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Extensible Delegation for DNS

Document Type Active Internet-Draft (individual)
Authors Tim April , Petr Špaček , Ralf Weber , David C Lawrence
Last updated 2024-01-23
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dnsop                                                           T. April
Updates: 1035 (if approved)                                    P. Špaček
Intended status: Standards Track                                     ISC
Expires: 26 July 2024                                           R. Weber
                                                     Akamai Technologies
                                                             D. Lawrence
                                                         23 January 2024

                     Extensible Delegation for DNS


   A delegation in the Domain Name System (DNS) is a mechanism that
   enables efficient and distributed management of the DNS namespace.
   It involves delegating authority over subdomains to specific DNS
   servers via NS records, allowing for a hierarchical structure and
   distributing the responsibility for maintaining DNS records.

   An NS record contains the hostname of the nameserver for the
   delegated namespace.  Any facilities of that nameserver must be
   discovered through other mechanisms.  This document proposes a new
   extensible DNS record type, DELEG, which contains additional
   information about the delegated namespace and the capabilities of
   authoritative nameservers for the delegated namespace.

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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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 July 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Motivation for DELEG
     1.3.  Introductory Examples
     1.4.  Goal of the DELEG record
       1.5.1.  Preventing downgrade attacks
     1.6.  Facilities
   2.  DELEG Record Type
     2.1.  Difference between the records
     2.2.  AliasMode Record Type
       2.2.1.  Multiple Service Providers
       2.2.2.  Loop Prevention
     2.3.  Deployment Considerations
       2.3.1.  AliasMode and ServiceMode in the Parent
       2.3.2.  Rollout
       2.3.3.  Availability
     2.4.  Response Size Considerations
   3.  Implementation
     3.1.  Including DELEG RRs in a Zone
       3.1.1.  Signing DELEG RRs
     3.2.  Authoritative Name Servers
       3.2.1.  Including DELEG RRs in a Response
       3.2.2.  Responding to Queries for Type DELEG
       3.2.3.  Priority of DELEG over NS and Glue Address records
   4.  Privacy Considerations
   5.  Security Considerations
     5.1.  Availability of Zones Without NS
     5.2.  Resolution Procedure
       5.2.1.  Failures when DELEG Delegation is Present
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Legacy Test Results
   Appendix B.  Acknowledgments {:unnumbered}
   Appendix C.  TODO
   Appendix D.  Change Log
   Authors' Addresses

1.  Introduction

   In the Domain Name System [STD13], subdomains within the domain name
   hierarchy are indicated by delegations to servers which are
   authoritative for their portion of the namespace.  The DNS records
   that do this, called NS records, contain hostnames of nameservers,
   which resolve to addresses.  No other information is available to the
   resolver.  It is limited to connect to the authoritative servers over
   UDP and TCP port 53.

   This limitation is a barrier for efficient introduction of new DNS
   technology.  New features come with additional overhead as they are
   constrained by the intersection of resolver and nameserver
   functionality.  New functionality could be discovered insecurely by
   trial and error, or negotiated after first connection, which is
   costly and unsafe.

   The proposed DELEG record type remedies this problem by providing
   extensible parameters to indicate capabilities that a resolver may
   use for the delegated authority, for example that it should be
   contacted using a transport mechanism other than DNS over UDP or TCP
   on port 53.

   DELEG records are served with NS and DS records in the Authority
   section of DNS delegation type responses.  Standard behavior of
   legacy DNS resolvers is to ignore the DELEG type and continue to rely
   on NS and DS records (see compliance testing described in
   Appendix A).  Resolvers that do understand DELEG and its associated
   parameters can efficiently switch to the new mechanism.

   The DELEG record leverages the Service Binding (SVCB) record format
   defined in [RFC9460], using a subset of the already defined service

   DELEG can use AliasMode, inherited from SVCB, to insert a level of
   indirection to ease the operational maintenance of multiple zones by
   the same servers.  For example, an operator can have numerous
   customer domains all aliased to nameserver sets whose operational
   characteristics can be easily updated without intervention from the
   customers.  Most notably, this provides a method for addressing the
   long-standing problem operators have with maintaining DS records on
   behalf of their customers.  This type of facility will be handled in
   separate documents.

1.1.  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.

   Terminology regarding the Domain Name System comes from [BCP219],
   with addition terms defined here:

   *  legacy name servers: An authoritative server that does not support
      the DELEG record.

   *  legacy resolvers: A resolver that does not support the DELEG

1.2.  Motivation for DELEG

   *  There is no secure way to signal capabilities or new features of
      an authoritative server, such as authenticated DNS-over-TLS.  A
      resolver must resort to trial-and-error methods that can
      potentially fall victim to downgrade attacks.

   *  Delegation point NS records and glue address records are, by
      design, not DNSSEC signed.  This presents a leap of faith.
      Spoofed delegation point NS records can be detected eventually if
      the delegated domain was signed, but only after traffic was sent
      to the (potentially) spoofed endpoint.

   *  The Registry, Registrar, Registrant (RRR) model has no formally
      defined role for DNS operators.  Consequently, registrants are the
      channel between DNS operators and registries/registrars on purely
      operational elements, such as adding NS records, modify DS records
      when rolling keys, etc.  Deleg's AliasMode allows the registrants
      to delegate these facilities to a DNS Operator.

1.3.  Introductory Examples

   To introduce DELEG record, this example shows the authority section
   of a DNS response that delegates a subdomain to another nameserver.  86400  IN DELEG  1 (
                   ipv4hint= ipv6hint=2001:DB8::1 )  86400  IN NS    86400   IN  A     86400   IN  AAAA    2001:DB8::1

   In this example, the authoritative nameserver is delegating using the
   same parameters as regular DNS, but the delegation as well as the
   glue can be signed.

   Like in SVCB, DELEG also offer the ability to use the Alias form of
   delegation.  The example below shows an example where is
   being delegated with a DELEG AliasMode record which can then be
   further resolved using standard SVCB to locate the actual parameters.  86400  IN DELEG 0  86400  IN NS

   The authoritative server may return the following SVCB
   records in response to a query as directed by the above records. 3600    IN SVCB . (
                   ipv6hint=2001:db8:2423::3,2001:db8:2423::4 )

   The above records indicate to the client that the actual
   configuration for the zone can be found at

   Later sections of this document will go into more detail on the
   resolution process using these records.

1.4.  Goal of the DELEG record

   The primary goal of the DELEG records is to provide zone owners a
   method to signal capabilities to clients how to connect and validate
   a subdomain.  This method coexists with NS records in the same zone.

   The DELEG record is authoritative in the parent zone and, if signed,
   has to be signed with the key of the parent zone.  The target of an
   alias record is an SVCB record that exists and can be signed in the
   zone it is pointed at, including the child zone.


   While DNSSEC is RECOMMENDED, unsigned DELEG records may be retrieved
   in a secure way from trusted, Privacy-enabling DNS servers using
   encrypted transports.

   FOR DISCUSSION: This will lead to cyclical dependencies.  A DELEG
   record can introduce a secure way to communicate with trusted,
   Privacy-enabling DNS servers.  For that, it needs to be DNSSEC

1.5.1.  Preventing downgrade attacks

   A flag in the DNSKEY record is used as a backwards compatible, secure
   signal to indicate to a resolver that DELEG records are present or
   that there is an authenticated denial of a DELEG record.  Legacy
   resolvers will ignore this flag and use the DNSKEY as is.

   Without this secure signal an on-path adversary can remove DELEG
   records and its RRsig from a response and effectively downgrade this
   to a legacy DNSSEC signed response.

1.6.  Facilities

   The DELEG record is extensible in such a way that future innovations
   in the domain name system, such as new methods of secure transport,
   message encoding, error reporting, etc, does not depend on a re-
   design of the DNS.

2.  DELEG Record Type

   The SVCB record allows for two types of records, the AliasMode and
   the ServiceMode.  The DELEG record takes advantage of both and each
   will be described below in depth.  The wire format of and the
   registry for the DELEG record is the same as SVCB record defined in

2.1.  Difference between the records

   This document uses two different resource record types.  Both records
   have the same functionality, with the difference between them being
   that the DELEG record MUST only be used at a delegation point, while
   the SVCB is used as a normal resource record and does not indicate
   that the label is being delegated.  For example, take the following
   DELEG record:

   Zone com.:  86400  IN DELEG 1

   When a client receives the above record, the resolver should send
   queries for any name under to the nameserver at unless further delegated.  By contrast, when
   presented with the records below:

Zone com.:  86400  IN DELEG 0

Zone  86400  IN SVCB 1 . ( ipv4hint=,
                ipv6hint=2001:db8:2423::3,2001:db8:2423::4 )

   A resolver trying to resolve a name under would get the
   first record above from the parent authoritative server, .COM,
   indicating that the SVCB records found at should
   be used to locate the authoritative nameservers of, and
   other parameters.

   The primary difference between the two records is that the DELEG
   record means that anything under the record label should be queried
   at the delegated server while the SVCB record is just for redirection
   purposes, and any names under the record's label should still be
   resolved using the same server unless otherwise delegated.

   It should be noted that both DELEG and SVCB records may exist for the
   same label, but they will be in different zones.  Below is an example
   of this:

   Zone com.:  86400  IN DELEG 0

   Zone  86400  IN DELEG  1 (
                               ipv6hint=2001:db8:2423::3 )  86400  IN NS 600 IN A

   Zone  86400  IN SVCB 1 (
                       ipv6hint=2001:db8:4567::4  )  86400  IN NS 600 IN A

   In the above case, the DELEG record for would only be
   used when trying to resolve names at or below  This
   is why when an AliasMode DELEG or SVCB record is encountered, the
   resolver MUST query for the SVCB record associated with the given

2.2.  AliasMode Record Type

   In order to take full advantage of the AliasMode of DELEG and SVCB,
   the parent, child, and resolver must support these records.  When
   supported, the use of the AliasMode will allow zone owners to
   delegate their zones to another operator with a single record in the
   parent.  If a resolver were to encounter an AliasMode DELEG or SVCB
   record, it would then resolve the name in the TargetName of the
   original record using SVCB RR type to receive either another
   AliasMode record or a ServiceMode SVCB record.

   For example, if the name was being resolved, the .com
   zone may issue a referral by returning the following record:    86400    IN  DELEG     0

   The above record would indicate to the resolver that in order to
   obtain the authoritative nameserver records for, the
   resolver should resolve the RR type SVCB for the name

2.2.1.  Multiple Service Providers

   Some zone owners may wish to use multiple providers to serve their
   zone, in which case multiple DELEG AliasMode records can be used.  In
   the event that multiple DELEG AliasMode records are encountered, the
   resolver SHOULD treat those as a union the same way this is done with
   NS records, picking one at random for the first lookup and eventually
   discovering the others.  How exactly DNS questions are directed and
   split between configuration sets is implementation specific:    86400    IN  DELEG     0    86400    IN  DELEG     0

   [ DRAFT NOTE: SVCB says that there "SHOULD only have a single RR".
   This ignores that but keeps the randomization part.  Section 2.4.2 of
   SVCB ]

2.2.2.  Loop Prevention

   The TargetName of an SVCB or DELEG record MAY be the owner of a CNAME
   record.  Resolvers MUST follow CNAMEs as well as further alias SVCB
   records as normal, but MUST not allow more then 4 total lookups per
   delegation, with the first one being the DELEG referral and then 3
   SVCB/CNAME lookups maximal.

   Special care should be taken by both the zone owner and the delegated
   zone operator to ensure that a lookup loop is not created by having
   two AliasMode records rely on each other to serve the zone.  Doing so
   may result in a resolution loop, and likely a denial of service.  The
   mechanism on following CNAME and SVCB alias above should prevent
   exhaustion of server resources.  If a resolution can not be found
   after 4 lookups the server should reply with a SERVFAIL error code.

2.3.  Deployment Considerations

   The DELEG and SVCB records are intended to replace the NS record
   while also adding additional functionality in order to support
   additional transports for the DNS.  Below are discussions of
   considerations for deployment.

2.3.1.  AliasMode and ServiceMode in the Parent

   Both the AliasMode and ServiceMode records can be returned for the
   DELEG record from the parent.  This is different from the SCVB
   [RFC9460] specification and only applies for the DELEG RRSet in the

2.3.2.  Rollout

   When introduced, the DELEG and SVCB records might not initially be
   supported by the DNS root or TLD operators.  Zone owners may place
   these records into their zones before the zones above them have done
   so.  However, doing so is only useful for further delegations down
   the tree as an SVCB record at the zone apex alone does not indicate a
   new delegation type.  The only way to discover new delegations is
   with the DELEG record at the parent.

2.3.3.  Availability

   If a zone operator removes all NS records before DELEG and SVCB
   records are implemented by all clients, the availability of their
   zones will be impacted for the clients that are using non-supporting
   resolvers.  In some cases, this may be a desired quality, but should
   be noted by zone owners and operators.

2.4.  Response Size Considerations

   For latency-conscious zones, the overall packet size of the
   delegation records from a parent zone to child zone should be taken
   into account when configuring the NS, DELEG and SVCB records.
   Resolvers that wish to receive DELEG and SVCB records in response
   SHOULD advertise and support a buffer size that is as large as
   possible, to allow the authoritative server to respond without
   truncating whenever possible.

3.  Implementation

   This document introduces the concept of signaling capabilities to
   clients on how to connect and validate a subdomain.  This section
   details the implementation specifics of the DELEG record for various
   DNS components.

3.1.  Including DELEG RRs in a Zone

   A DELEG RRset MAY be present at a delegation point.  The DELEG RRset
   MAY contain multiple records.  DELEG RRsets MUST NOT appear at a
   zone's apex.

   A DELEG RRset MAY be present with or without NS or DS RRsets at the
   delegation point.

   Construction of a DELEG RR requires knowledge which implies
   communication between the operators of the child and parent zones.
   This communication is an operational matter not covered by this

3.1.1.  Signing DELEG RRs

   A DELEG RRset MUST be DNSSEC signed if the zone is signed.

   If a signed zone contains DELEG records, the zone MUST be signed with
   a DNSKEY that has the DELEG flag set.

3.2.  Authoritative Name Servers

3.2.1.  Including DELEG RRs in a Response

   If a DELEG RRset is present at the delegation point, the name server
   MUST return both the DELEG RRset and its associated RRSIG RR in the
   Authority section along with the DS RRset and its associated RRSIG RR
   and the NS RRset.

   If no DELEG RRset is present at the delegation point, and the zone
   was signed with a DNSKEY that has the DELEG flag set, the name server
   MUST return the NSEC or NSEC3 RR that proves that the DELEG RRset is
   not present including its associated RRSIG RR along with the DS RRset
   and its associated RRSIG RR if present and the NS RRset, if present.

   Including these DELEG, DS, NSEC or NSEC3, and RRSIG RRs increases the
   size of referral messages.  If space does not permit inclusion of
   these records, including glue address records, the name server MUST
   set the TC bit on the response.

3.2.2.  Responding to Queries for Type DELEG

   DELEG records, when present, are included in referrals.  When a
   parent and child are served from the same authoritative server, this
   referral will not be sent because the authoritative server will
   respond with information from the child zone.  In that case, the
   resolver may query for type DELEG.

   The DELEG resource record type is unusual in that it appears only on
   the parent zone's side of a zone cut.  For example, the DELEG RRset
   for the delegation of "foo.example" is part of the "example" zone
   rather than in the "foo.example" zone.  This requires special
   processing rules for both name servers and resolvers because the name
   server for the child zone is authoritative for the name at the zone
   cut by the normal DNS rules, but the child zone does not contain the
   DELEG RRset.

   A DELEG-aware resolver sends queries to the parent zone when looking
   for a DELEG RR at a delegation point.  However, special rules are
   necessary to avoid confusing legacy resolvers which might become
   involved in processing such a query (for example, in a network
   configuration that forces a DELEG-aware resolver to channel its
   queries through a legacy recursive name server).  The rest of this
   section describes how a DELEG-aware name server processes DELEG
   queries in order to avoid this problem.

   The need for special processing by a DELEG-aware name server only
   arises when all the following conditions are met:

   *  The name server has received a query for the DELEG RRset at a zone

   *  The name server is authoritative for the child zone.

   *  The name server is not authoritative for the parent zone.

   *  The name server does not offer recursion.

   In all other cases, the name server either has some way of obtaining
   the DELEG RRset or could not have been expected to have the DELEG
   RRset, so the name server can return either the DELEG RRset or an
   error response according to the normal processing rules.

   If all the above conditions are met, however, the name server is
   authoritative for the domain name being searching for, but cannot
   supply the requested RRset.  In this case, the name server MUST
   return an authoritative "no data" response showing that the DELEG
   RRset does not exist in the child zone's apex.

3.2.3.  Priority of DELEG over NS and Glue Address records

   DELEG-aware resolvers SHOULD prioritize the information in DELEG
   records over NS and glue address records.

4.  Privacy Considerations

   All of the information handled or transmitted by this protocol is
   public information published in the DNS.

5.  Security Considerations

   TODO: Fill this section out

5.1.  Availability of Zones Without NS

5.2.  Resolution Procedure

   An example of a simplified DNS interaction after priming.  This is a
   query for type AAAA with DELEG-aware com and authoritative servers.

   *  Ask qtype AAAA to the answer
      is: Answer section: (empty) Authority section: com.  172800 IN NS  Additional section:
      172800 IN AAAA 2001:db8:a83e::2:30

   *  Ask qtype AAAA to the answer
      is: Answer section: (empty) Authority section:
      172800 IN NS  172800 IN DELEG 1  ( ipv6hint=2001:db8:440:1:1f::24 )
      Additional section: 172800 IN AAAA

   *  Ask qtype AAAA to
      (2001:db8:1:1f::24) the answer is: Answer section:  3600 IN AAAA 2001:db8:a0:322c::2 Authority
      section: (empty) Additional section: (empty)

   TODO: more resolution examples (e.g out of bailiwick)

5.2.1.  Failures when DELEG Delegation is Present

   When a delegation using DELEG to a child is present, the resolver
   MUST use it and SERVFAIL if none of the configurations provided work.

6.  IANA Considerations

   DELEG will use the SVCB IANA registry definitions in section 14.3 of

   The IANA has assigned a bit in the DNSKEY flags field (see Section 7
   of [RFC4034] for the DELEG bit (N).

7.  References

7.1.  Normative References

   [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,

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <>.

   [STD13]    Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

              Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.


7.2.  Informative References

   [BCP219]   Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, January 2019.


              April, T., "Parameterized Nameserver Delegation with NS2
              and NS2T", Work in Progress, Internet-Draft, draft-tapril-
              ns2-01, 13 July 2020,

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

Appendix A.  Legacy Test Results

   In December 2023, Roy Arends and Shumon Huque tested two distinct
   sets of requirements that would enable the approach taken in this

   *  legacy resolvers ignore unknown record types in the authority
      section of referrals.

   *  legacy resolvers ignore an unknown key flag in a DNSKEY.

   Various recent implmentations were tested (BIND, Akamai Cacheserve,
   Unbound, PowerDNS Recursor and Knot) in addition to various public
   resolver services (Cloudflare, Google, Packet Clearing House).  All
   possible variations of delegations were tested, and there were no
   issues.  Further details about the specific testing methodology,
   please see test-plan.

Appendix B.  Acknowledgments {:unnumbered}

   This document is heavily based on past work done by Tim April in
   [I-D.tapril-ns2] and thus extends the thanks to the people helping on
   this which are: John Levine, Erik Nygren, Jon Reed, Ben Kaduk,
   Mashooq Muhaimen, Jason Moreau, Jerrod Wiesman, Billy Tiemann, Gordon
   Marx and Brian Wellington.

Appendix C.  TODO


   *  Write a security considerations section

   *  worked out resolution example including alias form delegation

Appendix D.  Change Log




   Christian Elmerot

   Edward Lewis

   Roy Arends

   Shumon Huque

   Klaus Darilion

   Libor Peltan

   Vladimír Čunát

   Shane Kerr

   David Blacka

   George Michaelson

   Ben Schwartz

   Jan Včelák

   Peter van Dijk

   Philip Homburg
   NLnet Labs

   Erik Nygren
   Akamai Technologies

   Vandan Adhvaryu
   Team Internet

   Manu Bretelle

Authors' Addresses

   Tim April

   Petr Špaček

   Ralf Weber
   Akamai Technologies

   David C Lawrence