Internet DRAFT - draft-ietf-dnsext-trustupdate-timers


Network Working Group                                         M. StJohns
Internet-Draft                                               Independent
Expires: October 4, 2007                                   April 2, 2007

               Automated Updates of DNSSEC Trust Anchors

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document describes a means for automated, authenticated and
   authorized updating of DNSSEC "trust anchors".  The method provides
   protection against N-1 key compromises of N keys in the trust point
   key set.  Based on the trust established by the presence of a current
   anchor, other anchors may be added at the same place in the
   hierarchy, and, ultimately, supplant the existing anchor(s).

   This mechanism will require changes to resolver management behavior
   (but not resolver resolution behavior), and the addition of a single

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   flag bit to the DNSKEY record.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Compliance Nomenclature  . . . . . . . . . . . . . . . . .  3
   2.  Theory of Operation  . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Revocation . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Add Hold-Down  . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Active Refresh . . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Resolver Parameters  . . . . . . . . . . . . . . . . . . .  6
       2.4.1.  Add Hold-Down Time . . . . . . . . . . . . . . . . . .  6
       2.4.2.  Remove Hold-Down Time  . . . . . . . . . . . . . . . .  6
       2.4.3.  Minimum Trust Anchors per Trust Point  . . . . . . . .  6
   3.  Changes to DNSKEY RDATA Wire Format  . . . . . . . . . . . . .  6
   4.  State Table  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Events . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  States . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Trust Point Deletion . . . . . . . . . . . . . . . . . . . . .  8
   6.  Scenarios - Informative  . . . . . . . . . . . . . . . . . . .  9
     6.1.  Adding a Trust Anchor  . . . . . . . . . . . . . . . . . .  9
     6.2.  Deleting a Trust Anchor  . . . . . . . . . . . . . . . . .  9
     6.3.  Key Roll-Over  . . . . . . . . . . . . . . . . . . . . . . 10
     6.4.  Active Key Compromised . . . . . . . . . . . . . . . . . . 10
     6.5.  Stand-by Key Compromised . . . . . . . . . . . . . . . . . 10
     6.6.  Trust Point Deletion . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     8.1.  Key Ownership vs Acceptance Policy . . . . . . . . . . . . 11
     8.2.  Multiple Key Compromise  . . . . . . . . . . . . . . . . . 11
     8.3.  Dynamic Updates  . . . . . . . . . . . . . . . . . . . . . 11
   9.  Normative References . . . . . . . . . . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

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1.  Introduction

   As part of the reality of fielding DNSSEC (Domain Name System
   Security Extensions) [RFC4033] [RFC4034] [RFC4035], the community has
   come to the realization that there will not be one signed name space,
   but rather islands of signed name space each originating from
   specific points (i.e. 'trust points') in the DNS tree.  Each of those
   islands will be identified by the trust point name, and validated by
   at least one associated public key.  For the purpose of this document
   we'll call the association of that name and a particular key a 'trust
   anchor'.  A particular trust point can have more than one key
   designated as a trust anchor.

   For a DNSSEC-aware resolver to validate information in a DNSSEC
   protected branch of the hierarchy, it must have knowledge of a trust
   anchor applicable to that branch.  It may also have more than one
   trust anchor for any given trust point.  Under current rules, a chain
   of trust for DNSSEC-protected data that chains its way back to ANY
   known trust anchor is considered 'secure'.

   Because of the probable balkanization of the DNSSEC tree due to
   signing voids at key locations, a resolver may need to know literally
   thousands of trust anchors to perform its duties. (e.g., Consider an
   unsigned ".COM".)  Requiring the owner of the resolver to manually
   manage these many relationships is problematic.  It's even more
   problematic when considering the eventual requirement for key
   replacement/update for a given trust anchor.  The mechanism described
   herein won't help with the initial configuration of the trust anchors
   in the resolvers, but should make trust point key replacement/
   rollover more viable.

   As mentioned above, this document describes a mechanism whereby a
   resolver can update the trust anchors for a given trust point, mainly
   without human intervention at the resolver.  There are some corner
   cases discussed (e.g., multiple key compromise) that may require
   manual intervention, but they should be few and far between.  This
   document DOES NOT discuss the general problem of the initial
   configuration of trust anchors for the resolver.

1.1.  Compliance Nomenclature

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, [RFC2119].

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2.  Theory of Operation

   The general concept of this mechanism is that existing trust anchors
   can be used to authenticate new trust anchors at the same point in
   the DNS hierarchy.  When a zone operator adds a new SEP key (i.e. a
   DNSKEY with the Secure Entry Point bit set) (see [RFC4034], section
   2.1.1) to a trust point DNSKEY RRSet, and when that RRSet is
   validated by an existing trust anchor, then the resolver can add the
   new key to its set of valid trust anchors for that trust point.

   There are some issues with this approach which need to be mitigated.
   For example, a compromise of one of the existing keys could allow an
   attacker to add their own 'valid' data.  This implies a need for a
   method to revoke an existing key regardless of whether or not that
   key is compromised.  As another example, assuming a single key
   compromise, we need to prevent an attacker from adding a new key and
   revoking all the other old keys.

2.1.  Revocation

   Assume two trust anchor keys A and B. Assume that B has been
   compromised.  Without a specific revocation bit, B could invalidate A
   simply by sending out a signed trust point key set which didn't
   contain A. To fix this, we add a mechanism which requires knowledge
   of the private key of a DNSKEY to revoke that DNSKEY.

   A key is considered revoked when the resolver sees the key in a self-
   signed RRSet and the key has the REVOKE bit (see Section 7 below) set
   to '1'.  Once the resolver sees the REVOKE bit, it MUST NOT use this
   key as a trust anchor or for any other purposes except validating the
   RRSIG it signed over the DNSKEY RRSet specifically for the purpose of
   validating the revocation.  Unlike the 'Add' operation below,
   revocation is immediate and permanent upon receipt of a valid
   revocation at the resolver.

   A self-signed RRSet is a DNSKEY RRSet which contains the specific
   DNSKEY and for which there is a corresponding validated RRSIG record.
   It's not a special DNSKEY RRSet, just a way of describing the
   validation requirements for that RRSet.

   N.B. A DNSKEY with the REVOKE bit set has a different fingerprint
   than one without the bit set.  This affects the matching of a DNSKEY
   to DS records in the parent, or the fingerprint stored at a resolver
   used to configure a trust point.

   In the given example, the attacker could revoke B because it has
   knowledge of B's private key, but could not revoke A.

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2.2.  Add Hold-Down

   Assume two trust point keys A and B. Assume that B has been
   compromised.  An attacker could generate and add a new trust anchor
   key - C (by adding C to the DNSKEY RRSet and signing it with B), and
   then invalidate the compromised key.  This would result in both the
   attacker and owner being able to sign data in the zone and have it
   accepted as valid by resolvers.

   To mitigate but not completely solve this problem, we add a hold-down
   time to the addition of the trust anchor.  When the resolver sees a
   new SEP key in a validated trust point DNSKEY RRSet, the resolver
   starts an acceptance timer, and remembers all the keys that validated
   the RRSet.  If the resolver ever sees the DNSKEY RRSet without the
   new key but validly signed, it stops the acceptance process for that
   key and resets the acceptance timer.  If all of the keys which were
   originally used to validate this key are revoked prior to the timer
   expiring, the resolver stops the acceptance process and resets the

   Once the timer expires, the new key will be added as a trust anchor
   the next time the validated RRSet with the new key is seen at the
   resolver.  The resolver MUST NOT treat the new key as a trust anchor
   until the hold down time expires AND it has retrieved and validated a
   DNSKEY RRSet after the hold down time which contains the new key.

   N.B.: Once the resolver has accepted a key as a trust anchor, the key
   MUST be considered a valid trust anchor by that resolver until
   explictly revoked as described above.

   In the given example, the zone owner can recover from a compromise by
   revoking B and adding a new key D and signing the DNSKEY RRSet with
   both A and B.

   The reason this does not completely solve the problem has to do with
   the distributed nature of DNS.  The resolver only knows what it sees.
   A determined attacker who holds one compromised key could keep a
   single resolver from realizing that key had been compromised by
   intercepting 'real' data from the originating zone and substituting
   their own (e.g., using the example, signed only by B).  This is no
   worse than the current situation assuming a compromised key.

2.3.  Active Refresh

   A resolver which has been configured for automatic update of keys
   from a particular trust point MUST query that trust point (e.g., do a
   lookup for the DNSKEY RRSet and related RRSIG records) no less often
   than the lesser of 15 days or half the original TTL for the DNSKEY

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   RRSet or half the RRSIG expiration interval and no more often than
   once per hour.  The expiration interval is the amount of time from
   when the RRSIG was last retrieved until the expiration time in the
   RRSIG.  I.e.: queryInterval = MAX(1 hr, MIN (15 days, 1/2*OrigTTL,

   If the query fails, the resolver MUST repeat the query until
   satisfied no more often than once an hour and no less often than the
   lesser of 1 day or 10% of the original TTL or 10% of the original
   expiration interval.  I.e.: retryTime = MAX (1 hour, MIN (1 day, .1 *
   origTTL, .1 * expireInterval)).

2.4.  Resolver Parameters

2.4.1.  Add Hold-Down Time

   The add hold-down time is 30 days or the expiration time of the
   original TTL of the first trust point DNSKEY RRSet which contained
   the new key, whichever is greater.  This ensures that at least two
   validated DNSKEY RRSets which contain the new key MUST be seen by the
   resolver prior to the key's acceptance.

2.4.2.  Remove Hold-Down Time

   The remove hold-down time is 30 days.  This parameter is solely a key
   management database bookeeping parameter.  Failure to remove
   information about the state of defunct keys from the database will
   not adversely impact the security of this protocol, but may end up
   with a database cluttered with obsolete key information.

2.4.3.  Minimum Trust Anchors per Trust Point

   A compliant resolver MUST be able to manage at least five SEP keys
   per trust point.

3.  Changes to DNSKEY RDATA Wire Format

   Bit n of the DNSKEY Flags field is designated as the 'REVOKE' flag.
   If this bit is set to '1', AND the resolver sees an RRSIG(DNSKEY)
   signed by the associated key, then the resolver MUST consider this
   key permanently invalid for all purposes except for validating the

4.  State Table

   The most important thing to understand is the resolver's view of any

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   key at a trust point.  The following state table describes that view
   at various points in the key's lifetime.  The table is a normative
   part of this specification.  The initial state of the key is 'Start'.
   The resolver's view of the state of the key changes as various events

   This is the state of a trust point key as seen from the resolver.
   The column on the left indicates the current state.  The header at
   the top shows the next state.  The intersection of the two shows the
   event that will cause the state to transition from the current state
   to the next.

                             NEXT STATE
    FROM   |Start  |AddPend |Valid  |Missing|Revoked|Removed|
   Start   |       |NewKey  |       |       |       |       |
   AddPend |KeyRem |        |AddTime|       |       |       |
   Valid   |       |        |       |KeyRem |Revbit |       |
   Missing |       |        |KeyPres|       |Revbit |       |
   Revoked |       |        |       |       |       |RemTime|
   Removed |       |        |       |       |       |       |

                                State Table

4.1.  Events
   NewKey  The resolver sees a valid DNSKEY RRSet with a new SEP key.
      That key will become a new trust anchor for the named trust point
      after it's been present in the RRSet for at least 'add time'.
   KeyPres  The key has returned to the valid DNSKEY RRSet.
   KeyRem  The resolver sees a valid DNSKEY RRSet that does not contain
      this key.
   AddTime  The key has been in every valid DNSKEY RRSet seen for at
      least the 'add time'.
   RemTime  A revoked key has been missing from the trust point DNSKEY
      RRSet for sufficient time to be removed from the trust set.

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   RevBit  The key has appeared in the trust anchor DNSKEY RRSet with
      its "REVOKED" bit set, and there is an RRSig over the DNSKEY RRSet
      signed by this key.

4.2.  States
   Start  The key doesn't yet exist as a trust anchor at the resolver.
      It may or may not exist at the zone server, but either hasn't yet
      been seen at the resolver or was seen but was absent from the last
      DNSKEY RRSet (e.g., KeyRem event).
   AddPend  The key has been seen at the resolver, has its 'SEP' bit
      set, and has been included in a validated DNSKEY RRSet.  There is
      a hold-down time for the key before it can be used as a trust
   Valid  The key has been seen at the resolver and has been included in
      all validated DNSKEY RRSets from the time it was first seen up
      through the hold-down time.  It is now valid for verifying RRSets
      that arrive after the hold down time.  Clarification: The DNSKEY
      RRSet does not need to be continuously present at the resolver
      (e.g., its TTL might expire).  If the RRSet is seen, and is
      validated (i.e. verifies against an existing trust anchor), this
      key MUST be in the RRSet otherwise a 'KeyRem' event is triggered.
   Missing  This is an abnormal state.  The key remains as a valid trust
      point key, but was not seen at the resolver in the last validated
      DNSKEY RRSet.  This is an abnormal state because the zone operator
      should be using the REVOKE bit prior to removal.
   Revoked  This is the state a key moves to once the resolver sees an
      RRSIG(DNSKEY) signed by this key where that DNSKEY RRSet contains
      this key with its REVOKE bit set to '1'.  Once in this state, this
      key MUST permanently be considered invalid as a trust anchor.
   Removed  After a fairly long hold-down time, information about this
      key may be purged from the resolver.  A key in the removed state
      MUST NOT be considered a valid trust anchor.  (Note: this state is
      more or less equivalent to the "Start" state, except that it's bad
      practice to re-introduce previously used keys - think of this as
      the holding state for all the old keys for which the resolver no
      longer needs to track state.)

5.  Trust Point Deletion

   A trust point which has all of its trust anchors revoked is
   considered deleted and is treated as if the trust point was never
   configured.  If there are no superior configured trust points, data
   at and below the deleted trust point are considered insecure by the
   resolver.  If there ARE superior configured trust points, data at and
   below the deleted trust point are evaluated with respect to the
   superior trust point(s).

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   Alternately, a trust point which is subordinate to another configured
   trust point MAY be deleted by a resolver after 180 days where such
   subordinate trust point validly chains to a superior trust point.
   The decision to delete the subordinate trust anchor is a local
   configuration decision.  Once the subordinate trust point is deleted,
   validation of the subordinate zone is dependent on validating the
   chain of trust to the superior trust point.

6.  Scenarios - Informative

   The suggested model for operation is to have one active key and one
   stand-by key at each trust point.  The active key will be used to
   sign the DNSKEY RRSet.  The stand-by key will not normally sign this
   RRSet, but the resolver will accept it as a trust anchor if/when it
   sees the signature on the trust point DNSKEY RRSet.

   Since the stand-by key is not in active signing use, the associated
   private key may (and should) be provided with additional protections
   not normally available to a key that must be used frequently.  E.g.,
   locked in a safe, split among many parties, etc.  Notionally, the
   stand-by key should be less subject to compromise than an active key,
   but that will be dependent on operational concerns not addressed

6.1.  Adding a Trust Anchor

   Assume an existing trust anchor key 'A'.
   1.  Generate a new key pair.
   2.  Create a DNSKEY record from the key pair and set the SEP and Zone
       Key bits.
   3.  Add the DNSKEY to the RRSet.
   4.  Sign the DNSKEY RRSet ONLY with the existing trust anchor key -
   5.  Wait a while (i.e. for various resolvers' timers to go off and
       for them to retrieve the new DNSKEY RRSet and signatures).
   6.  The new trust anchor will be populated at the resolvers on the
       schedule described by the state table and update algorithm - see
       Section 2 and 4 above.

6.2.  Deleting a Trust Anchor

   Assume existing trust anchors 'A' and 'B' and that you want to revoke
   and delete 'A'.
   1.  Set the revocation bit on key 'A'.
   2.  Sign the DNSKEY RRSet with both 'A' and 'B'.
   'A' is now revoked.  The operator should include the revoked 'A' in
   the RRSet for at least the remove hold-down time, but then may remove

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   it from the DNSKEY RRSet.

6.3.  Key Roll-Over

   Assume existing keys A and B. 'A' is actively in use (i.e. has been
   signing the DNSKEY RRSet.)  'B' was the stand-by key. (i.e. has been
   in the DNSKEY RRSet and is a valid trust anchor, but wasn't being
   used to sign the RRSet.)
   1.  Generate a new key pair 'C'.
   2.  Add 'C' to the DNSKEY RRSet.
   3.  Set the revocation bit on key 'A'.
   4.  Sign the RRSet with 'A' and 'B'.
   'A' is now revoked, 'B' is now the active key, and 'C' will be the
   stand-by key once the hold-down expires.  The operator should include
   the revoked 'A' in the RRSet for at least the remove hold-down time,
   but may then remove it from the DNSKEY RRSet.

6.4.  Active Key Compromised

   This is the same as the mechanism for Key Roll-Over (Section 6.3)
   above assuming 'A' is the active key.

6.5.  Stand-by Key Compromised

   Using the same assumptions and naming conventions as Key Roll-Over
   (Section 6.3) above:
   1.  Generate a new key pair 'C'.
   2.  Add 'C' to the DNSKEY RRSet.
   3.  Set the revocation bit on key 'B'.
   4.  Sign the RRSet with 'A' and 'B'.
   'B' is now revoked, 'A' remains the active key, and 'C' will be the
   stand-by key once the hold-down expires.  'B' should continue to be
   included in the RRSet for the remove hold-down time.

6.6.  Trust Point Deletion

   To delete a trust point which is subordinate to another configured
   trust point (e.g. to .com) requires some juggling of the
   data.  The specific process is:
   1.  Generate a new DNSKEY and DS record and provide the DS record to
       the parent along with DS records for the old keys
   2.  Once the parent has published the DSs, add the new DNSKEY to the
       RRSet and revoke ALL of the old keys at the same time while
       signing the DNSKEY RRSet with all of the old and new keys.
   3.  After 30 days stop publishing the old, revoked keys and remove
       any corresponding DS records in the parent.
   Revoking the old trust point keys at the same time as adding new keys
   that chain to a superior trust prevents the resolver from adding the

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   new keys as trust anchors.  Adding DS records for the old keys avoids
   a race condition where either the subordinate zone becomes unsecure
   (because the trust point was deleted) or becomes bogus (because it
   didn't chain to the superior zone).

7.  IANA Considerations

   The IANA will need to assign a bit in the DNSKEY flags field (see
   section 7 of [RFC4034]) for the REVOKE bit.  There are no other IANA
   actions required.

8.  Security Considerations

   In addition to the following sections, see also Theory of Operation
   above and especially Section 2.2 for related discussions.

8.1.  Key Ownership vs Acceptance Policy

   The reader should note that, while the zone owner is responsible for
   creating and distributing keys, it's wholly the decision of the
   resolver owner as to whether to accept such keys for the
   authentication of the zone information.  This implies the decision to
   update trust anchor keys based on trust for a current trust anchor
   key is also the resolver owner's decision.

   The resolver owner (and resolver implementers) MAY choose to permit
   or prevent key status updates based on this mechanism for specific
   trust points.  If they choose to prevent the automated updates, they
   will need to establish a mechanism for manual or other out-of-band
   updates outside the scope of this document.

8.2.  Multiple Key Compromise

   This scheme permits recovery as long as at least one valid trust
   anchor key remains uncompromised.  E.g., if there are three keys, you
   can recover if two of them are compromised.  The zone owner should
   determine their own level of comfort with respect to the number of
   active valid trust anchors in a zone and should be prepared to
   implement recovery procedures once they detect a compromise.  A
   manual or other out-of-band update of all resolvers will be required
   if all trust anchor keys at a trust point are compromised.

8.3.  Dynamic Updates

   Allowing a resolver to update its trust anchor set based on in-band
   key information is potentially less secure than a manual process.

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   However, given the nature of the DNS, the number of resolvers that
   would require update if a trust anchor key were compromised, and the
   lack of a standard management framework for DNS, this approach is no
   worse than the existing situation.

9.  Normative References

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

   [RFC3755]  Weiler, S., "Legacy Resolver Compatibility for Delegation
              Signer (DS)", RFC 3755, May 2004.

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

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

Author's Address

   Michael StJohns


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