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rfc:rfc6781

Internet Engineering Task Force (IETF) O. Kolkman Request for Comments: 6781 W. Mekking Obsoletes: 4641 NLnet Labs Category: Informational R. Gieben ISSN: 2070-1721 SIDN Labs

                                                         December 2012
              DNSSEC Operational Practices, Version 2

Abstract

 This document describes a set of practices for operating the DNS with
 security extensions (DNSSEC).  The target audience is zone
 administrators deploying DNSSEC.
 The document discusses operational aspects of using keys and
 signatures in the DNS.  It discusses issues of key generation, key
 storage, signature generation, key rollover, and related policies.
 This document obsoletes RFC 4641, as it covers more operational
 ground and gives more up-to-date requirements with respect to key
 sizes and the DNSSEC operations.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6781.

Kolkman, et al. Informational [Page 1] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................4
    1.1. The Use of the Term 'key' ..................................5
    1.2. Time Definitions ...........................................6
 2. Keeping the Chain of Trust Intact ...............................6
 3. Key Generation and Storage ......................................7
    3.1. Operational Motivation for Zone Signing Keys 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 ..............................14
         3.4.1. Signature Algorithm ................................14
         3.4.2. Key Sizes ..........................................14
         3.4.3. Private Key Storage ................................16
         3.4.4. Key Generation .....................................17
         3.4.5. Differentiation for 'High-Level' Zones? ............17

Kolkman, et al. Informational [Page 2] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 4. Signature Generation, Key Rollover, and Related Policies .......18
    4.1. Key Rollovers .............................................18
         4.1.1. Zone Signing Key Rollovers .........................18
                4.1.1.1. Pre-Publish Zone Signing Key Rollover .....19
                4.1.1.2. Double-Signature Zone Signing Key Rollover 21
                4.1.1.3. Pros and Cons of the Schemes ..............23
         4.1.2. Key Signing Key Rollovers ..........................23
                4.1.2.1. Special Considerations for RFC 5011
                         KSK Rollover ..............................26
         4.1.3. Single-Type Signing Scheme Key Rollover ............26
         4.1.4. Algorithm Rollovers ................................28
                4.1.4.1. Single-Type Signing Scheme
                         Algorithm Rollover ........................32
                4.1.4.2. Algorithm Rollover, RFC 5011 Style ........32
                4.1.4.3. Single Signing Type Algorithm
                         Rollover, RFC 5011 Style ..................33
                4.1.4.4. NSEC-to-NSEC3 Algorithm Rollover ..........34
         4.1.5. Considerations for Automated Key Rollovers .........34
    4.2. Planning for Emergency Key Rollover .......................35
         4.2.1. KSK Compromise .....................................35
                4.2.1.1. Emergency Key Rollover Keeping the
                         Chain of Trust Intact .....................36
                4.2.1.2. Emergency Key Rollover Breaking
                         the Chain of Trust ........................37
         4.2.2. ZSK Compromise .....................................37
         4.2.3. Compromises of Keys Anchored in Resolvers ..........38
         4.2.4. Stand-By Keys ......................................38
    4.3. Parent Policies ...........................................39
         4.3.1. Initial Key Exchanges and Parental Policies
                Considerations .....................................39
         4.3.2. Storing Keys or Hashes? ............................40
         4.3.3. Security Lameness ..................................40
         4.3.4. DS Signature Validity Period .......................41
         4.3.5. Changing DNS Operators .............................42
                4.3.5.1. Cooperating DNS Operators .................42
                4.3.5.2. Non-Cooperating DNS Operators .............44
    4.4. Time in DNSSEC ............................................46
         4.4.1. Time Considerations ................................46
         4.4.2. Signature Validity Periods .........................48
                4.4.2.1. Maximum Value .............................48
                4.4.2.2. Minimum Value .............................49
                4.4.2.3. Differentiation between RRsets ............50

Kolkman, et al. Informational [Page 3] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 5. "Next Record" Types ............................................51
    5.1. Differences between NSEC and NSEC3 ........................51
    5.2. NSEC or NSEC3 .............................................52
    5.3. NSEC3 Parameters ..........................................53
         5.3.1. NSEC3 Algorithm ....................................53
         5.3.2. NSEC3 Iterations ...................................53
         5.3.3. NSEC3 Salt .........................................54
         5.3.4. Opt-Out ............................................54
 6. Security Considerations ........................................54
 7. Acknowledgments ................................................55
 8. Contributors ...................................................55
 9. References .....................................................56
    9.1. Normative References ......................................56
    9.2. Informative References ....................................56
 Appendix A. Terminology ...........................................59
 Appendix B. Typographic Conventions ...............................61
 Appendix C. Transition Figures for Special Cases of Algorithm
             Rollovers .............................................64
 Appendix D. Transition Figure for Changing DNS Operators ..........68
 Appendix E. Summary of Changes from RFC 4641 ......................70

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 [RFC1034] and RFC 1035 [RFC1035]) and want to
 deploy DNSSEC (RFC 4033 [RFC4033], RFC 4034 [RFC4034], RFC 4035
 [RFC4035], and RFC 5155 [RFC5155]).  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 relationship 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.
 Although the DNS Root has been signed since July 15, 2010 and now
 more than 80 secure delegations are provisioned in the root, at the
 time of this writing there still exists relatively little experience
 with DNSSEC in production environments below the Top-Level Domain
 (TLD) level; this document should therefore explicitly not be seen as
 representing 'Best Current Practices'.  Instead, it describes the
 decisions that should be made when deploying DNSSEC, gives the
 choices available for each one, and provides some operational
 guidelines.  The document does not give strong recommendations.  That
 may be the subject for a future version of this document.

Kolkman, et al. Informational [Page 4] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 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.  Sections 4.1 and 4.2 deal
 with the rollover, or replacement, of keys.  Section 4.3 discusses
 considerations on how parents deal with their children's public keys
 in order to maintain chains of trust.  Section 4.4 covers all kinds
 of timing issues around key publication.  Section 5 covers the
 considerations regarding selecting and using the NSEC or NSEC3
 [RFC5155] Resource Record.
 The typographic conventions used in this document are explained in
 Appendix B.
 Since we describe operational suggestions and there are no protocol
 specifications, the RFC 2119 [RFC2119] language does not apply to
 this document, though we do use quotes from other documents that do
 include the RFC 2119 language.
 This document obsoletes RFC 4641 [RFC4641].

1.1. The Use of the Term 'key'

 It is assumed that the reader is familiar with the concept of
 asymmetric cryptography, or public-key cryptography, on which DNSSEC
 is based (see the definition of 'asymmetric cryptography' in RFC 4949
 [RFC4949]).  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 (DNSKEY RR) and that it is the public
 part that is used in signature verification.

Kolkman, et al. Informational [Page 5] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

1.2. Time Definitions

 In this document, we will be using a number of time-related terms.
 The following definitions apply:
 Signature validity period:  The period that a signature is valid.  It
    starts at the (absolute) time specified in the signature inception
    field of the RRSIG RR and ends at the (absolute) time specified in
    the expiration field of the RRSIG RR.  The document sometimes also
    uses the term 'validity period', which means the same.
 Signature publication period:  The period that a signature is
    published.  It starts at the time the signature is introduced in
    the zone for the first time and ends at the time when the
    signature is removed or replaced with a new signature.  After one
    stops publishing an RRSIG in a zone, it may take a while before
    the RRSIG has expired from caches and has actually been removed
    from the DNS.
 Key effectivity period:  The period during which a key pair is
    expected to be effective.  It is defined as the time between the
    earliest 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.
 Maximum/Minimum Zone Time to Live (TTL):  The maximum or minimum
    value of the TTLs from the complete set of RRs in a zone, that are
    used by validators or resolvers.  Note that the minimum TTL is not
    the same as the MINIMUM field in the SOA RR.  See RFC 2308
    [RFC2308] 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
 RFC 4033 [RFC4033] 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.

Kolkman, et al. Informational [Page 6] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 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 name servers and
 clients may be fetching data from caching non-authoritative servers.
 In this light, note that the time until the data is available on the
 slave can be negligible when using NOTIFY [RFC1996] and Incremental
 Zone Transfer (IXFR) [RFC1995].  It increases when Authoritative
 (full) Zone Transfers (AXFRs) are used in combination with NOTIFY.
 It increases even more if you rely on the full zone transfers being
 based only on 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 name
 server, 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 tradeoff must be
 made between maintaining a valid chain of trust and replacing the
 compromised keys as soon as possible.  Then zone administrators will
 have to decide between keeping the chain of trust intact -- thereby
 allowing for attacks with the compromised key -- or deliberately
 breaking the chain of trust and making secured subdomains invisible
 to security-aware resolvers (also see Section 4.2).

3. Key Generation and Storage

 This section describes a number of considerations with respect to the
 use of keys.  For the design of an operational procedure for key
 generation and storage, a number of decisions need to be made:
 o  Does one differentiate between Zone Signing Keys 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
    [RFC4033]?
 o  What are the timing parameters that are allowed by the operational
    requirements?
 o  What are the cryptographic parameters that fit the operational
    need?

Kolkman, et al. Informational [Page 7] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

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

3.1. Operational Motivation for Zone Signing Keys and Key Signing Keys

 The DNSSEC validation protocol does not distinguish between different
 types of DNSKEYs.  The motivations to differentiate between keys are
 purely operational; validators will not make a distinction.
 For operational reasons, described below, it is possible to designate
 one or more keys to have the role of Key Signing Keys (KSKs).  These
 keys will only sign the apex DNSKEY RRset in a zone.  Other keys can
 be used to sign all the other RRsets in a zone that require
 signatures.  They are referred to as Zone Signing Keys (ZSKs).  In
 cases where the differentiation between the KSK and ZSK is not made,
 i.e., where keys have the role of both KSK and ZSK, we talk about a
 Single-Type Signing Scheme.
 If the two functions are separated, then for almost any method of key
 management and zone signing, the KSK is used less frequently than the
 ZSK.  Once a DNSKEY RRset is signed with the KSK, all the keys in the
 RRset can be used as ZSKs.  If there has been an event that increases
 the risk that a ZSK is compromised, it can be simply replaced with a
 ZSK rollover.  The new RRset is then re-signed with the KSK.
 Changing a key that is a Secure Entry Point (SEP) [RFC4034] for a
 zone can be relatively expensive, as it involves interaction with
 third parties: When a key is only pointed to by a Delegation Signer
 (DS) [RFC4034] record in the parent zone, one needs to complete the
 interaction with the parent and wait for the updated DS record to
 appear in the DNS.  In the case where a key is configured as a trust
 anchor, one has to wait until one has sufficient confidence that all
 trust anchors have been replaced.  In fact, it may be that one is not
 able to reach the complete user-base with information about the key
 rollover.
 Given the assumption that for KSKs the SEP flag is set, the KSK can
 be distinguished from a ZSK by examining the flag field in the DNSKEY
 RR: If the flag field is an odd number, it is a KSK; otherwise, it is
 a ZSK.
 There is also a risk that keys can be compromised through theft or
 loss.  For keys that are installed on file-systems of name servers
 that are connected to the network (e.g., for dynamic updates), that
 risk is relatively high.  Where keys are stored on Hardware Security
 Modules (HSMs) or stored off-line, such risk is relatively low.
 However, storing keys off-line or with more limitations on access
 control has a negative effect on the operational flexibility.  By

Kolkman, et al. Informational [Page 8] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 separating the KSK and ZSK functionality, these risks can be managed
 while making the tradeoff against the involved costs.  For example, a
 KSK can be stored off-line or with more limitations on access control
 than ZSKs, which need to be readily available for operational
 purposes such as the addition or deletion of zone data.  A KSK stored
 on a smartcard that is kept in a safe, combined with a ZSK stored on
 a file-system accessible by operators for daily routine use, may
 provide better protection against key compromise without losing much
 operational flexibility.  It must be said that some HSMs give the
 option to have your keys online, giving more protection and hardly
 affecting the operational flexibility.  In those cases, a KSK-ZSK
 split is not more beneficial than the Single-Type Signing Scheme.
 It is worth mentioning that there's not much point in obsessively
 protecting the key if you don't protect the zone files, which also
 live on the file-systems.
 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 be somewhere
 on the order of 10 to 100, the associated key sizes will vary only by
 about a byte in size for symmetric keys.  When translated to
 asymmetric keys, the size difference is still too insignificant to
 warrant a key-split; it only marginally affects the packet size and
 signing speed.
 The arguments for differentiation between the ZSK and KSK are weakest
 when:
 o  the exposure to risk is low (e.g., when keys are stored on HSMs);
 o  one can be certain that a key is not used as a trust anchor;
 o  maintenance of the various keys cannot be performed through tools
    (is prone to human error); and
 o  the interaction through the child-parent provisioning chain -- in
    particular, the timely appearance of a new DS record in the parent
    zone in emergency situations -- is predictable.

Kolkman, et al. Informational [Page 9] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 If the above arguments hold, then the costs of the operational
 complexity of a KSK-ZSK split may outweigh the costs of operational
 flexibility, and choosing a Single-Type Signing Scheme is a
 reasonable option.  In other cases, we advise that the separation
 between KSKs and ZSKs is made.

3.2. Practical Consequences of KSK and ZSK Separation

 A key that acts only as a Zone Signing Key is used to sign all the
 data except the DNSKEY RRset in a zone on a regular basis.  When a
 ZSK is to be rolled, no interaction with the parent is needed.  This
 allows for a relatively short key effectivity period.
 A key with only the Key Signing Key role is to be used to sign the
 DNSKEY RRs in a zone.  If a KSK is to be rolled, there may be
 interactions with other parties.  These can include the
 administrators of the parent zone or administrators of verifying
 resolvers that have the particular key configured as secure entry
 points.  In the latter case, everyone relying on the trust anchor
 needs to roll over to the new key, a process that may be subject to
 stability costs if automated trust anchor rollover mechanisms (e.g.,
 RFC 5011 [RFC5011]) 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 three schools of thought on rolling a KSK that is not a
 trust anchor:
 1.  It should be done frequently and regularly (possibly every few
     months), so that a key rollover remains an operational routine.
 2.  It should be done frequently but irregularly.  "Frequently" means
     every few months, again based on the argument that a rollover is
     a practiced and common operational routine; "irregular" means
     with a large jitter, so that third parties do not start to rely
     on the key and will not be tempted to configure it as a trust
     anchor.
 3.  It should only be done when it is known or strongly suspected
     that the key can be or has been compromised, or in conjunction
     with operator change policies and procedures, like when a new
     algorithm or key storage is required.
 There is no widespread agreement on which of these three schools of
 thought is better for different deployments of DNSSEC.  There is a
 stability cost every time a non-anchor KSK is rolled over, but it is
 possibly low if the communication between the child and the parent is

Kolkman, et al. Informational [Page 10] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 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 administrator 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 administrator, there may be stability costs for the validator
 administrator that (wrongfully) configured the trust anchor when the
 zone administrator rolls 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 administrator publishes a DNSSEC signing policy and/or a DNSSEC
 practice statement [DNSSEC-DPS], that policy or statement should be
 explicit regarding whether or not the existence of trust anchors will
 be taken into account.  There may be cases where local policies
 enforce the configuration of trust anchors on zones that 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 administrators 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 are smaller.  In fact, the costs for those
 users can be minimized by automating the rollover with RFC 5011
 [RFC5011] and by rolling the key regularly (and advertising such) so

Kolkman, et al. Informational [Page 11] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 that the operators of validating resolvers will put the appropriate
 mechanism in place to deal with these stability costs: In other
 words, budget for these costs instead of incurring them unexpectedly.
 It is therefore preferable to roll KSKs that are expected to be used
 as trust anchors on a regular basis if and only if those rollovers
 can be tracked using standardized (e.g., RFC 5011 [RFC5011])
 mechanisms.

3.2.3. The Use of the SEP Flag

 The so-called SEP [RFC4035] flag can be used to distinguish between
 keys that are intended to be used as the secure entry point into the
 zone when building chains of trust, i.e., they are (to be) pointed to
 by parental DS RRs or configured as a trust anchor.
 While the SEP flag does not play any role in validation, it is used
 in practice for operational purposes such as for the rollover
 mechanism described in RFC 5011 [RFC5011].  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 suggested that the SEP flag be set on keys
 that are used as KSKs and not on keys that are used as ZSKs, while in
 those cases where a distinction between a KSK and ZSK is not made
 (i.e., for a Single-Type Signing Scheme), it is suggested that the
 SEP flag be set on all keys.
 Note: Some 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 on the order
 of two 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 opts for a regular key rollover, a reasonable key
 effectivity period for KSKs that have a parent zone is one year,
 meaning you have the intent to replace them after 12 months.  The key
 effectivity period is merely a policy parameter and should not be

Kolkman, et al. Informational [Page 12] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 considered a constant value.  For example, the real key effectivity
 period may be a little bit longer than 12 months, because not all
 actions needed to complete the rollover could be finished in time.
 As argued above, this annual rollover gives an operational practice
 of rollovers for both the zone and validator administrators.
 Besides, in most environments a year is a time span that is easily
 planned and communicated.
 Where keys are stored online and the exposure to various threats of
 compromise is fairly high, an intended key effectivity period of a
 month is reasonable for Zone Signing Keys.
 Although very short key effectivity periods are theoretically
 possible, when replacing keys one has to take into account the
 rollover considerations discussed in Sections 4.1 and 4.4.  Key
 replacement endures for a couple of Maximum Zone TTLs, depending on
 the rollover scenario.  Therefore, a multiple of Maximum Zone TTL
 durations is a reasonable lower limit on the key effectivity period.
 Forcing a shorter key effectivity period will result in an
 unnecessary and inconveniently large DNSKEY RRset published in the
 zone.
 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.  Thus, in cases
 where the ZSK cannot be afforded the same level of protection as the
 KSK (such as when zone keys are kept online), and where the risk of
 unauthorized disclosure of the ZSK's private key is not negligible
 (e.g., when HSMs are not in use), the ZSK's effectivity period should
 be kept shorter than the KSK's effectivity period.
 In fact, if the risk of loss, theft, or other compromise is the same
 for a ZSK and a KSK, 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 in which the use of a Single-Type Signing
 Scheme with a long key effectivity period is a good choice, for
 example, where the costs and risks of compromise, and the costs and
 risks involved with having to perform an emergency roll, are low.

Kolkman, et al. Informational [Page 13] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

3.4. Cryptographic Considerations

3.4.1. Signature Algorithm

 At the time of this writing, there are three types of signature
 algorithms that can be used in DNSSEC: RSA, Digital Signature
 Algorithm (DSA), and GOST.  Proposals for other algorithms are in the
 making.  All three are fully specified in many freely available
 documents and are widely considered to be patent-free.  The creation
 of signatures with RSA and DSA takes roughly the same time, but DSA
 is about ten times slower for signature verification.  Also note
 that, in the context of DNSSEC, DSA is limited to a maximum of
 1024-bit keys.
 We suggest the use of RSA/SHA-256 as the preferred signature
 algorithm 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 be targeted for an effective attack.
 At the time of publication, it is known that the SHA-1 hash has
 cryptanalysis issues, and work is in progress to address them.  The
 use of public-key algorithms based on hashes stronger than SHA-1
 (e.g., SHA-256) is recommended, if these algorithms are available in
 implementations (see RFC 5702 [RFC5702] and RFC 4509 [RFC4509]).
 Also, at the time of publication, digital signature algorithms based
 on Elliptic Curve (EC) Cryptography with DNSSEC (GOST [RFC5933],
 Elliptic Curve Digital Signature Algorithm (ECDSA) [RFC6605]) are
 being standardized and implemented.  The use of EC has benefits in
 terms of size.  On the other hand, one has to balance that against
 the amount of validating resolver implementations that will not
 recognize EC signatures and thus treat the zone as insecure.  Beyond
 the observation of this tradeoff, we will not discuss this further.

3.4.2. Key Sizes

 This section assumes RSA keys, as suggested in the previous section.
 DNSSEC signing keys should be large enough to avoid all known
 cryptographic attacks during the effectivity period of the key.  To
 date, despite huge efforts, no one has broken a regular 1024-bit key;
 in fact, the best completed attack is estimated to be the equivalent
 of a 700-bit key.  An attacker breaking a 1024-bit signing key would
 need to expend phenomenal amounts of networked computing power in a

Kolkman, et al. Informational [Page 14] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 way that would not be detected in order to break a single key.
 Because of this, it is estimated that most zones can safely use
 1024-bit keys for at least the next ten years.  (A 1024-bit
 asymmetric key has an approximate equivalent strength of a symmetric
 80-bit key.)
 Depending on local policy (e.g., owners of keys that are used as
 extremely high value trust anchors, or non-anchor keys that may be
 difficult to roll over), it may be advisable 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 (RFC 3766 [RFC3766]).  Signing and verifying
 with a 2048-bit key takes longer than with a 1024-bit key.  The
 increase depends on software and hardware implementations, but public
 operations (such as verification) are about four times slower, while
 private operations (such as signing) are about eight times slower.
 Another way to decide on the size of a 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 Transport Layer
 Security (TLS) [RFC5246] 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 built into popular web browsers.  At that
 time, all 1024-bit keys (both ones with parent zones and ones that
 are trust anchors) can be rolled over and replaced with larger keys.
 Earlier documents (including the previous version of this document)
 urged the use of longer keys in situations where a particular key was
 "heavily used".  That advice may have been true 15 years ago, but it
 is not true today when using RSA algorithms and keys of 1024 bits or
 higher.

Kolkman, et al. Informational [Page 15] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

3.4.3. Private Key Storage

 It is preferred 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 to the master.
 When relying on dynamic update [RFC3007] or any other update
 mechanism that runs at a regular interval to manage a signed zone, be
 aware that at least one private key of the zone will have to reside
 on the master server or reside on an HSM to which the server has
 access.  This key is only as secure as the amount of exposure the
 server receives to unknown clients and on the level of security of
 the host.  Although not mandatory, one could administer a zone using
 a "hidden master" scheme to minimize the risk.  In this arrangement,
 the master name server that processes the updates is unavailable to
 general hosts on the Internet; it is not listed in the NS RRset.  The
 name servers 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 the master copy against unauthorized
 access in order to prevent modification of DNS data before it is
 signed.

Kolkman, et al. Informational [Page 16] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 Similarly, the choice for storing a private key in an HSM will be
 influenced by a tradeoff between various concerns:
 o  The risks that an unauthorized person has unnoticed read access to
    the private key.
 o  The remaining window of opportunity for the attacker.
 o  The economic impact of the possible attacks (for a TLD, that
    impact will typically be higher than for an individual user).
 o  The costs of rolling the (compromised) keys.  (The cost of rolling
    a ZSK is lowest, and the cost 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 [RFC3007], 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
 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 [RFC4086] and NIST SP 800-90A [NIST-SP-800-90A].  In
 particular, one should carefully assess whether the random number
 generator used during key generation adheres to these suggestions.
 Typically, HSMs tend to provide a good facility for key generation.
 Keys with a long effectivity period are particularly sensitive, as
 they will represent a more valuable target and be subject to attack
 for a longer time than short-period keys.  It is preferred 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?

 An earlier version of this document (RFC 4641 [RFC4641]) made a
 differentiation between key lengths for KSKs used for zones that are
 high in the DNS hierarchy and those for KSKs used lower down in the
 hierarchy.

Kolkman, et al. Informational [Page 17] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 This distinction is now considered irrelevant.  Longer key lengths
 for keys higher in the hierarchy are not useful because the
 cryptographic guidance is that everyone should use keys that no one
 can break.  Also, it is impossible to judge which zones are more or
 less valuable to an attacker.  An attack can only take place if the
 key compromise goes unnoticed and the attacker can act as a man-in-
 the-middle (MITM).  For example, if example.com is compromised, and
 the attacker forges answers for somebank.example.com. and sends them
 out during an MITM, when the attack is discovered it will be simple
 to prove that example.com has been compromised, and the KSK will be
 rolled.

4. Signature Generation, Key Rollover, and Related Policies

4.1. Key Rollovers

 Regardless of whether a zone uses periodic key rollovers or only
 rolls keys in case of an irregular event, 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 the fact 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.
 The typographic conventions used in the diagrams below are explained
 in Appendix B.

4.1.1. Zone Signing Key Rollovers

 If the choice for splitting ZSKs and KSKs has been made, then those
 two types of keys can be rolled separately, and ZSKs can be rolled
 without taking into account DS records from the parent or the
 configuration of such a key as the 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 scheme, described in Section 4.1.1.1, uses

Kolkman, et al. Informational [Page 18] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

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

4.1.1.1. Pre-Publish Zone Signing Key Rollover

 This section shows how to perform a ZSK rollover without the need to
 sign all the data in a zone twice -- the "Pre-Publish key rollover".
 This method has advantages in the case of a key compromise.  If the
 old key is compromised, the new key has already been distributed in
 the DNS.  The zone administrator is then able to quickly switch to
 the new key and remove the compromised key from the zone.  Another
 major advantage is that the zone size does not double, as is the case
 with the Double-Signature ZSK rollover.
 Pre-Publish key rollover from DNSKEY_Z_10 to DNSKEY_Z_11 involves
 four stages as follows:
  1. ———————————————————–

initial new DNSKEY new RRSIGs

  1. ———————————————————–

SOA_0 SOA_1 SOA_2

   RRSIG_Z_10(SOA)    RRSIG_Z_10(SOA)     RRSIG_Z_11(SOA)
   DNSKEY_K_1         DNSKEY_K_1          DNSKEY_K_1
   DNSKEY_Z_10        DNSKEY_Z_10         DNSKEY_Z_10
                      DNSKEY_Z_11         DNSKEY_Z_11
   RRSIG_K_1(DNSKEY)  RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)
  ------------------------------------------------------------
  1. ———————————————————–

DNSKEY removal

  1. ———————————————————–

SOA_3

   RRSIG_Z_11(SOA)
   DNSKEY_K_1
   DNSKEY_Z_11
   RRSIG_K_1(DNSKEY)
  ------------------------------------------------------------
                  Figure 1: Pre-Publish Key Rollover

Kolkman, et al. Informational [Page 19] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 initial:  Initial version of the zone: DNSKEY_K_1 is the Key Signing
    Key.  DNSKEY_Z_10 is used to sign all the data of the zone, i.e.,
    it is the Zone Signing Key.
 new DNSKEY:  DNSKEY_Z_11 is introduced into the key set (note that no
    signatures are generated with this key yet, but this does not
    secure against brute force attacks on its 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 the TTL
    value of the key set.
 new RRSIGs:  At the "new RRSIGs" stage, DNSKEY_Z_11 is used to sign
    the data in the zone exclusively (i.e., all the signatures from
    DNSKEY_Z_10 are removed from the zone).  DNSKEY_Z_10 remains
    published in the key set.  This way, data that was loaded into
    caches from the zone in the "new DNSKEY" step can still be
    verified with key sets fetched from this version of the zone.  The
    minimum time that the key set including DNSKEY_Z_10 is to be
    published is the time that it takes for zone data from the
    previous version of the zone to expire from old caches, i.e., the
    time it takes for this zone to propagate to all authoritative
    servers, plus the Maximum Zone TTL value of any of the data in the
    previous version of the zone.
 DNSKEY removal:  DNSKEY_Z_10 is removed from the zone.  The key set,
    now only containing DNSKEY_K_1 and DNSKEY_Z_11, is re-signed with
    DNSKEY_K_1.

Kolkman, et al. Informational [Page 20] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 The above scheme can be simplified by always publishing the "future"
 key immediately after the rollover.  The scheme would look as
 follows (we show two rollovers); the future key is introduced in "new
 DNSKEY" as DNSKEY_Z_12 and again a newer one, numbered 13, in "new
 DNSKEY (II)":
     initial             new RRSIGs          new DNSKEY
    -----------------------------------------------------------------
     SOA_0               SOA_1               SOA_2
     RRSIG_Z_10(SOA)     RRSIG_Z_11(SOA)     RRSIG_Z_11(SOA)
     DNSKEY_K_1          DNSKEY_K_1          DNSKEY_K_1
     DNSKEY_Z_10         DNSKEY_Z_10         DNSKEY_Z_11
     DNSKEY_Z_11         DNSKEY_Z_11         DNSKEY_Z_12
     RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)
     ----------------------------------------------------------------
  1. —————————————————————

new RRSIGs (II) new DNSKEY (II)

  1. —————————————————————

SOA_3 SOA_4

     RRSIG_Z_12(SOA)     RRSIG_Z_12(SOA)
     DNSKEY_K_1          DNSKEY_K_1
     DNSKEY_Z_11         DNSKEY_Z_12
     DNSKEY_Z_12         DNSKEY_Z_13
     RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)
     ----------------------------------------------------------------
           Figure 2: Pre-Publish Zone Signing Key Rollover,
                         Showing Two Rollovers
 Note that the key introduced in the "new DNSKEY" phase is not used
 for production yet; the private key can thus be stored in a
 physically secure manner and does not need to be 'fetched' every time
 a zone needs to be signed.

4.1.1.2. Double-Signature Zone Signing Key Rollover

 This section shows how to perform a ZSK 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 propagation delay plus the Maximum Zone TTL of previous versions
 of the zone.

Kolkman, et al. Informational [Page 21] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 Double-Signature ZSK rollover involves three stages as follows:
  1. —————————————————————

initial new DNSKEY DNSKEY removal

  1. —————————————————————

SOA_0 SOA_1 SOA_2

    RRSIG_Z_10(SOA)     RRSIG_Z_10(SOA)
                        RRSIG_Z_11(SOA)    RRSIG_Z_11(SOA)
    DNSKEY_K_1          DNSKEY_K_1         DNSKEY_K_1
    DNSKEY_Z_10         DNSKEY_Z_10
                        DNSKEY_Z_11        DNSKEY_Z_11
    RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)  RRSIG_K_1(DNSKEY)
    ----------------------------------------------------------------
         Figure 3: Double-Signature Zone Signing Key Rollover
 initial:  Initial version of the zone: DNSKEY_K_1 is the Key Signing
    Key.  DNSKEY_Z_10 is used to sign all the data of the zone, i.e.,
    it is the Zone Signing Key.
 new DNSKEY:  At the "new DNSKEY" stage, DNSKEY_Z_11 is introduced
    into the key set and all the data in the zone is signed with
    DNSKEY_Z_10 and DNSKEY_Z_11.  The rollover period will need to
    continue until all data from version 0 (i.e., the version of the
    zone data containing SOA_0) of the zone has been replaced in all
    secondary servers and then has expired from remote caches.  This
    will take at least the propagation delay plus the Maximum Zone TTL
    of version 0 of the zone.
 DNSKEY removal:  DNSKEY_Z_10 is removed from the zone, as are all
    signatures created with it.  The key set, now only containing
    DNSKEY_Z_11, is re-signed with DNSKEY_K_1 and DNSKEY_Z_11.
 At every instance, RRSIGs from the previous version of the zone can
 be verified with the DNSKEY RRset from the current version and vice
 versa.  The duration of the "new DNSKEY" phase and the period between
 rollovers should be at least the propagation delay to secondary
 servers plus the Maximum Zone TTL of the previous version of the
 zone.
 Note that in this example we assumed for simplicity that the zone was
 not modified during the rollover.  In fact, new data can be
 introduced at any time during this period, as long as it is signed
 with both keys.

Kolkman, et al. Informational [Page 22] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

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 stages.  Also, the Pre-Publish scheme involves more
    parental work when used for KSK rollovers, as explained in
    Section 4.1.2.
 Double-Signature ZSK rollover:  The drawback of this approach 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 stages.

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

Kolkman, et al. Informational [Page 23] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

  1. ——————————————————————–

initial new DNSKEY DS change DNSKEY removal

  1. ——————————————————————–

Parent:

  SOA_0 -----------------------------> SOA_1 ------------------------>
  RRSIG_par(SOA) --------------------> RRSIG_par(SOA) --------------->
  DS_K_1 ----------------------------> DS_K_2 ----------------------->
  RRSIG_par(DS) ---------------------> RRSIG_par(DS) ---------------->
 Child:
  SOA_0              SOA_1 -----------------------> SOA_2
  RRSIG_Z_10(SOA)    RRSIG_Z_10(SOA) -------------> RRSIG_Z_10(SOA)
  DNSKEY_K_1         DNSKEY_K_1 ------------------>
                     DNSKEY_K_2 ------------------> DNSKEY_K_2
  DNSKEY_Z_10        DNSKEY_Z_10 -----------------> DNSKEY_Z_10
  RRSIG_K_1(DNSKEY)  RRSIG_K_1 (DNSKEY) ---------->
                     RRSIG_K_2 (DNSKEY) ----------> RRSIG_K_2(DNSKEY)
 ---------------------------------------------------------------------
         Figure 4: Stages of Deployment for a Double-Signature
                       Key Signing Key Rollover
 initial:  Initial version of the zone.  The parental DS points to
    DNSKEY_K_1.  Before the rollover starts, the child will have to
    verify what the TTL is of the DS RR that points to DNSKEY_K_1 --
    it is needed during the rollover, and we refer to the value as
    TTL_DS.
 new DNSKEY:  During the "new DNSKEY" phase, the zone administrator
    generates a second KSK, DNSKEY_K_2.  The key is provided to the
    parent, and the child will have to wait until a new DS RR has been
    generated that points to DNSKEY_K_2.  After that DS RR has been
    published on all servers authoritative for the parent's zone, the
    zone administrator has to wait at least TTL_DS to make sure that
    the old DS RR has expired from caches.
 DS change:  The parent replaces DS_K_1 with DS_K_2.
 DNSKEY removal:  DNSKEY_K_1 has been removed.
 The scenario above puts the responsibility for maintaining a valid
 chain of trust with the child.  It also is based on the premise that
 the parent only has one DS RR (per algorithm) per zone.  An
 alternative mechanism has been considered.  Using an established
 trust relationship, the interaction can be performed in-band, and the
 removal of the keys by the child can possibly be signaled by the
 parent.  In this mechanism, there are periods where there are two DS

Kolkman, et al. Informational [Page 24] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 RRs at the parent.  This is known as a KSK Double-DS rollover and is
 shown in Figure 5.  This method has some drawbacks for KSKs.  We
 first describe the rollover scheme and then indicate these drawbacks.
  1. ——————————————————————-

initial new DS new DNSKEY DS removal

  1. ——————————————————————-

Parent:

   SOA_0           SOA_1 ------------------------> SOA_2
   RRSIG_par(SOA)  RRSIG_par(SOA) ---------------> RRSIG_par(SOA)
   DS_K_1          DS_K_1 ----------------------->
                   DS_K_2 -----------------------> DS_K_2
   RRSIG_par(DS)   RRSIG_par(DS) ----------------> RRSIG_par(DS)
 Child:
   SOA_0 -----------------------> SOA_1 ---------------------------->
   RRSIG_Z_10(SOA) -------------> RRSIG_Z_10(SOA) ------------------>
   DNSKEY_K_1 ------------------> DNSKEY_K_2 ----------------------->
   DNSKEY_Z_10 -----------------> DNSKEY_Z_10 ---------------------->
   RRSIG_K_1 (DNSKEY) ----------> RRSIG_K_2 (DNSKEY) --------------->
 --------------------------------------------------------------------
            Figure 5: Stages of Deployment for a Double-DS
                       Key Signing Key Rollover
 When the child zone wants to roll, it notifies the parent during the
 "new DS" phase and submits the new key (or the corresponding DS) to
 the parent.  The parent publishes DS_K_1 and DS_K_2, pointing to
 DNSKEY_K_1 and DNSKEY_K_2, respectively.  During the rollover ("new
 DNSKEY" phase), which can take place as soon as the new DS set
 propagated through the DNS, the child replaces DNSKEY_K_1 with
 DNSKEY_K_2.  If the old key has expired from caches, at the "DS
 removal" phase the parent can be notified that the old DS record can
 be deleted.
 The drawbacks of this scheme are that during the "new DS" phase, the
 parent cannot verify the match between the DS_K_2 RR and DNSKEY_K_2
 using the DNS, as DNSKEY_K_2 is not yet published.  Besides, we
 introduce a "security lame" key (see Section 4.3.3).  Finally, the
 child-parent interaction consists of two steps.  The "Double
 Signature" method only needs one interaction.

Kolkman, et al. Informational [Page 25] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

4.1.2.1. Special Considerations for RFC 5011 KSK Rollover

 The scenario sketched above assumes that the KSK is not in use as a
 trust anchor but that validating name servers exclusively depend on
 the parental DS record to establish the zone's security.  If it is
 known that validating name servers have configured trust anchors,
 then that needs to be taken into account.  Here, we assume that zone
 administrators will deploy RFC 5011 [RFC5011] style rollovers.
 RFC 5011 style rollovers increase the duration of key rollovers: The
 key to be removed must first be revoked.  Thus, before the DNSKEY_K_1
 removal phase, DNSKEY_K_1 must be published for one more Maximum Zone
 TTL with the REVOKE bit set.  The revoked key must be self-signed, so
 in this phase the DNSKEY RRset must also be signed with DNSKEY_K_1.

4.1.3. Single-Type Signing Scheme Key Rollover

 The rollover of a key when a Single-Type Signing Scheme is used is
 subject to the same requirement as the rollover of a KSK or ZSK:
 During any stage of the rollover, the chain of trust needs to
 continue to validate for any combination of data in the zone as well
 as data that may still live in distant caches.
 There are two variants for this rollover.  Since the choice for a
 Single-Type Signing Scheme is motivated by operational simplicity, we
 describe the most straightforward rollover scheme first.
  1. ——————————————————————

initial new DNSKEY DS change DNSKEY removal

  1. ——————————————————————

Parent:

   SOA_0 --------------------------> SOA_1 ---------------------->
   RRSIG_par(SOA) -----------------> RRSIG_par(SOA) ------------->
   DS_S_1 -------------------------> DS_S_2 --------------------->
   RRSIG_par(DS_S_1) --------------> RRSIG_par(DS_S_2) ---------->
 Child:
   SOA_0             SOA_1 ----------------------> SOA_2
   RRSIG_S_1(SOA)    RRSIG_S_1(SOA) ------------->
                     RRSIG_S_2(SOA) -------------> RRSIG_S_2(SOA)
   DNSKEY_S_1        DNSKEY_S_1 ----------------->
                     DNSKEY_S_2 -----------------> DNSKEY_S_2
   RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY) ---------->
                     RRSIG_S_2(DNSKEY) ----------> RRSIG_S_2(DNSKEY)
 -------------------------------------------------------------------
           Figure 6: Stages of the Straightforward Rollover
                    in a Single-Type Signing Scheme

Kolkman, et al. Informational [Page 26] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 initial:  Parental DS points to DNSKEY_S_1.  All RRsets in the zone
    are signed with DNSKEY_S_1.
 new DNSKEY:  A new key (DNSKEY_S_2) is introduced, and all the RRsets
    are signed with both DNSKEY_S_1 and DNSKEY_S_2.
 DS change:  After the DNSKEY RRset with the two keys had time to
    propagate into distant caches (that is, the key set exclusively
    containing DNSKEY_S_1 has been expired), the parental DS record
    can be changed.
 DNSKEY removal:  After the DS RRset containing DS_S_1 has expired
    from distant caches, DNSKEY_S_1 can be removed from the DNSKEY
    RRset.
 In this first variant, the new signatures and new public key are
 added to the zone.  Once they are propagated, the DS at the parent is
 switched.  If the old DS has expired from the caches, the old
 signatures and old public key can be removed from the zone.
 This rollover has the drawback that it introduces double signatures
 over all data of the zone.  Taking these zone size considerations
 into account, it is possible to not introduce the signatures made
 with DNSKEY_S_2 at the "new DNSKEY" step.  Instead, signatures of
 DNSKEY_S_1 are replaced with signatures of DNSKEY_S_2 in an
 additional stage between the "DS change" and "DNSKEY removal" step:
 After the DS RRset containing DS_S_1 has expired from distant caches,
 the signatures can be swapped.  Only after the new signatures made
 with DNSKEY_S_2 have been propagated can the old public key
 DNSKEY_S_1 be removed from the DNSKEY RRset.
 The second variant of the Single-Type Signing Scheme Key rollover is
 the Double-DS rollover.  In this variant, one introduces a new DNSKEY
 into the key set and submits the new DS to the parent.  The new key
 is not yet used to sign RRsets.  The signatures made with DNSKEY_S_1
 are replaced with signatures made with DNSKEY_S_2 at the moment that
 DNSKEY_S_2 and DS_S_2 have been propagated.

Kolkman, et al. Informational [Page 27] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012


 initial            new DS         new RRSIG         DS removal

———————————————————————– Parent:

 SOA_0              SOA_1 -------------------------> SOA_2
 RRSIG_par(SOA)     RRSIG_par(SOA) ----------------> RRSIG_par(SOA)
 DS_S_1             DS_S_1 ------------------------>
                    DS_S_2 ------------------------> DS_S_2
 RRSIG_par(DS)      RRSIG_par(DS) -----------------> RRSIG_par(DS)

Child:

 SOA_0              SOA_1          SOA_2             SOA_3
 RRSIG_S_1(SOA)     RRSIG_S_1(SOA) RRSIG_S_2(SOA)    RRSIG_S_2(SOA)
 DNSKEY_S_1         DNSKEY_S_1     DNSKEY_S_1
                    DNSKEY_S_2     DNSKEY_S_2        DNSKEY_S_2
 RRSIG_S_1 (DNSKEY)                RRSIG_S_2(DNSKEY) RRSIG_S_2(DNSKEY)

———————————————————————–

     Figure 7: Stages of Deployment for a Double-DS Rollover in a
                      Single-Type Signing Scheme

4.1.4. Algorithm Rollovers

 A special class of key rollovers is the one needed for a change of
 signature algorithms (either adding a new algorithm, removing an old
 algorithm, or both).  Additional steps are needed to retain integrity
 during this rollover.  We first describe the generic case; special
 considerations for rollovers that involve trust anchors and single-
 type keys are discussed later.
 There exist both a conservative and a liberal approach for algorithm
 rollover.  This has to do with Section 2.2 of RFC 4035 [RFC4035]:
    There MUST be an RRSIG for each RRset using at least one DNSKEY
    of each algorithm in the zone apex DNSKEY RRset.  The apex
    DNSKEY RRset itself MUST be signed by each algorithm appearing
    in the DS RRset located at the delegating parent (if any).
 The conservative approach interprets this section very strictly,
 meaning that it expects that every RRset has a valid signature for
 every algorithm signaled by the zone apex DNSKEY RRset, including
 RRsets in caches.  The liberal approach uses a more loose
 interpretation of the section and limits the rule to RRsets in the
 zone at the authoritative name servers.  There is a reasonable
 argument for saying that this is valid, because the specific section
 is a subsection of Section 2 ("Zone Signing") of RFC 4035.

Kolkman, et al. Informational [Page 28] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 When following the more liberal approach, algorithm rollover is just
 as easy as a regular Double-Signature KSK rollover (Section 4.1.2).
 Note that the Double-DS KSK rollover method cannot be used, since
 that would introduce a parental DS of which the apex DNSKEY RRset has
 not been signed with the introduced algorithm.
 However, there are implementations of validators known to follow the
 more conservative approach.  Performing a Double-Signature KSK
 algorithm rollover will temporarily make your zone appear as Bogus by
 such validators during the rollover.  Therefore, the rollover
 described in this section will explain the stages of deployment and
 will assume that the conservative approach is used.
 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 KSK rollover.
 When removing an old algorithm, the DS for the algorithm should be
 removed from the parent zone first, followed by the DNSKEY and the
 signatures (in the child zone).
 Figure 8 describes the steps.

Kolkman, et al. Informational [Page 29] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

  1. —————————————————————

initial new RRSIGs new DNSKEY

  1. —————————————————————

Parent:

  SOA_0 -------------------------------------------------------->
  RRSIG_par(SOA) ----------------------------------------------->
  DS_K_1 ------------------------------------------------------->
  RRSIG_par(DS_K_1) -------------------------------------------->
 Child:
  SOA_0                SOA_1                SOA_2
  RRSIG_Z_10(SOA)      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_K_2
  DNSKEY_Z_10          DNSKEY_Z_10          DNSKEY_Z_10
                                            DNSKEY_Z_11
  RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)
                                            RRSIG_K_2(DNSKEY)
  1. —————————————————————

new DS DNSKEY removal RRSIGs removal

  1. —————————————————————

Parent:

  SOA_1 ------------------------------------------------------->
  RRSIG_par(SOA) ---------------------------------------------->
  DS_K_2 ------------------------------------------------------>
  RRSIG_par(DS_K_2) ------------------------------------------->
 Child:
  -------------------> SOA_3                SOA_4
  -------------------> RRSIG_Z_10(SOA)
  -------------------> RRSIG_Z_11(SOA)      RRSIG_Z_11(SOA)
  1. ——————>
  2. ——————> DNSKEY_K_2 DNSKEY_K_2
  3. ——————>
  4. ——————> DNSKEY_Z_11 DNSKEY_Z_11
  5. ——————>
  6. ——————> RRSIG_K_2(DNSKEY) RRSIG_K_2(DNSKEY)
  7. —————————————————————
      Figure 8: Stages of Deployment during an Algorithm Rollover

Kolkman, et al. Informational [Page 30] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 initial:  Describes the state of the zone before any transition is
    done.  The number of the keys may vary, but all keys (in DNSKEY
    records) for the zone use the same algorithm.
 new RRSIGs:  The signatures made with the new key over all records in
    the zone are added, but the key itself is not.  This step is
    needed to propagate the signatures created with the new algorithm
    to the caches.  If this is not done, it is possible for a resolver
    to retrieve the new DNSKEY RRset (containing the new algorithm)
    but to have RRsets in its cache with signatures created by the old
    DNSKEY RRset (i.e., without the new algorithm).
    The RRSIG for the DNSKEY RRset does not need to be pre-published
    (since these records will travel together) and does not need
    special processing in order to keep them synchronized.
 new DNSKEY:  After the old data has expired from caches, the new key
    can be added to the zone.
 new DS:  After the cache data for the old DNSKEY RRset has expired,
    the DS record for the new key can be added to the parent zone and
    the DS record for the old key can be removed in the same step.
 DNSKEY removal:  After the cache data for the old DS RRset has
    expired, the old algorithm can be removed.  This time, the old key
    needs to be removed first, before removing the old signatures.
 RRSIGs removal:  After the cache data for the old DNSKEY RRset has
    expired, the old signatures can also be removed during this step.
 Below, we deal with a few special cases of algorithm rollovers:
 1: Single-Type Signing Scheme Algorithm rollover:  when there is no
    differentiation between ZSKs and KSKs (Section 4.1.4.1).
 2: RFC 5011 Algorithm rollover:  when trust anchors can track the
    roll via RFC 5011 style rollover (Section 4.1.4.2).
 3: 1 and 2 combined:  when a Single-Type Signing Scheme Algorithm
    rollover is performed RFC 5011 style (Section 4.1.4.3).
 In addition to the narrative below, these special cases are
 represented in Figures 12, 13, and 14 in Appendix C.

Kolkman, et al. Informational [Page 31] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

4.1.4.1. Single-Type Signing Scheme Algorithm Rollover

 If one key is used that acts as both ZSK and KSK, the same scheme and
 figure as above (Figure 8 in Section 4.1.4) applies, whereby all
 DNSKEY_Z_* records from the table are removed and all RRSIG_Z_* are
 replaced with RRSIG_S_*.  All DNSKEY_K_* records are replaced with
 DNSKEY_S_*, and all RRSIG_K_* records are replaced with RRSIG_S_*.
 The requirement to sign with both algorithms and make sure that old
 RRSIGs have the opportunity to expire from distant caches before
 introducing the new algorithm in the DNSKEY RRset is still valid.
 This is shown in Figure 12 in Appendix C.

4.1.4.2. Algorithm Rollover, RFC 5011 Style

 Trust anchor algorithm rollover is almost as simple as a regular
 RFC 5011-based rollover.  However, the old trust anchor must be
 revoked before it is removed from the zone.
 The timeline (see Figure 13 in Appendix C) is similar to that of
 Figure 8 above, but after the "new DS" step, an additional step is
 required where the DNSKEY is revoked.  The details of this step
 ("revoke DNSKEY") are shown in Figure 9 below.
  1. ——————————–

revoke DNSKEY

  1. ——————————–

Parent:

  1. —————————→
  2. —————————→
  3. —————————→
  4. —————————→
 Child:
   SOA_3
   RRSIG_Z_10(SOA)
   RRSIG_Z_11(SOA)
   DNSKEY_K_1_REVOKED
   DNSKEY_K_2
   DNSKEY_Z_11
   RRSIG_K_1(DNSKEY)
   RRSIG_K_2(DNSKEY)
 ---------------------------------
    Figure 9: The Revoke DNSKEY State That Is Added to an Algorithm
                   Rollover when RFC 5011 Is in Use

Kolkman, et al. Informational [Page 32] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 There is one exception to the requirement from RFC 4035 quoted in
 Section 4.1.4 above: While all zone data must be signed with an
 unrevoked key, it is permissible to sign the key set with a revoked
 key.  The somewhat esoteric argument is as follows:
 Resolvers that do not understand the RFC 5011 REVOKE flag will handle
 DNSKEY_K_1_REVOKED the same as if it were DNSKEY_K_1.  In other
 words, they will handle the revoked key as a normal key, and thus
 RRsets signed with this key will validate.  As a result, the
 signature matches the algorithm listed in the DNSKEY RRset.
 Resolvers that do implement RFC 5011 will remove DNSKEY_K_1 from the
 set of trust anchors.  That is okay, since they have already added
 DNSKEY_K_2 as the new trust anchor.  Thus, algorithm 2 is the only
 signaled algorithm by now.  That is, we only need RRSIG_K_2(DNSKEY)
 to authenticate the DNSKEY RRset, and we are still compliant with
 Section 2.2 of RFC 4035: There must be an RRSIG for each RRset using
 at least one DNSKEY of each algorithm in the zone apex DNSKEY RRset.

4.1.4.3. Single Signing Type Algorithm Rollover, RFC 5011 Style

 If a decision is made to perform an RFC 5011 style rollover with a
 Single Signing Scheme key, it should be noted that Section 2.1 of
 RFC 5011 states:
    Once the resolver sees the REVOKE bit, it MUST NOT use this key
    as a trust anchor or for any other purpose except to validate
    the RRSIG it signed over the DNSKEY RRset specifically for the
    purpose of validating the revocation.
 This means that once DNSKEY_S_1 is revoked, it cannot be used to
 validate its signatures over non-DNSKEY RRsets.  Thus, those RRsets
 should be signed with a shadow key, DNSKEY_Z_10, during the algorithm
 rollover.  The shadow key can be removed at the same time the revoked
 DNSKEY_S_1 is removed from the zone.  In other words, the zone must
 temporarily fall back to a KSK/ZSK split model during the rollover.
 In other words, the rule that at every RRset there must be at least
 one signature for each algorithm used in the DNSKEY RRset still
 applies.  This means that a different key with the same algorithm,
 other than the revoked key, must sign the entire zone.  Thus, more
 operations are needed if the Single-Type Signing Scheme is used.
 Before rolling the algorithm, a new key must be introduced with the
 same algorithm as the key that is a candidate for revocation.  That
 key can than temporarily act as a ZSK during the algorithm rollover.

Kolkman, et al. Informational [Page 33] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 As with algorithm rollover RFC 5011 style, while all zone data must
 be signed with an unrevoked key, it is permissible to sign the key
 set with a revoked key using the same esoteric argument given in
 Section 4.1.4.2.
 The lesson of all of this is that a Single-Type Signing Scheme
 algorithm rollover using RFC 5011 is as complicated as the name of
 the rollover implies: Reverting to a split-key scheme for the
 duration of the rollover may be preferable.

4.1.4.4. NSEC-to-NSEC3 Algorithm Rollover

 A special case is the rollover from an NSEC signed zone to an NSEC3
 signed zone.  In this case, algorithm numbers are used to signal
 support for NSEC3 but they do not mandate the use of NSEC3.
 Therefore, NSEC records should remain in the zone until the rollover
 to a new algorithm has completed and the new DNSKEY RRset has
 populated distant caches, at the end of the "new DNSKEY" stage.  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
 the 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 during the "new DS" step: increasing the
 serial number, introducing the NSEC3PARAM record to signal that
 NSEC3-authenticated data related to denial of existence should be
 served, and re-signing the zone.
 In summary, an NSEC-to-NSEC3 rollover is an ordinary algorithm
 rollover whereby NSEC is used all the time and only after that
 rollover finished NSEC3 needs to be deployed.  The procedures are
 also listed in Sections 10.4 and 10.5 of RFC 5155 [RFC5155].

4.1.5. Considerations for Automated Key Rollovers

 As keys must be renewed periodically, there is some motivation to
 automate the rollover process.  Consider the following:
 o  ZSK rollovers are easy to automate, as only the child zone is
    involved.
 o  A KSK rollover needs interaction between the 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.

Kolkman, et al. Informational [Page 34] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

4.2. Planning for Emergency Key Rollover

 This section deals with preparation for a possible key compromise.
 It is advisable to have a documented procedure ready for those times
 when a key compromise is suspected or confirmed.
 When the private material of one of a zone's keys is compromised, it
 can be used by an attacker 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, and
 o  as long as the DS RR in the parent zone points to the
    (compromised) key signing the DNSKEY RRset, and
 o  as long as the (compromised) 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 a zone's compromised key exists, your
 namespace is vulnerable to abuse by anyone who has obtained
 illegitimate possession of the key.  Zone administrators have to make
 a decision as to whether the abuse of the compromised key is worse
 than having data in caches that cannot be validated.  If the zone
 administrator chooses to break the trust chain to the compromised
 key, data in caches signed with this key cannot be validated.
 However, if the zone administrator chooses to take the path of a
 regular rollover, during the rollover the malicious key holder can
 continue to spoof data so that it appears to be valid.

4.2.1. KSK Compromise

 A compromised KSK can be used to sign the key set of an attacker's
 version of the zone.  That zone could be used to poison the DNS.
 A zone containing a DNSKEY RRset with a compromised KSK is vulnerable
 as long as the compromised KSK is configured as the trust anchor or a
 DS record in the parent zone points to it.
 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

Kolkman, et al. Informational [Page 35] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 is only one DS record at the parent.  If there are multiple DS
 records, this does not apply, although the chain of trust of this
 particular key is broken.
 Note that an attacker's version of the 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 version of
 the zone to appear as valid and the original zone as Bogus.
 Therefore, we advise administrators not to remove the KSK before the
 parent has a DS record for the new KSK in place.

4.2.1.1. Emergency Key Rollover Keeping the Chain of Trust Intact

 If it is desired to perform an emergency key rollover in a manner
 that keeps the chain of trust intact, 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.
 This means ensuring that the signature made with a new KSK over the
 key set that contains the compromised KSK expires just after the new
 DS appears at the parent.  Expiration of that signature will cause
 expiration of that key set from the caches.
 The procedure is as follows:
 1.  Introduce a new KSK into the key set; keep the compromised KSK in
     the key set.  Lower the TTL for DNSKEYs so that the DNSKEY RRset
     will expire from caches sooner.
 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 DSs have expired from caches.  This
     provides an upper limit on how long the compromised KSK can be
     used in a replay attack.
 3.  Upload the DS for this new key to the parent.
 4.  Follow the procedure of the regular KSK rollover: Wait for the DS
     to appear at the authoritative servers, and then wait as long as
     the TTL of the old DS RRs.  If necessary, re-sign the DNSKEY
     RRset and modify/extend the expiration time.
 5.  Remove the compromised DNSKEY RR from the zone, and re-sign the
     key set using your "normal" TTL and signature validity period.

Kolkman, et al. Informational [Page 36] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 An additional danger of a key compromise is that the compromised key
 could be used to facilitate a legitimate-looking DNSKEY/DS rollover
 and/or name server 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 to authenticate communication with the parent.

4.2.1.2. Emergency Key Rollover Breaking the Chain of Trust

 There are two methods to perform an emergency key rollover in a
 manner that breaks the chain of trust.  The first method causes the
 child zone to appear Bogus to validating resolvers.  The other causes
 the child zone to appear Insecure.  These are described below.
 In the method that causes the child zone to appear Bogus to
 validating resolvers, the child zone replaces the current KSK with a
 new one and re-signs the key set.  Next, it sends the DS of the new
 key to the parent.  Only after the parent has placed the new DS in
 the zone is the child's chain of trust repaired.  Note that until
 that time, the child zone is still vulnerable to spoofing: The
 attacker is still in possession of the compromised key that the DS
 points to.
 An alternative method of breaking the chain of trust is by removing
 the DS RRs from the parent zone altogether.  As a result, the child
 zone would become Insecure.  After the DS has expired from distant
 caches, the keys and signatures are removed from the child zone, new
 keys and signatures are introduced, and finally, a new DS is
 submitted to the parent.

4.2.2. ZSK Compromise

 Primarily because there is no interaction with the parent required
 when a ZSK is compromised, the situation is less severe than with a
 KSK compromise.  The zone must still be re-signed with a new ZSK as
 soon as possible.  As this is a local operation and requires no
 communication between the parent and child, this can be achieved
 fairly quickly.  However, one has to take into account that -- just
 as with a normal rollover -- the immediate disappearance of the old
 compromised key may lead to verification problems.  Also note that
 until the RRSIG over the compromised ZSK has expired, the zone may
 still be at risk.

Kolkman, et al. Informational [Page 37] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

4.2.3. Compromises of Keys Anchored in Resolvers

 A key can also be pre-configured in resolvers as a trust anchor.  If
 trust anchor keys are compromised, the administrators of resolvers
 using these keys should be notified of this fact.  Zone
 administrators may consider setting up a mailing list to communicate
 the fact that a SEP key is about to be rolled over.  This
 communication will of course need to be authenticated by some means,
 e.g., by using digital signatures.
 End-users faced with the task of updating an anchored key should
 always verify the new key.  New keys should be authenticated out-of-
 band, for example, through the use of an announcement website that is
 secured using Transport Layer Security (TLS) [RFC5246].

4.2.4. Stand-By Keys

 Stand-by keys are keys that are published in your zone but are not
 used to sign RRsets.  There are two reasons why someone would want to
 use stand-by keys.  One is to speed up the emergency key rollover.
 The other is to recover from a disaster that leaves your production
 private keys inaccessible.
 The way to deal with stand-by keys differs for ZSKs and KSKs.  To
 make a stand-by ZSK, you need to publish its DNSKEY RR.  To make a
 stand-by KSK, you need to get its DS RR published at the parent.
 Assuming you have your normal DNS operation, to prepare stand-by keys
 you need to:
 o  Generate a stand-by ZSK and KSK.  Store them safely in a location
    different than the place where the currently used ZSK and KSK are
    held.
 o  Pre-publish the DNSKEY RR of the stand-by ZSK in the zone.
 o  Pre-publish the DS of the stand-by KSK in the parent zone.
 Now suppose a disaster occurs and disables access to the currently
 used keys.  To recover from that situation, follow these procedures:
 o  Set up your DNS operations and introduce the stand-by KSK into the
    zone.
 o  Post-publish the disabled ZSK and sign the zone with the stand-by
    keys.

Kolkman, et al. Informational [Page 38] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 o  After some time, when the new signatures have been propagated, the
    old keys, old signatures, and the old DS can be removed.
 o  Generate a new stand-by key set at a different location and
    continue "normal" operation.

4.3. Parent Policies

4.3.1. Initial Key Exchanges and Parental Policies Considerations

 The initial key exchange is always subject to the policies set by the
 parent.  It is specifically important in a registry-registrar-
 registrant model where a registry maintains the parent zone, and the
 registrant (the user of the child-domain name) deals with the
 registry through an intermediary called a registrar (see [RFC3375]
 for a comprehensive definition).  The key material is to be passed
 from the DNS operator to the parent via a registrar, where both the
 DNS operator and registrar are selected by the registrant and might
 be different organizations.  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 the parent and child.
 That is, there is no implicit need in DNSSEC to make the
 authentication process stronger than it is for regular DNS.
 Using the DNS itself as the source for the actual DNSKEY material has
 the benefit that it reduces the chances of user error.  A DNSKEY
 query tool can make use of the SEP bit [RFC4035] to select the proper
 key(s) 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 that
 the child is secure.
 Note: Out-of-band verification is still needed when the key material
 is fetched for the first time, even via DNS.  The parent can never be
 sure whether or not the DNSKEY RRs have been spoofed.
 With some types of key rollovers, the DNSKEY is not pre-published,
 and a DNSKEY query tool is not able to retrieve the successor key.
 In this case, the out-of-band method is required.  This also allows
 the child to determine the digest algorithm of the DS record.

Kolkman, et al. Informational [Page 39] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

4.3.2. Storing Keys or Hashes?

 When designing a registry system, one should consider whether to
 store the DNSKEYs and/or the corresponding DSs.  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 suggest
 that registry systems should be able to store DS RRs, even if they
 also store DNSKEYs (see also "DNSSEC Trust Anchor Configuration and
 Maintenance" [DNSSEC-TRUST-ANCHOR]).
 The storage considerations also relate to the design of the customer
 interface and the method by which data is transferred between the
 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) [RFC5910], 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 to move 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 extensions to EPP may be applicable.
 Having an out-of-band mechanism such as a registry directory (e.g.,
 Whois) to find out which keys are used to generate DS Resource
 Records for specific owners and/or zones may also help with
 troubleshooting.

4.3.3. Security Lameness

 Security lameness is defined as the state whereby the parent has a DS
 RR pointing to a nonexistent DNSKEY RR.  Security lameness may occur
 temporarily during a Double-DS rollover scheme.  However, care should
 be taken that not all DS RRs are pointing to a nonexistent DNSKEY RR,
 which will 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 the child, the parental checks are limited to only
 comparing the key id.

Kolkman, et al. Informational [Page 40] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 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
 that 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 the 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 zone
 administrator unavailability during weekends shows the need for DS
 signature validity periods longer than two days.  Just like any
 signature validity period, we suggest an absolute minimum for the DS
 signature validity period of a few days.
 The maximum signature validity period of the DS record depends on how
 long child zones are willing to be vulnerable after a key compromise.
 On the other hand, shortening the DS signature validity period
 increases the operational risk for the parent.  Therefore, the parent
 may have a policy to use a signature validity period that is
 considerably longer than the child would hope for.
 A compromise between the policy/operational constraints of the parent
 and minimizing damage for the child may result in a DS signature
 validity period somewhere between a week and several months.
 In addition to the signature validity period, which sets a lower
 bound on the number of times the zone administrator will need to sign
 the zone data and an upper bound on the time that a child is
 vulnerable after key compromise, there is the TTL value on the DS
 RRs.  Shortening the TTL reduces the damage of a successful replay
 attack.  It does mean that the authoritative servers will see more
 queries.  But on the other hand, a short TTL lowers the persistence
 of DS RRsets in caches, thereby increasing the speed with which
 updated DS RRsets propagate through the DNS.

Kolkman, et al. Informational [Page 41] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

4.3.5. Changing DNS Operators

 The parent-child relationship is often described in terms of a
 registry-registrar-registrant model, where a registry maintains the
 parent zone and the registrant (the user of the child-domain name)
 deals with the registry through an intermediary called a registrar
 [RFC3375].  Registrants may outsource the maintenance of their DNS
 system, including the maintenance of DNSSEC key material, to the
 registrar or to another third party, referred to here as the 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 the losing operator has control over the DNS zone and its keys.
 The following sections describe the two cases: where the losing
 operator cooperates with the new operator (the gaining operator), and
 where the two do not cooperate.

4.3.5.1. Cooperating DNS Operators

 In this scenario, it is assumed that the 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, the change could be made with a Pre-Publish ZSK
 rollover, whereby the losing operator pre-publishes the ZSK of the
 gaining operator, combined with a Double-Signature KSK rollover where
 the two registrars exchange public keys and independently generate a
 signature over those key sets 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.

Kolkman, et al. Informational [Page 42] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

  1. ———————————————————–

initial | pre-publish |

  1. ———————————————————–

Parent:

   NS_A                            NS_A
   DS_A                            DS_A
  ------------------------------------------------------------
  Child at A:            Child at A:        Child at B:
   SOA_A0                 SOA_A1             SOA_B0
   RRSIG_Z_A(SOA)         RRSIG_Z_A(SOA)     RRSIG_Z_B(SOA)
   NS_A                   NS_A               NS_B
   RRSIG_Z_A(NS)          NS_B               RRSIG_Z_B(NS)
                          RRSIG_Z_A(NS)
   DNSKEY_Z_A             DNSKEY_Z_A         DNSKEY_Z_A
                          DNSKEY_Z_B         DNSKEY_Z_B
   DNSKEY_K_A             DNSKEY_K_A         DNSKEY_K_A
                          DNSKEY_K_B         DNSKEY_K_B
   RRSIG_K_A(DNSKEY)      RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)
                          RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)
  ------------------------------------------------------------
  1. ———————————————————–

re-delegation | post-migration |

  1. ———————————————————–

Parent:

            NS_B                           NS_B
            DS_B                           DS_B
  ------------------------------------------------------------
  Child at A:        Child at B:           Child at B:
   SOA_A1             SOA_B0                SOA_B1
   RRSIG_Z_A(SOA)     RRSIG_Z_B(SOA)        RRSIG_Z_B(SOA)
   NS_A               NS_B                  NS_B
   NS_B               RRSIG_Z_B(NS)         RRSIG_Z_B(NS)
   RRSIG_Z_A(NS)
   DNSKEY_Z_A         DNSKEY_Z_A
   DNSKEY_Z_B         DNSKEY_Z_B            DNSKEY_Z_B
   DNSKEY_K_A         DNSKEY_K_A
   DNSKEY_K_B         DNSKEY_K_B            DNSKEY_K_B
   RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)
   RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)     RRSIG_K_B(DNSKEY)
  ------------------------------------------------------------
             Figure 10: Rollover for Cooperating Operators

Kolkman, et al. Informational [Page 43] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 In this figure, A denotes the losing operator and B the gaining
 operator.  RRSIG_Z is the RRSIG produced by a ZSK, RRSIG_K is
 produced with a KSK, and the appended A or B indicates the producers
 of the key pair.  "Child at A" is how the zone content is represented
 by the losing DNS operator, and "Child at B" is how the zone content
 is represented by the gaining DNS operator.
 The zone is initially delegated from the parent to the name servers
 of operator A.  Operator A uses his own ZSK and KSK to sign the zone.
 The cooperating operator A will pre-publish the new NS record and the
 ZSK and KSK of operator B, including the RRSIG over the DNSKEY RRset
 generated by the KSK of operator B.  Operator B needs to publish the
 same DNSKEY RRset.  When that DNSKEY RRset has populated the caches,
 the re-delegation can be made, which involves adjusting the NS and DS
 records in the parent zone to point to operator B.  And after all
 DNSSEC records related to operator A have expired from the caches,
 operator B can stop publishing the keys and signatures belonging to
 operator A, and vice versa.
 The requirement to exchange signatures has a couple of drawbacks.  It
 requires more operational overhead, because not only do the operators
 have to exchange public keys but they also have to exchange the
 signatures of the new DNSKEY RRset.  This drawback does not exist if
 the Double-Signature KSK rollover is replaced with a Double-DS KSK
 rollover.  See Figure 15 in Appendix D for the diagram.
 Thus, if the registry and registrars allow DS records to be published
 that do not point to a published DNSKEY in the child zone, the
 Double-DS KSK rollover is preferred (see Figure 5), in combination
 with the Pre-Publish ZSK rollover.  This does not require sharing the
 KSK signatures between the operators, but both operators still have
 to publish each other's ZSKs.

4.3.5.2. Non-Cooperating DNS Operators

 In the non-cooperating case, matters are more complicated.  The
 losing operator may not cooperate and leave the data in the DNS as
 is.  In extreme cases, the losing operator may become obstructive and
 publish a DNSKEY RR with a high TTL and corresponding signature
 validity period so that registrar A's DNSKEY could end up in caches
 for (in theory at least) decades.
 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 re-delegation
 from the losing operator to the gaining operator has taken place.
 One could imagine a rollover scenario where the gaining operator
 takes a copy of all RRSIGs created by the losing operator and

Kolkman, et al. Informational [Page 44] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 publishes those in conjunction with its own signatures, but that
 would not allow any changes in the zone content.  Since a
 re-delegation took place, the NS RRset has by definition changed, so
 such a 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 the losing operator's RRSIGs have been
 copied.
 The only viable operation for the registrant is to have his zone go
 Insecure for the duration of the change.  The registry should be
 asked to remove the DS RR pointing to the losing operator's DNSKEY
 and to change the NS RRset to point to the gaining operator.  Once
 this has propagated through the DNS, the registry should be asked to
 insert the DS record pointing to the (newly signed) zone at
 operator B.
 Note that some behaviors of resolver implementations may aid in the
 process of changing DNS operators:
 o  TTL sanity checking, as described in RFC 2308 [RFC2308], will
    limit the impact of the actions of an obstructive losing operator.
    Resolvers that implement TTL sanity checking will use an upper
    limit for TTLs on RRsets in responses.
 o  If RRsets at the zone cut (are about to) expire, the resolver
    restarts its search above the zone cut.  Otherwise, the resolver
    risks continuing to use a name server that might be un-delegated
    by the parent.
 o  Limiting the time that DNSKEYs that seem to be unable to validate
    signatures are cached and/or trying to recover from cases where
    DNSKEYs do not seem to be able to validate data also reduce the
    effects of 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 seem 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 are referenced by that zone
 also change operators, possibly involving some downtime.
 In any case, to minimize such problems, the classic configuration is
 to have relatively short TTLs on all involved Resource Records.  That
 will solve many of the problems regarding changes to a zone,
 regardless of whether DNSSEC is used.

Kolkman, et al. Informational [Page 45] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

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
 that has elapsed after a slave server synchronized with a master
 server.  The TTL value and the SOA RR minimum TTL parameter [RFC2308]
 are used to determine how long a forwarder should cache data (or
 negative responses) 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 "DNSSEC Key
 Timing Considerations" [DNSSEC-KEY-TIMING].

4.4.1. Time Considerations

 Because of the expiration of signatures, one should consider the
 following:
 o  We suggest that the Maximum Zone TTL value of your zone data be
    smaller than your signature validity period.
       If the TTL duration was similar to that of the signature
       validity period, then all RRsets fetched during the validity
       period would be cached until the signature expiration time.
       Section 8.1 of RFC 4033 [RFC4033] 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, the query load on authoritative servers
       would peak at the signature expiration time, as this is also
       the time at which records simultaneously expire from caches.
       Having a TTL that is at least a few times smaller than your
       signature validity period avoids query load peaks.
 o  We suggest that the signature publication period end at least one
    Maximum Zone TTL duration (but preferably a minimum of a few days)
    before the end of the signature validity period.
       Re-signing a zone shortly before the end of the signature
       validity period may cause the 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

Kolkman, et al. Informational [Page 46] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

       whereby the zone is periodically visited for a re-signing
       operation, and those signatures that are within a so-called
       Refresh Period from signature expiration are recreated.  Also
       see Section 4.4.2 below.
       In the case of an operational error, you would have one Maximum
       Zone TTL duration to resolve the problem.  Re-signing a zone a
       few days before the end of the signature validity period
       ensures that the signatures will survive at least a (long)
       weekend in case of such operational havoc.  This is called the
       Refresh Period (see Section 4.4.2).
 o  We suggest that the Minimum Zone TTL be long enough to both fetch
    and verify all the RRs in the trust chain.  In workshop
    environments, it has been demonstrated [NIST-Workshop] 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 name servers.
        Data at delegation points, DS, DNSKEY, and RRSIG RRs benefits
        from caching.  The TTL on those should be relatively long.
        Data at the leaves in the DNS tree has less impact on
        recursive name servers.
 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.

Kolkman, et al. Informational [Page 47] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

       If the server serves a DNSSEC-secured zone, then it may 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 Refresh Period (see
       Section 4.4.2).
       The consequence of an authoritative server not being able to
       update a zone for an extended period of time is that signatures
       may expire.  In this case, non-secure resolvers will continue
       to be able to resolve data served by the particular slave
       servers, while security-aware resolvers will experience
       problems because of answers being marked as Bogus.
       We suggest that the SOA expiration timer be 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 signatures time out.
       We also suggest that operators of name servers 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 secondaries expire the zone?  How quickly can the
       administrators of the secondary servers be reached to load a
       valid zone?  These questions are not DNSSEC-specific but may
       influence the choice of your signature validity periods.

4.4.2. Signature Validity Periods

4.4.2.1. Maximum Value

 The first consideration for choosing a maximum signature validity
 period is the risk of a replay attack.  For low-value, long-term
 stable resources, the risks may be minimal, and the signature
 validity period may be several months.  Although signature validity
 periods of many years are allowed, the same "operational habit"
 arguments as those given in Section 3.2.2 play a role: When a zone is
 re-signed with some regularity, then zone administrators remain
 conscious of the operational necessity of re-signing.

Kolkman, et al. Informational [Page 48] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

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: At what time will a failure be noticed, and at what
 time is action expected to be taken?  By answering these questions,
 availability of zone administrators during (long) weekends or time
 taken to access backup media can be taken into account.  The result
 could easily suggest a minimum signature validity period of a few
 days.
 Note, however, that 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 within a given amount of time
 (the Refresh Period) of expiration.  The Re-Sign Period must be
 smaller than the Refresh Period in order for zone data to be signed
 in a timely fashion.
 If an operational problem occurs during re-signing, 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.
 To make matters slightly more complicated, some signers vary the
 signature validity period over a small range (the jitter interval) so
 that not all signatures expire at the same time.
 In other words, the minimum signature validity period 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
 Re-Sign Period, minus the maximum jitter sets the time in which
 operational havoc can be resolved.

Kolkman, et al. Informational [Page 49] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 The relationship between signature times is illustrated in Figure 11.
 Inception          Signing                                 Expiration
 time               time                                    time
 |                  |                                 |     |     |
 |------------------|---------------------------------|.....|.....|
 |                  |                                 |     |     |
                                                        +/-jitter
 | Inception offset |                                       |
 |<---------------->|            Validity Period            |
 |               |<---------------------------------------->|
 Inception          Signing Reuse   Reuse   Reuse   New     Expiration
 time               time                            RRSIG   time
 |                  |       |       |       |       |       |
 |------------------|-------------------------------|-------|
 |                  |       |       |       |       |       |
                     <-----> <-----> <-----> <----->
                   Re-Sign Period
                                              |   Refresh   |
                                              |<----------->|
                                              |   Period    |
                Figure 11: Signature Timing Parameters
 Note that in the figure the validity of the signature starts shortly
 before the signing time.  That is done to deal with validators that
 might have some clock skew.  This is called the inception offset, and
 it should be chosen so that false negatives are minimized to a
 reasonable level.

4.4.2.3. Differentiation between RRsets

 It is possible to vary signature validity periods between signatures
 over different RRsets 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) should be the leading factor in
 determining the signature validity period, since the TTLs on the data
 itself are still the primary parameter for cache expiry.
 In some cases, the risk of replaying existing data might be different
 from the risk of replaying the denial of data.  In those cases, the
 signature validity period on NSEC or NSEC3 records may be tweaked
 accordingly.

Kolkman, et al. Informational [Page 50] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 When a zone contains secure delegations, then a relatively short
 signature validity period protects the child against replay attacks
 in the case where 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 period and a higher operational risk for
 the child when choosing a long validity period, some (price)
 differentiation may occur for validity periods between individual DS
 RRs in a single zone.
 There seem to be no other arguments for differentiation in validity
 periods.

5. "Next Record" Types

 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 nonexistent names that lie between the names that do exist.
 There are two mechanisms to provide authenticated proof of
 nonexistence 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 nonexistence of RRTYPEs at a certain name;
 o  stores only the name for which the zone is authoritative (that is,
    glue in the zone is omitted); and
 o  uses a specific RRTYPE to store information about the RRTYPEs
    present at the name: The clear-text mechanism uses NSEC, and the
    obfuscated-data mechanism uses NSEC3.

5.1. Differences between NSEC and NSEC3

 The clear-text mechanism (NSEC) is implemented using a sorted linked
 list of names in the zone.  The obfuscated-data mechanism (NSEC3) is
 similar but first hashes the names using a one-way hash function,
 before creating a sorted linked list of the resulting (hashed)
 strings.
 The NSEC record requires no cryptographic operations aside from the
 validation of its associated signature record.  It is human readable
 and can be used in manual queries to determine correct operation.
 The disadvantage is that it allows for "zone walking", where one can
 request all the entries of a zone by following the linked list of
 NSEC RRs via the "Next Domain Name" field.  Though all agree that DNS

Kolkman, et al. Informational [Page 51] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 data is accessible through query mechanisms, for some zone
 administrators this behavior is undesirable for policy, regulatory,
 or other reasons.
 Furthermore, NSEC requires a signature over every RR in the zone
 file, thereby ensuring that any denial of existence is
 cryptographically signed.  However, in a large zone file 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 updates (a typical example might
 be a TLD operator with few registrants using secure delegations).
 NSEC3 allows intervals between two secure delegations to "opt out",
 in which case they may contain one or 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 name.  To
 increase the workload required to guess entries in the zone, the
 number of hashing iterations 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 (you can still derive the size of the zone,
 which RRTYPEs are in there, etc.).

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
 in-addr.arpa., ip6.arpa., and e164.arpa., can be trivially enumerated
 using ordinary DNS properties, while for small zones that only
 contain records in the apex of the zone and a few common names such
 as "www" or "mail", guessing zone content and proving completeness is
 also trivial when using NSEC3.  In these cases, the use of NSEC is
 preferred to ease the work required by signers and validating
 resolvers.
 For large zones where there is an implication of "not readily
 available" names, such as those where one has to sign a
 non-disclosure agreement before obtaining it, NSEC3 is preferred.
 The second reason to consider NSEC3 is "Opt-Out", which can reduce
 the number of NSEC3 records required.  This is discussed further
 below (Section 5.3.4).

Kolkman, et al. Informational [Page 52] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

5.3. NSEC3 Parameters

 NSEC3 is controlled by a number of parameters, some of which can be
 varied: This section discusses the choice of those parameters.

5.3.1. NSEC3 Algorithm

 The NSEC3 hashing algorithm is performed on the Fully Qualified
 Domain Name (FQDN) in its uncompressed form.  This ensures that brute
 force work done by an attacker for one FQDN cannot be reused for
 another FQDN attack, as these entries are by definition unique.
 At the time of this writing, there is only one NSEC3 hash algorithm
 defined.  [RFC5155] specifically states: "When specifying a new hash
 algorithm for use with NSEC3, 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 that a pre-calculated dictionary
 attack could be performed in order to assess whether or not certain
 domain names exist within a zone.  Two mechanisms are introduced in
 the NSEC3 specification to increase the costs of such dictionary
 attacks: iterations and salt.
 The iterations parameter defines the number of additional times the
 hash function has been performed.  A higher value results in greater
 resiliency against dictionary attacks, at a higher computational cost
 for both the server and resolver.
 RFC 5155 Section 10.3 [RFC5155] considers the tradeoffs between
 incurring cost during the signing process and imposing costs to the
 validating name server, while still providing a reasonable barrier
 against dictionary attacks.  It provides useful limits of iterations
 for a given RSA key size.  These are 150 iterations for 1024-bit
 keys, 500 iterations for 2048-bit keys, and 2,500 iterations for
 4096-bit keys.  Choosing a value of 100 iterations is deemed to be a
 sufficiently costly, yet not excessive, value: In the worst-case
 scenario, the performance of name servers would be halved, regardless
 of key size [NSEC3-HASH-PERF].

Kolkman, et al. Informational [Page 53] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

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
 administrator would cause an attacker to lose all pre-calculated work
 for that zone.
 There must be a complete NSEC3 chain using the same salt value, that
 matches the salt value in the NSEC3PARAM record.  NSEC3 salt changes
 do not need special rollover procedures.  Since changing the salt
 requires that all the NSEC3 records be regenerated and thus requires
 generating new RRSIGs over these NSEC3 records, it makes sense to
 align the change of the salt with a change of the Zone Signing Key,
 as that process in itself already usually requires that all RRSIGs be
 regenerated.  If there is no critical dependency on incremental
 signing and the zone can be signed with little effort, there is no
 need for such alignment.

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 proof that DNSSEC is not available for the
 delegations within the span.  This allows for the addition or removal
 of the delegations covered by the span without recalculating or
 re-signing RRs in the NSEC3 RR chain.
 Opt-Out is specified to be used only over delegation points and will
 therefore only bring relief to zones with a large number of insecure
 delegations.  This consideration typically holds for large TLDs and
 similar zones; in most other circumstances, Opt-Out should not be
 deployed.  Further considerations can be found in Section 12.2 of
 RFC 5155 [RFC5155].

6. Security Considerations

 DNSSEC adds data origin authentication and data integrity to the DNS,
 using digital signatures over Resource Record sets.  DNSSEC does not
 protect against denial-of-service attacks, nor does it provide
 confidentiality.  For more general security considerations related to
 DNSSEC, please see RFC 4033 [RFC4033], RFC 4034 [RFC4034], and
 RFC 4035 [RFC4035].

Kolkman, et al. Informational [Page 54] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 This document tries to assess the operational considerations to
 maintain a stable and secure DNSSEC service.  When performing key
 rollovers, it is important to keep in mind that it takes time for the
 data to be propagated to the verifying clients.  It is also important
 to note that this data may be cached.  Not taking into account the
 'data propagation' properties in the DNS may cause validation
 failures, because cached data may mismatch data fetched from the
 authoritative servers; this will make secured zones unavailable to
 security-aware resolvers.

7. Acknowledgments

 Significant parts of the text of this document are copied from
 RFC 4641 [RFC4641].  That document was edited by Olaf Kolkman and
 Miek Gieben.  Other people that contributed or were 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, Lindy Foster, and O. Courtay.
 For this version of the document, we would like to acknowledge people
 who were actively involved in the compilation of the document.  In
 random order: Mark Andrews, Patrik Faltstrom, Tony Finch, Alfred
 Hoenes, Bill Manning, Scott Rose, Wouter Wijngaards, Antoin
 Verschuren, Marc Lampo, George Barwood, Sebastian Castro, Suresh
 Krishnaswamy, Eric Rescorla, Stephen Morris, Olafur Gudmundsson,
 Ondrej Sury, and Rickard Bellgrim.

8. Contributors

 Significant contributions to this document were from:
    Paul Hoffman, who contributed on the choice of cryptographic
    parameters and addressing some of the trust anchor issues;
    Jelte Jansen, who provided the initial text in Section 4.1.4;
    Paul Wouters, who provided the initial text for Section 5, and
    Alex Bligh, who improved it.
 The figure in Section 4.4.2 was adapted from the OpenDNSSEC user
 documentation.

Kolkman, et al. Informational [Page 55] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

9. References

9.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, November 1987.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements",
            RFC 4033, March 2005.
 [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Resource Records for the DNS Security Extensions",
            RFC 4034, March 2005.
 [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Protocol Modifications for the DNS Security
            Extensions", RFC 4035, March 2005.
 [RFC4509]  Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
            (DS) Resource Records (RRs)", RFC 4509, May 2006.
 [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
            Security (DNSSEC) Hashed Authenticated Denial of
            Existence", RFC 5155, March 2008.
 [RFC5702]  Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
            and RRSIG Resource Records for DNSSEC", RFC 5702,
            October 2009.

9.2. Informative References

 [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
            August 1996.
 [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
            Changes (DNS NOTIFY)", RFC 1996, August 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
            NCACHE)", RFC 2308, March 1998.

Kolkman, et al. Informational [Page 56] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
            Update", RFC 3007, November 2000.
 [RFC3375]  Hollenbeck, S., "Generic Registry-Registrar Protocol
            Requirements", RFC 3375, September 2002.
 [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
            Public Keys Used For Exchanging Symmetric Keys", BCP 86,
            RFC 3766, April 2004.
 [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
            Requirements for Security", BCP 106, RFC 4086, June 2005.
 [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
            RFC 4641, September 2006.
 [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
            RFC 4949, August 2007.
 [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
            Trust Anchors", RFC 5011, September 2007.
 [RFC5910]  Gould, J. and S. Hollenbeck, "Domain Name System (DNS)
            Security Extensions Mapping for the Extensible
            Provisioning Protocol (EPP)", RFC 5910, May 2010.
 [RFC5933]  Dolmatov, V., Chuprina, A., and I. Ustinov, "Use of GOST
            Signature Algorithms in DNSKEY and RRSIG Resource Records
            for DNSSEC", RFC 5933, July 2010.
 [RFC6605]  Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital
            Signature Algorithm (DSA) for DNSSEC", RFC 6605,
            April 2012.
 [NIST-Workshop]
            Rose, S., "NIST DNSSEC workshop notes", July 2001,
            <http://www.ietf.org/mail-archive/web/dnsop/current/
            msg01020.html>.
 [NIST-SP-800-90A]
            Barker, E. and J. Kelsey, "Recommendation for Random
            Number Generation Using Deterministic Random Bit
            Generators", NIST Special Publication 800-90A,
            January 2012.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.

Kolkman, et al. Informational [Page 57] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 [DNSSEC-KEY-TIMING]
            Morris, S., Ihren, J., and J. Dickinson, "DNSSEC Key
            Timing Considerations", Work in Progress, July 2012.
 [DNSSEC-DPS]
            Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
            Framework for DNSSEC Policies and DNSSEC Practice
            Statements", Work in Progress, November 2012.
 [DNSSEC-TRUST-ANCHOR]
            Larson, M. and O. Gudmundsson, "DNSSEC Trust Anchor
            Configuration and Maintenance", Work in Progress,
            October 2010.
 [NSEC3-HASH-PERF]
            Schaeffer, Y., "NSEC3 Hash Performance", NLnet Labs
            document 2010-002, March 2010.

Kolkman, et al. Informational [Page 58] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

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 RFC 4033 [RFC4033].  An RRset in DNSSEC
    is marked "Bogus" when a signature of an RRset does not validate
    against a DNSKEY.
 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.
 Key Signing Key or KSK:  A Key Signing Key (KSK) is a key that is
    used exclusively for signing the apex key set.  The fact that a
    key is a KSK is only relevant to the signing tool.
 Key size:  The term 'key size' can be substituted by 'modulus size'
    throughout the document for RSA keys.  It is mathematically more
    correct to use modulus size for RSA keys, but as this is a
    document directed at operators we feel more at ease with the term
    'key size'.
 Private and public keys:  DNSSEC secures the DNS through the use of
    public-key cryptography.  Public-key cryptography is based on the
    existence of two (mathematically related) keys, a public key and a
    private key.  The public keys are published in the DNS by the use
    of the DNSKEY Resource Record (DNSKEY RR).  Private keys should
    remain private.
 Refresh Period:  The period before the expiration time of the
    signature, during which the signature is refreshed by the signer.
 Re-Sign Period:  This refers to the frequency with which a signing
    pass on the zone is performed.  The Re-Sign Period defines when
    the zone is exposed to the signer.  And on the signer, not all
    signatures in the zone have to be regenerated: That depends on the
    Refresh Period.

Kolkman, et al. Informational [Page 59] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 Secure Entry Point (SEP) key:  A KSK that has a DS record in the
    parent zone pointing to it or that is configured as a trust
    anchor.  Although not required by the protocol, we suggest that
    the SEP flag [RFC4034] be set on these keys.
 Self-signature:  This only applies to signatures over DNSKEYs; a
    signature made with DNSKEY x over DNSKEY x is called a self-
    signature.  Note: Without further information, self-signatures
    convey no trust.  They are useful to check the authenticity of the
    DNSKEY, i.e., they can be used as a hash.
 Signing jitter:  A random variation in the signature validity period
    of RRSIGs in a zone to prevent all of them from expiring at the
    same time.
 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.
 Singing the zone file:  The term used for the event where an
    administrator joyfully signs its zone file while producing melodic
    sound patterns.
 Single-Type Signing Scheme:  A signing scheme whereby the distinction
    between Zone Signing Keys and Key Signing Keys is not made.
 Zone administrator:  The 'role' that is responsible for signing a
    zone and publishing it on the primary authoritative server.
 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.

Kolkman, et al. Informational [Page 60] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

Appendix B. Typographic Conventions

 The following typographic conventions are used in this document:
 Key notation:  A key is denoted by DNSKEY_x_y, where x is an
    identifier for the type of key: K for Key Signing Key, Z for Zone
    Signing Key, and S when there is no distinction made between KSKs
    and ZSKs but the key is used as a secure entry point.  The 'y'
    denotes a number or an identifier; y could be thought of as the
    key id.
 RRsets ignored:  If the signatures of non-DNSKEY RRsets have the same
    parameters as the SOA, then 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.
 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.  An example of this would be "A1, A2", specifying the
    RRset containing two "A" records.  This could again be abbreviated
    to just "A".
 Signature notation:  Signatures are denoted as RRSIG_x_y(type), which
    means that the RRset with the specific RRTYPE 'type' is signed
    with DNSKEY_x_y.  Signatures in the parent zone are denoted as
    RRSIG_par(type).
 SOA representation:  SOAs are represented as SOA_x, where x is the
    serial number.
 DS representation:  DSs are represented as DS_x_y, where x and y are
    identifiers similar to the key notation: x is an identifier for
    the type of key the DS record refers to; y is the 'key id' of the
    key it refers to.
 Zone representation:  Using the above notation we have simplified the
    representation of a signed zone by leaving out all unnecessary
    details, such as the names, and by representing all data by
    "SOA_x".

Kolkman, et al. Informational [Page 61] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

 Using this notation, the following signed zone:
 example.com.  3600  IN SOA   ns1.example.com. olaf.example.net. (
                         2005092303 ; serial
                         450        ; refresh (7 minutes 30 seconds)
                         600        ; retry (10 minutes)
                         345600     ; expire (4 days)
                         300        ; minimum (5 minutes)
                         )
        3600    RRSIG    SOA 5 2 3600 20120824013000 (
                         20100424013000 14 example.com.
                         NMafnzmmZ8wevpCOI+/JxqWBzPxrnzPnSXfo
                         ...
                         OMY3rTMA2qorupQXjQ== )
        3600    NS       ns1.example.com.
        3600    NS       ns2.example.com.
        3600    NS       ns3.example.com.
        3600    RRSIG    NS 5 2 3600 20120824013000 (
                         20100424013000 14 example.com.
                         p0Cj3wzGoPFftFZjj3jeKGK6wGWLwY6mCBEz
                         ...
                         +SqZIoVHpvE7YBeH46wuyF8w4XknA4Oeimc4
                         zAgaJM/MeG08KpeHhg== )
        3600    TXT      "Net::DNS  domain"
        3600    RRSIG    TXT 5 2 3600 20120824013000 (
                         20100424013000 14 example.com.
                         o7eP8LISK2TEutFQRvK/+U3wq7t4X+PQaQkp
                         ...
                         BcQ1o99vwn+IS4+J1g== )
        300     NSEC     foo.example.com. NS SOA TXT RRSIG NSEC DNSKEY
        300     RRSIG    NSEC 5 2 300 20120824013000 (
                         20100424013000 14 example.com.
                         JtHm8ta0diCWYGu/TdrE1O1sYSHblN2i/IX+
                         ...
                         PkXNI/Vgf4t3xZaIyw== )
        3600    DNSKEY   256 3 5 (
                         AQPaoHW/nC0fj9HuCW3hACSGiP0AkPS3dQFX
                         ...
                         sAuryjQ/HFa5r4mrbhkJ
                         ) ; key id = 14
        3600    DNSKEY   257 3 5 (
                         AQPUiszMMAi36agx/V+7Tw95l8PYmoVjHWvO
                         ...
                         oy88Nh+u2c9HF1tw0naH
                         ) ; key id = 15

Kolkman, et al. Informational [Page 62] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

        3600    RRSIG    DNSKEY 5 2 3600 20120824013000 (
                         20100424013000 14 example.com.
                         HWj/VEr6p/FiUUiL70QQWtk+NBIlsJ9mdj5U
                         ...
                         QhhmMwV3tIxJk2eDRQ== )
        3600    RRSIG    DNSKEY 5 2 3600 20120824013000 (
                         20100424013000 15 example.com.
                         P47CUy/xPV8qIEuua4tMKG6ei3LQ8RYv3TwE
                         ...
                         JWL70YiUnUG3m9OL9w== )
foo.example.com.  3600  IN A 192.0.2.2
        3600    RRSIG    A 5 3 3600 20120824013000 (
                         20100424013000 14 example.com.
                         xHr023P79YrSHHMtSL0a1nlfUt4ywn/vWqsO
                         ...
                         JPV/SA4BkoFxIcPrDQ== )
        300     NSEC     example.com. A RRSIG NSEC
        300     RRSIG    NSEC 5 3 300 20120824013000 (
                         20100424013000 14 example.com.
                         Aaa4kgKhqY7Lzjq3rlPlFidymOeBEK1T6vUF
                         ...
                         Qe000JyzObxx27pY8A== )
 is reduced to the following representation:
          SOA_2005092303
          RRSIG_Z_14(SOA_2005092303)
          DNSKEY_K_14
          DNSKEY_Z_15
          RRSIG_K_14(DNSKEY)
          RRSIG_Z_15(DNSKEY)
 The rest of the zone data has the same signature as the SOA record,
 i.e., an RRSIG created with DNSKEY_K_14.

Kolkman, et al. Informational [Page 63] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

Appendix C. Transition Figures for Special Cases of Algorithm Rollovers

 The figures in this appendix complement and illustrate the special
 cases of algorithm rollovers as described in Section 4.1.4.
  1. —————————————————————

initial new RRSIGs new DNSKEY

  1. —————————————————————

Parent:

  SOA_0 -------------------------------------------------------->
  RRSIG_par(SOA) ----------------------------------------------->
  DS_S_1 ------------------------------------------------------->
  RRSIG_par(DS_S_1) -------------------------------------------->
 Child:
  SOA_0                SOA_1                SOA_2
  RRSIG_S_1(SOA)       RRSIG_S_1(SOA)       RRSIG_S_1(SOA)
                       RRSIG_S_2(SOA)       RRSIG_S_2(SOA)
  DNSKEY_S_1           DNSKEY_S_1           DNSKEY_S_1
                                            DNSKEY_S_2
  RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)
                       RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)
  1. —————————————————————

new DS DNSKEY removal RRSIGs removal

  1. —————————————————————

Parent:

  SOA_1 ------------------------------------------------------->
  RRSIG_par(SOA) ---------------------------------------------->
  DS_S_2 ------------------------------------------------------>
  RRSIG_par(DS_S_2) ------------------------------------------->
 Child:
  -------------------> SOA_3                SOA_4
  -------------------> RRSIG_S_1(SOA)
  -------------------> RRSIG_S_2(SOA)       RRSIG_S_2(SOA)
  1. ——————>
  2. ——————> DNSKEY_S_2 DNSKEY_S_2
  3. ——————> RRSIG_S_1(DNSKEY)
  4. ——————> RRSIG_S_2(DNSKEY) RRSIG_S_2(DNSKEY)
  5. —————————————————————
         Figure 12: Single-Type Signing Scheme Algorithm Roll
 Also see Section 4.1.4.1.

Kolkman, et al. Informational [Page 64] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

  1. —————————————————————

initial new RRSIGs new DNSKEY

  1. —————————————————————

Parent:

  SOA_0 -------------------------------------------------------->
  RRSIG_par(SOA) ----------------------------------------------->
  DS_K_1 ------------------------------------------------------->
  RRSIG_par(DS_K_1) -------------------------------------------->
 Child:
  SOA_0                SOA_1                SOA_2
  RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)
                       RRSIG_Z_2(SOA)       RRSIG_Z_2(SOA)
  DNSKEY_K_1           DNSKEY_K_1           DNSKEY_K_1
                                            DNSKEY_K_2
  DNSKEY_Z_1           DNSKEY_Z_1           DNSKEY_Z_1
                                            DNSKEY_Z_2
  RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)
                                            RRSIG_K_2(DNSKEY)
  1. —————————————————————

new DS revoke DNSKEY DNSKEY removal

  1. —————————————————————

Parent:

  SOA_1 ------------------------------------------------------->
  RRSIG_par(SOA) ---------------------------------------------->
  DS_K_2 ------------------------------------------------------>
  RRSIG_par(DS_K_2) ------------------------------------------->
 Child:
  -------------------> SOA_3                SOA_4
  -------------------> RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)
  -------------------> RRSIG_Z_2(SOA)       RRSIG_Z_2(SOA)
  1. ——————> DNSKEY_K_1_REVOKED
  2. ——————> DNSKEY_K_2 DNSKEY_K_2
  3. ——————>
  4. ——————> DNSKEY_Z_2 DNSKEY_Z_2
  5. ——————> RRSIG_K_1(DNSKEY)
  6. ——————> RRSIG_K_2(DNSKEY) RRSIG_K_2(DNSKEY)

Kolkman, et al. Informational [Page 65] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

  1. —————————————————————

RRSIGs removal

  1. —————————————————————

Parent:

  1. ————————————>
  2. ————————————>
  3. ————————————>
  4. ————————————>
 Child:
  SOA_5
  RRSIG_Z_2(SOA)
  DNSKEY_K_2
  DNSKEY_Z_2
  RRSIG_K_2(DNSKEY)
 ----------------------------------------------------------------
               Figure 13: RFC 5011 Style Algorithm Roll
 Also see Section 4.1.4.2.
  1. —————————————————————

initial new RRSIGs new DNSKEY

  1. —————————————————————

Parent:

  SOA_0 -------------------------------------------------------->
  RRSIG_par(SOA) ----------------------------------------------->
  DS_S_1 ------------------------------------------------------->
  RRSIG_par(DS_S_1) -------------------------------------------->
 Child:
  SOA_0                SOA_1                SOA_2
  RRSIG_S_1(SOA)
  RRSIG_Z_10(SOA)      RRSIG_Z_10(SOA)      RRSIG_Z_10(SOA)
                       RRSIG_S_2(SOA)       RRSIG_S_2(SOA)
  DNSKEY_S_1           DNSKEY_S_1           DNSKEY_S_1
  DNSKEY_Z_10          DNSKEY_Z_10          DNSKEY_Z_10
                                            DNSKEY_S_2
  RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)
                       RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)

Kolkman, et al. Informational [Page 66] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

  1. —————————————————————

new DS revoke DNSKEY DNSKEY removal

  1. —————————————————————

Parent:

  SOA_1 ------------------------------------------------------->
  RRSIG_par(SOA) ---------------------------------------------->
  DS_S_2 ------------------------------------------------------>
  RRSIG_par(DS_S_2) ------------------------------------------->
 Child:
  -------------------> SOA_3                SOA_4
  1. ——————> RRSIG_Z_10(SOA)
  2. ——————> RRSIG_S_2(SOA) RRSIG_S_2(SOA)
  1. ——————> DNSKEY_S_1_REVOKED
  2. ——————> DNSKEY_Z_10
  3. ——————> DNSKEY_S_2 DNSKEY_S_2
  4. ——————> RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY)
  5. ——————> RRSIG_S_2(DNSKEY) RRSIG_S_2(DNSKEY)
  1. —————————————————————

RRSIGs removal

  1. —————————————————————

Parent:

  1. ————————————>
  2. ————————————>
  3. ————————————>
  4. ————————————>
 Child:
  SOA_5
  RRSIG_S_2(SOA)
  DNSKEY_S_2
  RRSIG_S_2(DNSKEY)
 ----------------------------------------------------------------
          Figure 14: RFC 5011 Algorithm Roll in a Single-Type
                      Signing Scheme Environment
 Also see Section 4.1.4.3.

Kolkman, et al. Informational [Page 67] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

Appendix D. Transition Figure for Changing DNS Operators

 The figure in this Appendix complements and illustrates the special
 case of changing DNS operators as described in Section 4.3.5.1.

Kolkman, et al. Informational [Page 68] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

  1. ———————————————————–

new DS | pre-publish |

  1. ———————————————————–

Parent:

   NS_A                            NS_A
   DS_A DS_B                       DS_A DS_B
  ------------------------------------------------------------
  Child at A:            Child at A:        Child at B:
   SOA_A0                 SOA_A1             SOA_B0
   RRSIG_Z_A(SOA)         RRSIG_Z_A(SOA)     RRSIG_Z_B(SOA)
   NS_A                   NS_A               NS_B
   RRSIG_Z_A(NS)          NS_B               RRSIG_Z_B(NS)
                          RRSIG_Z_A(NS)
   DNSKEY_Z_A             DNSKEY_Z_A         DNSKEY_Z_A
                          DNSKEY_Z_B         DNSKEY_Z_B
   DNSKEY_K_A             DNSKEY_K_A         DNSKEY_K_B
   RRSIG_K_A(DNSKEY)      RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)
                          RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)
  ------------------------------------------------------------
  1. ———————————————————–

re-delegation | post-migration |

  1. ———————————————————–

Parent:

            NS_B                           NS_B
            DS_A DS_B                      DS_B
  ------------------------------------------------------------
  Child at A:        Child at B:           Child at B:
   SOA_A1             SOA_B0                SOA_B1
   RRSIG_Z_A(SOA)     RRSIG_Z_B(SOA)        RRSIG_Z_B(SOA)
   NS_A               NS_B                  NS_B
   NS_B               RRSIG_Z_B(NS)         RRSIG_Z_B(NS)
   RRSIG_Z_A(NS)
   DNSKEY_Z_A         DNSKEY_Z_A
   DNSKEY_Z_B         DNSKEY_Z_B            DNSKEY_Z_B
   DNSKEY_K_A         DNSKEY_K_B            DNSKEY_K_B
   RRSIG_K_A(DNSKEY)  RRSIG_K_B(DNSKEY)     RRSIG_K_B(DNSKEY)
  ------------------------------------------------------------
 Figure 15: An Alternative Rollover Approach for Cooperating Operators

Kolkman, et al. Informational [Page 69] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

Appendix E. Summary of Changes from RFC 4641

 This document differs from RFC 4641 [RFC4641] in the following ways:
 o  Addressed the errata listed on
    <http://www.rfc-editor.org/errata_search.php?rfc=4641>.
 o  Recommended RSA/SHA-256 in addition to RSA/SHA-1.
 o  Did a complete rewrite of Section 3.5 of RFC 4641 (Section 3.4.2
    of this document), removing the table and suggesting a key size of
    1024 for keys in use for less than 8 years, issued up to at least
    2015.
 o  Removed the KSK for high-level zones consideration.
 o  Added text on algorithm rollover.
 o  Added text on changing (non-cooperating) DNS registrars.
 o  Did a significant rewrite of Section 3, whereby the argument is
    made that the timescales for rollovers are made purely on
    operational arguments.
 o  Added Section 5.
 o  Introduced Single-Type Signing Scheme terminology and made the
    arguments for the choice of a Single-Type Signing Scheme more
    explicit.
 o  Added a section about stand-by keys.

Kolkman, et al. Informational [Page 70] RFC 6781 DNSSEC Operational Practices, Version 2 December 2012

Authors' Addresses

 Olaf M. Kolkman
 NLnet Labs
 Science Park 400
 Amsterdam  1098 XH
 The Netherlands
 EMail: olaf@nlnetlabs.nl
 URI:   http://www.nlnetlabs.nl
 W. (Matthijs) Mekking
 NLnet Labs
 Science Park 400
 Amsterdam  1098 XH
 The Netherlands
 EMail: matthijs@nlnetlabs.nl
 URI:   http://www.nlnetlabs.nl
 R. (Miek) Gieben
 SIDN Labs
 Meander 501
 Arnhem  6825 MD
 The Netherlands
 EMail: miek.gieben@sidn.nl
 URI:   http://www.sidn.nl

Kolkman, et al. Informational [Page 71]

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