Internet Engineering Task Force                                 S. Huque
Internet-Draft                                                   P. Aras
Intended status: Informational                                Salesforce
Expires: January 2, 2019                                    J. Dickinson
                                                               J. Vcelak
                                                            July 1, 2018

                      Multi Provider DNSSEC models


   Many enterprises today employ the service of multiple DNS providers
   to distribute their authoritative DNS service.  Deploying DNSSEC in
   such an environment can have some challenges depending on the
   configuration and feature set in use.  This document will present
   several deployment models that may be suitable.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 2, 2019.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   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.

Table of Contents

   1.  Introduction and Motivation . . . . . . . . . . . . . . . . .   2
   2.  Deployment Models . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Serve Only model  . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Sign and Serve model  . . . . . . . . . . . . . . . . . .   3
       2.2.1.  Model 1: Common KSK, Unique ZSK per provider  . . . .   4
       2.2.2.  Model 2: Unique KSK and ZSK per provider  . . . . . .   4
       2.2.3.  Model 3: Shared KSK/ZSK Signing Keys  . . . . . . . .   5
     2.3.  Inline Signing model  . . . . . . . . . . . . . . . . . .   5
     2.4.  Hybrid model  . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Signing Algorithm Considerations  . . . . . . . . . . . . . .   5
   4.  Authenticated Denial Considerations . . . . . . . . . . . . .   6
     4.1.  Single Method . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Mixing Methods  . . . . . . . . . . . . . . . . . . . . .   7
   5.  Validating Resolver Behavior  . . . . . . . . . . . . . . . .   7
   6.  Key Rollover Considerations . . . . . . . . . . . . . . . . .   8
     6.1.  Model 1: Common KSK, Unique ZSK per provider  . . . . . .   9
     6.2.  Model 2: Unique KSK and ZSK per provider  . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction and Motivation

   The source for this draft is maintained in GitHub at:

   Many enterprises today employ the service of multiple DNS providers
   to distribute their authoritative DNS service.  Two providers are
   fairly typical and this allows the DNS service to survive a complete
   failure of any single provider.  This document outlines some possible
   models of DNSSEC [RFC4033] [RFC4034] [RFC4035] deployment in such an

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2.  Deployment Models

   The two main models discussed are (1) where the zone owner runs a
   master signing server and essentially treats the managed DNS
   providers as secondary servers, the "Serve Only" model, and (2) where
   the managed DNS providers each act like primary servers, signing data
   received from the zone owner and serving it out to DNS queriers, the
   "Sign and Serve" model.  Inline signing and hybrid models are also
   briefly mentioned.  A large part of this document discusses the Sign
   and Serve models, which present novel challenges and requirements.

2.1.  Serve Only model

   The most straightforward deployment model is one in which the zone
   owner runs a primary master DNS server, and manages the signing of
   zone data.  The master server uses DNS zone transfer mechanisms
   (AXFR/IXFR) [RFC5936] [RFC1995] to distribute the signed zone to
   multiple DNS providers.

   This is also arguably the most secure model because the zone owner
   holds the private signing keys.  The managed DNS providers cannot
   serve bogus data (either maliciously or because of compromise of
   their systems) without detection by validating resolvers.

   One notable limitation of this model is that it may not work with DNS
   authoritative server configurations that use certain non-standardized
   DNS features.  Some of these features like DNS based Global Server
   Load Balancing (GSLB), dynamic failover pools, etc. rely on querier
   specific responses, or responses based on real-time state
   examination, and so, the answer and corresponding signature has to be
   determined at the authoritative server being queried, at the time of
   the query, or both.  (If all possible answer sets for these features
   are known in advance, it would be possible to pre-compute these
   answer sets and signatures, but the DNS zone transfer protocol cannot
   be used to distinguish or transfer such data sets, or the rules used
   to select among the possible answers.)

2.2.  Sign and Serve model

   In this category of models, multiple providers each independently
   sign and serve the same zone.  The zone owner typically uses
   provider-specific APIs to update zone content at each of the
   providers, and relies on the provider to perform signing of the data.
   A key requirement here is to manage the contents of the DNSKEY and DS
   RRset in such a way that validating resolvers always have a viable
   path to authenticate the DNSSEC signature chain no matter which
   provider they query and obtain responses from.  This requirement is

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   achieved by having each provider import the Zone Signing Keys of all
   other providers into their DNSKEY RRsets.

   These models can support DNSSEC even for the non-standard features
   mentioned previously, if the DNS providers have the capability of
   signing the response data generated by those features.  Since these
   responses are often generated dynamically at query time, one method
   is for the provider to perform online signing (also known as on-the-
   fly signing).  However, another possible approach is to pre-compute
   all the possible response sets and associated signatures and then
   algorithmically determine at query time which response set needs to
   be returned.

   In the first two of these models, the function of coordinating the
   DNSKEY or DS RRset does not involve the providers communicating
   directly with each other, which they are unlikely to do since they
   typically have a contractual relationship only with the zone owner.

   The following descriptions consider the case of two DNS providers,
   but the model is generalizable to any number.

2.2.1.  Model 1: Common KSK, Unique ZSK per provider

   o  Zone owner holds the KSK, manages the DS record, and is
      responsible for signing the DNSKEY RRset and distributing the
      signed DNSKEY RRset to the providers.

   o  Each provider has their own ZSK which is used to sign data.

   o  Providers have an API that owner uses to query the ZSK public key,
      and insert a combined DNSKEY RRset that includes both ZSKs and the
      KSK, signed by the KSK.

   o  Key rollovers need coordinated participation of the zone owner to
      update the DNSKEY RRset (for KSK or ZSK), and the DS RRset (for

2.2.2.  Model 2: Unique KSK and ZSK per provider

   o  Each provider has their own KSK and ZSK.

   o  Each provider offers an API that the Zone Owner uses to import the
      ZSK of the other provider into their DNSKEY RRset.

   o  DNSKEY RRset is signed independently by each provider using their
      own KSK.

   o  Zone Owner manages the DS RRset that includes both KSKs.

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   o  Key rollovers need coordinated participation of the zone owner to
      update the DS RRset (for KSK), and the DNSKEY RRset (for ZSK).

2.2.3.  Model 3: Shared KSK/ZSK Signing Keys

   Other possible models could involve the KSK and/or ZSK signing keys
   shared across providers.  Preliminary discussion with several
   providers has revealed that this is not a model they are comfortable
   with, again because they want to be independently responsible for
   securing the signing keys without involvement of other parties they
   don't have contractual relationships with.  A possible way to
   mitigate this concern might be for the zone owner to operate a
   networked Hardware Security Module (HSM) which houses the shared
   signing keys and performs the signing operations.  The signing
   instructions and results are communicated over a secure network
   channel between the provider and HSM.  This could work, but may also
   pose performance bottlenecks, particularly for providers that perform
   on-the-fly signing.  Due to open questions about the operational
   viability of this model, it is not discussed further.

2.3.  Inline Signing model

   In this model, the zone owner runs a master server but does not
   perform zone signing, instead pushing out the zone (typically via
   zone transfer mechanisms) to multiple providers, and relying on those
   providers to sign the zone data before serving them out.  This model
   has to address the same set of requirements as the Sign-and-Serve
   model regarding managing the DNSKEY and DS RRsets.  However, assuming
   standardized zone transfers mechanisms are being used to push out the
   zone to the providers, it likely also has the limitation that non-
   standardized DNS features cannot be supported or signed.  This model
   is not discussed further.

2.4.  Hybrid model

   In the hybrid model, the zone owner uses one provider as the primary,
   operating in Sign and Serve mode.  The other providers operate in
   Serve Only mode, i.e., they are configured as secondary servers,
   obtaining the signed zone from the primary provider using the DNS
   zone transfer protocol.  This model suffers from the same limitations
   as the Serve-Only model.  It additionally requires the signing keys
   to be held by the primary provider.

3.  Signing Algorithm Considerations

   In the Serve Only and Hybrid models, one entity (the Zone Owner in
   the former, and the primary provider in the latter) performs the
   signing and hence chooses the signing algorithm to be deployed.  The

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   more interesting case is the Sign and Serve model (Section 2.2),
   where multiple providers independently sign zone data.

   Ideally, the providers should be using a common signing algorithm
   (and common keysizes for algorithms that support variable key sizes).
   This ensures that the multiple providers have identical security
   postures and no provider is more vulnerable to cryptanalytic attack
   than the others.

   It may however be possible to deploy a configuration where different
   providers use different signing algorithms.  The main impediment is
   that current DNSSEC specifications require that if there are multiple
   algorithms in the DNSKEY RRset, then RRsets in the zone need to be
   signed with at least one DNSKEY of each algorithm, as described in
   RFC 4035 [RFC4035], Section 2.2.  However RFC 6781 [RFC6781],
   Section 4.1.4, also describes both a conservative and liberal
   interpretation of this requirement.  When validating DNS resolvers
   follow the liberal approach, they do not expect that zone RRsets are
   signed by every signing algorithm in the DNSKEY RRset, and responses
   with single algorithm signatures can be validated corectly assuming a
   valid chain of trust exists.  In fact, testing by the .BR Top Level
   domain for their planned algorithm rollover [BR-ROLLOVER],
   demonstrates that the liberal approach works.

4.  Authenticated Denial Considerations

   Authentiated denial of existence enables a resolver to validate that
   a record does not exist.  For this purpose, an authoritative server
   presents in a response to the resolver special NSEC (Section 3.1.3 of
   [RFC4035]) or NSEC3 (Section 7.2 of [RFC5155]) records.  The NSEC3
   method enhances NSEC by providing opt-out for signing insecure
   delegations and also adds limited protection against zone enumeration

   An authoritative server response carrying records for authenticated
   denial is always self-contained and the receiving resolver doesn't
   need to send additional queries to complete the denial proof data.
   For this reason, no rollover is needed when switching between NSEC
   and NSEC3 for a signed zone.

   Since authenticated denial responses are self-contained, NSEC and
   NSEC3 can be used by different providers to serve the same zone.
   Doing so however defeats the protection against zone enumeration
   provided by NSEC3.  A better configuration involves multiple
   providers using different authenticated denial of existence
   mechanisms that all provide zone enumeration defense, such as pre-
   computed NSEC3, NSEC3 White Lies [RFC7129], NSEC Black Lies
   [BLACKLIES], etc.  Note however that having multiple providers

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   offering different authenticated denial mechanisms may impact how
   effectively resolvers are able to make use of the caching of negative

4.1.  Single Method

   Usually, the NSEC and NSEC3 methods are used exclusively (i.e. the
   methods are not used at the same time by different servers).  This
   configuration is prefered because the behavior is well-defined and
   it's closest to the current operational practice.

4.2.  Mixing Methods

   Compliant resolvers should be able to serve zones when different
   authoritative servers for the same zone respond with different
   authentiated denial methods because this is normally observed when
   NSEC and NSEC3 are being switched or when NSEC3PARAM is updated.

   Resolver software may be however designed to handle a single
   transition between two authenticated denial configurations more
   optimally than permanent setup with mixed authenticated denial
   methods.  This could make caching on the resolver side less efficient
   and the authoritative servers may observe higher number of queries.
   This aspect should be considered especially in context of Aggresive
   Use of DNSSEC-Validated Cache [RFC8198].

   In case all providers cannot be configured for a matching
   authentiated denial, it is advised to find lowest number of possible
   configurations possible across all used providers.

   Note that NSEC3 configuration on all providers with different
   NSEC3PARAM values is considered a mixed setup.

5.  Validating Resolver Behavior

   From the point of view of the Validating Resolver, the Sign and Serve
   models (Section 2.2), that employ multiple providers signing the same
   zone data with distinct keys, are the most interesting.  In these
   models, for each provider, the Zone Signing Keys of the other
   providers are imported into the DNSKEY RRset and the DNSKEY RRset is
   re-signed.  If this is not done, the following situation can arise
   (assuming two providers A and B):

   o  The validating resolver follows a referral (delegation) to the
      zone in question.

   o  It retrieves the zone's DNSKEY RRset from one of provider A's

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   o  At some point in time, the resolver attempts to resolve a name in
      the zone, while the DNSKEY RRset received from provider A is still
      viable in its cache.

   o  It queries one of provider B's nameservers to resolve the name,
      and obtains a response that is signed by provider B's ZSK, which
      it cannot authenticate because this ZSK is not present in its
      cached DNSKEY RRset for the zone that it received from provider A.

   o  The resolver will not accept this response.  It may still be able
      to ultimately authenticate the name by querying other nameservers
      for the zone until it elicits a response from one of provider A's
      nameservers.  But it has incurred the penalty of additional
      roundtrips with other nameservers, with the corresponding latency
      and processing costs.  The exact number of additional roundtrips
      depends on details of the resolver's nameserver selection
      algorithm and the number of nameservers configured at provider B.

   o  It may also be the case that a resolver is unable to provide an
      authenticated response because it gave up after a certain number
      of retries or a certain amount of delay.  Or that downstream
      clients of the resolver that originated the query timed out
      waiting for a response.

   Zone owners will want to deploy a DNS service that responds as
   efficiently as possible with validatable answers only, and hence it
   is important that the DNSKEY RRset at each provider is maintained
   with the active ZSKs of all participating providers.  This ensures
   that resolvers can validate a response no matter which provider's
   nameservers it came from.

   Details of how the DNSKEY RRset itself is validated differs.  In Sign
   and Serve model 1 (Section 2.2.1), one unique KSK managed by the Zone
   Owner signs an identical DNSKEY RRset deployed at each provider, and
   the signed DS record in the parent zone refers to this KSK.  In Sign
   and Serve model 2 (Section 2.2.2), each provider has a distinct KSK
   and signs the DNSKEY RRset with it.  The Zone Owner deploys a DS
   RRset at the parent zone that contains multiple DS records, each
   referring to a distinct provider's KSK.  Hence it does not matter
   which provider's nameservers the resolver obtains the DNSKEY RRset
   from, the signed DS record in each model can authenticate the
   associated KSK.

6.  Key Rollover Considerations

   The Sign-and-Serve (Section 2.2) models introduce some new
   requirements for DNSSEC key rollovers.  Since this process
   necessarily involves co-ordinated actions on the part of providers

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   and the Zone Owner, one reasonable strategy is for the Zone Owner to
   initiate key rollover operations.  But other operationally plausible
   models may also suit, such as a DNS provider initiating a key
   rollover and signaling their intent to the Zone Owner in some manner.

   The descriptions in this section assume that KSK rollovers employ the
   commonly used Double Signature KSK Rollover Method, and that ZSK
   rollovers employ the Pre-Publish ZSK Rollover Method, as described in
   detail in [RFC6781].  With minor modifications, they can also be
   easily adapted to other models, such as Double DS KSK Rollover or
   Double Signature ZSK rollover, if desired.

6.1.  Model 1: Common KSK, Unique ZSK per provider

   o  Key Signing Key Rollover: In this model, the two managed DNS
      providers share a common KSK which is held by the Zone Owner.  To
      initiate the rollover, the Zone Owner generates a new KSK and
      obtains the DNSKEY RRset of each DNS provider using their
      respective APIs.  The new KSK is added to each provider's DNSKEY
      RRset and the RRset is re-signed with both the new and the old
      KSK.  This new DNSKEY RRset is then transferred to each provider.
      The Zone Owner then updates the DS RRset in the parent zone to
      point to the new KSK, and after the necessary DS record TTL period
      has expired, proceeds with updating the DNSKEY RRSet to remove the
      old KSK.

   o  Zone Signing Key Rollover: In this model, each DNS provider has
      separate Zone Signing Keys.  Each provider can choose to roll
      their ZSK independently by co-ordinating with the Zone Owner.
      Provider A would generate a new ZSK and communicate their intent
      to perform a rollover (note that Provider A cannot immediately
      insert this new ZSK into their DNSKEY RRset because the RRset has
      to be signed by the Zone Owner).  The Zone Owner obtains the new
      ZSK from Provider A.  It then obtains the current DNSKEY RRset
      from each provider (including Provider A), inserts the new ZSK
      into each DNSKEY RRset, re-signs the DNSKEY RRset, and sends it
      back to each provider for deployment via their respective key
      management APIs.  Once the necessary time period is elapsed (i.e.
      all zone data has been re-signed by the new ZSK and propagated to
      all authoritative servers for the zone, plus the maximum zone TTL
      value of any of the data in the zone signed by the old ZSK),
      Provider A and the zone owner can initiate the next phase of
      removing the old ZSK.

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6.2.  Model 2: Unique KSK and ZSK per provider

   o  Key Signing Key Rollover: In Model 2, each managed DNS provider
      has their own KSK.  A KSK roll for provider A does not require any
      change in the DNSKEY RRset of provider B, but does require co-
      ordination with the Zone Owner in order to get the DS record set
      in the parent zone updated.  The KSK roll starts with Provider A
      generating a new KSK and including it in their DNSKEY RRSet.  The
      DNSKey RRset would then be signed by both the new and old KSK.
      The new KSK is communicated to the Zone Owner, after which the
      Zone Owner updates the DS RRset to replace the DS record for the
      old KSK with a DS record for the new ZSK.  After the necessary DS
      RRset TTL period has elapsed, the old KSK can be removed from
      provider A's DNSKEY RRset.

   o  Zone Signing Key Rollover: In Model 2, each managed DNS provider
      has their own ZSK.  The ZSK roll for provider A would start with
      them generating new ZSK and including it in their DNSKEY RRset and
      re-signing the new DNSKEY RRset with their KSK.  The new ZSK of
      provider A would then be communicated to the Zone Owner, who will
      initiate the process of importing this ZSK into the DNSKEY RRsets
      of the other providers, using their respective APIs.  Once the
      necessary Pre-Publish key rollover time periods have elapsed,
      provider A and the Zone Owner can initiate the process of removing
      the old ZSK from the DNSKEY RRset of all providers.

7.  IANA Considerations

   This document includes no request to IANA.

8.  Security Considerations


9.  Acknowledgments

   This document benefited from discussions with and review from Duane
   Wessels and David Blacka.

10.  References

10.1.  Normative References

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              DOI 10.17487/RFC1995, August 1996, <https://www.rfc-

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   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,

   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781,
              DOI 10.17487/RFC6781, December 2012, <https://www.rfc-

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <>.

10.2.  Informative References

              Valsorda, F. and O. Gudmundsson, "Compact DNSSEC Denial of
              Existence or Black Lies", <

              Neves, F., ".br DNSSEC Algorithm Rollover Update",
              in ICANN 62 DNSSEC Workshop, June 2018,

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   [RFC7129]  Gieben, R. and W. Mekking, "Authenticated Denial of
              Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
              February 2014, <>.

Authors' Addresses

   Shumon Huque


   Pallavi Aras


   John Dickinson


   Jan Vcelak


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