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dnsop O. Kolkman
Internet-Draft 12 May 2026
Intended status: Informational
Expires: 13 November 2026
In-tree Hints for DNS Resiliency
draft-kolkman-in-tree-hints-pre-00
Abstract
By configuring so called in-tree hints in recursive nameservers and
by following operational practices, the resiliency against certain
types of DNS failures increases. We describe the approach, the
necessary operational practices, and the dilemmas this approach
introduces.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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 13 November 2026.
Copyright Notice
Copyright (c) 2026 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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The in-tree hints concept . . . . . . . . . . . . . . . . . . 4
2.1. Recursive nameserver . . . . . . . . . . . . . . . . . . 4
2.2. Domain Owner . . . . . . . . . . . . . . . . . . . . . . 5
3. Operational Considerations . . . . . . . . . . . . . . . . . 6
3.1. Signalling . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Achieving true resiliency of services within the
domain. . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Serving stale data . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Policy Considerations . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Appendix: Example configuration in Unbound . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The Domain Name System (DNS) is a remarkably stable and resilient
system. However, in many environments people are looking on how they
can remain in control over their own environments and reduce external
dependencies.
This memo documents an operational approach that, with minor support
of recursive nameserver can offer one of the elements towards greater
autonomy and resilience of infrastructure dependent on a specific
domain.
In an illustrative scenario, consider an enterprise operating under
the domain example.net that provides essential services, such as
logistics, to users on its campus. If the transit connection to the
broader Internet were to fail, the consequences could be significant.
Specifically, if the domain data for example.net is not cached within
the enterprise network, users will experience DNS resolution
failures. This means they will be unable to access critical services
because the necessary delegation from the .net top-level domain to
example.net is unavailable.
Moreover, and perhaps more importantly, this approach offers
protection against various attack vectors that could compromise the
delegation process. For instance, man-in-the-middle (MITM) attacks
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may attempt to alter delegation records, which could lead to denial
of service, particularly in systems utilizing DNSSEC (Domain Name
System Security Extensions). Additionally, threats such as DNS
supply chain attacks or inadvertent errors can result in unauthorized
changes to the delegation, including DS (Delegation Signer) records.
Finally, this method may offer some protection if, in an event that
would be catastrophic for the Internet, geopolitical tensions lead to
the de-delegation of a countries top-level domain.
Our approach is designed for proving resiliency for the Internet's
naming function and does not bring full resiliency by itself.
Instead, we this is as a building block for resiliency of critical
infrastructure or digital autonomy. The approach is complementary to
serving stale data from a resolvers cache [RFC8767] more on this in
section Section 3.3.
An important requirement of this approach is consistent with the
architecture, design, and operation of the DNS and the global
Internet. By following practices herein we avoid namespace
fragmentation. The approach avoids fundamental protocol changes, in
particular it avoids alternative roots.
We describe what parties that are critically dependent on a specific
domain and those that serve zones within that domain will need to do
in order to guarantee continuous operation. For instance, when their
parent nameservers are not reachable or there is a broken delegation
from the ancestor domain. Here, 'broken' means that DNS resolver
receives parental data that is inconsistent with the intent from the
(child) domain owner, i.e. receiving data that is inconsistent with
what is published on authoritative servers. Which includes not
receiving data at all.
In section Section 2 we describe the idea and the requirements for a
recursive DNS server and the requirements of the zone associated
with. In section Section 3.2 we shortly point to other measures that
must be taken in combination with this mechanism. In section
Section 5 we discuss some policy considerations and the dilemmas that
exist with respect to intentions of the DNS parent and child.
This document uses uppercase SHOULD, RECOMMENDED and MUST in the
meaning defined by [RFC2119]. Their lowercase equivalents do not
have normative meaning.
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2. The in-tree hints concept
[RFC9499] describes the root hints file "Operators who manage a DNS
recursive resolver typically need to configure a 'root hints file'.
This file contains the names and IP addresses of the authoritative
name servers for the root zone, so the software can bootstrap the DNS
resolution process. For many pieces of software, this list comes
built into the software."
The in-tree hints borrows this from this idea: by configuring a
'hints file' for a specific domain one allows oneself to bootstrap
from that domain down, even if its parents are not available. It
requires a modification in recursive nameservers and adherence to
some operational practices.
2.1. Recursive nameserver
Recursive nameserver software will need to be modified to deal to
work with in-tree hints.
An in-tree hints is configuration for a recursive resolver that
provides the names and IP addresses of authoritative name servers for
a specific domain. A recursive name server may be configured for in-
tree hints for multiple domains.
If there are no in-domain nameservers ([RFC9499]) in the NS set for
the domain then this mechanism MUST not be used. The reason for this
requirement is that when there is no in-domain nameserver the
resiliency properties cannot be achieved as there are external name
dependencies. This requirement can be enforced by the recursive
nameserver software at the moment of configuration parsing.
In-tree hints are only useful if the domain owner follows certain
practices and MAY only be followed if the domain owner indicates it
does so. Section Section 3.1 describes the RECOMMENDED way for
domain name owners to signaling their intent. This is also something
that the recursive nameserver can check and log.
In-tree hints MUST only be used in combination with a trust-anchor.
i.e. a trusted public DNSSEC key that is associated with the name.
The trust-anchor MUST be maintained. It SHOULD be maintained by the
mechanism described in [RFC5011]. Alternatively an appropriate and
trustworthy off-band mechanism MAY be used. The operator of a
recursive nameserver must validate that the domain associated with
the in-tree hints follows the operational practices described in this
memo. This can be achieved by out-of band mechanisms, or by querying
the TXT record as described in {#auth}
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When a recursive nameserver is configured with an in-tree hint then
the NS Resource Record set contained in the in-tree hint MUST be used
during the resolution process. When the NS RRset on the domain's
authoritative server changes and has been validated using DNSSEC
against configured key then the in-hints tree configuration SHOULD be
updated with the changed authoritative NS set. The recursive
nameserver should honor the TTLs to regular check a change of the
authoritative DNS RR set. Operators that implement in-tree hints
SHOULD use tooling, possibly implemented in the recursive nameserver,
to log and signal inconsistencies between information in the parents
and the in-tree configuration to the operators of the recursive
nameserver, these inconsistencies need to be well understood. They
could be the result of a bonafide redelegation (in which case the
parental records are likely a sub-set of the authoritative NS RR
set), the withdrawal of the delegation by the parent, or an error or
attack.
The trust anchor MUST be used for the validation of record within the
tree-hint's domain even when a parental DS record exists. Nota bene,
section 5 of [RFC5011] allows for deletion if a superior trust point
exists - when a trust anchor is part of an in-tree hint that deletion
with the motivation that a superior trust point exists MUST not
happen. When a tree-hint exists for a subordinate domain, that trust
anchor MUST take precedence.
Recursive nameservers that implement this mechanism should have a
fallback mechanism implemented that will eventually allow them to
reach the in-domain nameserver when other servers in the NS resource
record set fail.
2.2. Domain Owner
This section describes the operational practices that the domain
owner has to follow in order to achieve the resiliency within the
domain.
The domain owner MUST maintain its DNSSEC configuration using the
mechanism described in [RFC5011].
The domain owner MUST have at least one in-domain authoritative
nameserver in its NS set (e.g. ns.example.com for the example.com
domain). If that nameserver's name is within a delegated child
domain, then the nameservers for that delegated domain MUST also have
at least one in-domain authoritative nameserver. This requirement is
recursive for further delegation.
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In order to benefit from the resiliency properties provided by this
mechanism, the domain owner should require that zones within the
domain all have one in-domain nameserver. Note that delegated
domains do not have to maintain a trust anchor and can rely on there
being a chain of trust established using DS records from the trust-
anchor down.
Furthermore, the in-domain nameserver SHOULD be positioned in a
network that shares connectivity fate with the clients that rely on
the domain. For instance, in our enterprise example it should be in
the enterprise campus network. More generally the location is
subject to a risk based assessment about the likelihood of not being
able to obtain a network connection to the in-domain nameserver.
The domain owner should communicate to its community that it is using
this method. That communication MAY be out of band. A RECOMMENDED
in-band signalling mechanism in-band described in section
Section 3.1.
3. Operational Considerations
3.1. Signalling
It is RECOMMENDED that a domain owner (the owner of <domain>) signals
to its user community that they are using the mechanism described in
this memo. Signalling is done by putting a TXT resource record with
owner name _in-tree.<domain> containing an expiry timestamp in
[RFC3339] format. The expiry timestamp indicates the date to which
the owner is committed to follow the instructions in section
Section 2.2.
The recursive nameserver operator should at first opportunity, but
not longer than 30 days after the expiration, validate if a new
expiry record has been published by the domain owner. If not they
SHOULD disable the in-tree hints configuration for the domain.
_in-tree.<domain> TXT <expiry timestamp> [OMK: Alternatively we
create a trivial RR type for this. EXP RR containing a timestamp as
defined in RFC4034 section-3.1.5 ]
Out of band signalling is not in scope for this memo.
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3.2. Achieving true resiliency of services within the domain.
This memo describes a method to achieve resiliency of name resolution
for a community of interest of a particular domain. This is, by far,
not sufficient to achieve actual resiliency for services that are
provided within the domain. While further out of scope for this memo
we like to remind the reader of the following:
* The in-domain nameservers should run on IP addresses that can
reasonably be expected to be reachable by the community of use.
For example, if a service is critical for on-campus enterprise use
then the in-domain nameserver should run on the campus network.
* Any service provider that offers a service under a certain name
within the domain should make sure that those services itself can
be reasonably expected to be reachable by the community of use.
Any service dependencies should also be local.
* In an effort to create local resiliency one should not forget that
resiliency is also achieved by having no single source of failure.
Having in-domain nameservers, and having services in reach of the
community of interest does not mean that one deploys
infrastructure elsewhere.
* Running a local root [RFC8806] may be an additional method to
create resiliency against certain failure cases, mainly failure to
connect to DNS root-servers. When resolvers implement the local
root approach they MUST give prefer the information in the in-tree
hints file to the delegation information from the root. In other
words they should treat the local root as any other root server.
3.3. Serving stale data
In-tree hints are complementary to serving stale data [RFC8767].
Serving stale data will allow continuity for all zones when their
authoritative servers are not reachable and the data happens to be in
the resolvers cache. In-tree hints works for specific domains when
data does not happen to be available in recursive nameserver caches
or when the parent's server(s) deliver faulty delegation data.
In-tree hints is not scalable in the sense that there is significant
operational overhead for the domain owner, they have to run in-domain
nameservers and follow [RFC5011]. Similarly scalability concerns
exist for recursive nameserver operators as they will have to
troubleshoot inconsistencies. Serving stale data is highly scalable
as it only needs one configuration within the recursive nameserver
and then it applies for all domains.
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4. Security Considerations
In-tree hints can be used in recursive nameservers in combination
with protective block-lists and does therefore not debilitate the
available blocking mechanism available to protect the community of
users of a recursive nameserver.
Mallwares can use their own recursive nameservers configured with in-
trees for their command and control domains to circumvent de-
delegation by the parents. However, those recursive nameservers are
likely under the control of the mallware administrators and the risk
of disproportional damage for blocking these recursive nameservers
DNS after it has been established that they are used in command and
control seems proportionate.
This mechanism intends to provide resilience for network failures.
However, it adds complexity in software and operational procedures,
thereby increasing the fragility.
5. Policy Considerations
Inherently the approach described in this memo provides a mechanism
for a community of users of a domain to overwrite the policies from
the parent domain. For instance, it allows the community of users to
continue to use the domain even when e.g. the delegation for that
domain expires or it has been de-delegated after a court order. At
the same time, this in-tree approach can be a building block to
create resilience for a critical infrastructure. It can potentially
be applied to a country code top-level domain (CCTLD) and its user
community. While the failure mode at CCTLD level is extremely low,
this approach may add to confidence in the domain name system as a
whole in times of international tensions.
When an inconsistency exists between what is published in the parent
and what is used as in-tree-hints there is a fragmentation of the DNS
namespace. The operators of the recursive nameservers should
proactively restore the situation to consistency. Note that there is
no technical enforcement mechanism to aid that restoration, but it is
expected that if a recursive nameserver operator configures an in-
tree domain they are part of the community of interest and therefore
have out of band means to contact the domain administrator. Also
note that the operators of the domains usually do not have
communication mechanism that can enforce the use or non-use of in-
tree hints by recursive nameserver operators.
The authority for using or not using in-tree hints is with the
operator of the recursive nameserver - as a user agent for its
community. Users have historically been able to overwrite their DNS
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configuration. They can use a recursive nameserver that does not use
in-tree hints for a particular domain and therefore have the ability
opt-out of the mechanism.
6. IANA Considerations
No IANA considerations herein.
7. Acknowledgements
This document is inspired by a conversation about digital autonomy.
8. Disclaimer
The author is an employee of the Internet Society, this document does
not necessarily reflect the position of the Internet Society.
{olaf: source="olaf"}
9. Appendix: Example configuration in Unbound
[OMK: this example might be too vendor specific to maintain in an
RFC]
It is relatively trivial to configure this methodology in Unbound.
[OMK TODO: follow up with Willem for the example config]
10. References
10.1. 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/rfc/rfc2119>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/rfc/rfc3339>.
[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/rfc/rfc5011>.
[RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
DNSSEC Delegation Trust Maintenance", RFC 7344,
DOI 10.17487/RFC7344, September 2014,
<https://www.rfc-editor.org/rfc/rfc7344>.
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10.2. Informative References
[E-Gov-Resilience]
Sommese et al, "Assessing e-Government DNS Resilience",
IEEE Proceedings of the 2022 International Conference on
Network and Service Management (CNSM 2022), 2022.
[RFC8767] Lawrence, D., Kumari, W., and P. Sood, "Serving Stale Data
to Improve DNS Resiliency", RFC 8767,
DOI 10.17487/RFC8767, March 2020,
<https://www.rfc-editor.org/rfc/rfc8767>.
[RFC8806] Kumari, W. and P. Hoffman, "Running a Root Server Local to
a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,
<https://www.rfc-editor.org/rfc/rfc8806>.
[RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<https://www.rfc-editor.org/rfc/rfc9499>.
Author's Address
Olaf Kolkman
Email: olaf@xolx.nl
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