Improvements to DNS Resolvers for Resiliency, Robustness, and Responsiveness

Versions: 00                                                            
DNSEXT Working Group                                       P. Vixie, ISC
INTERNET-DRAFT                                      R. Joffe, Centergate
<draft-vixie-dnsext-resimprove-00.txt>                F. Neves, Registro
Intended Status: For Your Information                      June 22, 2010

                     Improvements to DNS Resolvers
             for Resiliency, Robustness, and Responsiveness

Status of this Memo
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   This document describes several mechanisms which can be employed by
   iterative caching DNS resolvers to improve resiliency, robustness,
   and responsiveness.  These improvements are optional and they require
   no changes to the protocol, or to authority servers, or to DNS stub
   resolver clients.

1. Introduction

   1.1. Iterative caching DNS resolvers can be both compliant and
   interoperable without also being optimal. Indeed a wide range of
   optimizations, mechanisms, and outright "tricks" can be employed
   without affecting correctness or interopability. Such optimizations
   could be recommended but never required.

   1.2. This document describes several practices which can improve the
   resilience, robustness, and responsiveness of iterative caching DNS
   resolvers.  These practices are not required for correctness or
   interoperability, and no other DNS protocol agent need be modified to
   gain the prospective benefits of implementing these practices.

   1.3. Three practices are described here:

      A. Revalidating a delegation when a parent NS RRset TTL expires.
      B. Stopping a downward cache search when an NXDOMAIN is encountered.
      C. Upgrading the credibility of NS RRsets upon delegation events.

   These practices are described in detail in later sections of this
   document.  While they are described together this single document for
   editorial convenience, and are known to work well together, they are
   in no way interdependent.

2. Delegation Revalidation Upon NS RRSet Expiry

   2.1. Because the delegating NS RRset at the bottom of the parent zone
   and the apex NS RRset in the child zone are unsynchronized, the TTL
   of the parent's delegating NS RRset is meaningless. A child zone's
   apex NS RRset is authoritative and thus has a higher cache
   credibility than the parent's delegating NS RRset, so, the NS RRset
   "below the cut" immediately replaces the parent's delegating NS RRset
   in cache when an iterative caching DNS resolver crosses a zone cut.

   2.2. The lowest TTL found in a parent zone's delegating NS RRset
   should be stored in the cache and used to trigger delegation

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   revalidation as follows.  Whenever a cached RRset is being considered
   for use in a response, the cache should be walked upward toward the
   root, looking for expired delegations. At the first expired
   delegation encountered while walking upward toward the root,
   revalidation should be triggered, putting the processing of dependent
   queries on hold until validation is complete.

   2.3. To revalidate a delegation, the iterative caching DNS resolver
   will forward the query that triggered revalidation to the nameservers
   at the closest enclosing zone cut above the revalidation point. While
   searching for these nameservers, additional revalidations may occur,
   perhaps placing an entire chain of dependent queries on hold,
   unwinding in downward order as revalidations closer to the root must
   be complete before revalidations further from the root can begin.

   2.4. If a delegation can be revalidated at the same node, then the
   old apex NS RRset should be deleted from cache and then the new
   delegating NS RRset should be stored in cache. The minimum TTL from
   the new delegating NS RRset should also be stored in cache to
   facilitate future revalidations. This order of operations ensures
   that the RRset credibility rules do not prevent the new delegating NS
   RRset from entering the cache. It is expected that the child's apex
   NS RRset will rapidly replace the parent's delegating NS RRset as
   soon as iteration restarts after the revalidation event.

   2.5. If the new delegating NS RRset cannot be found (RCODE=NXDOMAIN)
   or if there is a new zone cut at some different level of the
   hierarchy (insertion or deletion of a delegation point above the
   revalidation point) or if the new RRset shares no nameserver names in
   common with the old one (indicating some kind of redelegation, which
   is rare) then the cache should be purged of all names and RRsets at
   or below the revalidation point. This facilitates redelegation or
   revocation of a zone by a parent zone administrator, and also
   conserves cache storage by deleting unreachable data.

   2.6. To make the timing of a revalidation event unpredictable from
   the point of view of a potential cache-spoof attacker, the parent's
   delegating NS RRset TTL should be reduced by a random fraction of its
   value before being stored for use in revalidation activities.

3. Stopping Downward Cache Search on NXDOMAIN

   3.1. In virtually all existing resolvers, a cached NXDOMAIN is not
   considered "proof" that there can be no child domains underneath.
   This is due to an ambiguity in RFC 1034 that failed to distinguish

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   empty nonterminal domain names from nonexistent names.  For DNSSEC,
   the IETF had to distinguish this case, but the implication on non-
   DNSSEC resolvers wasn't fully realized.

   3.2. When searching downward in its cache, an iterative caching DNS
   resolver should stop searching if it encounters a cached NXDOMAIN.
   The response to the triggering query should be NXDOMAIN.

   3.3. When an iterative caching DNS resolver stores an NXDOMAIN in its
   cache, all names and RRsets at or below that node should be deleted
   since they will have become unreachable.

   3.4. By implication, a stream of queries FOO.TLD, BAR.FOO.TLD where
   FOO.TLD does not exist would normally cause both queries to be
   forwarded to TLD's nameservers. Following this recommended practice,
   the second query and indeed any other query for names at or below
   FOO.TLD would not be forwarded.

4. Upgrading NS RRset Credibility Upon Delegaton Events

   4.1. Noting that a parent's delegating NS RRset is nonauthoritative
   "glue" whereas a child's apex NS RRset is authoritative, the latter
   will replace the former in cache whenever the latter is encountered.
   However, it is not mandatory for the child zone's nameservers to
   include the apex NS RRset in responses, thus it is possible for an
   iterative caching DNS resolver to never learn the authoritative NS
   RRset for a zone.

   4.2. When a delegation response is received during iteration, a
   validation query should be sent in parallel with the forwarding of
   the triggering query to the delegated nameservers for the newly
   discovered zone cut. The response to the triggering query should be
   delayed until both the forwarded query and the validation query have
   been answered.

   4.3. A validation query consists of a query for the child's apex NS
   RRset, sent to the newly discovered delegation's nameservers. Normal
   iterative logic applies to the processing of responses to validation
   queries, including storing the results in cache, propagating NXDOMAIN
   back to the triggering query, trying the next server on SERVFAIL or
   timeout, and so on.

   4.4. If there are no nameserver names in common between the child's
   apex NS RRset and the parent's delegation NS RRset, then the
   responses received from forwarding the triggering query to the

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   parent's delegated nameservers should be discarded after validation,
   and this query should be forwarded again to the child's apex

5. Security Considerations

   5.1. A successful cache exploit which inserted a fake NXDOMAIN can
   deny more service from an iterative caching DNS resolver that
   implements the recommendation to stop downward searches when a cached
   NXDOMAIN is encountered.

   5.2. A perfectly timed cache exploit which inserted a fake NS RRset
   during a delegation validation event could cause one fewer good
   response to be heard (per TTL expiry interval) from an iterative
   caching DNS resolver that implements the recommendation to validate

IANA Considerations


Normative References

[RFC1035]  P. Mockapetris, "Domain Names - Implementation and
           Specification," RFC 1035, USC/Information Sciences Institute,
           November 1987.

Authors' Addresses

Paul Vixie

     Internet Systems Consortium
     950 Charter Street
     Redwood City, CA, USA

Rodney Joffe

     Centergate Research Group, LLC
     420 S Smith Rd
     Tempe, AZ 85281 USA

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Frederico A. C. Neves /
     Av. das Nacoes Unidas, 11541, 7
     Sao Paulo, SP  04578-000  BR

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