DNS Operations                                                 M. Larson
Internet-Draft                                                  VeriSign
Expires: May 16, 2008                                     O. Gudmundsson
                                                     OGUD Consulting LLC
                                                       November 13, 2007

           DNSSEC Trust Anchor Configuration and Maintenance

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   This document recommends a preferred format for specifying trust
   anchors in DNSSEC validating security-aware resolvers and describes
   how such a resolver should initialize trust anchors for use.  This
   document also describes different mechanisms for keeping trust
   anchors up to date over time.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Trust Anchor Format  . . . . . . . . . . . . . . . . . . . . .  4
   3.  Trust Anchor Priming . . . . . . . . . . . . . . . . . . . . .  5
   4.  Trust Anchor Maintenance . . . . . . . . . . . . . . . . . . .  7
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Internationalization considerations  . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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1.  Introduction

   The DNSSEC standards documents ([2], [3] and [4]) describe the need
   for trust anchors and how they are used.  A validating security-aware
   resolver (subsequently referred to as a "validating resolver") needs
   to be configured with one or more trust anchors, which specify the
   public keys of signed zones.  To authenticate DNS data, a validating
   resolver builds a chain of trust from a configured trust anchor to
   that data.

   In a widespread public DNSSEC deployment, the DNS root zone would be
   signed and a validating resolver would need to be configured with at
   least the root zone's trust anchor.  A validating resolver might need
   additional trust anchors configured to accommodate islands of
   security.  (An island of security is a signed, delegated zone that
   does not have an authentication chain from its delegating parent.)
   For example, consider the situation where the root zone is signed but
   a given top-level domain (TLD) zone is not.  Various second-level
   zones under this unsigned TLD might be signed and resolver operators
   might want to validate responses from those zones, requiring a
   validating resolver to be configured with those zones' trust anchors.

   Because many validating resolvers would be configured with trust
   anchors in a widespread DNSSEC deployment, there is a benefit to
   creating a common trust anchor format.  A similar situation has
   occurred with the "root hints", the list of root name server names
   and IP addresses: this information is distributed in standard master
   file format and many resolver implementations support this common

   To simplify this trust anchor configuration process that will occur
   on a large number of resolvers, this document offers guidance to
   validating resolver implementers by specifying a standardized format
   for describing trust anchors.  The document also describes how a
   validating resolver should initialize or "prime" trust anchors before
   first use.  Finally, the document lists options for keeping trust
   anchor information current over time.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

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2.  Trust Anchor Format

   A trust anchor is a DNSSEC public key configured in a validating
   resolver.  A validating resolver's configuration MUST allow one or
   more trust anchors to be specified.  According to the definition in
   Section 2 of RFC 4033 [2], a trust anchor can be specified as either
   a DNSKEY resource record (RR) or a DS RR, which contains the hash of
   the specific DNSKEY RR.  (DS records are defined in Section 5 of RFC
   4034 [3].)

   This document RECOMMENDS that a trust anchor be specified as a DS RR.
   A DS RR used to specify a trust anchor in this manner SHOULD use a
   digest algorithm of SHA-256 [5], which is DS digest type 2.  DS RRs
   using SHA-1 (DS digest type 1) to specify trust anchors are NOT
   RECOMMENDED: RFC 4509 encourages the use of DS RRs using SHA-256 over
   those using SHA-1.

   Specifying a trust anchor using a DS RR instead of a DNSKEY RR offers
   a slight advantage because it forces the resolver to make a DNS query
   to obtain the trust anchor's complete DNSKEY RRSet during a priming
   operation (described below).  If only a DNSKEY record were specified,
   a resolver implementers could conceivably avoid priming the trust
   anchor.  But priming is desirable because it causes the resolver to
   retrieve an up-to-date version of a zone's DNSKEY RRSet from one of
   the zone's authoritative servers.  It should be noted that in
   practice, priming is almost always required because data in the trust
   anchor zone will usually be signed with a different key than the one
   configured as the trust anchor, thus requiring the validating
   resolver to obtain all keys in the DNSKEY RRSet.

   Using a DS RR is also recommended because it is smaller than the
   DNSKEY RR and is easier to enter manually, either by typing or
   cutting and pasting.

   Another advantage of configuring a trust anchor using a DS RR is that
   the entire hash of the public key in the DS RDATA need not
   necessarily be specified.  A validating resolver MAY support
   configuration using a truncated DS hash value as a human-factors
   convenience: shorter strings are easier to type and less prone to
   error when entered manually.  Even with a truncated hash configured,
   a validating resolver can still verify that the corresponding DNSKEY
   is present in the trust anchor zone's apex DNSKEY RRSet.

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3.  Trust Anchor Priming

   A validating resolver needs to obtain and validate the DNSKEY RRSet
   corresponding to a configured DS RR for that trust anchor to be
   usable in DNSSEC validation.  This process is called "priming" the
   trust anchor.  Priming can occur when the validating resolver starts,
   but a validating resolver SHOULD defer priming of individual trust
   anchors until each is first needed for verification.  This priming on
   demand is especially important when a validating resolver is
   configured with a large number of trust anchors to avoid sending a
   large number of DNS queries on start-up.  This section adds
   additional details to the discussion of trust anchors in Section 5 of
   RFC 4035 [4].

   Following are the steps a validating resolver SHOULD take to prime a
   configured trust anchor:

   1.  Read the trust anchor's DS RR from the validating resolver's
       configuration (e.g., a text file).

   2.  Look up the DNSKEY RRSet corresponding to the owner name of the
       DS RR.  (The validating resolver can either perform iterative
       resolution or request recursive service from a recursive name
       server, depending on its capabilities.)

   3.  Verify that the DNSKEY RR corresponding to the configured DS RR
       (i.e., the DNSKEY whose hash appears in the DS record) appears in
       the DNSKEY RRSet and that the DNSKEY RR has the Zone Key Flag
       (DNSKEY RDATA bit 7) set.

   4.  Verify that the DNSKEY RRSet is signed by one of the DNSKEYs
       found in the previous step, i.e., that there exists a valid RRSIG
       (cryptographically and temporally) for the DNSKEY RRSet generated
       with the private key corresponding to the DNSKEY found in the
       previous step.

   If the validating resolver can successfully complete the steps above,
   all DNSKEY RRs in the RRSet ought to be considered authenticated and
   used authenticate RRSets at or below the trust anchor.

   If any of the steps above result in an error, the validating resolver
   SHOULD log them.

   If there are multiple trust anchors configured for a zone, any one of
   them is sufficient to validate data in the zone.  For this reason,
   old trust anchors SHOULD be removed from a validating resolver's
   trust anchor list soon after the corresponding keys are no longer
   used by the zone.  A validating resolver should remove a trust anchor

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   that has been revoked as indicated by the REVOKE bit in the
   corresponding DNSKEY record as described in RFC 5011.  RFC5011 [6]

   If a validating resolver is unable to to retrieve a signed DNSKEY
   RRSet corresponding to a trust anchor (i.e., prime the trust anchor),
   it SHOULD log this condition as an error.  Inability to prime a
   zone's trust anchor will likely result in the validating resolver's
   inability to validate data from the corresponding zone and cause the
   resolver to return an error in response to the original DNS query.

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4.  Trust Anchor Maintenance

   Trust anchors correspond to zones' key signing keys and these keys do
   change in the course of normal operation.  Validating resolver
   operators MUST ensure that configured trust anchor information
   remains current and does not go stale: each configured trust anchor
   DS RR SHOULD correspond to a DNSKEY RR in the trust anchor zone's
   apex DNSKEY RRSet.  This process is called trust anchor maintenance.
   (Initial trust anchor configuration requires human intervention to
   verify the trust anchor's authenticity using out-of-band means and is
   outside the scope of this document.)

   This section provides a brief overview of some possible mechanisms to
   keep trust anchor information current:

   Manual configuration:  The validating resolver operator MAY choose to
      maintain trust anchor information completely manually.  In this
      case, the operator assumes responsibility for noticing stale trust
      anchor information (i.e., DS records that no longer point to a
      corresponding DNSKEY RR in the trust anchor zone's apex DNSKEY
      RRSet) and updating that information.  This process MAY require
      the operator to use the same out-of-band verification mechanism
      used to initial configuration to ensure that the new trust anchor
      DS RR is trustworthy.  Because manual maintenance is burdensome
      and prone to error, and because other automated trust anchor
      maintenance processes either exist or are in development, manual
      trust anchor maintenance is NOT RECOMMENDED.

   DNSSEC In-band Update:  The IETF DNS Extensions Working Group has
      developed a protocol to automatically update DNSSEC trust anchors,
      which is described in RFC 5011.  RFC5011 [6] This protocol relies
      on a small DNSSEC protocol change (an additional flag in the
      DNSKEY record) and can be implemented either in a validating
      resolver itself or in an external program with access to the
      validating resolver's trust anchor configuration data.

   Trusted update mechanism:  Updated trust anchor information MAY be
      obtained via a trusted non-DNS update mechanism.  One possibility
      is the operating system update mechanism provided by most software
      vendors.  Operators already place considerable trust in this
      mechanism, so it is reasonable to extend this trust to allow
      distribution and update of DNSSEC public key material.  Another
      possibility is to obtain trust anchor configuration directly from
      the validating resolver software vendor.  This mechanism is
      realistically only feasible for updating a small number of trust
      anchors, such as for the top-level domains.  In a public DNSSEC
      deployment, the root zone would be signed and only the root's
      trust anchor would need updating.

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5.  Acknowledgments

   This work was undertaken at the suggestion of the DNSSEC Deployment
   working group (www.dnssec-deployment.org).

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6.  Security considerations

   This document proposes a standard format for documenting DNSSEC trust
   anchors.  Configuration of trust anchors, especially those obtained
   from third parties as part of an automated process, is a critical
   security operation.  The procedures described above describe the
   minimal checks that should be performed and reporting that should be
   done when configuring trust anchors.

   In a widespread DNSSEC deployment, the root zone and many TLD zones
   would be signed, thus greatly reducing the number trust anchors that
   validating resolvers would need to store and keep track of.

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7.  IANA considerations

   This document does not have any IANA actions.

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8.  Internationalization considerations

   There are no new internationalization considerations introduced by
   this memo.

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9.  References

9.1.  Normative References

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

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

   [3]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "Resource Records for the DNS Security Extensions", RFC 4034,
        March 2005.

   [4]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "Protocol Modifications for the DNS Security Extensions",
        RFC 4035, March 2005.

   [5]  Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS)
        Resource Records (RRs)", RFC 4509, May 2006.

   [6]  StJohns, M., "Automated Updates of DNS Security (DNSSEC) Trust
        Anchors", RFC 5011, September 2007.

9.2.  Informative References

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Authors' Addresses

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

   Email: mlarson@verisign.com

   Olafur Gudmundsson
   OGUD Consulting LLC
   3821 Village Park Drive
   Chevy Chase, MD  20815

   Email: ogud@ogud.com

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