Re: [savi] Potential issue for all SAVI mechanisms?

[Jean-Michel Combes <[e-mail]>]

Hi,

I would like to come back on this point and find a consensus.
Here is the text I propose to be added in the Security Considerations
section of any SAVI solution document (i.e., FCFS, DHCP, SEND and MIX
documents) as residential threats:

<TEXT: only keep ND for FCFS/SEND SAVI and only keep UDP(DHCP) for DHCP SAVI>
o SAVI limitations
In some cases, the SAVI device could not be able to process IP packets
and so to update correctly the Binding Table. For example, this could
be the case for encrypted packets (i.e., with IPsec/ESP) or fragmented
packets. To mitigate this last case, the SAVI device SHOULD proceed as
follows:
- If the Next Header value inside the Fragment header is ND/UDP(DHCP),
the packet is processed by the SAVI device,
- If the Next Header value inside the Fragment header is not
ND/UDP(DHCP) but there is a binding, the packet is processed by the
SAVI device,
- Else the packet is dropped and the incident is logged.
</TEXT>

Comments/alternative text (deadline: 8-september-2011) are welcome!

Thanks in advance.

Best regards.

JMC.

2011/6/28 Jean-Michel Combes <[e-mail]>:
> Hi Joel,
>
> 2011/6/28 Joel M. Halpern <[e-mail]>:
>> I don't think this works.
>> The question is not how often ND packets have that structure, but whether
>> 1) Hosts will accept ND packets with that structure
>> and
>> 2) Other protocols will use that structure.
>>
>> If both of those are true, which they currently are, then a SAVI device
>> can not detect an ND packet which is fragmented and hiding behind
>> destination options.
>> But, contrary to your resolution below, since other protocols use that
>> construct, SAVI devices can not simply drop all packets that are
>> fragmented and where the only visible next header is destination options
>> (or AH, or ESP.)  they would be dropping legitimate data packets.
>
> Yep, I missed this ... So, a new proposal:
> - if the Next Header value inside the Fragment header is ND/UDP(DHCP),
> the packet is processed by the SAVI device
> - if the Next Header value inside the Fragment header is not
> ND/UDP(DHCP) and there is a binding, the packet is processed by the
> SAVI device
> - else the packet is dropped and the incident is logged
>
>>
>> It does not matter whether any legitimate sender of ND messages would
>> ever use that construct.
>>
>> As far as I can tell, the only path to sanity here is for the hosts not
>> to accept such packets.
>> And that is a matter for 6man, not for SAVI.  We should simply document
>> this as a residual threat.
>
> IMHO, this is a matter for SAVI too, because, if the SAVI device is
> not able to process fragmented ND/DHCP packets from a node to
> correctly update the Binding Table, by default, any packet from this
> node will be dropped by the SAVI device even if hosts accept such
> packets.
>
> Cheers.
>
> JMC.
>
>>
>> Yours,
>> Joel
>>
>>
>> On 6/28/2011 1:26 PM, Jean-Michel Combes wrote:
>>> Based on RFC 2460:
>>> (1) the Next Header value inside the Fragment header identifies the
>>> first header of the Fragmentable Part of the original packet
>>> (2) The Unfragmentable Part consists of the IPv6 header plus any
>>> extension headers that must be processed by nodes en route to the
>>> destination, that is, all headers up to and including the Routing
>>> header if present, else the Hop-by-Hop Options header if present, else
>>> no extension headers.
>>> (3) Extension header order is: IPv6 header, Hop-by-Hop Options header,
>>> Destination Options header, Routing header, Fragment header,
>>> Authentication header, Encapsulating Security Payload header,
>>> Destination Options header, upper-layer header
>>>
>>> So, as far as I can see, there are only 3 scenarios where the Next
>>> Header value inside the Fragment header is not ND(including SEND, as
>>> this is just ND options)/DHCP:
>>> (a) AH
>>> (b) ESP
>>> (c) Destination Option
>>>
>>> IMHO, ND/DHCP messages with such extension headers are not common ...
>>>
>>>
>>>> >
>>>> >  Then the SAVI devices can assume and require that the known ULP is
>>>> >  contained in the first fragment, thus they can determine whether a
>>>> >  packet is ND/DHCP/SEND, and reject any such packets that include a
>>>> >  fragment header.
>>> Indeed, we could assume for each SAVI mechanism that:
>>> - if the Next Header value inside the Fragment header is ND/UDP(DHCP),
>>> the packet is processed
>>> - else the packet is dropped and the incident is logged
>>>
>>> As SAVI has a site/link scope, with the logs, IMHO, it should be
>>> easier for the SAVI admin to understand/solve the issue.
>>>
>>> Best regards.
>>>
>>> JMC.
>>>
>>>
>>>
>>
>