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Versions: 02
Network Working Group W. Hardaker
Internet-Draft USC/ISI
Intended status: Best Current Practice V. Dukhovni
Expires: August 23, 2021 Bloomberg, L.P.
February 19, 2021
Guidance for NSEC3 parameter settings
draft-hardaker-dnsop-nsec3-guidance-02
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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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 August 23, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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 Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Recommendation for zone publishers . . . . . . . . . . . . . 3
2.1. Algorithms . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. Iterations . . . . . . . . . . . . . . . . . . . . . . . 3
2.4. Salt . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Best-practice for zone publishers . . . . . . . . . . . . . . 5
4. Recommendation for validating resolvers . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Operational Considerations . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 6
Appendix B. Github Version of this document . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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 via SHA-1. (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. Opt-out
blocks 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
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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. Recommendation for zone publishers
The following sections describe recommendations for setting
parameters for NSEC3 and NSEC3PARAM.
2.1. Algorithms
The algorithm field is not discussed by this document.
2.2. Flags
The flags field currently contains a single flag, that of the "Opt-
Out" flag [RFC5155], which specifies whether or not NSEC3 records
provide 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 less 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. 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.
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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, 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 (tbd: insert ref), this document recommends
that zone owners SHOULD use an iteration value of 0 (zero),
indicating that only the initial hash value should be placed into a
DNS zone's NSEC3 records.
2.4. Salt
Salts add yet another layer of protection against offline, stored
dictionary attacks by combining the value to be hashed (in our case,
a DNS domainname) with a randomly generated value. This prevents
adversaries from building up and remembering a dictionary of values
that can translate a hash output back to the value that it derived
from.
In the case of DNS, it should be noted the hashed names placed in
NSEC3 records already include 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
required to compute a complete dictionary per zone, which is
expensive in both storage and CPU time.
To protect against a dictionary being built and used for a target
zone, an additional salt field can be included and changed on a
regular basis, forcing a would-be attacker to repeatedly compute a
new dictionary (or just do trial and error without the benefits of
precomputation).
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 incrementally signing it in the background where the new
salt is only activated once every name in the chain has been
completed.
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Most users of NSEC3 publish static salt values that never change.
This provides no added security benefit (because the complete fully
qualified domain name is already unique). If no rotation is planned,
operators are encouraged to forgo the salt entirely by using a zero-
length salt value instead (represented as a "-" in the presentation
format).
3. Best-practice for zone publishers
In short, for most zones, the recommended NSEC3 parameters are as
shown below:
; SHA-1, no opt-out, no extra iterations, empty salt:
;
bcp.example. IN NSEC3PARAM 1 0 0 -
For very large (e.g. 10 million plus unsigned delegations) and only
sparsely signed zones, where the majority of the records are insecure
delegations, use of opt-out may be justified. In such (large TLD or
similar) zones the alternative parameters are:
; SHA-1, with opt-out, no extra iterations, empty salt:
;
example. IN NSEC3PARAM 1 1 0 -
4. 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 should change their behaviour
with respect to large iteration values. Validating resolvers SHOULD
return a SERVFAIL when processing NSEC3 records with iterations
larger than 100. Note that this significantly decreases the
requirements originally specified in Section 10.3 of [RFC5155].
Validating resolvers returning a SERVFAIL in this situation SHOULD
return an Extended DNS Error {RFC8914} EDNS0 option of value [TBD].
5. Security Considerations
This entire document discusses security considerations with various
parameters selections of NSEC3 and NSEC3PARAM fields.
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6. Operational Considerations
This entire document discusses operational considerations with
various parameters selections of NSEC3 and NSEC3PARAM fields.
7. 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:
o INFO-CODE: TBD
o Purpose: Unsupported NSEC3 iterations value
o Reference: this document
8. References
8.1. 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>.
[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>.
8.2. Informative References
[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>.
Appendix A. Acknowledgments
dns-operations discussion participants
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Appendix B. 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
Authors' Addresses
Wes Hardaker
USC/ISI
Email: ietf@hardakers.net
Viktor Dukhovni
Bloomberg, L.P.
Email: ietf-dane@dukhovni.org
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