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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                                                NLnet Labs
Obsoletes: 2541 (if approved)                             August 1, 2010
Intended status: Informational
Expires: February 2, 2011


                DNSSEC Operational Practices, Version 2
                     draft-ietf-dnsop-rfc4641bis-04

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.

   [When approved] 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 February 2, 2011.

Copyright Notice



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   Copyright (c) 2010 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  The Use of the Term 'key'  . . . . . . . . . . . . . . . .  6
     1.2.  Time Definitions . . . . . . . . . . . . . . . . . . . . .  6
   2.  Keeping the Chain of Trust Intact  . . . . . . . . . . . . . .  7
   3.  Keys Generation and Storage  . . . . . . . . . . . . . . . . .  8
     3.1.  Operational Motivation for 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?  . . . . . . . 16
   4.  Signature Generation, Key Rollover, and Related Policies . . . 17
     4.1.  Key Rollovers  . . . . . . . . . . . . . . . . . . . . . . 17
       4.1.1.  Zone Signing Key Rollovers . . . . . . . . . . . . . . 17



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         4.1.1.1.  Pre-Publish Key Rollover . . . . . . . . . . . . . 17
         4.1.1.2.  Double Signature Zone Signing Key Rollover . . . . 20
         4.1.1.3.  Pros and Cons of the Schemes . . . . . . . . . . . 21
       4.1.2.  Key Signing Key Rollovers  . . . . . . . . . . . . . . 21
       4.1.3.  Difference Between ZSK and KSK Rollovers . . . . . . . 23
       4.1.4.  Rollover for a Single Type Signing Key rollover  . . . 24
       4.1.5.  Key algorithm rollover . . . . . . . . . . . . . . . . 26
       4.1.6.  Automated Key Rollovers  . . . . . . . . . . . . . . . 28
     4.2.  Planning for Emergency Key Rollover  . . . . . . . . . . . 29
       4.2.1.  KSK Compromise . . . . . . . . . . . . . . . . . . . . 29
         4.2.1.1.  Keeping the Chain of Trust Intact  . . . . . . . . 30
         4.2.1.2.  Breaking the Chain of Trust  . . . . . . . . . . . 31
       4.2.2.  ZSK Compromise . . . . . . . . . . . . . . . . . . . . 31
       4.2.3.  Compromises of Keys Anchored in Resolvers  . . . . . . 31
     4.3.  Parent Policies  . . . . . . . . . . . . . . . . . . . . . 32
       4.3.1.  Initial Key Exchanges and Parental Policies
               Considerations . . . . . . . . . . . . . . . . . . . . 32
       4.3.2.  Storing Keys or Hashes?  . . . . . . . . . . . . . . . 32
       4.3.3.  Security Lameness  . . . . . . . . . . . . . . . . . . 33
       4.3.4.  DS Signature Validity Period . . . . . . . . . . . . . 33
       4.3.5.  Changing DNS Operators . . . . . . . . . . . . . . . . 34
         4.3.5.1.  Cooperationg DNS operators . . . . . . . . . . . . 34
         4.3.5.2.  Non Cooperationg DNS Operators . . . . . . . . . . 36
     4.4.  Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . 37
       4.4.1.  Time Considerations  . . . . . . . . . . . . . . . . . 37
       4.4.2.  Signature Validation Periods . . . . . . . . . . . . . 40
         4.4.2.1.  Maximum Value  . . . . . . . . . . . . . . . . . . 40
         4.4.2.2.  Minimum Value  . . . . . . . . . . . . . . . . . . 40
         4.4.2.3.  Differentiation between RR sets  . . . . . . . . . 41
         4.4.2.4.  Other timing parameters in a zone  . . . . . . . . 42
   5.  Next Record type . . . . . . . . . . . . . . . . . . . . . . . 42
     5.1.  Differences between  NSEC and NSEC3  . . . . . . . . . . . 43
     5.2.  NSEC or NSEC3  . . . . . . . . . . . . . . . . . . . . . . 44
     5.3.  NSEC3 parameters . . . . . . . . . . . . . . . . . . . . . 44
       5.3.1.  NSEC3 Algorithm  . . . . . . . . . . . . . . . . . . . 44
       5.3.2.  NSEC3 Iterations . . . . . . . . . . . . . . . . . . . 44
       5.3.3.  NSEC3 Salt . . . . . . . . . . . . . . . . . . . . . . 45
       5.3.4.  Opt-out  . . . . . . . . . . . . . . . . . . . . . . . 45
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 46
   7.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 46
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 46
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 47
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 47
   Appendix A.  Terminology . . . . . . . . . . . . . . . . . . . . . 49
   Appendix B.  Typographic Conventions . . . . . . . . . . . . . . . 50
   Appendix C.  Document Editing History  . . . . . . . . . . . . . . 53
     C.1.  draft-ietf-dnsop-rfc4641-00  . . . . . . . . . . . . . . . 53



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     C.2.  version 0->1 . . . . . . . . . . . . . . . . . . . . . . . 53
     C.3.  version 1->2 . . . . . . . . . . . . . . . . . . . . . . . 54
     C.4.  version 2->3 . . . . . . . . . . . . . . . . . . . . . . . 54
     C.5.  version 3->4 . . . . . . . . . . . . . . . . . . . . . . . 55
     C.6.  Subversion infromation . . . . . . . . . . . . . . . . . . 55














































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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 is being signed and the first secure
   delegations are provisioned- there exists relatively little
   experience with DNSSEC in production environments; this document
   should therefore explicitly not be seen as representing 'Best Current
   Practices'.  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.  [OK: This is really a straw-man and causes a
   difference in tone that I believe was the instruction of the WG
   during the IETF 77 meeting.  The document could be made much shorter
   when particular recommendations are made?  Is there a general
   consensus that we should currently not make particular
   recommendations?]

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

   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.  Since these public keys
   appear in the DNS one has to take into account all kinds of timing
   issues, which are discussed in Section 4.4.  Section 4.1 and
   Section 4.2 deal with the rollover, or replacement, of keys.
   Finally, Section 4.3 discusses considerations on how parents deal
   with their children's public keys in order to maintain chains of
   trust.



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

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

   This document [OK: when approved] obsoletes RFC 4641 [14].

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

1.1.  The Use of the Term 'key'

   It is assumed that the reader is familiar with the concept of
   asymmetric keys on which DNSSEC is based (public key cryptography
   RFC4949 [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" Time after which a signature (made
      with a specific key) is replaced with a new signature (made with
      the same key) or removed.  This replacement takes place by
      publishing the relevant RRSIG in the master zone file.  After one
      stops publishing an RRSIG in a zone, it may take a while before
      the RRSIG has expired from caches and has actually been removed
      from the DNS.

   o  "Key effectivity period" The period during which a key pair is
      expected to be effective.  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.




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

2.  Keeping the Chain of Trust Intact

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




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3.  Keys Generation and Storage

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

   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 as Key Signing Keys (KSKs).  These keys will only
   sign the apex DNSKEY RRSet in a zone.  Other keys can be used to sign
   all the RRSets in a zone that require signatures.  They are referred
   to as Zone Signing Keys (ZSKs).  In case the differentiation between
   KSK and ZSK is not made we talk about a Single Type signing scheme.

   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 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 responsible registry 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-



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   base with information about the key rollover.

   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.  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.  For example, 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 more operational flexibility and higher computational
   performance than a single key (with combined KSK and ZSK
   functionality) stored on an HSM.

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

   If the above holds then the costs of the operational complexity of a
   KSK-ZSK split may outweigh the costs of operational flexibility and



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   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

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

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

   The Key Signing Key is only to be used to sign the DNSKEY RRs in a
   zone.  If a Key Signing Key is to be rolled, there will be
   interactions with parties other than the zone administrator.  If
   there is a parent zone, these can include the registry of the parent
   zone or administrators of verifying resolvers that have the
   particular key configured as secure entry points.  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



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   possibly low if the communication between the child and the parent is
   good.  On the other hand, the only completely effective way to tell
   if the communication is good is to test it periodically.  Thus,
   rolling a KSK with a parent is only done for two reasons: to test and
   verify the rolling system to prepare for an emergency, and in the
   case of (preventing) an actual emergency.

   Finally, 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,



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   budget for these costs instead of incurring them unexpectedly.

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

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, e.g 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 KSKs and not
   on 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 13 months,
   with the intent to replace them after 12 months.  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.




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

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

   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

   There are currently two types of signature algorithms that can be
   used in DNSSEC: RSA and DSA.  Both are fully specified in many
   freely-available documents, and both are widely considered to be
   patent-free.  The creation of signatures with RSA and DSA takes
   roughly the same time, but DSA is about ten times slower for
   signature verification.

   We suggest the use of RSA/SHA-256 as the preferred signature
   algorithms and RSA/SHA-1 as an alternative.  Both have advantages and
   disadvantages.  RSA/SHA-1 has been deployed for many years, while
   RSA/SHA-256 has only begun to be deployed.  On the other hand, it is
   expected that if effective attacks on either algorithm 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



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   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

   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.)

   Owners of keys that are used as extremely high value trust anchors,
   or non-anchor keys that may be difficult to roll over, may want to
   use lengths longer than 1024 bits.  Typically, the next larger key
   size used is 2048 bits, which has the approximate equivalent strength
   of a symmetric 112-bit key (e.g.  RFC3766 [12]).  In a standard CPU,
   it takes about four times as long to sign or verify with a 2048-bit
   key as it does with a 1024-bit key.

   Another way to decide on the size of key to use is to remember that
   the 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
   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 or DSA algorithms and keys of 1024
   bits or higher.




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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] 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 dynamic updates is unavailable from general hosts on
   the Internet; it is not listed in the NS RRSet, although its name
   appears in the SOA RRs MNAME field.  The nameservers in the NS RRSet
   are able to receive zone updates through 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.

   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 a HSM will be
   influenced by a tradeoff between various concerns:

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

   o  The remaining window of opportunity for the attacker.




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   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 roling
      a ZSK is lowest and the costs of rolling a KSK that is in wide use
      as a trust anchor is highest.)

   o  The costs of buying and maintaining an HSM.

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

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 800-900 [19].  In particular, one should
   carefully assess whether the random number generator used during key
   generation adheres to these suggestions.

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

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 te 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



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   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
   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 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



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   with the double signature ZSK rollover.

   Pre-publish key rollover involves four stages as follows:

    ----------------------------------------------------------
     initial            new DNSKEY          new RRSIGs
    ----------------------------------------------------------
     SOA0               SOA1                SOA2
     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)
     RRSIG_Z_10(DNSKEY) RRSIG_Z_10(DNSKEY)  RRSIG_Z_11(DNSKEY)
    ------------------------------------------------------------

    ------------------------------------------------------------
      DNSKEY removal
    ------------------------------------------------------------
     SOA3
     RRSIG_Z_11(SOA)

     DNSKEY_K_1
     DNSKEY_Z_11

     RRSIG_K_1(DNSKEY)
     RRSIG_Z_11(DNSKEY)
    ------------------------------------------------------------


   Pre-Publish Key Rollover

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

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

   new RRSIGs:  At the "new RRSIGs" stage (SOA serial 2), DNSKEY 11 is
      used to sign the data in the zone exclusively (i.e., all the
      signatures from DNSKEY 10 are removed from the zone).  DNSKEY 10
      remains published in the key set.  This way data that was loaded



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

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


       initial             new RRSIGs          new DNSKEY
      -----------------------------------------------------------------
       SOA0                SOA1                SOA2
       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)
       RRSIG_Z_10(DNSKEY)  RRSIG_Z_11(DNSKEY)  RRSIG_Z_11(DNSKEY)
       ----------------------------------------------------------------

       ----------------------------------------------------------------
       new RRSIGs (II)        new DNSKEY (II)
       ----------------------------------------------------------------
       SOA3                   SOA4
       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)
       RRSIG_Z_12(DNSKEY)     RRSIG_Z_12(DNSKEY)
       ----------------------------------------------------------------


   Pre-Publish Key Rollover, Showing Two Rollovers




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   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.

   Double signature ZSK rollover involves three stages as follows:

      ----------------------------------------------------------------
      initial             new DNSKEY         DNSKEY removal
      ----------------------------------------------------------------
      SOA0                SOA1               SOA2
      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)
      RRSIG_Z_10(DNSKEY)  RRSIG_Z_10(DNSKEY) RRSIG_Z_11(DNSKEY)
                          RRSIG_Z_11(DNSKEY)
      ----------------------------------------------------------------


   Double Signature Zone Signing Key Rollover

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

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







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

   At every instance, RRSIGs from the previous version of the zone can
   be verified with the DNSKEY RRSet from the current version and the
   other way around.  The data from the current version can be verified
   with the data from the previous version of the zone.  The duration of
   the "new DNSKEY" phase and the period between rollovers should be at
   least the Maximum Zone TTL.

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

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

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:
    SOA0              -------->          SOA1            -------->
    RRSIG_par(SOA)    -------->          RRSIG_par(SOA)  -------->
    DS_K_1            -------->          DS_K_2          -------->
    RRSIG_par(DS)     -------->          RRSIG_par(DS)   -------->



   Child:
    SOA0               SOA1               -------->   SOA2
    RRSIG_Z_10(SOA)    RRSIG_Z_10(SOA)    -------->   RRSIG_Z_10(SOA)
                                          -------->
    DNSKEY_K_1         DNSKEY_K_1         -------->   DNSKEY_K_2
                       DNSKEY_K_1         -------->
    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) -------->
    RRSIG_Z_10(DNSKEY) RRSIG_Z_10(DNSKEY) -------->   RRSIG_Z_10(DNSKEY)
  --------------------------------------------------------------------

   Stages of Deployment for a Double Signature Key Signing Key Rollover

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

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

   DS change:  The parent replaces DS1 with DS2.

   DNSKEY removal:  DNSKEY1 has been removed.

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



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   parent.  In this mechanism, there are periods where there are two DS
   RRs at the parent.  Since at the moment of writing the protocol for
   this interaction has not been developed, further discussion is out of
   scope for this document.

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

4.1.3.  Difference Between ZSK and KSK Rollovers

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

   A zone key rollover can be handled in two different ways: pre-publish
   (Section 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.  The pre-publish method would also work for a
   KSK rollover.  The records that are to be pre-published are the
   parental DS RRs.  The pre-publish method has some drawbacks for KSKs.
   We first describe the rollover scheme and then indicate these
   drawbacks.





















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   --------------------------------------------------------------------
     initial         new DS           new DNSKEY      DS/DNSKEY removal
   --------------------------------------------------------------------
   Parent:
     SOA0              SOA1             -------->        SOA2
     RRSIG_par(SOA)    RRSIG_par(SOA)   -------->        RRSIG_par(SOA)
     DS_K_1            DS_K_1           -------->        DS_K_2
                       DS_K_2           -------->
     RRSIGpar(DS)      RRSIG_par(DS)    -------->        RRSIG_par(DS)

   Child:
     SOA0               -------->     SOA1               SOA1
     RRSIG_Z_10(SOA)    -------->     RRSIG_Z_10(SOA)    RRSIG_Z_10(SOA)
                        -------->
     DNSKEY_K_1         -------->     DNSKEY_K_2         DNSKEY_K_2
                        -------->
     DNSKEY_Z_10        -------->     DNSKEY_Z_10        DNSKEY_Z_10
     RRSIG_K_1 (DNSKEY) -------->     RRSIG_K_2(DNSKEY)  RRSIG2 (DNSKEY)
     RRSIG_Z_10(DNSKEY) -------->     RRSIG_Z_10(DNSKEY) RRSIG10(DNSKEY)
   --------------------------------------------------------------------


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

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

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

4.1.4.  Rollover for a Single Type Signing Key rollover

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




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   There are two variants for this rollover.  Since the choice for a
   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:
     SOA0             -------->        SOA1            -------->
     RRSIG_par(SOA)   -------->        RRSIG_par(SOA)  -------->
     DS1              -------->        DS2             -------->
     RRSIG_par(DS)    -------->        RRSIG_par(DS)   -------->

   Child:
     SOA0              SOA1              ------------> SOA2
     RRSIG_S_1(SOA)    RRSIG_S_1(SOA)    ------------> RRSIG_S_2(SOA)
                       RRSIG_S_2(SOA1)   ------------>
     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) ------------>
     -----------------------------------------------------------------

   Stages of the Straightforward rollover in a Single Type Signing
   scheme.

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

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

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

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

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

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



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

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

4.1.5.  Key algorithm rollover

   A special class of 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.

   Because of the algorithm downgrade protection in RFC4035 section 2.2,
   you may not have a key of an algorithm for which you do not have
   signatures, and you may not have a DS record in the parent zone of an
   algorithm for which you don't have a corresponding key in the zone
   apex.

   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.

   The following figure describes the steps.  Whereby the trailing
   underscored number indicates the algorithm and ZSK and KSK indicate
   the obvious difference in key use.  For example DNSKEY_KSK_1 is a the
   DNSKEY RR representing the public part of the old key signing key of
   algorithm type 1 while RRSIG_ZSK_2(SOA3) is the RRSIG RR made with
   the private part of the new zone signing key of algorithm type 2 over
   a SOA RR (that has serial number 3).  It is assumed that the key that
   signes the SOA RR also signes all other non-DNSKEY RRset data.






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   ----------------------------------------------------------------
   1 Initial            2 New RRSIGS         3 New DNSKEY
   ----------------------------------------------------------------
   Parent:
    SOA0                 -------------- ( SOA ) -------------->
    RRSIG_par(SOA)       ------------------------------------->
    DS_K_1               ------------------------------------->
    RRSIG_par(DS_K_1)    ------------------------------------->

   Child:
    SOA0                 SOA1                 SOA2
    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_Z_1           DNSKEY_Z_1           DNSKEY_Z_1
    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)    DNSKEY_K_2
                         RRSIG_K_2(DNSKEY)    DNSKEY_Z_2
                                              RRSIG_K_1(DNSKEY)
                                              RRSIG_K_2(DNSKEY)
   ----------------------------------------------------------------
   4 Exchange DS         5 Remove DNSKEY      6 Remove RRSIGS
   ----------------------------------------------------------------
   Parent:
    SOA1                 -------------( SOA )---------------->
    RRSIG_par(SOA)       ------------------------------------->
    DS_K_2               ------------------------------------->
    RRSIG_par(DS_K_2)    ------------------------------------->

   Child:
    ---- (SOA2 ) --->    SOA3                 SOA4
    ---------------->    RRSIG_Z_1(SOA3)      RRSIG_Z_2(SOA4)
    ---------------->    RRSIG_Z_2(SOA3)

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


   Stages of Deployment during an Algorithm Rollover.

   Step 1 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.

   Step 2: the signatures made with the new key over all records in the



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

   Step 3: After the cache data has expired, the new key can be added to
   the zone.

   Step 4: 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.

   Step 5: 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.  The key is removed in this
   step , and after the cache data for the DNSKEY has expired, the
   signatures can also be removed during this step.

   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.

4.1.6.  Automated Key Rollovers

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

   o  ZSK rollovers are easy to automate as only the child zone is
      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
      be guaranteed in order to avoid attacks on the rollover.




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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 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
   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



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   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.

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

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

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

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

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

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





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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.

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

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




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

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

4.3.2.  Storing Keys or Hashes?

   When designing a registry system one should consider which of the
   DNSKEYs and/or the corresponding DSes to store.  Since a child zone
   might wish to have a DS published using a message digest algorithm
   not yet understood by the registry, the registry can't count on being
   able to generate the DS record from a raw DNSKEY.  Thus, we recommend
   that registry systems at least support storing DS records (also see
   draft-ietf-dnsop-dnssec-trut-anchor [26]).

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



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

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



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

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

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

4.3.5.  Changing DNS Operators

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

   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.  Cooperationg 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



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   operator, combined with a double signature KSK rollover where the two
   registrars exchange public KSKs and independently generate a
   signature over those keysets that they combine and both publish in
   their copy of the zone.  Once that is done they can use their own
   private keys to sign any of their zone content during the transfer.

    ------------------------------------------------------------
    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_K_A             DNSKEY_Z_B         DNSKEY_K_B
     RRSIG_Z_A(DNSKEY)      DNSKEY_K_A         DNSKEY_K_A
     RRSIG_K_A(DNSKEY)      DNSKEY_K_B         DNSKEY_K_B
                            RRSIG_Z_B(DNSKEY)  RRSIG_Z_B(DNSKEY)
                            RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)
                            RRSIG_Z_A(DNSKEY)  RRSIG_Z_A(DNSKEY)
                            RRSIG_K_A(DNSKEY)  RRSIG_K_A(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)



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     RRSIG_Z_A(NS)


     DNSKEY_Z_A         DNSKEY_Z_A            DNSKEY_Z_B
     DNSKEY_Z_B         DNSKEY_Z_B            DNSKEY_K_B
     DNSKEY_K_A         DNSKEY_K_A            RRSIG_Z_B(DNSKEY)
     DNSKEY_K_B         DNSKEY_K_B            RRSIG_K_B(DNSKEY)
     RRSIG_Z_B(DNSKEY)  RRSIG_Z_B(DNSKEY)
     RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)
     RRSIG_Z_A(DNSKEY)  RRSIG_Z_A(DNSKEY)
     RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)

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

   Rollover for non cooperating operators.

   In this figure A denotes the losing operator and B the gaining
   operator.  RRSIGZ is the RRSIG produced by a ZSK, RRSIGK 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.

4.3.5.2.  Non Cooperationg 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 loosing 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
   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



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   losing operator's DNSKEY for as long as the DNSKEY of the losing
   operator, or any of the signatures produced by it are likely to
   disappear in caches, which as mentioned above could in theory be for
   tens of years.

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

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



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

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

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

   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.





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   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.

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

         The consequence of an authoritative server not being able to
         update a zone 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



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         of your signature validity intervals.

4.4.2.  Signature Validation Periods

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

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
   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 intervall 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



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   resolved.

   To make matters slightly more complicated, some signers vary the
   signature validity period over a smal 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 [OK: The above needs careful review]


    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->|
                                                |                  |

   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.

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



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   itself still are the primary parameter for cache expiry.  [OK: are
   there strong arguments besides replay risks for varying signature
   validity]

   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.

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

4.4.2.4.  Other timing parameters in a zone

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

   The arguments for tuning minimum signature validity period are
   remarkably similar to the arguments used to set the SOA expiration
   timer.  It is advised to set timethis parameter to a value greater
   than the signature validity period.

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



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   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
   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 behaviour is acceptable or even desirable, for others
   it is undesirable for policy, regulatory or other reasons.  This is
   the first difference between NSEC and NSEC3.

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

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






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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
   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



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   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 two-thirds of the maximum is deemed to be a sufficiently
   costly yet not excessive value.

5.3.3.  NSEC3 Salt

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

   The salt of all NSEC3 records in a zone needs to be the same.  Since
   changing the salt requires all the NSEC3 records to be regenerated,
   and thus requires generating new RRSIG's over these NSEC3 records, it
   is recommended to align the change of the salt with a change of the
   Zone Signing Key, as that process in itself already requires all
   RRSIG's to be regenerated.  If there is no critical dependency on
   incremental signing and the whole zone can be signed with little
   effort there is no need for such alignment.  However, unlike Zone
   Signing Key changes, NSEC3 salt changes do not need special rollover
   procedures.  It is possible to change the salt each time the zone is
   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.  [Editors Note: One could make this
   construct more correct by talking about the hashed names and the
   hashed span, but I believe that is overkill].  This allows for the
   addition or removal of the delegations covered by the span without
   recalculating or re- signing RRs in the NSEC3 RR chain.

   Opt-Out is specified to be used only over delegation points and will
   therefore only bring 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



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   considerations can be found in RFC5155 section 12.2 [21].

6.  Security Considerations

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

7.  IANA considerations

   There are no IANA considerations with respect to this document

8.  Acknowledgments

   Most of the text of this document is copied from RFC4641 [14].  That
   document was edited by Olaf Kolkman and Miek Gieben.  Other people
   that contributed or where otherwise involved in that work were in
   random order: Rip Loomis, Olafur Gudmundsson, Wesley Griffin, Michael
   Richardson, Scott Rose, Rick van Rein, Tim McGinnis, Gilles Guette
   Olivier Courtay, Sam Weiler, Jelte Jansen, Niall O'Reilly, Holger
   Zuleger, Ed Lewis, Hilarie Orman, Marcos Sanz, Peter Koch, Mike
   StJohns, Emma Bretherick, Adrian Bedford, and Lindy Foster, G.
   Guette, and O. Courtay.

   For this version of the document we would like to acknowldge a few
   people for significant contributions: Paul Hoffman for his
   contribution on the choice of cryptographic paramenters and
   addressing some of the trust anchor issues; Jelte Jansen who provided
   the text in Section 4.1.5; Paul Wouters who provided the initial text
   for Section 5 and Alex Bligh who improved it; Erik Rescorla's whos
   blogpost on "the Security of ZSK rollovers" inspired text in
   Section 3.1; Stephen Morris who made a pass on English style and
   grammar; Olafur Gudmundsson and Onrej Sury who provided input on
   Section 4.1.5 based on actual operational experience;

   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.

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

9.  References




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9.1.  Normative References

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

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

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

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

   [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",



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         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.

   [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-00 (work in
         progress), July 2010.

   [25]  Ljunggren, F., Eklund-Lowinder, A., and T. Okubo, "DNSSEC
         Policy & Practice Statement Framework",
         draft-ietf-dnsop-dnssec-dps-framework-02 (work in progress),
         July 2010.

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







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Appendix A.  Terminology

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

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

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

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

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

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

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

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

   Re-Singing 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.



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

   Signing Jitter:  Jitter applied to the signature validty intervall.

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

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

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

   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 y 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 'x'
      denotes a number or an identifier, x could be thought of as the
      key id.

   RRSet notations:  RRs are only denoted by the type.  All other
      information -- owner, class, rdata, and TTL -- is left out.  Thus:
      "example.com 3600 IN A 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".







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   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 "SOAx"

   SOA representation:  SOAs are represented as SOAx, 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.



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                           JtHm8ta0diCWYGu/TdrE1O1sYSHblN2i/IX+
                           ...
                           PkXNI/Vgf4t3xZaIyw== )
          3600    DNSKEY   256 3 5 (
                           AQPaoHW/nC0fj9HuCW3hACSGiP0AkPS3dQFX
                           ...
                           sAuryjQ/HFa5r4mrbhkJ
                           ) ; 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:

            SOA2005092303
            RRSIG_Z_14(SOA2005092303)
            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.




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Appendix C.  Document Editing History

   [To be removed prior to publication as an RFC]

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

   Version 0 was differs from RFC4641 in the following ways.

   o  Status of this memo appropriate for I-D

   o  TOC formatting differs.

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

   o  References slightly reordered.

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

   o  Inserted trivial "IANA considertations" section.

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

C.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 cryptograpy with a
      reference to RFC4949.

   o  Removed the KSK for high level zones consideration

   Applied some differentiation with respect of the use of a KSK for
   parent or trust-anchor relation http://www.nlnetlabs.nl/svn/
   rfc4641bis/trunk/open-issues/differentiation_trustanchor_parent

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



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   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 Verschuur http://
   www.nlnetlabs.nl/svn/rfc4641bis/trunk/open-issues/
   non-cooperative-registrars

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

C.3.  version 1->2

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

   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.

C.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 keylength 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




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   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 paragrap 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

   o  Added Section 4.1.4.

   o  Added Section 4.4.2 to address issue "Signature_validity"

C.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 Onrej Sury.

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

C.6.  Subversion infromation

   www.nlnetlabs.nl/svn/rfc4641bis/

   $Id: draft-ietf-dnsop-rfc4641bis-04.txt 67 2010-08-02 15:30:13Z olaf $













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Author's Address

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

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









































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