Extension Mechanisms for DNS (EDNS) EXPIRE Option
Track how long DNS secondaries have been out of touch with the primary.
Andrews, M., "Extension Mechanisms for DNS (EDNS) EXPIRE Option", RFC 7314, DOI 10.17487/RFC7314, July 2014, <https://www.rfc-editor.org/info/rfc7314>.
@misc{rfc7314,
series = {Request for Comments},
number = 7314,
howpublished = {RFC 7314},
publisher = {RFC Editor},
doi = {10.17487/RFC7314},
url = {https://www.rfc-editor.org/info/rfc7314},
author = {M. Andrews},
title = {{Extension Mechanisms for DNS (EDNS) EXPIRE Option}},
pagetotal = 4,
year = 2014,
month = jul,
} RFC text
Contents
Independent Submission M. Andrews
Request for Comments: 7314 ISC
Category: Experimental July 2014
ISSN: 2070-1721
Extension Mechanisms for DNS (EDNS) EXPIRE Option
Abstract
This document specifies a method for secondary DNS servers to honour
the SOA EXPIRE field as if they were always transferring from the
primary, even when using other secondaries to perform indirect
transfers and refresh queries.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7314.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Reserved Words . . . . . . . . . . . . . . . . . . . . . 2
2. Expire EDNS Option (Query) . . . . . . . . . . . . . . . . . 3
3. Expire EDNS Option (Response) . . . . . . . . . . . . . . . . 3
3.1. Primary Server . . . . . . . . . . . . . . . . . . . . . 3
3.2. Secondary Server . . . . . . . . . . . . . . . . . . . . 3
3.3. Non-authoritative Server . . . . . . . . . . . . . . . . 3
4. Secondary Behaviour . . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. Normative References . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
The expire field of a DNS zone's SOA record [RFC1035] is supposed to
indicate when a secondary server shall discard the contents of the
zone when it has been unable to contact the primary [RFC1034].
Current practice only works when all the secondaries contact the
primary directly to perform refresh queries and zone transfers.
While secondaries are expected to be able to, and often are
configured to, transfer from other secondaries for robustness reasons
as well as reachability constraints, there is no mechanism provided
to preserve the expiry behaviour when using a secondary. Instead,
secondaries have to know whether they are talking directly to the
primary or another secondary and use that to decide whether or not to
update the expire timer. This, however, fails to take into account
delays in transferring from one secondary to another.
There are also zone-transfer graphs in which the secondary never
talks to the primary, so the effective expiry period becomes
multiplied by the length of the zone-transfer graph, which is
infinite when it contains loops.
This document provides a mechanism to preserve the expiry behaviour
regardless of what zone-transfer graph is constructed and whether the
secondary is talking to the primary or another secondary.
1.1. Reserved Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Expire EDNS Option (Query)
The EDNS [RFC6891] EXPIRE option has the value <9>. The EDNS EXPIRE
option MAY be included on any QUERY, though usually this is only done
on SOA, AXFR, and IXFR queries involved in zone maintenance. This is
done by adding a zero-length EDNS EXPIRE option to the options field
of the OPT record when the query is made.
3. Expire EDNS Option (Response)
3.1. Primary Server
When the query is directed to the primary server for the zone, the
response will be an EDNS EXPIRE option of length 4 containing the
value of the SOA EXPIRE field, in seconds and network byte order.
3.2. Secondary Server
When the query is directed to a secondary server for the zone, then
the response will be an EDNS EXPIRE option of length 4 containing the
value of the expire timer on that server, in seconds and network byte
order.
3.3. Non-authoritative Server
If an EDNS EXPIRE option is sent to a server that is not
authoritative for the zone, it MUST NOT add an EDNS EXPIRE option to
the response.
4. Secondary Behaviour
When a secondary server performs a zone-transfer request or a zone-
refresh query, it SHALL add an EDNS EXPIRE option to the query
message.
If a secondary receives an EDNS EXPIRE option in a response to an SOA
query, it SHALL update its expire timer to be the maximum of the
value returned in the EDNS EXPIRE option and the current timer value.
Similarly, if a secondary receives an EDNS EXPIRE option in its
response to an IXFR query that indicated the secondary is up to date
(serial matches current serial), the secondary SHALL update the
expire timer to be the maximum of the value returned in the EDNS
EXPIRE option and the current timer value.
If the zone is transferred or updated as the result of an AXFR or
IXFR query and there is an EDNS EXPIRE option with the response, then
the value of the EDNS EXPIRE option SHOULD be used instead of the
value of the SOA EXPIRE field to initialise the expire timer.
In all cases, if the value of the SOA EXPIRE field is less than the
value of the EDNS EXPIRE option, then the value of the SOA EXPIRE
field MUST be used and MUST be treated as a maximum when updating or
initialising the expire timer.
5. IANA Considerations
IANA has assigned an EDNS option code point for the EDNS EXPIRE
option specified in Section 2 with "Optional" status in the "DNS
EDNS0 Option Codes (OPT)" registry.
6. Security Considerations
The method described in this document ensures that servers that no
longer have a connection to the primary server, direct or indirectly,
cease serving the zone content when SOA EXPIRE timer is reached.
This prevents stale data from being served indefinitely.
The EDNS EXPIRE option exposes how long the secondaries have been out
of communication with the primary server. This is not believed to be
a problem and may provide some benefit to monitoring systems.
7. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
Author's Address
Mark P. Andrews
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
EMail: marka@isc.org
AI ELI5
On this page
§1The Zone Expiry Problem
In DNS, a primary server publishes a zone with an SOA EXPIRE field telling secondaries how long they may serve the zone if they lose contact with the primary. In a simple setup, secondaries refresh directly from the primary and the timeout works correctly. However, secondaries often transfer from other secondaries for robustness: secondary A copies from primary P, then secondary B copies from A. The problem: when B copies from A, there is no signal that both A and B should use the same expiry deadline based on the original primary's timing.
A photocopy of a list expires at a set date. If you make a copy of the copy, the new copy should expire on the same date, not later.
§2The EDNS EXPIRE Option
RFC 7314 solves this with a new EDNS option (code 9) that carries the remaining time before expiry. When a secondary makes a zone transfer request or refresh query to any server (primary or another secondary), it adds the zero-length option. The server responds with a 4-byte value showing either the original SOA EXPIRE (if it is the primary) or its current local expiry timer (if it is a secondary). The secondary then updates its own timer to the maximum of the value received and its current timer, never extending it, and following the rule that the SOA EXPIRE value itself acts as a cap.
When copying, ask "when does this expire?" and update your own expiry time to match the original, whichever is sooner.
§3Secondary Behavior and Stale Data
When a secondary receives an EDNS EXPIRE option in a response to an SOA query or a non-updating IXFR query (where the serial already matches), it updates its timer. After a zone transfer (AXFR or IXFR), the secondary prefers the EDNS EXPIRE value from the response over the SOA EXPIRE field in the zone itself, ensuring the expiry time is always the furthest original deadline. This mechanism prevents stale zone data from being served indefinitely when the secondary chain is broken from the primary.
A secondary stops serving the zone when the expiry time runs out, even if it has not heard from the primary in a long time.