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Network Working Group                                         M. Andrews
Internet-Draft                                                       ISC
Intended status: Experimental                             March 28, 2014
Expires: September 29, 2014


                           EDNS EXPIRE OPTION
                     draft-andrews-dnsext-expire-04

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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 29, 2014.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Reserved Words  . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Expire EDNS option (Query)  . . . . . . . . . . . . . . . . . . 3
   3.  Expire EDNS option (Response) . . . . . . . . . . . . . . . . . 3
     3.1.  Primary Server  . . . . . . . . . . . . . . . . . . . . . . 3
     3.2.  Secondary Server  . . . . . . . . . . . . . . . . . . . . . 4
     3.3.  Non-Authoritative Server  . . . . . . . . . . . . . . . . . 4
   4.  Secondary Behaviour . . . . . . . . . . . . . . . . . . . . . . 4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 5





































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1.  Introduction

   The expire field of a DNS zone's SOA record [RFC 1035] is supposed to
   indicate when a secondary server shall discard the contents of the
   zone when it has been unable to contact the primary [RFC 1034].
   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 was no mechanism provided
   to preserve the expiry behaviour when using a secondary.  Secondaries
   instead have to know whether they were talking directly to the
   primary or another secondary, and use that to decide whether to
   update the expire timer or not.  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 when it
   contains loops is infinite.

   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 [RFC 2119].


2.  Expire EDNS option (Query)

   The EDNS [RFC 6891] EXPIRE option has the value <TBA>.  The EDNS
   EXPIRE option MAY included set 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 a EDNS EXPIRE option of length 4 containing the



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   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 performs
   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 a 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 which 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 EXPIRE EDNS
   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 that of
   the SOA EXPIRE field to initialise the expire timer.

   In all cases, if the value of SOA EXPIRE field is less than the value
   of the EDNS EXPIRE option, then the value of SOA EXPIRE field MUST be
   used and MUST be treated as a maximum when updating or initialising
   the expire timer.


5.  IANA Considerations

   IANA is requested to assign a EDNS option code point (Registry Name:
   DNS EDNS0 Options) for the EDNS EXPIRE option specified in Section 2
   with "Optional" status.




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6.  Security Considerations

   This 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 prevent stale data 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

   [RFC 1034]
              Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
              STD 13, RFC 1034, November 1987.

   [RFC 1035]
              Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
              SPECIFICATION", STD 13, RFC 1035, November 1987.

   [RFC 2119]
              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC 6891]
              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











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