Internet-Draft dns-catalog-zones December 2020
van Dijk, et al. Expires 7 June 2021 [Page]
DNSOP Working Group
Intended Status:
Standards Track
P. van Dijk
L. Peltan
O. Sury
Internet Systems Consortium
W. Toorop
NLnet Labs
L. Vandewoestijne

DNS Catalog Zones


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

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

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This Internet-Draft will expire on 7 June 2021.

Table of Contents

1. Introduction

The data in a DNS zone is synchronized amongst its primary and secondary nameservers using AXFR and 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 nameserver farm not only has to add or remove the zone from the primary, they must also add/remove the zone from all secondaries, either manually or via an external application. This can be both inconvenient and error-prone; it is also dependent on the nameserver implementation.

This document describes a method in which the catalog is represented as a regular DNS zone (called a "catalog zone" here), and transferred using DNS zone transfers. As zones are added to or removed from the catalog zone, the changes are propagated to the secondary nameservers in the normal way. The secondary nameservers then add/remove/modify the zones they serve in accordance with the changes to the zone.

The contents and representation of catalog zones are described in Section 3. Nameserver behavior is described in Section 6.

2. Terminology

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.

Catalog zone
A DNS zone containing a DNS catalog, that is, a list of DNS zones.
Member zone
A DNS zone whose configuration is published inside a catalog zone.
Used in examples as a placeholder to represent the domain name of the catalog zone itself (c.f. $ORIGIN).

3. Description

A catalog zone is a specially crafted DNS zone that contains, as DNS zone data:

Implementations of catalog zones SHOULD ignore any RR in the catalog zone which is meaningless or useless to the implementation.

Authoritative servers may be preconfigured with multiple catalog zones, each associated with a different set of configurations. A member zone can as such be reconfigured with a different set of preconfigured settings by removing it as a member of one catalog zone and making it a member of another.

An implementation of catalog zones MAY allow the catalog to contain other catalog zones as member zones.

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

A catalog zone is primarily intended for the management of a farm of authoritative nameservers. It is not expected that the content of catalog zones will be accessible from any recursive nameserver.

4. Catalog Zone Structure

4.1. SOA and NS Records

As with any other DNS zone, a catalog zone MUST have a syntactically correct SOA record and at least one NS record at its apex.

The SOA record's SERIAL, REFRESH, RETRY and EXPIRE fields [RFC1035] are used during zone transfer. A catalog zone's SOA SERIAL field MUST increase when an update is made to the catalog zone's contents as per serial number arithmetic defined in [RFC1982]. Otherwise, secondary nameservers might not notice updates to the catalog zone's contents.

Should the zone be made available for querying, the SOA record's MINIMUM field's value is the negative cache time (as defined in [RFC2308]). Since recursive nameservers are not expected to be able to access (and subsequently cache) entries from a catalog zone a value of zero (0) is RECOMMENDED.

There is no requirement to be able to query the catalog zone via recursive nameservers. Implementations of catalog zones MUST ignore and MUST NOT assume or require NS records at the apex. However, at least one is still required so that catalog zones are syntactically correct DNS zones. A single NS RR with an NSDNAME field containing the absolute name "invalid." is RECOMMENDED [RFC2606].

4.2. Catalog Zone Schema Version

The catalog zone schema version is specified by an integer value embeded in a TXT RR named version.$CATZ. All catalog zones MUST have a TXT RRset named version.$CATZ with at least one RR. Primary and secondary nameservers MUST NOT use catalog zones without the expected value in one of the RRs in the version.$CATZ TXT RRset, but they may be transferred as ordinary zones. For this memo, the value of one of the RRs in the version.CATZ TXT RRset MUST be set to "2", i.e.

version.$CATZ 0 IN TXT "2"

NB: Version 1 was used in a draft version of this memo and reflected the implementation first found in BIND 9.11.

4.3. List of Member Zones

The list of member zones is specified as a collection of 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 (instead of as a part of owner names) so that all valid domain names may be represented regardless of their length [RFC1035].

For example, if a catalog zone lists three zones "", "" and "", the RRs would appear as follows:

<m-unique-1>.zones.$CATZ 0 IN PTR
<m-unique-2>.zones.$CATZ 0 IN PTR
<m-unique-3>.zones.$CATZ 0 IN PTR

where <m-unique-N> is a label that tags each record in the collection. Nameservers MUST accept catalog zones even with those labels not really unique; they MAY warn the user in such case.

Having a large number of member zones in a single RRset may cause the RRset to be too large to be conveyed via DNS messages which make up a zone transfer. Having the zones uniquely tagged with the <m-unique-N> label ensures the list of member zones can be split over multiple DNS messages in a zone transfer.

The <m-unique-N> label also enables the state for a zone to be reset. (see Section 6.1, Paragraph 9) As long as no zone state needs to be reset at the authoritative nameservers, the unique label associated with a zone SHOULD remain the same.

The CLASS field of every RR in a catalog zone MUST be IN (1).

The TTL field's value is not specially defined by this memo. Catalog zones are for authoritative nameserver management only and are not intended for general querying via recursive resolvers and therefore a value of zero (0) is RECOMMENDED.

Each RRSet of catalog zone, with the exception of the zone apex, SHOULD consist of just one RR. It's acceptable to generate owner names with the help of a sufficiently strong hash function, with small probablity that unrelated records fall within the same RRSet.

5. The Serial Property

The current default mechanism for prompting notifications of zone changes from a primary nameserver to the secondaries via DNS NOTIFY [RFC1996], can be unreliable due to packet loss, or secondary nameservers temporarily not being reachable. In such cases the secondary might pick up the change only after the refresh timer runs out, which might be long and out of the control of the nameserver operator. Low refresh values in the zones being served can alleviate update delays, but burdens the primary nameserver more severely with more refresh queries, especially with larger numbers of secondary nameservers serving large numbers of zones. Alternatively updates of zones MAY be signalled via catalog zones with the help of a serial property.

The serial number in the SOA record of the most recent version of a member zone MAY be provided by a serial property. When a serial property is present for a member zone, implementations of catalog zones MAY assume this number to be the current serial number in the SOA record of the most recent version of the member zone.

Nameservers that are secondary for that member zone, MAY compare the serial property with the SOA serial since the last time the zone was fetched. When the serial property is larger, the secondary MAY initiate a zone tranfer immediately without doing a SOA query first. The transfer MUST be aborted immediately when the serial number of the SOA resource record in the transfer is not larger than the SOA serial of the zone currently being served. In that case the zone transfer should be retried after the time given in the retry field of the SOA record of the member zone, or earlier if a new SOA serial number is learned via an updated serial property, or via NOTIFY [RFC1996].

When a serial property is present for a member zone and it matches the SOA serial of that member zone, implementations of catalog zones which are secondary for that member zone MAY ignore the refresh time in the SOA record of the member zone and rely on updates via the serial property of the member zone. A refresh timer of a catalog zone MUST not be ignored.

Primary nameservers MAY be configured to omit sending DNS NOTIFY messages to secondary nameservers which are known to process the serial property of the member zones in that catalog. However they MAY also combine signalling of zone changes with the serial property of a member zone, as well as sending DNS NOTIFY messages, to anticipate slow updates of the catalog zone (due to packet loss or other reasons) and to cater for secondaries that do not process the serial property.

All comparisons of serial numbers MUST use "Serial number arithmetic", as defined in [RFC1982]

Note to the DNSOP Working Group: In this section we present three ways to provide a serial property with a member zone. The first two ways make use of a new Resource Record type: SERIAL as described in Section 5.1, Section 5.2 and Section 5.3. The two different ways to provide a serial property with the SERIAL RR are described in Section 5.4 and Section 5.5 respectively. The third way is with a TXT RR and is described in Section 5.6.

5.1. The SERIAL Resource Record

The serial property value is provided with a SERIAL Resource Record. The Type value for the SERIAL RR is TBD. The SERIAL RR is class independent. The RDATA of the resource record consist of a single field: Serial.

5.2. SERIAL RDATA Wire Format

The SERIAL RDATA wire format is encoded as follows:

                     1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|                            Serial                             |

5.2.1. The Serial Field

The Serial field is a 32-bit unsigned integer in network byte order. It is the serial number of the member zone's SOA record ([RFC1035] section 3.3.13).

5.3. SERIAL Presentation Format

The presentation format of the RDATA portion is as follows:

The Serial fields is represented as an unsigned decimal integer.

5.4. SERIAL RR Usage - option 1

The serial property of a member zone is provided by a SERIAL RRset with a single SERIAL RR named serial.<m-unique-N>.zones.$CATZ.

For example, if a catalog zone lists three zones "", "" and "", and a serial property is provided for each of them, the RRs would appear as follows:

<m-unique-1>.zones.$CATZ        0 IN PTR
serial.<m-unique-1>.zones.$CATZ 0 IN SERIAL 2020111712
<m-unique-2>.zones.$CATZ        0 IN PTR
serial.<m-unique-2>.zones.$CATZ 0 IN SERIAL 2020111709
<m-unique-3>.zones.$CATZ        0 IN PTR
serial.<m-unique-3>.zones.$CATZ 0 IN SERIAL 2020112405

5.5. SERIAL RR Usage - option 2

The serial property of a member zone is provided by a SERIAL RRset on the same owner name as the PTR RR of the member zone.

For example, if a catalog zone lists three zones "", "" and "", and a serial property is provided for each of them, the RRs would appear as follows:

<m-unique-1>.zones.$CATZ 0 IN PTR
<m-unique-1>.zones.$CATZ 0 IN SERIAL 2020111712
<m-unique-2>.zones.$CATZ 0 IN PTR
<m-unique-2>.zones.$CATZ 0 IN SERIAL 2020111709
<m-unique-3>.zones.$CATZ 0 IN PTR
<m-unique-3>.zones.$CATZ 0 IN SERIAL 2020112405

5.6. Serial property as TXT RR - option 3

The serial property of a member zone is provided by a TXT RRset with a single TXT RR named serial.<m-unique-N>.zones.$CATZ. The TXT RR contains a single RDATA field consisting of the textual representation of the SOA serial number.

For example, if a catalog zone lists three zones "", "" and "", and a serial property is provided for each of them, the RRs would appear as follows:

<m-unique-1>.zones.$CATZ        0 IN PTR
serial.<m-unique-1>.zones.$CATZ 0 IN TXT 2020111712
<m-unique-2>.zones.$CATZ        0 IN PTR
serial.<m-unique-2>.zones.$CATZ 0 IN TXT 2020111709
<m-unique-3>.zones.$CATZ        0 IN PTR
serial.<m-unique-3>.zones.$CATZ 0 IN TXT 2020112405

6. Nameserver Behavior

6.1. General Requirements

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

Although they are regular DNS zones, catalog zones contain only information for the management of a set of authoritative nameservers. For this reason, operators may want to limit the systems able to query these zones. It may be inconvenient to serve some contents of catalog zones via DNS queries anyway due to the nature of their representation. A separate method of querying entries inside the catalog zone may be made available by nameserver implementations (see Section 7.1).

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

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

A catalog zone can be updated via DNS UPDATE on a reference primary nameserver, or via zone transfers. Nameservers MAY allow loading and transfer of broken zones with incorrect catalog zone syntax (as they are treated as regular zones), but nameservers MUST NOT process such broken zones as catalog zones. For the purpose of catalog processing, the broken catalogs MUST be ignored. If a broken catalog zone was transferred, the newly transferred catalog zone MUST be ignored (but the older copy of the catalog zone SHOULD be left running subject to values in SOA fields).

If there is a clash between an existing member zone's name 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.

When zones are introduced into a catalog zone, a primary SHOULD first make the new zones available for transfers before making the updated catalog zone available for transfer, or sending NOTIFY for the catalog zone to secondaries. Note that secondary nameservers may attempt to transfer the catalog zone upon refresh timeout, so care must be taken to make the member zones available before any update to the list of member zones is visible in the catalog zone.

When zones are deleted from a catalog zone, a primary MAY delete the member zone immediately after notifying secondaries. It is up to the secondary nameserver to handle this condition correctly.

When the <m-unique-N> label of a member zone changes, all its associated state MUST be reset, including the zone itself. This can be relevant for example when zone ownership is changed. In that case one does not want the new owner to inherit the metadata. Other situations might be resetting DNSSEC state, or forcing a new zone transfer. A simple removal followed by an addition of the member zone would be insufficient for this purpose because it is infeasible for secondaries to track, due to missed notifies or being offline during the removal/addition.

7. Updating Catalog Zones

TBD: Explain updating catalog zones using DNS UPDATE.

7.1. Implementation Notes

Catalog zones on secondary nameservers would have to be setup manually, perhaps as static configuration, similar to how ordinary DNS zones are configured. Members of such catalog zones will be automatically synchronized by the secondary after the catalog zone is configured.

An administrator may want to look at data inside a 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 multi-valued 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.

8. Implementation Status

Note to the RFC Editor: please remove this entire section before publication.

In the following implementation status descriptions, "DNS Catalog Zones" refers to DNS Catalog Zones as described in this document.

Interoperability between the above implementations has been tested during the hackathon at the IETF-109.

9. Security Considerations

As catalog zones are transmitted using DNS zone transfers, it is key for these transfers to be protected from unexpected modifications on the route. So, catalog zone transfers SHOULD be authenticated using TSIG [RFC8945]. A primary nameserver SHOULD NOT serve a catalog zone for transfer without using TSIG and a secondary nameserver SHOULD abandon an update to a catalog zone that was received without using TSIG.

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

Zone transfers of member zones SHOULD similarly be authenticated using TSIG [RFC8945]. The TSIG shared secrets used for member zones MUST NOT be mentioned anywhere in the catalog zone data. However, key identifiers may be shared within catalog zones.

Catalog zones do not need to be signed using DNSSEC, their zone transfers being authenticated by TSIG. Signed zones MUST be handled normally by nameservers, and their contents MUST NOT be DNSSEC- validated.

10. IANA Considerations

This document has no IANA actions.


11. Acknowledgements

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

Catalog zones originated as the chosen method among various proposals that were evaluated at 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 reviewed it. Catalog zones borrows some syntax ideas from Metazones, as both share this scheme of representing the catalog as a regular DNS zone.

Thanks to Brian Conry, Tony Finch, Evan Hunt, Patrik Lundin, Victoria Risk and Carsten Strotmann, for reviewing draft proposals and offering comments and suggestions.

Thanks to Klaus Darilion who came up with the idea for the serial property during the hackathon at the IETF-109. Thanks also to Shane Kerr, Petr Spacek, Brian Dickson for further brainstorming and discussing the serial property and how it would work best with catalog zones.

12. Normative References

Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, , <>.
Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982, DOI 10.17487/RFC1982, , <>.
Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
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, , <>.
Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, DOI 10.17487/RFC2308, , <>.
Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS Names", BCP 32, RFC 2606, DOI 10.17487/RFC2606, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
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, , <>.

13. Informative References

Vixie, P., "Federated Domain Name Service Using DNS Metazones", , <>.

Appendix A. Change History (to be removed before final publication)

Initial public draft.

Added Witold, Ray as authors. Fixed typos, consistency issues. Fixed references. Updated Area. Removed newly introduced custom RR TYPEs. Changed schema version to 1. Changed TSIG requirement from MUST to SHOULD. Removed restrictive language about use of DNS QUERY. When zones are introduced into a catalog zone, a primary SHOULD first make the new zones available for transfers first (instead of MUST). Updated examples, esp. use IPv6 in examples per Fred Baker. Add catalog zone example.

Addressed some review comments by Patrik Lundin.

Revision bump.

Reordering of sections into more logical order. Separation of multi-valued properties into their own category.

New authors to pickup the editor pen on this draft

Remove data type definitions for zone properties Removing configuration of member zones through zone properties altogether

Remove Open issues and discussion Appendix, which was about zone options (including primary/secondary relationships) only.

Authors' Addresses

Peter van Dijk
Den Haag
Libor Peltan
Ondrej Sury
Internet Systems Consortium
Willem Toorop
NLnet Labs
Science Park 400
1098 XH Amsterdam
Leo Vandewoestijne