Internet DRAFT - draft-ietf-dnsop-nsec3-guidance
draft-ietf-dnsop-nsec3-guidance
Network Working Group W. Hardaker
Internet-Draft USC/ISI
Updates: 5155 (if approved) V. Dukhovni
Intended status: Best Current Practice Bloomberg, L.P.
Expires: 26 November 2022 25 May 2022
Guidance for NSEC3 parameter settings
draft-ietf-dnsop-nsec3-guidance-10
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. This document updates [RFC5155] with guidance
about selecting NSEC3 iteration and salt parameters.
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
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This Internet-Draft will expire on 26 November 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
extracted from this document must include Revised BSD License text as
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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 Discussions . . . . . . . . . . . . . . 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 . 6
3.1. Best-practice for Zone Publishers . . . . . . . . . . . . 6
3.2. Recommendation for Validating Resolvers . . . . . . . . . 7
3.3. Recommendation for Primary / Secondary Relationships . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Operational Considerations . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Deployment measurements at time of publication . . . 10
Appendix B. Computational burdens of processing NSEC3
iterations . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . 10
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
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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 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 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 Discussions
The following sections describes the background of the parameters for
the NSEC3 and NSEC3PARAM resource record types.
2.1. Algorithms
The algorithm field is not discussed by this document. Readers are
encouraged to read [RFC8624] for guidance about DNSSEC algorithm
usage.
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2.2. Flags
The NSEC3PARAM flags field currently contains only reserved and
unassigned flags. Individual NSEC3 records, however, contain the
"Opt-Out" flag [RFC5155], which specifies whether that NSEC3 record
provides proof of non-existence. In general, NSEC3 with the Opt-Out
flag enabled should only be used in large, highly dynamic zones with
a small percentage of signed delegations. Operationally, this allows
for fewer signature creations when new delegations are inserted into
a zone. This is typically only necessary for extremely large
registration points providing zone updates faster than real-time
signing allows or when using memory-constrained hardware. Operators
considering the use of NSEC3 are advised to fully test their zones
deployment architectures and authoritative servers under both regular
operational loads to determine the tradeoffs using NSEC3 instead of
NSEC. Smaller zones, or large but relatively static zones, are
encouraged to not use a the opt-opt flag and to 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 using the same algorithm a number of times
based on the iteration parameter in the NSEC3PARM 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 to be "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.
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Most names published in the DNS are rarely secret or unpredictable.
They are published to be memorable, used and consumed by humans.
They are often recorded in many other network logs such as email
logs, certificate transparency logs, web page links, intrusion
detection systems, malware scanners, email archives, etc. Many times
a simple dictionary of commonly used domain names prefixes (www,
mail, imap, login, database, etc.) can be used to quickly reveal a
large number of labels within a zone. Because of this, there are
increasing performance costs yet diminishing returns associated with
applying additional hash iterations beyond the first.
Although Section 10.3 of [RFC5155] specifies 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 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.
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Changing a zone's salt value requires the construction of a complete
new NSEC3 chain. This is true both when re-signing the entire zone
at once, and when incrementally signing it in the background where
the new salt is only activated once every name in the chain has been
completed. As a result, re-salting is a very complex operation, with
significant CPU time, memory, and bandwidth consumption. This makes
very frequent re-salting impractical, and renders the additional salt
field functionally useless.
3. Recommendations for Deploying and Validating NSEC3 Records
The following subsections describe recommendations for the different
operating realms within the DNS.
3.1. Best-practice for Zone Publishers
First, if the operational or security features of NSEC3 are not
needed, then NSEC SHOULD be used in preference to NSEC3. NSEC3
requires greater computational power (see Appendix B) for both
authoritative servers and validating clients. Specifically, there is
a nontrivial complexity in finding matching NSEC3 records to randomly
generated prefixes within a DNS zone. NSEC mitigates this concern.
If NSEC3 must be used, then an iterations count of 0 MUST be used to
alleviate computational burdens. Note that extra iteration counts
other than 0 increase the impact of CPU-exhausting DoS attacks, and
also increase the risk of interoperability problems.
Note that deploying NSEC with minimally covering NSEC records
[RFC4470] also incurs a cost, and zone owners should measure the
computational difference in deploying either RFC4470 or NSEC3.
In short, for all zones, the recommended NSEC3 parameters are as
shown below:
; SHA-1, no extra iterations, empty salt:
;
bcp.example. IN NSEC3PARAM 1 0 0 -
For small zones, the use of opt-out based NSEC3 records is NOT
RECOMMENDED.
For very large and sparsely signed zones, where the majority of the
records are insecure delegations, opt-out MAY be used.
Operators SHOULD NOT use a salt by indicating a zero-length salt
value instead (represented as a "-" in the presentation format).
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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 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 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
but 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 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
[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.
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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 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,
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 First Come First
Served range of the "Extended DNS Error Codes" of the "Domain Name
System (DNS) Parameters" registration table with the following
characteristics:
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* 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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
Lawrence, "Extended DNS Errors", RFC 8914,
DOI 10.17487/RFC8914, October 2020,
<https://www.rfc-editor.org/info/rfc8914>.
7.2. Informative References
[GPUNSEC3] Wander, M., Schwittmann, L., Boelmann, C., and T. Weis,
"GPU-Based NSEC3 Hash Breaking", DOI 10.1109/NCA.2014.27,
2014, <https://doi.org/10.1109/NCA.2014.27>.
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[RFC8624] Wouters, P. and O. Sury, "Algorithm Implementation
Requirements and Usage Guidance for DNSSEC", RFC 8624,
DOI 10.17487/RFC8624, June 2019,
<https://www.rfc-editor.org/info/rfc8624>.
[ZONEENUM] Wang, Z., Xiao, L., and R. Wang, "An efficient DNSSEC zone
enumeration algorithm", 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 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 Cunat
* Tony Finch
* Paul Hoffman
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* Warren Kumari
* Alexander Mayrhofer
* Matthijs Mekking
* Florian Obser
* Petr Spacek
* Paul Vixie
* 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.
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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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