Guidance for NSEC3 parameter settings
draft-ietf-dnsop-nsec3-guidance-08

Network Working Group                                        W. Hardaker
Internet-Draft                                                   USC/ISI
Intended status: Best Current Practice                       V. Dukhovni
Expires: 18 October 2022                                 Bloomberg, L.P.
                                                           16 April 2022

                 Guidance for NSEC3 parameter settings
                   draft-ietf-dnsop-nsec3-guidance-08

Abstract

   NSEC3 is a DNSSEC mechanism providing proof of non-existence 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.

Status of This Memo

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Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   3
   2.  NSEC3 Parameter Value Considerations  . . . . . . . . . . . .   3
     2.1.  Algorithms  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Flags . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Iterations  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.4.  Salt  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Recommendations for Deploying and Validating NSEC3 Records  .   5
     3.1.  Best-practice for Zone Publishers . . . . . . . . . . . .   6
     3.2.  Recommendation for Validating Resolvers . . . . . . . . .   6
     3.3.  Recommendation for Primary / Secondary Relationships  . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Deployment measurements at time of publication . . .   9
   Appendix B.  Computational burdens of processing NSEC3
           iterations  . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix C.  Acknowledgments  . . . . . . . . . . . . . . . . . .  10
   Appendix D.  Github Version of This Document  . . . . . . . . . .  10
   Appendix E.  Implementation Notes . . . . . . . . . . . . . . . .  11
     E.1.  OpenDNSSEC  . . . . . . . . . . . . . . . . . . . . . . .  11
     E.2.  PowerDNS  . . . . . . . . . . . . . . . . . . . . . . . .  11
     E.3.  Knot DNS and Knot Resolver  . . . . . . . . . . . . . . .  11
     E.4.  Google Public DNS Resolver  . . . . . . . . . . . . . . .  11
     E.5.  Google Cloud DNS  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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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 (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.  Use of
   the opt-out feature allows 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.

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 Considerations

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

2.1.  Algorithms

   The algorithm field is not discussed by this document.

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2.2.  Flags

   The NSEC3PARAM flags field currently contains no flags, but
   individual NSEC3 records contain the "Opt-Out" flag [RFC5155], which
   specifies whether or not that NSEC3 record provides 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
   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.  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

   NSEC3 records are created by first hashing the input domain and then
   repeating that hashing algorithm a number of times based on the
   iteration parameter in the NSEC3PARM and NSEC3 records.  The first
   hash is typically sufficient to discourage zone enumeration performed
   by "zone walking" an NSEC or NSEC3 chain.  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, ftp,
   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 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].

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2.4.  Salt

   NSEC3 records provide an additional salt value, which can be combined
   with an 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 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 fully-qualified domain name 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 resigning 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.

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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 both 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 are encouraged to forgo using a salt entirely by using 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 [RFC5155] section 4), 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 zone resigned.

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 change their behavior with
   respect to large iteration values.  Specifically, validating resolver
   operators and validating resolver software implementers are

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   encouraged to continue evaluating NSEC3 iteration count deployments
   and lower their default acceptable limits over time.  Similarly,
   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 and acceptable limit for returning SERVFAIL when processing
   NSEC3 parameters containing large iteration count values.  See
   Appendix A for measurements taken near the time of publication 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
   [RFC5155] section 10.3).

   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 (RFC EDITOR: TBD).
   Validating resolvers that choose to ignore a response with an
   unsupported iteration count (and 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 for arguments on how to
   handle responses with non-zero iteration count.

3.3.  Recommendation for Primary / 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,

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   after primaries change their iteration counts, they should query
   their secondaries with known non-existent labels to verify the
   secondary servers are responding as expected.

4.  Security Considerations

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

   The point where a validating resolver returns insecure vs 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 was 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 counts to the same as the point where a
   validating resolver begins returning SERVFAIL.

5.  Operational Considerations

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

6.  IANA Considerations

   This document requests a new allocation in the "Extended DNS Error
   Codes" of the "Domain Name System (DNS) Parameters" registration
   table with the following characteristics:

   *  INFO-CODE: (RFC EDITOR: TBD)

   *  Purpose: Unsupported NSEC3 iterations value

   *  Reference: (RFC EDITOR: this document)

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>.

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   [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>.

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,
              2014, <https://doi.org/10.1109/NCA.2014.27>.

   [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>.

   [ZONEENUM] Wang, Z., Xiao, L., and R. Wang, "An efficient DNSSEC zone
              enumeration algorithm", n.d..

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 (Appendix C)
   below shows the drop in QPS for various iteration counts.

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

Appendix C.  Acknowledgments

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

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

   *  Vladimir &#268;unat

   *  Tony Finch

   *  Paul Hoffman

   *  Warren Kumari

   *  Alexander Mayrhofer

   *  Matthijs Mekking

   *  Florian Obser

   *  Petr &#352;pa&#269;ek

   *  Paul Vixie

   *  Tim Wicinski

Appendix D.  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

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Appendix E.  Implementation Notes

   The following implementations have implemented the guidance in this
   document.  They have graciously provided notes about the details of
   their implementation below.

E.1.  OpenDNSSEC

   The OpenDNSSEC configuration checking utility will alert the user
   about nsec3 iteration values larger than 100.

E.2.  PowerDNS

   PowerDNS 4.5.2 changed the default value of nsec3-max-iterations to
   150.

E.3.  Knot DNS and Knot Resolver

   Knot DNS 3.0.6 warns when signing with more than 20 NSEC3 iterations.
   Knot Resolver 5.3.1 treats NSEC3 iterations above 150 as insecure.

E.4.  Google Public DNS Resolver

   Google Public DNS treats NSEC3 iterations above 100 as insecure since
   September 2021.

E.5.  Google Cloud DNS

   Google Cloud DNS uses 1 iteration and 64-bits of fixed random salt
   for all zones using NSEC3.  These parameters cannot be adjusted by
   users.

Authors' Addresses

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

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

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