DNSOP                                                    P. Wouters, Ed.
Internet-Draft                                                   Red Hat
Updates: 4035 (if approved)                                       L. Xia
Intended status: Informational                                    Huawei
Expires: December 31, 2018                                   W. Hardaker
                                                           June 29, 2018

                    The Delegation_Only DNSKEY flag


   This document introduces a new DNSKEY flag called DELEGATION_ONLY
   that indicates that the particular zone will never sign zone data
   across a label.  That is, every label (dot) underneath is considered
   a zone cut and must have its own (signed) delegation.

Status of This Memo

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   This Internet-Draft will expire on December 31, 2018.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The Deep Link State problem . . . . . . . . . . . . . . . . .   3
   4.  Limiting the scope of a DNSKEY RRset to just delegations  . .   3
   5.  Parental Transparency . . . . . . . . . . . . . . . . . . . .   4
   6.  Marking the root key DELEGATION_ONLY  . . . . . . . . . . . .   4
   7.  Marking TLD keys DELEGATION_ONLY  . . . . . . . . . . . . . .   4
   8.  Migrating to and from DELEGATION_ONLY . . . . . . . . . . . .   5
   9.  Similarities to the Public Suffix List  . . . . . . . . . . .   5
   10. Operational Considerations  . . . . . . . . . . . . . . . . .   5
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   6
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     14.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     14.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The DNS Security Extensions [DNSSEC] use public key cryptography to
   create an hierarchical trust base with the DNSSEC root public keys at
   the top, followed by Top Level domain (TLD) keys one level
   underneath.  While the root and TLD zones are asumed to be almost
   exclusively delegation-only zones, there is currently no method to
   audit these zones to ensure they behave as a delegation-only zone.
   This creates an attractive target for malicious use of these zones -
   either by their owners or through coercion.  For example, the DNSSEC
   root key could simply sign an A record and TLSA record for
   "www.example.com", overriding the authority of "com" and
   "example.com".  If such a change is done in a targetted attack, the
   attack would be near impossible to detect without prior knowledge of
   what zone contents are legitimate within a given zone.  This document
   defines a mechanism for zone owners, at DNSKEY creation time, to
   indicate they will only delegate the remainder of the tree to lower-
   level zones, allowing easier logging and auditing of DNS responses
   they serve.

   This document introduces a new DNSKEY flag allowing zone owners to
   commit that the zone will never sign any DNS data that traverses a
   single label and if any such signed data is encountered by validating
   resolvers, that this data should be interpreted as BOGUS.

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2.  Terminology

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

3.  The Deep Link State problem

   The hierarchical model of DNS and DNSSEC ([RFC4033], [RFC4034] and
   [RFC4035]) comes with the property that a zone at one point in the
   hierarchy can define, and therefor override, everything in the DNS
   tree from their point and below.  For example, the DNSSEC root key
   could ignore the NS records for ".org" and "example.org" and could
   place a record "www.example.org" directly into its own zone, with a
   corresponding RRSIG signed by the root key itself.  Even if resolvers
   would defend against this attack by not allowing RRSIG's to span
   across a potential zone cut, the zone operator (any level higher in
   the hierarchy than the target victim) could briefly remove the NS and
   DS records, and create a "legitimate" DNS entry for
   "www.example.org", hiding the normal zonecuts.  The attacker can then
   publish DNS addresses records (e.g.  A and AAAA records), as well as
   records used for authentication (e.g.  TLSA, SMIME, OPENPGPKEY, SSHP
   or IPSECKEY records).

   Exposing such targetted attacks requires a transparency audit setup
   ([RFC6962]) that needs to log all signed DNS data to prove that data
   signed by a parental DNSKEY was out of expected policy.  The very
   distributed nature of DNS makes such transparency logs prohibitively
   expensive and nearly impossible to operate.  Additionally, it would
   expose all zone data to any public log operators, thereby exposing
   all DNS data to a public archive.  This data could then be used for
   other malicious purposes.

4.  Limiting the scope of a DNSKEY RRset to just delegations

   This document introduces a new DNSKEY flag called DELEGATION_ONLY.
   When this flag is set on a DNSKEY with SEP bit set (KSK), the zone
   owner commits to not sign any data that crosses a label down in the
   hierarchy.  This commits a parent in the DNS hierarchy to only sign
   NS and DS records (i.e. all non-glue, delegation records) for its
   child zones.  It will no longer be able to ignore (or briefly delete,
   see below) a child delegation and publish data crossing zone labels
   by pretending the next label is not a zone cut.

   For such a parent to take over data that belongs to its child zone,
   it has two choices.  It can (temporarilly) remove its own DNSKEY
   DELEGATION_ONLY flag or it can replace the NS and DS records of its
   child zone with its own data (destinations and key references) so it

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   can sign DNS data that belongs to its own child zone.  However, both
   of these actions cannot be hidden, thus exposing such malicious
   behavior when combined with public transparency logs.

5.  Parental Transparency

   A parent zone, such as the root zone, a TLD or any public suffix list
   delegation point, that has published a key with the DELEGATION_ONLY
   flag can no longer make an exception for a single delegated zone
   without removing the DELEGATION_ONLY flag, switching off its
   published policy.  This action would be highly visible, and for some
   domains such as the root or TLDs, require human interaction to notify
   the stake holders to prevent loss of trust.

   Removing the DELEGATION_ONLY flag from a DNSKEY requires that the
   zone signals a new DS record to its parent, as changing any DNSKEY
   flag requires changes to the DS record data for that corresponds to

   In the case of the root key, it would require updating out-of-band
   root key meta information and/or perform an [RFC5011] style rollover
   for the same key with updated DNSKEY flags.  Due to the timings of
   such a rollover, it would take at least 30 days for the first
   validating resolvers to even pick this policy change.  It would also
   be a highly visible event.

   Replacing the NS and DS records of a child zone can still be done in
   a targetted attack mode, but these events are something that can be
   easilly tracked by a transparency infrastructure similar to what is
   now in use for the WebPKI using [RFC6962](bis).  With client
   implementations of transparency, all records would be logged and
   become visible to the owner of attacked child zones, exposing a
   parent's malicious actions.

6.  Marking the root key DELEGATION_ONLY

   Once the root key is marked with a DELEGATION_ONLY flag, and deployed
   resolvers are configured with the new key, all TLDs will be ensured
   that the root key can no longer be abused to create "deep link" data.
   Until the root key sets this bit, software MAY imply this bit is
   always set, as this is the current expectation of the root zone.

7.  Marking TLD keys DELEGATION_ONLY

   Even before the root key has been marked with DELEGATION_ONLY, TLDs
   can already signal their own willingness to commit being
   DELEGATION_ONLY zones.  Any changes of that state in a TLD DNSKEY
   will require those TLDs to submit a new DS record to the root.

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8.  Migrating to and from DELEGATION_ONLY

   There might be multiple DNSKEYs with the SEP bit set in a zone.  For
   the purpose of delcaring a zone as DELEGATION_ONLY, only those
   DNSKEY's that have a corresponding DS record at the parent MUST be
   considered.  If multiple DS records appear at the parent, some of
   which point to DNSKEY's with and some of which point to DNSKEY's
   without the DELEGATION_ONLY flag set, the zone MUST be considered
   DELEGATION_ONLY.  This situation will occur when a zone is rolling
   its DNSKEY key at the same time as it is commiting to a
   DELEGATION_ONLY zone (or the reverse).

9.  Similarities to the Public Suffix List

   The DELEGATION_ONLY flag has a strong overlap in functionality with
   the Public Suffix List; both signal a formal split of authority
   between parent and child.  The DELEGATION_ONLY flag allows zones to
   formally state their intention.

10.  Operational Considerations

   Setting or unsetting the DELEGATION_ONLY flag must be handled like
   any other Key Signing Key rollover procedure, with the appropriate
   wait times to give resolvers the chance to update their caches.

   Some TLDs offer a service where small domains can be hosted in-zone
   at the TLD zone itself.  In that case, the TLD MUST NOT set the
   DELEGATION_ONLY flag.  Another solution for such TLDs is to create
   delegations for these child zones with the same or different DNSKEY
   as used in the parent zone itself.

   If a zone is publishing glue records for a number of zones, and the
   zone that contains the authoritative records for this glue is
   deleted, a resigning of the zone will make this orphaned glue
   authoritative within the zone.  However, with the DELEGATION_ONLY bit
   set, this (signed) DNSSEC data will be considered BOGUS as it
   violations the commitment to only delegate.  This may impact domains
   that depended on this unsigned glue.

   For example, if "example.com" and "example.net" use NS records
   pointing to "ns.example.net", then if "example.net" is deleted from
   the ".net" zone, and the previously unsigned glue of "ns.example.net"
   is now signed by the ".net" zone, the "example.com" zone will lose
   its NS records and fail to resolve.

   The bind DNS software has an option called "delegation_only zones"
   which is an option that means something completely different.  It

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   refers to ignoring wildcard records in specified zones that are
   deemed delegation-only zones.

11.  Security Considerations

   There are no negative security impacts of using the DELEGATION_ONLY

12.  IANA Considerations

   This document defines a new DNSKEY flag, the DELEGATION_ONLY flag,
   whose value [TBD] has been allocated by IANA from the DNSKEY FLAGS

13.  Acknowledgements

   The author wishes to thank Thomas H.  Ptacek for his insistence on
   this matter.

   Thanks to the following IETF participants: Viktor Dukhovni, Shumon
   Huque, Geoff Huston, Rick Lamb and Sam Weiler.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,

   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
              September 2007, <https://www.rfc-editor.org/info/rfc5011>.

14.2.  Informative References

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

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   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,

Authors' Addresses

   Paul Wouters (editor)
   Red Hat

   Email: pwouters@redhat.com

   Liang Xia

   Email: frank.xialiang@huawei.com

   Wes Hardaker
   P.O. Box 382
   Davis, CA  95617

   Email: ietf@hardakers.net

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