[ksk-change] How to tell which trust anchors are present at a DNS resolver.

Michael StJohns msj at nthpermutation.com
Tue Mar 24 21:55:47 UTC 2015

On 3/24/2015 5:41 PM, Wessels, Duane wrote:
>> On Mar 24, 2015, at 4:03 PM, Michael StJohns <msj at nthpermutation.com> wrote:
>> One of the discussions we've been having about 5011 roll overs is that there's no way to tell whether or not they are "taking" because there's no way to check the resolvers externally.
>> I was looking at various possibilities including locally significant RRs that could be queried to , but nothing clicked.
>> After a beer with Scott Rose - we came up with the following convention:
>> Querying a server with QNAME="." and QTYPE="DS" and with no recursion, gets you a set of DS records that represent the trust anchors for that server for the root.
>> This would have to be implemented, but given that I think it may take 2 years to get the rollover done, that may not be a problem.
>> Comments on this approach?  (Note comments of "this won't work because its too late" are understood and ignored).  What we;re looking for are comments on whether the convention has bad side effects or would be difficult to implement correctly.
> I think its reasonable enough to discuss.  Negative side effects that I can think of:
> - Special case in recursive resolver code for this name/type
Yes - unclear how large scale of a  problem this might be.
> - I'd expect to see "IN/./DS" queries become used in DDoS attacks
Probably not.  Basically, the root trust anchor set is pseudo-static.  
It changes, but over long long time.  This is no worse than retrieving 
any other static data record given smart programming.

> - Some (incorrect) implementations would almost certainly forward these to the roots.
If the "recurse" flag isn't set and the resolver is forwarding queries 
then you have a different problem.

> - Doesn't work for stubs.
You mean because they don't actually answer external queries. Probably 
not a big issue.

> I'm not sure it solves the problem of being able to check resolvers externally.  You would rely on them to have an open resolver path or use some other tricks.

What I think we're looking for is to survey a broad set of known 
recursive validating resolvers to see if they're paying attention to 
5011 signalling.  Unclear we need to worry about more than a large 
subset of the existing systems.

> Perhaps worth considering alternative proposals such as some kind of CHAOS TXT query?

> Or an EDNS extension whereby a client can transmit its trust anchor/DS keytag(s) along with a DNSKEY query?  This could work for all zones, not just
> root, but I suppose it assumes the validator knows the DS before it makes the DNSKEY query (top-down vs bottom-up).

Thought about this for a few minutes and decided it was probably a 
non-starter due to the possible size of the responses.  There's also 
this problem that the resolver may be gleaning data (e.g. the non-cached 
DNSKEY) from upstream when what we really want is the current state of 
the server-local trust anchor set.


> DW

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