Internet DRAFT - draft-ietf-dnsop-dnssec-validator-requirements
draft-ietf-dnsop-dnssec-validator-requirements
dnsop D. Migault
Internet-Draft Ericsson
Intended status: Informational E. Lewis
Expires: 29 July 2023 ICANN
D. York
ISOC
25 January 2023
Recommendations for DNSSEC Resolvers Operators
draft-ietf-dnsop-dnssec-validator-requirements-04
Abstract
The DNS Security Extensions (DNSSEC) define a process for validating
received data and assert them authentic and complete as opposed to
forged.
This document clarifies the scope and responsibilities of DNSSEC
Resolver Operators (DRO) as well as operational recommendations that
DNSSEC validators operators SHOULD put in place in order to implement
sufficient trust that makes DNSSEC validation output accurate. The
recommendations described in this document include, provisioning
mechanisms as well as monitoring and management mechanisms.
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 July 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. DNSSEC Validator Description . . . . . . . . . . . . . . . . 5
5. Recommendations Categories . . . . . . . . . . . . . . . . . 7
6. Time deviation and absence of Real Time Clock
Recommendations . . . . . . . . . . . . . . . . . . . . . 8
7. Trust Anchor Related Recommendations . . . . . . . . . . . . 9
7.1. Trust Anchor Configuration . . . . . . . . . . . . . . . 9
7.1.1. Definition of the Trust model . . . . . . . . . . . . 10
7.1.2. Trust Model Bootstrapping . . . . . . . . . . . . . . 12
7.1.3. Configuration Generation . . . . . . . . . . . . . . 13
7.1.4. DNSSEC Resolver Instantiation . . . . . . . . . . . . 13
7.2. Trust Anchor Update . . . . . . . . . . . . . . . . . . . 13
7.2.1. Automated Updates to DNSSEC Trust Anchors . . . . . . 14
7.2.2. Automated Trust Anchor Check . . . . . . . . . . . . 15
8. Negative Trust Anchors Related Recommendations . . . . . . . 16
9. ZSK / KSK (non TA) Related Recommendations . . . . . . . . . 17
10. DNSKEY Related Recommendations . . . . . . . . . . . . . . . 19
10.1. Automated Reporting . . . . . . . . . . . . . . . . . . 19
10.2. Interactions with the cached RRsets . . . . . . . . . . 20
11. Cryptography Deprecation Recommendations . . . . . . . . . . 20
12. Invalid Reporting Recommendations . . . . . . . . . . . . . . 21
13. Transport Recommendations . . . . . . . . . . . . . . . . . . 21
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
15. Security Considerations . . . . . . . . . . . . . . . . . . . 22
16. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 23
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
17.1. Normative References . . . . . . . . . . . . . . . . . . 24
17.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The purpose of a DNSSEC Resolver Operator (DRO) is to provide DNSSEC
validation in to their users.
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By validating with DNSSEC a received Resource Record Set (RRset), the
resolver provides a high level of certainty that the information
carried by the RRset is effectively as published by the legitimate
owner of the RRset. The act of DNSSEC validation [RFC4033][RFC4035]
can be broken into two part: A _Signature Validation_ which binds the
RRset to a private key as well as _Trust_ in the owner of the private
being the legitimate owner.
Signature Validation consists in checking the cryptographic signature
of a RRset and involves among other parameters a DNSKEY Resource
Record (RR) and RRSIG RR and the RRset itself. The signature
validation process results in designating the owner of the RRset as
the owner of the corresponding private part of the public key
contained in the DNSKEY RR. Cryptography provides a high level of
confidence in the binding to the private key. In that sense, a rogue
RRset likely results from the private key being exposed or guessed --
as opposed to signature or key collisions for example. As such
differ the confidence into the Trust to designate which DNSKEY RR is
legitimate.
Trust implicitly assumes the private keys used to sign are not shared
and as such can be associated their respective owners. The purpose
of Trust is to put significant confidence into designating the
legitimate DNSKEY RR - which also characterizes the corresponding
private key. Such trust is provided by a Trust Anchor (TA), and the
chain of trust established between the TA and the DNSKEY RR. The
chain of trust is obtained by recursively validating the DNSKEY RRs.
As a result, such trust results from the trust placed in the TA as
well as the delegation mechanism provided by DNSSEC and the Signature
Validation. As TAs need to be managed over time, the trust also
concerns the management procedure of the TA. This is the main
concern of this document.
Data's authenticity and integrity is tied to the operator of the key
that generates the signature. It is conceivable that a validator
could "know" the keys of each data source, but this is not practical
at large scale. To counter this, DNSSEC relied on securely chaining
keys in a manner isomorphic to the way names are delegated. Keys for
a name will "vouch for" keys at a name delegated via the signing of a
DS resource record set.
Using keys to vouch for keys, recursively, works when a manageable
set of key to name associations are determined to be "trusted" - and
are called trust anchors. In DNSSEC, a validator needs one or more
Trust Anchors from which to grow chains of verified keys.
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With operational experience, a twist has emerged. More often, to
date, failed validation is due to operator error or software bug
[ENT] and not an attempt to forge data. In general badly signed
RRsets or zone badly delegated are out of scope of the DRO's
responsibility. However, the DRO may reflect this operational error
with a temporary solution designated as Negative Trust Anchors (NTA)
[RFC7646]. A NTA instructs a validator to ignore the presence of
keys for a name, reacting as if the name is unsigned.
Once accurately validated the RRset is assumed to be accurately
validated and trusted during the time indicated by its TTL.
The responsibilities of a DRO are limited to the management of TAs as
well as providing the necessary infrastructure to perform the
signature validation, e.g. appropriated libraries and time. More
specifically, badly signed zones or insertion of malicious DNSKEY
fall out of the DRO's responsibilities. Even though these threats
fall out of these responsibilities, a DRO may collaborate with
authoritative servers to limit the damage of their operational
errors.
This document is focused on operational recommendations that a DRO
SHOULD put in place in order to implement sufficient trust that makes
DNSSEC validation output accurate. The recommendations described in
this document include provisioning mechanisms as well as monitoring
and management mechanisms.
The mechanisms provided are designed in accordance of the DNSSEC
trust model as to meet the current operations of DNSSEC. Such trust
model is briefly recapped in Section 4 so operators understand the
limits and motivations for such mechanisms.
2. 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.
3. Terminology
This document uses the following terminology:
DNSSEC validator: the entity that performs DNS resolution and
performs signature validation.
Accurate validation: validation that avoids false positives and
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catches true negatives.
Trust Anchor Data Store: a module (of code) implementing functions
related to the trust anchors used by the validator. This is
essentially a database allowing access, monitoring of, and changes
to trust anchors.
DNSSEC Resolver Operator (DRO): The operator providing DNSSEC
validation service and managing DNSSEC validators
4. DNSSEC Validator Description
This is a conceptual block diagram of the elements involved with
DNSSEC validation. This is not meant to be an architecture for code,
this is meant to be a framework for discussion and explanation.
+-------------+ +---------------+
| | | |
| Time Source | | Cryptographic |
| | | Libraries |
| | | |
+-------------+ +---------------+
| |
v v
+--------------------------------+ +--------------+
| | | |
| |<--| Trust Anchor |
| DNSSEC Validation Engine | | Manager & |
| |-->| Storage |
| | | |
+--------------------------------+ +--------------+
^ | ^ |
| v | |
+-------------+ +---------------+ |
| | | | |
| DNS Caches | | DNS Messages |<---------+
| | | |
+-------------+ +---------------+
Figure 1: DNSSEC Validator Description
Time Source: The wall clock time provides the DNSSEC Validation
Engine the current time. Time is one input used to validate the
RRSIG Signature and Inception Fields to provide some protection
against replay attacks.
Cryptograhic Libraries: The code performing the cryptographic
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functions providing the DNSSEC Validation Engine the ability to
check the Signature Field that contains the cryptographic
signature covering the RRSIG RDATA.
DNS Message: DNS responses are used to carry the information from
the DNS system. The receiver of the DNS message can be any kind
of application including DNS-related application such as in the
case of automated Trust Anchor update performed by the Trust
Anchor Manager & Storage. The DNSSEC Validator Engine accurately
validates the DNS responses before caching them in the DNS Cache
and forwarding them to the DNS receiver. In case of validation
failure, an error is returned and the information may be
negatively cached.
DNS Caches: Include positive and negative caches. The DNSSEC
Validation Engine fills DNS Caches with validated DNS RRsets. The
DNSSEC trust model considers that once a RRset has been accurately
validated by the DNSSEC Validator Engine, the RRset is considered
trusted (or untrusted) for its associated TTL. DNS Caches contain
RRsets that may contain information requested by the application
(RRset of type AAAA for example) as well as RRset necessary to
accurately validate the RRsets (RRsets of type DNSKEY or RRSIG for
example). This can also include RRsets that were not DNSSEC
signed at validation time.
Trust Anchor Manager: The database of trust anchors associated to
database management processes. This function provides the DNSSEC
Validation Engine Trust Anchor information when needed. When a TA
needs to be updated, the Trust Anchor Manager is also responsible
for handling the update procedure. This includes sending DNS
Messages as well as treating appropriately the DNS responses that
have been accurately validated by the DNSSEC Validator Engine.
This will require the DNSSEC Validator to update Trust Anchor
information, whether via methods like Automated Updates of DNSSEC
Trust Anchors [RFC5011], management of Negative Trust Anchors, or
other, possibly not yet defined, means.
DNSSEC Validation Engine: This follows local policy to approve data.
The approved data is returned to the requesting application as
well as to the DNS Caches. While the cryptographic computation of
the RRSIG signature may be the most visible step, the RRSIG record
also contains other information intended to help the validator
perform its work, in some cases sanity checks are performed. For
instance, the original TTL (needed to prepare the RR set for
validation) ought to be equal to or higher than the received TTL.
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Not shown - Name Server Process Management interfaces to elements,
handling of Checking Disabled request, responses, as well as all API
requests made of the name server.
5. Recommendations Categories
A DRO needs to be able to enable DNSSEC validation with sufficient
confidence they will not be held responsible in case their resolver
does not validate the DNSSEC response. The minimization of these
risks is provided by setting automated procedures, when a resolver is
started or while it is operating, as well as some on-demand
operations that enable the DRO to perform a specific operation. The
recommendations do not come with the same level of recommendations
and some are likely to be required. Others are optional and may be
followed by more advanced DROs. It is up to the DRO to determine
their applicability.
Regarding recommendations on the behavior of the resolver, some
recommendations may already be applied by default by the operated
resolver software, in which case the DRO does not have to perform any
action. Some recommendations may require to be activated via
configuration by the DRO. Some recommendations may simply not be
provided by the operated software, in which case we recommend the DRO
to contact the software vendor for further discussion.
The recommendations are voluntary restrictive to prevent side effects
of interventions that will be hard to predict, debug and understand.
The RECOMMENDATIONS in this document are subdivided into the
following categories:
Start-up recommendations (STARTUP): which describes RECOMMENDED
operations the DRO is expected to perform when the resolver is
started. These operations typically include health checks of the
infrastructure the resolver is instantiated on as well as
configuration check.
Run time recommendations (RUNTIME): which describes RECOMMENDED
operations the DRO is expected to perform on its running
resolvers. These operations typically include health checks of
the infrastructure as well as the resolvers.
On demand recommendations (ONDEMAND): which describes the
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RECOMMENDED operations a DRO may perform. This includes the
ability to operate health checks at a given time as well as
specific operations such as flushing the cache. The reason this
document mentions these recommendations is to enable DROs to have
the appropriates tools as well as to restrict their potential
interventions.
6. Time deviation and absence of Real Time Clock Recommendations
With M2M communication some devices are not expected to embed Real
Time Clock (Raspberry Pi is one example of such devices). With
Raspberry Pi for example, when these devices are re-plugged their
time is reset to some initial values like (January 1, 1970 for
example) until they get re synchronized via NTP. Other devices that
have clocks may suffer from time deviation. These devices cannot
rely on their time estimation to perform DNSSEC validation.
Time synchronization may be performed manually, but for the sake of
operations it is strongly RECOMMENDED to automate the time
synchronization on each resolver.
STARTUP:
* A DRO MUST provide a mean to update the time without relying on
DNSSEC when the DNSSEC validator is started. The resolver MUST
NOT start if the time synchronization does not succeed at start
time.
Note that updating time in order to be able to perform DNSSEC
validation may become a form of a chicken and egg problem when the
NTP server is designated by its FQDN. The update mechanism must
consider the DNSSEC validator may not able to validate the DNSSEC
queries. In other words, the mechanisms may have to update the time
over an unsecure DNSSEC resolution.
RUNTIME:
* While operating, a DRO MUST closely monitor time derivations of
the resolvers and maintain the time synchronized.
ONDEMAND:
* A DRO SHOULD be able to check and synchronize, on demand, the time
of the system of its resolver.
Note that time synchronization is a sensible operation. A DRO MUST
update time over an authenticated and secure channel.
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For all recommendations, it is strongly RECOMMENDED that
recommendations are supported by automated processes.
7. Trust Anchor Related Recommendations
A TA store maintains associations between domain names and keys
(whether stored as in a DNSKEY resource record or a DS resource
record) and domain names whose key are to be ignored (negative trust
anchors). The TA store is essentially a database, storing the
(positive) trust anchors. Management of the TA can be done manually
or in an automated way. Automatic management of the TA is
RECOMMENDED and can be subdivided into the following sub-categories:
1. Initial TA provisioning: that is the ability, for a DRO, to
ensure a starting resolver is automatically provisioned with
an up-to-date configuration. This includes defining a trust
model and ensuring the associated TAs values are valid at the
time the resolver is started. # This includes the TA
associated to the trust model established by the DRO.
2. TA update over time: while the trust model may not evolve, the
cryptographic keys associated to the security entry points are
subject to change and thus a TA needs to be updated
dynamically over time by a resolver. In this case, the DRO
needs to check that the resolver has properly updated the TAs.
3. TA reporting: The reporting of the TA used by the resolver is
made to the DRO as well as the authoritative servers which are
hold responsible for making their zone validated by DNSSEC
resolvers.
Note that TA update and TA reporting only concerns running resolvers.
This section is only considering TA and not NTA. The handling of NTA
is detailed in Section 8.
7.1. Trust Anchor Configuration
When a DRO starts a DNSSEC resolver, the DNSSEC resolver is
provisioned with the TAs as part of its configuration. As these TAs
change over time, the DRO MUST ensure the resolvers are always
provisioned with up-to-date TAs and detect deprecated configuration.
To do so, the TA configuration is considered as a trusted model
instantiated as follows:
1. Trust model definition: The DRO defines the domain names that
constitutes security entry point (TA) -- see Section 7.1.1 for
more details.
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2. Trust model bootstrapping: A bootstrapping procedure is applied
to each TA to retrieve their TA values - that is the
corresponding value of the key - in a trusted way. Such TA
MAY have a specific format not understood by the resolver --
see Section 7.1.2 for more details.
3. DNSSEC resolver configuration generation: The TA with associated
values are formatted appropriately for the DNSSEC resolver
that will be instantiated. In many cases the appropriated
format is the DNS RRset format - see Section 7.1.3 for more
details.
4. DNSSEC resolver instantiation: The DNSSEC resolver is started,
performs some checks before becoming operational, that is
checking compability between provisioned TAs and those found
online. These checks are implemented by the resolver and not
really in scope of the DRO -- see Section 7.1.4 for more
details.
While these steps may require some specific development for complex
trust models, no additional deployment is required when using the
default model where the root zone is defined as the only secure entry
point. In such cases, the trusted model bootstrapping is performed
by software-update that relies on the code signing key of the
software provider. The software provider also provides the
configuration to the appropriated format and the checks at
instantiation -- see Section 7.1.2.1.
7.1.1. Definition of the Trust model
The DRO defines its trust model by explicitly mentioning the domain
name that constitutes security entry point as well as domain name
that are known to be unsecured.
This document does not provide recommendations regarding the number
of TAs a DRO needs to configure its DNSSEC resolver with. There are
many reasons a DRO may be willing to consider multiple TAs as opposed
to a single Root Zone Trust Anchor. In fact it is not always
possible to build a trusted delegation between the Root Zone and a
sub zone. This may happen, for example, if one of the upper zones
does not handle the secure delegation or improperly implements it.
Typically, a DS RRset may not be properly filled or its associated
signature cannot be validated. As the chain of trust between a zone
and the root zone may not be validated, the DNSSEC validation for the
zone requires a TA. Such DNS(SEC) resolutions may be critical for
infrastructure management. A company may, for example, address all
its devices under the domain example.com and may not want disruption
to happen if the .com delegation cannot be validated for any reason.
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Such companies may operate DNSSEC with a TA for the zone example.com
in addition to the regular DNSSEC delegation. Similarly some domains
may present different views such as a "private" view and a "public
view". These zones may have some different content, and may use a
different KSK for each view.
The domain name chosen as the TA security entry point may be a child
zone from another TA security entry point. For example, the DRO may
use seperate TAs for example.com, .com, and the root zone. The TA
associated with a domain name is determined by the longest match.
However, the trust model MUST at least ensure that any domain name in
the DNS be covered by at least one TA. As the number of top level
domains is evolving overtime, it remains safe to keep the root zone
as a security entry point in order to cover the full domain name
space.
The default trust model remains rooted in the root zone as a security
entry point, and no zones being considered as unsecured. By default,
the instantiation of this trusted model is delegated by the DRO to
the software vendor that is responsible to update the the default
trust model with the up-to-date value of the TA. In such cases, the
trusted model is implemented by software-update that relies on the
code signing key of the software provider.
The use of a different trust model must be carefully considered and
NOT RECOMMENDED for DRO without a strong DNSSEC expertise. A DRO
implementing another trust model MUST be able to ensure that the
trust model is properly implemented - that is with up-to-date TAs.
The DRO MUST carefully balance the implications, the risks, the
necessary involved mechanisms to achieve this goal with the potential
benfits. More specifically, it is expected updating the trust
model's instantiation is performed in an automated way with strong
validation validation and checks being put in place. It is also
likely that the DRO and the owner of the TA will have a specific
relationship and TA update will be provided via a specific out-of-
band channel. More specifically, [RFC5011] that we recommend to
update TA by a running instance of resolver, is likely not to be
sufficent, as it is completly automated and relies on in band
signaling which comes with risks detailled in [RFC5011], Section 8.
A DRO need to understand these risks. Note also that even when
[RFC5011] the owner of the TA needs to respect the [RFC5011] timings
-- see Section 7.2.1.
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7.1.2. Trust Model Bootstrapping
The purpose of the boostrapping step is clearly to securely retrieve
DNSKEY as well as DS RRsets with a valid and authentic RDATA to
implement the trust model (see Section 7.1.1). Authentic data
includes data that are up-to-date at the time these are requested,
provided with securely and verified. In particular the TA MUST NOT
be retrieved from a local source that is not known to be up to date.
This typically includes software or any local store of data for which
there is not a dedicated and automated updating system. Similarly,
TA MUST NOT be retrieved from untrusted communication such as a DNS
resolution that cannot be verified or validated.
The authenticity of the RDATA is usually based on the authentication
of the source which can take multiple forms, but the principle is
that a chain of signature ends up being validated by a trusted key.
For TA that are not the root zone KSK, DNSSEC may be used to retrieve
and validate the TA. Note that such means to validate the
authenticity, the TA as a subdomain mostly prevents potential
disruptions of the parent domains. In many cases, an alternative
trusted source will be preferred such as TLS which is likely to rely
on the Web PKI, or the signature of a file.
Although some bootstrapping mechanisms to securely retrieve publish
[RFC7958] and retrieve [UNBOUND-ANCHOR] the Root Zone Trust Anchor
have been defined, it is believed these mechanisms should be extended
to other KSKs or Trust Anchors. Such bootstrapping process enables a
DRO to start a DNSSEC resolver from a configuration file, that
reflects the trust model of the DRO.
STARTUP:
* DRO SHOULD only rely on TA associated with a bootstrapping
mechanism.
7.1.2.1. IANA Trust Anchor Bootstrapping
For validators that may be used on the global public Internet (with
"may be" referring to general purpose, general release code),
handling the IANA managed root zone KSK trust anchor is a
consideration.
The IANA managed root zone KSK is an operationally significant trust
point in the global public Internet. Attention to the trust anchor
for this point is paramount. Trust anchor management ought to
recognize that the majority of operators deploying DNSSEC validators
will need to explicitly or implicitly rely on this trust anchor.
Trust anchor management needs to recognize that there may be other
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trust anchors of interest to operators. Besides deployments in
networks other than the global public Internet (hence a different
root), operators may want to configure other trust points.
The IANA managed root zone KSK is managed and published as described
in "DNSSEC Trust Anchor Publication for the Root Zone" [RFC7958].
That document is written as specific to that trust point. Other
trust points may adopt the technique describe (or may use other
approaches).
This represents a consideration for implementations. On one hand,
operators will place special emphasis on how the root zone DNSSEC KSK
is managed. On the other hand, implementations ought to accommodate
trust anchors in a general manner, despite the odds that other trust
anchors will not be configured in a specific deployment.
Because of this, it is RECOMMENDED that implementations make the root
zone trust anchor obvious to the operator while still enabling
configuration of general trust points.
7.1.3. Configuration Generation
The generation of a configuration file associated to the TA is
expected to be implementation independent. The necessity of tweaking
the data depending of the software implementer or eventually the
software version introduces a vector for configuration errors.
7.1.4. DNSSEC Resolver Instantiation
STARTUP:
* DNS resolver MUST validate the TA before starting the DNSSEC
resolver, and a failure of TA validity check MUST prevent the
DNSSEC resolver to be started. Validation of the TA includes
coherence between out-out band values, values stored in the DNS as
well as corresponding DS RRsets.
7.2. Trust Anchor Update
Updating the TA reflects the evolution of the trust. It is important
to understand that this section does not consider the trust model to
be updated by the DRO. This has been defined in section Section 7.1.
Instead, it considers the evolution over time of the instantiation of
the trust model, that is the update of the values associated to the
TA, by a running instance of resolver. Such updates need to be
operated in a reliable and trusted way. Note that you may not need
to proceed to such TA update if the DRO can guarantee that any
instance has been instantiated with a configuration the reflects the
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current value of the TAs. Given that the roll-over phases for a TA
update are relatively long, for DRO that can ensure a fast deployment
of the latest release of resolver, with the defult trust model being
implemented by the software vendor, TA update is likely to be
achieved via software updates.
The remaining of this section is left to DRO willing to ensure their
instance have the capacity to update their TAs while running. This
may be a specific feature or a complementary feature to software
update. The value associated to the TA may be updated over time
which is part of the maintenance of the configuration and needs to be
performed by the DNSSEC resolver without any intervention of the DRO.
This is the purpose of this section.
TA update is expected to be transparent to the DRO (see
Section 7.2.1. However, a DRO MAY wish to ensure its resolvers
operate according to the provisioned configurations and are updated
normally (see Section 7.2.2. This includes for a DRO the ability to
check which TA are in used as well as to resolve in collaboration of
authoritative servers and report the used TAs.
7.2.1. Automated Updates to DNSSEC Trust Anchors
Trust is inherently a matter of an operations policy. As such, a DRO
will need to be able to update the list of Trust Anchors. TA updates
are not expected to be handled manually. This introduces a
potentially huge vector for configuration errors, due to human
intervention as well as potential misunderstanding of ongoing
operations. Instead DRO will rely on "Automated Updates to DNSSEC
Trust Anchors" [RFC5011] when the TA signers is known to respect the
[RFC5011] timing.
STARTUP:
* DRO SHOULD check TA signers commits to respect "Automated Updates
to DNSSEC Trust Anchors" [RFC5011] timings.
* DRO SHOULD enable "Automated Updates to DNSSEC Trust Anchors"
[RFC5011] [I-D.ietf-dnsop-rfc5011-security-considerations].
Note that while automation prevents the risks associated to manual
intervention, but managing TA on its own - that is via other means
than software updates, does not come without any risks. A DRO need
to understand these risks.
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7.2.2. Automated Trust Anchor Check
A DRO SHOULD regularly check the trust anchor used by the DNSSEC
resolver is up-to-date and that values used by the resolvers are
conform to the ones in the configuration (see Section 7.1). Such
check is designated as TA health check.
Note that retrieving in an automated way the value of the TA removes
old values from the configuration and ensures that resolvers are
always started with up-to-date values. In the case of a key roll
over, the resolver is moving from an old value to an up-to date
value. This up-to-date value does not need to survive reboot, and
there is no need to update the configuration file of the running
instances - configuration is updated by a separate process. To put
it in other words, the updated value of the TA is only expected to be
stored in the resolver's memory. Avoiding the configuration file to
be updated prevents old configuration file to survive to writing
error on read-only file systems.
The TA used by a resolver may be part of a configuration parameter as
well as part of an internal state of the resolver. It is NOT
RECOMMENDED a DRO accesses configuration or internal state of a
resolver as it may open the resolver to other vulnerabilities and
provides privileged access to a potential attacker.
STARTUP:
* DRO SHOULD enable "Signaling Trust Anchor Knowledge in DNS
Security Extensions (DNSSEC)" [RFC8145] to provide visibility to
the TA used by the resolver. The TA can be queried using a DNSKEY
query. The channel MAY be protected and restricted to the DRO.
Note also that [RFC8145] does not only concern Trust Anchor but is
instead generic to DNSKEY RRsets. As a result, unless for the root
zone, it is not possible to determine if the KSK/ZSK or DS is a Trust
Anchor or a KSK/ZSK obtained from regular DNSSEC resolutions.
TA health check includes validating DNSKEY RRsets and associated DS
RRsets in the resolver, on the DNS authoritative servers as well as
those obtained out-of-band. TA health check results MUST be logged.
The check SHOULD evaluate if the mismatch resulted from an ongoing
normal roll over, a potential emergency key roll over, failed roll
over or any other envisioned cases. Conflicts are not inherently a
problem as some keys may be withheld from distribution via the DNS.
A failed key roll over or any other abnormal situation MUST trigger
an alarm.
RUNTIME:
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* A DRO SHOULD regularly run TA health checks.
If the mismatch is due to a failed key roll-over, this SHOULD be
considered as a bug by the DRO. The DRO MUST restart the resolver
with updated TA.
ONDEMAND:
* A DRO SHOULD be able to check the status of a TA as defined in
Section 3 of [RFC7583].
8. Negative Trust Anchors Related Recommendations
When the DNSSEC Resolver is not able to validate signatures because a
key or DS has been published with an error, the DNSSEC Operator MAY
temporarily disable the signature check for that key until the time
the error is addressed. This is performed using NTA[RFC7646]. NTA
represents the only permitted intervention in the resolving process
for a DRO.
NTA are considered as temporary fix for a known unsecured domain,
which is different from an TA that would not be trusted. The
designation of NTA might be misleading, but NTA are not expected to
be part of the trust model. This does not prevent a DRO to provision
NTA as a configuration parameters NTA. The management of the
configuration SHOULD be automated as described in Section 7.1.
Handling NTA is described in [RFC7646] and a DRO SHOULD follow these
guidelines. The intent of this section is to position these
guidelines toward the operational recommendations provided in this
document.
STARTUP:
* DRO SHOULD be able to automatically configure NTA when starting
DNSSEC resolvers.
ONDEMAND:
* DRO SHOULD set automated procedures to determine the NTA of DNSSEC
resolvers.
* DRO SHOULD be able to handle NTA as defined in [RFC7646].
Note that adding a Negative Trust Anchor only requires the domain
name to be specified. Note also that NTA can disable any sort of
DNSKEY and is not restricted to TA.
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A failure in signaling validation is associated to a mismatch between
the key and the signature. DNSKEY/DS RRsets for TA have a higher
level of trust then regular KSK/ZSK. In addition, DRO are likely to
have specific communication channel with TA maintainer which eases
trouble shooting.
A signature validation failure is either an attack or a failure in
the signing operation on the authoritative servers. The DRO is
expected to confirm this off line before introducing the NTA. This
is likely to happen via a human confirmation. As a result here are
the following recommendations:
RUNTIME:
* DRO SHOULD monitor the number of signature failure associated to
each DNSKEY. These number are only hints and MUST NOT trigger
automated insertion of NTA.
* A DRO MAY collect additional information associated each DNSKEY
RRSets. This information may be useful to follow-up roll over
when these happen and evaluate when a key roll over is not
performed appropriately on the resolver side or on the
authoritative server. It would provide some means to the DRO to
take action with full knowledge without necessary asking for a
confirmation. In other cases it could prevent invalidation to
happen. These check may be performed for a limited subset of
domains or generalized.
9. ZSK / KSK (non TA) Related Recommendations
KSK / ZSK are not part of the DNSSEC validator configuration. Their
values in the DNS Caches may not reflect those published by the
authoritative servers or may be incoherent with the RRset in the DNS
Cache they are validating. However, such incoherence primary results
from error in the management of the authoritative servers. As a
result, it is not expected that the DNSSEC validator provides complex
management facilities to address these issues as this will modify the
DNS architecture and add complexity that is not proved to be
beneficial. As a result, recommendations always belong to the run
time or on demand recommendations. The main difference between TA
and KSK/ZSK is that the DRO does not necessarily have an out of band
mechanism to retrieve the RRsets. As a result, the DRO has less
information to determine and confirm what is happening. The default
recommendation is to let things go.
A number of reasons may result in inconsistencies between the RRsets
stored in the cache and those published by the authoritative server.
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An emergency KSK / ZSK rollover may result in a new KSK / ZSK with
associated new RRSIG published in the authoritative zone, while
DNSSEC validator may still cache the old value of the ZSK / KSK. For
a RRset not cached, the DNSSEC validator performs a DNSSEC query to
the authoritative server that returns the RRset signed with the new
KSK / ZSK. The DNSSEC validator may not be able to retrieve the new
KSK / ZSK while being unable to validate the signature with the old
KSK / ZSK. This either results in a bogus resolution or in an
invalid signature check. Note that by comparing the Key Tag Fields,
the DNSSEC validator is able to notice the new KSK / ZSK used for
signing differs from the one used to generate the received generated
signature. However, the DNSSEC validator is not expected to retrieve
the new ZSK / KSK, as such behavior could be used by an attacker.
Instead, ZSK / KSK key roll over procedures are expected to avoid
such inconsistencies.
Similarly, a KSK / ZSK roll over may be performed normally, that is
as described in [RFC6781] and [RFC7583]. While the KSK / ZSK roll
over is performed, there is no obligation to flush the RRsets in the
cache that have been associated with the old key. In fact, these
RRsets may still be considered as trusted and be removed from the
cache as their TTL timeout. With very long TTL, these RRsets may
remain in the cache while the ZSK / KSK with a shorter TTL is no
longer published nor in the cache. In such situations, the purpose
of the KSK / ZSK used to validate the data is considered trusted at
the time it enters the cache, and such trust may remain after the KSK
/ ZSK is being rolled over. Note also that even though the data may
not be associated to the KSK / ZSK that has been used to validate the
data, the link between the KSK / ZSK and the data is still stored in
the cache using the RRSIG. Note also that inconsistencies between
the ZSK / KSK stored in the cache and those published on the
authoritative server, may lead to inconsistencies to downstream
DNSSEC validators that rely on multiple cache over time.
Typically, a request for the KSK / ZSK may have been provided by a
cache that is storing the new published value, while the data and
associated signatures may be associated to the old KSK / ZSK.
Incoherence between RRsets and DNSKEYs is not the responsibility of
the DRO. Instead, it is the responsibility of authoritative server
publishing these data. This includes insuring the coherence between
TTLs and signature validation periods as well as small variations of
the resolvers clocks. Section 4.4.1 of [RFC6781] provides some
recommendations that can be implemented by the authoritative server
which puts the responsibility of failure of signature validation
under the responsibility of the authoritative server. A DRO MAY
however limit the risks for these inconsistencies to happen by
configuring the DNSSEC validator with generic rules that applies to
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the validation process. Typically, the TTL associate to the DNSKEY
is an engagement from the authoritative server that the DNSKEY will
remain valid over this period. As this engagement supersedes the
validation of any RRSIG and by extension to any RRset in the zone,
this TTL value may be used as the maximum value for the TTL
associated to FQDNs in the zone. Section 8.1 of [RFC4033] mention
the ability by the resolver to set the upper bound of the TTL to the
remaining signature validity period. In addition, the DNSSEC
validator should also be able to provide a maximum values for TTLs.
These values MAY also consider the small inaccuracy of the local
clock.
RUNTIME:
* To limit the risks of incoherent data in the cache, it is
RECOMMENDED DRO enforce TTL policies of RRsets based on the TTL of
the DS, KSK and ZSK. RRsets TTL SHOULD NOT exceed the DS, KSK or
ZSK initial TTL value, that TTL SHOULD trigger delegation
revalidation as described in [I-D.ietf-dnsop-ns-revalidation].
TTL SHOULD NOT exceed the signature validity.
10. DNSKEY Related Recommendations
This section considers the recommendations that are common to TA as
well as non TA DNSKEY RRsets.
10.1. Automated Reporting
A DRO MAY regularly report the Trust Anchor used to the authoritative
server. This would at least provide insight to the authoritative
server and provide some context before moving a key roll over
further.
The purpose of reporting the currently used Trust Anchor for a domain
name is to establish an informational channel between the resolver
and the authoritative server. This data may not directly be useful
for the DNSSEC Resolver, but instead to the authoritative server. In
return it is likely the authoritative server will take the
appropriate steps in operating the authoritative server and consider
this information. This results in the following recommendation:
RUNNING:
* A DRO SHOULD enable TA reporting to the authoritative server as
specified in "Signaling Trust Anchor Knowledge in DNS Security
Extensions (DNSSEC)" [RFC8145]
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10.2. Interactions with the cached RRsets
The purpose of automated checks is to enable early detection of
failed operations, which provides enough time to the DRO to react
without any consequences. On the other hand, these checks MAY reveal
as well that a rogue TA has been placed and that the resolver is
corrupted. Similarly, a DRO may be informed by other channel a rogue
or unwilling DNSKEY has been emitted.
In such situation, the DRO SHOULD be able to remove the RRsets
validated by the rogue DNSKEY.
ONDEMAND:
* A DRO MUST be able to flush the cached data subtree associated to
a DNSKEY
11. Cryptography Deprecation Recommendations
As mentioned in [RFC8247] and [RFC8221] cryptography used one day is
expected over the time to be replaced by new and more robust
cryptographic mechanisms. In the case of DNSSEC signature protocols
are likely to be updated over time. In order to anticipate the
sunset of one of the signature scheme, a DNSSEC validator may be
willing to estimate the impact of deprecating one signature scheme.
Currently, interoperability and security are enforced via
cryptographic recommendations [RFC8624] that are followed by both
resolvers and authoritative servers. The implementation of such
guidance is ensured by the software vendors and the compliance of
their releases.
To safely deprecate one signature scheme, the DNSSEC validator
operator is expected to follow the recommendation below:
STARTUP: * DRO MUST ensure recent software releases that comply with
the most recent cryptographic guidances are being used
RUNTIME:
* A DRO SHOULD regularly request and monitor the signature scheme
supported by an authoritative server.
* A DRO SHOULD report a "Unsupported DNSKEY Algorithm" as defined in
[RFC8914] when a deprecated algorithm is used for validation.
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One inconvenient to such strategy i sthat it does not let one DRO to
take advantage of more recent cryptographic. While currently not
being widely used, a DRO MAY share information with authoritative
server in the hope that future deployment of authoritative servers
will be able to leverage it. [RFC6975] provides the ability for a
DNSSEC validator to announce an authoritative server the supported
signature schemes. However, a DNSSEC validator is not able to
determine other than by requesting and monitoring DNSKEY RRsets as
well as RRSIG. These RRsets are received by enabling DNSSEC
validation by default which is obviously the case for DNSSEC
validator.
12. Invalid Reporting Recommendations
A DNSSEC validator receiving a DNS response cannot make the
difference between receiving an non-secure response versus an attack.
Dropping DNSSEC fields by a misconfigured middle boxes, such as DS,
RRRSIG is considered as an attack. A DNSSEC validator is expected to
perform secure DNS resolution and as such protects its stub client.
An invalid response may be the result of an attack or a
misconfiguration, and the DNSSEC validator may play an important role
in sharing this information with the authoritative server or domain
name owner.
RUNTIME:
* DRO SHOULD monitor and report DNSSEC validation error. Reporting
may take various means but the DRO SHOULD implement [RFC8914] to
inform the DNS client.
13. Transport Recommendations
DNSSEC validation requires that the validator is able to reliably
obtain necessary records, especially DNSKEY records. This should be
done at initial configuration, and tested periodically.
This means the validator MUST ensure it is configured so that the UDP
and TCP transports, and DNS resolver components, are compatible with
the network paths that the majority of DNS queries traverse - which
includes compatibility between DNS and transport parameters with the
Maximum Transmission Unit (MTU).
In other words, make sure that: 1. DNS UDP bufsize (EDNS parameter)
is set to a value compatible with network MTUs the queries and
responses will encounter. If the validator advertises a bufsize >>
MTU, responses with the IPv4 Don't Fragment (DF) bit set whose size R
where MTU < R <= bufsize exceeds the MTU will be dropped by the
router with MTU < R.
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1. The validator's OS TCP configuration has its advertised Maximum
Segment Size (MSS) set to a value compatible with network MTUs
the queries and responses will encounter. * Having an advertised
MSS set to a value < MTU ensures that Path MTU Discovery is not
required * If PMTUD fails for any reason, or if the server
responding does not maintain or use PMTUD, and advertised MSS >
MTU at any point in the path, TCP may encounter problems caused
by IP fragmentation and reassembly. * This is particularly
relevant if there are any NAT type devices in the path, as those
may not properly handle fragmentation and reassembly * If all TCP
segments are smaller than the path MTU, TCP will work reliably.
The avoidance of fragmentation in order to address known
fragmentation-related security issues with DNS (leading to cache
poisoning, for example) has resulted in the need to set the DF bit on
UDP. Validators will need to ensure their local environment can
reliably get any critical DNSSEC records (notably DNSKEY) over UDP,
or reliably get responses with TC=1 if overly large responses cannot
be sent over UDP due to answers not fitting within the advertised
bufsize payload. Validators also need to ensure TCP works if it is
needed, for the same situations.
STARTUP: * DRO MUST ensure UDP and TCP transport are enabled. * DRO
MUST ensure DNS and TCP parameters will not exceed the expected MTU
RUNTIME: * DRO SHOULD regularly discover MTU and ensure the DNS and
TCP parameters are aligned wit this value. * DRO SHOULD closely
monitor the absence of responses, or ICMP PTB message as a potential
incompatibility between configured parameters and the MTU.
14. IANA Considerations
There are no IANA consideration for this document.
15. Security Considerations
The recommendations listed in this document have two goals. First
ensuring the DNSSEC validator has appropriated information to
appropriately perform DNSSEC validation. Second, monitoring the
necessary elements that would enable a DNSSEC validator operator to
ease a potential analysis. The recommendations provide very limited
ability for a DNSSEC validator operator to alter or directly
interfere on the validation process and the main purpose in providing
the recommendations was to let the protocol run as much as possible.
Providing inappropriate information can lead to misconfiguring the
DNSSEC validator, and thus disrupting the DNSSEC resolution service.
As a result, enabling the setting of configuration parameters by a
third party may open a wide surface of attacks. In addition, such
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changes may lead to unexpected corner cases that would result in
making analysis and trouble shooting very hard.
As an appropriate time value is necessary to perform signature check,
an attacker may provide rogue time value to prevent the DNSSEC
validator to check signatures.
An attacker may also affect the resolution service by regularly
asking the DNSSEC validator to flush the KSK/ZSK from its cache. All
associated data will also be flushed. This generates additional
DNSSEC resolution and additional validations, as RRSet that were
cached require a DNSSEC resolution over the Internet. This affects
the resolution service by slowing down responses, and increases the
load on the DNSSEC validator.
An attacker may ask the DNSSEC validator to consider a rogue KSK/ZSK,
thus hijacking the DNS zone. Similarly, an attacker may inform the
DNSSEC validator not to trust a given KSK in order to prevent DNSSEC
validation to be performed.
An attacker (cf. Section 7) can advertise a "known insecure" KSK or
ZSK is "back to secure" to prevent signature check to be performed
correctly.
As a result, information considered by the DNSSEC validator should be
from a trusted party. This trust party should have been
authenticated, and the channel used to exchange the information
should also be protected and authenticated.
The software used for DNSSEC validator is not immune to bugs and may
become vulnerable independently of how it is operated. As a result a
DRO SHOULD NOT depend on a single implementation or version of a
given software and SHOULD instead run at least two independent pieces
of software.
16. Acknowledgment
The need to address DNSSEC issues on the resolver occured during
multiple discussions including among others Ted Lemon, Ralph Weber,
Normen Kowalewski, Mikael Abrahamsson, Jim Gettys, Paul Wouters, Joe
Abley, Michael Richardson, Vladimír Čunát, James Gannon, Andrew
McConachie, Peter Thomassen, Florian Obser and Brian Dickson.
We also appreciated the support of the DNSOP chairs Tim Wicinski,
Suzanne Woolf and Benno Overeinder.
17. References
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17.1. Normative References
[I-D.ietf-dnsop-ns-revalidation]
Huque, S., Vixie, P. A., and R. Dolmans, "Delegation
Revalidation by DNS Resolvers", Work in Progress,
Internet-Draft, draft-ietf-dnsop-ns-revalidation-03, 6
September 2022, <https://www.ietf.org/archive/id/draft-
ietf-dnsop-ns-revalidation-03.txt>.
[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>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[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>.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
September 2007, <https://www.rfc-editor.org/info/rfc5011>.
[RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
Operational Practices, Version 2", RFC 6781,
DOI 10.17487/RFC6781, December 2012,
<https://www.rfc-editor.org/info/rfc6781>.
[RFC6975] Crocker, S. and S. Rose, "Signaling Cryptographic
Algorithm Understanding in DNS Security Extensions
(DNSSEC)", RFC 6975, DOI 10.17487/RFC6975, July 2013,
<https://www.rfc-editor.org/info/rfc6975>.
[RFC7583] Morris, S., Ihren, J., Dickinson, J., and W. Mekking,
"DNSSEC Key Rollover Timing Considerations", RFC 7583,
DOI 10.17487/RFC7583, October 2015,
<https://www.rfc-editor.org/info/rfc7583>.
[RFC7646] Ebersman, P., Kumari, W., Griffiths, C., Livingood, J.,
and R. Weber, "Definition and Use of DNSSEC Negative Trust
Anchors", RFC 7646, DOI 10.17487/RFC7646, September 2015,
<https://www.rfc-editor.org/info/rfc7646>.
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[RFC8145] Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
Anchor Knowledge in DNS Security Extensions (DNSSEC)",
RFC 8145, DOI 10.17487/RFC8145, April 2017,
<https://www.rfc-editor.org/info/rfc8145>.
[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>.
[RFC8221] Wouters, P., Migault, D., Mattsson, J., Nir, Y., and T.
Kivinen, "Cryptographic Algorithm Implementation
Requirements and Usage Guidance for Encapsulating Security
Payload (ESP) and Authentication Header (AH)", RFC 8221,
DOI 10.17487/RFC8221, October 2017,
<https://www.rfc-editor.org/info/rfc8221>.
[RFC8247] Nir, Y., Kivinen, T., Wouters, P., and D. Migault,
"Algorithm Implementation Requirements and Usage Guidance
for the Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 8247, DOI 10.17487/RFC8247, September 2017,
<https://www.rfc-editor.org/info/rfc8247>.
[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>.
[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>.
17.2. Informative References
[ENT] Levigneron, V., "ENT was here !!!", n.d.,
<https://indico.dns-
oarc.net/event/25/contributions/403/attachments/378/647/
AFNIC_OARC_Dallas.pdf>.
[I-D.ietf-dnsop-rfc5011-security-considerations]
Hardaker, W. and W. A. Kumari, "Security Considerations
for RFC5011 Publishers", Work in Progress, Internet-Draft,
draft-ietf-dnsop-rfc5011-security-considerations-13, 16
July 2018, <https://www.ietf.org/archive/id/draft-ietf-
dnsop-rfc5011-security-considerations-13.txt>.
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[RFC7958] Abley, J., Schlyter, J., Bailey, G., and P. Hoffman,
"DNSSEC Trust Anchor Publication for the Root Zone",
RFC 7958, DOI 10.17487/RFC7958, August 2016,
<https://www.rfc-editor.org/info/rfc7958>.
[UNBOUND-ANCHOR]
"unbound-anchor - Unbound anchor utility", n.d.,
<https://nlnetlabs.nl/documentation/unbound/unbound-
anchor/>.
Authors' Addresses
Daniel Migault
Ericsson
8275 Trans Canada Route
Saint Laurent, QC 4S 0B6
Canada
Email: daniel.migault@ericsson.com
Edward Lewis
ICANN
United States of America
Email: edward.lewis@icann.org
Dan York
ISOC
United States of America
Email: york@isoc.org
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