1. RFC 9432
Internet Engineering Task Force (IETF)                       P. van Dijk
Request for Comments: 9432                                      PowerDNS
Category: Standards Track                                      L. Peltan
ISSN: 2070-1721                                                   CZ.NIC
                                                                 O. Sury
                                             Internet Systems Consortium
                                                               W. Toorop
                                                              NLnet Labs
                                                         C.R. Monshouwer
                                                            P. Thomassen
                                 deSEC, SSE - Secure Systems Engineering
                                                             A. Sargsyan
                                             Internet Systems Consortium
                                                               July 2023

                           DNS Catalog Zones


   This document describes a method for automatic DNS zone provisioning
   among DNS primary and secondary name servers by storing and
   transferring the catalog of zones to be provisioned as one or more
   regular DNS zones.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Description
   4.  Catalog Zone Structure
     4.1.  Member Zones
     4.2.  Properties
       4.2.1.  Schema Version (version property)
     4.3.  Member Zone Properties
       4.3.1.  Change of Ownership (coo property)
       4.3.2.  Groups (group property)
     4.4.  Custom Properties (*.ext properties)
   5.  Name Server Behavior
     5.1.  General Requirements
     5.2.  Member Zone Name Clash
     5.3.  Member Zone Removal
     5.4.  Member Node Name Change
     5.5.  Migrating Member Zones between Catalogs
     5.6.  Zone-Associated State Reset
   6.  Implementation and Operational Notes
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Catalog Zone Example
   Authors' Addresses

1.  Introduction

   The content of a DNS zone is synchronized among its primary and
   secondary name servers using Authoritative Transfer (AXFR) and
   Incremental Zone Transfer (IXFR).  However, the list of zones served
   by the primary (called a "catalog" in [RFC1035]) is not automatically
   synchronized with the secondaries.  To add or remove a zone, the
   administrator of a DNS name server farm has to not only add or remove
   the zone from the primary but must also add or remove configuration
   for the zone from all secondaries.  This can be both inconvenient and
   error-prone.  In addition, the steps required are dependent on the
   name server implementation.

   This document describes a method in which the list of zones is
   represented as a regular DNS zone (called a "catalog zone" here) and
   transferred using DNS zone transfers.  When entries are added to or
   removed from the catalog zone, it is distributed to the secondary
   name servers just like any other zone.  Secondary name servers can
   then add, remove, or modify the zones they serve in accordance with
   the changes to the catalog zone.  Other use cases of name server
   remote configuration by catalog zones are possible where the catalog
   consumer might not be a secondary.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   Catalog zone:  A DNS zone containing a DNS catalog, which is a list
      of DNS zones and associated properties.

   Member zone:  A DNS zone whose configuration is published inside a
      catalog zone.

   Member node:  A DNS name in the catalog zone representing a member

   $CATZ:  Used in examples as a placeholder to represent the domain
      name of the catalog zone itself. $OLDCATZ and $NEWCATZ are used to
      discuss migration of a member zone from one catalog zone
      ($OLDCATZ) to another catalog zone ($NEWCATZ).

   Catalog producer:  An entity that generates and is responsible for
      the contents of the catalog zone.

   Catalog consumer:  An entity that extracts information from the
      catalog zone (such as a DNS server that configures itself
      according to the catalog zone's contents).

   This document makes use of terminology for transfer mechanisms (AXFR
   and IXFR), record types (SOA, NS, and PTR), and other technical terms
   (such as RDATA) that are specific to the DNS.  Since these terms have
   specific meanings in the DNS, they are not expanded upon first use in
   this document.  For definitions of these and other terms, see

3.  Description

   A catalog zone is a DNS zone whose contents are specially crafted.
   Its resource records (RRs) primarily constitute a list of PTR records
   referencing other DNS zones (so-called "member zones").  The catalog
   zone may contain other records indicating additional metadata (so-
   called "properties") associated with these member zones.

   Catalog consumers MUST ignore any RRs in the catalog zone for which
   no processing is specified or which are otherwise not supported by
   the implementation.

   Authoritative servers may be pre-configured with multiple catalog
   zones, each associated with a different set of configurations.

   Although the contents of a catalog zone are interpreted and acted
   upon by name servers, a catalog zone is a regular DNS zone and must
   adhere to the standards for DNS zones.

   A catalog zone is primarily intended for the management of a farm of
   authoritative name servers and should not be expected to be
   accessible from any recursive name server.

4.  Catalog Zone Structure

   A catalog zone MUST follow the usual rules for DNS zones.  In
   particular, SOA and NS record sets MUST be present and adhere to
   standard requirements (such as [RFC1982]).

   Although catalog zones are not intended to be queried via recursive
   resolution (see Section 7), at least one NS RR is still required so
   that a catalog zone is a syntactically correct DNS zone.  A single NS
   RR with a NSDNAME field containing the absolute name "invalid." is
   RECOMMENDED [RFC2606] [RFC6761].

4.1.  Member Zones

   The list of member zones is specified as a collection of member nodes
   represented by domain names under the owner name "zones" where
   "zones" is a direct child domain of the catalog zone.

   The names of member zones are represented on the RDATA side of a PTR
   record (instead of being represented as a part of owner names) so
   that all valid domain names may be represented regardless of their
   length [RFC1035].  This PTR record MUST be the only record in the PTR
   RRset with the same name.  The presence of more than one record in
   the RRset indicates a broken catalog zone that MUST NOT be processed
   (see Section 5.1).

   For example, if a catalog zone lists three zones ("example.com.",
   "example.net.", and "example.org."), the member node RRs would appear
   as follows:

   <unique-1>.zones.$CATZ 0 IN PTR example.com.
   <unique-2>.zones.$CATZ 0 IN PTR example.net.
   <unique-3>.zones.$CATZ 0 IN PTR example.org.

   where <unique-N> is a label that tags each record in the collection
   and has a unique value.  When different <unique-N> labels hold the
   same PTR value (i.e., point to the same member zone), the catalog
   zone is broken and MUST NOT be processed (see Section 5.1).

   Member node labels carry no informational meaning beyond labeling
   member zones.  A changed label may indicate that the state for a zone
   needs to be reset (see Section 5.6).

   Having the zones uniquely tagged with the <unique-N> label ensures
   that additional RRs can be added below the member node (see
   Section 4.2).

   The CLASS field of every RR in a catalog zone MUST be IN (1).  The
   TTL field's value has no meaning in this context and SHOULD be

4.2.  Properties

   Catalog zone information is stored in the form of "properties".

   Properties are identified by their name, which is used as an owner
   name prefix for one or more record sets underneath a member node (or
   underneath the catalog zone apex), with RR type(s) as appropriate for
   the respective property.

   Known properties that have the correct RR type but are for some
   reason invalid (for example, because of an impossible value or
   because of an illegal number of RRs in the RRset) denote a broken
   catalog zone, which MUST NOT be processed (see Section 5.1).

   This document includes a set of initial properties that can be
   extended via the IANA registry defined and created in Section 8.
   Some properties are defined at the global level; others are scoped to
   apply only to a specific member zone.  This document defines a
   mandatory global property in Section 4.2.1.  The "zones" label from
   Section 4.1 can also be seen as a global property and is listed as
   such in the IANA registry in Section 8.  Member-specific properties
   are described in Section 4.3.

   Implementers may store additional information in the catalog zone
   with custom properties; see Section 4.4.  The meaning of such custom
   properties is determined by the implementation in question.

4.2.1.  Schema Version (version property)

   The catalog zone schema version is specified by an integer value
   embedded in a TXT RR named version.$CATZ.  All catalog zones MUST
   have a TXT RRset named version.$CATZ with exactly one RR.

   Catalog consumers MUST NOT apply catalog zone processing to:

   *  zones without the version property

   *  zones with a version property with more than one RR in the RRset

   *  zones with a version property without an expected value in the
      version.$CATZ TXT RR

   *  zones with a version property with a schema version value that is
      not implemented by the consumer (e.g., version "1")

   These conditions signify a broken catalog zone, which MUST NOT be
   processed (see Section 5.1).

   For this memo, the value of the version.$CATZ TXT RR MUST be set to
   "2"; that is:

   version.$CATZ 0 IN TXT "2"

   Note that Version 1 was used in an earlier draft version of this memo
   and reflected the implementation first found in BIND 9.11.

4.3.  Member Zone Properties

   Each member zone MAY have one or more additional properties that are
   described in this section.  The member properties described in this
   document are all optional, and implementations MAY choose to
   implement all, some, or none of them.  Member zone properties are
   represented by RRsets below the corresponding member node.

4.3.1.  Change of Ownership (coo property)

   The coo property facilitates controlled migration of a member zone
   from one catalog to another.

   A Change Of Ownership is signaled by the coo property in the catalog
   zone currently "owning" the zone.  The name of the new catalog is the
   value of a PTR record in the relevant coo property in the old
   catalog.  For example, if member "example.com." migrates from catalog
   zone $OLDCATZ to catalog zone $NEWCATZ, this will appear in the
   $OLDCATZ catalog zone as follows:

   <unique-N>.zones.$OLDCATZ 0 IN PTR example.com.
   coo.<unique-N>.zones.$OLDCATZ 0 IN PTR $NEWCATZ

   The PTR RRset MUST consist of a single PTR record.  The presence of
   more than one record in the RRset indicates a broken catalog zone,
   which MUST NOT be processed (see Section 5.1).

   When a consumer of a catalog zone $OLDCATZ receives an update that
   adds or changes a coo property for a member zone in $OLDCATZ, it does
   _not_ migrate the member zone immediately.  The migration has to wait
   for an update of $NEWCATZ in which the member zone is present.
   Before the actual migration, the consumer MUST verify that the coo
   property pointing to $NEWCATZ is still present in $OLDCATZ.

   Unless the member node label (i.e., <unique-N>) for the member is the
   same in $NEWCATZ, all its associated state for a just migrated zone
   MUST be reset (see Section 5.6).  Note that the owner of $OLDCATZ
   allows for the zone-associated state to be taken over by the owner of
   $NEWCATZ by default.  To prevent the takeover of the zone-associated
   state, the owner of $OLDCATZ must remove this state by updating the
   associated properties or by performing a zone state reset (see
   Section 5.6) before or simultaneous with adding the coo property (see
   Section 7).

   The old owner may remove the member zone containing the coo property
   from $OLDCATZ once it has been established that all its consumers
   have processed the Change of Ownership.

4.3.2.  Groups (group property)

   With a group property, a consumer(s) can be signaled to treat some
   member zones within the catalog zone differently.

   The consumer MAY apply different configuration options when
   processing member zones, based on the value of the group property.  A
   group property value is stored as the entire RDATA of a TXT record
   directly below the member node.  The exact handling of the group
   property value is left to the consumer's implementation and

   The producer MAY assign a group property to all, some, or none of the
   member zones within a catalog zone.  The producer MAY assign more
   than one group property to one member zone.  This will make it
   possible to transfer group information for different consumer
   operators in a single catalog zone.  Implementations MAY facilitate
   mapping of a specific group value to a specific configuration
   configurable _on a per catalog zone basis_ to allow for producers
   that publish their catalog zone at multiple consumer operators.
   Consumer operators SHOULD namespace their group values to reduce the
   risk of having to resolve clashes.

   The consumer MUST ignore group values it does not understand.  When a
   consumer encounters multiple group values for a single member zone,
   it MAY choose to process all, some, or none of them.  This is left to
   the implementation.  Example

   group properties are represented by TXT RRs.  The record content has
   no pre-defined meaning.  Their interpretation is purely a matter of
   agreement between the producer and the consumer(s) of the catalog.

   For example, the "foo" group could be agreed to indicate that a zone
   not be signed with DNSSEC.  Conversely, an agreement could define
   that group names starting with "operator-" indicate in which way a
   given DNS operator should set up certain aspects of the member zone's
   DNSSEC configuration.

   Assuming that the catalog producer and consumer(s) have established
   such agreements, consider the following catalog zone (snippet) that
   signals to a consumer(s) how to treat DNSSEC for the zones
   "example.net." and "example.com.":

   <unique-1>.zones.$CATZ        0 IN PTR    example.com.
   group.<unique-1>.zones.$CATZ  0 IN TXT    "foo"
   <unique-2>.zones.$CATZ        0 IN PTR    example.net.
   group.<unique-2>.zones.$CATZ  0 IN TXT    "operator-x-foo"
   group.<unique-2>.zones.$CATZ  0 IN TXT    "operator-y" "bar"

   In this scenario, a consumer(s) shall, by agreement, not sign the
   member zone "example.com." with DNSSEC.  For "example.net.", the
   consumers, at two different operators, will configure the member zone
   to be signed with a specific combination of settings.  The group
   value designated to indicate this combination of settings is
   prearranged with each operator ("operator-x-foo" vs. "operator-y"

4.4.  Custom Properties (*.ext properties)

   Implementations and operators of catalog zones may choose to provide
   their own properties.  Custom properties can occur globally or for a
   specific member zone.  To prevent a name clash with future
   properties, such properties MUST be represented below the label

   "ext" is not a placeholder.  A custom property is named as follows:

   ; a global custom property:

   ; a member zone custom property:

   <property-prefix> may consist of one or more labels.

   Implementations SHOULD namespace their custom properties to limit
   risk of clashes with other implementations of catalog zones.  This
   can be achieved by using two labels as the <property-prefix> so that
   the name of the implementation is included in the prefix: <some-

   Implementations MAY use such properties on the member zone level to
   store additional information about member zones (e.g., to flag them
   for specific treatment).

   Further, implementations MAY use custom properties on the global
   level to store additional information about the catalog zone itself.
   While there may be many use cases for this, a plausible one is to
   store default values for custom properties on the global level, then
   override them using a property of the same name on the member level
   (= under the ext label of the member node) if so desired.  A property
   agreement between producer and consumer should clearly define what
   semantics apply and whether a property is global, member, or both.

   The meaning of the custom properties described in this section is
   determined by the implementation alone without expectation of

5.  Name Server Behavior

5.1.  General Requirements

   As it is a regular DNS zone, a catalog zone can be transferred using
   DNS zone transfers among name servers.

   Catalog updates should be automatic; i.e., when a name server that
   supports catalog zones completes a zone transfer for a catalog zone,
   it SHOULD apply changes to the catalog within the running name server
   automatically without any manual intervention.

   Name servers MAY allow loading and transfer of broken zones with
   incorrect catalog zone syntax (as they are treated as regular zones).
   The reason a catalog zone is considered broken SHOULD be communicated
   clearly to the operator (e.g., through a log message).

   When a previously correct catalog zone becomes a broken catalog zone,
   it loses its catalog meaning because of an update through an
   incremental transfer or otherwise.  No special processing occurs.
   Member zones previously configured by this catalog MUST NOT be
   removed or reconfigured in any way.

   If a name server restarts with a broken catalog zone, the broken
   catalog SHOULD NOT prevent the name server from starting up and
   serving the member zones in the last valid version of the catalog

   Processing of a broken catalog SHALL start (or resume) when the
   catalog turns into a correct catalog zone, e.g., by an additional
   update (through zone transfer or updates) fixing the catalog zone.

   Similarly, when a catalog zone expires, it loses its catalog meaning
   and MUST no longer be processed as such.  No special processing
   occurs until the zone becomes fresh again.

5.2.  Member Zone Name Clash

   If there is a clash between an existing zone's name (from either an
   existing member zone or an otherwise configured zone) and an incoming
   member zone's name (via transfer or update), the new instance of the
   zone MUST be ignored and an error SHOULD be logged.

   A clash between an existing member zone's name and an incoming member
   zone's name (via transfer or update) may be an attempt to migrate a
   zone to a different catalog, but it should not be treated as one
   except as described in Section 4.3.1.

5.3.  Member Zone Removal

   When a member zone is removed from a specific catalog zone, a
   consumer MUST NOT remove the zone and associated state data if the
   zone was not configured from that specific catalog zone.  The zone
   and associated state (such as zone data and DNSSEC keys) MUST be
   removed from the consumer when and only when the zone was configured
   initially from the same catalog.  Consumer operators may consider
   temporarily archiving associated state to facilitate mistake

5.4.  Member Node Name Change

   When the member node's label value (<unique-N>) changes via a single
   update or transfer, catalog consumers MUST process this as a member
   zone removal, including the removal of all the zone's associated
   state (as described in Section 5.3), and then immediately process the
   member as a newly added zone to be configured in the same catalog.

5.5.  Migrating Member Zones between Catalogs

   If all consumers of the catalog zones involved support the coo
   property, it is RECOMMENDED to perform migration of a member zone by
   following the procedure described in Section 4.3.1.  Otherwise, the
   migration of a member zone from a catalog zone $OLDCATZ to a catalog
   zone $NEWCATZ has to be done by first removing the member zone from
   $OLDCATZ and then adding the member zone to $NEWCATZ.

   If in the process of a migration some consumers of the involved
   catalog zones did not catch the removal of the member zone from
   $OLDCATZ yet (because of a lost packet or downtime or otherwise) but
   already saw the update of $NEWCATZ containing the addition of that
   member zone, they may consider this update to be a name clash (see
   Section 5.2) and, as a consequence, the member is not migrated to
   $NEWCATZ.  This possibility needs to be anticipated with a member
   zone migration.  Recovery from such a situation is out of the scope
   of this document.  For example, it may entail a manually forced
   retransfer of $NEWCATZ to consumers after they have been detected to
   have received and processed the removal of the member zone from

5.6.  Zone-Associated State Reset

   It may be desirable to reset state (such as zone data and DNSSEC
   keys) associated with a member zone.

   A zone state reset may be performed by a change of the member node's
   name (see Section 5.4).

6.  Implementation and Operational Notes

   Although any valid domain name can be used for the catalog name
   $CATZ, a catalog producer MUST NOT use names that are not under the
   control of the catalog producer (with the exception of reserved
   names).  It is RECOMMENDED to use either a domain name owned by the
   catalog producer or a domain name under a suitable name such as
   "invalid."  [RFC6761].

   Catalog zones on secondary name servers would have to be set up
   manually, perhaps as static configuration, similar to how ordinary
   DNS zones are configured when catalog zones or another automatic
   configuration mechanism are not in place.  Additionally, the
   secondary needs to be configured as a catalog consumer for the
   catalog zone to enable processing of the member zones in the catalog,
   such as automatic synchronization of the member zones for secondary

   Operators of catalog consumers should note that secondary name
   servers may receive DNS NOTIFY messages [RFC1996] for zones before
   they are seen as newly added member zones to the catalog from which
   that secondary is provisioned.

   Although they are regular DNS zones, catalog zones only contain
   information for the management of a set of authoritative name
   servers.  To prevent unintended exposure to other parties, operators
   SHOULD limit the systems able to query these zones.

   Querying/serving catalog zone contents may be inconvenient via DNS
   due to the nature of their representation.  Therefore, an
   administrator may want to use a different method for looking at data
   inside the catalog zone.  Typical queries might include dumping the
   list of member zones, dumping a member zone's effective
   configuration, querying a specific property value of a member zone,
   etc.  Because of the structure of catalog zones, it may not be
   possible to perform these queries intuitively, or in some cases at
   all, using DNS QUERY.  For example, it is not possible to enumerate
   the contents of a multivalued property (such as the list of member
   zones) with a single QUERY.  Implementations are therefore advised to
   provide a tool that uses either the output of AXFR or an out-of-band
   method to perform queries on catalog zones.

   Great power comes with great responsibility.  Catalog zones simplify
   zone provisioning by orchestrating zones on secondary name servers
   from a single data source: the catalog.  Hence, the catalog producer
   has great power and changes must be treated carefully.  For example,
   if the catalog is generated by some script and this script generates
   an empty catalog, millions of member zones may get deleted from their
   secondaries within seconds, and all the affected domains may be
   offline in a blink of an eye.

7.  Security Considerations

   As catalog zones are transmitted using DNS zone transfers, it is
   RECOMMENDED that catalog zone transfers be protected from unexpected
   modifications by way of authentication, e.g., by using a Transaction
   Signature (TSIG) [RFC8945] or Strict or Mutual TLS authentication
   with DNS zone transfer over TLS or QUIC [RFC9103].

   Use of DNS UPDATE [RFC2136] to modify the content of catalog zones
   SHOULD similarly be authenticated.

   Zone transfers of member zones SHOULD similarly be authenticated.
   TSIG shared secrets used for member zones SHOULD NOT be mentioned in
   the catalog zone data.  However, key identifiers may be shared within
   catalog zones.

   Catalog zones reveal the zones served by their consumers, including
   their properties.  To prevent unintentional exposure of catalog zone
   contents, it is RECOMMENDED to limit the systems able to query them
   and to conduct catalog zone transfers confidentially [RFC9103].

   As with regular zones, primary and secondary name servers for a
   catalog zone may be operated by different administrators.  The
   secondary name servers may be configured as a catalog consumer to
   synchronize catalog zones from the primary, but the primary's
   administrators may not have any administrative access to the

   Administrative control over what zones are served from the configured
   name servers shifts completely from the server operator (consumer) to
   the "owner" (producer) of the catalog zone content.  To prevent
   unintended provisioning of zones, a consumer(s) SHOULD scope the set
   of admissible member zones by any means deemed suitable (such as
   statically via regular expressions, or dynamically by verifying
   against another database before accepting a member zone).

   With migration of member zones between catalogs using the coo
   property, it is possible for the owner of the target catalog (i.e.,
   $NEWCATZ) to take over all its associated state with the zone from
   the original owner (i.e., $OLDCATZ) by maintaining the same member
   node label (i.e., <unique-N>).  To prevent the takeover of the zone-
   associated state, the original owner has to enforce a zone state
   reset by changing the member node label (see Section 5.6) before or
   simultaneously with adding the coo property.

8.  IANA Considerations

   IANA has created the "DNS Catalog Zones Properties" registry under
   the "Domain Name System (DNS) Parameters" registry as follows:

   Registry Name:  DNS Catalog Zones Properties

   Assignment Policy:  Expert Review, except for property prefixes
      ending in the label "ext", which are for Private Use [RFC8126].

   Reference:  RFC 9432

   Note:  This registry applies to Catalog Zones schema version "2" as
      specified in RFC 9432.

    | Property Prefix | Description          | Status    | Reference |
    | zones           | List of member zones | Standards | RFC 9432  |
    |                 |                      | Track     |           |
    | version         | Schema version       | Standards | RFC 9432  |
    |                 |                      | Track     |           |
    | coo             | Change of Ownership  | Standards | RFC 9432  |
    |                 |                      | Track     |           |
    | group           | Group                | Standards | RFC 9432  |
    |                 |                      | Track     |           |
    | *.ext           | Custom properties    | Private   | RFC 9432  |
    |                 |                      | Use       |           |

              Table 1: DNS Catalog Zones Properties Registry

   The meanings of the fields are as follows:

   Property prefix:  One or more domain name labels.

   Description:  A human-readable short description or name for the

   Status:  IETF Stream RFC status or "External" if not documented in an
      IETF Stream RFC.

   Reference:  A stable reference to the document in which this property
      is defined.

9.  References

9.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
              August 1996, <https://www.rfc-editor.org/info/rfc1996>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,

   [RFC2606]  Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, DOI 10.17487/RFC2606, June 1999,

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,

   [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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [RFC8945]  Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D.,
              Gudmundsson, O., and B. Wellington, "Secret Key
              Transaction Authentication for DNS (TSIG)", STD 93,
              RFC 8945, DOI 10.17487/RFC8945, November 2020,

   [RFC9103]  Toorop, W., Dickinson, S., Sahib, S., Aras, P., and A.
              Mankin, "DNS Zone Transfer over TLS", RFC 9103,
              DOI 10.17487/RFC9103, August 2021,

9.2.  Informative References

   [FOSDEM20] Vandewoestijne, L., "Extending Catalog zones - another
              approach in automating maintenance", February 2020,

              Vixie, P., "Federated Domain Name Service Using DNS
              Metazones", DOI 10.1093/ietcom/e89-b.4.1144, April 2006,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

Appendix A.  Catalog Zone Example

   The following is a full example of a catalog zone containing three
   member zones with various properties:

   catalog.invalid.                                0  SOA   invalid. (
                           invalid. 1625079950 3600 600 2147483646 0 )
   catalog.invalid.                                0  NS    invalid.
   example.vendor.ext.catalog.invalid.             0  CNAME example.net.
   version.catalog.invalid.                        0  TXT   "2"
   nj2xg5b.zones.catalog.invalid.                  0  PTR   example.com.
   nvxxezj.zones.catalog.invalid.                  0  PTR   example.net.
   group.nvxxezj.zones.catalog.invalid.            0  TXT   (
                           "operator-x-foo" )
   nfwxa33.zones.catalog.invalid.                  0  PTR   example.org.
   coo.nfwxa33.zones.catalog.invalid.              0  PTR   (
                           newcatz.invalid. )
   group.nfwxa33.zones.catalog.invalid.            0  TXT   (
                           "operator-y-bar" )
   metrics.vendor.ext.nfwxa33.zones.catalog.invalid. 0  CNAME (
                           collector.example.net. )


   Our deepest thanks and appreciation go to Stephen Morris, Ray Bellis,
   and Witold Krecicki who initiated this document and did the bulk of
   the work.

   Catalog zones originated as the chosen method among various proposals
   that were evaluated at Internet Systems Consortium (ISC) for easy
   zone management.  The chosen method of storing the catalog as a
   regular DNS zone was proposed by Stephen Morris.

   The initial authors discovered that Paul Vixie's earlier [Metazones]
   proposal implemented a similar approach, and they reviewed it.
   Catalog zones borrow some syntax ideas from [Metazones], as both
   share this scheme of representing the catalog as a regular DNS zone.

   Thanks to Leo Vandewoestijne.  Leo's presentation in the DNS devroom
   at FOSDEM'20 [FOSDEM20] was one of the motivations to take up and
   continue the effort of standardizing catalog zones.

   Thanks to Joe Abley, David Blacka, Brian Conry, Klaus Darilion, Brian
   Dickson, Tony Finch, Evan Hunt, Shane Kerr, Warren Kumari, Patrik
   Lundin, Matthijs Mekking, Victoria Risk, Josh Soref, Petr Spacek,
   Michael StJohns, Carsten Strotmann, and Tim Wicinski for reviewing
   earlier draft versions and offering comments and suggestions.

Authors' Addresses

   Peter van Dijk
   Den Haag
   Email: peter.van.dijk@powerdns.com

   Libor Peltan
   Czech Republic
   Email: libor.peltan@nic.cz

   Ondrej Sury
   Internet Systems Consortium
   Czech Republic
   Email: ondrej@isc.org

   Willem Toorop
   NLnet Labs
   Science Park 400
   1098 XH Amsterdam
   Email: willem@nlnetlabs.nl

   Kees Monshouwer
   Email: mind@monshouwer.eu

   Peter Thomassen
   deSEC, SSE - Secure Systems Engineering
   Email: peter@desec.io

   Aram Sargsyan
   Internet Systems Consortium
   Email: aram@isc.org
  1. RFC 9432