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DNSext Working Group                                             O. Sury
Internet-Draft                                                    CZ.NIC
Updates: 1995 (if approved)                                 S. Kerr, Ed.
Intended status: Standards Track                                     ISC
Expires: August 30, 2010                               February 26, 2010

               IXFR-ONLY to Prevent IXFR Fallback to AXFR


   This documents proposes a new QTYPE (Query pseudo RRtype) for the
   Domain Name System (DNS).  IXFR-ONLY is a variant of IXFR (RFC 1995)
   that allows an authoritative server to incrementally update zone
   content from another (primary) server without falling back from IXFR
   to AXFR.  This way, alternate peers can be contacted more quickly and
   convergence of zone content may be achieved much faster in important,
   resilient operational scenarios.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  IXFR Server Side  . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  IXFR Client Side  . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Applicability of IXFR-ONLY  . . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  Introduction

   For large DNS zones, RFC 1995 [RFC1995] defines Incremental Zone
   Transfer (IXFR), which allows only to transfer the changed portion(s)
   of a zone.

   In the document, an IXFR client and an IXFR server is defined as in
   RFC 1995 [RFC1995], a secondary name server which requests IXFR is
   called an IXFR client and a primary or secondary name server which
   responds to the request is called an IXFR server.

   IXFR is an efficient way to transfer changes in zones from IXFR
   servers to IXFR clients.  However, when an IXFR client has multiple
   IXFR servers for a single zone, it is possible that not all IXFR
   servers have the zone with same serial number for that zone.  In this
   case, if an IXFR client attempts an IXFR from an IXFR server which
   does not have zone with the serial number used by the IXFR client,
   the IXFR server will fall back to a full zone transfer (AXFR) when it
   has a version of the zone with serial number greater than the serial
   requested by the IXFR client.

   For example, IXFR server NS1 may have serial numbers 1, 2, and 3 for
   a zone, and IXFR server NS2 may have serial numbers 1 and 3 for the
   same zone.  An IXFR client that has the zone with serial number 2
   which sends an IXFR request to IXFR server NS2 will get a full zone
   transfer (AXFR) of the zone at serial number 3.  This is because NS2
   does not know the zone with serial number 2, and therefore does not
   know what the differences are between zone with serial number 2 and

   If the IXFR client in this example had known to send the query to
   IXFR server NS1, then it could have gotten an incremental transfer
   (IXFR).  But IXFR clients can only know what the latest version of
   the zone is at a IXFR server (this information is available via an
   SOA query).

   The IXFR-ONLY query type provides a way for the IXFR client to ask
   each IXFR server to return an error instead of sending the current
   version of the zone via full zone transfer (AXFR).  By using this, a
   IXFR client can check each IXFR server until it finds one able to
   provide IXFR.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2.  IXFR Server Side

   A IXFR server receiving a DNS message requesting IXFR-ONLY will reply
   as described in RFC 1995 [RFC1995] if it is able to produce an IXFR
   for the serial number requested.

   If the IXFR server is is not able to reply with an IXFR it MUST NOT
   reply with an AXFR unless AXFR result is smaller than IXFR result.
   Instead, it MUST reply with RCODE CannotIXFR. (!FIXME)

   If the IXFR result is larger than an AXFR, then an IXFR server MAY
   reply with an AXFR result instead.  This is an optimization, and IXFR
   servers MAY only reply with AXFR if they are certain that the reply
   using AXFR is smaller than an equivalent IXFR reply.

3.  IXFR Client Side

   An IXFR client who wishes to use IXFR-ONLY will send a message to one
   of the IXFR servers.  The format is exactly the same as for IXFR,
   except the IXFR-ONLY QTYPE code is used instead of the IXFR QTYPE

   If the IXFR server replies with IXFR, then the IXFR client is done.

   If the IXFR server replies with an RCODE of CannotIXFR, then the IXFR
   client proceeds on to a different IXFR server.  In this case the IXFR
   server implements IXFR-ONLY, but does not have information about zone
   with the serial number requested.

   If the IXFR server replies with any RCODE other than CannotIXFR or
   NoError, then the IXFR client proceeds on to a different IXFR server.
   In this case the IXFR server does not implement IXFR-ONLY.

   If the IXFR client attempts IXFR-ONLY to each IXFR server and none of
   them reply with an incremental transfer (IXFR), then it should
   attempt an IXFR as described in RFC 1995 [RFC1995] to each of the
   IXFR servers which replied with an RCODE other than CannotIXFR or

   The method described above allows IXFR clients to operate normally in
   situatians where some of the IXFR servers do support IXFR-ONLY, and
   some who do not.  IXFR clients MAY remember which IXFR servers
   support IXFR-ONLY and query those IXFR servers first.  However since
   IXFR servers may change software or even run a mix of software, IXFR
   clients MUST attempt to query each IXFR server periodically when they
   attempt to get new versions of a zone.

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   Implementations MAY allow IXFR clients to disable IXFR-ONLY for a
   given IXFR server, if this is known in advance.  These IXFR servers
   are treated as if they replied with an RCODE other than CannotIXFR or
   NoError, although no query with IXFR-ONLY is actually sent.

4.  Applicability of IXFR-ONLY

   Implementations SHOULD allow IXFR clients to disable IXFR-ONLY

   Implementations MAY allow IXFR clients to disable IXFR-ONLY for a
   specific zone.  This may be useful for small zones, where fallback to
   AXFR is cheap, or in other cases where IXFR-ONLY is causing problems.

   Usage of IXFR-ONLY may cause IXFR clients to prefer particular IXFR
   servers, by shifting load to ones that support IXFR-ONLY.  If this a
   problem, then administrators can disable IXFR-ONLY in implementations
   that allow it.

   If a IXFR client has a single IXFR server for a zone, it SHOULD use
   IXFR rather than IXFR-ONLY.

5.  IANA Considerations

   IANA allocates the new IXFR-ONLY QTYPE, which means "incremental
   transfer only".  IANA allocates the CannotIXFR RCODE, which means
   "Server cannot provide IXFR for zone".

6.  Security Considerations

   IXFR-ONLY may be used by someone to get information about the state
   of IXFR servers by providing a quick and efficient way to check which
   versions of a zone each IXFR server supports.  Zones should be
   secured via TSIG [RFC2845] to prevent unauthorized information
   exposure.  However, even administrators of IXFR servers may not want
   this information given to IXFR clients, in which case they will need
   to disable IXFR-ONLY.

7.  Normative References

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              August 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate

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              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, May 2000.

Authors' Addresses

   Ondrej Sury
   Americka 23
   120 00 Praha 2

   Phone: +420 222 745 110
   Email: ondrej.sury@nic.cz

   Shane Kerr (editor)
   Bennebrokestraat 17-I
   1015 PE Amsterdam

   Phone: +31 64 6336297
   Email: shane@isc.org

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