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

[Joe Touch <[e-mail]>]

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.

>
>>>>>
>>>>> 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