Re: Last Call: <draft-ietf-6man-rfc1981bis-04.txt> (Path MTU Discovery for IP version 6) to Internet Standard

Gorry Fairhurst <gorry@erg.abdn.ac.uk> Thu, 16 February 2017 17:13 UTC

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Subject: Re: Last Call: <draft-ietf-6man-rfc1981bis-04.txt> (Path MTU Discovery for IP version 6) to Internet Standard
References: <148599312602.18643.4886733052828400859.idtracker@ietfa.amsl.com> <2d36e28c-ee7d-20fc-3fec-54561e520691@si6networks.com> <C0A114C1-5E4A-4B8E-A408-55AF1E30873F@netapp.com> <3A5429F6-0EA6-436A-AF30-E55C9026F456@employees.org> <8cf1fe7d-bdfd-5e81-e61f-55d9ecd5d28a@isi.edu> <7E9AB9E8-3FCB-4475-BEEB-F18CFC4BC752@employees.org> <8076a1ea-182d-9cbe-f954-3e50f0fc53d9@isi.edu> <E11F9A4D-DE9E-4BFD-8D0D-252842719FC5@employees.org> <619f0dc52a514f07a70b44126aeb66f3@XCH15-06-08.nw.nos.boeing.com> <da3de0a5-fe7f-c874-db1d-da2684619213@si6networks.com> <706163b815ef439bbd9e0a17eba83512@XCH15-06-08.nw.nos.boeing.com> <e201c72e-b7c1-5a5f-eacb-93896cd7a7bb@si6networks.com> <58A33D08.4090505@erg.abdn.ac.uk> <ed4e3ab5-e93e-d9ca-28c0-43f8bf22039a@isi.edu> <2fc3beb1-86d1-2436-71e1-a90c525cb0d6@erg.abdn.ac.uk> <2eb7f087-571e-d2be-2c57-1d287f443ad7@isi.edu> <f4050249-f606-c38b-2081-0303f5fa85b9@erg.abdn.ac.uk> <68d2bffb-825a-5eee-e6c8-2c4241c98ae9@isi.edu>
To: Joe Touch <touch@isi.edu>
From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Date: Thu, 16 Feb 2017 17:13:05 +0000
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I think we agreed.

Gorry

On 16/02/2017 17:05, Joe Touch wrote:
> Hi, Gorry,
>
>
> On 2/16/2017 8:47 AM, Gorry Fairhurst wrote:
>>>
>> The point was that according to this spec (as currently written), an
>> off-path attacker can trivially inject an ICMPv6 message into the
>> traffic, which then causes a host to accept a different PathMTU.
>> Normally a transport design would expect ICMP messages to be at least
>> checked against the list of known connections, so that successfully
>> mounting this attack required the  packet to correspond to ports that
>> are in use. (Usually unknown to an off-path attacker).
>
> Agreed - but IMO this has nothing to do with "encapsulation" or
> tunneling, AFAICT.
>
Yes. This simply to do wth input processing by the PMTUD routine.

>>
>>>>>>
>>>>>> Moreover, other layers view ICMP messages with suspicion and have
>>>>>> long
>>>>>> noted the need to check ICMP payload and match only packets that
>>>>>> relate to actual 5-tuples in use (effectively reducing vulnerability
>>>>>> to off-path attacks). For example, the Guidelines for UDP,
>>>>>> rfc5405bis,
>>>>>> state:
>>>>>>
>>>>>> " Applications SHOULD appropriately validate the payload of ICMP
>>>>>>    messages to ensure these are received in response to transmitted
>>>>>>    traffic (i.e., a reported error condition that corresponds to a
>>>>>> UDP
>>>>>>    datagram actually sent by the application). …“
>>>>
>>>> The comment below could easily be handled by something that clearly
>>>> indicates the problem and points to the tunnel draft for guidance, I
>>>> agree no need to go into algorithms/methods here.
>>>
>>> The problem isn't unique to tunnels - it happens on any link whose MTU
>>> can vary, and IMO the solution is the same. React to the change in
>>> subsequent traffic, rather than attempting to rely in ICMP relaying from
>>> signaling inside the link layer -- regardless of that link layer.
>>>
>>> Joe
>>>
>> I'd be fine with recommending that way of working - but if the host
>> reacts to ICMP, it is important to try to verify ICMPv6 messages
>> before accepting them.
>
> I think that's a fine punchline. The key is what "verification" means -
> as you note, a transport connection might not want to react to
> conflicting information and no entity (transport, OS, etc.) ought to
> react to nonsensical info (attempts to push MTU below required minimums).
>
> Joe
>

Gorry