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DNSOP                                                            D. York
Internet-Draft                                          Internet Society
Intended status: Informational                                   O. Sury
Expires: January 4, 2018                                          CZ.NIC
                                                              P. Wouters
                                                                 Red Hat
                                                          O. Gudmundsson
                                                              CloudFlare
                                                            July 3, 2017


     Observations on Deploying New DNSSEC Cryptographic Algorithms
            draft-york-dnsop-deploying-dnssec-crypto-algs-05

Abstract

   As new cryptographic algorithms are developed for use in DNSSEC
   signing and validation, this document captures the steps needed for
   new algorithms to be deployed and enter general usage.  The intent is
   to ensure a common understanding of the typical deployment process
   and potentially identify opportunities for improvement of operations.

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 January 4, 2018.

Copyright Notice

   Copyright (c) 2017 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



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Aspects of Deploying New Algorithms . . . . . . . . . . . . .   3
     2.1.  DNS Resolvers Performing Validation . . . . . . . . . . .   4
       2.1.1.  Resolvers and Unknown Algorithms  . . . . . . . . . .   4
     2.2.  Authoritative DNS Servers . . . . . . . . . . . . . . . .   5
     2.3.  Signing Software  . . . . . . . . . . . . . . . . . . . .   5
       2.3.1.  NSEC3 Iterations  . . . . . . . . . . . . . . . . . .   5
     2.4.  Registries  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  Registrars  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.6.  DNS Hosting Operators . . . . . . . . . . . . . . . . . .   8
     2.7.  Applications  . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Changes  . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The DNS Security Extensions (DNSSEC), broadly defined in [RFC4033],
   [RFC4034] and [RFC4035], make use of cryptographic algorithms in both
   the signing of DNS records and the validation of DNSSEC signatures by
   recursive resolvers.

   The current list of cryptographic algorithms can be found in the IANA
   "Domain Name System Security (DNSSEC) Algorithm Numbers" registry
   located at <http://www.iana.org/assignments/dns-sec-alg-numbers/>
   Algorithms are added to this IANA registry through a process defined
   in [RFC6014].  Note that [RFC6944] provides some guidance as to which
   of these algorithms should be implemented and supported.

   Historically DNSSEC signatures have primarily used cryptographic
   algorithms based on RSA keys.  As deployment of DNSSEC has increased
   there has been interest in using newer and more secure algorithms,
   particularly those using elliptic curve cryptography.



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   The ECDSA algorithm [RFC6605] has seen some adoption and the more
   recent [RFC8080] specifies the Edwards-curve Digital Signature
   Algorithm (EdDSA) using a choice of two curves, Ed25519 and Ed448.

   The challenge is that the deployment of a new cryptographic algorithm
   for DNSSEC is not a simple process.  DNSSEC algorithms are used
   throughout the DNS infrastructure for tasks such as:

   o  Generation of keys ("DNSKEY" record) for signing

   o  Creation of DNSSEC signatures in zone files ("RRSIG")

   o  Usage in a Delegation Signer ("DS") record [RFC3658] for the
      "chain of trust" connecting back to the root of DNS

   o  Generation of NSEC/NSEC3 responses by authoritative DNS servers

   o  Validation of DNSSEC signatures by DNS resolvers

   In order for a new cryptographic algorithm to be fully deployed, all
   aspects of the DNS infrastructure that interact with DNSSEC must be
   updated to use the new algorithm.

   This document outlines the current understanding of the components of
   the DNS infrastructure that need to be updated to deploy a new
   cryptographic algorithm.

   It should be noted that DNSSEC is not alone in complexity of
   deployment.  The IAB documented "Guidelines for Cryptographic
   Algorithm Agility" in [RFC7696] to highlight the importance of this
   issue.

1.1.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119].

2.  Aspects of Deploying New Algorithms

   For a new cryptographic algorithm to be deployed in DNSSEC, the
   following aspects of the DNS infrastructure must be updated:

   o  DNS resolvers performing validation

   o  Authoritative DNS servers




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   o  Signing software

   o  Registries

   o  Registrars

   o  DNS Hosting Operators

   o  Applications

   Each of these aspects is discussed in more detail below.

2.1.  DNS Resolvers Performing Validation

   DNS recursive resolvers perform "validation" to check the DNSSEC
   signatures of records received in a DNS query.  To validate the
   signatures, the resolvers need to be able to understand the algorithm
   used to create the signatures.

   In the case of a new algorithm, the resolver software needs to be
   updated.  In some cases this could require waiting until an
   underlying library is updated to support the new algorithm.

   Once the software is updated, the updates need to be deployed to all
   resolvers using that software.  This can be challenging in cases of
   customer-premises equipment (CPE) that does not have any mechanism
   for automatic updating.

2.1.1.  Resolvers and Unknown Algorithms

   It should be noted that section 5.2 of [RFC4035] states:

      "If the resolver does not support any of the algorithms listed
      in an authenticated DS RRset, then the resolver will not be
      able to verify the authentication path to the child zone.
      In this case, the resolver SHOULD treat the child zone as
      if it were unsigned."

   This means that signing a zone with a new algorithm that is not
   widely supported by DNS resolvers would result in the signatures
   being ignored and the zone treated as unsigned until resolvers were
   updated to recognize the new algorithm.

   Note that in at least one 2016 case the resolver software deployed on
   customer premises by an Internet service provider (ISP) turned out
   not to be compliant with RFC 4035.  Instead of ignoring the
   signatures using unknown algorithms and treating the zones as
   unsigned, the validating resolver rejected the signatures and



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   returned a SERVFAIL to the DNS query.  This resulted in the ISP
   turning off DNSSEC validation on the equipment.  Further
   investigation showed that a newer version of the resolver software
   did correctly support ECDSA, but now all customer premises equipment
   must be updated to this new version.

   The point is that it is not safe to assume all resolver software will
   correctly implement this part of RFC 4035.

2.2.  Authoritative DNS Servers

   Authoritative DNS servers serve out signed DNS records.  Serving new
   DNSSEC signing algorithms should not be a problem as a well-written
   authoritative DNS server implementation should be agnostic to the RR
   DATA they serve.

   The one exception is if the new cryptographic algorithms are used in
   the creation of NSEC/NSEC3 responses.  In the case of new NSEC/NSEC3
   algorithms, the authoritative DNS server software would need to be
   updated to be able to use the new algorithms.

   Note that some authoritative server implementations could include
   DNSSEC signing as part of the server and thus also fall into the
   "Signing Software" category below.

2.3.  Signing Software

   The software performing the signing of the records needs to be
   updated with the new cryptographic algorithm.

   User interfaces that allow users to interact with the DNSSEC signing
   software may also need to be updated to reflect the existence of the
   new algorithm.

   Note that the key and signatures with the new algorithm will need to
   co-exist with the existing key and signatures for some period of
   time.  This will have an impact on the size of the DNS records.

   One issue that has been identified is that not all commonly-used
   signing software releases include support for an algorithm rollover.
   This software would need to be updated to support rolling an
   algorithm before any new algorithms could be deployed.

2.3.1.  NSEC3 Iterations

   Implementation experience has shown that the [RFC5155] NSEC3
   iteration count limits are poorly understood and are fragile in the
   context of adoption of elliptic curve(EC)-based algorithms.



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   A simple design would have constrained the iteration count only by
   the bit width of the iteration count field (perhaps 12 bits for up
   4096 iterations), with all representable values supported by both
   signers and resolvers.  Instead, the iteration count limit was made
   dependent on key size.  When the original text of Section 10.3 of
   [RFC5155] was written, the only commonly used DNSSEC key algorithms
   were RSA and DSA.  These had similar key sizes with comparable
   security, with DSA slower than RSA.  A decision was made to specify
   iteration count limits roughly commensurate with the cost of RSA
   operations for a given key size, and to use the same limits for both
   RSA and DSA.  The essential features of the specification are:

       The limits, therefore, are based on the size of the smallest
       zone signing key, rounded up to the nearest table value (or
       rounded down if the key is larger than the largest table
       value).
       ...
       Therefore the values in the table MUST be used independent of
       the key algorithm.

   While the specified key-size-dependent limits made some sense for
   both RSA and DSA, they map poorly to elliptic-curve-based (EC) DNSSEC
   algorithms, which only use keys shorter than 1024 bits.
   Nevertheless, popular DNS resolvers apply the specified table of
   limits to EC algorithms, and so zones with EC keys need to cap their
   NSEC3 iteration counts at 150.

   This requirement is surprising to some operators migrating from RSA
   to EC keys.  They continue to use iteration counts that work for RSA-
   2048, but which exceed the 150 limit for the smaller EC keys.  This
   renders denial-of-existence "Insecure" for the zones in question.

   Some signer implementations allow maximums that are higher than the
   specified key-size-dependent limits, resulting again in resolvers
   possibly returning these answers as "Insecure".

   To avoid surprises, such as downgrade attacks against "SMTP Security
   via Opportunistic DANE TLS" [RFC7672], DNSSEC signers should not use
   an iteration count higher than 150: such iteration counts are prone
   to fail when configuration changes introduce new algorithms.

   Similarly, resolvers should not support configurations with iteration
   count limits below 150, as lower limits may lead to insecure denial
   of existence, even for compliant zones.







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2.4.  Registries

   The registry for a top-level domain (TLD) needs to accept DS records
   using the new cryptographic algorithm.

   Observations to date have shown that some registries only accept DS
   records with certain algorithms.  Registry representatives have
   indicated that they verify the accuracy of DS records to reduce
   technical support incidents and ensure customers do not mistakenly
   create any outages.

   However, this means that registries who perform this level of
   checking must be able to understand new algorithms in order to
   successfully verify the DS records.

   Separately, feedback from registrars has indicated that they do not
   currently have any mechanism to understand what DNSSEC algorithms a
   registry can accept.

2.5.  Registrars

   Registrars perform a critical role in the DNSSEC "chain of trust" of
   passing the DS record up to the Registry to ensure that the signed
   zone can be authenticated from the root of DNS all the way to the
   zone.

   If the registrar is also providing the DNS hosting services for a
   domain, the registrar can easily create the "DS" record from the
   "DNSKEY" record and pass the DS record up to the registry.

   However, if the authoritative servers for a domain are not with the
   registrar, then the registrar needs to provide some mechanism to
   accept a DS record to pass that up to the registry.  Typically this
   is done through a web interface.

   An issue is that many registrar web interfaces only allow the input
   of DS records using a listed set of DNSSEC algorithms.  Any new
   cryptographic algorithms need to be added to the web interface in
   order to be accepted into the registrar's system.

   Additionally, in a manner similar to registries, many registrars
   perform some level of verification on the DS record to ensure it was
   entered "correctly".  To do this verification, the registrar's
   software needs to understand the algorithm used in the DS record.
   This requires the software to be updated to support the new
   algorithm.





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   Note that [RFC8078] defines an automated mechanism to update the DS
   records with a registry.  If this method becomes widely adopted,
   registrar web interfaces may no longer be needed.

2.6.  DNS Hosting Operators

   DNS hosting operators are entities that are operating the
   authoritative DNS servers for domains and with DNSSEC are also
   providing the signing of zones.  In many cases they may also be the
   registrar for domain names, but in other cases they are a separate
   entity providing DNS services to customers.

   DNS hosting operators need to update their authoritative DNS server
   software to understand new cryptographic algorithms, but they also
   need to update their web interfaces and provisioning software to
   allow configuration and support of new algorithms.

2.7.  Applications

   Beyond the recursive resolvers, authoritative servers, web interfaces
   and provisioning software, it has been observed that some
   applications (or "apps"), particularly in the mobile environment, are
   including their own DNS resolvers within the app itself.  These
   recursive resolvers are used by the app instead of the recursive
   resolver included with the underlying operating system.  These
   applications that perform DNSSEC validation would need to also be
   updated to understand a new algorithm.

   In many cases, it may be that an underlying developer library needs
   to be updated first.  It will then depend upon how long it takes the
   application developer to pull in the updated library.

   Outside of applications, these developer libraries are also typically
   used by recursive resolver software and signing software.

3.  Conclusion

   This document provides a view into the steps necessary for the
   deployment of new cryptographic algorithms in DNSSEC at the time of
   this publication.  In order to more rapidly roll out new DNSSEC
   algorithms, these steps must be understood and hopefully improved
   over time.

   It should be noted that a common theme to emerge from all discussions
   is a general reluctance to update or change any DNS-related software.
   "If it isn't broken, don't fix it" is a common refrain.  While
   perhaps understandable from a stability point of view, this attitude
   creates a challenge for deploying new algorithms.



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   One potential idea suggested during discussions was for some kind of
   web-based testing tool that could assist people in understanding what
   algorithms are supported by different servers and sites.

   It is also quite clear that any deployment of new algorithms for
   DNSSEC use will take a few years to propagate throughout the
   infrastructure.  This needs to be factored in as new algorithms are
   proposed.

4.  IANA Considerations

   This document does not make any requests of IANA.

5.  Security Considerations

   No new security considerations are created by this document.

   It should be noted that there are security considerations regarding
   changing DNSSEC algorithms mentioned in both [RFC6781] and [RFC7583].

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc4033>.

   [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,
              <http://www.rfc-editor.org/info/rfc4034>.

   [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,
              <http://www.rfc-editor.org/info/rfc4035>.

   [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,
              <http://www.rfc-editor.org/info/rfc5155>.



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   [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", RFC 7672,
              DOI 10.17487/RFC7672, October 2015,
              <http://www.rfc-editor.org/info/rfc7672>.

   [RFC8078]  Gudmundsson, O. and P. Wouters, "Managing DS Records from
              the Parent via CDS/CDNSKEY", RFC 8078,
              DOI 10.17487/RFC8078, March 2017,
              <http://www.rfc-editor.org/info/rfc8078>.

   [RFC8080]  Sury, O. and R. Edmonds, "Edwards-Curve Digital Security
              Algorithm (EdDSA) for DNSSEC", RFC 8080,
              DOI 10.17487/RFC8080, February 2017,
              <http://www.rfc-editor.org/info/rfc8080>.

6.2.  Informative References

   [RFC3658]  Gudmundsson, O., "Delegation Signer (DS) Resource Record
              (RR)", RFC 3658, DOI 10.17487/RFC3658, December 2003,
              <http://www.rfc-editor.org/info/rfc3658>.

   [RFC6014]  Hoffman, P., "Cryptographic Algorithm Identifier
              Allocation for DNSSEC", RFC 6014, DOI 10.17487/RFC6014,
              November 2010, <http://www.rfc-editor.org/info/rfc6014>.

   [RFC6605]  Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital
              Signature Algorithm (DSA) for DNSSEC", RFC 6605,
              DOI 10.17487/RFC6605, April 2012,
              <http://www.rfc-editor.org/info/rfc6605>.

   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781,
              DOI 10.17487/RFC6781, December 2012,
              <http://www.rfc-editor.org/info/rfc6781>.

   [RFC6944]  Rose, S., "Applicability Statement: DNS Security (DNSSEC)
              DNSKEY Algorithm Implementation Status", RFC 6944,
              DOI 10.17487/RFC6944, April 2013,
              <http://www.rfc-editor.org/info/rfc6944>.

   [RFC7583]  Morris, S., Ihren, J., Dickinson, J., and W. Mekking,
              "DNSSEC Key Rollover Timing Considerations", RFC 7583,
              DOI 10.17487/RFC7583, October 2015,
              <http://www.rfc-editor.org/info/rfc7583>.






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   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <http://www.rfc-editor.org/info/rfc7696>.

Appendix A.  Acknowledgements

   The information in this document evolved out of several mailing list
   discussions and also through engagement with participants in the
   following sessions or events:

   o  DNSSEC Workshop at ICANN 53 (Buenos Aires)

   o  DNSSEC Workshop at ICANN 55 (Marrakech)

   o  Spring 2016 DNS-OARC meeeting (Buenos Aires)

   o  various IETF 95 working groups (Buenos Aires)

   o  Panel session at RIPE 72 (Copenhagen)

   o  DNSSEC Workshop at ICANN 56 (Helsinki)

   The authors thank the participants of the various sessions for their
   feedback.

   The authors thank Viktor Dukhovni for contributing the text for the
   section on NSEC3 Iterations.

Appendix B.  Changes

   NOTE TO RFC EDITOR - Please remove this "Changes" section prior to
   publication.  Thank you.

   o  Revision -05 corrected typos around two other references that did
      not appear in -04.  It also added the new section on "NSEC3
      Iterations" contributed by Paul Wouters and Viktor Dukhovni.

   o  Revision -04 corrected the references which did not appear in -03
      due to an error in the markdown source.

   o  Revision -03 removed the reference to the location of the ISP in
      the text added in version -02.

   o  Revision -02 added text to the resolver section about an example
      where resolver software did not correctly follow RFC 4035 and
      treat packets with unknown algorithms as unsigned.  The markdown




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      source of this I-D was also migrated to the markdown syntax
      favored by the 'mmark' tool.

   o  Revision -01 adds text about authoritative servers needing an
      update if the algorithm is for NSEC/NSEC3.  Also expands
      acknowledgements.

Authors' Addresses

   Dan York
   Internet Society

   Email: york@isoc.org
   URI:   https://www.internetsociety.org/


   Ondrej Sury
   CZ.NIC

   Email: ondrej.sury@nic.cz


   Paul Wouters
   Red Hat

   Email: pwouters@redhat.com


   Olafur Gudmundsson
   CloudFlare

   Email: olafur+ietf@cloudflare.com



















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