netdev - Re: [PATCH net-next] ipv4: introduce new IP_MTU_DISCOVER mode IP_PMTUDISC

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Message-Id: <20131029.000844.1092862708536984032.davem@davemloft.net>
Date:	Tue, 29 Oct 2013 00:08:44 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	netdev@...r.kernel.org, fweimer@...hat.com
Subject: Re: [PATCH net-next] ipv4: introduce new IP_MTU_DISCOVER mode
 IP_PMTUDISC_INTERFACE

From: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Sat, 26 Oct 2013 22:11:58 +0200

> Sockets marked with IP_PMTUDISC_INTERFACE won't do path mtu discovery,
> their sockets won't accept and install new path mtu information and they
> will always use the interface mtu for outgoing packets. It is guaranteed
> that the packet is not fragmented locally. But we won't set the DF-Flag
> on the outgoing frames.
> 
> Florian Weimer had the idea to use this flag to ensure DNS servers are
> never generating outgoing fragments. They may well be fragmented on the
> path, but the server never stores or usees path mtu values, which could
> well be forged in an attack.
> 
> (The root of the problem with path MTU discovery is that there is
> no reliable way to authenticate ICMP Fragmentation Needed But DF Set
> messages because they are sent from intermediate routers with their
> source addresses, and the IMCP payload will not always contain sufficient
> information to identify a flow.)

I do not like this reasoning.  You have several more acceptable paths to take
to resolve this problem:

1) "I don't trust path MTU information at all"

   Just turn it off globally, end of story.  It has the same effect as your
   new per-application mode.

2) "I don't trust path MTU information unless the full socket ID is available
    in the ICMP packets quoted headers"

   Then simply implement a policy as such and submit it to me.
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