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

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Date:	Mon, 13 Jan 2014 22:28:08 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	John Heffner <johnwheffner@...il.com>
Cc:	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 04:08:22PM -0500, John Heffner wrote:
> 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.

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.

Greetings,

  Hannes

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