Re: Non-Last Small IPv6 Fragments

Fernando Gont <fgont@si6networks.com> Sat, 12 January 2019 04:49 UTC

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Subject: Re: Non-Last Small IPv6 Fragments
To: Tom Herbert <tom@herbertland.com>
Cc: David Farmer <farmer@umn.edu>, IPv6 List <ipv6@ietf.org>
References: <CAOSSMjV0Vazum5OKztWhAhJrjLjXc5w5YGxdzHgbzi7YVSk7rg@mail.gmail.com> <BYAPR05MB4245B9305E6EC57EDD45509FAE840@BYAPR05MB4245.namprd05.prod.outlook.com> <7453645f-ff91-e866-b087-e7d4f1450ab6@gmail.com> <0e792b48-4360-6977-9ae8-9cdfdc78c7b8@gmail.com> <16A642DC-D3A4-452C-B7D1-20CA0EEEDDA2@lists.zabbadoz.net> <CAOSSMjWS9po2XuBHJ5hbN9hfNDKZ1diecH08Kt697-15jRtAvg@mail.gmail.com> <0e0c3141-889e-ff60-2787-2889b3a9af6b@si6networks.com> <748DA428-5AB2-4487-A4FB-F2DABF5BF8BE@thehobsons.co.uk> <8b43af81-1c49-5cea-6472-97703674e661@si6networks.com> <CAN-Dau1HwG5RndacpSA+si+zKuTdpSvA=QA1A11A==rMNe=4+w@mail.gmail.com> <CALx6S35KNhV2gFp9OdU+M1zy5WUuEAEvXkDXNDWWxi7uQ4e_cw@mail.gmail.com> <CAN-Dau0rTdiiF2SjByxcMG6nhPCEjUH2pYBCOeK_FSGJ_ucDQw@mail.gmail.com> <CALx6S34AyV9OpvnjQhQc56n5vfeVgU5Zd3kheP0g+XvsMbBV9g@mail.gmail.com> <1b2e318e-1a9f-bb5d-75a5-04444c42ef20@si6networks.com> <CALx6S37TJr++fC=pVoeS=mrO1fHc4gL_Wtu-XkVTswzs2XxXCA@mail.gmail.com>
From: Fernando Gont <fgont@si6networks.com>
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Date: Sat, 12 Jan 2019 01:37:31 -0300
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On 12/1/19 00:47, Tom Herbert wrote:
> On Fri, Jan 11, 2019 at 7:32 PM Fernando Gont <fgont@si6networks.com> wrote:
>>
>> On 11/1/19 19:47, Tom Herbert wrote:
>>> On Fri, Jan 11, 2019 at 2:33 PM David Farmer <farmer@umn.edu> wrote:
>>>>
>>>>
>>>> On Fri, Jan 11, 2019 at 3:27 PM Tom Herbert <tom@herbertland.com> wrote:
>>>>>
>>>>> On Fri, Jan 11, 2019 at 1:13 PM David Farmer <farmer@umn.edu> wrote:
>>>>>>
>>>>>> On Fri, Jan 11, 2019 at 1:58 PM Fernando Gont <fgont@si6networks.com> wrote:
>>>>>>>
>>>>>>> On 11/1/19 15:38, Simon Hobson wrote:
>>>>>>>> Fernando Gont <fgont@si6networks.com> wrote:
>>>>>>>>
>> [....]
>>>>>>
>>>>>> I just had a thought for the guidance;
>>>>>>
>>>>>> 1. System-wide limit the number of fragments based on the overall system resources available.
>>>>>> 2. As you near said limit, maybe 75% of the limit, drop outright, or at a high probability, non-final fragments smaller than (1280/2) 640 bytes.
>>>>>>
>>>>>> The reasonable fragmentation algorithms I can think of do not generate non-final fragments that are less than 640 bytes. Nevertheless, such fragments SHOULD NOT be dropped except for when the system is under stress.
>>>>>>
>>>>>> What do others think?
>>>>>
>>>>> I don't see any practical purpose of sending tiny fragments with eight
>>>>> byte payloads in non-first fragments, these can only happens under DOS
>>>>> attack or synthetic testing. So I do believe it's prudent to have some
>>>>> guidance on setting limits for minimal sizes of non-first fragments.
>>>>> The limit should probably be configurable with some recommended
>>>>> default value. I believe the 1280 limit in Linux is too high, but
>>>>> there are some compelling arguments for 640.
>>>>
>>>>
>>>> Why non-first fragments? Did you mean non-final? Maybe both first and last should be exempted?
>>>>
>>> Yes.
>>
>> So why should I, as an attacker, send you first fragments, then? If for
>> some reason I just want to send you, I'd send fragments such that
>> Offset!=0 and M=0, with small size, WHat have you gained with the check
>> you propose?
> 
> That sort of attack would exhaust the limit on how many packets are
> allowed to be in reassembly at which point such packets would start to
> be dropped. It's also easy enough to prioritize packets for reassmbly
> for which the first fragment has already been received when
> considering eviction from the reassembly queue.

I agree with the above (please see the ref I posted to RFC6274, which
basically argues about this). HOowever, it is still not clear to me
what's the problem you are addressing by enforcing a minimum fragment
size. If you allow MIN_MTU/2, then you require first frags to be 640
bytes.. and teh attacker is required to send 640 as opposed to 60 bytes
or  so. Doesn't seem to be much of a difference to me.
And as noted by Bob, I still don't see what's the big deal with the
small fragments.

There are some DoS attacks where you send a last frag and possibly a
first frag, and the implementation allocates a buffer that would fit the
rest of the missing fragments -- obviously exacerbating the problem a
bit. BUt still i this cases, the size of the first frag is not a big deal.

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492