The DNS Zone Version (ZONEVERSION) Option

The DNS Zone Version (ZONEVERSION) Option NIC Chile

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OPS dnsop zoneversion The DNS ZONEVERSION option is a way for DNS clients to request, and for authoritative DNS servers to provide, information regarding the version of the zone from which a response is generated. The SERIAL field from the Start of Authority (SOA) resource record (RR) is a good example of a zone's version, and it is the only one defined by this specification. Additional version types may be defined by future specifications. Including zone version data in a response simplifies and improves the quality of debugging and diagnostics since the version and the DNS answer are provided atomically. This can be especially useful for zones and DNS providers that leverage IP anycast or multiple backend systems. It functions similarly to the DNS Name Server Identifier (NSID) option described in RFC 5001.

Introduction The ZONEVERSION option allows DNS queriers to request, and authoritative DNS servers to provide, a token representing the version of the zone from which a DNS response was generated. It is similar to the NSID option , which can be used to convey the identification of a name server that generates a response. The Domain Name System allows data to be loosely coherent , because synchronization can never be instantaneous, and some uses of DNS do not require strong coherency anyway. This means that a record obtained by one response could be out of sync with other authoritative sources of the same data at the same point in time. This can make it difficult to debug some problems when there is a need to couple the data with the version of the zone it came from. Furthermore, in today's Internet, it is common for high volume and important DNS zones to utilize IP anycast () and/or load-balanced backend servers. In general, there is no way to ensure that two separate queries are delivered to the same server. The ZONEVERSION option both simplifies and improves DNS monitoring and debugging by directly associating the data and the version together in a single response. The SOA SERIAL field () is one example of zone versioning. Its purpose is to facilitate the distribution of zone data between primary and secondary name servers. It is also often useful in DNS monitoring and debugging. This document specifies the SOA SERIAL as one type of ZONEVERSION data. Some DNS zones may use other distribution and synchronization mechanisms that are not based on the SOA SERIAL number, such as relational databases or other proprietary methods. In those cases, the SOA SERIAL field may not be relevant with respect to the versioning of its content. To accommodate these use cases, new ZONEVERSION types could be defined in future specifications. Alternatively, zone operators may use one of the Private Use ZONEVERSION code points allocated by this specification. The ZONEVERSION option is OPTIONAL to implement by DNS clients and name servers. It is designed for use only when a name server provides authoritative response data. It is intended only for hop-to-hop communication and is not transitive.

Requirements Language 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.

Terminology In this document, "original QNAME" is used to mean what the DNS terminology document calls "QNAME (original)":

The name actually sent in the Question section in the original query, which is always echoed in the (final) reply in the Question section when the QR bit is set to 1.

In this document, an "enclosing zone" of a domain name means a zone in which the domain name is present as an owner name or any parent of that zone. For example, if B.C.EXAMPLE and EXAMPLE are zones but C.EXAMPLE is not, the domain name A.B.C.EXAMPLE has B.C.EXAMPLE, EXAMPLE, and the root as enclosing zones.

The ZONEVERSION Option This document specifies a new EDNS(0) option, ZONEVERSION, which can be used by DNS clients and servers to provide information regarding the version of the zone from which a response is generated.

Wire Format The ZONEVERSION option is encoded as follows: OPTION-CODE for the ZONEVERSION option is 19. OPTION-LENGTH for the ZONEVERSION option MUST have a value of 0 for queries and MUST have the value of the length (in octets) of the OPTION-DATA for responses. OPTION-DATA for the ZONEVERSION option is omitted in queries. For responses, it is composed of three fields:

an unsigned 1-octet Label Count (LABELCOUNT) indicating the number of labels for the name of the zone that VERSION value refers to
an unsigned 1-octet type number (TYPE) distinguishing the format and meaning of VERSION
an opaque octet string conveying the zone version data (VERSION)

Diagram with the OPTION-DATA Format for the ZONEVERSION Option The LABELCOUNT field indicates the name of the zone that the ZONEVERSION option refers to, by means of taking the last LABELCOUNT labels of the original QNAME. For example, an answer with QNAME "a.b.c.example.com" and a ZONEVERSION option with a LABELCOUNT of value 2 indicates that the zone name in which this ZONEVERSION refers to is "example.com.". In the case of a downward referral response, the LABELCOUNT number helps to differentiate between the parent and child zones, where the parent is authoritative for some portion of the QNAME above a zone cut. Also, if the ANSWER section has more than one RR set with different zones (like a CNAME and a target name in another zone), the number of labels in the QNAME disambiguates such a situation. The value of the LABELCOUNT field MUST NOT count the null (root) label that terminates the original QNAME. The value of the LABELCOUNT field MUST be less than or equal to the number of labels in the original QNAME. The Root zone (".") has a LABELCOUNT field value of 0.

Presentation Format The presentation format of the ZONEVERSION option is as follows: The OPTION-CODE field MUST be represented as the mnemonic value ZONEVERSION. The OPTION-LENGTH field MAY be omitted, but if present, it MUST be represented as an unsigned decimal integer. The LABELCOUNT value of the OPTION-DATA field MAY be omitted, but if present, it MUST be represented as an unsigned decimal integer. The corresponding zone name SHOULD be displayed (i.e., LABELCOUNT labels of the original QNAME) for easier human consumption. The TYPE and VERSION fields of the option SHOULD be represented according to each specific TYPE.

ZONEVERSION Processing

Initiators A DNS client MAY signal its support and desire for zone version information by including an empty ZONEVERSION option in the EDNS(0) OPT pseudo-RR of a query to an authoritative name server. An empty ZONEVERSION option has OPTION-LENGTH set to zero. A DNS client SHOULD NOT send the ZONEVERSION option to non-authoritative name servers. A DNS client MUST NOT include more than one ZONEVERSION option in the OPT pseudo-RR of a DNS query.

Responders A name server that (a) understands the ZONEVERSION option, (b) receives a query with the ZONEVERSION option, (c) is authoritative for one or more enclosing zones of the original QNAME, and (d) chooses to honor a particular ZONEVERSION request responds by including a TYPE and corresponding VERSION value in a ZONEVERSION option in an EDNS(0) OPT pseudo-RR in the response message. Otherwise, a server MUST NOT include a ZONEVERSION option in the response. A name server MAY include more than one ZONEVERSION option in the response if it supports multiple TYPEs. A name server MAY also include more than one ZONEVERSION option in the response if it is authoritative for more than one enclosing zone of the original QNAME. A name server MUST NOT include more than one ZONEVERSION option for a given TYPE and LABELCOUNT. Note: the ZONEVERSION option should be included for any response satisfying the criteria above including, but not limited to, the following:

Downward referral (see "Referrals" in ), even though the response's Authoritative Answer bit is not set. In this case, the ZONEVERSION data MUST correspond to the version of the referring zone.
Name error (NXDOMAIN), even though the response does not include any Answer section RRs.
NODATA (), even though the response does not include any Answer section RRs.
Server failure (SERVFAIL) when the server is authoritative for the original QNAME.

Responding to Invalid ZONEVERSION Queries A name server that understands the ZONEVERSION option MUST return a FORMERR response when:

The ZONEVERSION OPTION-LENGTH is not zero.
More than one ZONEVERSION option is present.

ZONEVERSION Is Not Transitive The ZONEVERSION option is not transitive. A name server (recursive or otherwise) MUST NOT blindly copy the ZONEVERSION option from a query it receives into a subsequent query that it sends onward to another server. A name server MUST NOT send a ZONEVERSION option back to a client that did not request it.

The SOA-SERIAL ZONEVERSION Type The first and only ZONEVERSION option TYPE defined in this document is a zone's serial number as found in the Start of Authority (SOA) RR. As mentioned previously, some DNS zones may use alternative distribution and synchronization mechanisms that are not based on the SOA SERIAL number, and the SERIAL field may not be relevant with respect to the versioning of zone content. In those cases, a name server SHOULD NOT include a ZONEVERSION option with type SOA-SERIAL in a reply. The value for this type is "0". The mnemonic for this type is "SOA-SERIAL". The EDNS(0) OPTION-LENGTH for this type MUST be set to "6" in responses. The VERSION value for the SOA-SERIAL type MUST be a copy of the unsigned 32-bit SERIAL field of the SOA RR, as defined in .

Type SOA-SERIAL Presentation Format The presentation format of this type content is as follows:

The TYPE field MUST be represented as the mnemonic value "SOA-SERIAL".
The VERSION field MUST be represented as an unsigned decimal integer.

Example Usage A name server that (a) implements this specification, (b) receives a query with the ZONEVERSION option, (c) is authoritative for the zone of the original QNAME, and (d) utilizes the SOA SERIAL field for versioning of said zone should include a ZONEVERSION option in its response. In the response's ZONEVERSION option, the EDNS(0) OPTION-LENGTH would be set to 6 and the OPTION-DATA would consist of the 1-octet LABELCOUNT, the 1-octet TYPE with value 0, and the 4-octet SOA-SERIAL value. The example below demonstrates expected output of a diagnostic tool that implements the ZONEVERSION option, displaying a response from a compliant authoritative DNS server:

Example Usage and Dig Output >HEADER<<- opcode: QUERY, status: NOERROR, id: 7077 ;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 2 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 1232 ; ZONEVERSION: 02 00 78 95 a4 e9 ("SOA-SERIAL: 2023073001 \ ; (example.com.)") ;; QUESTION SECTION: ;www.example.com. IN AAAA ;; ANSWER SECTION: www.example.com. 43200 IN AAAA 2001:db8::80 ;; AUTHORITY SECTION: example.com. 43200 IN NS ns.example.com. ;; ADDITIONAL SECTION: ns.example.com. 43200 IN AAAA 2001:db8::53 ;; Query time: 15 msec ;; SERVER: 2001:db8::53#53(2001:db8::53) (UDP) ;; WHEN: dom jul 30 19:51:04 -04 2023 ;; MSG SIZE rcvd: 129 ]]>

IANA Considerations

DNS EDNS(0) Option Code Registration This document defines a new EDNS(0) option, entitled "ZONEVERSION" (see ), with the assigned value of 19 from the "DNS EDNS0 Option Codes (OPT)" registry: DNS EDNS0 Option Codes (OPT) Registry

Value	Name	Status	Reference
19	ZONEVERSION	Standard	RFC 9660

ZONEVERSION TYPE Values Registry IANA has created and will maintain a new registry called "ZONEVERSION TYPE Values" in the "Domain Name System (DNS) Parameters" registry group as follows: Registration Procedures for the ZONEVERSION TYPE Values Registry

Range	Registration Procedures
0-245	Specification Required
246-254	Private Use
255	Reserved

Initial values for the "ZONEVERSION TYPE Values" registry are given below; future assignments in the 1-245 values range are to be made through Specification Required . Assignments consist of a TYPE value as an unsigned 8-bit integer recorded in decimal, a Mnemonic name as an uppercase ASCII string with a maximum length of 15 characters, and the required document reference. ZONEVERSION TYPE Values Registry

ZONEVERSION TYPE	Mnemonic	Reference
0	SOA-SERIAL	RFC 9660
1-245	Unassigned
246-254	Reserved for Private Use	RFC 9660
255	Reserved	RFC 9660

The change controller for this registry is IETF.

Designated Expert Review Directives The allocation procedure for new code points in the "ZONEVERSION TYPE Values" registry is Specification Required, thus review is required by a designated expert as stated in . When evaluating requests, the expert should consider the following:

Duplication of code point allocations should be avoided.
A Presentation Format section should be provided with a clear code point mnemonic.
The referenced document and stated use of the new code point should be appropriate for the intended use of a ZONEVERSION TYPE assignment. In particular, the reference should state clear instructions for implementers about the syntax and semantic of the data. Also, the length of the data must have proper limits.

The expert reviewing the request MUST respond within 10 business days.

Security Considerations The EDNS extension data is not covered by RRSIG records, so there's no way to verify its authenticity nor integrity using DNSSEC, and it could theoretically be tampered with by a person in the middle if the transport is made by insecure means. Caution should be taken to use the EDNS ZONEVERSION data for any means besides troubleshooting and debugging. If there's a need to certify the trustworthiness of ZONEVERSION, it will be necessary to use an encrypted and authenticated DNS transport, a transaction signature (TSIG) , or SIG(0) . If there's a need to authenticate the data origin for the ZONEVERSION value, an answer with the SOA-SERIAL type as defined above could be compared to a separate regular SOA query with a DO flag, whose answer shall be DNSSEC signed. Since these are separate queries, the caveats about loose coherency already stated in the () would apply. With the SOA-SERIAL type defined above, there's no risk on disclosure of private information, as the SERIAL of the SOA record is already publicly available. Please note that the ZONEVERSION option cannot be used for checking the correctness of an entire zone in a server. For such cases, the ZONEMD record might be better suited for such a task. ZONEVERSION can help identify and correlate a specific answer with a version of a zone, but it has no special integrity or verification function besides a normal field value inside a zone, as stated above.

Normative References Informative References Zoneversion Implementations commit f5f68a0

Implementation Considerations With very few exceptions, EDNS(0) option values in a response are independent of the queried name. This is not the case for ZONEVERSION, so its implementation may be more or less difficult, depending on how EDNS(0) options are handled in the name server.

Implementation References There is a patched NSD server (version 4.7.0) with support for ZONEVERSION as well as live test servers installed for compliance tests. Also, there is a client command "dig" with added zoneversion support, along with test libraries in Perl, Python, and Go. See for more information.

Acknowledgements The authors are thankful for all the support and comments made in the DNSOP Working Group mailing list, chats, and discussions. A special thanks for suggestions to generalize the option using a registry of types from and , suggestions for implementation from , clarifications on security from , zone name disambiguation from and , and reviews from and .