Andrew,
I don't want to stretch this discussion out too much because I
think the point has been made, but a few comments below.
--On Friday, 28 November, 2008 10:58 -0500 Andrew Sullivan
<ajs@xxxxxxxxxxxx> wrote:
> On Fri, Nov 28, 2008 at 10:09:16AM -0500, John C Klensin wrote:
>
>> ones) are the most likely targets of attacks. If they are,
>> then having DNSSEC verification only to those servers, with
>> client machines trusting the nearby caching servers without
>> DNSSEC protection, provides very little protection at all.
>> Put differently, if we cannot extend DNSSEC protection and
>> verification to the desktop, DNSSEC provides very little
>> marginal security advantage.
>
> This doesn't actually follow, because there could be another
> way to validate the link between the end host and the
> validating recursive resolver. For instance, we could use
> TSIG between a non-validating stub resolver and a validating
> recursive resolver in order to ensure that attacks between
> those two points aren't successful. If I know I have the
> right node doing the validation for me, then attacks against
> the ISP's validating recursive resolver require complete
> takeover of that machine: by no means impossible, for sure,
> but a bigger deal than just spoofing answers to a stub
> resolver.
Sure. But I suspect that the number of systems that fully
support TSIG that do not support client validation are few. I'd
be happy to be proven wrong about that. One could also run the
DNS queries between stub resolver and validating recursive
resolver over a properly-validated and secured tunnel, but the
number of those isn't huge either. We could also debate what
is, and isn't difficult -- depending on network topology and
operations quality, it is often much easier and more effective
in practice to mount an attack against a server than against the
network.
> That said, I don't want to make light of the end-point
> problem, since TSIG between a stub and a recursor isn't a
> trivial problem today either. Moreover, since end nodes in
> many environments get their recursor's address(es) via DHCP,
> and since that path is pretty easy to compromise, the whole
> edifice rests on a sandy foundation.
Exactly.
> Nevertheless, I just want
> to be clear that having every end node in the world doing RFC
> 4035-and-friends validation is not the only path to useful
> DNSSEC.
I would never go so far as to say "only path to useful...".
I'm actually a big believer, in the present environment, in
LAN-local validating caching resolvers. But that is not a
popular setup, especially for the residential, SOHO, and small
business setups, that are often at the greatest risk. But,
unless one can either take advantage of special cases or harden
the servers and data paths well past current norms, I don't see
the potential of DNSSEC living up to the expectations and hype
unless one has end node (or at least end-network) validation.
>> As several people have pointed out, effective use of DNSSEC to
>> the desktop requires sufficient work on APIs and UIs that an
>> application, or the user, can distinguish between "signed and
>> validated", "signed but does not validate", and "unsigned".
>
> Why? It seems to me that acceptable definitions of "works" and
> "doesn't work" in a security-aware context could include
> "validated or insecure delegation" and "bogus delegation"
> respectively. In my opinion, any plans that involve users
> making sensible security trade-offs due to validation failures
> will get us right back where we are with self-signed or
> expired (or both) certificates for https. It seems a
> perfectly good idea to me that "bogus" means exactly the same
> thing as "site off the air".
We are in agreement about end users doing security validation
and decision-making. But, unless you can deploy DNSSEC, with
signing of all relevant zones, on a flag day basis, the end user
software needs to be able to distinguish between "address
validated with DNNSEC" and "address accepted because no
signatures are present". Otherwise, one has to treat every
address as equally untrusted and that is more or less equivalent
to DNSSEC not being present.
Whether it is appropriate to treat "failed validation" was
equivalent to "no domain" or "no server response" is a much more
subtle question, one I'm much more comfortable trying to answer
with a signed root and tree than I am with lookaside.
>...
>> the middlebox problem, with servers not under the user's
>> control making decisions about whether or not particular
>> strings are resolved or reported to the user machine as
>> non-existent. I have not been following the DNSSEC protocol
>> work closely enough to be sure, but my impression is that
>> such protocol work has not even been started, much less
>> concluded and standardized.
>
> You have exactly two options: allow the recursive server to
> make the decisions you seem to dislike -- and I think people
> who like that approach think it's a feature, not a bug -- or
> else to do validation out at the end nodes. The end node gets
> a bit to tell upstream validators that it is going to do all
> validation itself, and those upstream systems are required to
> pass along all the data necessary for such validation. So
> it's still possible to do everything at the end node.
I don't either like or dislike that recursive server model. I
just think that the quality of security/trust improvement it
provides is questionable given current operational realities
and, perhaps more important, that only a very small number of
successful attacks on such servers that people are depending on
could bring the whole DNSSEC concept into serious disrepute.
> This is quite independent of the question of whether
> applications have the ability to understand the results from
> the validator. I agree that OS APIs seem to be missing. I'm
> not sure that's something the IETF ought to be solving, but
> I'd happily entertain arguments either way.
IMO, the dividing line is precisely between doing validation at
the endpoints and doing it somewhere else. If the answer is
"endpoints", then it is perfectly sensible and consistent with
IETF history to say "local problem". On the other hand, if
validation is at the caching resolver, then it seems to me that
the model for communicating between the stub resolver and that
system is precisely an IETF problem (again, if only to be sure
that the application can tell the difference between "validated"
and "unsigned".
>> several of them, do we need search rules for look-aside
>> databases
>
> My personal reading of the current specifications is that, if
> you have at least one path to validation, then validation is
> supposed to work. So search rules ought not to be needed.
> What the implementations actually do is currently at variance
> with my interpretation, however.
Again, I'm much more concerned about the current operational
practice and how it is evolving than I am about the theory in
the specs. I know that, in any situation like this, "single
authoritative tree" is a lot easier, and a lot harder for either
a bad guy or carelessness to diddle in subtle ways without being
caught and held accountable, than having multiple arrangements.
And it is quite clear that we don't have that tree today.
To paraphrase Bill, if there are two possible validation paths,
using two different sets of lists on different servers, there is
the possibility of different answers on different paths. And,
precisely because this mechanism is supposed to provide security
and trust validation, playing "see no evil, hear no evil,
anticipate no evil" with that particular risk is wildly
inappropriate.
I'm in favor of getting things signed just as quickly as that is
feasible -- either from the root down or using look-aside
mechanisms that are, themselves, fully validated and with good
tools for dealing with potential conflicts. But my reading of
Russ's proposed experiment had to do with demonstrating that
DNSSEC is actually useful in dealing with threats and plausible
attack scenarios, not just demonstrating that one can safely
sign zones and deploy validating software in some places on the
network. For that demonstration of effectiveness, we are not,
IMO, quite there yet and it is frightening that we are only
having the discussion now (from that point of view, the proposed
experiment has already succeeded because we [finally] are having
the discussion).
john
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