Internet-Draft Generalized Notifications August 2023
Stenstam, et al. Expires 8 February 2024 [Page]
DNSOP Working Group
Intended Status:
Standards Track
J. Stenstam
The Swedish Internet Foundation
P. Thomassen
deSEC, Secure Systems Engineering
J. Levine
Standcore LLC

Generalized DNS Notifications


Changes in CDS/CDNSKEY, CSYNC, and other records related to delegation maintenance are usually detected through scheduled scans run by the consuming party (e.g. top-level domain registry), incurring an uncomfortable trade-off between scanning cost and update latency.

A similar problem exists when scheduling zone transfers, and has been solved using the well-known DNS NOTIFY mechanism ([RFC1996]). This mechanism enables a primary nameserver to proactively inform secondaries about zone changes, allowing the secondary to initiate an ad-hoc transfer independently of when the next SOA check would be due.

This document extends the use of DNS NOTIFY beyond conventional zone transfer hints, bringing the benefits of ad-hoc notifications to DNS delegation maintenance in general. Use cases include DNSSEC key rollovers hints via NOTIFY(CDS) and NOTIFY(DNSKEY) messages, and quicker changes to a delegation's NS record set via NOTIFY(CSYNC) messages.

Furthermore, this document proposes a new DNS record type, tentatively referred to as "NOTIFY record", which is used to publish details about where generalized notifications should be sent.

TO BE REMOVED: This document is being collaborated on in Github at: The most recent working version of the document, open issues, etc. should all be available there. The authors (gratefully) accept pull requests.

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

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 8 February 2024.

1. Introduction

This document introduces a generalization of the DNS NOTIFY mechanism described in [RFC1996].

Traditional DNS notifications, which are here referred to as "NOTIFY(SOA)", are sent from a primary server to a secondary server to minimize the latter's convergence time to a new version of the zone. This mechanism successfully addresses a significant inefficiency in the original protocol.

Today there are several new cases where similar inefficiencies cause significant problems. However, just as in the NOTIFY(SOA) case, a new set of notification types will have a major positive benefit by allowing the DNS infrastructure to completely sidestep these inefficiencies.

There are no DNS protocol changes introduced by this document.

There are, however, proposals for how to interpret a wider range of DNS messages that are already allowed (but not used) by the protocol.

Readers are expected to be familiar with DNSSEC, including [RFC4033], [RFC4034], [RFC4035], [RFC6781], [RFC7344], [RFC7477], [RFC7583], and [RFC8901].

1.1. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.2. Terminology

In the text below there are two different uses of the term "NOTIFY". One refers to the NOTIFY message, sent from a DNSSEC signer or name server to a notification target (for subsequent processing). We refer to this message as NOTIFY(RRtype) where the RRtype indicates the type of NOTIFY message (CDS, CSYNC or DNSKEY respectively).

The second is a proposed new DNS record type, with the suggested mnemonic "NOTIFY". This record is used to publish the location of the notification target. We refer to this as the "NOTIFY record".

2. Costs and Dangers of Slow Convergence

[RFC1996] introduced the concept of a DNS Notify message which was used to improve the convergence time for secondary servers when a DNS zone had been updated in the primary. The basic idea was to augment the traditional "pull" mechanism (a periodic SOA query) with a "push" mechanism (a Notify) for a common case that was otherwise very inefficient (due to either slow convergence or wasteful overly frequent scanning of the primary for changes).

Today we have similar issues with slow updates of DNS data in spite of the data having been published. The two most obvious cases are the scalability issues of modern CDS and CSYNC scanners that are beginning to be deployed in a growing number of TLD registries. Because of the large number of child delegations, scanning for CDS and CSYNC records is rather slow (as in infrequent).

Another use case is for so-called "multi-signer" setups where there are multiple, semi-independent, "signers" that each sign the same zone, but with individual sets of keys. One requirement for multi-signer setups to work is the ability to insert additional DNSKEY records in each signer's version of the zone. (This is not the only multi-signer requirement, but it is the one that matters here.) The problem here is that modern signers will commonly roll DNSSEC Zone Signing Keys automatically and without informing anyone, assuming a single-signer setup where ZSK rollovers are a local matter (as opposed to KSK rollovers that involve the parent). As a result, when the ZSKs of one signer are rolled, the other signers will be unaware of this event. However, to enable validation of signatures using a new ZSK, it needs to be announced by all nameservers that are authoritative for the child. To achieve this, it is therefore necessary to bring the signers' DNSKEY RRsets back in sync. Such re-synchronization can either happen based on a schedule, or on demand, given a suitable trigger.

3. Efficiency Issues with DNS Scanning vs. Notification

The original problem that [RFC1996] addressed was the problem of optimization between frequent checking for new versions of a zone by a secondary and infrequent checking. While it is possible to indicate how frequently checks should occur (via the SOA Refresh parameter), these checks did not allow catching zone changes that fall between checkpoints. NOTIFY replaces scheduled scanning with a more efficient mechanism.

In modern DNS infrastructure there are several new scanning-based inefficiencies that did not exist at the time of the writing of [RFC1996].

3.1. CDS and CDNSKEY Scanners

An increasing number of TLD registries are implementing periodic scanning for CDS and CDNSKEY records in child zones. Because of the large number of delegations this is rather costly, especially as it is only a very small number of the signed delegations that will have updated the CDS or CDNSKEY record between two scanning runs.

Frequent scanning is costly. Infrequent scanning causes slower convergence (i.e. delay until the DS in the parent is updated).

Not every zone runs a CDS or CSYNC scanner (today). However, for those that do, the problem is actually worse than in the traditional primary to secondary case. The reason is that in the original case the primary is able to indicate how frequently changes are to be expected (via the SOA Refresh parameter). No equivalent mechanism exist for the new scanning services.

3.2. CSYNC Scanners

This is basically the same problem as for the CDS / CDNSKEY scanners: large number of delegations vs infrequent updates to the CSYNC record in the child zones.

Frequent scanning is costly. Infrequent scanning causes slower convergence (i.e. delay until the delegation information in the parent is updated).

3.3. Multi-Signer Setups

[I-D.wisser-dnssec-automation] describes processes for managing signed zones using multiple semi-independent "signers" (i.e. services that take an unsigned zone, sign it using unique DNSKEYs and publish the zone on the public Internet). The setup most commonly discussed for multi-signer uses a "multi-signer controller" which is a separate service responsible for keeping the signing and delegation information in sync across multiple signers.

To keep information in sync, the signers need to be scanned for current information about the DNSKEY RRset in each signer. The problem is that modern "signers" will typically do DNSKEY rollovers (especially ZSK rollovers) automatically without informing anyone else, because a single-signer setup is often assumed (in which case the ZSK rollover is a matter completely internal to the signer).

In the multi-signer case, this is not a correct assumption. It is therefore necessary to run polls frequently to minimize the time window between one signer changing its version of the DNSKEY RRset and the controller noticing and re-synchronizing all signers.

Frequent scanning: costly. Infrequent scanning: slower convergence (i.e. longer potential inconsistency across signers).

4. CDS/CDNSKEY and CSYNC Notifications

The [RFC1996] NOTIFY message sends a SOA record in the Query Section. We refer to this as a NOTIFY(SOA). By generalizing the concept of DNS NOTIFY it is possible to address not only the original inefficiency (primary name server to secondary nameserver convergence) but also the problems with CDS/CDNSKEY, CSYNC and Multi-Signer scanning for zone updates.

The CDS/CDNSKEY inefficiency may be addressed by the child sending a NOTIFY(CDS) to an address where the parent listens for such notifications.

While the receiving side will often be a scanning service provided by the registry itself, it is expected that in the ICANN RRR model, some registries will prefer registrars to conduct CDS/CDNSKEY scans. For such parents, the receiving service should notify the appropriate registrar, over any communication channel deemed suitable between registry and registrar (such as EPP, or possibly by forwarding the actual NOTIFY(CDS) or NOTIFY(CSYNC) directly). Such internal processing is inconsequential from the perspective of the child: the NOTIFY packet is simply sent to the notification address.

To address the CDS/CDNSKEY dichotomy, NOTIFY(CDS) is defined to indicate any child-side changes pertaining to a upcoming update of DS records. Upon receipt of NOTIFY(CDS), the parent SHOULD initiate the same scan that would otherwise be triggered based on a timer.

The CSYNC inefficiency may similarly be addressed by the child sending a NOTIFY(CSYNC) to an address where the parent is listening to CSYNC notifications.

In both cases the notification will speed up the CDS and CSYNC scanning by providing the parent with a hint that a particular child zone has published new CDS, CDNSKEY and/or CSYNC records.

The DNS protocol already allows these new notification types, so no protocol change is required. The only thing needed is specification of where to send the new types of Notifies and how to interpret them in the receiving end.

4.1. Where to send CDS and CSYNC Notifications

In the case of NOTIFY(CDS) and NOTIFY(CSYNC) the ultimate recipient of the notification is the parent, to improve the speed and efficiency of the parent's CDS/CSYNC scanning. Because the name of the parent zone is known, and because the parent obviously controls the contents of its own zone the simplest solution is for the parent to publish the address where it prefers to have notifications sent.

However, there may exist cases where this scheme (sending the notification to the parent) is not sufficient and a more general design is needed. At the same time, it is strongly desireable that the child is able to figure out where to send the NOTIFY via a single query.

By adding the following to the zone parent.:

parent.   IN NOTIFY CDS   scheme port scanner.parent.
parent.   IN NOTIFY CSYNC scheme port scanner.parent.

where the only scheme defined here is scheme=1 with the interpretation that when a new CDS (or CDNSKEY or CSYNC) is published, a NOTIFY(CDS) or NOTIFY(CSYNC) should be sent to the address and port listed in the corresponding NOTIFY RRset.


parent.   IN NOTIFY CDS   1 5359 cds-scanner.parent.
parent.   IN NOTIFY CSYNC 1 5360 csync-scanner.parent.

Other schemes are possible, but are out of scope for this document.

The suggested mnemonic for the new record type is "NOTIFY" and it is further described below.

4.2. How to Interpret CDS and CSYNC Notifications

Upon receipt of a NOTIFY(CDS) for a particular child zone at the published address for CDS notifications, the parent has two options:

  1. Schedule an immediate check of the CDS and CDNSKEY RRsets as published by that particular child zone.

    If the check finds that the CDS/CDNSKEY RRset has indeed changed, the parent MAY reset the scanning timer for children for which NOTIFY(CDS) is received, or reduce the periodic scanning frequency accordingly (e.g. to every two weeks). This will decrease the scanning effort for the parent. If a CDS/CDNSKEY change is then detected (without having received a notification), the parent SHOULD clear that state and revert to the default scanning schedule.

    Parents introducing CDS/CDNSKEY scanning support at the same time as NOTIFY(CDS) support are not in danger of breaking children's scanning assumption, and MAY therefore use a low-frequency scanning schedule in default mode.

  2. Ignore the notification, in which case the system works exactly as before.

If the parent implements the first option, the convergence time (time between publication of a new CDS and propagation of the resulting DS) will decrease significantly, thereby providing improved service to the child zone.

If the parent, in addition to scheduling an immediate check for the child zone of the notification, also choses to modify the scanning schedule (to be less frequent), the cost of providing the scanning service will be reduced.

Upon receipt of a NOTIFY(CSYNC) to the published address for CSYNC notifications, the parent has exactly the same options to choose among as for the NOTIFY(CDS).

5. DNSKEY Notifications

In the multi-signer case the problem is slightly different, because it is not the parent that should be notified, but rather a third party.

5.1. Where to send DNSKEY Notifications

The question is how the multi-signer controller should inform the signer about where to send the notification. In the NOTIFY(CDS) and NOTIFY(CSYNC) cases the child (the service that signs the child zone) knows the name of the parent zone and can look up the notification address there. In the NOTIFY(DNSKEY) case, there is no trivial target.

However, it is possible to look up the notification address in the child zone itself. This translates into a requirement for multi-signer setups, namely that at least one external party (typically a multi-signer controller) has the ability to insert or modify RRsets in the child zone. Therefore the controller should be able to insert a record that documents the wanted notification address for NOTIFY(DNSKEY) messages.

Also in the NOTIFY(DNSKEY) case there is the possibility that this scheme will not be sufficient for all cases. And therefore the proposed design (in analogy with the NOTIFY(CDS) and NOTIFY(CSYNC) cases) is:

child.parent. IN NOTIFY DNSKEY scheme port scanner.signerA.
child.parent. IN NOTIFY DNSKEY scheme port scanner.signerB.

with the only defined scheme at this time being scheme=1 with the interpretation that whenever there are changes to the DNSKEY RRset in a signer it should send a corresponding NOTIFY(DNSKEY) to all notification addresses listed in the "child.parent. NOTIFY" RRset.


child.parent. IN NOTIFY DNSKEY 1 5361 scanner.signerA.
child.parent. IN NOTIFY DNSKEY 1 5361 scanner.signerB.
child.parent. IN NOTIFY DNSKEY 1 5361 ctrl.multi-signer.example.

Other schemes are possible, but are out of scope for this document.

5.2. How to Interpret DNSKEY Notifications

The receipt of a NOTIFY(DNSKEY) is a hint that the DNSKEY RRset at the sending signer has been updated. If the child zone is not part of a multi-signer setup this should be a no-op that does not cause any action. In a multi-signer setup, though, this hint provides the recipient of the notification with the same options as in the NOTIFY(CDS) and NOTIFY(CSYNC) cases: schedule an immediate check, or ignore.

6. Who Should Send the Notifications?

Because of the security model where a notification by itself never causes a change (it can only speed up the time until the next check for the same thing), the sender's identity is not crucial.

This opens up the possibility of having the controller in a multi-signer setup send the CDS and CSYNC notifications to the parent, thereby enabling this new functionality even before the emergence of support for generalized DNS notifications in the name servers used for signing.

However, as multi-signer is strongly dependent on DNSKEY notifications (which, in case of an automatic ZSK rollover, can only be generated by the signing software), this does not alleviate the need for modifications also to signing software to support NOTIFY(DNSKEY).

7. Timing Considerations

When a primary name server publishes a new CDS RRset there will be a time delay until all copies of the zone world-wide will have been updated. Usually this delay is short (on the order of seconds), but it is larger than zero. If the primary sends a NOTIFY(CDS) at the exact time of publication of the new zone there is a potential for the parent to schedule the CDS check that the notification triggers faster than the zone propagates to all secondaries. In this case the parent may draw the wrong conclusion ("the CDS RRset has not been updated").

Having a delay between the publication of the new data and the check for the new data would alleviate this issue. However, as the parent has no way of knowing how quickly the child zone propagates, the appropriate amount of delay is uncertain. It is therefore RECOMMENDED that the child delays sending NOTIFY packets to the parent until a consistent public view of the pertinent records is ensured.

NOTIFY(CSYNC) has the same timing consideration as NOTIFY(CDS) while NOTIFY(DNSKEY) does not.

8. The Format of the NOTIFY Record

Here is the format of the NOTIFY RR, whose DNS type code is not yet defined.

    Name TTL Class NOTIFY RRtype Scheme Port Target

Name The zone this RR refers to. In the case of NOTIFY(CDS) and NOTIFY(CSYNC) this is the name of the parent zone. In the case of NOTIFY(DNSKEY) this is the name of the zone in which a change in the DNSKEY RRset is taking place.

TTL Standard DNS meaning [RFC 1035].

Class Standard DNS meaning [RFC 1035]. NOTIFY records occur in the IN Class.

RRtype The type of generalized NOTIFY that this NOTIFY RR defines the desired target address for. Currently only the types CDS, CSYNC and DNSKEY are suggested to be supported, but there is no protocol issue should a use case for additional types of notifications arise in the future.

Scheme The scheme for locating the desired notification address. The range is 0-255. This is a 16 bit unsigned integer in network byte order. The value 0 is an error. The value 1 is described in this document and all other values are currently unspecified.

Port The port on the target host of the notification service. The range is 0-65535. This is a 16 bit unsigned integer in network byte order.

Target The domain name of the target host providing the service of listening for generalized notifications of the specified type. This name MUST resolve to one or more address records.

9. Open Questions

9.1. Rationale for a new record type, "NOTIFY"

It is technically possible to store the same information in an SRV record as in the proposed NOTIFY record. This would, however, require name space pollution (indicating the RRtype via a label in the owner name) and also changing the semantics of one of the integer fields of the SRV record.

Overloading semantics on a single record type has not been a good idea in the past. Furthermore, as the generalized notifications are a new proposal with no prior deployment on the public Internet there is an opportunity to avoid repeating previous mistakes.

In the case of the "vertical" NOTIFY(CDS) and NOTIFY(CSYNC), no special processing needs to be applied by the authoritative nameserver upon insertion of the record indicating the notification target. The nameserver can be "unaware"; a conventional SRV record would therefore suffice from a processing point of view. In the case of the more "horizontal" NOTIFY(DNSKEY), however, the nameserver will have to act on the insertion of a

    zone.example.   IN NOTIFY DNSKEY ...


A new record type would therefore make it possible to more easily associate the special processing with the record's insertion. The NOTIFY record type would provide a cleaner solution to all the new types of notification signaling. Eg.:

parent.         IN NOTIFY  CDS     1  59   scanner.parent.
parent.         IN NOTIFY  CSYNC   1  59   scanner.parent.
child.parent.   IN NOTIFY  DNSKEY  1  5900 ctrl.multi-signer.example.

9.2. Rationale for Keeping DNS Message Format and Transport

In the most common cases of using generalized notifications the recipient is expected to not be a nameserver, but rather some other type of service, like a CDS/CSYNC scanner.

However, this will likely not always be true. In particular it seems likely that in cases where the parent is not a large delegation-centric zone like a TLD, but rather a smaller zone with a small number of delegations there will not be separate services for everything and the recipient of the NOTIFY(CDS) or NOTIFY(CSYNC) will be an authoritative nameserver for the parent zone.

Another case where this seems likely is in controller-less multi-signer setups where there is no central controller. Instead the multi-signer algorithms will be implemented inside or near each participating signer. Also in these cases it seems likely that the recipient in some cases will be an authoritative nameserver.

For this reason it seems most reasonable to stay within the the well documented and already supported message format specified in RFC 1996 and delivered over normal DNS transport, although not necessarily to port 53.

9.3. Open Question For DNSKEY Notifications

In a multi-signer setup there are multiple signers. How will the multi-signer controller know which signer sent the notification? As the number of signers for a particular zone is low (typically 2 or possibly 3), there is no major problem caused by not knowing which signer sent the notification and instead always check all the signers for updates to the DNSKEY RRset.

10. Out of Scope

To accommodate ICANN's RRR model, the parent's designated notification target may forward NOTIFY(CDS) messages to the registrar, e.g. via EPP or by forwarding the NOTIFY(CDS) message directly. The same is true also for NOTIFY(CSYNC).

From the perspective of this protocol, the NOTIFY(CDS) packet is simply sent to the parent's listener address. However, should this turn out not to be sufficient, it is possible to define a new "scheme" that specifies alternative logic for dealing with such requirements. Description of internal processing in the recipient end or for locating the recipient are out of scope of this document.

In the multi-signer case, where the same zone is signed by multiple semi-independent "signers" it is obviously necessary to devise means of keeping the non-DNSSEC contents of the zone in sync. That is out of scope of the multi-signer work and also out of scope of this document.

A corner case of this scoping regards making changes to the NS RRset. Individual signers (as in DNS service providers) may want to be able to make such changes independently of the other contents of the zone.

11. Security Considerations

The original NOTIFY specification ([RFC1996]) sidesteps most security issues by not relying on the information in the NOTIFY message in any way but instead only use it to set the timer until the next scheduled check (for the SOA record) to zero. Therefore it is impossible to affect the behaviour of the recipient of the NOTIFY other than by changing the timing for when different checks are initiated.

This mechanism and security model is reused for all the generalized NOTIFY messages.

Another consideration is whether generalized DNS Notifications can be used as an amplification attack. The answer seems to be "NO", because the size of the generalized NOTIFY messages is mostly equal to the size of the response and it is also mostly equal to the size of the resulting outbound query (for the child zone's CDS, CSYNC or DNSKEY RRset).

Hence the amplification attack potential of generalized Notifications is the same as for the original NOTIFY(SOA), which has never been found to be useful for amplification attacks.

In any case, NOTIFY consumers MAY configure rate limits to address concerns about the impact of unsolicited NOTIFY messages.

12. IANA Considerations

Per [RFC8552], IANA is requested to create a new registry on the "Domain Name System (DNS) Parameters" IANA web page as follows:

Name: Generalized DNS Notifications

Assignment Policy: Expert Review

Reference: [this document]

Table 1
NOTIFY type Scheme Location Reference
CDS 1 parent. [this document]
CSYNC 1 parent. [this document]
DNSKEY 1 zone. [this document]

13. Acknowledgements

Joe Abley, Mark Andrews, Christian Elmerot, Olafur Gu[eth]mundsson, Paul Wouters, Brian Dickson

14. Normative References

Wisser, U. and S. Huque, "DNSSEC automation", Work in Progress, Internet-Draft, draft-wisser-dnssec-automation-03, , <>.
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, , <>.
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, , <>.
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, , <>.
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, , <>.
Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC Operational Practices, Version 2", RFC 6781, DOI 10.17487/RFC6781, , <>.
Kumari, W., Gudmundsson, O., and G. Barwood, "Automating DNSSEC Delegation Trust Maintenance", RFC 7344, DOI 10.17487/RFC7344, , <>.
Hardaker, W., "Child-to-Parent Synchronization in DNS", RFC 7477, DOI 10.17487/RFC7477, , <>.
Morris, S., Ihren, J., Dickinson, J., and W. Mekking, "DNSSEC Key Rollover Timing Considerations", RFC 7583, DOI 10.17487/RFC7583, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, , <>.
Huque, S., Aras, P., Dickinson, J., Vcelak, J., and D. Blacka, "Multi-Signer DNSSEC Models", RFC 8901, DOI 10.17487/RFC8901, , <>.

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

  • draft-thomassen-dnsop-generalized-dns-notify-02
  • Add rationale for staying in band

  • Add John as an author

  • draft-thomassen-dnsop-generalized-dns-notify-01
  • Mention Ry-to-Rr forwarding to accommodate RRR model

  • Add port number flexiblity

  • Add scheme parameter

  • Drop SRV-based alternative in favour of new NOTIFY RR

  • Editorial improvements

  • draft-thomassen-dnsop-generalized-dns-notify-00
  • Initial public draft.

Authors' Addresses

Johan Stenstam
The Swedish Internet Foundation
Peter Thomassen
deSEC, Secure Systems Engineering
John Levine
Standcore LLC