The "RRSERIAL" EDNS option for the SOA serial of a RR's zone

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Table of Contents

1. Introduction

The "RRSERIAL" 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 the SOA serial number field of the zone associated to the answered Resource Record.¶

This "RRSERIAL" 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 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 RRSERIAL EDNS extension doesn't offer much relevance for zones served by an Authoritative server that don't use the SOA serial versioning as a meaning to its content. There are cases where nameservers use different backends for its data sources, like relational databases or by using a different off-DNS synchronicity. In such cases this extension has no benefit or utility to use in debugging or analysis of a response.¶

4. Example usage

  $ dig @ns.example.com www.example.com AAAA +rrserial +norec +nocmd

  ; (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
  ; RRSERIAL: 2019073001
  ;; 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

5. Acknowledgements

The authors thanks all the comments and support made in the DNSOPS mailing list, chats and discussions.¶

6. IANA Considerations

7. 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 RRSERIAL data for any means besides troubleshooting and debugging. If there's a need to certify the RRSERIAL trustwortiness, it will be necessary to use an encrypted and authenticated DNS transport. If there's a need to authenticate data origin for the RRSERIAL value, it should 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.¶

There's no risk on disclosure of private information, as the SERIAL of the SOA record is already publicly available.¶

Appendix A. Implementation References

There's a patched NSD server 4.1.23 with support for RRSERIAL with the experimental opcode 65024 maintained in https://github.com/huguei/nsd/tree/rrserial , 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