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Versions: (RFC 4641) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 6781

DNSOP                                                         O. Kolkman
Internet-Draft                                                W. Mekking
Obsoletes: 2541 (if approved)                                 NLnet Labs
Intended status: Informational                              Oct 31, 2011
Expires: May 3, 2012


                DNSSEC Operational Practices, Version 2
                     draft-ietf-dnsop-rfc4641bis-08

Abstract

   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.

   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.

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on May 3, 2012.

Copyright Notice



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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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  . . . . . . . . . . . . . . . . .  7
     3.1.  Operational Motivation for Zone Signing and Key
           Signing Keys . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Practical Consequences of KSK and ZSK Separation . . . . . 10
       3.2.1.  Rolling a KSK that is not a trust-anchor . . . . . . . 10
       3.2.2.  Rolling a KSK that is a trust-anchor . . . . . . . . . 11
       3.2.3.  The use of the SEP flag  . . . . . . . . . . . . . . . 12
     3.3.  Key Effectivity Period . . . . . . . . . . . . . . . . . . 12
     3.4.  Cryptographic Considerations . . . . . . . . . . . . . . . 13
       3.4.1.  Key Algorithm  . . . . . . . . . . . . . . . . . . . . 13
       3.4.2.  Key Sizes  . . . . . . . . . . . . . . . . . . . . . . 14
       3.4.3.  Private Key Storage  . . . . . . . . . . . . . . . . . 15
       3.4.4.  Key Generation . . . . . . . . . . . . . . . . . . . . 16
       3.4.5.  Differentiation for 'High-Level' Zones?  . . . . . . . 17
   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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         4.1.1.1.  Pre-Publish Zone Signing Key Rollover  . . . . . . 18
         4.1.1.2.  Double Signature Zone Signing Key Rollover . . . . 20
         4.1.1.3.  Pros and Cons of the Schemes . . . . . . . . . . . 22
       4.1.2.  Key Signing Key Rollovers  . . . . . . . . . . . . . . 22
         4.1.2.1.  Special Considerations for RFC5011 KSK rollover  . 24
       4.1.3.  Difference Between ZSK and KSK Rollovers . . . . . . . 24
       4.1.4.  Rollover for a Single Type Signing Key rollover  . . . 25
       4.1.5.  Algorithm rollovers  . . . . . . . . . . . . . . . . . 27
         4.1.5.1.  Single Type Signing Scheme Algorithm Rollover  . . 31
         4.1.5.2.  Algorithm rollover, RFC5011 style  . . . . . . . . 31
         4.1.5.3.  Single Signing Type Algorithm Rollover,
                   RFC5011 style  . . . . . . . . . . . . . . . . . . 32
         4.1.5.4.  NSEC to NSEC3 algorithm rollover . . . . . . . . . 33
       4.1.6.  Considerations for Automated Key Rollovers . . . . . . 33
     4.2.  Planning for Emergency Key Rollover  . . . . . . . . . . . 34
       4.2.1.  KSK Compromise . . . . . . . . . . . . . . . . . . . . 34
         4.2.1.1.  Keeping the Chain of Trust Intact  . . . . . . . . 35
         4.2.1.2.  Breaking the Chain of Trust  . . . . . . . . . . . 36
       4.2.2.  ZSK Compromise . . . . . . . . . . . . . . . . . . . . 36
       4.2.3.  Compromises of Keys Anchored in Resolvers  . . . . . . 36
       4.2.4.  Stand-by keys  . . . . . . . . . . . . . . . . . . . . 37
     4.3.  Parent Policies  . . . . . . . . . . . . . . . . . . . . . 37
       4.3.1.  Initial Key Exchanges and Parental Policies
               Considerations . . . . . . . . . . . . . . . . . . . . 37
       4.3.2.  Storing Keys or Hashes?  . . . . . . . . . . . . . . . 38
       4.3.3.  Security Lameness  . . . . . . . . . . . . . . . . . . 39
       4.3.4.  DS Signature Validity Period . . . . . . . . . . . . . 39
       4.3.5.  Changing DNS Operators . . . . . . . . . . . . . . . . 40
         4.3.5.1.  Cooperating DNS operators  . . . . . . . . . . . . 40
         4.3.5.2.  Non Cooperating DNS Operators  . . . . . . . . . . 42
     4.4.  Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . 43
       4.4.1.  Time Considerations  . . . . . . . . . . . . . . . . . 44
       4.4.2.  Signature Validation Periods . . . . . . . . . . . . . 46
         4.4.2.1.  Maximum Value  . . . . . . . . . . . . . . . . . . 46
         4.4.2.2.  Minimum Value  . . . . . . . . . . . . . . . . . . 46
         4.4.2.3.  Differentiation between RR sets  . . . . . . . . . 48
   5.  Next Record type . . . . . . . . . . . . . . . . . . . . . . . 49
     5.1.  Differences between  NSEC and NSEC3  . . . . . . . . . . . 49
     5.2.  NSEC or NSEC3  . . . . . . . . . . . . . . . . . . . . . . 50
     5.3.  NSEC3 parameters . . . . . . . . . . . . . . . . . . . . . 51
       5.3.1.  NSEC3 Algorithm  . . . . . . . . . . . . . . . . . . . 51
       5.3.2.  NSEC3 Iterations . . . . . . . . . . . . . . . . . . . 51
       5.3.3.  NSEC3 Salt . . . . . . . . . . . . . . . . . . . . . . 51
       5.3.4.  Opt-out  . . . . . . . . . . . . . . . . . . . . . . . 52
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 52
   7.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 52
   8.  Contributors and Acknowledgments . . . . . . . . . . . . . . . 52
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 53



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     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 53
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 54
   Appendix A.  Terminology . . . . . . . . . . . . . . . . . . . . . 55
   Appendix B.  Typographic Conventions . . . . . . . . . . . . . . . 57
   Appendix C.  Transition Figures for Special Case Algorithm
                Rollovers . . . . . . . . . . . . . . . . . . . . . . 59
   Appendix D.  Document Editing History  . . . . . . . . . . . . . . 63
     D.1.  draft-ietf-dnsop-rfc4641-00  . . . . . . . . . . . . . . . 63
     D.2.  version 0->1 . . . . . . . . . . . . . . . . . . . . . . . 64
     D.3.  version 1->2 . . . . . . . . . . . . . . . . . . . . . . . 64
     D.4.  version 2->3 . . . . . . . . . . . . . . . . . . . . . . . 65
     D.5.  version 3->4 . . . . . . . . . . . . . . . . . . . . . . . 66
     D.6.  version 4->5 . . . . . . . . . . . . . . . . . . . . . . . 66
     D.7.  version 5->6 . . . . . . . . . . . . . . . . . . . . . . . 66
     D.8.  version 6->7 . . . . . . . . . . . . . . . . . . . . . . . 66
     D.9.  version 7->8 . . . . . . . . . . . . . . . . . . . . . . . 67
     D.10. Subversion information . . . . . . . . . . . . . . . . . . 67


































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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
   (RFC 4033 [3], RFC 4034 [4], and RFC 4035 [5]).  The focus of the
   document is on serving authoritative DNS information and is aimed at
   zone owners, name server operators, registries, registrars and
   registrants.  It assumes that there is no direct relation between
   those entities and the operators of validating recursive name servers
   (validators).

   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 has just been signed and the first secure
   delegations are provisioned- there exists relatively little
   experience with DNSSEC in production environments below the TLD
   level; this document should therefore explicitly not be seen as
   representing 'Best Current Practices'.  Instead, it describes the
   decisions that should be made when deploying DNSSEC, gives the
   choices available for each one, and provides some operational
   guidelines.  The document does not give strong recommendations, that
   may be subject for a future version of this document.

   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.

   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 keys are discussed in
   Section 3; the focus in this section is mainly on the security of the
   private part of the key(s).  Section 4 describes considerations
   concerning the public part of the keys.  Section 4.1 and Section 4.2
   deal with the rollover, or replacement, of keys.  Section 4.3
   discusses considerations on how parents deal with their children's
   public keys in order to maintain chains of trust.  Section 4.4 covers
   all kinds of timing issues around keys publication.  Section 5 covers
   the considerations regarding selecting and using NSEC and NSEC3.

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

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



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   This document obsoletes RFC 4641 [14].

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 [15]).  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" The period that a signature is
      published.  It starts at the time the signature is introduced in
      the zone for the first time and ends at the time when the
      signature is removed or replaced with a new signature.  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.  It 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.

   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.








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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
   need to realize that to verifying clients their zone is, 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
   Internet.

   Administrators of secured zones will need 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 can be 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 must be
   made between maintaining a valid chain of trust and replacing the
   compromised keys as soon as possible.  Then zone administrators will
   have to decide, 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.)

3.  Keys Generation and Storage

   This section describes a number of considerations with respect to the
   use of keys.  For the design of a operational procedure for key
   generation and storage then a number of decisions need to be made:



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   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?

   o  What are the cryptographic parameters that fit the operational
      need?

   The following section discusses the considerations that need to be
   taken into account when making those choices.

3.1.  Operational Motivation for Zone Signing and Key Signing Keys

   The DNSSEC validation protocol does not distinguish between different
   types of DNSKEYs.  The motivations to differentiate between keys are
   purely operational; validators will not make a distinction.

   For operational reasons, described below, it is possible to designate
   one or more keys to have the role of 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 other RRSets in a zone that require
   signatures.  They are referred to as Zone Signing Keys (ZSKs).  In
   case the differentiation between KSK and ZSK is not made, keys have
   both the role of KSK and ZSK, we talk about a Single Type signing
   scheme.

   If the two functions are separated then, for almost any method of key
   management and zone signing, the KSK is used less frequently than 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 that is a secure entry point (SEP) for a zone can be
   relatively expensive as it involves interaction with 3rd parties:
   When a key is only pointed to by a DS record in the parent zone, one
   needs to complete the interaction with the parent and wait for the
   updated DS record to appear in the DNS.  In the case where a key is
   configured 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.

   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



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   RR: If the flag field is an odd number the RR is a KSK; otherwise it
   is a ZSK.

   There is also a risk that keys are compromised through theft or loss.
   For keys that are installed on file-systems of nameservers that are
   connected to the network (e.g. for dynamic updates) that risk is
   relatively high.  Where keys are stored on Hardware Security Modules
   (HSMs) or stored off-line, such risk is relatively low.  However,
   storing keys off-line or with more limitation on access control has a
   negative effect on the operational flexibility.  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.  A KSK stored on a smartcard,
   that is kept in a safe, combined with a ZSK stored on a filesystem
   accessible by operators for daily routine may provide a better
   protection against key compromise, without losing much operational
   flexibility.  It must be said that some HSMs give the option to have
   your keys online, giving more protection and hardly affecting the the
   operational flexibility.  In those cases, a KSK-ZSK split is not more
   beneficial than the Single-Type signing scheme.

   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.  When translated to asymmetric
   keys, is still too insignificant a size difference to warrant a key-
   split; it only marginally affects the packet size and signing speed.

   The arguments for differentiation between the ZSK and KSK are weakest
   when:

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

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

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

   o  the interaction through the child-parent provisioning chain -- in
      particular the timely appearance of a new DS record in the parent



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      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 a single type signing scheme is 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.

3.2.  Practical Consequences of KSK and ZSK Separation

   A key that acts only as a Zone Signing Key can be used to sign all
   the data but the DNSKEY RRset in a zone on a regular basis.  When a
   ZSK is to be rolled, no interaction with the parent is needed.  This
   allows for signature validity periods on the order of days.

   A key with only the Key Signing Key role is to be used to sign the
   DNSKEY RRs in a zone.  If a KSK is to be rolled, there may be
   interactions with other parties.  These can include the
   administrators of the parent zone or administrators of verifying
   resolvers that have the particular key configured as secure entry
   points.  In the latter case, everyone relying on the trust anchor
   needs to roll over to the new key, a process that may be subject to
   stability costs if automated trust-anchor rollover mechanisms (such
   as e.g.  RFC5011 [16]) are not in place.  Hence, the key effectivity
   period of these keys can and should be made much longer.

3.2.1.  Rolling a KSK that is not a trust-anchor

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

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

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

   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



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   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, in most cases a zone owner cannot be fully certain that the
   zone's KSK is not in use as a trust-anchor somewhere.  While the
   configuration of trust-anchors is not the responsibility of the zone
   owner there may be stability costs for the validator administrator
   that (wrongfully) configured the trust-anchor when the zone owner
   roles a KSK.

3.2.2.  Rolling a KSK that is a trust-anchor

   The same operational concerns apply to the rollover of KSKs that are
   used as trust-anchors: if a trust anchor replacement is done
   incorrectly, the entire domain 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 one's 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
   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' validators.  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 [16] 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 incurring them unexpectedly.




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   It is therefore recommended, to roll KSKs that are likely to be used
   as trust-anchors, on a regular basis if and only if those rollovers
   can be tracked using standardized (e.g.  RFC5011) mechanisms.

3.2.3.  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, i.e they are
   (to be) pointed to by parental DS RRs or configured as a trust-
   anchor.

   While the SEP flag does not play any role in the failure it is used
   in practice for operational purposes such as for the rollover
   mechanism described in RFC5011 [16].  The common convention is to set
   the SEP flag on any key that is used for key exchanges with the
   parent and/or potentially used for configuration as a trust anchor.
   Therefore it is recommended that the SEP flag is set on keys that are
   used as KSKs and not on keys that are used as ZSKs, while in those
   cases where a distinction between KSK and ZSK is not made (i.e. for a
   Single Type signing scheme) it is recommended that the SEP flag is
   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.

3.3.  Key Effectivity Period

   In general the available key length 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 key length to that value.  Ignoring the
   operational perspective, a reasonable effectivity period for KSKs
   that have corresponding DS records in the 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 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 one year,
   meaning you have the intent to replace them after 12 months.  The key
   effectivity period is merely a policy parameter, and should not be
   considered a constant value.  For example, the real key effectivity
   period may be a little bit longer than 12 months, because not all
   actions needed to complete the rollover could be finished in time.




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   As argued above, this annual rollover gives operational practice of
   rollovers for both the zone and validator administrators.  Besides,
   in most environments a year is a time-span that is easily planned and
   communicated.

   Where 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.

   Although very short key effectivity periods are theoretically
   possible, when replacing keys one has to take into account the
   rollover considerations from Section 4.1 and Section 4.4.  Key
   replacement endures for a couple of Zone TTLs, depending on the
   rollover scenario.  Therefore, a multiple of Zone TTL is a reasonable
   lower limit on the key effectivity period.  Forcing a smaller key
   effectivity period will result your zone to have a ever-growing
   keyset.

   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 weakened.

   In fact, if the risk of loss, theft or other compromise is the same
   for a zone and key signing key there is little reason to choose
   different effectivity periods for ZSKs and KSKs.  And when the split
   between ZSKs and KSKs is not made, the argument is redundant.

   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.

3.4.  Cryptographic Considerations

3.4.1.  Key Algorithm

   At the time of writing, there are three types of signature algorithms
   that can be used in DNSSEC: RSA, DSA and GOST.  Proposals for other
   algorithms are in the making.  All three are fully specified in many
   freely-available documents, and are widely considered to be patent-
   free.  The creation of signatures with RSA and DSA takes roughly the
   same time, but DSA is about ten times slower for signature
   verification.  Also, DSA in context of DNSSEC is limited to the
   maximum of 1024 bit keys.




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   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 appear, 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 and work is in progress on addressing them.  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 implementations (see RFC5702 [23] and RFC4509 [20]).

3.4.2.  Key Sizes

   This section assumes RSA keys, as suggested in the previous section.

   DNSSEC signing keys should be large enough to avoid all known
   cryptographic attacks during the effectivity period 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 to expend phenomenal 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).

   Depending on local policy (e.g. owners of keys that are used as
   extremely high value trust anchors, or non-anchor keys that may be
   difficult to roll over), you may want to use lengths longer than 1024
   bits.  Typically, the next larger key size used is 2048 bits, which
   has the approximate equivalent strength of a symmetric 112-bit key
   (e.g.  RFC3766 [12]).  Signing and verifiying with a 2048-bit key
   takes of course longer than with a 1024-bit key.  The increase
   depends on software and hardware implementations, but public
   operations (such as verification) are about four times slower, while
   private operations (such as signing) slow down about eight times.

   Another way to decide on the size of key to use is to remember that
   the 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 currently installed in web browsers.  If the value of a
   DNSSEC key is lower to the attacker than the value of a TLS trust



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   anchor, the attacker will use the resources to attack the latter.

   It is possible that there will be an unexpected improvement in the
   ability for attackers to break 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 algorithms and keys of 1024 bits or
   higher.

3.4.3.  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/NSEC3 RRs.  Then the augmented file can
   be transferred.

   When relying on dynamic update [10], or any other update mechanism
   that runs at a regular interval to manage a signed zone, be aware
   that at least one private key of the zone will have to reside on the
   master server (or reside on an HSM to which the server has access).
   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 a zone using a "hidden master"
   scheme that minimize the risk.  In this arrangement the master that
   processes the updates is unavailable from general hosts on the
   Internet; it is not listed in the NS RRSet.  The nameservers in the
   NS RRSet are able to receive zone updates through IXFR, AXFR, or an
   out-of-band distribution mechanism, possibly in combination with
   NOTIFY or another mechanism to trigger zone replication.

   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 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.



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   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
   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 an 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 typically be higher than for an individual users).

   o  The costs of rolling the (compromised) keys.  (The costs of
      rolling 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.

3.4.4.  Key Generation

   Careful generation of all keys is a sometimes overlooked but is an
   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 800-90 [19].  In particular, one should
   carefully assess whether the random number generator used during key
   generation adheres to these suggestions.  Typically, HSMs tend to
   provide a good facility for key generation.

   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.



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3.4.5.  Differentiation for 'High-Level' Zones?

   In an earlier version of this document (RFC4641 [14]) we made a
   differentiation between key lengths for KSKs used for zones that are
   high in the DNS hierarchy and those for KSKs used low down.

   This distinction is now considered not relevant.  Longer key lengths
   for keys higher in the hierarchy are not useful because the
   cryptographic 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 take place if the
   key compromise goes unnoticed and the attacker can act as a man-in-
   the-middle (MITM).  For example if example.com is compromised and the
   attacker forges answers for somebank.example.com. and sends them out
   during an MITM, when the attack is discovered it will be simple to
   prove that example.com has been compromised and the KSK will be
   rolled.  Designing 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 separately and zone
   signing keys can be rolled without taking into account 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



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   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 4.1.1.2, uses
   double signatures; the other uses key pre-publication
   (Section 4.1.1.1).  The pros, cons, and recommendations are described
   in Section 4.1.1.3.

4.1.1.1.  Pre-Publish Zone Signing 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.

   Pre-Publish key rollover involves four stages as follows:

    ----------------------------------------------------------
     initial            new DNSKEY          new RRSIGs
    ----------------------------------------------------------
     SOA_0              SOA_1               SOA_2
     RRSIG_Z_10(SOA)    RRSIG_Z_10(SOA)     RRSIG_Z_11(SOA)

     DNSKEY_K_1         DNSKEY_K_1          DNSKEY_K_1
     DNSKEY_Z_10        DNSKEY_Z_10         DNSKEY_Z_10
                        DNSKEY_Z_11         DNSKEY_Z_11
     RRSIG_K_1(DNSKEY)  RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)
    ------------------------------------------------------------

    ------------------------------------------------------------
     DNSKEY removal
    ------------------------------------------------------------
     SOA_3
     RRSIG_Z_11(SOA)

     DNSKEY_K_1
     DNSKEY_Z_11

     RRSIG_K_1(DNSKEY)
    ------------------------------------------------------------


                    Figure 1: Pre-Publish Key Rollover





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   initial:  Initial version of the zone: DNSKEY_K_1 is the Key Signing
      Key. DNSKEY_Z_10 is used to sign all the data of the zone, the
      Zone Signing Key.

   new DNSKEY:  DNSKEY_Z_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_Z_11 is
      used to sign the data in the zone exclusively (i.e., all the
      signatures from DNSKEY_Z_10 are removed from the zone).
      DNSKEY_Z_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 that the key set including DNSKEY_Z_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 zone.

   DNSKEY removal:  DNSKEY_Z_10 is removed from the zone.  The key set,
      now only containing DNSKEY_K_1 and DNSKEY_Z_11, is re-signed with
      the DNSKEY_K_1 and DNSKEY_Z_11.

   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_Z_12 and again a newer one, numbered 13, in "new DNSKEY
   (II)":


















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       initial             new RRSIGs          new DNSKEY
      -----------------------------------------------------------------
       SOA_0               SOA_1               SOA_2
       RRSIG_Z_10(SOA)     RRSIG_Z_11(SOA)     RRSIG_Z_11(SOA)

       DNSKEY_K_1          DNSKEY_K_1          DNSKEY_K_1
       DNSKEY_Z_10         DNSKEY_Z_10         DNSKEY_Z_11
       DNSKEY_Z_11         DNSKEY_Z_11         DNSKEY_Z_12
       RRSIG_K_1(DNSKEY)   RRSIG_K_1 (DNSKEY)  RRSIG_K_1(DNSKEY)
       ----------------------------------------------------------------

       ----------------------------------------------------------------
       new RRSIGs (II)        new DNSKEY (II)
       ----------------------------------------------------------------
       SOA_3                  SOA_4
       RRSIG_Z_12(SOA)        RRSIG_Z_12(SOA)

       DNSKEY_K_1             DNSKEY_K_1
       DNSKEY_Z_11            DNSKEY_Z_12
       DNSKEY_Z_12            DNSKEY_Z_13
       RRSIG_K_1(DNSKEY)      RRSIG_K_1(DNSKEY)
       ----------------------------------------------------------------


       Figure 2: Pre-Publish Zone Signing 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
   a zone needs to be signed.

4.1.1.2.  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.










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   Double Signature ZSK rollover involves three stages as follows:

      ----------------------------------------------------------------
      initial             new DNSKEY         DNSKEY removal
      ----------------------------------------------------------------
      SOA_0               SOA_1              SOA_2
      RRSIG_Z_10(SOA)     RRSIG_Z_10(SOA)    RRSIG_Z_11(SOA)
                          RRSIG_Z_11(SOA)
      DNSKEY_K_1          DNSKEY_K_1         DNSKEY_K_1
      DNSKEY_Z_10         DNSKEY_Z_10
                          DNSKEY_Z_11        DNSKEY_Z_11
      RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)  RRSIG_K_1(DNSKEY)
      ----------------------------------------------------------------


           Figure 3: Double Signature Zone Signing Key Rollover

   initial:  Initial Version of the zone: DNSKEY_K_1 is the Key Signing
      Key. DNSKEY_Z_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_Z_11 is
      introduced into the key set and all the data in the zone is signed
      with DNSKEY_Z_10 and DNSKEY_Z_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_Z_10 is removed from the zone.  All the
      signatures from DNSKEY_Z_10 are removed from the zone.  The key
      set, now only containing DNSKEY_Z_11, is re-signed with DNSKEY_K_1
      and DNSKEY_Z_11.

   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
   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



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   as it is signed with both keys.

4.1.1.3.  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
   ---------------------------------------------------------------------
   Parent:
    SOA_0 -----------------------------> SOA_1 ------------------------>
    RRSIG_par(SOA) --------------------> RRSIG_par(SOA) --------------->
    DS_K_1 ----------------------------> DS_K_2 ----------------------->
    RRSIG_par(DS) ---------------------> RRSIG_par(DS) ---------------->

   Child:
    SOA_0              SOA_1 -----------------------> SOA_2
    RRSIG_Z_10(SOA)    RRSIG_Z_10(SOA) -------------> RRSIG_Z_10(SOA)

    DNSKEY_K_1         DNSKEY_K_1 ------------------>
                       DNSKEY_K_2 ------------------> DNSKEY_K_2
    DNSKEY_Z_10        DNSKEY_Z_10 -----------------> DNSKEY_Z_10
    RRSIG_K_1(DNSKEY)  RRSIG_K_1 (DNSKEY) ---------->
                       RRSIG_K_2 (DNSKEY) ----------> RRSIG_K_2(DNSKEY)
   ---------------------------------------------------------------------

    Figure 4: Stages of Deployment for a Double Signature Key  Signing
                               Key Rollover

   initial:  Initial version of the zone.  The parental DS points to
      DNSKEY_K_1.  Before the rollover starts, the child will have to
      verify what the TTL is of the DS RR that points to DNSKEY_K_1 --
      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, DNSKEY_K_2.  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 DNSKEY_K_2.  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 DS_K_1 with DS_K_2.

   DNSKEY removal:  DNSKEY_K_1 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.

4.1.2.1.  Special Considerations for RFC5011 KSK rollover

   The scenario sketched above assumes that the KSK is not in use as a
   trust-anchor too but that validating nameservers exclusively depend
   on the parental DS record to establish the zone's security.  If it is
   known that validating nameservers have configured trust-anchors then
   such needs to be taken into account.  Here we assume that operators
   of zones will deploy RFC5011 [16] style rollovers.

   RFC5011 style rollovers increase the duration of key rollovers: the
   key to be removed must first be revoked.  Thus, before the DNSKEY_K_1
   removal phase, DNSKEY_K_1 must be published for one more Maximum Zone
   TTL with the REVOKE bit set.  The revoked key must be self-signed, so
   in this phase the DNSKEY RRset must also be signed with DNSKEY_K_1.

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 ZSK rollover can be handled in two different ways, meaningful: Pre-
   Publish (Section 4.1.1.1) and Double Signature (Section 4.1.1.2).

   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.  A Pre-Publish method is also possible for KSKs,
   known as the Double-DS rollover.  The name being a give away, the
   record that needs to be pre-published is the DS RR at the parent.
   The Pre-Publish method has some drawbacks for KSKs.  We first
   describe the rollover scheme and then indicate these drawbacks.

















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   --------------------------------------------------------------------
     initial         new DS         new DNSKEY       DS removal
   --------------------------------------------------------------------
   Parent:
     SOA_0           SOA_1 ------------------------> SOA_2
     RRSIG_par(SOA)  RRSIG_par(SOA) ---------------> RRSIG_par(SOA)
     DS_K_1          DS_K_1 ----------------------->
                     DS_K_2 -----------------------> DS_K_2
     RRSIG_par(DS)   RRSIG_par(DS) ----------------> RRSIG_par(DS)

   Child:
     SOA_0 -----------------------> SOA_1 ---------------------------->
     RRSIG_Z_10(SOA) -------------> RRSIG_Z_10(SOA) ------------------>

     DNSKEY_K_1 ------------------> DNSKEY_K_2 ----------------------->
     DNSKEY_Z_10 -----------------> DNSKEY_Z_10 ---------------------->
     RRSIG_K_1 (DNSKEY) ----------> RRSIG_K_2 (DNSKEY) --------------->
   --------------------------------------------------------------------


      Figure 5: Stages of Deployment for a Double-DS 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 DS_S_1 and DS_S_2, pointing to
   DNSKEY_S_1 and DNSKEY_S_2, 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 DNSKEY_S_1 with
   DNSKEY_S_2.  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 DS_S_2 RR and DNSKEY_S_2
   using the DNS -- as DNSKEY_S_2 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
     ----------------------------------------------------------------
   Parent:
     SOA_0 --------------------------> SOA_1 ---------------------->
     RRSIG_par(SOA) -----------------> RRSIG_par(SOA) ------------->
     DS_S_1 -------------------------> DS_S_2 --------------------->
     RRSIG_par(DS_S_1) --------------> RRSIG_par(DS_S_2) ---------->

   Child:
     SOA_0             SOA_1 ----------------------> SOA_2
     RRSIG_S_1(SOA)    RRSIG_S_1(SOA) ------------->
                       RRSIG_S_2(SOA) -------------> RRSIG_S_2(SOA)
     DNSKEY_S_1        DNSKEY_S_1 ----------------->
                       DNSKEY_S_2 -----------------> DNSKEY_S_2
     RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY) ---------->
                       RRSIG_S_2(DNSKEY) ----------> RRSIG_S_2(DNSKEY)
     -----------------------------------------------------------------

     Figure 6: Stages of the Straightforward rollover in a Single Type
                              Signing Scheme

   initial:  Parental DS points to DNSKEY_S_1.  All RR sets in the zone
      are signed with DNSKEY_S_1.

   new DNSKEY:  A new key (DNSKEY_S_2) is introduced and all the RR sets
      are signed with both DNSKEY_S_1 and DNSKEY_S_2.

   DS change:  After the DNSKEY RRset with the two keys had time to
      propagate into distant caches (that is the key set exclusively
      containing DNSKEY_S_1 has been expired) the parental DS record can
      be changed.

   DNSKEY removal:  After the DS RRset containing DS_S_1 has expired
      from distant caches DNSKEY_S_1 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
   DNSKEY_S_1.  One replaces the DNSKEY_S_1 signatures with signatures
   made with DNSKEY_S_2 at the moment of DNSKEY_S_1 removal.

   The second variety of this rollover can be considered when zone size



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   considerations prevent the introduction of double signatures over all
   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 double
   signatures or a double keyset.  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.

   There is also a second variety of the Double-DS rollover during which
   one introduces a new DNSKEY into the key set and submit the new DS to
   the parent.  The new key is not yet used to sign RRsets.  One
   replaces the DNSKEY_S_1 signatures with signatures made with
   DNSKEY_S_2 at the moment that DNSKEY_S_2 and DS_S_2 have been
   propagated.

   Again, this second variety of this rollover can be considered when
   zone size considerations prevent the introduction of double
   signatures over all of the zone data although also in this case,
   choosing for a KSK/ZSK split may be a better option.

4.1.5.  Algorithm rollovers

   A special class of key rollover is the one needed for a change of key
   algorithms (either adding a new algorithm, removing an old algorithm,
   or both).  Additional steps are needed to retain integrity during
   this rollover.  We first describe the generic case, special
   considerations for rollovers that involve trust-anchors and single
   type keys are discussed below.

   There exist a conservative and a liberal approach for algorithm
   rollover.  This has to do with section 2.2 in RFC4035 [5]:


    There MUST be an RRSIG for each RRset using at least one DNSKEY of
    each algorithm in the zone apex DNSKEY RRset.  The apex DNSKEY RRset
    itself MUST be signed by each algorithm appearing in the DS RRset
    located at the delegating parent (if any).


   The conservative approach interprets this section very strict,
   meaning that it expects that all RRset has a valid signature for
   every algorithm signalled by the zone apex DNSKEY RRset, no matter
   where this RRset is kept.  Important to know is that this also
   includes the resolvers cache.  The liberal approach uses a more loose
   interpretation of the section and limits the rule to RRsets in the



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   zone at the authoritative name servers.  There is a reasonable
   argument for saying that this is valid, because the specific section
   is a subsection of section 2. in RFC4035: Zone Signing.

   When following the more liberal approach, algorithm rollover is just
   as easy as a regular Double-Signature KSK rollover (Section 4.1.2).
   Note that the Double-DS rollover method cannot be used, since that
   would introduce a parental DS of which the apex DNSKEY RRset has not
   been signed with the introduced algorithm.

   However, there are implementations of validators known that follow
   the more conservative approach.  Performing a Double-Signature KSK
   algorithm rollover will temporarily make your zone appear as Bogus by
   such validators during the rollover.  Therefore, the rollover in this
   section will explain the stages of deployment assuming the
   conservative approach.

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

   After the new algorithm has been added, the DS record can be
   exchanged using Double Signature Key Rollover.  You cannot use Pre-
   Publish key rollover method when you do key algorithm rollover.

   When removing an old algorithm, the DNSKEY should be removed first,
   but only after the DS for the old algorithm was removed from the
   parent zone.

   Figure 7 describes the steps.  The underscored number indicates the
   algorithm and ZSK and KSK indicate the obvious difference in key use.
   For example DNSKEY_K_1 is a the DNSKEY RR representing the public
   part of the old key signing key of algorithm type 1 while
   RRSIG_Z_2(SOA) is the RRSIG RR made with the private part of the new
   zone signing key of algorithm type 2 over a SOA RR.  It is assumed
   that the key that signs the SOA RR also signes all other non-DNSKEY
   RRset data.













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   ----------------------------------------------------------------
    initial              new RRSIGs           new DNSKEY
   ----------------------------------------------------------------
   Parent:
    SOA_0 -------------------------------------------------------->
    RRSIG_par(SOA) ----------------------------------------------->
    DS_K_1 ------------------------------------------------------->
    RRSIG_par(DS_K_1) -------------------------------------------->

   Child:
    SOA_0                SOA_1                SOA_2
    RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)
                         RRSIG_Z_2(SOA)       RRSIG_Z_2(SOA)

    DNSKEY_K_1           DNSKEY_K_1           DNSKEY_K_1
                                              DNSKEY_K_2
    DNSKEY_Z_1           DNSKEY_Z_1           DNSKEY_Z_1
                                              DNSKEY_Z_2
    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)
                                              RRSIG_K_2(DNSKEY)

   ----------------------------------------------------------------
    new DS               DNSKEY removal       RRSIGs removal
   ----------------------------------------------------------------
   Parent:
    SOA_0 ------------------------------------------------------->
    RRSIG_par(SOA) ---------------------------------------------->
    DS_K_2 ------------------------------------------------------>
    RRSIG_par(DS_K_2) ------------------------------------------->

   Child:
    -------------------> SOA_3                SOA_4
    -------------------> RRSIG_Z_1(SOA)
    -------------------> RRSIG_Z_2(SOA)       RRSIG_Z_2(SOA)

    ------------------->
    -------------------> DNSKEY_K_2           DNSKEY_K_2
    ------------------->
    -------------------> DNSKEY_Z_2           DNSKEY_Z_2
    ------------------->
    -------------------> RRSIG_K_2(DNSKEY)    RRSIG_K_2(DNSKEY)
   ----------------------------------------------------------------


        Figure 7: Stages of Deployment during an Algorithm Rollover






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   initial:  Describes state of the zone before any transition is done.
      Number of the keys may vary, but the algorithm of keys in the zone
      is same for all DNSKEY records.

   new RRSIGs:  The signatures made with the new key over all records in
      the zone are added, but the key itself is not.  This step is
      needed to propagate the signatures created with the new algorithm
      to the caches.  If you do not do that, it might happen that the
      resolver picks up the new DNSKEY RRset (with the new algorithm
      included), but does not yet have the corresponding new signatures,
      because it has a previous version of the RRset plus signatures in
      the cache.

      The RRSIG for the DNSKEY RRset does not need to be pre-published,
      since these records will travel together and does not need special
      processing in order to keep them synchronized.

   new DNSKEY:  After the cache data has expired, the new key can be
      added to the zone.

   new DS:  After the cache data for the DNSKEY has expired, the DS
      record for the new key can be added to the parent zone and the DS
      record for the old key can be removed in the same step.

   DNSKEY removal:  After the cache data for the DS has expired, the old
      algorithm can be removed.  This time the key needs to be removed
      first, before removing the signatures.

   RRSIGs removal:  After the cache data for the DNSKEY has expired, the
      signatures can also be removed during this step.

   Below we deal with a few special cases of algorithm rollovers.

   1: Single Type Signing Scheme Algorithm Rollover  : when you have
      chosen not to differentiate between Zone and Key signing keys
      (Section 4.1.5.1)

   2: RFC5011 Algorithm Rollover  : when trust-anchors can track the
      roll via RFC5011 style rollover (Section 4.1.5.2)

   3: 1 and 2 combined  : when a Single Type Signing Scheme Algorithm
      rollover is RFC5011-enabled (Section 4.1.5.3)

   In addition to the narrative below these special cases are
   represented in Figure 11, Figure 12 and Figure 13 in Appendix C.






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4.1.5.1.  Single Type Signing Scheme Algorithm Rollover

   If one key is used that acts both as ZSK and KSK, the same scheme and
   figure as above applies whereby all DNSKEY_Z_* records from the table
   are removed and all RRSIG_Z_* are replaced with RRSIG_K_*.  The
   requirement to sign with both algorithms and make sure that old
   RRSIGS have the opportunity to expire from distant caches before
   introducing the new algorithm in the DNSKEY RRset is still valid.

   Also see Figure 11 in Appendix C.

4.1.5.2.  Algorithm rollover, RFC5011 style

   Trust anchor algorithm rollover is almost as simple as a regular
   RFC5011 based rollover.  However, the old trust anchor must be
   revoked before it is removed from the zone.

   Take a look at the Figure 7 above.  After the "new DS" step, we need
   an additional step where the DNSKEY is revoked (revoke DNSKEY):


   ---------------------------------
     revoke DNSKEY
   ---------------------------------
   Parent:
     ----------------------------->
     ----------------------------->
     ----------------------------->
     ----------------------------->

   Child:
     SOA_3
     RRSIG_Z_1(SOA)
     RRSIG_Z_2(SOA)

     DNSKEY_K_1_REVOKED
     DNSKEY_K_2
     DNSKEY_Z_1
     DNSKEY_Z_2
     RRSIG_K_1(DNSKEY)
     RRSIG_K_2(DNSKEY)
   --------------------------------

      Figure 8: The Revoke DNSKEY state that is added to an algorithm
                     rollover when RFC5011 is in use.

   There is one exception to the rule above.  While all zone data must
   be signed with an unrevoked key, it is permissible to sign the keyset



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   with a revoked key.  The somewhat esoteric argument follows.

   Resolvers that do not understand the RFC5011 Revoke flag will handle
   DNSKEY_K_1_REVOKED the same as if it was DNSKEY_K_1.  In other words,
   they will handle the revoked key as a normal key, and thus RRsets
   signed with this key will validate.  As a result, the signature
   matches the algorithm listed in the DNSKEY RRset.  Resolvers that do
   implement RFC5011 will remove DNSKEY_K_1 from the set of trust
   anchors.  That is okay, since they have already added DNSKEY_K_2 as
   the new trust anchor.  Thus, algorithm 2 is the only signaled
   algorithm by now.  That means, we only need RRSIG_K_2(DNSKEY) to
   authenticate the DNSKEY RRset, and we still are compliant with
   section 2.2 from RFC 4035: There must be a RRSIG for each RRset using
   at least one DNSKEY of each algorithm in the zone apex DNSKEY RRset.

   Also see Figure 12 in Appendix C.

4.1.5.3.  Single Signing Type Algorithm Rollover, RFC5011 style

   Combining the Single Signing Type Scheme Algorithm Rollover and
   RFC5011 style rollovers is not trivial.

   Should you choose to perform an RFC5011 style rollover with a Single
   Signing Type key then remember that section 2.1, RFC 5011 states:


       Once the resolver sees the REVOKE bit, it MUST NOT use this key
       as a trust anchor or for any other purpose except to validate
       the RRSIG it signed over the DNSKEY RRSet specifically for the
       purpose of validating the revocation.


   This means that if you revoke DNSKEY_S_1, it cannot be used to
   validate its signatures over non-DNSKEY RRsets.  Thus, those RRsets
   should be signed with a shadow key, DNSKEY_Z_1, during the algorithm
   rollover.  This shadow key can be introduced at the same time the
   signatures are pre-published, in step 2 (new RRSIGs).  The shadow key
   must be removed at the same time the revoked DNSKEY_S_1 is removed
   from the zone.  De-facto you temporarily falling back to a KSK/ZSK
   split model.

   In other words, the rule that at every RRset there must be at least
   one signature for each algorithm used in the DNSKEY RRset still
   applies.  This means that a different key with the same algorithm,
   other than the revoked key, must sign the entire zone.  This can be
   the ZSK.  Thus, more operations are needed if the Single Type Signing
   Scheme is used.  Before rolling the algorithm, a new key must be
   introduced with the same algorithm as the key that is candidate for



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   revocation.  That key can than temporarily act as ZSK during the
   algorithm rollover.

   Just like with algorithm rollover RFC5011 style, while all zone data
   must be signed with an unrevoked key, it is permissible to sign the
   keyset with a revoked key, for the same esoteric argument described
   in Section 4.1.5.2.

   The lesson of all of this is that a Single Type Signing scheme
   algorithm rollover using RFC5011 is as complicated as the name of the
   rollover implies, one is better off explicitly using a split key
   temporarily.

   Also see Figure 12 in Appendix C.

4.1.5.4.  NSEC to NSEC3 algorithm rollover

   A special case is the rollover from an NSEC signed zone to an NSEC3
   signed zone.  In this case algorithm numbers are used to signal
   support for NSEC3 but they do not mandate the use of NSEC3.
   Therefore NSEC records should remain in the zone until 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 point 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 changing from
   zone serial number, 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
   existence.

   Summarizing, an NSEC to NSEC3 rollover is an ordinary algorithm
   rollover whereby NSEC is used all the time and only after that
   rollover finished NSEC3 needs to be deployed.

4.1.6.  Considerations for 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
      involved.

   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



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      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,

   o  as long as the DS RR in the parent zone 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
      update).

   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, during the rollover the
   malicious key holder can continue to 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 DS
   record in the parent zone 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
   parent DS record 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
   record pointing to the compromised KSK (the assumption is that there



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   is only one DS record at the parent.  If there are multiple DS
   records 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 the corresponding DS record in the parent 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 record for the new KSK in place.

4.2.1.1.  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.  We therefore have to
   make sure that the signature made with a new KSK over the key set
   that contains the compromised KSK expires just after the new DS
   appears at the parent.  Expiration of that signature will cause
   expiration of that key set from the caches.

   The procedure is as follows:

   1.  Introduce a new KSK into the key set, keep the compromised KSK in
       the key set.  Lower the TTL for DNSKEYs so that it will expire
       faster from caches.

   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.  This
       provides an upper limit on how long the compromised KSK can be
       used in a replay attack.

   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" TTL and signature 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.



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

4.2.1.2.  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.  Note that until
   that time, the child zone is still vulnerable to spoofing: the
   attacker is still in possesion of the compromised key that the DS
   points to.

   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.

4.2.2.  ZSK Compromise

   Primarily because there is no interaction with the parent 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
   until the RRSIG over the compromised ZSK has 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 as a trust-anchor.  If
   trust-anchor keys are compromised, the administrators of 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 by some means,
   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-



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   of-band, for example, through the use of an announcement website that
   is secured using secure sockets (TLS) [22].

4.2.4.  Stand-by keys

   Stand-by keys are keys that are published in your zone, but are not
   used to sign RRsets.  There are two reasons why someone would want to
   use stand-by keys.  One is to speed up the emergency key rollover.
   The other is to recover from a disaster that leaves your production
   private keys inaccessible.

   The way to deal with stand-by keys differs for ZSKs and KSKs.  To
   make a stand-by ZSK, you need to publish its DNSKEY RR.  To make a
   stand-by KSK, you need to get its DS RR published at the parent.

   Assuming you have your DNS operation at location A, to prepare
   stand-by keys you need to:

   o  Generate a stand-by ZSK and KSK.  Store them safely in a different
      location (B) than the currently used ZSK and KSK (that are at
      location A).

   o  Pre-publish DNSKEY RR of the stand-by ZSK in the zone.

   o  Pre-publish DS of the stand-by KSK in the parent zone.

   Now suppose a disaster occurs at location A, that disables the access
   to your currently used keys.  To recover from that situation, follow
   these procedures:

   o  Set up your DNS operations and import the stand-by keys from
      location B.

   o  Post-publish the old ZSK and sign the zone with the stand-by keys.

   o  After some time, when the new signatures have been propagated, the
      old ZSK and DS can be removed.

   o  Generate a new stand-by keyset at a different location and
      continue "normal" operation.

4.3.  Parent 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.  It is specifically important in a registry-registrar model
   where the key material is to be passed from the DNS operator, to the



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   (parent) registry via a registrar, where both DNS operator and
   registrar are selected by the registrant and 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 is
   for regular DNS.

   Using the DNS itself as the source for the actual DNSKEY material 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.

   With some type of key rollovers, the DNSKEY is not pre-published and
   a DNSKEY query tool is not able to retrieve the successor key.  In
   this case, the out-of-band method is required.  This also allows the
   child to determine the digest algorithm of the DS record.

4.3.2.  Storing Keys or Hashes?

   When designing a registry system one should consider whether to store
   the DNSKEYs and/or the corresponding DSes.  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 (also see
   draft-ietf-dnsop-dnssec-trust-anchor [26]).

   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) [17], that can be used for registrar-registry
   interactions since that protocol allows the transfer of both DS and
   optionally DNSKEY RRs.  There is no standardized way for moving the
   data between the customer and the registrar.  Different registrars



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   have different mechanisms, ranging from simple web interfaces to
   various APIs.  In some cases the use of the DNSSEC extensions to EPP
   may be applicable.

   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.

4.3.3.  Security Lameness

   Security lameness is defined as the state whereby the parent has a DS
   RR pointing to a non-existing DNSKEY RR.  Security lameness may occur
   temporarily during a Double-DS rollover scheme.  However care should
   be taken that not all DS RRs are security lame which may cause the
   child's zone to 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 for the DS RRSIG 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 (this is a generic argument also
   see Section 4.4.2).  Something as mundane as operator unavailability
   during weekends shows the need for DS signature validity periods
   longer than two 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



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   considerably longer than the child would hope for.

   A compromise between the policy/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 reduces the damage of a successful replay
   attack.  It does mean 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.

4.3.5.  Changing DNS Operators

   The parent-child relation is often described in terms of a registry-
   registrar-registrant 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.

   For various reasons, a registrant may want to move between DNS
   operators.  How easy this move will be depends principally on the DNS
   operator from which the registrant is moving (the losing operator),
   as they have control over the DNS zone and its keys.  The following
   sections describe the two cases: where the losing operator cooperates
   with the new operator (the gaining operator), and where the two do
   not cooperate.

4.3.5.1.  Cooperating DNS operators

   In this scenario, it is assumed that losing operator will not pass
   any private key material to the gaining operator (that would
   constitute a trivial case) but is otherwise fully cooperative.

   In this environment one could proceed with a Pre-Publish ZSK rollover
   whereby the losing operator pre-publishes the ZSK of the gaining
   operator, combined with a Double Signature KSK rollover where the two
   registrars exchange public keys and independently generate a
   signature over those keysets that they combine and both publish in



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   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.

    ------------------------------------------------------------
    initial            |        pre-publish                    |
    ------------------------------------------------------------
    Parent:
     NS_A                            NS_A
     DS_A                            DS_A
    ------------------------------------------------------------
    Child at A:            Child at A:       Child at B:
     SOA_A0                 SOA_A1            SOA_B0
     RRSIG_Z_A(SOA)         RRSIG_Z_A(SOA)    RRSIG_Z_B(SOA)

     NS_A                   NS_A              NS_B
     RRSIG_Z_A(NS)          NS_B              RRSIG_Z_B(NS)
                            RRSIG_Z_A(NS)




     DNSKEY_Z_A             DNSKEY_Z_A         DNSKEY_Z_A
                            DNSKEY_Z_B         DNSKEY_Z_B
     DNSKEY_K_A             DNSKEY_K_A         DNSKEY_K_A
                            DNSKEY_K_B         DNSKEY_K_B
     RRSIG_K_A(DNSKEY)      RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)
                            RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)
    ------------------------------------------------------------

    ------------------------------------------------------------
          Redelegation                 |   post migration      |
    ------------------------------------------------------------
    Parent:
              NS_B                           NS_B
              DS_B                           DS_B
    ------------------------------------------------------------
    Child at A:       Child at B:             Child at B:

     SOA_A2             SOA_B1                SOA_B2
     RRSIG_Z_A(SOA)     RRSIG_Z_B(SOA)        RRSIG_Z_B(SOA)

     NS_A               NS_B                  NS_B
     NS_B               RRSIG_Z_B(NS)         RRSIG_Z_B(NS)
     RRSIG_Z_A(NS)


     DNSKEY_Z_A         DNSKEY_Z_A
     DNSKEY_Z_B         DNSKEY_Z_B            DNSKEY_Z_B



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     DNSKEY_K_A         DNSKEY_K_A
     DNSKEY_K_B         DNSKEY_K_B            DNSKEY_K_B
     RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)
     RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)     RRSIG_K_B(DNSKEY)

    ------------------------------------------------------------

               Figure 9: Rollover for cooperating operators

   In this figure A denotes the losing operator and B the gaining
   operator.  RRSIG_Z is the RRSIG produced by a ZSK, RRSIG_K is
   produced with a KSK, the appended A or B indicates the producers of
   the key pair.  Child at A is how the zone content is represented by
   the losing DNS operator and Child at B is how the zone content is
   represented by the gaining DNS operator.

   The requirement to exchange signatures has a couple of drawbacks.  It
   requires more operational overhead, because not only the operators
   have to exchange public keys, they also have to exchange the
   signatures of the new DNSKEY RRset.  Also, it disallows the children
   to refresh the signatures when they expire for a certain period.
   Both drawbacks do not exist if you replace the Double Signature KSK
   rollover with a Double-DS KSK rollover.

   Thus, if the registry and registrars allow for DS records to be
   published, that do not point to a published DNSKEY in the child zone,
   the Double-DS KSK rollover is preferred (also known as Pre-
   Publication KSK Rollover, see Figure 5), in combination with the Pre-
   Publish ZSK rollover.  This does not require to share the KSK
   signatures between the operators.  Both the losing and the gaining
   operator still need to publish the public ZSK of each other.

4.3.5.2.  Non Cooperating DNS Operators

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

   The problem arises when a validator tries to validate with the losing
   operator's key and there is no signature material produced with the
   losing operator available in the delegation path after redelegation
   from the losing operator to the gaining operator has taken place.
   One could imagine a rollover scenario where the gaining operator
   pulls all RRSIGs created by the losing operator and publishes those
   in conjunction with its own signatures, but that would not allow any



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   changes in the zone content.  Since a redelegation took place the NS
   RRset has - by definition - changed so such rollover scenario will
   not work.  Besides if zone transfers are not allowed by the losing
   operator and NSEC3 is deployed in the losing operator's zone, then
   the gaining operator's zone will not have certainty that all of A's
   RRSIGs are transferred.

   The only viable option for the registrant is to publish its zone
   unsigned and ask the registry to remove the DS RR pointing to the
   losing operator's DNSKEY.

   Note that some behavior of resolver implementations may aid in the
   process of changing DNS operators:

   o  TTL sanity checking, as described in RFC2308 [9], will limit the
      impact the actions of an obstructive, losing operator.  Resolvers
      that implement TTL sanity checking will use an upper limit for
      TTLs on RRsets in responses.

   o  If RRsets at the zone cut (are about to) expire, the resolver
      restarts its search above the zone cut.  Otherwise, the resolver
      risks to keep using a nameserver that might be undelegated by the
      parent.

   o  Limiting the time DNSKEYS that seem to be unable to validate
      signatures are cached and/or trying to recover from cases where
      DNSKEYs do not seem to be able to validate data, also reduces the
      effects of the problem of non-cooperating registars.

   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
   operators are changed the services that that zone references also
   change operator, possibly involving some downtime.

   In any case, to minimise such problems, the classic recommendation 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



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   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 thorough 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
   following:

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

         If the TTL was 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
         8.1 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 (but preferably a few days) before the
      end of the signature validity period.

         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 interval from signature expiration are recreated.  Also
         see Section 4.4.2 below.




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         In case of an operational error, you would have one Maximum
         Zone TTL duration to resolve the problem.  Re-signing a zone a
         few days before the end of the signature validity period
         ensures the signatures will survive a weekend in case of such
         operational havoc.  This is called the Refresh period (see
         Section 4.4.2).

   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
      problems:

         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: DS, DNSKEY, RRSIG, and the
         final answers, i.e., the RRSet that is returned for the initial
         query.

         2.  Frequent verification causes load on recursive nameservers.
         Data at delegation points, DS, DNSKEY, and RRSIG RRs benefit
         from caching.  The TTL on those should be relatively long.
         Data at the leafs in the DNS tree has less impact on recursive
         nameservers.

   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 synchronization 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 by modifying 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 Refresh period (see
         Section 4.4.2).

         The consequence of an authoritative server not being able to
         update a zone for an extended period of time is that signatures
         may expire.  In this case 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 a quarter 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 systems to identify upcoming
         signature expirations in zones they slave and take appropriate
         action where such an event is detected.

         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

4.4.2.1.  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 several months.  Although signature validity periods of
   many years are allowed the same operational habit arguments as in
   Section 3.2.2 play a role: when a zone is re-signed with some
   regularity then operators remain conscious about the operational
   necessity of re-signing.

4.4.2.2.  Minimum Value

   The minimum value of the signature validity period is set for the
   time by which one would like to survive operational failure in
   provisioning: what is the time that a failure will be noticed, what
   is the time that action is expected to be taken?  By answering these
   questions availability of operators during (long) weekends or time



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   taken to access to backup media can be taken into account.  The
   result 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 Re-
   Sign 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 Re-Sign Period must be smaller than
   the Refresh Period in order for zone data to be signed in timely
   fashion.

   If an operational problem occurs during resigning then the signatures
   in the zone to expire first are the ones that have been generated
   longest ago.  In the worst case these signatures are the Refresh
   Period minus the Re-Sign Period away from signature expiration.

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

   To make matters slightly more complicated, some signers vary the
   signature validity period over a small range (the jitter interval) 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.




















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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->|
                                                |                  |

                  Figure 10: Signature Timing Parameters

   Note that in the figure the validity of the signature starts shortly
   before the inception time.  That is done to deal with validators that
   might have some clock skew.  The inception offset should be chosen so
   that you minimize the false negatives to a reasonable level.

4.4.2.3.  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.

   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
   accordingly.




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   When a zone contains secure delegations then a relatively short
   signature validity interval protects the child against 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 interval and a higher
   operational risk for the child when choosing a long validity period
   some (price) 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.

5.  Next Record type

   One of the design tradeoffs made during the development of DNSSEC was
   to separate the signing and serving operations instead of performing
   cryptographic operations as DNS requests are being serviced.  It is
   therefore necessary 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: a 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) is
   similar but first hashes the names using a one-way hash function,
   before creating a sorted linked list of the resulting (hashed)
   strings.

   The NSEC record requires no cryptographic operations aside from the
   validation of 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
   request all the entries of a zone by following the linked list of



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   NSEC RRs via the "Next Domain Name" field.

   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 behavior 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 a non-
   disclosure agreement before obtaining it, NSEC3 is recommended.

   The considerations for the second reason to deploy NSEC3 are



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   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, by
   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 consider NSEC3 hash
   algorithm transition.

5.3.2.  NSEC3 Iterations

   One of the concerns with NSEC3 is a pre-calculated dictionary attack
   could be made 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 Section 10.3 [21] considers the trade-offs between incurring
   cost during the signing process and imposing costs to the validating
   nameserver, while still providing a reasonable barrier against
   dictionary attacks.  It provides useful limits of iterations for a
   given 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 a value of 100 iterations is deemed to be a sufficiently
   costly yet not excessive value: In the worst case scenario, the
   performance of your nameservers would be halved, regardless of key
   size [27].

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
   zone.

   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 usually requires
   all RRSIG's to be regenerated (you can have a smooth ZSK rollover by
   honoring the Refresh period).  If there is no critical dependency on
   incremental signing and the 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
   updated.

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.  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 relief to 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.  Contributors and Acknowledgments

   Significant parts of the text of this document is copied from RFC4641
   [14].  That document was edited by Olaf Kolkman and Miek Gieben.
   Other people that contributed or where otherwise involved in that



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   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, and O. Courtay.

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

   Paul Hoffman  for his contribution on the choice of cryptographic
      parameters and addressing some of the trust anchor issues;

   Jelte Jansen  who provided the initial text in Section 4.1.5;

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

   Erik Rescorla  whose blogpost on "the Security of ZSK rollovers"
      inspired text in Section 3.1;

   Stephen Morris  who made a pass on English style and grammar;

   Matthijs Mekking  thorougly reviewed and provided concrete
      improvements on the specific types of keyrollovers (e.g. he
      provided the tables in Appendix C); and

   Olafur Gudmundsson and Ondrej Sury  who provided input on
      Section 4.1.5 based on actual operational experience.

   Rickard Bellgrim  reviewed the document extensively.

   The figure in Section 4.4.2 was adapted from the OpenDNSSEC user
   documentation.

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

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.



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   [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.

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

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

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




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   [17]  Gould, J. and S. Hollenbeck, "Domain Name System (DNS) Security
         Extensions Mapping for the Extensible Provisioning Protocol
         (EPP)", RFC 5910, May 2010.

   [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
         (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-01 (work in
         progress), October 2010.

   [25]  Ljunggren, F., Eklund-Lowinder, A., and T. Okubo, "DNSSEC
         Policy & Practice Statement Framework",
         draft-ietf-dnsop-dnssec-dps-framework-03 (work in progress),
         November 2010.

   [26]  Larson, M. and O. Gudmundsson, "DNSSEC Trust Anchor
         Configuration and Maintenance",
         draft-ietf-dnsop-dnssec-trust-anchor-04 (work in progress),
         October 2010.

   [27]  Schaeffer, Y., "NSEC3 Hash Performance", NLnet Labs
         document 2010-02, 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.




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   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 for RSA keys.  It is mathematically more
      correct to use modulus size for RSA keys, but as this is a
      document directed at operators we feel more at ease with the term
      key size.

   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 period before the expiration time of the
      signature, during which the signature is refreshed by the signer.

   Re-Signing frequency:  Frequency with which a signing pass on the
      zone is performed.  Alternatively expressed as "Re-Signing
      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/interval.

   Secure Entry Point (SEP) key:  A KSK that has a DS record in the
      parent zone 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.




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   Signing Jitter:  Jitter applied to the signature validty interval.

   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 Signing 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.

   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.  Typographic Conventions

   The following typographic conventions are used in this document:

   Key notation:  A key is denoted by DNSKEY_x_y, where x is an
      identifier for the type of key: K for Keys Signing Key, Z for Zone
      Signing Key and S when there is no distinction made between KSK
      and ZSKs but the key is used as a secure entry point.  The 'y'
      denotes a number or an identifier, y 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 192.0.2.1" 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".

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

   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 "SOA_x"






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

   RRsets ignored:  If the signature of non DNSKEY RRsets have the same
      parameters as the SOA than those are not mentioned. e.g.  In the
      example below the SOA is signed with the same parameters as the
      foo.example.com A RRset and the latter is therefore ignored in the
      abbreviated notation.

   Using this notation the following signed zone:

   example.com.  3600  IN SOA   ns1.example.com. olaf.example.net. (
                           2005092303 ; serial
                           450        ; refresh (7 minutes 30 seconds)
                           600        ; retry (10 minutes)
                           345600     ; expire (4 days)
                           300        ; minimum (5 minutes)
                           )
          3600    RRSIG    SOA 5 2 3600 20120824013000 (
                           20100424013000 14 example.com.
                           NMafnzmmZ8wevpCOI+/JxqWBzPxrnzPnSXfo
                           ...
                           OMY3rTMA2qorupQXjQ== )
          3600    NS    ns1.example.com.
          3600    NS    ns2.example.com.
          3600    NS    ns3.example.com.
          3600    RRSIG    NS 5 2 3600 20120824013000 (
                           20100424013000 14 example.com.
                           p0Cj3wzGoPFftFZjj3jeKGK6wGWLwY6mCBEz
                           ...
                           +SqZIoVHpvE7YBeH46wuyF8w4XknA4Oeimc4
                           zAgaJM/MeG08KpeHhg== )
          3600    TXT      "Net::DNS  domain"
          3600    RRSIG    TXT 5 2 3600 20120824013000 (
                           20100424013000 14 example.com.
                           o7eP8LISK2TEutFQRvK/+U3wq7t4X+PQaQkp
                           ...
                           BcQ1o99vwn+IS4+J1g== )
          300    NSEC      foo.example.com. NS SOA TXT RRSIG NSEC DNSKEY
          300    RRSIG     NSEC 5 2 300 20120824013000 (
                           20100424013000 14 example.com.
                           JtHm8ta0diCWYGu/TdrE1O1sYSHblN2i/IX+
                           ...
                           PkXNI/Vgf4t3xZaIyw== )
          3600    DNSKEY   256 3 5 (
                           AQPaoHW/nC0fj9HuCW3hACSGiP0AkPS3dQFX
                           ...
                           sAuryjQ/HFa5r4mrbhkJ



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                           ) ; key id = 14
          3600    DNSKEY   257 3 5 (
                           AQPUiszMMAi36agx/V+7Tw95l8PYmoVjHWvO
                           ...
                           oy88Nh+u2c9HF1tw0naH
                           ) ; key id = 15
          3600    RRSIG    DNSKEY 5 2 3600 20120824013000 (
                           20100424013000 14 example.com.
                           HWj/VEr6p/FiUUiL70QQWtk+NBIlsJ9mdj5U
                           ...
                           QhhmMwV3tIxJk2eDRQ== )
          3600    RRSIG    DNSKEY 5 2 3600 20120824013000 (
                           20100424013000 15 example.com.
                           P47CUy/xPV8qIEuua4tMKG6ei3LQ8RYv3TwE
                           ...
                           JWL70YiUnUG3m9OL9w== )
  foo.example.com.  3600  IN A 192.0.2.2
          3600    RRSIG    A 5 3 3600 20120824013000 (
                           20100424013000 14 example.com.
                           xHr023P79YrSHHMtSL0a1nlfUt4ywn/vWqsO
                           ...
                           JPV/SA4BkoFxIcPrDQ== )
          300    NSEC      example.com. A RRSIG NSEC
          300    RRSIG     NSEC 5 3 300 20120824013000 (
                          20100424013000 14 example.com.
                           Aaa4kgKhqY7Lzjq3rlPlFidymOeBEK1T6vUF
                           ...
                           Qe000JyzObxx27pY8A== )

   is reduced to the following representation:

            SOA_2005092303
            RRSIG_Z_14(SOA_2005092303)
            DNSKEY_K_14
            DNSKEY_Z_15
            RRSIG_K_14(DNSKEY)
            RRSIG_Z_15(DNSKEY)

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

Appendix C.  Transition Figures for Special Case Algorithm Rollovers

   The figures appendix complement and illustrate the special cases of
   algorithm rollovers as described in Section 4.1.5






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   ----------------------------------------------------------------
    initial              new RRSIGs           new DNSKEY
   ----------------------------------------------------------------
   Parent:
    SOA_0 -------------------------------------------------------->
    RRSIG_par(SOA) ----------------------------------------------->
    DS_S_1 ------------------------------------------------------->
    RRSIG_par(DS_S_1) -------------------------------------------->

   Child:
    SOA_0                SOA_1                SOA_2
    RRSIG_S_1(SOA)       RRSIG_S_1(SOA)       RRSIG_S_1(SOA)
                         RRSIG_S_2(SOA)       RRSIG_S_2(SOA)

    DNSKEY_S_1           DNSKEY_S_1           DNSKEY_S_1
                                              DNSKEY_S_2
    RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)
                         RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)

   ----------------------------------------------------------------
    new DS               DNSKEY removal       RRSIGs removal
   ----------------------------------------------------------------
   Parent:
    SOA_1 ------------------------------------------------------->
    RRSIG_par(SOA) ---------------------------------------------->
    DS_S_2 ------------------------------------------------------>
    RRSIG_par(DS_S_2) ------------------------------------------->

   Child:
    -------------------> SOA_3                SOA_4
    -------------------> RRSIG_S_1(SOA)
    -------------------> RRSIG_S_2(SOA)       RRSIG_S_2(SOA)

    ------------------->
    -------------------> DNSKEY_S_2           DNSKEY_S_2
    -------------------> RRSIG_S_1(DNSKEY)
    -------------------> RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)
   ----------------------------------------------------------------


   Also see Section 4.1.5.1.

           Figure 11: Single Type Signing Scheme Algorithm Roll


   ----------------------------------------------------------------
    initial              new RRSIGs           new DNSKEY
   ----------------------------------------------------------------



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   Parent:
    SOA_0 -------------------------------------------------------->
    RRSIG_par(SOA) ----------------------------------------------->
    DS_K_1 ------------------------------------------------------->
    RRSIG_par(DS_K_1) -------------------------------------------->

   Child:
    SOA_0                SOA_1                SOA_2
    RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)
                         RRSIG_Z_2(SOA)       RRSIG_Z_2(SOA)

    DNSKEY_K_1           DNSKEY_K_1           DNSKEY_K_1
                                              DNSKEY_K_2
    DNSKEY_Z_1           DNSKEY_Z_1           DNSKEY_Z_1
                                              DNSKEY_Z_2
    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)
                                              RRSIG_K_2(DNSKEY)

   ----------------------------------------------------------------
    new DS               revoke DNSKEY        DNSKEY removal
   ----------------------------------------------------------------
   Parent:
    SOA_0 ------------------------------------------------------->
    RRSIG_par(SOA) ---------------------------------------------->
    DS_K_2 ------------------------------------------------------>
    RRSIG_par(DS_K_2) ------------------------------------------->

   Child:
    -------------------> SOA_3                SOA_4
    -------------------> RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)
    -------------------> RRSIG_Z_2(SOA)       RRSIG_Z_2(SOA)

    -------------------> DNSKEY_K_1_REVOKED
    -------------------> DNSKEY_K_2           DNSKEY_K_2
    ------------------->
    -------------------> DNSKEY_Z_2           DNSKEY_Z_2
    -------------------> RRSIG_K_1(DNSKEY)
    -------------------> RRSIG_K_2(DNSKEY)    RRSIG_K_2(DNSKEY)
   ----------------------------------------------------------------

    RRSIGs removal
   ----------------------------------------------------------------
   Parent:
    ------------------------------------->
    ------------------------------------->
    ------------------------------------->
    ------------------------------------->




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   Child:
    SOA_5
    RRSIG_Z_2(SOA)

    DNSKEY_K_2
    DNSKEY_Z_2
    RRSIG_K_2(DNSKEY)
   ----------------------------------------------------------------


   Also see Section 4.1.5.2.

                  Figure 12: RFC5011 Style algorithm roll


   ----------------------------------------------------------------
    initial              new RRSIGs           new DNSKEY
   ----------------------------------------------------------------
   Parent:
    SOA_0 -------------------------------------------------------->
    RRSIG_par(SOA) ----------------------------------------------->
    DS_S_1 ------------------------------------------------------->
    RRSIG_par(DS_S_1) -------------------------------------------->

   Child:
    SOA_0                SOA_1                SOA_2
    RRSIG_S_1(SOA)
    RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)
                         RRSIG_S_2(SOA)       RRSIG_S_2(SOA)

    DNSKEY_S_1           DNSKEY_S_1           DNSKEY_S_1
    DNSKEY_Z_1           DNSKEY_Z_1           DNSKEY_Z_1
                                              DNSKEY_S_2
    RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)
                         RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)

   ----------------------------------------------------------------
    new DS               revoke DNSKEY        DNSKEY removal
   ----------------------------------------------------------------
   Parent:
    SOA_0 ------------------------------------------------------->
    RRSIG_par(SOA) ---------------------------------------------->
    DS_S_2 ------------------------------------------------------>
    RRSIG_par(DS_S_2) ------------------------------------------->

   Child:
    -------------------> SOA_3                SOA_4
    -------------------> RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)



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    -------------------> RRSIG_S_2(SOA)       RRSIG_S_2(SOA)

    -------------------> DNSKEY_S_1_REVOKED
    -------------------> DNSKEY_S_2           DNSKEY_S_2
    -------------------> DNSKEY_Z_1
    -------------------> RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)
    -------------------> RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)

   ----------------------------------------------------------------
    RRSIGs removal
   ----------------------------------------------------------------
   Parent:
    ------------------------------------->
    ------------------------------------->
    ------------------------------------->
    ------------------------------------->

   Child:
    SOA_5
    RRSIG_S_2(SOA)

    DNSKEY_S_2
    RRSIG_S_2(DNSKEY)
   ----------------------------------------------------------------


   Also see Section 4.1.5.3.

     Figure 13: RFC5011 algorithm roll in a Single Type Signing Scheme
                                Environment

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.




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   o  Applied the errata from
      http://www.rfc-editor.org/errata_search.php?rfc=4641

   o  Inserted trivial "IANA considerations" section.

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

D.2.  version 0->1

   Cryptography details rewritten.  (See http://www.nlnetlabs.nl/svn/
   rfc4641bis/trunk/open-issues/cryptography_flawed)

   o  Reference to NIST 800-90 added

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

   o  Complete rewrite of Section 3.4.2 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 cryptography 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/
   rfc4641bis/trunk/open-issues/differentiation_trustanchor_parent

   http://www.nlnetlabs.nl/svn/rfc4641bis/trunk/open-issues/
   rollover_assumptions

   Added Section 4.1.5 as suggested by Jelte Jansen in http://
   www.nlnetlabs.nl/svn/rfc4641bis/trunk/open-issues/Key_algorithm_roll

   Added Section 4.3.5.1 Issue identified by Antoin Verschuren http://
   www.nlnetlabs.nl/svn/rfc4641bis/trunk/open-issues/
   non-cooperative-registrars

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

D.3.  version 1->2

   o  Significant rewrite of Section 3 whereby the argument is made that
      the timescales for rollovers are made purely on operational
      arguments hopefully resolving http://www.nlnetlabs.nl/svn/
      rfc4641bis/trunk/open-issues/discussion_of_timescales



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   o  Added Section 5 based on http://www.nlnetlabs.nl/svn/rfc4641bis/
      trunk/open-issues/NSEC-NSEC3

   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.

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:

      *  tried to motivate that key length is not a strong motivation
         for KSK ZSK split (based on http://www.educatedguesswork.org/
         2009/10/on_the_security_of_zsk_rollove.html)

      *  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/
      trunk/open-issues/timing_terminology

   o  Added paragraph 2 and clarified the second but last paragraph of
      Section 3.2.2.

   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




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   o  Added Section 4.1.4.

   o  Added Section 4.4.2 to address issue "Signature_validity"

D.5.  version 3->4

   o  Stephen Morris submitted a large number of language, style and
      editorial nits.

   o  Section 4.1.5 improved based on comments from Olafur Gudmundsson
      and Ondrej Sury.

   o  Tried to improve consistency of notation in the various rollover
      figures

D.6.  version 4->5

   o  Improved consistency of notation

   o  Matthijs Mekking provided substantive feedback on algorithm
      rollover and suggested the content of the subsections of
      Section 4.1.5 and the content of the figures in Appendix C

D.7.  version 5->6

   o  More improved consistency of notation and some other nits

   o  Review of Rickard Bellgrim

   o  Review of Sebastian Castro

   o  Added a section about Stand-by keys

   o  Algorithm rollover: Conservative or Liberal Approach

   o  Added a reference to NSEC3 hash performance report

   o  More clarifications on the topic of non cooperating operators

D.8.  version 6->7

   o  Fixed minor nits.

   o  Clarified the Double DS Rollover in Changing DNS Operator
      sections.

   o  Adjusted STSS Rollover Figures.




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   o  Remove the ZSK RRSIGs over DNSKEY RRset in Figures.

   o  Added text: second variety on STSS Double DS Rollover.

   o  Reviewed by Antoin Verschuren, Marc Lampo, George Barwood.

D.9.  version 7->8

   o  Signatures over DNSKEY RRset does not need to be propagated in the
      new RRSIGS step.

D.10.  Subversion information

   www.nlnetlabs.nl/svn/rfc4641bis/

   $Id: draft-ietf-dnsop-rfc4641bis.xml 110 2011-10-28 14:15:25Z matje $

Authors' Addresses

   Olaf M. Kolkman
   NLnet Labs
   Science Park 400
   Amsterdam  1098 XH
   The Netherlands

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


   W. (Matthijs) Mekking
   NLnet Labs
   Science Park 400
   Amsterdam  1098 XH
   The Netherlands

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














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