[dns-operations] NSEC3PARAM iteration count update

Lanlan Pan abbypan at gmail.com
Mon Jan 8 06:05:43 UTC 2018


Viktor Dukhovni <ietf-dane at dukhovni.org>于2018年1月8日周一 上午4:00写道:

>
>
> > On Jan 7, 2018, at 1:55 PM, Olafur Gudmundsson <ogud at ogud.com> wrote:
> >
> > Sorry for the late response,
>
> The issue is longstanding so there's no rush, your follow-up is
> appreciated.
>
> >> Of the 453 domains with iteration counts above 150 only 4 have counts
> >> in excess of 2500, which are unsupported by many resolvers with the
> >> default RFC5155 iteration count limits.  The remaining "interesting"
> >> domains are the 449 with iterations in the interval [151,2500].
> >
> > RFC5155 advice is in hindsight bad, it was written from the point of
> > “more work is better protection”.
> > Advances in graphics cards have shown that NSEC3 is nothing but an
> > obfuscation mechanism, [...]
>
> The sparse signing (opt-out bit) feature of NSEC3 was and I think still is
> useful, despite the fact that it is sometimes misused by small "leaf"
> domains, that don't have a large number of insecure delegations.
>
> The salt value and iteration counts above 0 (i.e. 1 as you note below)
> turned out to be largely counter-productive.  It seems that Verisign,
> for example, understand this quite clearly.  The ".com" zone has an
> empty salt, 0 iterations, but uses opt-out:
>
>    CK0POJMG874LJREF7EFN8430QVIT8BSM.com. NSEC3 1 1 0 -
> CK0Q1GIN43N1ARRC9OSM6QPQR81H5M9A  NS SOA RRSIG DNSKEY NSEC3PARAM
>

"empty salt, 0 iteration"  will be close to NSEC , just a simple hash map.


>
> > The harm from NSEC3 iterations if mainly felt be resolvers, but it is
> > easy to generate an attack against authoritative servers that serve
> > zones with high iteration counts causing them to fall over.
>
> Yes, I expect the zone with an iteration count of 65535 would take
> a noticeable CPU hit at very modest query rates:
>
>    $ openssl speed sha1
>    ...
>    Doing sha1 for 3s on 64 size blocks: 10499108 sha1's in 3.01s
>    ...
>
> So, on e.g. my CPU, 65536 iterations of sha1 would take 18ms, so the
> server consumes ~1 CPU for just ~54 queries / sec!
>
>
> > I would like to see someone write an RFC saying Max iteration count of
> <= 10 for all algorithms.
>
> If the number is to be variable (and not just "0", really 1, like ".com")
> then the upper bound should be a power of 2.  So the draft could choose:
>
>    0 - Accept the reality that further hashing is futile.
>   15 - Support a modest additional work-factor.
>  127 - This covers the vast majority of deployed systems as counts above
>        100 are quite infrequent.
>
> That said, the reduced limit would for quite some time apply only to
> servers,
> resolvers would still need to support the RFC5155 limits for some time, as
> it
> will be a while before the message gets out to all the server operators and
> domain owners who'd need to make changes.
>
> >> Of these:
> >>
> >> * 258 have 512-bit P256 (algorithm 13) keys and 300 iterations.  This
> >>   exceeds the RFC5155 iteration limits and breaks secure DoE for many
> >>   resolvers.  All these domains are hosted at "ns1.desec.io”.
> > Nit: P256 keys are 256 bit long but have two 256 bit numbers in the key,
> but
> > equivalent to 3100 bit RSA key.
>
> Yes, I know.  I am not sure how resolvers determine the key length.  Is it
> from the bit count of the published key, or the algorithm bit strength?
> Either way though, the number is < 1024, so the limit of 150 is applied
> by many resolvers, in particular "unbound" (and IIRC BIND) in their default
> configurations.
>
> >> [...] So the problem described in the draft exists in the wild,
> >> but is, for the moment at least, quite infrequent.  The vast majority of
> >> domains use sensibly low counts (with 1 being the most popular value,
> though
> >> frankly 0 would have done just as well, but is perhaps not as well
> understood).
> >
> > I think NSEC3 spec is counterproductive in specifying the iteration count
> > as additional iteration so value 1 is actually two iterations.
>
> Yes, I think this has some operators unsure of the meaning, and so they
> use "1".  I think more would have chosen "0" (that is actually "1") if
> the meaning were more obvious.
>
> >> With a bit of luck, better documentation and tools that warn users to
> >> not exceed 150 (regardless of key size) will keep the problem largely
> >> in check.
> >
> > 150 is still to high IMHO,
>
> As I mentioned above, sadly resolvers still need to support the
> RFC5155 limits for some time.  You're right of course that if
> we're clarifying the limits for authoritative servers we may as
> well make the changes that make most sense in hindsight.  So
> do you think that the server limits be should be "0", "15" or
> "127"?
>
> One might even encourage the maintainers of authoritative server
> software to artificially cap the user-specified iteration count
> to the recommended value unless an additional "I really mean it"
> override is also configured.  With that, the next zone resigning
> would have an iteration count of max(configured, 5155bis limit).
>
> >> So an update to RFC5155 that sets a flat iteration limit of 127 and
> >> reserves the leading 9 bits of the iteration count would IMHO be a
> >> good idea.
> >
> > ^^^ Retiring NSEC3 is a better idea,
>
> I don't think that's practical, it is too widely deployed, and retiring
> it would be too disruptive.  I want DNSSEC adoption to grow, and not be
> disrupted by another major protocol change.
>
> NSEC3 can be simplified:
>
>   1.  Encourage users to not bother with "salt", an empty salt saves some
>       bandwidth and CPU and is just as effective.  It is too difficult to
>       change salt values anyway once a zone is signed.
>
>   2.  Encourage users to set the iteration count to 0 (really 1), cap it
>       at one of 0, 15 or 127 (based on WG/IETF consensus).
>
> > and NSEC5 is not the solution,
>
> Indeed, in my view that falls into the "disrupted by another major protocol
> change" category.  Cool crypto research, but not operationally sound.
>
> >> In any case, protocols with integral fields where only a subset of the
> >> values is supported, and the supported set depends on other parameters
> >> is a design feature that should be avoided.
> >
> > +1
>
> Thanks for the moral support.
>
> --
>         Viktor.
>
>
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-- 
致礼  Best Regards

潘蓝兰  Pan Lanlan
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