@...essinduktion.org>
>> Date: Mon, 6 Jan 2014 09:48:27 +0100
>> 
>> > +ip_forward_use_pmtu - BOOLEAN
>> > +	By default we don't trust protocol path MTUs while forwarding
>> > +	because they could be easily forged and can lead to unwanted
>> > +	fragmentation by the router.
>> > +	You only need to enable this if you have user-space software
>> > +	which tries to discover path mtus by itself and depends on the
>> > +	kernel honoring this information. This is normally not the
>> > +	case.
>> > +	Default: 0 (disabled)
>> > +	Possible values:
>> > +	0 - disabled
>> > +	1 - enabled
>> 
>> You made this default to off, great, but the description text still says
>> that we don't trust PMTU information by default :-)
> 
> Hm, sorry, but I don't see the contradiction.

You say "By default we don't trust protocol path MTUs", but if the
default value of ip_forward_use_pmtu is zero, we in fact do.
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