Internet Engineering Task Force                                 S. Huque
Internet-Draft                                                   P. Aras
Intended status: Informational                                Salesforce
Expires: September 20, 2018                                 J. Dickinson
                                                               J. Vcelak
                                                          March 19, 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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   This Internet-Draft will expire on September 20, 2018.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction and Motivation . . . . . . . . . . . . . . . . .   2
   2.  Deployment Models . . . . . . . . . . . . . . . . . . . . . .   2
     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.  Validating Resolver Behavior  . . . . . . . . . . . . . . . .   6
   5.  Key Rollover Considerations . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction and Motivation

   The source for this draft is maintained in GitHub at: https://

   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

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

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   briefly mentioned.  A large part of this document discusses the Sign
   and Serve models, which present novel challenges.

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.

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   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.

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

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

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   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.  [TODO: investigate resolver
   implementations to see what they actually do.]

4.  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

   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

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      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.

5.  Key Rollover Considerations


6.  IANA Considerations

   This document includes no request to IANA.

7.  Security Considerations


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8.  Acknowledgments

   This document benefited from discussions with and review from Duane

9.  References

9.1.  Normative References

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

   [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.

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

9.2.  Informative References

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552, July

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

Authors' Addresses

   Shumon Huque


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   Pallavi Aras


   John Dickinson


   Jan Vcelak


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