rfc9276.original   rfc9276.txt 
Network Working Group W. Hardaker Internet Engineering Task Force (IETF) W. Hardaker
Internet-Draft USC/ISI Request for Comments: 9276 USC/ISI
Updates: 5155 (if approved) V. Dukhovni BCP: 236 V. Dukhovni
Intended status: Best Current Practice Bloomberg, L.P. Updates: 5155 Bloomberg, L.P.
Expires: 26 November 2022 25 May 2022 Category: Best Current Practice August 2022
ISSN: 2070-1721
Guidance for NSEC3 parameter settings Guidance for NSEC3 Parameter Settings
draft-ietf-dnsop-nsec3-guidance-10
Abstract Abstract
NSEC3 is a DNSSEC mechanism providing proof of non-existence by NSEC3 is a DNSSEC mechanism providing proof of nonexistence by
asserting that there are no names that exist between two domain names asserting that there are no names that exist between two domain names
within a zone. Unlike its counterpart NSEC, NSEC3 avoids directly within a zone. Unlike its counterpart NSEC, NSEC3 avoids directly
disclosing the bounding domain name pairs. This document provides disclosing the bounding domain name pairs. This document provides
guidance on setting NSEC3 parameters based on recent operational guidance on setting NSEC3 parameters based on recent operational
deployment experience. This document updates [RFC5155] with guidance deployment experience. This document updates RFC 5155 with guidance
about selecting NSEC3 iteration and salt parameters. about selecting NSEC3 iteration and salt parameters.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This memo documents an Internet Best Current Practice.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
This Internet-Draft will expire on 26 November 2022. 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 Notice
Copyright (c) 2022 IETF Trust and the persons identified as the Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Notation
2. NSEC3 Parameter Value Discussions . . . . . . . . . . . . . . 3 2. NSEC3 Parameter Value Discussions
2.1. Algorithms . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Algorithms
2.2. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Flags
2.3. Iterations . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Iterations
2.4. Salt . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Salt
3. Recommendations for Deploying and Validating NSEC3 Records . 6 3. Recommendations for Deploying and Validating NSEC3 Records
3.1. Best-practice for Zone Publishers . . . . . . . . . . . . 6 3.1. Best Practice for Zone Publishers
3.2. Recommendation for Validating Resolvers . . . . . . . . . 7 3.2. Recommendation for Validating Resolvers
3.3. Recommendation for Primary / Secondary Relationships . . 8 3.3. Recommendation for Primary and Secondary Relationships
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations
5. Operational Considerations . . . . . . . . . . . . . . . . . 8 5. Operational Considerations
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 7. References
7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.1. Normative References
7.2. Informative References . . . . . . . . . . . . . . . . . 9 7.2. Informative References
Appendix A. Deployment measurements at time of publication . . . 10 Appendix A. Deployment Measurements at Time of Publication
Appendix B. Computational burdens of processing NSEC3 Appendix B. Computational Burdens of Processing NSEC3 Iterations
iterations . . . . . . . . . . . . . . . . . . . . . . . 10 Acknowledgments
Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . 10 Authors' Addresses
Appendix D. GitHub Version of This Document . . . . . . . . . . 11
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 . . . . . . . . . . . . . . . 12
E.5. Google Cloud DNS . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
As with NSEC [RFC4035], NSEC3 [RFC5155] provides proof of non- As with NSEC [RFC4035], NSEC3 [RFC5155] provides proof of
existence that consists of signed DNS records establishing the non- nonexistence that consists of signed DNS records establishing the
existence of a given name or associated Resource Record Type (RRTYPE) nonexistence of a given name or associated Resource Record Type
in a DNSSEC [RFC4035] signed zone. In the case of NSEC3, however, (RRTYPE) in a DNSSEC-signed zone [RFC4035]. However, in the case of
the names of valid nodes in the zone are obfuscated through (possibly NSEC3, the names of valid nodes in the zone are obfuscated through
multiple iterations of) hashing (currently only SHA-1 is in use on (possibly multiple iterations of) hashing (currently only SHA-1 is in
the Internet). use on the Internet).
NSEC3 also provides "opt-out support", allowing for blocks of NSEC3 also provides "opt-out support", allowing for blocks of
unsigned delegations to be covered by a single NSEC3 record. Use 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 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; NSEC3 records as there are signed DS or other Resource Record sets
with opt-out, unsigned delegations don't require additional NSEC3 (RRsets) in the zone; with opt-out, unsigned delegations don't
records. This sacrifices the tamper-resistance proof of non- require additional NSEC3 records. This sacrifices the tamper-
existence offered by NSEC3 in order to reduce memory and CPU resistance of the proof of nonexistence offered by NSEC3 in order to
overheads. reduce memory and CPU overheads.
NSEC3 records have a number of tunable parameters that are specified NSEC3 records have a number of tunable parameters that are specified
via an NSEC3PARAM record at the zone apex. These parameters are the via an NSEC3PARAM record at the zone apex. These parameters are the
hash algorithm, processing flags, the number of hash iterations and hash algorithm, the processing flags, the number of hash iterations,
the salt. Each of these has security and operational considerations and the salt. Each of these has security and operational
that impact both zone owners and validating resolvers. This document considerations that impact both zone owners and validating resolvers.
provides some best-practice recommendations for setting the NSEC3 This document provides some best-practice recommendations for setting
parameters. the NSEC3 parameters.
1.1. Requirements Notation 1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. NSEC3 Parameter Value Discussions 2. NSEC3 Parameter Value Discussions
The following sections describes the background of the parameters for The following sections describe the background of the parameters for
the NSEC3 and NSEC3PARAM resource record types. the NSEC3 and NSEC3PARAM RRTYPEs.
2.1. Algorithms 2.1. Algorithms
The algorithm field is not discussed by this document. Readers are The algorithm field is not discussed by this document. Readers are
encouraged to read [RFC8624] for guidance about DNSSEC algorithm encouraged to read [RFC8624] for guidance about DNSSEC algorithm
usage. usage.
2.2. Flags 2.2. Flags
The NSEC3PARAM flags field currently contains only reserved and The NSEC3PARAM flags field currently contains only reserved and
unassigned flags. Individual NSEC3 records, however, contain the unassigned flags. However, individual NSEC3 records contain the
"Opt-Out" flag [RFC5155], which specifies whether that NSEC3 record "Opt-Out" flag [RFC5155] that specifies whether that NSEC3 record
provides proof of non-existence. In general, NSEC3 with the Opt-Out provides proof of nonexistence. In general, NSEC3 with the Opt-Out
flag enabled should only be used in large, highly dynamic zones with flag enabled should only be used in large, highly dynamic zones with
a small percentage of signed delegations. Operationally, this allows a small percentage of signed delegations. Operationally, this allows
for fewer signature creations when new delegations are inserted into for fewer signature creations when new delegations are inserted into
a zone. This is typically only necessary for extremely large a zone. This is typically only necessary for extremely large
registration points providing zone updates faster than real-time registration points providing zone updates faster than real-time
signing allows or when using memory-constrained hardware. Operators signing allows or when using memory-constrained hardware. Operators
considering the use of NSEC3 are advised to fully test their zones considering the use of NSEC3 are advised to carefully weigh the costs
deployment architectures and authoritative servers under both regular and benefits of choosing NSEC3 over NSEC. Smaller zones, or large
operational loads to determine the tradeoffs using NSEC3 instead of but relatively static zones, are encouraged to not use the opt-opt
NSEC. Smaller zones, or large but relatively static zones, are flag and to take advantage of DNSSEC's authenticated denial of
encouraged to not use a the opt-opt flag and to take advantage of existence.
DNSSEC's proof-of-non-existence support.
2.3. Iterations 2.3. Iterations
NSEC3 records are created by first hashing the input domain and then NSEC3 records are created by first hashing the input domain and then
repeating that hashing using the same algorithm a number of times repeating that hashing using the same algorithm a number of times
based on the iteration parameter in the NSEC3PARM and NSEC3 records. based on the iteration parameter in the NSEC3PARAM and NSEC3 records.
The first hash with NSEC3 is typically sufficient to discourage zone The first hash with NSEC3 is typically sufficient to discourage zone
enumeration performed by "zone walking" an unhashed NSEC chain. enumeration performed by "zone walking" an unhashed NSEC chain.
Note that [RFC5155] describes the Iterations field to be "The Note that [RFC5155] describes the Iterations field as follows
Iterations field defines the number of additional times the hash
function has been performed." This means that an NSEC3 record with | The Iterations field defines the number of additional times the
an Iterations field of 0 actually requires one hash iteration. | 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 Only determined parties with significant resources are likely to try
and uncover hashed values, regardless of the number of additional and uncover hashed values, regardless of the number of additional
iterations performed. If an adversary really wants to expend iterations performed. If an adversary really wants to expend
significant CPU resources to mount an offline dictionary attack on a 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 zone's NSEC3 chain, they'll likely be able to find most of the
"guessable" names despite any level of additional hashing iterations. "guessable" names despite any level of additional hashing iterations.
Most names published in the DNS are rarely secret or unpredictable. Most names published in the DNS are rarely secret or unpredictable.
They are published to be memorable, used and consumed by humans. They are published to be memorable, used and consumed by humans.
They are often recorded in many other network logs such as email They are often recorded in many other network logs such as email
logs, certificate transparency logs, web page links, intrusion logs, certificate transparency logs, web page links, intrusion-
detection systems, malware scanners, email archives, etc. Many times detection systems, malware scanners, email archives, etc. Many times
a simple dictionary of commonly used domain names prefixes (www, a simple dictionary of commonly used domain names prefixes (www,
mail, imap, login, database, etc.) can be used to quickly reveal a mail, imap, login, database, etc.) can be used to quickly reveal a
large number of labels within a zone. Because of this, there are large number of labels within a zone. Because of this, there are
increasing performance costs yet diminishing returns associated with increasing performance costs yet diminishing returns associated with
applying additional hash iterations beyond the first. applying additional hash iterations beyond the first.
Although Section 10.3 of [RFC5155] specifies upper bounds for the Although Section 10.3 of [RFC5155] specifies the upper bounds for the
number of hash iterations to use, there is no published guidance for number of hash iterations to use, there is no published guidance for
zone owners about good values to select. Recent academic studies zone owners about good values to select. Recent academic studies
have shown that NSEC3 hashing provides only moderate protection have shown that NSEC3 hashing provides only moderate protection
[GPUNSEC3][ZONEENUM]. [GPUNSEC3] [ZONEENUM].
2.4. Salt 2.4. Salt
NSEC3 records provide an additional salt value, which can be combined NSEC3 records provide an additional salt value, which can be combined
with an FQDN to influence the resulting hash, but properties of this with a Fully Qualified Domain Name (FQDN) to influence the resulting
extra salt are complicated. hash, but properties of this extra salt are complicated.
In cryptography, salts generally add a layer of protection against In cryptography, salts generally add a layer of protection against
offline, stored dictionary attacks by combining the value to be offline, stored dictionary attacks by combining the value to be
hashed with a unique "salt" value. This prevents adversaries from hashed with a unique "salt" value. This prevents adversaries from
building up and remembering a single dictionary of values that can building up and remembering a single dictionary of values that can
translate a hash output back to the value that it derived from. 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 In the case of DNS, the situation is different because the hashed
names placed in NSEC3 records are always implicitly "salted" by names placed in NSEC3 records are always implicitly "salted" by
hashing the fully-qualified domain name from each zone. Thus, no hashing the FQDN from each zone. Thus, no single pre-computed table
single pre-computed table works to speed up dictionary attacks works to speed up dictionary attacks against multiple target zones.
against multiple target zones. An attacker is always required to An attacker is always required to compute a complete dictionary per
compute a complete dictionary per zone, which is expensive in both zone, which is expensive in both storage and CPU time.
storage and CPU time.
To understand the role of the additional NSEC3 salt field, we have to To understand the role of the additional NSEC3 salt field, we have to
consider how a typical zone walking attack works. Typically, the consider how a typical zone walking attack works. Typically, the
attack has two phases - online and offline. In the online phase, an attack has two phases: online and offline. In the online phase, an
attacker "walks the zone" by enumerating (almost) all hashes listed attacker "walks the zone" by enumerating (almost) all hashes listed
in NSEC3 records and storing them for the offline phase. Then, in in NSEC3 records and storing them for the offline phase. Then, in
the offline cracking phase, the attacker attempts to crack the the offline cracking phase, the attacker attempts to crack the
underlying hash. In this phase, the additional salt value raises 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 cost of the attack only if the salt value changes during the online
phase of the attack. In other words, an additional, constant salt phase of the attack. In other words, an additional, constant salt
value does not change the cost of the attack. value does not change the cost of the attack.
Changing a zone's salt value requires the construction of a complete 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 new NSEC3 chain. This is true both when re-signing the entire zone
at once, and when incrementally signing it in the background where at once and when incrementally signing it in the background where the
the new salt is only activated once every name in the chain has been 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 completed. As a result, re-salting is a very complex operation, with
significant CPU time, memory, and bandwidth consumption. This makes significant CPU time, memory, and bandwidth consumption. This makes
very frequent re-salting impractical, and renders the additional salt very frequent re-salting impractical and renders the additional salt
field functionally useless. field functionally useless.
3. Recommendations for Deploying and Validating NSEC3 Records 3. Recommendations for Deploying and Validating NSEC3 Records
The following subsections describe recommendations for the different The following subsections describe recommendations for the different
operating realms within the DNS. operating realms within the DNS.
3.1. Best-practice for Zone Publishers 3.1. Best Practice for Zone Publishers
First, if the operational or security features of NSEC3 are not First, if the operational or security features of NSEC3 are not
needed, then NSEC SHOULD be used in preference to NSEC3. NSEC3 needed, then NSEC SHOULD be used in preference to NSEC3. NSEC3
requires greater computational power (see Appendix B) for both requires greater computational power (see Appendix B) for both
authoritative servers and validating clients. Specifically, there is authoritative servers and validating clients. Specifically, there is
a nontrivial complexity in finding matching NSEC3 records to randomly a nontrivial complexity in finding matching NSEC3 records to randomly
generated prefixes within a DNS zone. NSEC mitigates this concern. 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 If NSEC3 must be used, then an iterations count of 0 MUST be used to
alleviate computational burdens. Note that extra iteration counts alleviate computational burdens. Note that extra iteration counts
other than 0 increase the impact of CPU-exhausting DoS attacks, and other than 0 increase the impact of CPU-exhausting DoS attacks, and
also increase the risk of interoperability problems. also increase the risk of interoperability problems.
Note that deploying NSEC with minimally covering NSEC records Note that deploying NSEC with minimally covering NSEC records
[RFC4470] also incurs a cost, and zone owners should measure the [RFC4470] also incurs a cost, and zone owners should measure the
computational difference in deploying either RFC4470 or NSEC3. computational difference in deploying either [RFC4470] or NSEC3.
In short, for all zones, the recommended NSEC3 parameters are as In short, for all zones, the recommended NSEC3 parameters are as
shown below: shown below:
; SHA-1, no extra iterations, empty salt: ; SHA-1, no extra iterations, empty salt:
; ;
bcp.example. IN NSEC3PARAM 1 0 0 - bcp.example. IN NSEC3PARAM 1 0 0 -
For small zones, the use of opt-out based NSEC3 records is NOT For small zones, the use of opt-out-based NSEC3 records is NOT
RECOMMENDED. RECOMMENDED.
For very large and sparsely signed zones, where the majority of the For very large and sparsely signed zones, where the majority of the
records are insecure delegations, opt-out MAY be used. records are insecure delegations, opt-out MAY be used.
Operators SHOULD NOT use a salt by indicating a zero-length salt Operators SHOULD NOT use a salt by indicating a zero-length salt
value instead (represented as a "-" in the presentation format). value instead (represented as a "-" in the presentation format).
If salts are used, note that since the NSEC3PARAM RR is not used by If salts are used, note that since the NSEC3PARAM RR is not used by
validating resolvers (see [RFC5155] section 4), the iterations and validating resolvers (see Section 4 of [RFC5155]), the iterations and
salt parameters can be changed without the need to wait for RRsets to salt parameters can be changed without the need to wait for RRsets to
expire from caches. A complete new NSEC3 chain needs to be expire from caches. A complete new NSEC3 chain needs to be
constructed and the full zone needs to be re-signed. constructed and the full zone needs to be re-signed.
3.2. Recommendation for Validating Resolvers 3.2. Recommendation for Validating Resolvers
Because there has been a large growth of open (public) DNSSEC Because there has been a large growth of open (public) DNSSEC
validating resolvers that are subject to compute resource constraints validating resolvers that are subject to compute resource constraints
when handling requests from anonymous clients, this document when handling requests from anonymous clients, this document
recommends that validating resolvers change their behavior with recommends that validating resolvers reduce their iteration count
respect to large iteration values. Specifically, validating resolver limits over time. Specifically, validating resolver operators and
operators and validating resolver software implementers are validating resolver software implementers are encouraged to continue
encouraged to continue evaluating NSEC3 iteration count deployments evaluating NSEC3 iteration count deployment trends and lower their
but lower their default acceptable limits over time. Similarly, acceptable iteration limits over time. Because treating a high
because treating a high iterations count as insecure leaves zones iterations count as insecure leaves zones subject to attack,
subject to attack, validating resolver operators and validating validating resolver operators and validating resolver software
resolver software implementers are further encouraged to lower their implementers are further encouraged to lower their default limit for
default and acceptable limit for returning SERVFAIL when processing returning SERVFAIL when processing NSEC3 parameters containing large
NSEC3 parameters containing large iteration count values. See iteration count values. See Appendix A for measurements taken near
Appendix A for measurements taken near the time of publication of the time of publication of this document and potential starting
this document and potential starting points. points.
Validating resolvers MAY return an insecure response to their clients Validating resolvers MAY return an insecure response to their clients
when processing NSEC3 records with iterations larger than 0. Note when processing NSEC3 records with iterations larger than 0. Note
also that a validating resolver returning an insecure response MUST also that a validating resolver returning an insecure response MUST
still validate the signature over the NSEC3 record to ensure the still validate the signature over the NSEC3 record to ensure the
iteration count was not altered since record publication (see iteration count was not altered since record publication (see
[RFC5155] section 10.3). Section 10.3 of [RFC5155]).
Validating resolvers MAY also return a SERVFAIL response when Validating resolvers MAY also return a SERVFAIL response when
processing NSEC3 records with iterations larger than 0. Validating processing NSEC3 records with iterations larger than 0. Validating
resolvers MAY choose to ignore authoritative server responses with resolvers MAY choose to ignore authoritative server responses with
iteration counts greater than 0, which will likely result in iteration counts greater than 0, which will likely result in
returning a SERVFAIL to the client when no acceptable responses are returning a SERVFAIL to the client when no acceptable responses are
received from authoritative servers. received from authoritative servers.
Validating resolvers returning an insecure or SERVFAIL answer to Validating resolvers returning an insecure or SERVFAIL answer to
their client after receiving and validating an unsupported NSEC3 their client after receiving and validating an unsupported NSEC3
parameter from the authoritative server(s) SHOULD return an Extended parameter from the authoritative server(s) SHOULD return an Extended
DNS Error (EDE) [RFC8914] EDNS0 option of value (RFC EDITOR: TBD). DNS Error (EDE) [RFC8914] EDNS0 option of value 27. Validating
Validating resolvers that choose to ignore a response with an resolvers that choose to ignore a response with an unsupported
unsupported iteration count (and do not validate the signature) MUST iteration count (and that do not validate the signature) MUST NOT
NOT return this EDE option. return this EDE option.
Note that this specification updates [RFC5155] by significantly Note that this specification updates [RFC5155] by significantly
decreasing the requirements originally specified in Section 10.3 of decreasing the requirements originally specified in Section 10.3 of
[RFC5155]. See the Security Considerations for arguments on how to [RFC5155]. See the Security Considerations (Section 4) for arguments
handle responses with non-zero iteration count. on how to handle responses with non-zero iteration count.
3.3. Recommendation for Primary / Secondary Relationships 3.3. Recommendation for Primary and Secondary Relationships
Primary and secondary authoritative servers for a zone that are not Primary and secondary authoritative servers for a zone that are not
being run by the same operational staff and/or using the same being run by the same operational staff and/or using the same
software and configuration must take into account the potential software and configuration must take into account the potential
differences in NSEC3 iteration support. differences in NSEC3 iteration support.
Operators of secondary services should advertise the parameter limits Operators of secondary services should advertise the parameter limits
that their servers support. Correspondingly, operators of primary that their servers support. Correspondingly, operators of primary
servers need to ensure that their secondaries support the NSEC3 servers need to ensure that their secondaries support the NSEC3
parameters they expect to use in their zones. To ensure reliability, parameters they expect to use in their zones. To ensure reliability,
after primaries change their iteration counts, they should query after primaries change their iteration counts, they should query
their secondaries with known non-existent labels to verify the their secondaries with known nonexistent labels to verify the
secondary servers are responding as expected. secondary servers are responding as expected.
4. Security Considerations 4. Security Considerations
This entire document discusses security considerations with various This entire document discusses security considerations with various
parameters selections of NSEC3 and NSEC3PARAM fields. parameter selections of NSEC3 and NSEC3PARAM fields.
The point where a validating resolver returns insecure vs the point The point where a validating resolver returns insecure versus the
where it returns SERVFAIL must be considered carefully. point where it returns SERVFAIL must be considered carefully.
Specifically, when a validating resolver treats a zone as insecure Specifically, when a validating resolver treats a zone as insecure
above a particular value (say 100) and returns SERVFAIL above a above a particular value (say 100) and returns SERVFAIL above a
higher point (say 500), it leaves the zone subject to attacker-in- higher point (say 500), it leaves the zone subject to attacker-in-
the-middle attacks as if it was unsigned between these values. Thus, the-middle attacks as if it were unsigned between these values.
validating resolver operators and software implementers SHOULD set Thus, validating resolver operators and software implementers SHOULD
the point above which a zone is treated as insecure for certain 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 values of NSEC3 iterations to the same as the point where a
validating resolver begins returning SERVFAIL. validating resolver begins returning SERVFAIL.
5. Operational Considerations 5. Operational Considerations
This entire document discusses operational considerations with This entire document discusses operational considerations with
various parameters selections of NSEC3 and NSEC3PARAM fields. various parameter selections of NSEC3 and NSEC3PARAM fields.
6. IANA Considerations 6. IANA Considerations
This document requests a new allocation in the First Come First IANA has allocated the following code in the First Come First Served
Served range of the "Extended DNS Error Codes" of the "Domain Name range [RFC8126] of the "Extended DNS Error Codes" registry within the
System (DNS) Parameters" registration table with the following "Domain Name System (DNS) Parameters" registry:
characteristics:
* INFO-CODE: (RFC EDITOR: TBD)
* Purpose: Unsupported NSEC3 iterations value
* Reference: (RFC EDITOR: this document) INFO-CODE: 27
Purpose: Unsupported NSEC3 iterations value
Reference: RFC 9276
7. References 7. References
7.1. Normative References 7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 9, line 48 skipping to change at line 389
[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D. [RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
Lawrence, "Extended DNS Errors", RFC 8914, Lawrence, "Extended DNS Errors", RFC 8914,
DOI 10.17487/RFC8914, October 2020, DOI 10.17487/RFC8914, October 2020,
<https://www.rfc-editor.org/info/rfc8914>. <https://www.rfc-editor.org/info/rfc8914>.
7.2. Informative References 7.2. Informative References
[GPUNSEC3] Wander, M., Schwittmann, L., Boelmann, C., and T. Weis, [GPUNSEC3] Wander, M., Schwittmann, L., Boelmann, C., and T. Weis,
"GPU-Based NSEC3 Hash Breaking", DOI 10.1109/NCA.2014.27, "GPU-Based NSEC3 Hash Breaking", DOI 10.1109/NCA.2014.27,
2014, <https://doi.org/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 [RFC8624] Wouters, P. and O. Sury, "Algorithm Implementation
Requirements and Usage Guidance for DNSSEC", RFC 8624, Requirements and Usage Guidance for DNSSEC", RFC 8624,
DOI 10.17487/RFC8624, June 2019, DOI 10.17487/RFC8624, June 2019,
<https://www.rfc-editor.org/info/rfc8624>. <https://www.rfc-editor.org/info/rfc8624>.
[ZONEENUM] Wang, Z., Xiao, L., and R. Wang, "An efficient DNSSEC zone [ZONEENUM] Wang, Z., Xiao, L., and R. Wang, "An efficient DNSSEC zone
enumeration algorithm", n.d.. enumeration algorithm", DOI 10.2495/MIIT130591, April
2014, <https://doi.org/10.2495/MIIT130591>.
Appendix A. Deployment measurements at time of publication Appendix A. Deployment Measurements at Time of Publication
At the time of publication, setting an upper limit of 100 iterations At the time of publication, setting an upper limit of 100 iterations
for treating a zone as insecure is interoperable without significant for treating a zone as insecure is interoperable without significant
problems, but at the same time still enables CPU-exhausting DoS problems, but at the same time still enables CPU-exhausting DoS
attacks. attacks.
At the time of publication, returning SERVFAIL beyond 500 iterations At the time of publication, returning SERVFAIL beyond 500 iterations
appears to be interoperable without significant problems. appears to be interoperable without significant problems.
Appendix B. Computational burdens of processing NSEC3 iterations Appendix B. Computational Burdens of Processing NSEC3 Iterations
The queries per second (QPS) of authoritative servers will decrease The queries per second (QPS) of authoritative servers will decrease
due to computational overhead when processing DNS requests for zones due to computational overhead when processing DNS requests for zones
containing higher NSEC3 iteration counts. The table below shows the containing higher NSEC3 iteration counts. The table below shows the
drop in QPS for various iteration counts. drop in QPS for various iteration counts.
| Iterations | QPS [% of 0 iterations QPS] | +============+=============================+
|------------+-----------------------------| | Iterations | QPS [% of 0 Iterations QPS] |
| 0 | 100 % | +============+=============================+
| 10 | 89 % | | 0 | 100% |
| 20 | 82 % | +------------+-----------------------------+
| 50 | 64 % | | 10 | 89% |
| 100 | 47 % | +------------+-----------------------------+
| 150 | 38 % | | 20 | 82% |
+------------+-----------------------------+
| 50 | 64% |
+------------+-----------------------------+
| 100 | 47% |
+------------+-----------------------------+
| 150 | 38% |
+------------+-----------------------------+
Appendix C. Acknowledgments Table 1: Drop in QPS for Various
Iteration Counts
The authors would like to thank the dns-operations discussion Acknowledgments
participants, which took place on mattermost hosted by DNS-OARC.
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 Additionally, the following people contributed text or review
comments to the draft: comments to this document:
* Vladimir Cunat * Vladimir Cunat
* Tony Finch * Tony Finch
* Paul Hoffman * Paul Hoffman
* Warren Kumari * Warren Kumari
* Alexander Mayrhofer * Alexander Mayrhofer
* Matthijs Mekking * Matthijs Mekking
* Florian Obser * Florian Obser
* Petr Spacek * Petr Spacek
* Paul Vixie * Paul Vixie
* Tim Wicinski * Tim Wicinski
Appendix D. GitHub Version of This Document
(RFCEditor: remove this section)
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
Appendix E. Implementation Notes
(RFCEditor: remove this section)
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 Authors' Addresses
Wes Hardaker Wes Hardaker
USC/ISI USC/ISI
Email: ietf@hardakers.net Email: ietf@hardakers.net
Viktor Dukhovni Viktor Dukhovni
Bloomberg, L.P. Bloomberg, L.P.
Email: ietf-dane@dukhovni.org Email: ietf-dane@dukhovni.org
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