



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

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   This Internet-Draft will expire on 13 November 2026.

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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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