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This document defines a new AXFR message type for DNS NOTIFY messages, together with an accompanying subtype for the ZONEVERSION EDNS(0) option. The message instructs a secondary server to perform an AXFR zone transfer of a zone.

DNS NOTFY 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 or AXFR 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, a signer implementation refreshing DNSSEC signatures or adding a ZONEMD RRset without incrementing the SOA SERIAL, or an operator incidentally wanting to propagate changes to a zone without updating the SOA record. 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. Additionally, a loop prevention mechanism is specified, using a new ZONETIMESTAMP subtype for the ZONEVERSION EDNS(0) option.

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 when, and only when, they appear in all capitals, as shown here.

The below sections describe the implementation of forced AXFR zone transfer support on primary and secondary servers. Although the QTYPE used is AXFR, implementations SHOULD transfer the zone over whatever protocol they have been configured to use, which includes both AXFR over DNS/TCP and AXFR over TLS. Note that IXFR transfers, which requires serial number alignment, MUST NOT be used.

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 The server SHOULD locally store the current number of seconds since the Unix Epoch at the moment the NOTIFY was generated. This value is from here on referred to as the ZONETIMESTAMP.

When a primary server receives an AXFR request with an empty ZONEVERSION option in the EDNS(0) OPT pseudo-RR of the query and a stored ZONETIMESTAMP is available, it MUST include this value in a ZONEVERSION option in the response (see ). This updates section 2.2 of .

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. The AXFR query SHOULD contain an empty ZONEVERSION option in the EDNS(0) OPT pseudo-RR. The secondary server 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 AXFR response from the primary server includes a ZONEVERSION option of type ZONETIMESTAMP, the following procedure MUST be followed.

• If no locally stored ZONETIMESTAMP value is available or if the received ZONETIMESTAMP value is higher than the local value:
  1. Process the AXFR response
  2. Replace the locally stored ZONETIMESTAMP value with the ZONETIMESTAMP value from the AXFR response
  3. If the secondary server, for the zone enclosing the given QNAME, is configured as a primary server for other secondary servers, send a NOTIFY(AXFR) request to each of its configured secondaries
• If the received ZONETIMESTAMP value is lower than or equal to the local value:
  1. Do not process the AXFR response
  2. If any other primary servers are available that have not yet been tried, attempt an AXFR zone transfer from the next primary

If no ZONETIMESTAMP is included in the AXFR response, the response is processed but no configured secondaries are notified. The above procedure ensures that an entire chain of secondary servers is synchronized in concert, while preventing a loop of AXFR queries. Secondary servers MAY optimize this procedure by sending a SOA query with an empty ZONEVERSION option to a primary server and comparing the returned ZONETIMESTAMP value with their locally stored value, before attempting a full AXFR.

This document defines a new ZONEVERSION option TYPE with the following characteristics.

• The value for this type is "<TBD>"
• The mnemonic for this type is "ZONETIMESTAMP"
• The EDNS(0) OPTION-LENGTH for this type MUST be set to "8" in responses
• The VERSION value for the ZONETIMESTAMP type MUST be a 64-bit unsigned integer, representing a number of seconds since the Unix Epoch

This document registers the following entry in the "ZONEVERSION TYPE Values Registry", following the format defined in :

ZONEVERSION TYPE	Mnemonic	Reference
<TBD>	ZONETIMESTAMP	RFCYYYY

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, NOTIFY(AXFR) requests MUST be secured using TSIG. 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. Secondary servers SHOULD rate-limit how often they will request an AXFR from their primaries.

&RFC1995; &RFC1996; &RFC5936; &RFC8945; &RFC9660; &IEEE_1003.1_2024; &RFC2119; &RFC8174; &RFC9103;

The authors would like to thank the following people for contributing ideas and feedback to this document:

• Anand Buddhdev
• Peter van Dijk
• Philip Homburg
• Shane Kerr
• Stefan Ubbink
• Wes Hardaker