DNSOP                                                         O. Kolkman
Internet-Draft                                                NLnet Labs
Obsoletes: 2541 (if approved)                              July 12, 2010
Intended status: Informational
Expires: January 13, 2011

                DNSSEC Operational Practices, Version 2


   This document describes a set of practices for operating the DNS with
   security extensions (DNSSEC).  The target audience is zone
   administrators deploying DNSSEC.

   The document discusses operational aspects of using keys and
   signatures in the DNS.  It discusses issues of key generation, key
   storage, signature generation, key rollover, and related policies.

   [When apporoved] This document obsoletes RFC 4641 as it covers more
   operational ground and gives more up-to-date requirements with
   respect to key sizes and the DNSSEC operations.

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   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on January 13, 2011.

Copyright Notice

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  The Use of the Term 'key'  . . . . . . . . . . . . . . . .  6
     1.2.  Time Definitions . . . . . . . . . . . . . . . . . . . . .  6
   2.  Keeping the Chain of Trust Intact  . . . . . . . . . . . . . .  7
   3.  Keys Generation and Storage  . . . . . . . . . . . . . . . . .  8
     3.1.  Operational motivation for ZSKs and KSKs . . . . . . . . .  8
       3.1.1.  Motivations for the KSK and ZSK Separation . . . . . .  8
       3.1.2.  Practical concequences of KSK and ZSK Separation . . . 10  Rolling a KSK that is not a trust-anchor . . . . . 10  Rolling a KSK that is a trust-anchor . . . . . . . 11  The use of the SEP flag  . . . . . . . . . . . . . 12
       3.1.3.  Key Effectivity Period . . . . . . . . . . . . . . . . 12
       3.1.4.  Cryptographic Considerations . . . . . . . . . . . . . 13  Key Algorithm  . . . . . . . . . . . . . . . . . . 13  Key Sizes  . . . . . . . . . . . . . . . . . . . . 14  Private Key Storage  . . . . . . . . . . . . . . . 15  Key Generation . . . . . . . . . . . . . . . . . . 16  Differentiation for 'High-Level' Zones?  . . . . . 16
   4.  Signature Generation, Key Rollover, and Related Policies . . . 17
     4.1.  Key Rollovers  . . . . . . . . . . . . . . . . . . . . . . 17
       4.1.1.  Zone Signing Key Rollovers . . . . . . . . . . . . . . 17

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  Pre-Publish Key Rollover . . . . . . . . . . . . . 18  Double Signature Zone Signing Key Rollover . . . . 20  Pros and Cons of the Schemes . . . . . . . . . . . 21
       4.1.2.  Key Signing Key Rollovers  . . . . . . . . . . . . . . 21
       4.1.3.  Difference Between ZSK and KSK Rollovers . . . . . . . 23
       4.1.4.  Rollover for a Single Type Signing Key rollover  . . . 24
       4.1.5.  Key algorithm rollover . . . . . . . . . . . . . . . . 26
       4.1.6.  Automated Key Rollovers  . . . . . . . . . . . . . . . 28
     4.2.  Planning for Emergency Key Rollover  . . . . . . . . . . . 28
       4.2.1.  KSK Compromise . . . . . . . . . . . . . . . . . . . . 29  Keeping the Chain of Trust Intact  . . . . . . . . 29  Breaking the Chain of Trust  . . . . . . . . . . . 30
       4.2.2.  ZSK Compromise . . . . . . . . . . . . . . . . . . . . 31
       4.2.3.  Compromises of Keys Anchored in Resolvers  . . . . . . 31
     4.3.  Parental Policies  . . . . . . . . . . . . . . . . . . . . 31
       4.3.1.  Initial Key Exchanges and Parental Policies
               Considerations . . . . . . . . . . . . . . . . . . . . 31
       4.3.2.  Storing Keys or Hashes?  . . . . . . . . . . . . . . . 32
       4.3.3.  Security Lameness  . . . . . . . . . . . . . . . . . . 32
       4.3.4.  DS Signature Validity Period . . . . . . . . . . . . . 33
       4.3.5.  Changing DNS Operators . . . . . . . . . . . . . . . . 34  Cooperationg DNS operators . . . . . . . . . . . . 34  Non Cooperationg DNS Operators . . . . . . . . . . 36
     4.4.  Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . 37
       4.4.1.  Time Considerations  . . . . . . . . . . . . . . . . . 37
       4.4.2.  Signature Validation Periods . . . . . . . . . . . . . 39  Maximum Value  . . . . . . . . . . . . . . . . . . 39  Minimum Value  . . . . . . . . . . . . . . . . . . 39  differentiation between RR sets  . . . . . . . . . 41  Other timing parameters in a zone  . . . . . . . . 42
   5.  Next Record type . . . . . . . . . . . . . . . . . . . . . . . 42
     5.1.  Differences between  NSEC and NSEC3  . . . . . . . . . . . 42
     5.2.  NSEC or NSEC3  . . . . . . . . . . . . . . . . . . . . . . 43
     5.3.  NSEC3 parameters . . . . . . . . . . . . . . . . . . . . . 44
       5.3.1.  NSEC3 Algorithm  . . . . . . . . . . . . . . . . . . . 44
       5.3.2.  NSEC3 Iterations . . . . . . . . . . . . . . . . . . . 44
       5.3.3.  NSEC3 Salt . . . . . . . . . . . . . . . . . . . . . . 44
       5.3.4.  Opt-out  . . . . . . . . . . . . . . . . . . . . . . . 45
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 45
   7.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 45
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 45
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 46
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 47
   Appendix A.  Terminology . . . . . . . . . . . . . . . . . . . . . 48
   Appendix B.  Zone Signing Key Rollover How-To  . . . . . . . . . . 50
   Appendix C.  Typographic Conventions . . . . . . . . . . . . . . . 50
   Appendix D.  Document Editing History  . . . . . . . . . . . . . . 52

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     D.1.  draft-ietf-dnsop-rfc4641-00  . . . . . . . . . . . . . . . 52
     D.2.  version 0->1 . . . . . . . . . . . . . . . . . . . . . . . 52
     D.3.  version 1->2 . . . . . . . . . . . . . . . . . . . . . . . 53
     D.4.  version 2->3 . . . . . . . . . . . . . . . . . . . . . . . 54

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1.  Introduction

   This document describes how to run a DNS Security (DNSSEC)-enabled
   environment.  It is intended for operators who have knowledge of the
   DNS (see RFC 1034 [1] and RFC 1035 [2]) and want to deploy DNSSEC.
   The focus of the document is on serving authoritative DNS information
   and concerns zone owners, name server operators, registry, registrar
   and registrants whereby it is assumed that there is no direct
   relation between those entities and the operators of validating
   recursive name servers (validators).  See RFC 4033 [3] for an
   introduction to DNSSEC, RFC 4034 [4] for the newly introduced
   Resource Records (RRs), and RFC 4035 [5] for the protocol changes.

   During workshops and early operational deployment, operators and
   system administrators have gained experience about operating the DNS
   with security extensions (DNSSEC).  This document translates these
   experiences into a set of practices for zone administrators.  At the
   time of writing -the root is being signed and the first secure
   delegations are provisioned- there exists relatively little
   experience with DNSSEC in production environments; this document
   should therefore explicitly not be seen as representing 'Best Current
   Practices'.  The document therefore targets to provide a number of
   choices with the considerations that motivate a particular choice.
   It then gives the operational guidelines that relate to a choice.
   The document does not give strong recommendations, that may be
   subject for a future version of this document.  [OK: This is really a
   straw-man and causes a difference in tone that I believe was the
   instruction of the WG during the IETF 77 meeting.  The document could
   be made much shorter when particular recommendations are made?  Is
   there a general consensus that we should currently not make
   particular recommendations?]

   The procedures herein are focused on the maintenance of signed zones
   (i.e., signing and publishing zones on authoritative servers).  It is
   intended that maintenance of zones such as re-signing or key
   rollovers be transparent to any verifying clients on the Internet.

   The structure of this document is as follows.  In Section 2, we
   discuss the importance of keeping the "chain of trust" intact.
   Aspects of key generation and storage of private keys are discussed
   in Section 3; the focus in this section is mainly on the private part
   of the key(s).  Section 4 describes considerations concerning the
   public part of the keys.  Since these public keys appear in the DNS
   one has to take into account all kinds of timing issues, which are
   discussed in Section 4.4.  Section 4.1 and Section 4.2 deal with the
   rollover, or supercession, of keys.  Finally, Section 4.3 discusses
   considerations on how parents deal with their children's public keys
   in order to maintain chains of trust.

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   The typographic conventions used in this document are explained in
   Appendix C.

   Since this is a document with operational suggestions and there are
   no protocol specifications, the RFC 2119 [6] language does not apply.

   This document [OK: when approved] obsoletes RFC 4641 [15].

   [OK: Editorial comments and questions are indicated by square
   brackets and editor innitials]

1.1.  The Use of the Term 'key'

   It is assumed that the reader is familiar with the concept of
   asymmetric keys on which DNSSEC is based (public key cryptography
   RFC4949 [16]).  Therefore, this document will use the term 'key'
   rather loosely.  Where it is written that 'a key is used to sign
   data' it is assumed that the reader understands that it is the
   private part of the key pair that is used for signing.  It is also
   assumed that the reader understands that the public part of the key
   pair is published in the DNSKEY Resource Record and that it is the
   public part that is used in key exchanges.

1.2.  Time Definitions

   In this document, we will be using a number of time-related terms.
   The following definitions apply:

   o  "Signature validity period" The period that a signature is valid.
      It starts at the time specified in the signature inception field
      of the RRSIG RR and ends at the time specified in the expiration
      field of the RRSIG RR.

   o  "Signature publication period" Time after which a signature (made
      with a specific key) is replaced with a new signature (made with
      the same key).  This replacement takes place by publishing the
      relevant RRSIG in the master zone file.  After one stops
      publishing an RRSIG in a zone, it may take a while before the
      RRSIG has expired from caches and has actually been removed from
      the DNS.

   o  "Key effectivity period" The period during which a key pair is
      expected to be effective.  This period is defined as the time
      between the first inception time stamp and the last expiration
      date of any signature made with this key, regardless of any
      discontinuity in the use of the key.  The key effectivity period
      can span multiple signature validity periods.

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   o  "Maximum/Minimum Zone Time to Live (TTL)" The maximum or minimum
      value of the TTLs from the complete set of RRs in a zone.  Note
      that the minimum TTL is not the same as the MINIMUM field in the
      SOA RR.  See RFC2308 [9] for more information.

2.  Keeping the Chain of Trust Intact

   Maintaining a valid chain of trust is important because broken chains
   of trust will result in data being marked as Bogus (as defined in
   RFC4033 [3] Section 5), which may cause entire (sub)domains to become
   invisible to verifying clients.  The administrators of secured zones
   have to realize that their zone is, to verifying clients, part of a
   chain of trust.

   As mentioned in the introduction, the procedures herein are intended
   to ensure that maintenance of zones, such as re-signing or key
   rollovers, will be transparent to the verifying clients on the

   Administrators of secured zones will have to keep in mind that data
   published on an authoritative primary server will not be immediately
   seen by verifying clients; it may take some time for the data to be
   transferred to other secondary authoritative nameservers and clients
   may be fetching data from caching non-authoritative servers.  In this
   light, note that the time for a zone transfer from master to slave is
   negligible when using NOTIFY [8] and incremental transfer (IXFR) [7].
   It increases when full zone transfers (AXFR) are used in combination
   with NOTIFY.  It increases even more if you rely on full zone
   transfers based on only the SOA timing parameters for refresh.

   For the verifying clients, it is important that data from secured
   zones can be used to build chains of trust regardless of whether the
   data came directly from an authoritative server, a caching
   nameserver, or some middle box.  Only by carefully using the
   available timing parameters can a zone administrator ensure that the
   data necessary for verification can be obtained.

   The responsibility for maintaining the chain of trust is shared by
   administrators of secured zones in the chain of trust.  This is most
   obvious in the case of a 'key compromise' when a trade-off between
   maintaining a valid chain of trust and replacing the compromised keys
   as soon as possible must be made.  Then zone administrators will have
   to make a trade-off, between keeping the chain of trust intact --
   thereby allowing for attacks with the compromised key -- or
   deliberately breaking the chain of trust and making secured
   subdomains invisible to security-aware resolvers.  Also see
   Section 4.2.

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3.  Keys Generation and Storage

   This section describes a number of considerations with respect to the
   use of keys.  For the desgin of a operational procedure for key
   generation and storage the following decisions need to be taken in

   o  Does one differentiate between Zone Signing and Key Signing Keys
      or is the use of one type of key sufficient;

   o  Are Key Signing Keys (likely to be) in use as Trust Anchors;

   o  What are the timing parameters that are allowed by the operational
      requirements; and finally

   o  What are the cryptographic parameters that fit the operational

   Below are the considerations that need to be taken into account when
   making decisions.

3.1.  Operational motivation for ZSKs and KSKs

   The DNSSEC validation protocol does not distinguish between different
   types of DNSKEYs.  All DNSKEYs can be used during the validation.
   The motivations to differentiate between keys are purely operational.

   For operational reasons, motivated below, it is possible to designate
   one or more keys as Key Signing Keys (KSKs).  These keys will only
   sign the apex DNSKEY RRSet in a zone.  Other keys can be used to sign
   all the RRSets in a zone and are referred to as Zone Signing Keys
   (ZSKs).  In case the differentiation between KSK and ZSK is not made
   we talk about a Single Type signing scheme

3.1.1.  Motivations for the KSK and ZSK Separation

   Differentiating between the KSK and ZSK is mostly done for
   operational purposes whereby

   If the two functions are separated then, for almost any method of key
   management and zone signing, the KSK is used less frequently then the
   ZSK.  Once a key set is signed with the KSK, all the keys in the key
   set can be used as ZSKs.  If there has been an event that increases
   the risk that a ZSK is compromised it can be simply dropped from the
   key set.  The new key set is then re-signed with the KSK.

   Changing a key with secure entry point (SEP) functionality can be
   relatively expensive as it involves interaction with 3rd parties:

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   When a key is only pointed to by the parental DS one needs to
   complete the interaction with the parental registry and wait for the
   transaction to appear in the DNS.  In the case that a SEP key that is
   in use as a trust-anchor one has to wait until one has sufficient
   confidence that all trust anchors have been replaced.  In fact, it
   may be that one is not able to reach the complete user-base with
   information about the key rollover.

   There is also a risk that keys are compromised through theft or loss.
   For keys that are needed to sign dynamically updated records and are
   therefore installed on file-systems of nameservers that are connected
   to the network that risk is relatively high.  For keys that are
   stored on Hardware Signing Modules such risk is relatively low.  By
   separating the KSK and ZSK functionality these risks can be managed
   while making the tradeoff against the costs involved.  For example, a
   KSK can be stored off-line or with more limitation on access control
   than ZSKs which need to be readily available for operational purposes
   such as the addition or deletion of zone data.  More concretely a KSK
   stored on a smartcard, that is kept in a safe, combinded with a ZSK
   stored on a filesystem accessible by operators for daily routine may
   provide more operational flexibility and higher computational
   performance than a single key (with combined KSK and ZSK
   functionality) stored on an HSM.

   [OK: The following paragraph is new in version 3 of the draft, does
   it need to be more detailed e.g. by referring to RFC 3766?  Also the
   argument is made on http://www.educatedguesswork.org/2009/10/
   on_the_security_of_zsk_rollove.html, not sure about how about
   providing a stable reference to that.  Should we add an informative
   reference here? ]

   Finally there is a risk of cryptanalysis of the key material.  The
   costs of such analysis are correlated to the length of the key.
   However, cryptanalysis arguments provide no strong motivation for a
   KSK/ZSK split.  Suppose one differentiates between a KSK and a ZSK
   whereby the KSK effectivity period is X times the ZSK effectivity
   period.  Then, in order for the resistance to cryptanalysis to be the
   same for the KSK and the ZSK, the KSK needs to be X times stronger
   than the ZSK.  Since for all practical purposes X will somewhere of
   the order of 10 to 100 the associated key sizes will vary only about
   a byte in size for symmetric keys.  Which, translated to asymmetric
   keys, is still an insignificant enough size difference to warrant a
   key-split; not for packet size or signing speed arguments.

   The arguments for differentiation between the ZSK and KSK are weakest

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      the exposure to risk is low (e.g. when keys are stored on HSMs);

      if one can be certain that a key is not used as a trust-anchor;

      maintenance of the various keys cannot be performed through tools
      (is prone to human error); and

      the interaction through the registrar-registry provisioning chain
      -- in particular the timely appearance of a new DS record in the
      parent zone in emergency situations -- is predictable.

   If the above holds then the costs of the operational complexity of a
   KSK-ZSK split may outweigh the costs of operational flexibility and
   choosing for a single type signing scheme is then a reasonable
   option.  In other cases we advise that the separation between KSKs
   and ZSKs is made and that the SEP flag is exclusively set on KSKs.

3.1.2.  Practical concequences of KSK and ZSK Separation

   Given the assumption that for KSKs the SEP flag is set, the KSK can
   be distinguished from a ZSK by examining the flag field in the DNSKEY
   RR: If the flag field is an odd number it is a KSK.  If it is an even
   number it is a ZSK.

   The Zone Signing Key can be used to sign all the data in a zone on a
   regular basis.  When a Zone Signing Key is to be rolled, no
   interaction with the parent is needed.  This allows for signature
   validity periods on the order of days.

   The Key Signing Key is only to be used to sign the DNSKEY RRs in a
   zone.  If a Key Signing Key is to be rolled over, there will be
   interactions with parties other than the zone administrator.  If
   there is a parent zone, these can include the registry of the parent
   zone or administrators of verifying resolvers that have the
   particular key configured as secure entry points.  If this is a trust
   anchor, everyone relying on the trust anchor needs to roll over to
   the new key.  The latter may be subject to stability costs if
   automated trust-anchor rollover mechanisms (such as e.g.  RFC5011
   [17]) are not in place.  Hence, the key effectivity period of these
   keys can and should be made much longer.  Rolling a KSK that is not a trust-anchor

   There are 3 schools of thought on rolling a KSK that is not a trust

   o  It should be done frequently and regularly (possibly every few
      months) so that a key rollover remains an operational routine.

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   o  It should be done frequently but irregularly.  Frequently meaning
      every few monthts, again based on the argument that a rollover is
      a practiced and common operational routine, but irregular, i.e.
      with a large jitter, so that 3rd parties do not start to rely on
      the key and will not be tempted to configure those as a trust-

   o  It should only be done when it is known or strongly suspected that
      the key can be or has been compromised.

   There is no widespread agreement on which of these three schools of
   thought is better for different deployments of DNSSEC.  There is a
   stability cost every time a non-anchor KSK is rolled over, but it is
   possibly low if the communication between the child and the parent is
   good.  On the other hand, the only completely effective way to tell
   if the communication is good is to test it periodically.  Thus,
   rolling a KSK with a parent is only done for two reasons: to test and
   verify the rolling system to prepare for an emergency, and in the
   case of (preventing) an actual emergency.

   Finally, a zone ower can, in most cases, not be fully certain that
   the zone's KSK is not in use as a trust-anchor and while the
   configuration of trust-anchors is not the responsibility of the zone
   owner there may be stabilty costs for the validator administrator
   that (wrongfully) configured the trust-anchor when the zone owner
   roles a KSK.  Rolling a KSK that is a trust-anchor

   The same operational concerns apply to the rollover of KSKs that are
   used as trust-anchors.  But remember: if a trust anchor replacement
   is done incorrectly, the entire zone that the trust anchor covers
   will become bogus until the trust anchor is corrected.

   In a large number of cases it will be safe to work from the
   assumption that ones keys are not in use as trust-anchors.  If a zone
   owner publishes a "DNSSEC Signing Policy and Practice Statement" [25]
   that should be explicit about the fact whether the existence of trust
   anchors will be taken into account in any way or not.  There may be
   cases where local policies enforce the configuration of trust-anchors
   on zones which are mission critical (e.g. in enterprises where the
   trust-anchor for the enterprise domain is configured in the
   enterprise's validator) It is expected that the zone owners are aware
   of such circumstances.

   One can argue that because of the difficulty of getting all users of
   a trust anchor to replace an old trust anchor with a new one, a KSK
   that is a trust anchor should never be rolled unless it is known or

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   strongly suspected that the key has been compromised.  In other words
   the costs of a KSK rollover are prohibitively high because some users
   cannot be reached.

   However, the "operational habit" argument also applies to trust
   anchor reconfiguration at the clients validator.  If a short key
   effectivity period is used and the trust anchor configuration has to
   be revisited on a regular basis, the odds that the configuration
   tends to be forgotten is smaller.  In fact, the costs for those users
   can be minimized by automating the rollover RFC5011 [17] and by
   rolling the key regularly, and advertising such, so that the
   operators of recursive nameservers will put the appropriate mechanism
   in place to deal with these stability costs, or, in other words,
   budget for these costs instead of incuring them unexpectedly.

   It is therefore recommended to roll KSKs (that are likely to be used
   as trust-anchors) if and only if those rollovers can be tracked using
   standardized (e.g.  RFC5011) mechanisms.  The use of the SEP flag

   The so-called Secure Entry Point (SEP) [5] flag can be used to
   distinguish between keys that are intended to be used as the secure
   entry point into the zone when building chains of trust, e.g they are
   (to be) pointed to by parental DS RRs or configured as a trust-

   While the SEP flag does not play any role in the validation failure
   it is used in parctice for operational purposes such as for the
   rollover mechanism discribed in RFC5011 [17].  The comon convention
   is to set the SEP flag on any key that is used for key exchanges with
   the parent and potentially for configuration as trust anchors.
   Therefore it is recomended that the SEP flag is set on KSKs and not
   on ZSKs while in those cases that a distinction between KSK and ZSK
   is not made (i.e. for a Single Type signing scheme) the SEP flag is
   recommended be set on all keys.

   Note that signing tools may assume a KSK/ZSK split and use the (non)
   presence of the SEP flag to determine which key is to be used for
   signing zone data, these tools may get confused when a single type
   signing scheme is used.

3.1.3.  Key Effectivity Period

   In general the available key lenght sets an upper limit on the Key
   Effectivity Period.  For all practical purposes it is sufficient to
   define the Key Effectivity Period based on purely operational
   requirements and match the keylength to that value; Ignoring the

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   operational perspective, a reasonable effectivity period for KSKs
   that have a parent zone is of the order of 2 decades or longer.  That
   is, if one does not plan to test the rollover procedure, the key
   should be effective essentially forever, and then only rolled over in
   case of emergency.

   When one chooses for a regular key-rollover, a reasonable key
   effectivity period for KSKs that have a parent zone is 13 months,
   with the intent to replace them after 12 months.  As argued above,
   this annual rollover gives operational practice to rollovers for both
   the zone as the validator administrators.  Besided, in most
   environments a year is a timespan that is easily planned and

   In case keys are stored on on-line systems and the exposure to
   various threats of compromise is fairly high an intended key
   effectivity period of a month is reasonable for Zone Signing Keys.

   Key effectivity periods can be made very short, as in a few minutes.
   But when replacing keys one has to take the considerations from
   Section 4.4 and Section 4.1 into account.

   The motivation for having the ZSK's effectivity period shorter than
   the KSK's effectivity period is rooted in the operational
   consideration that it is more likely that operators have more
   frequent read access to the ZSK than to the KSK.  If ZSK's are
   maintained on cryptographic Hardware Security Modules (HSM) than the
   motivation to have different key effectivity periods is weakend.

   However, if the risk of loss, theft or other compromise is the same
   for a zone and key signing key there are little arguments to choose
   different effectivity periods for ZSKs and KSKs.  And in fact, when
   the split between ZSKs and KSKs is not made than all considerations
   above apply.

   There are certainly cases (e.g. where the the costs and risk of
   compromise, and the costs and risks involved with having to perform
   an emergency roll are also low) that the use of a single type signing
   scheme with a long key effectivity period is a good choice.

3.1.4.  Cryptographic Considerations  Key Algorithm

   There are currently two types of signature algorithms that can be
   used in DNSSEC: RSA and DSA.  Both are fully specified in many
   freely-available documents, and both are widely considered to be
   patent-free.  The creation of signatures wiht RSA and DSA takes

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   roughly the same time, but DSA is about ten times slower for
   signature verification.

   We suggest the use of RSA/SHA-256 as the preferred signature
   algorithms and RSA/SHA-1 as an alternative.  Both have advantages and
   disadvantages.  RSA/SHA-1 has been deployed for many years, while
   RSA/SHA-256 has only begun to be deployed.  On the other hand, it is
   expected that if effective attacks on either algorithm appeark, they
   will appear for RSA/SHA-1 first.  RSA/MD5 should not be considered
   for use because RSA/MD5 will very likely be the first common-use
   signature algorithm to have an effective attack.

   At the time of publication, it is known that the SHA-1 hash has
   cryptanalysis issues.  There is work in progress on addressing these
   issues.  We recommend the use of public key algorithms based on
   hashes stronger than SHA-1 (e.g., SHA-256), as soon as these
   algorithms are available in protocol specifications (see RFC5702 [23]
   and RFC4509 [20]) and implementations.  Key Sizes

   DNSSEC signing keys should be large enough to avoid all know
   cryptographic attacks during the lifetime of the key.  To date,
   despite huge efforts, no one has broken a regular 1024-bit key; in
   fact, the best completed attack is estimated to be the equivalent of
   a 700-bit key.  An attacker breaking a 1024-bit signing key would
   need expend phenominal amounts of networked computing power in a way
   that would not be detected in order to break a single key.  Because
   of this, it is estimated that most zones can safely use 1024-bit keys
   for at least the next ten years.  A 1024-bit asymmetric key has an
   approximate equivalent strength of a symmetric 80-bit key.

   Keys that are used as extremely high value trust anchors, or non-
   anchor keys that may be difficult to roll over, may want to use
   lengths longer than 1024 bits.  Typically, the next larger key size
   used is 2048 bits, which have the approximate equivalent strength of
   a symmetric 112-bit key.  In a standard CPU, it takes about four
   times as long to sign or verify with a 2048-bit key as it does with a
   1024-bit key.

   Another way to decide on the size of key to use is to remember that
   the phenominal effort it takes for an attacker to break a 1024-bit
   key is the same regardless of how the key is used.  If an attacker
   has the capability of breaking a 1024-bit DNSSEC key, he also has the
   capability of breaking one of the many 1024-bit TLS trust anchor keys
   that are installed with web browsers.  If the value of a DNSSEC key
   is lower to the attacker than the value of a TLS trust anchor, the
   attacker will use the resources to attack the TLS trust anchor.

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   It is possible that there is a unexpected improvement in the ability
   for attackers to beak keys, and that such an attack would make it
   feasible to break 1024-bit keys but not 2048-bit keys.  If such an
   improvement happens, it is likely that there will be a huge amount of
   publicity, particularly because of the large number of 1024-bit TLS
   trust anchors build into popular web browsers.  At that time, all
   1024-bit keys (both ones with parent zones and ones that are trust
   anchors) can be rolled over and replaced with larger keys.

   Earlier documents (including the previous version of this document)
   urged the use of longer keys in situations where a particular key was
   "heavily used".  That advice may have been true 15 years ago, but it
   is not true today when using RSA or DSA algorithms and keys of 1024
   bits or higher.  Private Key Storage

   It is recommended that, where possible, zone private keys and the
   zone file master copy that is to be signed be kept and used in off-
   line, non-network-connected, physically secure machines only.
   Periodically, an application can be run to add authentication to a
   zone by adding RRSIG and NSEC RRs.  Then the augmented file can be

   When relying on dynamic update [10] to manage a signed zone, be aware
   that at least one private key of the zone will have to reside on the
   master server.  This key is only as secure as the amount of exposure
   the server receives to unknown clients and the security of the host.
   Although not mandatory, one could administer the DNS in the following
   way.  The master that processes the dynamic updates is unavailable
   from generic hosts on the Internet, it is not listed in the NS RRSet,
   although its name appears in the SOA RRs MNAME field.  The
   nameservers in the NS RRSet are able to receive zone updates through
   NOTIFY, IXFR, AXFR, or an out-of-band distribution mechanism.  This
   approach is known as the "hidden master" setup.

   The ideal situation is to have a one-way information flow to the
   network to avoid the possibility of tampering from the network.
   Keeping the zone master file on-line on the network and simply
   cycling it through an off-line signer does not do this.  The on-line
   version could still be tampered with if the host it resides on is
   compromised.  For maximum security, the master copy of the zone file
   should be off-net and should not be updated based on an unsecured
   network mediated communication.

   The ideal situation may not be achievable because of economic
   tradeoffs between risks and costs.  For instance, keeping a zone file
   off-line is not practical and will increase the costs of operating a

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   DNS zone.  So in practice the machines on which zone files are
   maintained will be connected to a network.  Operators are advised to
   take security measures to shield unauthorized access to the master
   copy in order to prevent modification of DNS data before its signed.

   Similarly the choice for storing a private key in a HSM will be
   influenced by a tradeoff between various concerns:

   o  The risks that an unauthorized person has unnoticed read-access to
      the private key

   o  The remaining window of opportunity for the attacker.

   o  The economic impact of the possible attacks (for a TLD that impact
      will in most cases be higher than for an individual users).

   o  The costs of rolling the (compromised) keys: whereby the costs of
      roling a ZSK is lowest and the costs of rolling a KSK that is in
      wide use as a trust anchor is highest

   o  The costs of buying and maintaining an HSM.

   For dynamically updated secured zones [10], both the master copy and
   the private key that is used to update signatures on updated RRs will
   need to be on-line.  Key Generation

   Careful generation of all keys is a sometimes overlooked but
   absolutely essential element in any cryptographically secure system.
   The strongest algorithms used with the longest keys are still of no
   use if an adversary can guess enough to lower the size of the likely
   key space so that it can be exhaustively searched.  Technical
   suggestions for the generation of random keys will be found in RFC
   4086 [13] and NIST SP 800-900 [19].  One should carefully assess if
   the random number generator used during key generation adheres to
   these suggestions.

   Keys with a long effectivity period are particularly sensitive as
   they will represent a more valuable target and be subject to attack
   for a longer time than short-period keys.  It is strongly recommended
   that long-term key generation occur off-line in a manner isolated
   from the network via an air gap or, at a minimum, high-level secure
   hardware.  Differentiation for 'High-Level' Zones?

   In an earlier version of this document (RFC4641 [15]) we made a

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   differentiation between KSKs used for zones that are high in the DNS
   hierarchy versus KSKs used for zones low in that hierarchy.

   Longer keys are not useful because the crypto guidance is that
   everyone should use keys that no one can break.  Also, it is
   impossible to judge which zones are more or less valuable to an
   attacker.  An attack can only be used if the compromise is unnoticed
   and the attacker can act as an man-in-the-middle attack (MITM) in an
   unnoticed way.  If example. is compromised and the attacker forges
   answers for somebank.example. and sends them out as an MITM, when the
   attack is discovered it will be simple to prove that example. has
   been compromised and the KSK will be rolled.  Defining a long-term
   successful attack is difficult for keys at any level.

4.  Signature Generation, Key Rollover, and Related Policies

4.1.  Key Rollovers

   Regardless of whether a zone uses periodic key rollovers in order to
   practice for emergencies, or only rolls over keys in an emergency,
   key rollovers are a fact of life when using DNSSEC.  Zone
   administrators who are in the process of rolling their keys have to
   take into account that data published in previous versions of their
   zone still lives in caches.  When deploying DNSSEC, this becomes an
   important consideration; ignoring data that may be in caches may lead
   to loss of service for clients.

   The most pressing example of this occurs when zone material signed
   with an old key is being validated by a resolver that does not have
   the old zone key cached.  If the old key is no longer present in the
   current zone, this validation fails, marking the data "Bogus".
   Alternatively, an attempt could be made to validate data that is
   signed with a new key against an old key that lives in a local cache,
   also resulting in data being marked "Bogus".

4.1.1.  Zone Signing Key Rollovers

   If the choice for splitting zone and key signing keys has been made
   than those two types of keys can be rolled seperatly and zone signing
   keys can be rolled without taking into acount DS records from the
   parent or the configuration of such a key as trust-anchor.

   For "Zone Signing Key rollovers", there are two ways to make sure
   that during the rollover data still cached can be verified with the
   new key sets or newly generated signatures can be verified with the
   keys still in caches.  One schema, described in Section, uses
   double signatures; the other uses key pre-publication
   (Section  The pros, cons, and recommendations are described

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   in Section  Pre-Publish Key Rollover

   This section shows how to perform a ZSK rollover without the need to
   sign all the data in a zone twice -- the "pre-publish key rollover".
   This method has advantages in the case of a key compromise.  If the
   old key is compromised, the new key has already been distributed in
   the DNS.  The zone administrator is then able to quickly switch to
   the new key and remove the compromised key from the zone.  Another
   major advantage is that the zone size does not double, as is the case
   with the double signature ZSK rollover.  A small "how-to" for this
   kind of rollover can be found in Appendix B.

   Pre-publish key rollover involves four stages as follows:

    initial         new DNSKEY       new RRSIGs      DNSKEY removal
    SOA0            SOA1             SOA2            SOA3
    RRSIG10(SOA0)   RRSIG10(SOA1)    RRSIG11(SOA2)   RRSIG11(SOA3)

    DNSKEY1         DNSKEY1          DNSKEY1         DNSKEY1
    DNSKEY10        DNSKEY10         DNSKEY10        DNSKEY11
                    DNSKEY11         DNSKEY11

   Pre-Publish Key Rollover

   initial:  Initial version of the zone: DNSKEY 1 is the Key Signing
      Key. DNSKEY 10 is used to sign all the data of the zone, the Zone
      Signing Key.

   new DNSKEY:  DNSKEY 11 is introduced into the key set.  Note that no
      signatures are generated with this key yet, but this does not
      secure against brute force attacks on the public key.  The minimum
      duration of this pre-roll phase is the time it takes for the data
      to propagate to the authoritative servers plus TTL value of the
      key set.

   new RRSIGs:  At the "new RRSIGs" stage (SOA serial 2), DNSKEY 11 is
      used to sign the data in the zone exclusively (i.e., all the
      signatures from DNSKEY 10 are removed from the zone).  DNSKEY 10
      remains published in the key set.  This way data that was loaded
      into caches from version 1 of the zone can still be verified with
      key sets fetched from version 2 of the zone.  The minimum time

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      that the key set including DNSKEY 10 is to be published is the
      time that it takes for zone data from the previous version of the
      zone to expire from old caches, i.e., the time it takes for this
      zone to propagate to all authoritative servers plus the Maximum
      Zone TTL value of any of the data in the previous version of the

   DNSKEY removal:  DNSKEY 10 is removed from the zone.  The key set,
      now only containing DNSKEY 1 and DNSKEY 11, is re-signed with the
      DNSKEY 1.

   The above scheme can be simplified by always publishing the "future"
   key immediately after the rollover.  The scheme would look as follows
   (we show two rollovers); the future key is introduced in "new DNSKEY"
   as DNSKEY 12 and again a newer one, numbered 13, in "new DNSKEY

       initial             new RRSIGs          new DNSKEY
       SOA0                SOA1                SOA2
       RRSIG10(SOA0)       RRSIG11(SOA1)       RRSIG11(SOA2)

       DNSKEY1             DNSKEY1             DNSKEY1
       DNSKEY10            DNSKEY10            DNSKEY11
       DNSKEY11            DNSKEY11            DNSKEY12

       new RRSIGs (II)     new DNSKEY (II)
       SOA3                SOA4
       RRSIG12(SOA3)       RRSIG12(SOA4)

       DNSKEY1             DNSKEY1
       DNSKEY11            DNSKEY12
       DNSKEY12            DNSKEY13

   Pre-Publish Key Rollover, Showing Two Rollovers

   Note that the key introduced in the "new DNSKEY" phase is not used
   for production yet; the private key can thus be stored in a
   physically secure manner and does not need to be 'fetched' every time

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   a zone needs to be signed.  Double Signature Zone Signing Key Rollover

   This section shows how to perform a ZSK key rollover using the double
   zone data signature scheme, aptly named "double signature rollover".

   During the "new DNSKEY" stage the new version of the zone file will
   need to propagate to all authoritative servers and the data that
   exists in (distant) caches will need to expire, requiring at least
   the Maximum Zone TTL.

   Double signature ZSK rollover involves three stages as follows:

      initial             new DNSKEY         DNSKEY removal
      SOA0                SOA1               SOA2
      RRSIG10(SOA0)       RRSIG10(SOA1)      RRSIG11(SOA2)
      DNSKEY1             DNSKEY1            DNSKEY1
      DNSKEY10            DNSKEY10           DNSKEY11

   Double Signature Zone Signing Key Rollover

   initial:  Initial Version of the zone: DNSKEY 1 is the Key Signing
      Key. DNSKEY 10 is used to sign all the data of the zone, the Zone
      Signing Key.

   new DNSKEY:  At the "New DNSKEY" stage (SOA serial 1) DNSKEY 11 is
      introduced into the key set and all the data in the zone is signed
      with DNSKEY 10 and DNSKEY 11.  The rollover period will need to
      continue until all data from version 0 of the zone has expired
      from remote caches.  This will take at least the Maximum Zone TTL
      of version 0 of the zone.

   DNSKEY removal:  DNSKEY 10 is removed from the zone.  All the
      signatures from DNSKEY 10 are removed from the zone.  The key set,
      now only containing DNSKEY 11, is re-signed with DNSKEY 1.

   At every instance, RRSIGs from the previous version of the zone can
   be verified with the DNSKEY RRSet from the current version and the
   other way around.  The data from the current version can be verified

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   with the data from the previous version of the zone.  The duration of
   the "new DNSKEY" phase and the period between rollovers should be at
   least the Maximum Zone TTL.

   Making sure that the "new DNSKEY" phase lasts until the signature
   expiration time of the data in the initial version of the zone is
   recommended.  This way all caches are cleared of the old signatures.
   However, this duration could be considerably longer than the Maximum
   Zone TTL, making the rollover a lengthy procedure.

   Note that in this example we assumed that the zone was not modified
   during the rollover.  New data can be introduced in the zone as long
   as it is signed with both keys.  Pros and Cons of the Schemes

   Pre-publish key rollover:  This rollover does not involve signing the
      zone data twice.  Instead, before the actual rollover, the new key
      is published in the key set and thus is available for
      cryptanalysis attacks.  A small disadvantage is that this process
      requires four steps.  Also the pre-publish scheme involves more
      parental work when used for KSK rollovers as explained in
      Section 4.1.3.

   Double signature ZSK rollover:  The drawback of this signing scheme
      is that during the rollover the number of signatures in your zone
      doubles; this may be prohibitive if you have very big zones.  An
      advantage is that it only requires three steps.

4.1.2.  Key Signing Key Rollovers

   For the rollover of a Key Signing Key, the same considerations as for
   the rollover of a Zone Signing Key apply.  However, we can use a
   double signature scheme to guarantee that old data (only the apex key
   set) in caches can be verified with a new key set and vice versa.
   Since only the key set is signed with a KSK, zone size considerations
   do not apply.

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       initial        new DNSKEY        DS change       DNSKEY removal
       SOA0           -------->         SOA1            -------->
       RRSIGpar(SOA0) -------->         RRSIGpar(SOA1)  -------->
       DS1            -------->         DS2             -------->
       RRSIGpar(DS)   -------->         RRSIGpar(DS)    -------->

       SOA0            SOA1             -------->       SOA2
       RRSIG10(SOA0)   RRSIG10(SOA1)    -------->       RRSIG10(SOA2)
       DNSKEY1         DNSKEY1          -------->       DNSKEY2
                       DNSKEY2          -------->
       DNSKEY10        DNSKEY10         -------->       DNSKEY10
       RRSIG1 (DNSKEY) RRSIG1 (DNSKEY)  -------->       RRSIG2 (DNSKEY)
                       RRSIG2 (DNSKEY)  -------->
       RRSIG10(DNSKEY) RRSIG10(DNSKEY)  -------->       RRSIG10(DNSKEY)

   Stages of Deployment for a Double Signature Key Signing Key Rollover

   initial:  Initial version of the zone.  The parental DS points to
      DNSKEY1.  Before the rollover starts, the child will have to
      verify what the TTL is of the DS RR that points to DNSKEY1 -- it
      is needed during the rollover and we refer to the value as TTL_DS.

   new DNSKEY:  During the "new DNSKEY" phase, the zone administrator
      generates a second KSK, DNSKEY2.  The key is provided to the
      parent, and the child will have to wait until a new DS RR has been
      generated that points to DNSKEY2.  After that DS RR has been
      published on all servers authoritative for the parent's zone, the
      zone administrator has to wait at least TTL_DS to make sure that
      the old DS RR has expired from caches.

   DS change:  The parent replaces DS1 with DS2.

   DNSKEY removal:  DNSKEY1 has been removed.

   The scenario above puts the responsibility for maintaining a valid
   chain of trust with the child.  It also is based on the premise that
   the parent only has one DS RR (per algorithm) per zone.  An
   alternative mechanism has been considered.  Using an established
   trust relation, the interaction can be performed in-band, and the
   removal of the keys by the child can possibly be signaled by the
   parent.  In this mechanism, there are periods where there are two DS

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   RRs at the parent.  Since at the moment of writing the protocol for
   this interaction has not been developed, further discussion is out of
   scope for this document.

   The scenario scetched above assumes that the KSK is not in use as a
   trust-anchor too.  If that is the case then special care need to be
   taken.  For instance, when RFC5011 type rollover is in use then the
   DNSKEY1 removal phase above is the moment that the revoke flag is set
   on DNSKEY1 while it is still published, at least as long as the
   RFC5011 holdback timer proscribes.  Only after that timer expired
   DNSKEY1 can be removed.

4.1.3.  Difference Between ZSK and KSK Rollovers

   Note that KSK rollovers and ZSK rollovers are different in the sense
   that a KSK rollover requires interaction with the parent (and
   possibly replacing of trust anchors) and the ensuing delay while
   waiting for it.

   A zone key rollover can be handled in two different ways: pre-publish
   (Section and double signature (Section

   As the KSK is used to validate the key set and because the KSK is not
   changed during a ZSK rollover, a cache is able to validate the new
   key set of the zone.  The pre-publish method would also work for a
   KSK rollover.  The records that are to be pre-published are the
   parental DS RRs.  The pre-publish method has some drawbacks for KSKs.
   We first describe the rollover scheme and then indicate these

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     initial         new DS           new DNSKEY      DS/DNSKEY removal
     SOA0            SOA1             -------->       SOA2
     RRSIGpar(SOA0)  RRSIGpar(SOA1)   -------->       RRSIGpar(SOA2)
     DS1             DS1              -------->       DS2
                     DS2              -------->
     RRSIGpar(DS)    RRSIGpar(DS)     -------->       RRSIGpar(DS)

     SOA0            -------->        SOA1            SOA1
     RRSIG10(SOA0)   -------->        RRSIG10(SOA1)   RRSIG10(SOA1)
     DNSKEY1         -------->        DNSKEY2         DNSKEY2
     DNSKEY10        -------->        DNSKEY10        DNSKEY10
     RRSIG1 (DNSKEY) -------->        RRSIG2(DNSKEY)  RRSIG2 (DNSKEY)
     RRSIG10(DNSKEY) -------->        RRSIG10(DNSKEY) RRSIG10(DNSKEY)

   Stages of Deployment for a Pre-Publish Key Signing Key Rollover

   When the child zone wants to roll, it notifies the parent during the
   "new DS" phase and submits the new key (or the corresponding DS) to
   the parent.  The parent publishes DS1 and DS2, pointing to DNSKEY1
   and DNSKEY2, respectively.  During the rollover ("new DNSKEY" phase),
   which can take place as soon as the new DS set propagated through the
   DNS, the child replaces DNSKEY1 with DNSKEY2.  Immediately after that
   ("DS/DNSKEY removal" phase), it can notify the parent that the old DS
   record can be deleted.

   The drawbacks of this scheme are that during the "new DS" phase the
   parent cannot verify the match between the DS2 RR and DNSKEY2 using
   the DNS -- as DNSKEY2 is not yet published.  Besides, we introduce a
   "security lame" key (see Section 4.3.3).  Finally, the child-parent
   interaction consists of two steps.  The "double signature" method
   only needs one interaction.

4.1.4.  Rollover for a Single Type Signing Key rollover

   The rollover of a DNSKEY when a Single Type Signing scheme is used is
   subject to the same requirement as the rollover of a KSK or ZSK:
   During any stage of the rollover the chain of trust needs to continue
   to validate for any combination of data in the zone as well as data
   that may still live in distant caches.

   There are two variants for this rollover.  Since the choice for a

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   Single Type Signing scheme is motivated by operational simplicity we
   first describe the most straightforward rollover scheme first.

      initial           new DNSKEY       DS change      DNSKEY removal
      SOA0             -------->        SOA1            -------->
      RRSIGpar(SOA0)   -------->        RRSIGpar(SOA1)  -------->
      DS1              -------->        DS2             -------->
      RRSIGpar(DS)     -------->        RRSIGpar(DS)    -------->

      SOA0             SOA1             ------------>    SOA2
      RRSIG1(SOA0)     RRSIG1(SOA1)     ------------>    RRSIG2(SOA2)
                       RRSIG2(SOA1)     ------------>
      DNSKEY1          DNSKEY1          ------------>    DNSKEY2
                       DNSKEY2          ------------>
      RRSIG1(DNSKEY)   RRSIG1(DNSKEY)   ------------>    RRSIG2(DNSKEY)
                       RRSIG2(DNSKEY)   ------------>

   Stages of the Straightforward rollover in a Single Type Signing

   initial:  Parental DS points to DNSKEY1.  All RR sets in the zone are
      signed with DNSKEY1.

   new DNSKEY:  A new key (DNSKEY2) is introduced and all the RR sets
      are signed with both DNSKEY1 and DNSKEY2.

   DS change:  After the DNSKEY RRset with the two keys had time to
      propagate into distant caches (that is the key set exclusively
      containing DNSKEY1 has been expired) the parental DS record can be

   DNSKEY removal:  After the DS RRset containing DS1 has expired from
      distant caches DNSKEY1 can be removed from the DNSKEY RRset .

   There is a second variety of this rollover during which one
   introduces a new DNSKEY into the key set and signs the keyset with
   both keys while signing the zone data with only the original DNSKEY1.
   One replaces the DNSKEY1 signatures with signatures made with DNSKEY2
   at the moment of DNSKEY1 removal.

   The second variety of this rollover can be considered when zone size
   considerations prevent the introduction of double signatures over all

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   of the zone data although in that case choosing for a KSK/ZSK split
   may be a better option.

   A double DS rollover scheme is compatible with a rollover using a
   Single Type signing scheme although in order to maintain a valid
   chain of trust the zone data would need to be published with a double
   signatures or a double key key set would need to be published.  Since
   this leads to increase in zone and packet size at both child and
   parent there are little benefits to a double DS rollover with a
   Single Type signing scheme.

4.1.5.  Key algorithm rollover

   A special class of keyrollover is the rollover of key algorithms
   (either adding a new algorithm, removing an old algorithm, or both),
   additional steps are needed to retain integrity during the rollover.

   Because of the algorithm downgrade protection in RFC4035 section 2.2,
   you may not have a key of an algorithm for which you do not have

   When adding a new algorithm, the signatures should be added first.
   After the TTL has expired, and caches have dropped the old data
   covered by those signatures, the DNSKEY with the new algorithm can be
   added.  When removing an old algorithm, the DNSKEY should be removed

   The following figure describes the steps.  Whereby the trailing
   underscored number indicates the algorithm and ZSK and KSK indicate
   the obvious difference in key use.  For example DNSKEY_KSK_1 is a the
   DNSKEY RR representing the public part of a key signing key of
   algorithm type 1 while RRSIG_ZSK_2(SOA3) is the RRSIG RR made with
   the private part of a zone signing key of algorithm type 2 over a SOA
   RR (that has serialnumber 3).

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   1 Initial            2 New RRSIGS         3 New DNSKEY
   SOA0                 SOA1                 SOA2
                        RRSIG_ZSK_2(SOA1)    RRSIG_ZSK_2(SOA2)

   DNSKEY_KSK_1         DNSKEY_KSK_1         DNSKEY_KSK_1
   DNSKEY_ZSK_1         DNSKEY_ZSK_1         DNSKEY_ZSK_1
                        RRSIG_KSK_2(DNSKEY)  DNSKEY_ZKS_2
   4 Remove DNSKEY      5 Remove RRSIGS
   SOA3                 SOA4

   DNSKEY_KSK_2         DNSKEY_KSK_2
   DNSKEY_ZSK_2         DNSKEY_ZSK_2

   Stages of Deployment during an Algorithm Rollover.

   In step 2, the signatures for the new key are added, but the key
   itself is not.  While in theory, the signatures of the keyset should
   always be synchronized with the keyset itself, it can be possible
   that RRSIGS are requested separately, so it might be prudent to also
   sign the DNSKEY set with the new signature.

   After the cache data has expired, the new key can be added to the
   zone, as done in step 3.

   The next step is to remove the old algorithm.  This time the key
   needs to be removed first, before removing the signatures.  The key
   is removed in step 4, and after the cache data has expired, the
   signatures can be removed in step 5.

   If a parent needs to roll a DS record that step can be done during
   stage 3.  First one has to wait until caches are populated with the
   content of the DNSKEY RRset that was created at the beginning of
   stage 3, then the parent can swap the DS from pointing to
   DNSKEY_KSK_1 to DNSKEY_KSK_2, and then one has to wait for the old DS

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   to expire from distant caches before taking step 4.

   [OK:The following paragrapgh is provisional and needs careful

   A special case is the rollover from an NSEC signed zone to an NSEC3
   signed zone.  In this case algortihm numbers are used to signal
   support for NSEC3 but they do not mandate the use of NSEC3.
   Therefore NSEC records should remain to be served untill the rollover
   to a new algorithm has completed and the new DNSKEY RR set has
   populated distant caches(at least one TTL into stage 4, or at any
   time during stage 5).  At that stage the validators that have not
   implemented NSEC3 will treat the zone as unsecured as soon as they
   follow the chain of trust to DS that points to a DNSKEY of the new
   algorithm while validators that support NSEC3 will happily validate
   using NSEC.  Turning on NSEC3 can then be done when moving from one
   serial to another, realizing that that involves a resigning of the
   zone and the introduction of the NSECPARAM record in order to signal
   authoritative servers to start serving NSEC3 authenticated denial of

4.1.6.  Automated Key Rollovers

   As keys must be renewed periodically, there is some motivation to
   automate the rollover process.  Consider the following:

   o  ZSK rollovers are easy to automate as only the child zone is

   o  A KSK rollover needs interaction between parent and child.  Data
      exchange is needed to provide the new keys to the parent;
      consequently, this data must be authenticated and integrity must
      be guaranteed in order to avoid attacks on the rollover.

4.2.  Planning for Emergency Key Rollover

   This section deals with preparation for a possible key compromise.
   Our advice is to have a documented procedure ready for when a key
   compromise is suspected or confirmed.

   When the private material of one of your keys is compromised it can
   be used for as long as a valid trust chain exists.  A trust chain
   remains intact for

   o  as long as a signature over the compromised key in the trust chain
      is valid,

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   o  as long as a parental DS RR (and signature) points to the
      compromised key,

   o  as long as the key is anchored in a resolver and is used as a
      starting point for validation (this is generally the hardest to

   While a trust chain to your compromised key exists, your namespace is
   vulnerable to abuse by anyone who has obtained illegitimate
   possession of the key.  Zone operators have to make a trade-off if
   the abuse of the compromised key is worse than having data in caches
   that cannot be validated.  If the zone operator chooses to break the
   trust chain to the compromised key, data in caches signed with this
   key cannot be validated.  However, if the zone administrator chooses
   to take the path of a regular rollover, the malicious key holder can
   spoof data so that it appears to be valid.

4.2.1.  KSK Compromise

   A zone containing a DNSKEY RRSet with a compromised KSK is vulnerable
   as long as the compromised KSK is configured as trust anchor or a
   parental DS points to it.

   A compromised KSK can be used to sign the key set of an attacker's
   zone.  That zone could be used to poison the DNS.

   Therefore, when the KSK has been compromised, the trust anchor or the
   parental DS should be replaced as soon as possible.  It is local
   policy whether to break the trust chain during the emergency
   rollover.  The trust chain would be broken when the compromised KSK
   is removed from the child's zone while the parent still has a DS
   pointing to the compromised KSK (the assumption is that there is only
   one DS at the parent.  If there are multiple DSes this does not apply
   -- however the chain of trust of this particular key is broken).

   Note that an attacker's zone still uses the compromised KSK and the
   presence of a parental DS would cause the data in this zone to appear
   as valid.  Removing the compromised key would cause the attacker's
   zone to appear as valid and the child's zone as Bogus.  Therefore, we
   advise not to remove the KSK before the parent has a DS to a new KSK
   in place.  Keeping the Chain of Trust Intact

   If we follow this advice, the timing of the replacement of the KSK is
   somewhat critical.  The goal is to remove the compromised KSK as soon
   as the new DS RR is available at the parent.  And also make sure that
   the signature made with a new KSK over the key set with the

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   compromised KSK in it expires just after the new DS appears at the
   parent, thus removing the old cruft in one swoop.

   The procedure is as follows:

   1.  Introduce a new KSK into the key set, keep the compromised KSK in
       the key set.

   2.  Sign the key set, with a short validity period.  The validity
       period should expire shortly after the DS is expected to appear
       in the parent and the old DSes have expired from caches.

   3.  Upload the DS for this new key to the parent.

   4.  Follow the procedure of the regular KSK rollover: Wait for the DS
       to appear in the authoritative servers and then wait as long as
       the TTL of the old DS RRs.  If necessary re-sign the DNSKEY RRSet
       and modify/extend the expiration time.

   5.  Remove the compromised DNSKEY RR from the zone and re-sign the
       key set using your "normal" validity interval.

   An additional danger of a key compromise is that the compromised key
   could be used to facilitate a legitimate DNSKEY/DS rollover and/or
   nameserver changes at the parent.  When that happens, the domain may
   be in dispute.  An authenticated out-of-band and secure notify
   mechanism to contact a parent is needed in this case.

   Note that this is only a problem when the DNSKEY and or DS records
   are used for authentication at the parent.  Breaking the Chain of Trust

   There are two methods to break the chain of trust.  The first method
   causes the child zone to appear 'Bogus' to validating resolvers.  The
   other causes the child zone to appear 'insecure'.  These are
   described below.

   In the method that causes the child zone to appear 'Bogus' to
   validating resolvers, the child zone replaces the current KSK with a
   new one and re-signs the key set.  Next it sends the DS of the new
   key to the parent.  Only after the parent has placed the new DS in
   the zone is the child's chain of trust repaired.

   An alternative method of breaking the chain of trust is by removing
   the DS RRs from the parent zone altogether.  As a result, the child
   zone would become insecure.

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4.2.2.  ZSK Compromise

   Primarily because there is no parental interaction required when a
   ZSK is compromised, the situation is less severe than with a KSK
   compromise.  The zone must still be re-signed with a new ZSK as soon
   as possible.  As this is a local operation and requires no
   communication between the parent and child, this can be achieved
   fairly quickly.  However, one has to take into account that just as
   with a normal rollover the immediate disappearance of the old
   compromised key may lead to verification problems.  Also note that as
   long as the RRSIG over the compromised ZSK is not expired the zone
   may be still at risk.

4.2.3.  Compromises of Keys Anchored in Resolvers

   A key can also be pre-configured in resolvers.  For instance, if
   DNSSEC is successfully deployed the root key may be pre-configured in
   most security aware resolvers.

   If trust-anchor keys are compromised, the resolvers using these keys
   should be notified of this fact.  Zone administrators may consider
   setting up a mailing list to communicate the fact that a SEP key is
   about to be rolled over.  This communication will of course need to
   be authenticated, e.g., by using digital signatures.

   End-users faced with the task of updating an anchored key should
   always validate the new key.  New keys should be authenticated out-
   of-band, for example, through the use of an announcement website that
   is secured using secure sockets (TLS) [22].

4.3.  Parental Policies

4.3.1.  Initial Key Exchanges and Parental Policies Considerations

   The initial key exchange is always subject to the policies set by the
   parent.  The initial key exchange is always subject to the policies
   set by the parent.  This is specifically important in a registry-
   registrar model where the key material is to be passed from the DNS
   operator, via a registrar to the (parent) registry, where both DNS
   operator and registrar is selected by the registrant and they might
   be different organisations.  When designing a key exchange policy one
   should take into account that the authentication and authorization
   mechanisms used during a key exchange should be as strong as the
   authentication and authorization mechanisms used for the exchange of
   delegation information between parent and child.  That is, there is
   no implicit need in DNSSEC to make the authentication process
   stronger than it was in DNS.

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   Using the DNS itself as the source for the actual DNSKEY material,
   with an out-of-band check on the validity of the DNSKEY, has the
   benefit that it reduces the chances of user error.  A DNSKEY query
   tool can make use of the SEP bit [5] to select the proper key from a
   DNSSEC key set, thereby reducing the chance that the wrong DNSKEY is
   sent.  It can validate the self-signature over a key; thereby
   verifying the ownership of the private key material.  Fetching the
   DNSKEY from the DNS ensures that the chain of trust remains intact
   once the parent publishes the DS RR indicating the child is secure.

   Note: the out-of-band verification is still needed when the key
   material is fetched via the DNS.  The parent can never be sure
   whether or not the DNSKEY RRs have been spoofed.

4.3.2.  Storing Keys or Hashes?

   When designing a registry system one should consider which of the
   DNSKEYs and/or the corresponding DSes to store.  Since a child zone
   might wish to have a DS published using a message digest algorithm
   not yet understood by the registry, the registry can't count on being
   able to generate the DS record from a raw DNSKEY.  Thus, we recommend
   that registry systems at least support storing DS records.

   It may also be useful to store DNSKEYs, since having them may help
   during troubleshooting and, as long as the child's chosen message
   digest is supported, the overhead of generating DS records from them
   is minimal.  Having an out-of-band mechanism, such as a registry
   directory (e.g., Whois), to find out which keys are used to generate
   DS Resource Records for specific owners and/or zones may also help
   with troubleshooting.

   The storage considerations also relate to the design of the customer
   interface and the method by which data is transferred between
   registrant and registry; Will the child zone administrator be able to
   upload DS RRs with unknown hash algorithms or does the interface only
   allow DNSKEYs?  When Registries support the Extensible Provisioning
   Protocol (EPP) [14] then that can be used for registrar-registry
   interactions since that protocol allows the transfer of DS and
   optionally DNSKEY RRs.  For moving data between DNS operator and
   registrar there is no standardized way for moving the data.
   Different registrars have different mechanisms, ranging from simple
   web interfaces to various APIs.  In some cases the use of the DNSSEC
   extentions to EPP may be aplicable to.

4.3.3.  Security Lameness

   Security lameness is defined as what happens when a parent has a DS
   RR pointing to a non-existing DNSKEY RR.  When this happens, the

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   child's zone may be marked "Bogus" by verifying DNS clients.

   As part of a comprehensive delegation check, the parent could, at key
   exchange time, verify that the child's key is actually configured in
   the DNS.  However, if a parent does not understand the hashing
   algorithm used by child, the parental checks are limited to only
   comparing the key id.

   Child zones should be very careful in removing DNSKEY material,
   specifically SEP keys, for which a DS RR exists.

   Once a zone is "security lame", a fix (e.g., removing a DS RR) will
   take time to propagate through the DNS.

4.3.4.  DS Signature Validity Period

   Since the DS can be replayed as long as it has a valid signature, a
   short signature validity period over the DS minimizes the time a
   child is vulnerable in the case of a compromise of the child's
   KSK(s).  A signature validity period that is too short introduces the
   possibility that a zone is marked "Bogus" in case of a configuration
   error in the signer.  There may not be enough time to fix the
   problems before signatures expire.  Something as mundane as operator
   unavailability during weekends shows the need for DS signature
   validity periods longer than 2 days.  We recommend an absolute
   minimum for a DS signature validity period of a few days.

   The maximum signature validity period of the DS record depends on how
   long child zones are willing to be vulnerable after a key compromise.
   On the other hand, shortening the DS signature validity interval
   increases the operational risk for the parent.  Therefore, the parent
   may have policy to use a signature validity interval that is
   considerably longer than the child would hope for.

   A compromise between the operational constraints of the parent and
   minimizing damage for the child may result in a DS signature validity
   period somewhere between a week and months.

   In addition to the signature validity period, which sets a lower
   bound on the number of times the zone owner will need to sign the
   zone data and which sets an upper bound to the time a child is
   vulnerable after key compromise, there is the TTL value on the DS
   RRs.  Shortening the TTL means that the authoritative servers will
   see more queries.  But on the other hand, a short TTL lowers the
   persistence of DS RRSets in caches thereby increasing the speed with
   which updated DS RRSets propagate through the DNS.

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4.3.5.  Changing DNS Operators

   [OK: No feedback received between version 2 and 3.  This is likely to
   remain]  Cooperationg DNS operators

   The parent-child relation is often described in terms of a (thin)
   registry model.  Where a registry maintains the parent zone, and the
   registrant (the user of the child-domain name), deals with the
   registry through an intermediary called a registrar.  (See [11] for a
   comprehensive definition).  Registrants may out-source the
   maintenance of their DNS system, including the maintenance of DNSSEC
   key material, to the registrar or to another third party, which we
   will call the DNS operator.  The DNS operator that has control over
   the DNS zone and its keys may prevent the registrant to make a timely
   move to a different DNS operator.

   Suppose that the registrant wants to move from a losing DNS operator
   (loosing operator for short) to a gaining DNS operator (gaining
   operator).  Let us first look what would happen in a cooperative
   environment.  The assumption is that the loosing operator will not
   hand off any private key material to the gaining operator, that would
   constitute a trivial case.

   In a cooperating environment one could proceed with a pre-publish ZSK
   rollover whereby the loosing operator pre-publishes the ZSK of the
   gaining operator, combined with a double signature KSK rollover where
   the two registrars exchange public KSKs and independently generate a
   signature over those keysets that they combine and both publish in
   their copy of the zone.  Once that is done they can use their own
   private keys to sign any of their zone content during the transfer.

    intitial          |        pre-publish             |
     NSA                            NSA
     DSA                            DSA

   Child at A:             Child at A:    Child at B:
     ZSKA                   ZSKA            ZSKA
     KSKA                   ZSKB            ZSKB
     RRSIGZA(DNSKEY)        KSKA            KSKA
     RRSIGKA(DNSKEY)        KSKB            KSKB
                            RRSIGZB         RRSIGZB

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                            RRSIGKB         RRSIGKB
                            RRSIGZA         RRSIGZA
                            RRSIGKA         RRSIGKA

     SOAA                   SOAA             SOAB

     NSA                    NSA              NSB
     RRSIGZA(NS)            NSB              RRSIGZB(NS)

          Redelegation          |        post migration        |

              NSB                             NSB
              DSB                             DSB

    Child at A:    Child at B:             Child at B:
     ZSKA            ZSKA                   ZSKB
     ZSKB            ZSKB                   KSKB
     KSKA            KSKA                   RRSIGZB(DNSKEY)
     KSKB            KSKB                   RRSIGKB(DNSKEY)
     RRSIGZB         RRSIGZB
     RRSIGKB         RRSIGKB
     RRSIGZA         RRSIGZA
     RRSIGKA         RRSIGKA

     SOAA             SOAB                  SOAB

     NSA              NSB                   NSB
     NSB              RRSIGZB(NS)           RRSIGZB(NS)

   In this figure A denotes the loosing DNS operator and B the gaining
   DNS operator.  RRSIGZ is the RRSIG produced by a ZSK, RRSIGK is
   produced with a KSK, the appendend A or B indicate the producers of
   the key pair.  Child at A is how the zone content is represented by
   the loosing DNS operator and Child at B is how the zone content is
   represented by the gaining DNS operator.

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   In the non-cooperative case matters are more complicated.  The
   loosing operator may not cooperate and leave the data in the DNS as
   is.  In the extreme case the loosing operator may become obstructive
   and publish a DNSKEY RR with a high TTL and corresponding signature
   validity so that registrar A's DNSKEY, would end up in caches for, in
   theory, tens of years.

   The problem arises when a validator tries to validate with the
   loosing operator's key and there is no signature material produced
   with the loosing operator available in the delegation path after
   redelegation from the loosing operator to the gaiing operator has
   taken place.  One could imagine a rollover scenario where the gaining
   operator pulls all RRSIGs created by the loosing operator and
   publishes those in conjunction with its own signatures, but that
   would not allow any changes in the zone content.  Since a
   redelegation took place the NS RRset has -- per definition-- changed
   so such rollover scenario will not work.  Besides if zone transfers
   are not allowed by the loosing operator and NSEC3 is deployed in the
   loosing operator's zone then the gainging operator's zone will not
   have certainty that all of A's RRSIGs are transfered.

   The only viable option for the registrant is to publish its zone
   unsigned and ask the registry to remove the DS pointing to the
   loosing operator's DNSKEY for as long as the DNSKEY of the loosing
   operator, or any of the signatures produced by it are likely to
   disappear in caches, which as mentioned above could in theory be for
   tens of years.

   Note that some [OK: most/all ?] implementations limit the time
   DNSKEYs that seem to be unable to validate signatures are cached
   and/or will try to recover from cases where DNSKEYs do not seem to be
   able to validate data.  Although that is not a protocol requirement
   it seems that that practice may limit the impact of this problem the
   problem of non-cooperating registrars.

   However, there is no operational methodology to work around this
   business issue, and proper contractual relationships between all
   involved parties seems to be the only solution to cope with these
   problems.  It should be noted that in many cases, the problem with
   temporary broken delegations already exists when a zone changes from
   one DNS operator to another.  Besides, it is often the case that when
   the DNS moves from one operator to another the services that that
   zone references also change operator, possibly involving some

   In any case.  To minimise such problems, the classic recommendation

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   is to have relative short TTL on all involved resource records.  That
   will solve many of the problems regarding changes to a zone
   regardless of whether DNSSEC is used.

4.4.  Time in DNSSEC

   Without DNSSEC, all times in the DNS are relative.  The SOA fields
   REFRESH, RETRY, and EXPIRATION are timers used to determine the time
   elapsed after a slave server synchronized with a master server.  The
   Time to Live (TTL) value and the SOA RR minimum TTL parameter [9] are
   used to determine how long a forwarder should cache data after it has
   been fetched from an authoritative server.  By using a signature
   validity period, DNSSEC introduces the notion of an absolute time in
   the DNS.  Signatures in DNSSEC have an expiration date after which
   the signature is marked as invalid and the signed data is to be
   considered Bogus.

   The considerations in this section are all qualitative and focused on
   the operational and managerial issues.  A more thourhough
   quantitative analysis of rollover timing parameters can be found in
   draft-ietf-dnsop-dnssec-key-timing [24]

4.4.1.  Time Considerations

   Because of the expiration of signatures, one should consider the

   o  We suggest the Maximum Zone TTL of your zone data to be a fraction
      of your signature validity period.

         If the TTL would be of similar order as the signature validity
         period, then all RRSets fetched during the validity period
         would be cached until the signature expiration time.  Section
         7.1 of RFC4033 [3] suggests that "the resolver may use the time
         remaining before expiration of the signature validity period of
         a signed RRSet as an upper bound for the TTL".  As a result,
         query load on authoritative servers would peak at signature
         expiration time, as this is also the time at which records
         simultaneously expire from caches.

         To avoid query load peaks, we suggest the TTL on all the RRs in
         your zone to be at least a few times smaller than your
         signature validity period.

   o  We suggest the signature publication period to end at least one
      Maximum Zone TTL duration before the end of the signature validity

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         Re-signing a zone shortly before the end of the signature
         validity period may cause simultaneous expiration of data from
         caches.  This in turn may lead to peaks in the load on
         authoritative servers.  To avoid this schemes are deployed
         whereby the zone is periodically visited for a resigning
         operation and those signatures that are within a so called
         refresh intervall from signature expiration are recreated.
         Also see Section 4.4.2 below.

   o  We suggest the Minimum Zone TTL to be long enough to both fetch
      and verify all the RRs in the trust chain.  In workshop
      environments, it has been demonstrated [18] that a low TTL (under
      5 to 10 minutes) caused disruptions because of the following two

         1.  During validation, some data may expire before the
         validation is complete.  The validator should be able to keep
         all data until it is completed.  This applies to all RRs needed
         to complete the chain of trust: DSes, DNSKEYs, RRSIGs, and the
         final answers, i.e., the RRSet that is returned for the initial

         2.  Frequent verification causes load on recursive nameservers.
         Data at delegation points, DSes, DNSKEYs, and RRSIGs benefit
         from caching.  The TTL on those should be relatively long.

   o  Slave servers will need to be able to fetch newly signed zones
      well before the RRSIGs in the zone served by the slave server pass
      their signature expiration time.

         When a slave server is out of sync with its master and data in
         a zone is signed by expired signatures, it may be better for
         the slave server not to give out any answer.

         Normally, a slave server that is not able to contact a master
         server for an extended period will expire a zone.  When that
         happens, the server will respond differently to queries for
         that zone.  Some servers issue SERVFAIL, whereas others turn
         off the 'AA' bit in the answers.  The time of expiration is set
         in the SOA record and is relative to the last successful
         refresh between the master and the slave servers.  There exists
         no coupling between the signature expiration of RRSIGs in the
         zone and the expire parameter in the SOA.

         If the server serves a DNSSEC zone, then it may well happen
         that the signatures expire well before the SOA expiration timer
         counts down to zero.  It is not possible to completely prevent
         this from happening by tweaking the SOA parameters.

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         However, the effects can be minimized where the SOA expiration
         time is equal to or shorter than the signature validity period.

         The consequence of an authoritative server not being able to
         update a zone, whilst that zone includes expired signatures, is
         that non-secure resolvers will continue to be able to resolve
         data served by the particular slave servers while security-
         aware resolvers will experience problems because of answers
         being marked as Bogus.

         We suggest the SOA expiration timer being approximately one
         third or one fourth of the signature validity period.  It will
         allow problems with transfers from the master server to be
         noticed before the actual signature times out.

         We also suggest that operators of nameservers that supply
         secondary services develop 'watch dogs' to spot upcoming
         signature expirations in zones they slave, and take appropriate

         When determining the value for the expiration parameter one has
         to take the following into account: What are the chances that
         all my secondaries expire the zone?  How quickly can I reach an
         administrator of secondary servers to load a valid zone?  These
         questions are not DNSSEC specific but may influence the choice
         of your signature validity intervals.

4.4.2.  Signature Validation Periods

   [OK: This section is newly introduced and needs a check on
   consistency with the rest of the document]  Maximum Value

   The first consideration for choosing a maximum signature validity
   period is the risk of a replay attack.  For low-value, long-term
   stable resources the risks may be minimal and the signature validity
   period may be multiple months.  Although signature validity periods
   of many years are allowed the same operational habit arguments as in
   Section play a role: when a zone is resigned with some
   regularity then operators remain conscious about the operational
   necessity of resigning.  Minimum Value

   The minimum value of the signature validity period is set by the time
   by which one would like to survive operational failure in
   provisioning: what is the time that a failure will be noticed, what

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   is the time that action is expected to be taken?  By answering these
   questions availability of operators during (long) weekends or access
   to backup media needs to be taken into account which could easily
   suggest a minimum Signature Validity period of a few days.

   Note however, the argument above is assuming that zone data has just
   been signed and published when the problem occurred.  In practice it
   may be that a zone is signed according to a frequency set by the
   Resign Period whereby the signer visits the zone content and only
   refreshes signatures that are close to expiring: the signer will only
   refresh signatures if they are within the Refresh Period from the
   signature expiration time.  The Resign Period must be smaller than
   the Refresh Period in order for zone data to be signed timely.

   If an operational problem occurs during resigning then the signatures
   in the zone wich will expire first are the ones that have been
   generated longest ago.  And in the worst case these signatures are
   the Refresh Period minus the Resign Period away from signature

   In other words, the minimum Signature Validity intervall is set by
   first choosing the Refresh Period (usually a few days), then defining
   the Resign period in such a way that the Refresh Period minus the
   Resign period sets the time in which operational havoc can be

   To make matters slightly mor complicated.  Some signers apply some
   jitter to the signature expiry time so that not all signatures expire
   at the same time.  The jitter should not influence your calculation
   as long as it is smaller than the refresh period and the resign
   period is at least half the refresh period [OK: The above needs
   careful review]

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    Inception      Signing                                 Expiration
    time             time                                     time
    |                  |                              |       |       |
    |                  |                              |       |       |
                                                          +/- jitter

    | Inception offset |         Validity Period              |
    | <--------------->|<------------------------------------>|

    Inception      Signing  reuse  reuse  reuse    new      Expiration
    time             time                          signature     time
    |                  |      |      |      |      |             |
    |                  |      |      |      |      |             |
                        <----> <----> <----> <---->
                    Resign Period

                                                |                  |
                                                |<-Refresh Period->|
                                                |                  |  differentiation between RR sets

   It is possible to vary signature validity periods between signatures
   over different RR sets in the zone.  In practice this could be done
   when zones contain highly volatile data (which may be the case in
   dynamic update environments).  Note however that the risk of replay
   (e.g. by stale secondary servers) is what should be leading in
   determining the signature validity period since the TTL on the data
   itself still are the primary parameter for cache expiry.  [OK: are
   there strong arguments besides replay risks for varying signature

   In some cases the risk of replaying existing data might be different
   from the risk of replaying the denial of data.  In those cases the
   signature validity period on NSEC or NSEC3 records may be tweaked

   When a zone contains secure delegations then a relatively short
   signature validity interval protects the child agains replay attacks,
   in the case the child's key is compromised (see Section 4.3.4).
   Since there is a higher operational risk for the parent registry when
   choosing a short validity intervall and a higher operational risk for
   the child when choosing a long validity period some (price)

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   differentiation may occur for validity periods between individual DS
   RRs in a single zone.

   There seem to be no other arguments for differentiation in validity
   periods.  Other timing parameters in a zone

   [OK: Isn't the following not to vague?  Is it sufficient?]

   The arguments for tuning minimum signature validity period are
   remarkably similar to the arguments used to set the SOA expiration
   timer.  It is advised to set the expiration time longer than the
   signature validity period.

5.  Next Record type

   One of the design tradeofs made during the development of DNSSEC has
   been to seperate the signing and serving operations instead of
   performing cryptographic operations on the fly.  It is therefore
   necessry to create records that cover the very large number of non-
   existent names that lie between the names that do exist.

   There are two mechanisms to provide authenticated proof of non-
   existence of domain names in DNSSEC: clear text one and an
   obfuscated-data one.  Each mechanism

   o  includes a list of all the RRTYPEs present which can be used to
      prove the non-existence of RRTYPEs at a certain name;

   o  stores only the name for which the zone is authoritative (that is,
      glue in the zone is omitted); and

   o  uses a specific RRTYPE to store information about the RRTYPEs
      present at the name: the clear-text mechanism uses NSEC, and the
      obfuscated-data mechanism uses NSEC3.

5.1.  Differences between  NSEC and NSEC3

   The clear text mechanism (NSEC) is implemented using a sorted linked
   list of names in the zone.  The obfuscated-data mechanism (NSEC3)
   first hashes the names using a one-way hash function, and then sorts
   the resulting (hashed) strings.

   The NSEC record requires no cryptographic operations aside from
   validating its associated signature record.  It is human readable and
   can be used in manual queries to determine correct operation.  The
   disadvantage is that it allows for "zone walking", where one can

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   request all the entries of a zone by following the next RRlabel
   pointed to in each subsequent NSEC record.

   Though all agree DNS data is accessible through query mechanisms, a
   side effect of NSEC is that it allows the contents of a zone file to
   be enumerated in full by sequential queries.  Whilst for some
   operators this behaviour is acceptable or even desirable, for others
   it is undesirable for policy, regulatory or other reasons.  This is
   the first difference between NSEC and NSEC3.

   The second difference between NSEC and NSEC3 is that NSEC requires a
   signature over every RR in the zonefile, thereby ensuring that any
   denial of existence is cryptographically signed.  However, in a large
   zonefile containing many delegations very few of which are to signed
   zones, this may produce unacceptable additional overhead especially
   where insecure delegations are subject to frequent update (a typical
   example might be a TLD operator with few registrants using secure
   delegations).  NSEC3 allows intervals between two such delegations to
   "Opt-out" in which case they may contain one more more insecure
   delegations, thus reducing the size and cryptographic complexity of
   the zone at the expense of the ability to cryptographically deny the
   existence of names in a specific span.

   The NSEC3 record uses a hashing method of the requested RRlabel.  To
   increase the workload required to guess entries in the zone, the
   number of hashing iteration's can be specified in the NSEC3 record.
   Additionally, a salt can be specified that also modifies the hashes.
   Note that NSEC3 does not give full protection against information
   leakage from the zone.

5.2.  NSEC or NSEC3

   The first motivation to deploy NSEC3, prevention of zone enumeration,
   only makes sense when zone content is not highly structured or
   trivially guessable.  Highly structured zones such as the in-
   addr.arpa, ip6.arpa and e164.arpa can be trivially enumerated using
   ordinary DNS properties while for small zones that only contain
   contain records in the APEX and a few common RRlabels such as "www"
   or "mail" guessing zone content and proving completeness is also
   trivial when using NSEC3.

   In those cases the use of NSEC is recommended to ease the work
   required by signers and validating resolvers.

   For large zones where there is an implication of "not readily
   available" RRlabels, such as those where one has to sign an NDA
   before obtaining it, NSEC3 is recommended.

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   The considerations for the second reason to deploy NSEC3 are
   discussed below (Section 5.3.4).

5.3.  NSEC3 parameters

   The NSEC3 hashing algorithm is performed on the Fully Qualified
   Domain Name (FQDN) in its uncompressed form.  This ensures brute
   force work done by an attacker for one (FQDN) RRlabel cannot be re-
   used for another (FQDN) RRlabel attack, as these entries are per
   definition unique.

5.3.1.  NSEC3 Algorithm

   At the moment of writing there is only one NSEC3 Hashing algorithm
   defined. [21] specifically calls out that when a new hash algorithm
   for use with NSEC3 is specified, a transition mechanism MUST also be
   defined.  Therefore this document does not considder NSEC3 hash
   algorithm transition.

5.3.2.  NSEC3 Iterations

   One of the concerns with NSEC3 is that bad actors could perform a
   pre-calculated dictionary attack in order to assess if certain domain
   names exist within the zones or not.  Two mechanisms are introduced
   in the NSEC3 specification to increase the costs of such dictionary
   attacks: Iterations and Salt.

   RFC5155 [21] considers the trade-offs between incuring cost during
   the signing process, imposing costs to the validating nameserver,
   while still providing a reasonable barrier against dictionary
   attacks.  It provides useful limits of iterations compared to RSA key
   size.  These are 150 iterations for 1024 bit keys, 500 iterations for
   2048 bit keys and 2,500 iterations for 4096 bit keys.  Choosing 2/3rd
   of the maximum is deemed to be a sufficiently costly yet not
   excessive value.

5.3.3.  NSEC3 Salt

   While the NSEC3 iterations parameter increases the cost of hashing a
   dictionary word, the NSEC3 salt reduces the lifetime for which that
   calculated hash can be used.  A change of the salt value by the zone
   owner would cause an attacker to lose all precalculated work for that

   The FQDN RRlabel, which is part of the value that is hashed, already
   ensures that brute force work for one RRlabel can not be re-used to
   attack other RRlabel (e.g. in other domains) due to their uniqueness.

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   The salt of all NSEC3 records in a zone needs to be the same.  Since
   changing the salt requires all the NSEC3 records to be regenerated,
   and thus requires generating new RRSIG's over these NSEC3 records, it
   is recommended to align the change of the salt with a change of the
   Zone Signing Key, as that process in itself already requires all
   RRSIG's to be regenerated.  If there is no critical dependency on
   incremental signing and the whole zone can be signed with little
   effort there is no need for such alignment.  However, unlike Zone
   Signing Key changes, NSEC3 salt changes do not need special rollover
   procedures.  It is possible to change the salt each time the zone is

5.3.4.  Opt-out

   The Opt-Out mechanism was introduced to allow for a gradual
   introduction of signed records in zones that contain mostly
   delegation records.  The use of the OPT-OUT flag changes the meaning
   of the NSEC3 span from authoritative denial of the existence of names
   within the span to a proof that DNSSEC is not available for the
   delegations within the span.  [Editors Note: One could make this
   construct more correct by talking about the hashed names and the
   hashed span, but I believe that is overkill].  This allows for the
   addition or removal of the delegations covered by the span without
   recalculating or re- signing RRs in the NSEC3 RR chain.

   Opt-Out is specified to be used only over delegation points and will
   therefore only bring relieve in zones with a large number of zones
   and where the number of secure delegations is small.  This
   consideration typically holds for large top-level-domains and similar
   zones, in most other circumstances Opt-Out should not be deployed.
   Further considerations can be found in RFC5155 section 12.2 [21].

6.  Security Considerations

   DNSSEC adds data integrity to the DNS.  This document tries to assess
   the operational considerations to maintain a stable and secure DNSSEC
   service.  Not taking into account the 'data propagation' properties
   in the DNS will cause validation failures and may make secured zones
   unavailable to security-aware resolvers.

7.  IANA considerations

   There are no IANA considerations with respect to this document

8.  Acknowledgments

   Most of the text of this document is copied from RFC4641 [15].  That
   document was edited by Olaf Kolkman and Miek Gieben.  Other people

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   that contributed or where otherwise involved in that work were in
   random order: Rip Loomis, Olafur Gudmundsson, Wesley Griffin, Michael
   Richardson, Scott Rose, Rick van Rein, Tim McGinnis, Gilles Guette
   Olivier Courtay, Sam Weiler, Jelte Jansen, Niall O'Reilly, Holger
   Zuleger, Ed Lewis, Hilarie Orman, Marcos Sanz, Peter Koch, Mike
   StJohns, Emma Bretherick, Adrian Bedford, and Lindy Foster, G.
   Guette, and O. Courtay.

   For this version of the document we would like to acknowldge a few
   people for significant contributions:

   o  Paul Hoffman for his contribution on the choice of cryptographic
      paramenters and addressing some of the trust anchor issues.

   o  Jelte Jansen who provided the text in Section 4.1.5

   o  Paul Wouters who provided the initial text for Section 5 and Alex
      Bligh who improved it.

   o  Erik Rescorla's blogpost on "the Security of ZSK rollovers"
      inspired text in Section 3.1.1.

   o  The figure in Section 4.4.2 was adapted from the OpenDNSSEC user

   In addition valuable contributions in the form of text, comments, or
   review where provided by Mark Andrews, Patrik Faltstrom, Tony Finch,
   Olafur Gudmundsson, Alfred Hines, Bill Manning, Scott Rose.

   [EDITOR NOTE: please let me know if there is an oversight here]

9.  References

9.1.  Normative References

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

   [2]   Mockapetris, P., "Domain names - implementation and
         specification", STD 13, RFC 1035, November 1987.

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

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

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

9.2.  Informative References

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

   [7]   Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
         August 1996.

   [8]   Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
         (DNS NOTIFY)", RFC 1996, August 1996.

   [9]   Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)",
         RFC 2308, March 1998.

   [10]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
         Update", RFC 3007, November 2000.

   [11]  Hollenbeck, S., "Generic Registry-Registrar Protocol
         Requirements", RFC 3375, September 2002.

   [12]  Orman, H. and P. Hoffman, "Determining Strengths For Public
         Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
         April 2004.

   [13]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
         Requirements for Security", BCP 106, RFC 4086, June 2005.

   [14]  Hollenbeck, S., "Domain Name System (DNS) Security Extensions
         Mapping for the Extensible Provisioning Protocol (EPP)",
         RFC 4310, December 2005.

   [15]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
         RFC 4641, September 2006.

   [16]  Shirey, R., "Internet Security Glossary, Version 2", RFC 4949,
         August 2007.

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

   [18]  Rose, S., "NIST DNSSEC workshop notes",  , June 2001.

   [19]  Barker, E. and J. Kelsey, "Recommendation for Random Number
         Generation Using Deterministic Random Bit Generators

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         (Revised)", Nist Special Publication 800-90, March 2007.

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

   [21]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
         Security (DNSSEC) Hashed Authenticated Denial of Existence",
         RFC 5155, March 2008.

   [22]  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
         Protocol Version 1.2", RFC 5246, August 2008.

   [23]  Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY and
         RRSIG Resource Records for DNSSEC", RFC 5702, October 2009.

   [24]  Morris, S., Ihren, J., and J. Dickinson, "DNSSEC Key Timing
         Considerations", draft-ietf-dnsop-dnssec-key-timing-00 (work in
         progress), July 2010.

   [25]  Ljunggren, F., Eklund-Lowinder, A., and T. Okubo, "DNSSEC
         Signing Policy & Practice Statement Framework",
         draft-ietf-dnsop-dnssec-dps-framework-01 (work in progress),
         March 2010.

Appendix A.  Terminology

   In this document, there is some jargon used that is defined in other
   documents.  In most cases, we have not copied the text from the
   documents defining the terms but have given a more elaborate
   explanation of the meaning.  Note that these explanations should not
   be seen as authoritative.

   Anchored key:  A DNSKEY configured in resolvers around the globe.
      This key is hard to update, hence the term anchored.

   Bogus:  Also see Section 5 of RFC4033 [3].  An RRSet in DNSSEC is
      marked "Bogus" when a signature of an RRSet does not validate
      against a DNSKEY.

   Key Signing Key or KSK:  A Key Signing Key (KSK) is a key that is
      used exclusively for signing the apex key set.  The fact that a
      key is a KSK is only relevant to the signing tool.

   Key size:  The term 'key size' can be substituted by 'modulus size'
      throughout the document.  It is mathematically more correct to use
      modulus size, but as this is a document directed at operators we
      feel more at ease with the term key size.

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   Private and public keys:  DNSSEC secures the DNS through the use of
      public key cryptography.  Public key cryptography is based on the
      existence of two (mathematically related) keys, a public key and a
      private key.  The public keys are published in the DNS by use of
      the DNSKEY Resource Record (DNSKEY RR).  Private keys should
      remain private.

   Key rollover:  A key rollover (also called key supercession in some
      environments) is the act of replacing one key pair with another at
      the end of a key effectivity period.

   Refresh Period:  The time at the end of the Signature Validity Period
      during which signatures are refreshed.

   Resinging frequency:  Frequency with wich a signing pass on the zone
      is performed.  Alternatively expressed as "Resigning Period".  It
      defines when the zone is exposed to the signer.  During a signing
      pass not all signatures in the zone may be refreshed, that depend
      refresh frequency/intervall.

   Secure Entry Point (SEP) key:  A KSK that has a parental DS record
      pointing to it or is configured as a trust anchor.  Although not
      required by the protocol, we recommend that the SEP flag [5] is
      set on these keys.

   Self-signature:  This only applies to signatures over DNSKEYs; a
      signature made with DNSKEY x, over DNSKEY x is called a self-
      signature.  Note: without further information, self-signatures
      convey no trust.  They are useful to check the authenticity of the
      DNSKEY, i.e., they can be used as a hash.

   Signing Jitter:  Jitter applied to the signature validty intervall.

   Signer:  The system that has access to the private key material and
      signs the Resource Record sets in a zone.  A signer may be
      configured to sign only parts of the zone, e.g., only those RRSets
      for which existing signatures are about to expire.

   Single Type Signing Scheme:  A signing scheme whereby the distinction
      between Zone Signing Keys and Key Singing Keys is not made.

   Zone Signing Key (ZSK):  A key that is used for signing all data in a
      zone (except, perhaps, the DNSKEY RRSet).  The fact that a key is
      a ZSK is only relevant to the signing tool.

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   Singing the zone file:  The term used for the event where an
      administrator joyfully signs its zone file while producing melodic
      sound patterns.

   Zone administrator:  The 'role' that is responsible for signing a
      zone and publishing it on the primary authoritative server.

Appendix B.  Zone Signing Key Rollover How-To

   Using the pre-published signature scheme and the most conservative
   method to assure oneself that data does not live in caches, here
   follows the "how-to".

   Step 0:  The preparation: Create two keys and publish both in your
      key set.  Mark one of the keys "active" and the other "published".
      Use the "active" key for signing your zone data.  Store the
      private part of the "published" key, preferably off-line.  The
      protocol does not provide for attributes to mark a key as active
      or published.  This is something you have to do on your own,
      through the use of a notebook or key management tool.

   Step 1:  Determine expiration: At the beginning of the rollover make
      a note of the highest expiration time of signatures in your zone
      file created with the current key marked as active.  Wait until
      the expiration time marked in Step 1 has passed.

   Step 2:  Then start using the key that was marked "published" to sign
      your data (i.e., mark it "active").  Stop using the key that was
      marked "active"; mark it "rolled".

   Step 3:  It is safe to engage in a new rollover (Step 1) after at
      least one signature validity period.

Appendix C.  Typographic Conventions

   The following typographic conventions are used in this document:

   Key notation:  A key is denoted by DNSKEYx, where x is a number or an
      identifier, x could be thought of as the key id.

   RRSet notations:  RRs are only denoted by the type.  All other
      information -- owner, class, rdata, and TTL -- is left out.  Thus:
      "example.com 3600 IN A" is reduced to "A".  RRSets are a
      list of RRs.  A example of this would be "A1, A2", specifying the
      RRSet containing two "A" records.  This could again be abbreviated
      to just "A".

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   Signature notation:  Signatures are denoted as RRSIGx(RRSet), which
      means that RRSet is signed with DNSKEYx.

   Zone representation:  Using the above notation we have simplified the
      representation of a signed zone by leaving out all unnecessary
      details such as the names and by representing all data by "SOAx"

   SOA representation:  SOAs are represented as SOAx, where x is the
      serial number.

   Using this notation the following signed zone:

   example.net.      86400  IN SOA  ns.example.net. bert.example.net. (
                            2006022100   ; serial
                            86400        ; refresh (  24 hours)
                            7200         ; retry   (   2 hours)
                            3600000      ; expire  (1000 hours)
                            28800 )      ; minimum (   8 hours)
                     86400  RRSIG   SOA 5 2 86400 20130522213204 (
                                  20130422213204 14 example.net.
                                  cmL62SI6iAX46xGNQAdQ... )
                     86400  NS      a.example.net.
                     86400  NS      b.example.net.
                     86400  RRSIG   NS 5 2 86400 20130507213204 (
                                  20130407213204 14 example.net.
                                  SO5epiJei19AjXoUpFnQ ... )
                     86400  DNSKEY  256 3 5 (
                                  EtRB9MP5/AvOuVO0I8XDxy0... ) ; id = 14
                     86400  DNSKEY  257 3 5 (
                                  gsPW/Yy19GzYIY+Gnr8HABU... ) ; id = 15
                     86400  RRSIG   DNSKEY 5 2 86400 20130522213204 (
                                  20130422213204 14 example.net.
                                  J4zCe8QX4tXVGjV4e1r9... )
                     86400  RRSIG   DNSKEY 5 2 86400 20130522213204 (
                                  20130422213204 15 example.net.
                                  keVDCOpsSeDReyV6O... )
                     86400  RRSIG   NSEC 5 2 86400 20130507213204 (
                                  20130407213204 14 example.net.
                                  obj3HEp1GjnmhRjX... )
   a.example.net.    86400  IN TXT  "A label"
                     86400  RRSIG   TXT 5 3 86400 20130507213204 (
                                  20130407213204 14 example.net.
                                  IkDMlRdYLmXH7QJnuF3v... )
                     86400  NSEC    b.example.com. TXT RRSIG NSEC
                     86400  RRSIG   NSEC 5 3 86400 20130507213204 (
                                  20130407213204 14 example.net.
                                  bZMjoZ3bHjnEz0nIsPMM... )

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   is reduced to the following representation:



   The rest of the zone data has the same signature as the SOA record,
   i.e., an RRSIG created with DNSKEY 14.

Appendix D.  Document Editing History

   [To be removed prior to publication as an RFC]

D.1.  draft-ietf-dnsop-rfc4641-00

   Version 0 was differs from RFC4641 in the following ways.

   o  Status of this memo appropriate for I-D

   o  TOC formatting differs.

   o  Whitespaces, linebreaks, and pagebreaks may be slightly different
      because of xml2rfc generation.

   o  References slightly reordered.

   o  Applied the errata from

   o  Inserted trivial "IANA considertations" section.

   In other words it should not contain substantive changes in content
   as intended by the workinggroup for the original RFC4641.

D.2.  version 0->1

   Cryptography details rewritten.  (See http://www.nlnetlabs.nl/svn/

   o  Reference to NIST 800-90 added

   o  RSA/SHA256 is being recommended in addition to RSA/SHA1.

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   o  Complete rewrite of Section removing the table and
      suggesting a keysize of 1024 for keys in use for less than 8
      years, issued up to at least 2015.

   o  Replaced the reference to Schneiers' applied cryptograpy with a
      reference to RFC4949.

   o  Removed the KSK for high level zones consideration

   Applied some differentiation with respect of the use of a KSK for
   parent or trust-anchor relation http://www.nlnetlabs.nl/svn/


   Added Section 4.1.5 as suggested by Jelte Jansen in http://

   Added Section Issue identified by Antoin Verschuur http://

   In Appendix A: ZSK does not nescessarily sign the DNSKEY RRset.

D.3.  version 1->2

   o  Significant rewrite of Section 3 whereby the argument is made that
      the timescakes for rollovers are made purely on operational
      arguments hopefully resolving http://www.nlnetlabs.nl/svn/

   o  Added Section 5 based on http://www.nlnetlabs.nl/svn/rfc4641bis/

   o  Added a reference to draft-morris-dnsop-dnssec-key-timing [24] for
      the quantitative analysis on keyrolls

   o  Updated Section 4.3.5 to reflect that the problem occurs when
      changing DNS operators, and not DNS registrars, also added the
      table indicating the redelegation procedure.  Added text about the
      fact that implementations will dismiss keys that fail to validate
      at some point.

   o  Updated a number of references.

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D.4.  version 2->3

   o  Added bulleted list to serve as an introduction on the decision
      tree in Section 3.

   o  In section Section 3.1.1:

      *  tried to motivate that keylength is not a strong motivation for
         KSK ZSK split (based on http://www.educatedguesswork.org/2009/

      *  Introduced Common Signing Key terminology and made the
         arguments for the choice of a Common Signing Key more explicit.

      *  Moved the SEP flag considerations to its own paragraph

   o  In a few places in the document, but section Section 4 in
      particular the comments from Patrik Faltstrom (On Mar 24, 2010) on
      the clarity on the roles of the registrant, dns operator,
      registrar and registry was addressed.

   o  Added some terms based on http://www.nlnetlabs.nl/svn/rfc4641bis/

   o  Added paragrap 2 and clarified the second but last paragraph of

   o  Clarified the table and some text in Section 4.1.5.  Also added
      some text on what happens when the algorithm rollover also
      involves a roll from NSEC to NSEC3.

   o  Added a paragraph about rolling KSKs that are also configured as
      trust-anchors in Section 4.1.2

   o  Added Section 4.1.4.

   o  Added Section 4.4.2 to address issue "Signature_validity"

   $Id: draft-ietf-dnsop-rfc4641bis-03.txt 60 2010-07-12 19:33:51Z olaf $

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Author's Address

   Olaf M. Kolkman
   NLnet Labs
   Kruislaan 419
   Amsterdam  1098 VA
   The Netherlands

   EMail: olaf@nlnetlabs.nl
   URI:   http://www.nlnetlabs.nl

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