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Internet Engineering Task Force (IETF)                        S. CrockerRequest for Comments: 6975                                 Shinkuro Inc.Category: Standards Track                                        S. RoseISSN: 2070-1721                                                     NIST                                                               July 2013Signaling Cryptographic Algorithm Understanding inDNS Security Extensions (DNSSEC)Abstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6975.Crocker & Rose               Standards Track                    [Page 1]

RFC 6975                    Algorithm-Signal                   July 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .32.  Requirements Language . . . . . . . . . . . . . . . . . . . . .4   3.  Signaling DNSSEC Algorithm Understood (DAU), DS Hash       Understood (DHU), and NSEC3 Hash Understood (N3U) Using EDNS  . 44.  Client Considerations . . . . . . . . . . . . . . . . . . . . .54.1.  Stub Resolvers  . . . . . . . . . . . . . . . . . . . . . .54.1.1.  Validating Stub Resolvers . . . . . . . . . . . . . . .54.1.2.  Non-validating Stub Resolvers . . . . . . . . . . . . .64.2.  Recursive Resolvers . . . . . . . . . . . . . . . . . . . .64.2.1.  Validating Recursive Resolvers  . . . . . . . . . . . .64.2.2.  Non-validating Recursive Resolvers  . . . . . . . . . .65.  Intermediate System Considerations  . . . . . . . . . . . . . .66.  Server Considerations . . . . . . . . . . . . . . . . . . . . .77.  Traffic Analysis Considerations . . . . . . . . . . . . . . . .78.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .89.  Security Considerations . . . . . . . . . . . . . . . . . . . .810. Normative References  . . . . . . . . . . . . . . . . . . . . .8Crocker & Rose               Standards Track                    [Page 2]

RFC 6975                    Algorithm-Signal                   July 20131.  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   registries.   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 inSection 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.Crocker & Rose               Standards Track                    [Page 3]

RFC 6975                    Algorithm-Signal                   July 20132.  Requirements Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described inRFC2119 [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   are:   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   octet.Crocker & Rose               Standards Track                    [Page 4]

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   query.Crocker & Rose               Standards Track                    [Page 5]

RFC 6975                    Algorithm-Signal                   July 20134.1.2.  Non-validating Stub Resolvers   The DAU, DHU, and N3U EDNS0 options MUST NOT be included by   non-validating stub resolvers.4.2.  Recursive Resolvers4.2.1.  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   query.5.  Intermediate System Considerations   Intermediate proxies (seeSection 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.Crocker & Rose               Standards Track                    [Page 6]

RFC 6975                    Algorithm-Signal                   July 20136.  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   option(s).   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.Crocker & Rose               Standards Track                    [Page 7]

RFC 6975                    Algorithm-Signal                   July 20138.  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   way.   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.Crocker & Rose               Standards Track                    [Page 8]

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   USA   EMail: steve@shinkuro.com   Scott Rose   NIST   100 Bureau Dr.   Gaithersburg, MD  20899   USA   Phone: +1-301-975-8439   EMail: scottr.nist@gmail.comCrocker & Rose               Standards Track                    [Page 9]