From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hannes Frederic Sowa Subject: Re: [PATCH net-next v4 0/3] path mtu hardening patches Date: Mon, 13 Jan 2014 22:28:08 +0100 Message-ID: <20140113212808.GJ6586@order.stressinduktion.org> References: <1389258077-23282-1-git-send-email-hannes@stressinduktion.org> <20140113.112504.922587457597727366.davem@davemloft.net> <20140113204253.GI6586@order.stressinduktion.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: David Miller , Netdev , Eric Dumazet , steffen.klassert@secunet.com, fweimer@redhat.com To: John Heffner Return-path: Received: from order.stressinduktion.org ([87.106.68.36]:33571 "EHLO order.stressinduktion.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751236AbaAMV2K (ORCPT ); Mon, 13 Jan 2014 16:28:10 -0500 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: 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 > 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