Guidance for NSEC3 parameter settings
draft-hardaker-dnsop-nsec3-guidance-00

Network Working Group                                        W. Hardaker
Internet-Draft                                                   USC/ISI
Intended status: Best Current Practice                       V. Dukhovni
Expires: August 22, 2021                                 Bloomberg, L.P.
                                                       February 18, 2021

                 Guidance for NSEC3 parameter settings
                 draft-hardaker-dnsop-nsec3-guidance-00

Abstract

   NSEC3 is a DNSSEC mechanism providing proof of non-existence by
   promising there are no names that exist between two domainnames
   within a zone.  Unlike its counterpart NSEC, NSEC3 avoids directly
   disclosing the bounding domainname pairs.  This document provides
   guidance on setting NSEC3 parameters based on recent operational
   deployment experience.

Status of This Memo

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   This Internet-Draft will expire on August 22, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements notation . . . . . . . . . . . . . . . . . .   3
   2.  Recommendation for zone publishers  . . . . . . . . . . . . .   3
     2.1.  Algorithms  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Flags . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Iterations  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.4.  Salt  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Recommendation for validating resolvers . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   5
   Appendix B.  Github Version of this document  . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   As with NSEC [RFC4035], NSEC3 [RFC5155] provides proof of non-
   existence that consists of signed DNS records establishing the non-
   existence of a given name or associated Resource Record Type (RRTYPE)
   in a DNSSEC [RFC4035] signed zone.  In the case of NSEC3, however,
   the names of valid nodes in the zone are obfuscated through (possibly
   multiple iterations of) hashing via SHA-1. (currently only SHA-1 is
   in use within the Internet).

   NSEC3 also provides "opt-out support", allowing for blocks of
   unsigned delegations to be covered by a single NSEC3 record.  Opt-out
   blocks allow large registries to only sign as many NSEC3 records as
   there are signed DS or other RRsets in the zone - with opt-out,
   unsigned delegations don't require additional NSEC3 records.  This
   sacrifices the tamper-resistance proof of non-existence offered by
   NSEC3 in order to reduce memory and CPU overheads.

   NSEC3 records have a number of tunable parameters that are specified
   via an NSEC3PARAM record at the zone apex.  These parameters are the
   Hash Algorithm, processing Flags, the number of hash Iterations and
   the Salt.  Each of these has security and operational considerations
   that impact both zone owners and validating resolvers.  This document
   provides some best-practice recommendations for setting the NSEC3
   parameters.

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1.1.  Requirements notation

   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 in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Recommendation for zone publishers

   The following sections describe recommendations for setting
   parameters for NSEC3 and NSEC3PARAM.

2.1.  Algorithms

   The algorithm field is not discussed by this document.

2.2.  Flags

   The flags field currently contains a single flag, that of the "Opt-
   Out" flag [RFC5155], which specifies whether or not NSEC3 records
   provide proof of non-existence or not.  In general, NSEC3 with the
   Opt-Out flag enabled should only be used in large, highly dynamic
   zones with a small percentage of signed delegations.  Operationally,
   this allows for less signature creations when new delegations are
   inserted into a zone.  This is typically only necessary for extremely
   large registration points providing zone updates faster than real-
   time signing allows.  Smaller zones, or large but relatively static
   zones, are encouraged to use a Flags value of 0 (zero) and take
   advantage of DNSSEC's proof-of-non-existence support.

2.3.  Iterations

   Generally increasing the number of iterations offers little improved
   protections for modern machinery.  Although Section 10.3 of [RFC5155]
   specifies upper bounds for the number hash iterations to use, there
   is no published guidance on good values to select.  Because hashing
   provides only moderate protection, as shown recently in academic
   studies of NSEC3 protected zones (tbd: insert ref), this document
   recommends using an iteration value of 0 (zero).  This leaves the
   creating and verifying hashes with just one application of the
   hashing algorithm.

2.4.  Salt

   Salts add yet another layer of protection against offline, stored
   dictionary attacks by using randomly generated values when creating
   new records.  The length and usage of a salt value has little

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   operational concerns beyond bandwidth requirements for transmitting
   the salt.  Thus, the primary consideration is whether or not there is
   a security benefit to deploying signed zones with salt values.
   Operators may choose to use a salt for this reason, though it should
   be noted that the use of salts doesn't prevent against guess based
   approaches in offline attacks - only against memorization hash based
   lookups.  Thus, the added value is minimal enough that operators may
   wish to deploy zones without a hash value at all.

3.  Recommendation for validating resolvers

   Because there has been a large growth of open (public) DNSSEC
   validating resolvers that are subject to compute resource constraints
   when handling requests from anonymous clients, this document
   recommends that validating resolvers should change their behaviour
   with respect to large iteration values.  Validating resolvers SHOULD
   return a SERVFAIL when processing NSEC3 records with iterations
   larger than 100.  Note that this significantly decreases the
   requirements originally specified in Section 10.3 of [RFC5155].

4.  Security Considerations

   This entire document discusses security considerations with various
   parameters selections of NSEC3 and NSEC3PARAM fields.

5.  Operational Considerations

   This entire document discusses operational considerations with
   various parameters selections of NSEC3 and NSEC3PARAM fields.

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,
              <https://www.rfc-editor.org/info/rfc2119>.

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

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6.2.  Informative References

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Appendix A.  Acknowledgments

   dns-operations discussion participants

Appendix B.  Github Version of this document

   While this document is under development, it can be viewed, tracked,
   issued, pushed with PRs, ... here:

   https://github.com/hardaker/draft-hardaker-dnsop-nsec3-guidance

Authors' Addresses

   Wes Hardaker
   USC/ISI

   Email: ietf@hardakers.net

   Viktor Dukhovni
   Bloomberg, L.P.

   Email: ietf-dane@dukhovni.org

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