netdev - Re: [PATCH net-next v4 0/3] path mtu hardening patches

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Date:	Mon, 13 Jan 2014 16:08:22 -0500
From:	John Heffner <johnwheffner@...il.com>
To:	John Heffner <johnwheffner@...il.com>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	steffen.klassert@...unet.com, fweimer@...hat.com
Subject: Re: [PATCH net-next v4 0/3] path mtu hardening patches

On Mon, Jan 13, 2014 at 3:42 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Mon, Jan 13, 2014 at 02:35:31PM -0500, John Heffner wrote:
>> My only comment would be not to look to me as the only source of
>> reason not to include this change.  I've been largely disconnected
>> from Linux development for several years and don't have time to get
>> into a protracted discussion on this topic.
>>
>> FWIW, I still have doubts as to whether this is the best approach to
>> solving the underlying problem.  I still haven't heard any reason why
>> firewall rules and other administrative best practices, such as using
>
> Because we currently cannot easily filter icmp payloads and check whether
> it is in a response for a local socket or a malicious one.
>
>> separate management and forwarding interfaces on a router, don't
>> practically solve this problem.
>
> I don't think this is practiable, especially in times of small devices
> doing routing (e.g. smartphones).
>
>> I'd also be curious to hear what
>> dedicated routing operating systems do, and why I haven't heard about
>> widespread fragmentation DoS attacks.
>
> My old Cisco didn't honour those pmtu packets (at least in default
> configuration) and FreeBSD only accepts pmtu information for TCP sockets
> where it also verifies the sequence number. It does not react to pmtu
> notifications in response to icmp or udp payloads.
>
> Routing path does use the pmtu values on FreeBSD, though. But it is much
> harder to inject path mtu packets there because, as said, they are only
> accepted for tcp.

Would it be sufficient to allow Linux to be configured in a way that
matches FreeBSD's behavior?  (I believe you can do this easily with
stateful firewall rules now, or possibly in the ICMP processing code
with a sysctl switch.)  I feel this would be a much cleaner approach.

Thanks,
  -John
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