Guidance for NSEC3 Parameter Settings

Stream: Internet Engineering Task Force (IETF)

RFC: 9276

BCP: 236

Updates: 5155

Category: Best Current Practice

Published: August 2022

ISSN: 2070-1721

Authors: W. Hardaker V. Dukhovni
USC/ISI Bloomberg, L.P.

RFC 9276
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 howto 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.

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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
. Security Considerations
. Operational Considerations

. IANA Considerations

I Aw Bb

. 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

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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

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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 reveala
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.

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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:

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; SHA-1, no extra iterations, empty salt:
bcp.example. IN NSEC3PARAM 1 8 @ -

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] EDNSO 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.

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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 howto 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

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7. References

7.1. Normative References

[RFC2119]

[RFC4035]

[RFC4470]

[RFC5155]

[RFC8174]

[RFC8914]

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>.

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>.

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>.

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>.

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>.

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]

[RFC8126]

[RFC8624]

[ZONEENUM]

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>.

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>.

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>.

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>.

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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[% of0 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:

e Vladimir Cunat
e Tony Finch

e Paul Hoffman

e Warren Kumari

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Alexander Mayrhofer
e Matthijs Mekking

e Florian Obser

e Petr Spacek

e Paul Vixie

e Tim Wicinski

Authors' Addresses

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

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

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