Guidance for NSEC3 parameter settings
draft-ietf-dnsop-nsec3-guidance-04
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9276.
|
|
|---|---|---|---|
| Authors | Wes Hardaker , Viktor Dukhovni | ||
| Last updated | 2022-02-25 | ||
| Replaces | draft-hardaker-dnsop-nsec3-guidance | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 9276 (Best Current Practice) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-dnsop-nsec3-guidance-04
Network Working Group W. Hardaker
Internet-Draft USC/ISI
Intended status: Best Current Practice V. Dukhovni
Expires: 29 August 2022 Bloomberg, L.P.
25 February 2022
Guidance for NSEC3 parameter settings
draft-ietf-dnsop-nsec3-guidance-04
Abstract
NSEC3 is a DNSSEC mechanism providing proof of non-existence by
promising there are no names that exist between two domainnames
within a zone. Unlike its counterpart NSEC, NSEC3 avoids directly
disclosing the bounding domainname pairs. This document provides
guidance on setting NSEC3 parameters based on recent operational
deployment experience.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 29 August 2022.
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
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provided without warranty as described in the Revised BSD License.
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 . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . . 9
Appendix D. Github Version of This Document . . . . . . . . . . 10
Appendix E. Implementation Notes . . . . . . . . . . . . . . . . 10
E.1. OpenDNSSEC . . . . . . . . . . . . . . . . . . . . . . . 10
E.2. PowerDNS . . . . . . . . . . . . . . . . . . . . . . . . 10
E.3. Knot DNS and Knot Resolver . . . . . . . . . . . . . . . 10
E.4. Google Public DNS Resolver . . . . . . . . . . . . . . . 10
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 allow large registries to only sign as many NSEC3
records as there are signed DS or other RRsets in the zone - with
opt-out, unsigned delegations don't require additional NSEC3 records.
This sacrifices the tamper-resistance proof of non-existence offered
by NSEC3 in order to reduce memory and CPU overheads.
NSEC3 records have a number of tunable parameters that are specified
via an NSEC3PARAM record at the zone apex. These parameters are the
Hash Algorithm, processing Flags, the number of hash Iterations and
the Salt. Each of these has security and operational considerations
that impact both zone owners and validating resolvers. This document
provides some best-practice recommendations for setting the NSEC3
parameters.
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
iterations 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. Because hashing provides
only moderate protection, as shown recently in academic studies of
NSEC3 protected zones [GPUNSEC3][ZONEENUM].
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2.4. Salt
Operators are encouraged to forget the salt entirely by using a zero-
length salt value instead (represented as a "-" in the presentation
format).
NSEC3 records provide an 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, or 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 a is 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 non trivial 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. Please note that extra
iteration counts other than 0 increase impact of resource CPU-
exhausting DoS attacks, and also increase 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.
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
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
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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 when processing
NSEC3 records with iterations larger than 0. Validating resolvers
MAY also return SERVFAIL when processing NSEC3 records with
iterations larger than 0. Validating resolvers MAY choose to not
respond to NSEC3 records with iterations larger than 0. This
significantly decreases 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.
Validating resolvers returning an insecure or SERVFAIL answer because
of unsupported NSEC3 parameter values SHOULD return an Extended DNS
Error (EDE) {RFC8914} EDNS0 option of value (RFC EDITOR: TBD).
Note 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).
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,
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
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above a particular value (say 100) and returns SERVFAIL above a
higher point (say 500), it leaves the zone subject to man-it-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 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>.
[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>.
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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.
As 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.
| 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 Čunat
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* Tony Finch
* Paul Hoffman
* Alexander Mayrhofer
* Matthijs Mekking
* Florian Obser
* Petr Špaček
* Paul Vixie
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
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.
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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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