NSEC3 Guidance: Guidance for NSEC3 Parameter Settings

Internet Engineering Task Force (IETF)                       W. Hardaker
Request for Comments: 9276                                       USC/ISI
BCP: 236                                                     V. Dukhovni
Updates: 5155                                            Bloomberg, L.P.
Category: Best Current Practice                              August 2022
ISSN: 2070-1721


                 Guidance for NSEC3 Parameter Settings

Abstract

   NSEC3 is a DNSSEC mechanism providing proof of nonexistence by
   asserting that there are no names that exist between two domain names
   within a zone.  Unlike its counterpart NSEC, NSEC3 avoids directly
   disclosing the bounding domain name pairs.  This document provides
   guidance on setting NSEC3 parameters based on recent operational
   deployment experience.  This document updates RFC 5155 with guidance
   about selecting NSEC3 iteration and salt parameters.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9276.

Copyright Notice

   Copyright (c) 2022 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
   (https://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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Notation
   2.  NSEC3 Parameter Value Discussions
     2.1.  Algorithms
     2.2.  Flags
     2.3.  Iterations
     2.4.  Salt
   3.  Recommendations for Deploying and Validating NSEC3 Records
     3.1.  Best Practice for Zone Publishers
     3.2.  Recommendation for Validating Resolvers
     3.3.  Recommendation for Primary and Secondary Relationships
   4.  Security Considerations
   5.  Operational Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Deployment Measurements at Time of Publication
   Appendix B.  Computational Burdens of Processing NSEC3 Iterations
   Acknowledgments
   Authors' Addresses

1.  Introduction

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

   NSEC3 also provides "opt-out support", allowing for blocks of
   unsigned delegations to be covered by a single NSEC3 record.  Use of
   the opt-out feature allows large registries to only sign as many
   NSEC3 records as there are signed DS or other Resource Record sets
   (RRsets) in the zone; with opt-out, unsigned delegations don't
   require additional NSEC3 records.  This sacrifices the tamper-
   resistance of the proof of nonexistence 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, the 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.

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.  NSEC3 Parameter Value Discussions

   The following sections describe the background of the parameters for
   the NSEC3 and NSEC3PARAM RRTYPEs.

2.1.  Algorithms

   The algorithm field is not discussed by this document.  Readers are
   encouraged to read [RFC8624] for guidance about DNSSEC algorithm
   usage.

2.2.  Flags

   The NSEC3PARAM flags field currently contains only reserved and
   unassigned flags.  However, individual NSEC3 records contain the
   "Opt-Out" flag [RFC5155] that specifies whether that NSEC3 record
   provides proof of nonexistence.  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 fewer 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 or when using memory-constrained hardware.  Operators
   considering the use of NSEC3 are advised to carefully weigh the costs
   and benefits of choosing NSEC3 over NSEC.  Smaller zones, or large
   but relatively static zones, are encouraged to not use the opt-opt
   flag and to take advantage of DNSSEC's authenticated denial of
   existence.

2.3.  Iterations

   NSEC3 records are created by first hashing the input domain and then
   repeating that hashing using the same algorithm a number of times
   based on the iteration parameter in the NSEC3PARAM and NSEC3 records.
   The first hash with NSEC3 is typically sufficient to discourage zone
   enumeration performed by "zone walking" an unhashed NSEC chain.

   Note that [RFC5155] describes the Iterations field as follows

   |  The Iterations field defines the number of additional times the
   |  hash function has been performed.

   This means that an NSEC3 record with an Iterations field of 0
   actually requires one hash iteration.

   Only determined parties with significant resources are likely to try
   and uncover hashed values, regardless of the number of additional
   iterations performed.  If an adversary really wants to expend
   significant CPU resources to mount an offline dictionary attack on a
   zone's NSEC3 chain, they'll likely be able to find most of the
   "guessable" names despite any level of additional hashing iterations.

   Most names published in the DNS are rarely secret or unpredictable.
   They are published to be memorable, used and consumed by humans.
   They are often recorded in many other network logs such as email
   logs, certificate transparency logs, web page links, intrusion-
   detection systems, malware scanners, email archives, etc.  Many times
   a simple dictionary of commonly used domain names prefixes (www,
   mail, imap, login, database, etc.) can be used to quickly reveal a
   large number of labels within a zone.  Because of this, there are
   increasing performance costs yet diminishing returns associated with
   applying additional hash iterations beyond the first.

   Although Section 10.3 of [RFC5155] specifies the upper bounds for the
   number of hash iterations to use, there is no published guidance for
   zone owners about good values to select.  Recent academic studies
   have shown that NSEC3 hashing provides only moderate protection
   [GPUNSEC3] [ZONEENUM].

2.4.  Salt

   NSEC3 records provide an additional salt value, which can be combined
   with a Fully Qualified Domain Name (FQDN) to influence the resulting
   hash, but properties of this extra salt are complicated.

   In cryptography, salts generally add a layer of protection against
   offline, stored dictionary attacks by combining the value to be
   hashed with a unique "salt" value.  This prevents adversaries from
   building up and remembering a single dictionary of values that can
   translate a hash output back to the value that it was derived from.

   In the case of DNS, the situation is different because the hashed
   names placed in NSEC3 records are always implicitly "salted" by
   hashing the FQDN from each zone.  Thus, no single pre-computed table
   works to speed up dictionary attacks against multiple target zones.
   An attacker is always required to compute a complete dictionary per
   zone, which is expensive in both storage and CPU time.

   To understand the role of the additional NSEC3 salt field, we have to
   consider how a typical zone walking attack works.  Typically, the
   attack has two phases: online and offline.  In the online phase, an
   attacker "walks the zone" by enumerating (almost) all hashes listed
   in NSEC3 records and storing them for the offline phase.  Then, in
   the offline cracking phase, the attacker attempts to crack the
   underlying hash.  In this phase, the additional salt value raises the
   cost of the attack only if the salt value changes during the online
   phase of the attack.  In other words, an additional, constant salt
   value does not change the cost of the attack.

   Changing a zone's salt value requires the construction of a complete
   new NSEC3 chain.  This is true both when re-signing the entire zone
   at once and when incrementally signing it in the background where the
   new salt is only activated once every name in the chain has been
   completed.  As a result, re-salting is a very complex operation, with
   significant CPU time, memory, and bandwidth consumption.  This makes
   very frequent re-salting impractical and renders the additional salt
   field functionally useless.

3.  Recommendations for Deploying and Validating NSEC3 Records

   The following subsections describe recommendations for the different
   operating realms within the DNS.

3.1.  Best Practice for Zone Publishers

   First, if the operational or security features of NSEC3 are not
   needed, then NSEC SHOULD be used in preference to NSEC3.  NSEC3
   requires greater computational power (see Appendix B) for both
   authoritative servers and validating clients.  Specifically, there is
   a nontrivial complexity in finding matching NSEC3 records to randomly
   generated prefixes within a DNS zone.  NSEC mitigates this concern.
   If NSEC3 must be used, then an iterations count of 0 MUST be used to
   alleviate computational burdens.  Note that extra iteration counts
   other than 0 increase the impact of CPU-exhausting DoS attacks, and
   also increase the risk of interoperability problems.

   Note that deploying NSEC with minimally covering NSEC records
   [RFC4470] also incurs a cost, and zone owners should measure the
   computational difference in deploying either [RFC4470] or NSEC3.

   In short, for all zones, the recommended NSEC3 parameters are as
   shown below:

   ; SHA-1, no extra iterations, empty salt:
   ;
   bcp.example. IN NSEC3PARAM 1 0 0 -

   For small zones, the use of opt-out-based NSEC3 records is NOT
   RECOMMENDED.

   For very large and sparsely signed zones, where the majority of the
   records are insecure delegations, opt-out MAY be used.

   Operators SHOULD NOT use a salt by indicating a zero-length salt
   value instead (represented as a "-" in the presentation format).

   If salts are used, note that since the NSEC3PARAM RR is not used by
   validating resolvers (see Section 4 of [RFC5155]), the iterations and
   salt parameters can be changed without the need to wait for RRsets to
   expire from caches.  A complete new NSEC3 chain needs to be
   constructed and the full zone needs to be re-signed.

3.2.  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 reduce their iteration count
   limits over time.  Specifically, validating resolver operators and
   validating resolver software implementers are encouraged to continue
   evaluating NSEC3 iteration count deployment trends and lower their
   acceptable iteration limits over time.  Because treating a high
   iterations count as insecure leaves zones subject to attack,
   validating resolver operators and validating resolver software
   implementers are further encouraged to lower their default limit for
   returning SERVFAIL when processing NSEC3 parameters containing large
   iteration count values.  See Appendix A for measurements taken near
   the time of publication of this document and potential starting
   points.

   Validating resolvers MAY return an insecure response to their clients
   when processing NSEC3 records with iterations larger than 0.  Note
   also that a validating resolver returning an insecure response MUST
   still validate the signature over the NSEC3 record to ensure the
   iteration count was not altered since record publication (see
   Section 10.3 of [RFC5155]).

   Validating resolvers MAY also return a SERVFAIL response when
   processing NSEC3 records with iterations larger than 0.  Validating
   resolvers MAY choose to ignore authoritative server responses with
   iteration counts greater than 0, which will likely result in
   returning a SERVFAIL to the client when no acceptable responses are
   received from authoritative servers.

   Validating resolvers returning an insecure or SERVFAIL answer to
   their client after receiving and validating an unsupported NSEC3
   parameter from the authoritative server(s) SHOULD return an Extended
   DNS Error (EDE) [RFC8914] EDNS0 option of value 27.  Validating
   resolvers that choose to ignore a response with an unsupported
   iteration count (and that do not validate the signature) MUST NOT
   return this EDE option.

   Note that this specification updates [RFC5155] by significantly
   decreasing the requirements originally specified in Section 10.3 of
   [RFC5155].  See the Security Considerations (Section 4) for arguments
   on how to handle responses with non-zero iteration count.

3.3.  Recommendation for Primary and Secondary Relationships

   Primary and secondary authoritative servers for a zone that are not
   being run by the same operational staff and/or using the same
   software and configuration must take into account the potential
   differences in NSEC3 iteration support.

   Operators of secondary services should advertise the parameter limits
   that their servers support.  Correspondingly, operators of primary
   servers need to ensure that their secondaries support the NSEC3
   parameters they expect to use in their zones.  To ensure reliability,
   after primaries change their iteration counts, they should query
   their secondaries with known nonexistent labels to verify the
   secondary servers are responding as expected.

4.  Security Considerations

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

   The point where a validating resolver returns insecure versus the
   point where it returns SERVFAIL must be considered carefully.
   Specifically, when a validating resolver treats a zone as insecure
   above a particular value (say 100) and returns SERVFAIL above a
   higher point (say 500), it leaves the zone subject to attacker-in-
   the-middle attacks as if it were unsigned between these values.
   Thus, validating resolver operators and software implementers SHOULD
   set the point above which a zone is treated as insecure for certain
   values of NSEC3 iterations to the same as the point where a
   validating resolver begins returning SERVFAIL.

5.  Operational Considerations

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

6.  IANA Considerations

   IANA has allocated the following code in the First Come First Served
   range [RFC8126] of the "Extended DNS Error Codes" registry within the
   "Domain Name System (DNS) Parameters" registry:

   INFO-CODE:  27
   Purpose:  Unsupported NSEC3 iterations value
   Reference:  RFC 9276

7.  References

7.1.  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>.

   [RFC4470]  Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
              and DNSSEC On-line Signing", RFC 4470,
              DOI 10.17487/RFC4470, April 2006,
              <https://www.rfc-editor.org/info/rfc4470>.

   [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>.

   [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>.

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,
              <https://www.rfc-editor.org/info/rfc8914>.

7.2.  Informative References

   [GPUNSEC3] Wander, M., Schwittmann, L., Boelmann, C., and T. Weis,
              "GPU-Based NSEC3 Hash Breaking", DOI 10.1109/NCA.2014.27,
              August 2014, <https://doi.org/10.1109/NCA.2014.27>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8624]  Wouters, P. and O. Sury, "Algorithm Implementation
              Requirements and Usage Guidance for DNSSEC", RFC 8624,
              DOI 10.17487/RFC8624, June 2019,
              <https://www.rfc-editor.org/info/rfc8624>.

   [ZONEENUM] Wang, Z., Xiao, L., and R. Wang, "An efficient DNSSEC zone
              enumeration algorithm", DOI 10.2495/MIIT130591, April
              2014, <https://doi.org/10.2495/MIIT130591>.

Appendix A.  Deployment Measurements at Time of Publication

   At the time of publication, setting an upper limit of 100 iterations
   for treating a zone as insecure is interoperable without significant
   problems, but at the same time still enables CPU-exhausting DoS
   attacks.

   At the time of publication, returning SERVFAIL beyond 500 iterations
   appears to be interoperable without significant problems.

Appendix B.  Computational Burdens of Processing NSEC3 Iterations

   The queries per second (QPS) of authoritative servers will decrease
   due to computational overhead when processing DNS requests for zones
   containing higher NSEC3 iteration counts.  The table below shows the
   drop in QPS for various iteration counts.

               +============+=============================+
               | Iterations | QPS [% of 0 Iterations QPS] |
               +============+=============================+
               | 0          | 100%                        |
               +------------+-----------------------------+
               | 10         | 89%                         |
               +------------+-----------------------------+
               | 20         | 82%                         |
               +------------+-----------------------------+
               | 50         | 64%                         |
               +------------+-----------------------------+
               | 100        | 47%                         |
               +------------+-----------------------------+
               | 150        | 38%                         |
               +------------+-----------------------------+

                     Table 1: Drop in QPS for Various
                             Iteration Counts

Acknowledgments

   The authors would like to thank the participants in the dns-
   operations discussion, which took place on mattermost hosted by DNS-
   OARC.

   Additionally, the following people contributed text or review
   comments to this document:

   *  Vladimir Cunat

   *  Tony Finch

   *  Paul Hoffman

   *  Warren Kumari

   *  Alexander Mayrhofer

   *  Matthijs Mekking

   *  Florian Obser

   *  Petr Spacek

   *  Paul Vixie

   *  Tim Wicinski

Authors' Addresses

   Wes Hardaker
   USC/ISI
   Email: ietf@hardakers.net


   Viktor Dukhovni
   Bloomberg, L.P.
   Email: ietf-dane@dukhovni.org