Network Working Group M. Pels Internet-Draft F. Obser Intended status: Standards Track RIPE NCC Expires: 19 November 2026 18 May 2026 AXFR message type for DNS NOTIFY draft-pels-dnsop-axfr-notify-00 Abstract This document defines a new AXFR message type for DNS NOTIFY messages. The message instructs a secondary server to perform an AXFR zone transfer of a zone. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 19 November 2026. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Pels & Obser Expires 19 November 2026 [Page 1] Internet-Draft AXFR NOTIFY May 2026 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Protocol sequence . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Initiation . . . . . . . . . . . . . . . . . . . . . . . 3 3.2. Secondary server behaviour . . . . . . . . . . . . . . . 3 4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 3 5. Security considerations . . . . . . . . . . . . . . . . . . . 4 6. Normative References . . . . . . . . . . . . . . . . . . . . 4 7. Informative References . . . . . . . . . . . . . . . . . . . 4 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 4 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction DNS NOTFY[RFC1996] describes a method to inform secondary name servers that a zone has changed. Upon receipt of a NOTIFY(SOA) message, a secondary will query the primary server for the SOA of the zone, and perform an IXFR[RFC1995] or AXFR[RFC5936] zone transfer if the SOA SERIAL has been incremented since the last time the zone was fetched. There are cases when it is desirable to force a secondary server to perform an AXFR zone transfer. Example scenarios are an incorrectly synchronized database on a secondary server due to a software bug, or an operator incidentally wanting to propagate changes to a zone without updating the SOA SERIAL. Most authoritative DNS server software supports performing a forced AXFR zone transfer. However, this can only be initiated by the operator of the DNS server, and secondary servers for a zone may not be managed by the same operator as the primary server(s). This document defines a new NOTIFY(AXFR) message type, used to initiate forced AXFR transfers by secondary servers from a primary server of a zone. 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. Pels & Obser Expires 19 November 2026 [Page 2] Internet-Draft AXFR NOTIFY May 2026 3. Protocol sequence 3.1. Initiation When a primary server wants its secondary servers to perform an AXFR zone transfer, it sends a NOTIFY message with the following characteristics to each secondary server: query ID: (new) op: NOTIFY (4) resp: NOERROR flags: AA qcount: 1 qname: (zone name) qclass: (zone class) qtype: T_AXFR 3.2. Secondary server behaviour When a secondary server receives a NOTIFY(AXFR) request from one of its locally designated primaries for the zone enclosing the given QNAME, with QTYPE=AXFR and QR=0, it SHOULD perform an AXFR zone transfer from one of its configured primary servers. It will also send a NOTIFY response back to the NOTIFY request's source, with the following characteristics: query ID: (same) op: NOTIFY (4) resp: NOERROR flags: QR AA qcount: 1 qname: (zone name) qclass: (zone class) qtype: T_AXFR If the secondary server, for the zone enclosing the given QNAME, is configured as a primary server for other secondary servers, it MUST also send a NOTIFY(AXFR) request to each of its configured secondaries. This ensures that an entire chain of secondary servers is synchronized in concert. 4. IANA considerations This document has no IANA actions. Pels & Obser Expires 19 November 2026 [Page 3] Internet-Draft AXFR NOTIFY May 2026 5. Security considerations A NOTIFY(AXFR) request with a forged IP/UDP source address can cause a secondary to send spurious AXFR requests to its primary servers, leading to a denial of service attack if the forged requests are sent very often. To counter this, secondary servers MUST rate-limit how often they will request an AXFR from their primaries. If a primary server has many secondaries, requesting all of them to perform an AXFR simultaneously may cause the primary to become overloaded. The sending out of NOTIFY(AXFR) requests to secondary servers SHOULD be splayed to reduce this thundering herd effect. 6. Normative References [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, DOI 10.17487/RFC1995, August 1996, . [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996, August 1996, . [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010, . 7. Informative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Appendix A. Acknowledgements The authors would like to thank Anand Buddhdev for contributing ideas and feedback to this document. Authors' Addresses Martin Pels RIPE NCC Netherlands Pels & Obser Expires 19 November 2026 [Page 4] Internet-Draft AXFR NOTIFY May 2026 Email: mpels@ripe.net Florian Obser RIPE NCC Netherlands Email: fobser@ripe.net Pels & Obser Expires 19 November 2026 [Page 5]