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EXPERIMENTAL
Independent Submission                                        M. AndrewsRequest for Comments: 7314                                           ISCCategory: Experimental                                         July 2014ISSN: 2070-1721Extension Mechanisms for DNS (EDNS) EXPIRE OptionAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Andrews                       Experimental                      [Page 1]

RFC 7314    Extension Mechanisms for DNS (EDNS) EXPIRE Option  July 2014Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Reserved Words  . . . . . . . . . . . . . . . . . . . . .22.  Expire EDNS Option (Query)  . . . . . . . . . . . . . . . . .33.  Expire EDNS Option (Response) . . . . . . . . . . . . . . . .33.1.  Primary Server  . . . . . . . . . . . . . . . . . . . . .33.2.  Secondary Server  . . . . . . . . . . . . . . . . . . . .33.3.  Non-authoritative Server  . . . . . . . . . . . . . . . .34.  Secondary Behaviour . . . . . . . . . . . . . . . . . . . . .35.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .46.  Security Considerations . . . . . . . . . . . . . . . . . . .47.  Normative References  . . . . . . . . . . . . . . . . . . . .41.  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].Andrews                       Experimental                      [Page 2]

RFC 7314    Extension Mechanisms for DNS (EDNS) EXPIRE Option  July 20142.  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.Andrews                       Experimental                      [Page 3]

RFC 7314    Extension Mechanisms for DNS (EDNS) EXPIRE Option  July 2014   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 inSection 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.orgAndrews                       Experimental                      [Page 4]

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