DNS Extensions Working Group                                  S. Crocker
Internet-Draft                                             Shinkuro Inc.
Intended status: Standards Track                                 S. Rose
Expires: March 28, 2013                                             NIST
                                                      September 24, 2012

       Signaling Cryptographic Algorithm Understanding in 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 draft sets out to specify a way for
   validating end-system resolvers to signal to a server which digital
   signature and hash algorithms they support.

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].

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
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   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 March 28, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

   3.  Client Considerations . . . . . . . . . . . . . . . . . . . . . 5
     3.1.  Stub Resolvers  . . . . . . . . . . . . . . . . . . . . . . 5
       3.1.1.  Validating Stub Resolvers . . . . . . . . . . . . . . . 5
       3.1.2.  Non-Validating Stub Resolvers . . . . . . . . . . . . . 5
     3.2.  Recursive Resolvers . . . . . . . . . . . . . . . . . . . . 5
       3.2.1.  Validating Recursive Resolvers  . . . . . . . . . . . . 5
       3.2.2.  Non-validating Recursive Resolvers  . . . . . . . . . . 6

   4.  Intermediate System Considerations  . . . . . . . . . . . . . . 6

   5.  Server Considerations . . . . . . . . . . . . . . . . . . . . . 6

   6.  Traffic Analysis Considerations . . . . . . . . . . . . . . . . 6

   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7

   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7

   9.  Normative References  . . . . . . . . . . . . . . . . . . . . . 7

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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 RR (RRSIG) contains an algorithm code number.
   These algorithm codes tell validators which cryptographic algorithm
   was used to generate the digital signature.

   Likewise, Delegation Signer (DS) RRs and NSEC3 RRs use a hashed value
   as part of their 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 draft sets out to specify 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 EDNS
   options specified below in Section 2 for use in the OPT meta-RR
   [I-D.ietf-dnsext-rfc2671bis-edns0].  These three new EDNS option
   codes are all OPTIONAL to implement and use.

   These proposed EDNS 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 newly deployed digital signature
   algorithm, DS hash and 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 the period of years
   before a tipping point is seen.

   This draft 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 which it

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

   The EDNS0 specification outlined in
   [I-D.ietf-dnsext-rfc2671bis-edns0] 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
   EDNS options for a client to signal which digital signature and/or

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   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 [I-D.ietf-dnsext-rfc2671bis-edns0]:

       0                       8                      16
       |                 OPTION-CODE (TBD)             |
       |                  LIST-LENGTH                  |
       |       ALG-CODE        |        ...            \

   OPTION-CODE is the code for the given signaling option.  They are:

   o  DNSSEC Algorithm Understood (DAU) option for DNSSEC digital
      signing algorithms.  Its value is fixed at TBD1.

   o  DS Hash Understood (DHU) option for DS RR hash algorithms.  Its
      value is fixed at TBD2.

   o  NSEC3 Hash Understood (N3U) option for NSEC3 hash algorithms.  Its
      value is fixed at TBD3.

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

   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
   EDNS option codes and option lengths in a potential future example.

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3.  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

   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.

3.1.  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.

3.1.1.  Validating Stub Resolvers

   A validating stub resolver already (usually) sets the 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.

3.1.2.  Non-Validating Stub Resolvers

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

3.2.  Recursive Resolvers

3.2.1.  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 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).

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   If the client did include the DO and 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.

3.2.2.  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

4.  Intermediate System Considerations

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

5.  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 DNSSEC-OK (OK) bit is not set, the
   server does not do any DNSSEC processing, including any recording of
   the option(s).

6.  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.  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

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   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 which would also not implement any newly
   deployed algorithm.

7.  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.

   This draft seeks to update the "DNS EDNS Options" registry by adding
   the DAU, DHU and N3U options and referencing this document.  The code
   for these options are TBD1, TBD2 and TBD3 respectively.

8.  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 are 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.

9.  Normative References

   [I-D.ietf-dnsext-rfc2671bis-edns0]  Damas, J., Graff, M., and P.
                                       Vixie, "Extension Mechanisms for
                                       DNS (EDNS0)", draft-ietf-dnsext-
                                       rfc2671bis-edns0-09 (work in
                                       progress), August 2012.

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

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   [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.

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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