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Versions: (draft-koch-dnsop-resolver-priming) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 8109

Network Working Group                                            P. Koch
Internet-Draft                                                  DENIC eG
Intended status: BCP                                           M. Larson
Expires: April 29, 2010                                   VeriSign, Inc.
                                                        October 26, 2009


            Initializing a DNS Resolver with Priming Queries
                  draft-ietf-dnsop-resolver-priming-02

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Abstract

   This document describes the initial queries a DNS resolver is
   supposed to emit to initialize its cache with a current NS RRSet for



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   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
   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 four
   address changes affecting the L (twice), 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 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
   feature.




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   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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 6.1.3.2 of [RFC1123]).
   The UDP source port SHOULD be randomly selected [RFC5452].  The RD
   bit MUST NOT be set.  The resolver SHOULD also use EDNS0 [RFC2671]
   and announce and handle a reassembly size of at least 1024 octets
   [RFC3226].

   [[Do we need a fallback strategy for EDNS unfriendly environments?]]

2.2.  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.  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 the A/AAAA
   RRSets' TTL, whichever is lower) have passed.

   Should the priming query time out, the resolver should try another
   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.

   [[Is it OK to send multiple priming queries to multiple targets in
   parallel?]]





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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 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.

   [[Is there any special consideration needed for those cases where



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   someone managed to cause a signed root NS RRSet being fed into the
   cache?]]

3.3.  Completeness of the Response

   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.  To ensure equal
   availability the A and AAAA RRSets should have identical TTL values
   at the authoritative source.  [[There might still be some degenerate
   cases of response sizes between 512 and 1024.]]

   [[If the resolver did not announce a reassembly size larger than 512
   octets, this assumption is invalid.  How to acquire the remaining
   address RRSets is TBD.  Simple re-issueing of the priming query does
   not help with those root name servers that respond with a fixed order
   of addresses in the additional section.]]


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
   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., even though only a subset
   received an official delegation.]]

   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.

   [[EDNS0 is used as an indication of AAAA understanding on the side of
   the client.  What to do with small payload sizes indicated by EDNS0
   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 [RFC4472].]]




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   [[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 implementation.]]


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 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.

   [[Any recommendation on the "."  NS RRSet TTL or the TTLs of the
   respective A and/or AAAA RRSets would go here.]]

   [[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.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",



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              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.

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

7.2.  Informative References

   [BLKT2004]
              Barber, P., Larson, M., Kosters, M., and P. Toscano, "Life
              and Times of J-Root", NANOG 32, October 2004.

   [I-D.ietf-dnsop-dnssec-trust-anchor]
              Larson, M. and O. Gudmundsson, "DNSSEC Trust Anchor
              Configuration and Maintenance",
              draft-ietf-dnsop-dnssec-trust-anchor-03 (work in
              progress), March 2009.

   [I-D.koch-dns-glue-clarifications]
              Koch, P., "DNS Glue RR Survey and Terminology
              Clarification", draft-koch-dns-glue-clarifications-03
              (work in progress), November 2007.

   [Mann2006]
              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 2006.

   [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.



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Appendix A.  Document Revision History

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

   $Id: draft-ietf-dnsop-resolver-priming.xml,v 1.4 2009/10/26 20:18:33
   pk Exp $

A.1.  -02 WG Document

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

A.2.  -01 WG Document

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

A.3.  -00 WG Document

   Reposted as WG document with minor edits.

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

A.4.  Initial Document

   First draft


Authors' Addresses

   Peter Koch
   DENIC eG
   Kaiserstrasse 75-77
   Frankfurt  60329
   DE

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


   Matt Larson
   VeriSign, Inc.
   21345 Ridgetop Circle
   Dulles, VA  20166-6503
   USA

   Email: mlarson@verisign.com





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