Network Working Group                                            P. Koch
Internet-Draft                                                  DENIC eG
Expires: August 30, 2007                               February 26, 2007

            Initializing a DNS Resolver with Priming Queries

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


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

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

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   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.''

   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''

   Today's practice generally seperates serving and resolving
   functionality, so the servers ``for the host's domain'' might no
   longer be an appropriate choice, even if they were only intended to
   resolve ``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
   ten years.  After the last four servers had been added and moved to
   their final (network) destinations in 1997, there have been only
   three address changes affecting the L, J, and B 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
   updates to be applied by their operator.

   The recent 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.larson-dnsop-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].

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   Comments should be sent to the IETF DNSOP WG mailing list.

2.  Parameters of a Priming Query

   This document only deals with QCLASS IN.  A priming query SHOULD use
   a QNAME of "." and a QTYPE of NS.  It SHOULD also use EDNS0 [RFC2671]
   and announce and handle a reassembly size of at least 1024 octets

   A priming query MUST be sent over UDP (section of [RFC1123]).
   The RD bit MUST NOT be set in the query.

2.1.  Target Selection

   A resolver MUST select the target for a priming query randomly from
   its list of available addresses 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.

2.2.  DNSSEC with Priming Queries

   The resolver MAY choose to use DNSSEC OK [RFC4033], in which case it
   MUST announce and handle a message size of at least 1220 octets.

   Discussion: Delegations in referral responses are not signed, so
   following this model there would be no need to require a signed root
   NS RRSet and, equally important, signed A and AAAA RRSet for the root
   name servers' names.  On the other hand, a poisoned priming response
   could drastically influence the resolver's operations.  If the
   priming response should be secured by DNSSEC, then it should also be
   self contained, i.e., the whole validation chain should be present in
   the priming response.  This might call for a different naming scheme
   (see section 5.1 of [I-D.koch-dns-glue-clarifications]).

2.3.  Repeating Priming Queries

   A resolver SHOULD NOT originate a priming query more often than once
   per day (or whenever it starts).  It SHOULD adhere to the TTL values
   given in the priming response. {May be useful to proactively re-prime
   before the old root NS RRSet expires from the cache.}

3.  Expected Properties of a Priming Response

   The 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

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   (since the NS RRSet originates from the root zone), an empty
   authority section and an additional section with A and AAAA RRSets
   for the root name servers pointed at by the NS RRSet. {Note that the
   number 13 does not appear here.  It might be necessary to consider
   "internal" root server setups in split DNS configurations.}

3.1.  Use of the Priming Response

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

3.2.  Completeness of the 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 a strawman only.  TTL synchronization is an issue here.}

4.  Root Name Server Requirements

   The operational requirements for root name servers are described in

   All DNS root name servers need to be able to provide for all
   addresses of all root name servers.  This can easily achieved by
   making all root name servers authoritative for the zone containing
   the servers' names. {At the time of writing, all but one root name
   server were authoritative for ROOT-SERVERS.NET.}

   If the response packet does not provide for more than 512 octets due
   to lack of EDNS0 support, AAAA RRSets should be omitted from the
   response. {What to do with small payloads indicated by EDNS0 is open
   to discussion.}

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 2.2

   Spoofing a response to a priming query can be used to redirect all
   queries originating from a victim 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

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   random target selection choses the address of a retired root name
   server.  Operational measures to prevent reuse of these addresses are
   out of the scope of this document.

   {This section needs more work.}

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.

   {This section needs more work.}

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.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

   [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.

7.2.  Informative References

              Barber, P., Larson, M., Kosters, M., and P. Toscano, "Life

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              and Times of J-Root", NANOG 32, October 2004.

              Koch, P., "DNS Glue RR Survey and Terminology
              Clarification", draft-koch-dns-glue-clarifications-02
              (work in progress), October 2006.

              Larson, M. and O. Gudmundsson, "DNSSEC Trust Anchor
              Configuration and Maintenance",
              draft-larson-dnsop-trust-anchor-00 (work in progress),
              January 2007.

              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.

   [SSAC016]  ICANN Security and Stability Advisory Committee, "Testing
              Firewalls for IPv6 and EDNS0 Support", SSAC 016,
              January 2007.

   [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.

A.1.  Initial Document

   First draft

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Author's Address

   Peter Koch
   Wiesenhuettenplatz 26
   Frankfurt  60329

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

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