Internet Engineering Task Force S. Huque Internet-Draft P. Aras Intended status: Informational Salesforce Expires: September 20, 2018 J. Dickinson Sinodun J. Vcelak NS1 March 19, 2018 Multi Provider DNSSEC models draft-huque-dnsop-multi-provider-dnssec-02 Abstract 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 20, 2018. Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Huque, et al. Expires September 20, 2018 [Page 1] Internet-Draft Multi Provider DNSSEC models March 2018 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 . . . . . . . . . . . . . . . . . . . . . . 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 RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING: The source for this draft is maintained in GitHub at: https:// github.com/shuque/multi-provider-dnssec 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 environment. 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 Huque, et al. Expires September 20, 2018 [Page 2] Internet-Draft Multi Provider DNSSEC models March 2018 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. Huque, et al. Expires September 20, 2018 [Page 3] Internet-Draft Multi Provider DNSSEC models March 2018 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 KSK). 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). Huque, et al. Expires September 20, 2018 [Page 4] Internet-Draft Multi Provider DNSSEC models March 2018 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. Huque, et al. Expires September 20, 2018 [Page 5] Internet-Draft Multi Provider DNSSEC models March 2018 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 nameservers. 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 Huque, et al. Expires September 20, 2018 [Page 6] Internet-Draft Multi Provider DNSSEC models March 2018 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 TBD 6. IANA Considerations This document includes no request to IANA. 7. Security Considerations [TBD] Huque, et al. Expires September 20, 2018 [Page 7] Internet-Draft Multi Provider DNSSEC models March 2018 8. Acknowledgments This document benefited from discussions with and review from Duane Wessels. 9. References 9.1. Normative References [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, DOI 10.17487/RFC1995, August 1996, . [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 2003. [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 Salesforce Email: shuque@gmail.com Huque, et al. Expires September 20, 2018 [Page 8] Internet-Draft Multi Provider DNSSEC models March 2018 Pallavi Aras Salesforce Email: paras@salesforce.com John Dickinson Sinodun Email: jad@sinodun.com Jan Vcelak NS1 Email: jvcelak@ns1.com Huque, et al. Expires September 20, 2018 [Page 9]