The "ZONEVERSION" EDNS option for the version token of a RR'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, 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, 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). Resolver and forwarder behavior is undefined.¶

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 Resource Record of the zone, a classic behaviour defined in Section 4.3.5 of [RFC1034].¶

As 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 flag 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 length in octects for the next OPTION-DATA for responses.¶
• The OPTION-DATA for the ZONEVERSION option is composed of the concatenation of an unsigned 1 byte Flag number (FLAG-NUMBER), an unsigned 2 bytes number with the Length of the next field in bytes (FLAG-LENGTH), and the final Data value of the ZONEVERSION field as an opaque bit value, with the previous specified length (FLAG-DATA).¶

[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 Resource Record as a number of the zone version release, should answer a ZONEVERSION request with an EDNS option code ZONEVERSION, an OPTION-DATA with a Flag byte with value 0, Length value 4 and a copy of the unsigned 32 bit version number of the SERIAL field of its SOA zone Resourse Record, and an OPTION-LENGTH with value 7.¶

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 +norec +cmd

  ; (1 server found)
  ;; global options: +cmd
  ;; Got answer:
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16429
  ;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

  ;; OPT PSEUDOSECTION:
  ; EDNS: version: 0, flags:; udp: 4096
  ; ZONEVERSION: 2019073001 (SOA-SERIAL)
  ;; QUESTION SECTION:
  ;www.example.com.                    IN      AAAA

  ;; ANSWER SECTION:
  www.example.com.             900     IN      AAAA

  ;; Query time: 53 msec
  ;; SERVER: ns.example.com#53(2001:DB8::53)
  ;; WHEN: Tue Aug 07 16:54:05 -04 2018
  ;; MSG SIZE  rcvd: 71

6. Acknowledgements

The authors thanks all the comments and support made in the DNSOPS mailing list, chats and discussions. In special for the suggestions to generalize the option using a registry of flags from Petr Špaček, and suggestions for implementators from Stéphane Bortzmeyer.¶

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 theoricatelly be tampered by a person-in-the-middle if the transport is made by unsecure 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 trustwortiness, 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 flag 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 flag defined above, there's no risk on disclosure of private information, as the SERIAL of the SOA record is already publicly available.¶

Appendix A. Implementation Considerations

With very few exceptions, EDNS options which elicit an EDNS option in the response are independant 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 4.3.7 with support for ZONEVERSION with the experimental opcode 65024 maintained in https://github.com/huguei/nsd/tree/rrserial with SOA-SERIAL flag support, and installed for live testing in 200.1.122.30 address with configured zones dateserial.example.com. and incserial.example.com.; with MX, TXT and AAAA apex records.¶

Authors' Addresses