Network Working Group                                            P. Koch
Internet-Draft                                                  DENIC eG
Intended status: Best Current Practice                         M. Larson
Expires: August 18, 2014 September 10, 2015                                    Dyn, Inc.
                                                       February 14, 2014
                                                           March 9, 2015

            Initializing a DNS Resolver with Priming Queries


   This document describes the initial queries a DNS resolver is
   supposed to emit in order to initialize its cache with both a current
   NS RRSet for the root zone as well as and the necessary address information.

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   This Internet-Draft will expire on August 18, 2014. September 10, 2015.

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1.  Introduction

   Domain Name System (DNS) resolvers need a starting point to resolve
   queries.  [RFC1034], section 5.3.2, defines the SBELT structure in a
   full resolver as:


      a "safety belt" structure of the same form as SLIST, which is
      initialized from a configuration file, and lists servers which
      should be used when the resolver doesn't have any local
      information to guide name server selection.  The match count will
      be -1 to indicate that no labels are known to match.'' match.

   Section 5.3.3 of [RFC1034] adds


      the usual choice is two of the root servers and two of the servers
      for the host's domain'' domain

   Today's practice generally seperates serving and resolving
   functionalities, so the servers ``for "for the host's domain'' domain" might no
   longer be an appropriate choice, even if they were only intended to
   resolve ``local'' "local" names, especially since the SBELT structure does not
   distinguish between local and global information.  In addition, DNS
   server implementations have for a long time been seeded with not only
   two but an exhaustive list of the root servers' addresses.  This list
   is either supplied as a configuration file (root "hints", an excerpt
   of the DNS root zone) or even compiled into the software.

   The list of root name servers has been rather stable over the last
   fifteen years.  After the last four servers had been added and moved
   to their final (network) destinations in 1997, there have been only
   five address changes affecting the L (twice), J, B, and D servers.
   Research is available for B [Mann2006] and J [BLKT2004], which shows
   that several months or even years after the change had become
   effective, traffic is still received on the old addresses.
   Therefore, it is important that resolvers be able to cope with
   change, even without relying upon configuration updates to be applied
   by their operator.

   Work by the ICANN SSAC and RSSAC committees, [SSAC016] and [SSAC017],
   aiming at adding AAAA RRs for the root name servers' names, deals
   with priming queries and so does a draft on DNSSEC Trust Anchor
   maintenance [I-D.ietf-dnsop-dnssec-trust-anchor].  However, it turned
   out that despite having been practiced for a long time, priming
   queries have not yet been documented as an important resolver

   The following sections cover parameters of both the priming query and
   the response to be sent by a root name server.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Priming Queries

   This document only deals with recursive name servers (recursive
   resolvers, resolvers) for the IN CLASS.

2.1.  Parameters of a Priming Query

   A priming query SHOULD use a QNAME of "." and a QTYPE of NS.  The
   priming query MUST be sent over UDP (section of [RFC1123]).
   The UDP source port SHOULD be randomly selected [RFC5452].  The RD
   bit MUST NOT be set.

   The resolver SHOULD also use EDNS0 [RFC6891] and SHOULD announce and
   handle a reassembly size of at least 1024 octets [RFC3226].  This is
   to cover the size of a full priming response (see Section 3.3).

2.2.  Repeating Priming Queries

   A resolver SHOULD NOT originate a priming query more often than once
   per day (or whenever the resolver starts).  It SHOULD adhere to the
   TTL values given in the priming response.  To avoid amnesia, the
   resolver MAY proactively re-prime before the old root NS RRSet
   expires from the cache, but only after 75 percent of the NS RRSet's
   TTL (or of the A/AAAA RRSets' TTL, whichever is lower) have passed.

   Should the priming query time out, the resolver SHOULD retry with a
   different target address.

2.3.  Target Selection

   A resolver MUST select the target for a priming query randomly from
   the list of addresses (IPv4 and IPv6) available in its SBELT
   structure and it MUST ensure that all targets are selected with equal
   probability even upon startup.  For resending the priming query to a
   different server the random selection SHOULD also be used.

2.4.  DNSSEC with Priming Queries

   The resolver SHOULD NOT set the DNSSEC OK [RFC4033] bit.

   Discussion: Delegations in referral responses are not signed, so
   consequently the priming response is not validated, either.  For that
   to work, the priming response would also have to be self-contained in
   that it would allow the resolver to not only validate the NS RRSet
   (with the root DNSKEY RRSet and the root NS RRSet's signature), but
   also the A and AAAA RRSets.  All this information cannot be
   guaranteed to be either present at the root name servers or fit into
   the priming reponse even with the largest feasible EDNS0 buffer size.
   In fact, in today's Internet, with the root name servers' names under
   "ROOT-SERVERS.NET.", this isn't even true for the top level domain
   involved.  So, even though a poisoned priming response could
   drastically influence the resolver's operations, there is little a
   DNSSEC enhanced priming response could achieve without the whole
   validation chain.  This would probably call for a different naming
   scheme (see section 6.1 of [I-D.koch-dns-glue-clarifications]).

3.  Priming Responses

   A root name server cannot distinguish a priming query from any other
   query for the root NS RRSet, except that QTYPE NS would not usually
   be part of the DNS resolution process.

3.1.  Expected Properties of the Priming Response

   The priming response can be expected to have an RCODE of NOERROR and
   the AA bit set.  Also, there should be an NS RRSet in the answer
   section (since the NS RRSet originates from the root zone), an empty
   authority section (since the NS RRSet already appears in the answer
   section) and an additional section with A and AAAA RRSets for the
   root name servers pointed at by the NS RRSet.  Resolver software
   SHOULD NOT expect a fixed number of 13 NS RRs, since "internal" root
   server setups in split DNS configurations might use a different
   number of servers.  Resolver software SHOULD warn the operator about
   any change in the number or names of name servers or their addresses
   compared to the SBELT information.

3.2.  Use of the Priming Response

   A resolver MAY use the priming response as it would use any other
   data fed to its cache.  However, it SHOULD NOT use the SBELT
   information directly in any responses it hands out.

3.3.  Completeness of the Response

   Assuming an upper bound of thirteen root name servers and one address
   each for IPv4 and IPv6, the combined size of all the A and AAAA
   RRSets is 13 * (16 + 28) == 572, independent of the naming scheme.
   Not even counting the NS RRSet, this value exceeds the original 512
   octet payload limit.

   For an EDNS response, a resolver SHOULD consider the address
   information found in the additional section complete for any
   particular server that appears at all.  In other words: if the
   additional section only has an A RRSet for a server, the resolver
   SHOULD assume that no AAAA RRSet exists.  This is to avoid repeated
   unnecessary queries for names of name servers that do not or do not
   yet offer IPv6 service, or, in perspective, will have ceased IPv4

   If the resolver did not announce a reassembly size larger than 512
   octets, this assumption is invalid.  Simple re-issuing of the priming
   query does not help with those root name servers that respond with a
   fixed order of addresses in the additional section.  Instead the
   resolver ought to issue direct queries for A and AAAA RRSets for the
   remaining names.  In today's environment these RRSets would be
   authoritatively available from the root name servers.

4.  Requirements for Root Name Servers and the Root Zone

   The operational requirements for root name servers are described in
   [RFC2870].  This section specifies additional guidance for the
   configuration of and software deployed at the root name servers.

   All DNS root name servers need to be able to provide for all
   addresses of all root name servers.  This can easily achieved by
   keeping all root name server names in a single zone and by making all
   root name servers authoritative for that zone.

   If the response packet does not provide for more than 512 octets due
   to lack of EDNS0 support, A RRSets SHOULD be given preference over
   AAAA RRSets when filling the additional section.

   [[Issue 1: EDNS0 is used as an indication of AAAA understanding on
   the side of the client.  What to do with payload sizes indicated by
   EDNS0 that are smaller than 1024, is open to discussion.  At the time
   of writing, some root name servers will fill the additional section
   with all available A RRSets, only adding some AAAA RRSets, when
   queried over IPv4 without EDNS0.  Other servers will deliver more
   AAAA RRSets, therefore withholding some A RRSets completely

   To ensure equal availability the A and AAAA RRSets for the root name
   servers' names SHOULD have identical TTL values at the authoritative

   [[Issue 2: Do the TTLs for the root NS RRSet and address RRSets in
   the root and the ROOT-SERVERS.NET. zones need to be aligned?  In real
   life responses, the address RRSet's TTL values vary by name server
   implementation.  Is this diversity we can live with?  Should the
   authoritative source prevail?  Is it therefore a protocol issue
   rather than an operational choice of parameters?]]

5.  Security Considerations

   This document deals with priming a DNS resolver's cache.  The usual
   DNS caveats apply.  Use of DNSSEC with priming queries is discussed
   in section 2.4.

   Spoofing a response to a priming query can be used to redirect all
   queries originating from a victim resolver, therefore resolver.  Therefore, any
   difference between the inital SBELT list and the priming response
   SHOULD be brought to the operators' attention.  There is also a
   chance that the random target selection chooses the address of a
   retired root name server.  Operational measures to prevent reuse of
   these addresses are out of the scope of this document.

6.  IANA Considerations

   This document does not propose any new IANA registry nor does it ask
   for any allocation from an existing IANA registry.

   However, this document deals with requirements for the root zone and
   root server operations.

   [[Issue3: related to issue 2 - any recommendation on the "."  NS
   RRSet TTL or the TTLs of the respective A and/or AAAA RRSets might go

7.  References

7.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3226]  Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
              message size requirements", RFC 3226, December 2001.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452, January 2009.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.

7.2.  Informative References

              Barber, P., Larson, M., Kosters, M., and P. Toscano, "Life
              and Times of J-Root", NANOG 32, October 2004.

              Larson, M. and O. Gudmundsson, "DNSSEC Trust Anchor
              Configuration and Maintenance", draft-ietf-dnsop-dnssec-
              trust-anchor-04 (work in progress), October 2010.

              Koch, P., "DNS Glue RR Survey and Terminology
              Clarification", draft-koch-dns-glue-clarifications-04
              (work in progress), July 2010.

              Manning, B., "persistent queries and phantom nameservers",
              WIDE/CAIDA Workshop , October 2006.

   [RFC2870]  Bush, R., Karrenberg, D., Kosters, M., and R. Plzak, "Root
              Name Server Operational Requirements", BCP 40, RFC 2870,
              June 2000.

   [RFC4472]  Durand, A., Ihren, J., and P. Savola, "Operational
              Considerations and Issues with IPv6 DNS", RFC 4472, April

   [RFC4697]  Larson, M. and P. Barber, "Observed DNS Resolution
              Misbehavior", BCP 123, RFC 4697, October 2006.

   [SSAC016]  ICANN Security and Stability Advisory Committee, "Testing
              Firewalls for IPv6 and EDNS0 Support", SSAC 016, January

   [SSAC017]  ICANN Security and Stability Advisory Committee, "Testing
              Recursive Name Servers for IPv6 and EDNS0 Support", SSAC
              017, February 2007.

Appendix A.  Document Revision History

   This section is to be removed should the draft be published.

   $Id: draft-ietf-dnsop-resolver-priming.xml,v 1.7 2014/02/14 23:27:23 1.8 2015/03/09 20:22:52
   pk Exp $

A.1.  -05 WG Document

   Revived.  Minor edits for readability - Warren.

A.2.  -04 WG Document

   Revived.  Updated EDNS0 reference.  Minor edits for clarity.


A.3.  -03 WG Document

   Revived.  Resolved most open issues [[]] as per previous WG
   discussion.  Minor edits on history and wording.  All root servers
   authoritative for ROOT-SERVERS.NET.


A.4.  -02 WG Document

   Revived.  Changed use of DNSSEC OK in the priming query as per the WG


A.5.  -01 WG Document

   Revived with minor edits.  Open issues marked [[]].


A.6.  -00 WG Document

   Reposted as WG document with minor edits.

   Added re-primimg proposal and A/AAAA TTL considerations.


A.7.  Initial Document

   First draft

Authors' Addresses
   Peter Koch
   Kaiserstrasse 75-77
   Frankfurt  60329

   Phone: +49 69 27235 0
   Email: pk@DENIC.DE

   Matt Larson
   Dyn, Inc.
   150 Dow St
   Manchester, NH  03101