Movatterモバイル変換

Internet Engineering Task Force (IETF)                        O. KolkmanRequest for Comments: 6781                                    W. MekkingObsoletes:4641                                               NLnet LabsCategory: Informational                                        R. GiebenISSN: 2070-1721                                                SIDN Labs                                                           December 2012DNSSEC Operational Practices, Version 2Abstract   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 obsoletesRFC 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6781.Kolkman, et al.               Informational                     [Page 1]

RFC 6781         DNSSEC Operational Practices, Version 2   December 2012Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................41.1. The Use of the Term 'key' ..................................51.2. Time Definitions ...........................................62. Keeping the Chain of Trust Intact ...............................63. Key Generation and Storage ......................................7      3.1. Operational Motivation for Zone Signing Keys and           Key Signing Keys ...........................................83.2. Practical Consequences of KSK and ZSK Separation ..........103.2.1. Rolling a KSK That Is Not a Trust Anchor ...........103.2.2. Rolling a KSK That Is a Trust Anchor ...............113.2.3. The Use of the SEP Flag ............................123.3. Key Effectivity Period ....................................123.4. Cryptographic Considerations ..............................143.4.1. Signature Algorithm ................................143.4.2. Key Sizes ..........................................143.4.3. Private Key Storage ................................163.4.4. Key Generation .....................................173.4.5. Differentiation for 'High-Level' Zones? ............17Kolkman, et al.               Informational                     [Page 2]

RFC 6781         DNSSEC Operational Practices, Version 2   December 20124. Signature Generation, Key Rollover, and Related Policies .......184.1. Key Rollovers .............................................184.1.1. Zone Signing Key Rollovers .........................184.1.1.1. Pre-Publish Zone Signing Key Rollover .....19                  4.1.1.2. Double-Signature Zone Signing Key Rollover 214.1.1.3. Pros and Cons of the Schemes ..............234.1.2. Key Signing Key Rollovers ..........................23                  4.1.2.1. Special Considerations forRFC 5011                           KSK Rollover ..............................264.1.3. Single-Type Signing Scheme Key Rollover ............264.1.4. Algorithm Rollovers ................................28                  4.1.4.1. Single-Type Signing Scheme                           Algorithm Rollover ........................324.1.4.2. Algorithm Rollover,RFC 5011 Style ........32                  4.1.4.3. Single Signing Type Algorithm                           Rollover,RFC 5011 Style ..................334.1.4.4. NSEC-to-NSEC3 Algorithm Rollover ..........344.1.5. Considerations for Automated Key Rollovers .........344.2. Planning for Emergency Key Rollover .......................354.2.1. 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 ........................374.2.2. ZSK Compromise .....................................374.2.3. Compromises of Keys Anchored in Resolvers ..........384.2.4. Stand-By Keys ......................................384.3. Parent Policies ...........................................39           4.3.1. Initial Key Exchanges and Parental Policies                  Considerations .....................................394.3.2. Storing Keys or Hashes? ............................404.3.3. Security Lameness ..................................404.3.4. DS Signature Validity Period .......................414.3.5. Changing DNS Operators .............................424.3.5.1. Cooperating DNS Operators .................424.3.5.2. Non-Cooperating DNS Operators .............444.4. Time in DNSSEC ............................................464.4.1. Time Considerations ................................464.4.2. Signature Validity Periods .........................484.4.2.1. Maximum Value .............................484.4.2.2. Minimum Value .............................494.4.2.3. Differentiation between RRsets ............50Kolkman, et al.               Informational                     [Page 3]

RFC 6781         DNSSEC Operational Practices, Version 2   December 20125. "Next Record" Types ............................................515.1. Differences between NSEC and NSEC3 ........................515.2. NSEC or NSEC3 .............................................525.3. NSEC3 Parameters ..........................................535.3.1. NSEC3 Algorithm ....................................535.3.2. NSEC3 Iterations ...................................535.3.3. NSEC3 Salt .........................................545.3.4. Opt-Out ............................................546. Security Considerations ........................................547. Acknowledgments ................................................558. Contributors ...................................................559. References .....................................................569.1. Normative References ......................................569.2. Informative References ....................................56Appendix A. Terminology ...........................................59Appendix B. Typographic Conventions ...............................61Appendix C. Transition Figures for Special Cases of Algorithm               Rollovers .............................................64Appendix D. Transition Figure for Changing DNS Operators ..........68Appendix E. Summary of Changes fromRFC 4641 ......................701.  Introduction   This document describes how to run a DNS Security (DNSSEC)-enabled   environment.  It is intended for operators who have knowledge of the   DNS (seeRFC 1034 [RFC1034] andRFC 1035 [RFC1035]) and want to   deploy DNSSEC (RFC 4033 [RFC4033],RFC 4034 [RFC4034],RFC 4035   [RFC4035], andRFC 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.  InSection 2, we   discuss the importance of keeping the "chain of trust" intact.   Aspects of key generation and storage of keys are discussed inSection 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.  Sections4.1 and4.2 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 inAppendix B.   Since we describe operational suggestions and there are no protocol   specifications, theRFC 2119 [RFC2119] language does not apply to   this document, though we do use quotes from other documents that do   include theRFC 2119 language.   This document obsoletesRFC 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' inRFC 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 20121.2.  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.  SeeRFC 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 inRFC 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 seeSection 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.  ByKolkman, 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 isKolkman, 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 withRFC 5011   [RFC5011] and by rolling the key regularly (and advertising such) soKolkman, 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 inRFC 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 beKolkman, 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 Sections4.1 and4.4.  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 20123.4.  Cryptographic Considerations3.4.1.  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 (seeRFC 5702 [RFC5702] andRFC 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 aKolkman, 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 20123.4.3.  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 inRFC 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 Policies4.1.  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   inAppendix 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 inSection 4.1.1.1, usesKolkman, et al.               Informational                    [Page 18]

RFC 6781         DNSSEC Operational Practices, Version 2   December 2012   key pre-publication; the other uses double signatures, as described   inSection 4.1.1.2.  The pros and cons are described inSection 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:    ------------------------------------------------------------     initial            new DNSKEY          new RRSIGs    ------------------------------------------------------------     SOA_0              SOA_1               SOA_2     RRSIG_Z_10(SOA)    RRSIG_Z_10(SOA)     RRSIG_Z_11(SOA)     DNSKEY_K_1         DNSKEY_K_1          DNSKEY_K_1     DNSKEY_Z_10        DNSKEY_Z_10         DNSKEY_Z_10                        DNSKEY_Z_11         DNSKEY_Z_11     RRSIG_K_1(DNSKEY)  RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)    ------------------------------------------------------------    ------------------------------------------------------------     DNSKEY removal    ------------------------------------------------------------     SOA_3     RRSIG_Z_11(SOA)     DNSKEY_K_1     DNSKEY_Z_11     RRSIG_K_1(DNSKEY)    ------------------------------------------------------------                    Figure 1: Pre-Publish Key RolloverKolkman, 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)       ----------------------------------------------------------------       ----------------------------------------------------------------       new RRSIGs (II)     new DNSKEY (II)       ----------------------------------------------------------------       SOA_3               SOA_4       RRSIG_Z_12(SOA)     RRSIG_Z_12(SOA)       DNSKEY_K_1          DNSKEY_K_1       DNSKEY_Z_11         DNSKEY_Z_12       DNSKEY_Z_12         DNSKEY_Z_13       RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)       ----------------------------------------------------------------             Figure 2: Pre-Publish Zone Signing Key Rollover,                           Showing Two Rollovers   Note that the key introduced in the "new DNSKEY" phase is not used   for production yet; the private key can thus be stored in a   physically secure manner and does not need to be 'fetched' every time   a zone needs to be signed.4.1.1.2.  Double-Signature Zone Signing Key Rollover   This section shows how to perform a ZSK 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:      ----------------------------------------------------------------      initial             new DNSKEY         DNSKEY removal      ----------------------------------------------------------------      SOA_0               SOA_1              SOA_2      RRSIG_Z_10(SOA)     RRSIG_Z_10(SOA)                          RRSIG_Z_11(SOA)    RRSIG_Z_11(SOA)      DNSKEY_K_1          DNSKEY_K_1         DNSKEY_K_1      DNSKEY_Z_10         DNSKEY_Z_10                          DNSKEY_Z_11        DNSKEY_Z_11      RRSIG_K_1(DNSKEY)   RRSIG_K_1(DNSKEY)  RRSIG_K_1(DNSKEY)      ----------------------------------------------------------------           Figure 3: Double-Signature Zone Signing Key Rollover   initial:  Initial version of the zone: DNSKEY_K_1 is the Key Signing      Key.  DNSKEY_Z_10 is used to sign all the data of the zone, 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 20124.1.1.3.  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 inSection 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   ---------------------------------------------------------------------    initial            new DNSKEY        DS change    DNSKEY removal   ---------------------------------------------------------------------   Parent:    SOA_0 -----------------------------> SOA_1 ------------------------>    RRSIG_par(SOA) --------------------> RRSIG_par(SOA) --------------->    DS_K_1 ----------------------------> DS_K_2 ----------------------->    RRSIG_par(DS) ---------------------> RRSIG_par(DS) ---------------->   Child:    SOA_0              SOA_1 -----------------------> SOA_2    RRSIG_Z_10(SOA)    RRSIG_Z_10(SOA) -------------> RRSIG_Z_10(SOA)    DNSKEY_K_1         DNSKEY_K_1 ------------------>                       DNSKEY_K_2 ------------------> DNSKEY_K_2    DNSKEY_Z_10        DNSKEY_Z_10 -----------------> DNSKEY_Z_10    RRSIG_K_1(DNSKEY)  RRSIG_K_1 (DNSKEY) ---------->                       RRSIG_K_2 (DNSKEY) ----------> RRSIG_K_2(DNSKEY)   ---------------------------------------------------------------------           Figure 4: Stages of Deployment for a Double-Signature                         Key Signing Key Rollover   initial:  Initial version of the zone.  The parental DS points to      DNSKEY_K_1.  Before the rollover starts, the child will have to      verify what the TTL is of the DS RR that points to DNSKEY_K_1 --      it is needed during the rollover, and we refer to the value as      TTL_DS.   new DNSKEY:  During the "new DNSKEY" phase, the zone administrator      generates a second KSK, DNSKEY_K_2.  The key is provided to the      parent, and the child will have to wait until a new DS RR has been      generated that points to DNSKEY_K_2.  After that DS RR has been      published on all servers authoritative for the parent's zone, the      zone administrator has to wait at least TTL_DS to make sure that      the old DS RR has expired from caches.   DS change:  The parent replaces DS_K_1 with DS_K_2.   DNSKEY removal:  DNSKEY_K_1 has been removed.   The scenario above puts the responsibility for maintaining a valid   chain of trust with the child.  It also is based on the premise that   the parent only has one DS RR (per algorithm) per zone.  An   alternative mechanism has been considered.  Using an established   trust 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 DSKolkman, 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.   --------------------------------------------------------------------     initial         new DS         new DNSKEY       DS removal   --------------------------------------------------------------------   Parent:     SOA_0           SOA_1 ------------------------> SOA_2     RRSIG_par(SOA)  RRSIG_par(SOA) ---------------> RRSIG_par(SOA)     DS_K_1          DS_K_1 ----------------------->                     DS_K_2 -----------------------> DS_K_2     RRSIG_par(DS)   RRSIG_par(DS) ----------------> RRSIG_par(DS)   Child:     SOA_0 -----------------------> SOA_1 ---------------------------->     RRSIG_Z_10(SOA) -------------> RRSIG_Z_10(SOA) ------------------>     DNSKEY_K_1 ------------------> DNSKEY_K_2 ----------------------->     DNSKEY_Z_10 -----------------> DNSKEY_Z_10 ---------------------->     RRSIG_K_1 (DNSKEY) ----------> RRSIG_K_2 (DNSKEY) --------------->   --------------------------------------------------------------------              Figure 5: Stages of Deployment for a Double-DS                         Key Signing Key Rollover   When the child zone wants to roll, it notifies the parent during the   "new DS" phase and submits the new key (or the corresponding DS) to   the parent.  The parent publishes DS_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 (seeSection 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 20124.1.2.1.  Special Considerations forRFC 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 deployRFC 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.   -------------------------------------------------------------------     initial           new DNSKEY      DS change     DNSKEY removal   -------------------------------------------------------------------   Parent:     SOA_0 --------------------------> SOA_1 ---------------------->     RRSIG_par(SOA) -----------------> RRSIG_par(SOA) ------------->     DS_S_1 -------------------------> DS_S_2 --------------------->     RRSIG_par(DS_S_1) --------------> RRSIG_par(DS_S_2) ---------->   Child:     SOA_0             SOA_1 ----------------------> SOA_2     RRSIG_S_1(SOA)    RRSIG_S_1(SOA) ------------->                       RRSIG_S_2(SOA) -------------> RRSIG_S_2(SOA)     DNSKEY_S_1        DNSKEY_S_1 ----------------->                       DNSKEY_S_2 -----------------> DNSKEY_S_2     RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY) ---------->                       RRSIG_S_2(DNSKEY) ----------> RRSIG_S_2(DNSKEY)   -------------------------------------------------------------------             Figure 6: Stages of the Straightforward Rollover                      in a Single-Type Signing SchemeKolkman, 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 Scheme4.1.4.  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 withSection 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 ofSection 2 ("Zone Signing") ofRFC 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   ----------------------------------------------------------------    initial              new RRSIGs           new DNSKEY   ----------------------------------------------------------------   Parent:    SOA_0 -------------------------------------------------------->    RRSIG_par(SOA) ----------------------------------------------->    DS_K_1 ------------------------------------------------------->    RRSIG_par(DS_K_1) -------------------------------------------->   Child:    SOA_0                SOA_1                SOA_2    RRSIG_Z_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)   ----------------------------------------------------------------    new DS               DNSKEY removal       RRSIGs removal   ----------------------------------------------------------------   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)    ------------------->    -------------------> DNSKEY_K_2           DNSKEY_K_2    ------------------->    -------------------> DNSKEY_Z_11          DNSKEY_Z_11    ------------------->    -------------------> RRSIG_K_2(DNSKEY)    RRSIG_K_2(DNSKEY)   ----------------------------------------------------------------        Figure 8: Stages of Deployment during an Algorithm RolloverKolkman, 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 viaRFC 5011 style rollover (Section 4.1.4.2).   3: 1 and 2 combined:  when a Single-Type Signing Scheme Algorithm      rollover is performedRFC 5011 style (Section 4.1.4.3).   In addition to the narrative below, these special cases are   represented in Figures 12, 13, and 14 inAppendix C.Kolkman, et al.               Informational                    [Page 31]

RFC 6781         DNSSEC Operational Practices, Version 2   December 20124.1.4.1.  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 inSection 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 inAppendix C.4.1.4.2.  Algorithm Rollover,RFC 5011 Style   Trust anchor algorithm rollover is almost as simple as a regularRFC 5011-based rollover.  However, the old trust anchor must be   revoked before it is removed from the zone.   The timeline (see Figure 13 inAppendix 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.   ---------------------------------     revoke DNSKEY   ---------------------------------   Parent:     ----------------------------->     ----------------------------->     ----------------------------->     ----------------------------->   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 whenRFC 5011 Is in UseKolkman, et al.               Informational                    [Page 32]

RFC 6781         DNSSEC Operational Practices, Version 2   December 2012   There is one exception to the requirement fromRFC 4035 quoted inSection 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 theRFC 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 implementRFC 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 withSection 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 anRFC 5011 style rollover with a   Single Signing Scheme key, it should be noted thatSection 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 rolloverRFC 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 inSection 4.1.4.2.   The lesson of all of this is that a Single-Type Signing Scheme   algorithm rollover usingRFC 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 Sections10.4 and10.5 ofRFC 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 20124.2.  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 thereKolkman, 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 20124.2.3.  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 Policies4.3.1.  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 20124.3.2.  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 seeSection 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 20124.3.5.  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    ------------------------------------------------------------    initial            |        pre-publish                    |    ------------------------------------------------------------    Parent:     NS_A                            NS_A     DS_A                            DS_A    ------------------------------------------------------------    Child at A:            Child at A:        Child at B:     SOA_A0                 SOA_A1             SOA_B0     RRSIG_Z_A(SOA)         RRSIG_Z_A(SOA)     RRSIG_Z_B(SOA)     NS_A                   NS_A               NS_B     RRSIG_Z_A(NS)          NS_B               RRSIG_Z_B(NS)                            RRSIG_Z_A(NS)     DNSKEY_Z_A             DNSKEY_Z_A         DNSKEY_Z_A                            DNSKEY_Z_B         DNSKEY_Z_B     DNSKEY_K_A             DNSKEY_K_A         DNSKEY_K_A                            DNSKEY_K_B         DNSKEY_K_B     RRSIG_K_A(DNSKEY)      RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)                            RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)    ------------------------------------------------------------    ------------------------------------------------------------          re-delegation                |   post-migration      |    ------------------------------------------------------------    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 OperatorsKolkman, 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 inAppendix 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 andKolkman, 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 inRFC 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 20124.4.  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 deployedKolkman, 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         seeSection 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 (seeSection 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 (seeSection 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 Periods4.4.2.1.  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 inSection 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 20124.4.2.2.  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 (seeSection 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 DNSKolkman, 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 20125.3.  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 20125.3.3.  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 inSection 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 seeRFC 4033 [RFC4033],RFC 4034 [RFC4034], andRFC 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 fromRFC 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 inSection 4.1.4;      Paul Wouters, who provided the initial text forSection 5, and      Alex Bligh, who improved it.   The figure inSection 4.4.2 was adapted from the OpenDNSSEC user   documentation.Kolkman, et al.               Informational                    [Page 55]

RFC 6781         DNSSEC Operational Practices, Version 2   December 20129.  References9.1.  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 2012Appendix 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 seeSection 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 2012Appendix 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 = 15Kolkman, 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 2012Appendix 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 inSection 4.1.4.   ----------------------------------------------------------------    initial              new RRSIGs           new DNSKEY   ----------------------------------------------------------------   Parent:    SOA_0 -------------------------------------------------------->    RRSIG_par(SOA) ----------------------------------------------->    DS_S_1 ------------------------------------------------------->    RRSIG_par(DS_S_1) -------------------------------------------->   Child:    SOA_0                SOA_1                SOA_2    RRSIG_S_1(SOA)       RRSIG_S_1(SOA)       RRSIG_S_1(SOA)                         RRSIG_S_2(SOA)       RRSIG_S_2(SOA)    DNSKEY_S_1           DNSKEY_S_1           DNSKEY_S_1                                              DNSKEY_S_2    RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)                         RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)   ----------------------------------------------------------------    new DS               DNSKEY removal       RRSIGs removal   ----------------------------------------------------------------   Parent:    SOA_1 ------------------------------------------------------->    RRSIG_par(SOA) ---------------------------------------------->    DS_S_2 ------------------------------------------------------>    RRSIG_par(DS_S_2) ------------------------------------------->   Child:    -------------------> SOA_3                SOA_4    -------------------> RRSIG_S_1(SOA)    -------------------> RRSIG_S_2(SOA)       RRSIG_S_2(SOA)    ------------------->    -------------------> DNSKEY_S_2           DNSKEY_S_2    -------------------> RRSIG_S_1(DNSKEY)    -------------------> RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)   ----------------------------------------------------------------           Figure 12: Single-Type Signing Scheme Algorithm Roll   Also seeSection 4.1.4.1.Kolkman, et al.               Informational                    [Page 64]

RFC 6781         DNSSEC Operational Practices, Version 2   December 2012   ----------------------------------------------------------------    initial              new RRSIGs           new DNSKEY   ----------------------------------------------------------------   Parent:    SOA_0 -------------------------------------------------------->    RRSIG_par(SOA) ----------------------------------------------->    DS_K_1 ------------------------------------------------------->    RRSIG_par(DS_K_1) -------------------------------------------->   Child:    SOA_0                SOA_1                SOA_2    RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)       RRSIG_Z_1(SOA)                         RRSIG_Z_2(SOA)       RRSIG_Z_2(SOA)    DNSKEY_K_1           DNSKEY_K_1           DNSKEY_K_1                                              DNSKEY_K_2    DNSKEY_Z_1           DNSKEY_Z_1           DNSKEY_Z_1                                              DNSKEY_Z_2    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)    RRSIG_K_1(DNSKEY)                                              RRSIG_K_2(DNSKEY)   ----------------------------------------------------------------    new DS               revoke DNSKEY        DNSKEY removal   ----------------------------------------------------------------   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)    -------------------> DNSKEY_K_1_REVOKED    -------------------> DNSKEY_K_2           DNSKEY_K_2    ------------------->    -------------------> DNSKEY_Z_2           DNSKEY_Z_2    -------------------> RRSIG_K_1(DNSKEY)    -------------------> RRSIG_K_2(DNSKEY)    RRSIG_K_2(DNSKEY)Kolkman, et al.               Informational                    [Page 65]

RFC 6781         DNSSEC Operational Practices, Version 2   December 2012   ----------------------------------------------------------------    RRSIGs removal   ----------------------------------------------------------------   Parent:    ------------------------------------->    ------------------------------------->    ------------------------------------->    ------------------------------------->   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 seeSection 4.1.4.2.   ----------------------------------------------------------------    initial              new RRSIGs           new DNSKEY   ----------------------------------------------------------------   Parent:    SOA_0 -------------------------------------------------------->    RRSIG_par(SOA) ----------------------------------------------->    DS_S_1 ------------------------------------------------------->    RRSIG_par(DS_S_1) -------------------------------------------->   Child:    SOA_0                SOA_1                SOA_2    RRSIG_S_1(SOA)    RRSIG_Z_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   ----------------------------------------------------------------    new DS               revoke DNSKEY        DNSKEY removal   ----------------------------------------------------------------   Parent:    SOA_1 ------------------------------------------------------->    RRSIG_par(SOA) ---------------------------------------------->    DS_S_2 ------------------------------------------------------>    RRSIG_par(DS_S_2) ------------------------------------------->   Child:    -------------------> SOA_3                SOA_4    -------------------> RRSIG_Z_10(SOA)    -------------------> RRSIG_S_2(SOA)       RRSIG_S_2(SOA)    -------------------> DNSKEY_S_1_REVOKED    -------------------> DNSKEY_Z_10    -------------------> DNSKEY_S_2           DNSKEY_S_2    -------------------> RRSIG_S_1(DNSKEY)    RRSIG_S_1(DNSKEY)    -------------------> RRSIG_S_2(DNSKEY)    RRSIG_S_2(DNSKEY)   ----------------------------------------------------------------    RRSIGs removal   ----------------------------------------------------------------   Parent:    ------------------------------------->    ------------------------------------->    ------------------------------------->    ------------------------------------->   Child:    SOA_5    RRSIG_S_2(SOA)    DNSKEY_S_2    RRSIG_S_2(DNSKEY)   ----------------------------------------------------------------            Figure 14:RFC 5011 Algorithm Roll in a Single-Type                        Signing Scheme Environment   Also seeSection 4.1.4.3.Kolkman, et al.               Informational                    [Page 67]

RFC 6781         DNSSEC Operational Practices, Version 2   December 2012Appendix D.  Transition Figure for Changing DNS Operators   The figure in this Appendix complements and illustrates the special   case of changing DNS operators as described inSection 4.3.5.1.Kolkman, et al.               Informational                    [Page 68]

RFC 6781         DNSSEC Operational Practices, Version 2   December 2012    ------------------------------------------------------------    new DS             |        pre-publish                    |    ------------------------------------------------------------    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)    ------------------------------------------------------------    ------------------------------------------------------------          re-delegation                |   post-migration      |    ------------------------------------------------------------    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 OperatorsKolkman, et al.               Informational                    [Page 69]

RFC 6781         DNSSEC Operational Practices, Version 2   December 2012Appendix E.  Summary of Changes fromRFC 4641   This document differs fromRFC 4641 [RFC4641] in the following ways:   o  Addressed the errata listed on      <http://www.rfc-editor.org/errata_search.php./rfc4641>.   o  Recommended RSA/SHA-256 in addition to RSA/SHA-1.   o  Did a complete rewrite ofSection 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 ofSection 3, whereby the argument is      made that the timescales for rollovers are made purely on      operational arguments.   o  AddedSection 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 2012Authors' 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.nlKolkman, et al.               Informational                    [Page 71]