The "ZONEVERSION" EDNS option for the version token of a Resource Record's zone

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Table of Contents

1. Introduction

The "ZONEVERSION" EDNS option [RFC6891] allows a DNS querier to request to a DNS authoritative server to add an EDNS option in the answer of such query with a token field representing the version of the zone associated to the answered Resource Record (RR), such as the SOA serial field in zones when this number corresponds to the zone version.¶

This "ZONEVERSION" data allows to help debug by recognizing the data source of an answer, associating this answer with a respective zone version.¶

DNS data is of loose coherent nature, meaning that a record obtained by a response could be out-of-sync with other authoritative sources of the same data. This makes it difficult to debug responses, because you'd need to couple an answer with the same version of the zone used to obtain such data. Even when in zones where the SOA serial field have the meaning of zone version you could use a separate query to ask for the SOA RR of the zone and therefore know its SOA serial, but such separate query is performed in a different time and could arrive from another authoritative source (for example, in the case the server is anycasted as described in Section 4.9 of [RFC4786]), so it's not directly correlated with the original query.¶

This EDNS option is aimed to be used only on authoritative servers for a zone. It's intended for hop-to-hop communication (not transitive).¶

The ZONEVERSION EDNS extension can have different meaning depending on the semantics of the zone maintainer and implementation of nameservers. This document defines one possible value, when the zone version corresponds to the serial field of the SOA RR of the zone, a classic behaviour defined in Section 4.3.5 of [RFC1034].¶

As of the writing of this document, we recognize there are cases where nameservers use different backends for its data sources (like relational databases or by using a different off-DNS synchronicity among others) therefore, the SOA version field doesn't offer much relevance as a versioning to its content, and in those cases the ZONEVERSION EDNS extension SHOULD be extended with a different type and have an opaque value for its data token.¶

2. The ZONEVERSION Option

The variable part of RDATA in the OPT RR Section 6.1.2 of [RFC6891] for ZONEVERSION is defined as follows:¶

• The OPTION-CODE for the ZONEVERSION option is <TBD>.¶

• The OPTION-LENGTH for the ZONEVERSION option MUST have a value of 0 for queries, and MUST have the value of the length in octets for the next OPTION-DATA for responses.¶

• The OPTION-DATA for the ZONEVERSION option is composed of the concatenation of¶

  • An unsigned 1 byte Label Count number (LABELCOUNT) indicating the number of labels in the QNAME owner name this answer's zone name refers to¶
  • An unsigned 1 byte type number (TYPE) that distingishes the format and meaning of TYPEDATA¶
  • And the final Data value of the ZONEVERSION field as an opaque bit value (TYPEDATA).¶

                    +0 (MSB)                       +1 (LSB)
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    0: |           LABELCOUNT          |            TYPE               |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    2: |                           TYPEDATA                            |
       /                                                               /
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

The LABELCOUNT number helps to disambiguate the name of the zone that this ZONEVERSION refers to. For example if the response is a downward referral and the server is authoritative for some portion of the QNAME that differs from a server that is below the zone cut and is completely authoritative for a longer match of the labels in the QNAME. 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 disambiguate such situation.¶

The value of the LABELCOUNT field MUST NOT count the null (root) label that terminates the QNAME owner name. The value of the LABELCOUNT field MUST be less than or equal to the number of labels in the QNAME owner name. For example, a QNAME "www.example.com." or "a.b.example.com" or "a.b.c.example.com" inside a "example.com." zone, that is not delegated not authoritative for those sub-zones in the same nameserver, has a LABELCOUNT field value of 2 for all such cases. Root zone (".") has a LABELCOUNT field value of 0.¶

[RFC Editor: change <TBD> to the proper code when assigned by IANA.]¶

5. Example usage

A zone which utilizes the serial field of the SOA RR as a number of the zone version release, should answer a ZONEVERSION request with an EDNS option code ZONEVERSION, an OPTION-LENGTH with value 6, and an OPTION-DATA with a 1-byte LABELCOUNT with the number of labels that corresponds to the zone apex name, a 1-byte TYPE with value 0, and a copy of the unsigned 32 bit version number of the SERIAL field of its SOA zone Resource Record.¶

An example of a proper diagnostic tool that implements ZONEVERSION EDNS extension towards a compliant authoritative DNS server could be:¶

  $ dig @ns.example.com www.example.com aaaa +zoneversion

  ; <<>> DiG 9.17.14-patched <<>> @ns.example.com www.example.com aaaa +zoneversion
  ; (1 server found)
  ;; global options: +cmd
  ;; Got answer:
  ;; ->>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 00 04 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

6. Acknowledgements

The authors thanks all the comments and support made in the DNSOP mailing list, chats and discussions. In special for the suggestions to generalize the option using a registry of types from Petr Špaček and Florian Obser, suggestions for implementation from Stéphane Bortzmeyer, security clarifications from George Michaelson, zone name disambiguation from Joe Abley and Brian Dickson, and reviews from Tim Wicinski and Peter Thomassen.¶

7. IANA Considerations

8. Security Considerations

The EDNS extension data it's not covered by RRSIG records, so there's no way to verify its authenticity nor integrity using DNSSEC and could theoretically be tampered 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 ZONEVERSION trustworthiness, it will be necessary to use an encrypted and authenticated DNS transport.¶

If there's a need to authenticate 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 DO flag, whose answer shall be DNSSEC signed, with the cautions about Anycast and others as already stated in Introduction.¶

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 can not be used for checking the correctness of an entire zone in a server. For such cases, the ZONEMD record [RFC8976] might be better suited at such task. ZONEVERSION can help identify and correlate a certain 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.¶

Appendix A. Implementation Considerations

With very few exceptions, EDNS options which elicit an EDNS option in the response are independent of the QNAME. This is not the case of ZONEVERSION, so its implementation may be more or less difficult depending on how EDNS options are handled in the name server.¶

Appendix B. Implementation References

There's a patched NSD server version 4.7.0 with support for ZONEVERSION with an experimental opcode, with 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. More information in the working document [ImplRef].¶

Authors' Addresses