DNSOP                                                         P. Wouters
Internet-Draft                                                   Red Hat
Updates: 4035 (if approved)                                  W. Hardaker
Intended status: Informational                                   USC/ISI
Expires: November 7, 2020                                    May 6, 2020

                    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.  Additionally,
   it indicates the zone is expecting its parent to never bypass or
   override the zone.  DNSSEC Validating Resolvers can use this flag to
   mark any data that violates the DELEGATION_ONLY policy as BOGUS.

Status of This Memo

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   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 7, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of
   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 Signing problem  . . . . . . . . . . . . . . . . . .   3
     3.1.  Affected parties and their roles  . . . . . . . . . . . .   4
   4.  The DELEGATION_ONLY DNSKEY flag . . . . . . . . . . . . . . .   5
   5.  _underscore label exception . . . . . . . . . . . . . . . . .   6
   6.  Parental Transparency . . . . . . . . . . . . . . . . . . . .   6
   7.  Marking zone keys DELEGATION_ONLY . . . . . . . . . . . . . .   6
     7.1.  Marking the Root DNSKEY DELEGATION_ONLY . . . . . . . . .   7
     7.2.  Migrating to and from DELEGATION_ONLY . . . . . . . . . .   7
   8.  Operational Considerations  . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   11. Human Rights Considerations . . . . . . . . . . . . . . . . .   9
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     14.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 most TLD zones are assumed to be
   exclusively delegation-only zones, there is currently no
   interoperable and automatable mechanism for auditing these zones to
   ensure they behave as a delegation-only zone.  This creates a target
   for malicious use of these zones - either by their owners or through

   This document defines a mechanism for delegation-only zone owners to
   create a DNSKEY that indicate they will only delegate the remainder
   of the DNS tree to lower-level zones.  This allows for easier
   delegation policy verification and logging and auditing of DNS
   responses served by their infrastructure.

   Specifically, this document introduces a new DNSKEY flag allowing
   zone owners to commit to only signing records relating to delegation.

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   If a DNSSEC validator discovers any non-delegation zone data signed
   by a flagged key, this data can be interpreted as BOGUS.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  The Deep Signing problem

   The hierarchical model of DNS and DNSSEC ([RFC1034], [RFC1035],
   [RFC4033], [RFC4034] and [RFC4035]) comes with the property that a
   zone at one point in the hierarchy can define, and therefor override,
   everything below it in the DNS tree.  And this is possible to achieve
   on a per-client basis.

   For example, the owner of the DNSSEC root key could generate a
   specially crafted zone file that ignores the intended NS records for
   ".org" and "example.org".  It could place a AAAA record for
   "www.example.org" directly into the specially crafted zone, with a
   corresponding RRSIG signed by the root DNSKEY itself.  Validating
   resolvers would find this record perfectly acceptable, as it was
   signed by a key within the proper chain of trust (in this case, a
   root DNSKEY).  This specially crafted zone could then even be served
   to specific clients in an effort to subvert a portion of the DNS
   ecosystem for a portion of the Internet.

   Similarly, the TLD "example" could circumvent company.example for
   certain clients.  It is important to note that this can be done
   without changing the upstream DS record or trust anchor -- the DNSKEY
   is (again) already in the trust path and is merely signing deeper DNS
   records than the zone owner's clients may have expected it to sign.

   It is important to note that this "feature" has always been present.
   Since the creation of the DNS, it has always been possible to serve
   "split zones".  Specifically, it is not a problem created by DNSSEC
   -- DNSSEC was not designed to protect against this use case.

   Exposing such targeted attacks requires a transparency audit
   infrastructure similar to what is deployed for PKIX ([RFC6962]).
   However, a DNSSEC version would need to log significantly more data,
   as all signed DNS data used by a DNSKEY must be recorded in order to
   prove that data signed by a parent zone's DNSKEY was out of expected
   policy.  The very distributed nature of the DNS combined with the

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   typically frequent zone resigning rate makes such transparency logs
   prohibitively expensive and nearly impossible to operate.

   Additionally, it would require zone owners to expose all their zone
   data to any public log operators, thereby introducing privacy
   implications and exposing all relevant DNS data to a public archive.
   This may be acceptable for some domains, such as the root, where DNS
   data is already considered public.  However, other delegation domains
   have legal implications that prohibit them from participating in such
   a system.

   Furthermore, there is no signalling mechanism that lets validating
   resolvers know which zones are supposedly delegation-only, and what
   zones can be logged.  Today there are over 1500 TLDs in the root
   zone, some of which may be considered delegation-only while others
   may not be.  At the time of this writing, the list of entries in the
   public suffix list contains over 8800 entries as well, with 73 wild-
   card entries (prefixed with a "*") indicating that all of their
   (unknown number of) children are public registration points.  In the
   absence of an interoperable mechanism (like this draft provides), it
   is infeasible that a validating resolver or auditing log could know
   which of these zones are delegation-only without individual policy
   statements from each of them.  [todo: xref psl]

3.1.  Affected parties and their roles

   Upon deployment of this specification, the following parties would be
   potentially benefit or be affected by:

   Authoritative parent: If (and only if) an authoritative parent is a
   "delegation only" zone, it could generate a DNSKEY with the
   DELEGATION_ONLY flag set, indicating a verifiable promise to the
   world that will not sign any records other than delegation records.

   Authoritative Child / Delegated Zone: child zones existing underneath
   a marked delegation-only zone get the added benefit of knowing their
   parent will not (and cannot) sign DNS records within the child's
   portion of the DNS tree using the marked DNSKEY.

   Validating Resolver: A validating that supports verifying the
   DELEGATION_ONLY flag is capable of rejecting or discarding any data
   from an authoritative parent that incorrectly signs non-delegation
   records too low in the DNS tree.  If the validating resolver supports
   a (future-defined) DNSSEC transparency audit log as well, it may
   submit the appropriate data to a DNSSEC transparency log that
   appropriately tracks DNSSEC signatures.

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   DNSSEC Transparency Log (optional future): a DNSSEC transparency log
   would create a non-modifiable trace of log entries submitted to it,
   for public verification, similar to [RFC6962].  What it chooses to
   accept into its log might be only certain zone data, or any zone with
   a marked DNSKEY.

   Note that without a DNSSEC Log, the DELEGATION_ONLY flag is still
   useful per the discussion in the Validating Resolvers role: the
   resolver will reject incorrectly signed, non-delegation data.
   However, malicious parent zones are still capable of creating two (or
   more) DNSKEYs, one with the DELEGATION_ONLY flag and one without.
   However, they would also have to publish those DS records as well,
   which is detectable by DNSSEC monitoring platforms, regardless of the
   existence of a DNSSEC Transparency Log.  Any zone with multiple DS
   records that link to both a DELEGATION_ONLY marked and an unmarked
   DNSKEY would be considered suspicious (or at least in transition).
   Circumventing this through obfuscation would require the collusion of
   their parent as well.  Finally, a DELEGATION_ONLY flagged DNSKEY for
   the root zone cannot be overridden easily, as it would require a
   trust anchor update in all validating resolvers.


   This document introduces a new DNSKEY flag called DELEGATION_ONLY.
   When this flag is set on a DNSKEY with its Secure Entry Point (SEP)
   flag set, 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 publish signed DS records and unsigned glue records
   (NS and A/AAAA) 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

   For such a parent to take over data that belongs to its child zone,
   it has two choices.  It can (temporarily) 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
   can sign DNS data that belongs to its own child zone.  However, both
   of these actions cannot be hidden, thus exposing such malicious
   behaviour when combined with DNSSEC Transparency logs.

   A zone that publishes a DNSKEY with the DELEGATION_ONLY flag also
   signifies that it is not expecting its own parent to skip it, thereby
   bypassing its DELEGATION_ONLY flag.

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5.  _underscore label exception

   Some protocols, such as the DANE protocol [RFC6698] use a number of
   labels that start with an underscore (_) prefix to publish
   information about the zone itself.  For example, the TLSA record for
   www.example.com is published at the location
   _443._tcp.www.example.com.  These records are semantically part of
   the zone itself and are not delegated child zones.  Any chain of
   labels that each start with an underscore (_) is not considered to
   violate the DELEGATION_ONLY flag limitation of being DELEGATION_ONLY,
   as this data is logically part of the zone itself and is never meant
   to be interpreted as an independent delegated child zone.

6.  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 first publishes an additional updated DS record to its parent.

   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 pick up 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 targeted attack mode, but these events are something that can be
   easily tracked by a transparency infrastructure similar to what is
   now in use for the WebPKI using [RFC6962](bis).  With client
   implementations of transparency, all DELEGATION_ONLY flag changes
   would be logged and become visible to the owner of attacked child
   zones, exposing a parent's malicious behaviour.

7.  Marking zone keys DELEGATION_ONLY

   Even before a parent DNSKEY (or the root key) has set the
   DELEGATION_ONLY flag, zones can already signal their own willingness
   to commit to being DELEGATION_ONLY zones.  Any changes of that state
   in a zone DNSKEY will require those zones to submit a new DS record
   to their parent.  Setting the DELEGATION_ONLY flag would trigger

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   DNSSEC Transparency clients to start monitoring for actions by the
   zone or its parents that would be bypassing the DELEGATION_ONLY
   policy of the zone.  Validating resolvers would mark any data in
   violation of the DELEGATION_ONLY policy as BOGUS.

7.1.  Marking the Root DNSKEY DELEGATION_ONLY

   Once the Root DNSKEY is marked with a DELEGATION_ONLY flag and
   deployed resolvers are configured with the new DNSKEY, all TLDs will
   be ensured that the Root DNSKEY can no longer be abused to override
   child zone data.  Until the Root KSK DNSKEY sets this flag, software
   SHOULD imply this flag is always set, as this is the current
   expectation of the Root Zone.

7.2.  Migrating to and from DELEGATION_ONLY

   There might be multiple DNSKEYs with the SEP flag set in a zone.  For
   the purpose of declaring 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 DNSKEYs with and some of which point to DNSKEYs
   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 committing to a
   DELEGATION_ONLY zone (or the reverse).

8.  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
   flag 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.  Note that glue handling differs
   per zone.  Some TLDs already remove the glue records if no
   authoritative child is left in its zone that matches these glue

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   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.

   If a domain uses Empty Non Terminals (ENT), that is uses multiple
   labels where the label is not covered by its own delegation, then the
   DELEGATION_ONLY flag cannot be set.  For example, some domains allow
   registrations straight into their zone (eg "child.example") while
   others use an ENT to categorize these (eg "child.co.example" and
   "child.ac.example").  Some TLDs contain a few ENTs marking some
   administrative or geographic region.  Such TLDs must first migrate
   their ENT to fully delegated child zones before enabling the

   Some TLDs publish their nameserver (NS) records straight within their
   TLD (eg "ns1.example") which makes these names indistinguishable from
   real delegations with respect to the DELEGATION_ONLY flag.  These NS
   entries would have to be moved to their own delegation zone (eg

   Some TLDs have a requirement for certain Fully Qualified Domain Names
   (FQDN) within their TLD, such as "whois.example" or "nic.example".
   These usually appear as signed data of the TLD and not as a delegated
   child zone.  These names would have to be converted to delegated
   zones before enabling the DELEGATION_ONLY flag.

   The bind DNS software has an option called "delegation_only zones"
   which is an option that means something completely different.  It
   refers to ignoring wildcard records in specified zones that are
   deemed delegation-only zones.

9.  Security Considerations

   Some parental attacks cannot be detected when the validating
   resolver's cache is empty.  Care should be taken by resolvers to not
   unnecessarily empty their cache.  This is specifically important for
   roaming clients that re-connect frequently to different wireless or
   mobile data networks.

   This DELEGATION_ONLY mechanism is not designed to completely foil
   attacks (since parent's can simply change a child's referral data),
   but rather to empower transparency logging mechanisms.

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10.  Privacy Considerations

   Some of the protection offered by the DELEGATION_ONLY flag is only
   available when DNS resolvers report changes in the signing depth of
   high level (root or TLD) DNSKEYs to gain DNSSEC Transparency.  This
   reporting can reveal that a particular node is trying to access a
   certain DNS name.  Defensive measures to prevent exposing users
   should be taken when implementing DNSSEC Transparency.  It is
   expected that DNSSEC Transparency behaviour will be written up in a
   separate document.

11.  Human Rights Considerations

   The DNS protocol's hierarchy limits zones authority to themselves and
   their child zones only.  While this provides a finer grained trust
   model compared to a simple list of trusted entities, such as used in
   the WebPKI, it consolidates a lot of power in the top of the DNS
   hierarchy.  With the increased reliance on DNSSEC for securely
   identifying resources, such as DANE records, it becomes very
   important to audit those entities high up in the hierarchy to not
   abuse or be co-erced into abusing control of the independent child
   zones.  The protocol extension specifically aims at increasing
   parental transparency and blocks some parental attacks from those
   parents who have publicly claimed to never override their child zone

   Parents using the DELEGATION_ONLY flag publication to increase their
   public trust are still able to remove child zones from their zone,
   for example in cases of legal compliance or to prevent malicious
   activity happening in its child zone.  But these parents can only do
   so publicly and can no longer surreptitiously take control of their
   own child zones.

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 authors 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.

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14.  References

14.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

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

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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,

   [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,

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

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

Authors' Addresses

   Paul Wouters
   Red Hat

   Email: pwouters@redhat.com

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

   Email: ietf@hardakers.net

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