EDNS EXPIRE OPTION
draft-andrews-dnsext-expire-00

Network Working Group                                         M. Andrews
Internet-Draft                                                       ISC
Intended status: Standards Track                         August 25, 2008
Expires: February 26, 2009

                           EDNS EXPIRE OPTION
                     draft-andrews-dnsext-expire-00

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Abstract

   Provide a method for slave DNS servers to honour the SOA EXPIRE field
   as if they were always transferring from the master, even when using
   other slaves to perform indirect transfers and refresh queries.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Reserved Words  . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Expire EDNS option  . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Slave Behaviour . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   Intellectual Property and Copyright Statements  . . . . . . . . . . 6

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1.  Introduction

   The expire field of a DNS zone's SOA record [RFC 1035] is supposed to
   indicate when a slave server shall discard the contents of the zone
   when it has been unable to contact the master [RFC 1034].  Current
   practice works when all the slaves contact the master directly to
   perform refresh queries and zone transfers.

   While slaves are expected to be able to transfer from other slaves
   for robustness reasons as well as reachability constraints, there was
   no mechanism provided to preserve the expiry behaviour when using a
   slave.  Slaves instead have to know whether they were talking
   directly to the master or another slave, and use that to decide
   whether to update the expiry timer or not.  This however fails to
   take into account delays in transferring from one slave to another.

   There are also zone transfer graphs in which the slave never talks to
   the master, so the effective expiry period becomes multiplied 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 or whether the
   slave is talking to the master or another slave.

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

   The EDNS [RFC 2671] EXPIRE option has the value <TBD>.  The EDNS
   EXPIRE option MAY be 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.

   When the query is directed to the master server for the zone, the
   response will be a EDNS EXPIRE option of length 4 containing the
   current value of the SOA EXPIRE field, in network order.  When the
   query is directed to a slave server for the zone, then the response
   will be an EDNS EXPIRE option of length 4 containing the value of the
   the EXPIRE timer, in network order.

   If an EDNS EXPIRE option is sent to a server that is not

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   authoritative for the zone then there will be no EDNS EXPIRE option
   added to the response.

3.  Slave Behaviour

   When a slave 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 slave receives an EDNS EXPIRE option in its 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 its current value.
   Similarly, if a slave receives an EDNS EXPIRE option in its response
   to an IXFR query which indicated the slave is up to date (serial
   matches current serial) the slave SHALL update the expire timer to be
   the maximum of the value returned in the EXPIRE EDNS option and its
   current value.

   If the zone is transferred or updated as the result of a AXFR or IXFR
   query and there is a 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.

4.  IANA Considerations

   IANA is requested to assign a EDNS option code point.

5.  Security Considerations

   The EDNS EXPIRE option exposes how long the slaves have been out of
   communication with the master server.  This is not believed to be a
   problem and may provide some benefit to monitoring systems.

6.  Normative References

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

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   [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 2671]
              Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

Author's Address

   Mark P. Andrews
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA  94063
   US

   Email: Mark_Andrews@isc.org

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