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Domain Name System Operations Working Group                   G. Barwood
Intended status: Standards Track                            12 June 2011

                            DNS Transport


   This document describes a new resource record type that allows a
   child zone to update the parent DS RRset for a DNS zone.

Status of this Memo

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

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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1.  Introduction

  This document defines a new resource record that may be used to
  update the parent the DS RRset [RFC4034]. A new resource record type
  is used, because the DS RR appears only on the upper (parental) side
  of a delegation.

  The DNSSEC DS RRset for a zone is defined by the child zone but stored
  in the parent zone. After creating a new key signing key (or before an
  existing key is to be withdrawn), the child zone needs to update the
  parent zone.

  There is currently no DNS protocol mechanism for accomplishing this.
  It is assumed that the DS RRset is transferred by some out-of-band

  The mnenomic for the new resource record type is "CDS", which is
  intended to stand for "Child DS".

  In particular the CDS RR MAY be used to securely automate the rollover
  of the key signing key for a zone.

  A new resource record type is preferred to using flags in the DNSKEY
  RRset. It allows the DS to be published without revealing the public
  key, delaying the time at which an attacker can start cryptanalysis;
  the size of the DNSKEY RRset is not changed, which avoids potential
  transport problems with large responses; it allows an algorithm to be
  retired; and it allows arbitrary DS records to be published which may
  have no corresponding DNSKEY, which might be useful in future for
  defining transport parameters.

2.  Definitions

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

3.  Resource Record Format

  The wire and presentation format is identical to the DS record.

  However no special processing is performed by servers or clients when
  serving or resolving.

  The CDS record MUST be signed with a key that has the Secure Entry
  Point flag set.

3.  Usage

  The CDS RRset MAY be used by the parent zone to create or update the
  DS RRset. The parent zone MAY periodically check the child zone to see
  if the CDS RRset has changed. The child zone MAY send a NOTIFY message
  [RFC1996] to a name server for the parent zone to expidite the
  process. The child zone SHOULD take into account timing considerations

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  to ensure that validation failures do not occur.

  The parent zone SHOULD attempt to authenticate [RFC4033] the CDS
  RRset. If the authentication succeeds extra security checks are not
  needed. If the result is insecure, extra checks MAY be performed
  according to the parent zone policy. If the authentication fails (the
  result is  Bogus), no action is taken, other than appropriate alerts
  to inform operators or administrators that there is a problem.

  The parent zone SHOULD check that the signing key(s) have the Secure
  Entry Point flag set.

  The parent zone SHOULD ensure that old versions of the CDS RRset do
  not overwrite newer versions, which can occur if there is a delay
  updating secondary name servers for the child zone. This MAY be
  accomplished by checking that the signature inception in the RRSIG has
  increased - that is the minimum inception of the new signatures
  is greater than the maximum inception of the old signatures.

  If the CDS RRset does not exist, the parent MUST take no action.
  Specifically it MUST NOT delete the existing DS RRset.

  If the child zone loses the secret key(s) for the zone, and needs to
  reset the parent DS RRset, this must be accomplished by an out-of-band
  mechanism not defined here.

  To mitigate situations where a key signing key has been compromised,
  the parent zone MAY take extra security measures, for example
  informing ( by email or other methods ) the zone administrator of the
  change,  and delaying the acceptance of the new DS RRset for some
  period of time. However the precise out-of-band measures that a parent
  zone SHOULD take are outside the scope of this document.

4.  IANA Considerations

  IANA has assigned RR Type code 59 for CDS.

5.  Security considerations

  The CDS RRtype should allow for enhanced security. Since rollover is
  automated, updating a DS RRset by other means may be regarded as
  unusual and subject to extra security checks.

6.  Acknowledgements

  This document was created following discussion on automation of KSK
  rollover on the DNS Operations Working Group mailing list.

  Thanks to the people who provided review and suggestions:
  Mark Andrews, Richard Doty, Olafur Gudmundsson, Shane Kerr,
  Stephan Lagerholm, Chris Thompson.

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7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, August 1996.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

Appendix A. Example KSK rollover

  The example given is a simple single signature rollover. Other
  schemes are also possible.

  Suppose the child zone is secure.

  Step 1.
  A new Key Signing Key is generated, and a new CDS record is added
  to the child CDS RRset.

  Step 2.
  The parent zone retrieves the new CDS RRset from the child zone, and
  updates the published DS RRset.

  Step 3.
  The child zone, after seeing the new DS record in the parent zone,
  publishes the new DNSKEY. Note: the child zone may also publish the
  new DNSKEY at Step 1.

  Step 4.
  The child zone waits for the new DNSKEY and DS records to fully
  propagate to caches.

  Step 5.
  The child zone stops signing with the old Key Signing Key, and starts
  signing with the new Key Signing Key.

  Step 6.
  The child zone waits for the old DNSKEY and any associated RRSIGs
  to expire from caches.

  Step 7.
  The child zone removes the old CDS record from the child CDS RRset.

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  Step 8.
  The parent zone retrieves the final CDS RRset from the child zone
  and publishes the final DS RRset.

  Note: when signing a zone for the first time, the DNSKEY RRset must
  be published first, followed by a delay to allow the non-existence of
  the DNSKEY RRset to expire from caches, before the CDS RRset is

Author's Address

   George Barwood
   33 Sandpiper Close
   GL2 4LZ
   United Kingdom

   Email: george.barwood@blueyonder.co.uk

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