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

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Date:	Mon, 13 Jan 2014 16:50:38 -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 4:28 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Mon, Jan 13, 2014 at 04:08:22PM -0500, John Heffner wrote:
>> 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.
>
> Actually, this is part of this series. The hardened path mtu mode provides
> exactly that (Patch 3).
>
> But because we cannot switch this on by default, I also protected the
> forwarding path. UDP path mtu discovery has been too long available on
> Linux and, I guess, a lot of applications, especially running on routers,
> depend on that.

Perhaps I misunderstood your description of FreeBSD then.  It seems
hard for me to believe that MTU discovery for UDP is broken by default
in FreeBSD.  It was not as of a couple years ago...

The nice thing about stateful firewall rules is that they give you
fine-grained policies over which ICMP messages you want to trust, and
can filter out messages that don't match "connections" with existing
state across a wide variety of protocols (including TCP, UDP and
ICMP).

  -John
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