RFC 6975

Internet Engineering Task Force (IETF)                        S. Crocker
Request for Comments: 6975                                 Shinkuro Inc.
Category: Standards Track                                        S. Rose
ISSN: 2070-1721                                                     NIST
                                                               July 2013

           Signaling Cryptographic Algorithm Understanding in
                    DNS Security Extensions (DNSSEC)


   The DNS Security Extensions (DNSSEC) were developed to provide origin
   authentication and integrity protection for DNS data by using digital
   signatures.  These digital signatures can be generated using
   different algorithms.  This document specifies a way for validating
   end-system resolvers to signal to a server which digital signature
   and hash algorithms they support.  The extensions allow the signaling
   of new algorithm uptake in client code to allow zone administrators
   to know when it is possible to complete an algorithm rollover in a
   DNSSEC-signed zone.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 6975                    Algorithm-Signal                   July 2013

Copyright Notice

   Copyright (c) 2013 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
   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  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . . . 4
   3.  Signaling DNSSEC Algorithm Understood (DAU), DS Hash
       Understood (DHU), and NSEC3 Hash Understood (N3U) Using EDNS  . 4
   4.  Client Considerations . . . . . . . . . . . . . . . . . . . . . 5
     4.1.  Stub Resolvers  . . . . . . . . . . . . . . . . . . . . . . 5
       4.1.1.  Validating Stub Resolvers . . . . . . . . . . . . . . . 5
       4.1.2.  Non-validating Stub Resolvers . . . . . . . . . . . . . 6
     4.2.  Recursive Resolvers . . . . . . . . . . . . . . . . . . . . 6
       4.2.1.  Validating Recursive Resolvers  . . . . . . . . . . . . 6
       4.2.2.  Non-validating Recursive Resolvers  . . . . . . . . . . 6
   5.  Intermediate System Considerations  . . . . . . . . . . . . . . 6
   6.  Server Considerations . . . . . . . . . . . . . . . . . . . . . 7
   7.  Traffic Analysis Considerations . . . . . . . . . . . . . . . . 7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   10. Normative References  . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   The DNS Security Extensions (DNSSEC), [RFC4033], [RFC4034], and
   [RFC4035], were developed to provide origin authentication and
   integrity protection for DNS data by using digital signatures.  Each
   digital signature (RRSIG) Resource Record (RR) contains an algorithm
   code number that corresponds to a DNSSEC public key (DNSKEY) RR.
   These algorithm codes tell validators which cryptographic algorithm
   was used to generate the digital signature.

   Likewise, the Delegation Signer (DS) RRs and Hashed Authenticated
   Denial of Existence (NSEC3) RRs use a hashed value as part of their
   resource record data (RDATA) and, like digital signature algorithms,
   these hash algorithms have code numbers.  All three algorithm codes
   (RRSIG/DNSKEY, DS, and NSEC3) are maintained in unique IANA

   This document sets specifies a way for validating end-system
   resolvers to tell a server in a DNS query which digital signature
   and/or hash algorithms they support.  This is done using the new
   Extension Mechanisms for DNS (EDNS0) options specified below in
   Section 2 for use in the OPT meta-RR [RFC6891].  These three new
   EDNS0 option codes are all OPTIONAL to implement and use.

   These proposed EDNS0 options serve to measure the acceptance and use
   of new digital signing algorithms.  These signaling options can be
   used by zone administrators as a gauge to measure the successful
   deployment of code that implements the newly deployed digital
   signature algorithm, DS hash, and the NSEC3 hash algorithm used with
   DNSSEC.  A zone administrator is able to determine when to stop
   signing with a superseded algorithm when the server sees that a
   significant number of its clients signal that they are able to accept
   the new algorithm.  Note that this survey may be conducted over a
   period of years before a tipping point is seen.

   This document does not seek to introduce another process for
   including new algorithms for use with DNSSEC.  It also does not
   address the question of which algorithms are to be included in any
   official list of mandatory or recommended cryptographic algorithms
   for use with DNSSEC.  Rather, this document specifies a means by
   which a client query can signal the set of algorithms and hashes that
   it implements.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

3.  Signaling DNSSEC Algorithm Understood (DAU), DS Hash Understood
    (DHU), and NSEC3 Hash Understood (N3U) Using EDNS

   The EDNS0 specification outlined in [RFC6891] defines a way to
   include new options using a standardized mechanism.  These options
   are contained in the RDATA of the OPT meta-RR.  This document defines
   three new EDNS0 options for a client to signal which digital
   signature and/or hash algorithms the client supports.  These options
   can be used independently of each other and MAY appear in any order
   in the OPT RR.  Each option code can appear only once in an OPT RR.

   The figure below shows how each option is defined in the RDATA of the
   OPT RR specified in [RFC6891]:

       0                       8                      16
       |                  OPTION-CODE                  |
       |                  LIST-LENGTH                  |
       |       ALG-CODE        |        ...            /

   OPTION-CODE is the code for the given signaling option.  The options

   o  DNSSEC Algorithm Understood (DAU) option for DNSSEC digital
      signing algorithms.  Its value is fixed at 5.

   o  DS Hash Understood (DHU) option for DS RR hash algorithms.  Its
      value is fixed at 6.

   o  NSEC3 Hash Understood (N3U) option for NSEC3 hash algorithms.  Its
      value is fixed at 7.

   LIST-LENGTH is the length of the list of digital signatures or hash
   algorithm codes in octets.  Each algorithm code occupies a single

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RFC 6975                    Algorithm-Signal                   July 2013

   ALG-CODE is the list of assigned values of DNSSEC zone signing
   algorithms, DS hash algorithms, or NSEC3 hash algorithms (depending
   on the OPTION-CODE in use) that the client declares to be supported.
   The order of the code values can be arbitrary and MUST NOT be used to
   infer preference.

   If all three options are included in the OPT RR, there is a potential
   for the OPT RR to take up considerable size in the DNS message.
   However, in practical terms, including all three options is likely to
   take up 22-32 octets (average of 6-10 digital signature algorithms,
   3-5 DS hash algorithms, and 1-5 NSEC3 hash algorithms) including the
   EDNS0 option codes and option lengths in potential future examples.

4.  Client Considerations

   A validating end-system resolver sets the DAU, DHU, and/or N3U
   option, or combination thereof, in the OPT meta-RR when sending a
   query.  The validating end-system resolver MUST also set the DNSSEC
   OK bit [RFC4035] to indicate that it wishes to receive DNSSEC RRs in
   the response.

   Note that the PRIVATEDNS (253) and/or the PRIVATEOID (254) digital
   signature codes both cover a potentially wide range of algorithms and
   are likely not useful to a server.  There is no compelling reason for
   a client to include these codes in its list of the DAU.  Likewise,
   clients MUST NOT include RESERVED codes in any of the options.
   Additionally, a client is under no obligation to list every algorithm
   it implements and MAY choose to only list algorithms the client
   wishes to signal as understood.

   Since the DAU, DHU, and/or N3U options are only set in the query, if
   a client sees these options in the response, no action needs to be
   taken and the client MUST ignore the option values.

4.1.  Stub Resolvers

   Typically, stub resolvers rely on an upstream recursive server (or
   cache) to provide a response.  So optimal setting of the DAU, DSU,
   and N3U options depends on whether the stub resolver elects to
   perform its own validation.

4.1.1.  Validating Stub Resolvers

   A validating stub resolver sets the DNSSEC OK (DO) bit [RFC4035] to
   indicate that it wishes to receive additional DNSSEC RRs (i.e., RRSIG
   RRs) in the response.  Such validating resolvers SHOULD include the
   DAU, DHU, and/or the N3U option(s) in the OPT RR when sending a

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4.1.2.  Non-validating Stub Resolvers

   The DAU, DHU, and N3U EDNS0 options MUST NOT be included by
   non-validating stub resolvers.

4.2.  Recursive Resolvers

4.2.1.  Validating Recursive Resolvers

   A validating recursive resolver sets the DAU, DHU, and/or N3U
   option(s) when performing recursion based on its list of algorithms
   and any DAU, DHU, and/or N3U option lists in the stub client query.
   When the recursive server receives a query with one or more of the
   options set, the recursive server MUST set the algorithm list for any
   outgoing iterative queries for that resolution chain to a union of
   the stub client's list and the validating recursive resolver's list.
   For example, if the recursive resolver's algorithm list for the DAU
   option is (3, 5, 7) and the stub's algorithm list is (7, 8), the
   final DAU algorithm list would be (3, 5, 7, 8).

   If the client included the DO and Checking Disabled (CD) bits, but
   did not include the DAU, DHU, and/or N3U option(s) in the query, the
   validating recursive resolver MAY include the option(s) with its own
   list in full.  If one or more of the options are missing, the
   validating recursive resolver MAY include the missing options with
   its own list in full.

   Validating recursive resolvers MUST NOT set the DAU, DHU, and/or N3U
   option(s) in the final response to the stub client.

4.2.2.  Non-validating Recursive Resolvers

   Recursive resolvers that do not do validation MUST copy the DAU, DHU,
   and/or N3U option(s) seen in received queries as they represent the
   wishes of the validating downstream resolver that issued the original

5.  Intermediate System Considerations

   Intermediate proxies (see Section 4.4.2 of [RFC5625]) that understand
   DNS are RECOMMENDED to behave like a comparable recursive resolver
   when dealing with the DAU, DHU, and N3U options.

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6.  Server Considerations

   When an authoritative server sees the DAU, DHU, and/or N3U option(s)
   in the OPT meta-RR in a request, the normal algorithm for servicing
   requests is followed.  The options MUST NOT trigger any special
   processing (e.g., RRSIG filtering in responses) on the server side.

   If the options are present but the DO bit is not set, the server does
   not do any DNSSEC processing, which includes any recording of the

   If the server sees one (or more) of the options set with RESERVED
   values, the server MAY ignore recoding of those values.

   Authoritative servers MUST NOT set the DAU, DHU, and/or N3U option(s)
   on any responses.  These values are only set in queries.

7.  Traffic Analysis Considerations

   Zone administrators that are planning or are in the process of a
   cryptographic algorithm rollover operation should monitor DNS query
   traffic and record the number of queries, the presence of the OPT RR
   in queries, and the values of the DAU/DHU/N3U option(s) (if present).
   This monitoring can be used to measure the deployment of client code
   that implements (and signals) specific algorithms.  A description of
   the techniques used to capture DNS traffic and measure new algorithm
   adoption is beyond the scope of this document.

   Zone administrators that need to comply with changes to their
   organization's security policy (with regards to cryptographic
   algorithm use) can use this data to set milestone dates for
   performing an algorithm rollover.  For example, zone administrators
   can use the data to determine when older algorithms can be phased out
   without disrupting a significant number of clients.  In order to keep
   this disruption to a minimum, zone administrators should wait to
   complete an algorithm rollover until a large majority of clients
   signal that they recognize the new algorithm.  This may be in the
   order of years rather than months.

   Note that clients that do not implement these options are likely to
   be older implementations that would also not implement any newly
   deployed algorithm.

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8.  IANA Considerations

   The algorithm codes used to identify DNSSEC algorithms, DS RR hash
   algorithms, and NSEC3 hash algorithms have already been established
   by IANA.  This document does not seek to alter that registry in any

   IANA has allocated option codes 5, 6, and 7 for the DAU, DHU, and N3U
   options, respectively, in the "DNS EDNS0 Option Codes (OPT)"
   registry.  The three options have a status of "standard".

9.  Security Considerations

   This document specifies a way for a client to signal its digital
   signature and hash algorithm knowledge to a cache or server.  It is
   not meant to be a discussion on algorithm superiority.  The signals
   are optional codes contained in the OPT meta-RR used with EDNS.  The
   goal of these options is to signal new algorithm uptake in client
   code to allow zone administrators to know when it is possible to
   complete an algorithm rollover in a DNSSEC-signed zone.

   There is a possibility that an eavesdropper or server could infer the
   validator in use by a client by the presence of the AU options and/or
   algorithm code list.  This information leakage in itself is not very
   useful to a potential attacker, but it could be used to identify the
   validator or narrow down the possible validator implementations in
   use by a client, which could have a known vulnerability that could be
   exploited by the attacker.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",
              BCP 152, RFC 5625, August 2009.

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RFC 6975                    Algorithm-Signal                   July 2013

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.

Authors' Addresses

   Steve Crocker
   Shinkuro Inc.
   5110 Edgemoor Lane
   Bethesda, MD  20814

   EMail: steve@shinkuro.com

   Scott Rose
   100 Bureau Dr.
   Gaithersburg, MD  20899

   Phone: +1-301-975-8439
   EMail: scottr.nist@gmail.com

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