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Internet Engineering Task Force                                  L. Song
Internet-Draft                                Beijing Internet Institute
Intended status: Informational                              May 17, 2016
Expires: November 18, 2016


                     An IXFR Fallback to AXFR Case
                   draft-song-dnsop-ixfr-fallback-01

Abstract

   This memo introduces an IXFR issue observed during a multiple signers
   experiment conducted in Yeti DNS project.  In the experiment IXFR
   client is designed to pull the zone from three IXFR servers who used
   their own key to sign the zone and produce different RRSIG records
   intentionally.  The configuration of multiple signers cause the
   failure of IXFR in client side.

   REMOVE BEFORE PUBLICATION: The source of the document is currently
   placed at GitHub [xml-file].  Comments and pull request are welcome.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 18, 2016.

Copyright Notice

   Copyright (c) 2016 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
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   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The IXFR issues observed in MZSK experiment . . . . . . . . .   3
   3.  Possible solution . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   In DNS specifications authoritative name server uses full zone
   transfer (AXFR) [RFC5936], incremental Zone Transfer (IXFR)[RFC1995],
   and NOTIFY [RFC1996] to achieve coherency of the zone contents.  IXFR
   is an optimization for large DNS zone transfer, which allows server
   only transfer the changed portion(s) to client.

   AXFR fallback usually happens at server side by simply returning IXFR
   client the entire new zone in condition that IXFR server cannot
   fulfill the given delta-update request.  It is because an IXFR client
   may has multiple IXFR servers for a single zone.  It is not a
   protocol defect but do stimulate people to find optimization avoiding
   full zone transfer [I-D.kerr-ixfr-only] and trying to make a new IXFR
   protocol [I-D.ietf-dnsext-rfc1995bis-ixfr].

   [RFC5936] suggests that if its upstream servers have different ideas
   of the zone contents with the same (zone, serial) pair, the client
   can stop adding records that already exist or deleting records that
   do not exist.  However, if an IXFR incoherence error is spotted by
   that client, it is not clear whether the client should stop IXFR
   process and ask for AXFR as a fallback.  To the author's knowledge,
   there is such recommendation so far for AXFR fallback initiated by
   client in formal document.

   This memo introduces an IXFR problem observed during a DNS root
   experiment in Yeti project[Yeti-DNS-Project] which involves multiple
   root zone distribution master (DM).  It is designed that three DMs do
   have different "ideas" on how their keys are managed and produce
   different RRSIG records.  In this scenario, it is observed that
   different DNS implementations have different behaviors due to the
   ambiguity in understanding of IXFR and fallback to AXFR.





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   REMOVE BEFORE Publication: The motivation of this memo is to ask for
   discussion in the community whether this specific fallback or non-
   fallback is viewed as a problem.  Is it worthwhile developing IXFR
   protocol further towards rfc1996bis.  Or this memo can serve
   providing some information and guidance to operators who do run
   authoritative servers in similar situation as it is done in Yeti
   experiment.

2.  The IXFR issues observed in MZSK experiment

   As a background for this memo, the introduction of Yeti testbed and
   experiments can be found in [I-D.song-yeti-testbed-experience], and
   section 3.1 and 4.2 are relevant.  Conceptually, Yeti DNS intends to
   break one signer/DM role into three which buys some properties in
   loosely cooperative environment , such as resilience to single point
   of failure, more independent choice for slave server, and certain
   degree of transparency and management coordination for important zone
   (root zone in Yeti's case).

   One experiment in Yeti is designed to test multiple signers with
   Multiple ZSKs (MZSK).  It is required that all public ZSKs used by
   DMs are included in the zone as a key set; and resolver can validate
   the message by picking one key from the key set.  From DNSSEC point
   of view, it is technically workable.  However, different signers do
   produce different RRSIG RR which introduces zone inconsistency from
   beginning in this case.  In current setting of Yeti experiment, it is
   possible that one client does AXFR/IXFR from one server and later
   asks for IXFR from another server.

   It is observed that when the IXFR client switched from one IXFR
   server to another, it received a IXFR response deleting RRSIG record
   that does not exist.  One IXFR client running NSD 4.1.7 rejected IXFR
   response, made a log indicating a bad data and then asked for full
   zone transfer.  Luckily, Yeti root zone is relatively small (691K),
   so the fallback to AXFR does not cause significant performance
   degeneration.  But if operator does host big zone with MZSK model, it
   will cause problem based on current IXFR.

   Another observation is that another IXFR client running Knot 2.1.0 in
   similar situation just accepts the IXFR response, ignores the
   differences and generates a merged zone with two RRSIG RRs.  It not
   only produces larger response, but also causes DNSSEC failure when a
   new zone is generated given that old RRSIG is the signature of old
   zone RRs.







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3.  Possible solution

   Generally, there are three considerations to this issue.

   o  Asking for development of RRSIG-aware IXFR format in which the
      RRSIG is treated as a special and RRSIG RR should always be
      transfered in full (like it does in AXFR).  In the case of MZSK
      experiment, the old RRSIG record(s) is replaced by the new RRSIG
      record(s) and no specific deleted RRSIG is sent.  Compared to the
      first case in NSD 4.1.7, it is helpful to reduce the cost for full
      zone transfer if the zone is fairly large.

   o  Adopting the behavior of NSD 4.1.7 as a improvement for IXFR
      protocol in which an IXFR client should fall back to AXFR
      automatically in the event of an IXFR incoherence error.  To avoid
      unnecessary full zone transfer, it is desirable that the IXFR
      client is more "sticky" to the server who transfers the zone to
      the client last time.

   o  Without modification of IXFR protocol, asking each IXFR client
      (root slave in Yeti case) to be tied to a specific IXFR
      server(Yeti DM).  This would decrease redundancy and resiliency
      but would allow normal IXFR, and may make debugging easier.  But
      the MZSK operation introduce regional DM which is not desirable
      for Yeti case.

4.  Acknowledgments

   Specially thanks to Stephane Bortzmeyer who first spotted the IXFR
   issues from his NSD and Knot authoritative servers.  Acknowledgment
   to Paul Vixie and Shane Kerr who contributed a lot to this technical
   finding and possible solutions in this memo.  Thanks to Antonio Prado
   who helped to make the language more readable.

5.  References

   [I-D.ietf-dnsext-rfc1995bis-ixfr]
              Hoenes, A., Sury, O., and S. Kerr, "DNS Incremental Zone
              Transfer Protocol (IXFR)", draft-ietf-dnsext-rfc1995bis-
              ixfr-01 (work in progress), April 2012.

   [I-D.kerr-ixfr-only]
              Sury, O. and S. Kerr, "IXFR-ONLY to Prevent IXFR Fallback
              to AXFR", draft-kerr-ixfr-only-01 (work in progress),
              February 2010.






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   [I-D.song-yeti-testbed-experience]
              Song, D., Kerr, S., and D. Liu, "Experiences from Root
              Testbed in the Yeti DNS Project", draft-song-yeti-testbed-
              experience-01 (work in progress), December 2015.

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              DOI 10.17487/RFC1995, August 1996,
              <http://www.rfc-editor.org/info/rfc1995>.

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
              August 1996, <http://www.rfc-editor.org/info/rfc1996>.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
              <http://www.rfc-editor.org/info/rfc5936>.

   [xml-file]
              "XML source file of IXFR Case draft", 2016,
              <https://github.com/songlinjian/ietf-draft-ixfr-ps>.

   [Yeti-DNS-Project]
              "Website of Yeti DNS Project", <http://www.yeti-dns.org>.

Author's Address

   Linjian Song
   Beijing Internet Institute
   2508 Room, 25th Floor, Tower A, Time Fortune
   Beijing  100028
   P. R. China

   Email: songlinjian@gmail.com
   URI:   http://www.biigroup.com/

















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