



Internet Engineering Task Force (IETF)                        D. Wessels
Request for Comments: 8976                                     P. Barber
Category: Standards Track                                       Verisign
ISSN: 2070-1721                                              M. Weinberg
                                                                  Amazon
                                                               W. Kumari
                                                                  Google
                                                             W. Hardaker
                                                                 USC/ISI
                                                           February 2021


                      Message Digest for DNS Zones

Abstract

   This document describes a protocol and new DNS Resource Record that
   provides a cryptographic message digest over DNS zone data at rest.
   The ZONEMD Resource Record conveys the digest data in the zone
   itself.  When used in combination with DNSSEC, ZONEMD allows
   recipients to verify the zone contents for data integrity and origin
   authenticity.  This provides assurance that received zone data
   matches published data, regardless of how the zone data has been
   transmitted and received.  When used without DNSSEC, ZONEMD functions
   as a checksum, guarding only against unintentional changes.

   ZONEMD does not replace DNSSEC: DNSSEC protects individual RRsets
   (DNS data with fine granularity), whereas ZONEMD protects a zone's
   data as a whole, whether consumed by authoritative name servers,
   recursive name servers, or any other applications.

   As specified herein, ZONEMD is impractical for large, dynamic zones
   due to the time and resources required for digest calculation.
   However, the ZONEMD record is extensible so that new digest schemes
   may be added in the future to support large, dynamic zones.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8976.

Copyright Notice

   Copyright (c) 2021 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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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
     1.1.  Motivation
     1.2.  Alternative Approaches
     1.3.  Design Overview
     1.4.  Use Cases
       1.4.1.  Root Zone
       1.4.2.  Providers, Secondaries, and Anycast
       1.4.3.  Response Policy Zones
       1.4.4.  Centralized Zone Data Service
       1.4.5.  General Purpose Comparison Check
     1.5.  Terminology
   2.  The ZONEMD Resource Record
     2.1.  Non-apex ZONEMD Records
     2.2.  ZONEMD RDATA Wire Format
       2.2.1.  The Serial Field
       2.2.2.  The Scheme Field
       2.2.3.  The Hash Algorithm Field
       2.2.4.  The Digest Field
     2.3.  ZONEMD Presentation Format
     2.4.  ZONEMD Example
     2.5.  Including ZONEMD RRs in a Zone
   3.  Calculating the Digest
     3.1.  Add ZONEMD Placeholder
     3.2.  Optionally, Sign the Zone
     3.3.  Scheme-Specific Processing
       3.3.1.  The SIMPLE Scheme
         3.3.1.1.  SIMPLE Scheme Inclusion/Exclusion Rules
         3.3.1.2.  SIMPLE Scheme Digest Calculation
     3.4.  Update ZONEMD RR
   4.  Verifying Zone Digest
   5.  IANA Considerations
     5.1.  ZONEMD RRtype
     5.2.  ZONEMD Scheme
     5.3.  ZONEMD Hash Algorithms
   6.  Security Considerations
     6.1.  Using Zone Digest without DNSSEC
     6.2.  Attacks against the Zone Digest
     6.3.  Use of Multiple ZONEMD Hash Algorithms
     6.4.  DNSSEC Timing Considerations
     6.5.  Attacks Utilizing ZONEMD Queries
     6.6.  Resilience and Fragility
   7.  Performance Considerations
     7.1.  SIMPLE SHA384
   8.  Privacy Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Example Zones with Digests
     A.1.  Simple EXAMPLE Zone
     A.2.  Complex EXAMPLE Zone
     A.3.  EXAMPLE Zone with Multiple Digests
     A.4.  The URI.ARPA Zone
     A.5.  The ROOT-SERVERS.NET Zone
   Appendix B.  Implementation Status
     B.1.  Authors' Implementation
     B.2.  Shane Kerr's Implementation
     B.3.  NIC Chile Lab's Implementation
   Acknowledgments
   Authors' Addresses

1.  Introduction

   In the DNS, a zone is the collection of authoritative resource
   records (RRs) sharing a common origin ([RFC8499]).  Zones are often
   stored as files in the so-called "master file format" ([RFC1034]).
   Zones are generally distributed among name servers using the zone
   transfer (AXFR) ([RFC5936]) and incremental zone transfer (IXFR)
   ([RFC1995]) protocols.  They can also be distributed outside of the
   DNS with any file transfer protocol such as FTP, HTTP, and rsync, or
   even as email attachments.  Currently, there is no standard way to
   compute a hash or message digest for a stand-alone zone.

   This document specifies an RR type that provides a cryptographic
   message digest of the data in a zone.  It allows a receiver of the
   zone to verify the zone's integrity and authenticity when used in
   combination with DNSSEC.  The digest RR is a part of the zone itself,
   allowing verification of the zone, no matter how it is transmitted.
   The digest uses the wire format of zone data in a canonical ordering.
   Thus, it is independent of presentation format such as whitespace,
   capitalization, and comments.

   This specification is OPTIONAL to implement by both publishers and
   consumers of zone data.

1.1.  Motivation

   The primary motivation for this protocol enhancement is the desire to
   verify the data integrity and origin authenticity of a stand-alone
   zone, regardless of how it is transmitted.  A consumer of zone data
   should be able to verify that it is as published by the zone
   operator.

   Note, however, that integrity and authenticity can only be assured
   when the zone is signed.  DNSSEC provides three strong security
   guarantees relevant to this protocol:

   1.  whether or not to expect DNSSEC records in the zone,

   2.  whether or not to expect a ZONEMD record in a signed zone, and

   3.  whether or not the ZONEMD record has been altered since it was
       signed.

   A secondary motivation is to provide the equivalent of a checksum,
   allowing a zone recipient to check for unintended changes and
   operational errors such as accidental truncation.

1.2.  Alternative Approaches

   One approach to preventing data tampering and corruption is to secure
   the distribution channel.  The DNS has a number of features that are
   already used for channel security.  Perhaps the most widely used is
   DNS transaction signatures (TSIGs) ([RFC8945]).  A TSIG uses shared
   secret keys and a message digest to protect individual query and
   response messages.  It is generally used to authenticate and validate
   UPDATE ([RFC2136]), AXFR ([RFC5936]), and IXFR ([RFC1995]) messages.

   DNS Request and Transaction Signatures (SIG(0)) ([RFC2931]) is
   another protocol extension that authenticates individual DNS
   transactions.  Whereas SIG records normally cover specific RR types,
   SIG(0) is used to sign an entire DNS message.  Unlike TSIG, SIG(0)
   uses public key cryptography rather than shared secrets.

   The Transport Layer Security protocol suite also provides channel
   security.  The DPRIVE Working Group is in the process of specifying
   DNS Zone Transfer-over-TLS ([DPRIVE-XFR-OVER-TLS]).  One can also
   easily imagine the distribution of zones over HTTPS-enabled web
   servers as well as DNS-over-HTTPS ([RFC8484]).

   Unfortunately, the protections provided by these channel security
   techniques are (in practice) ephemeral and are not retained after the
   data transfer is complete.  They ensure that the client receives the
   data from the expected server and that the data sent by the server is
   not modified during transmission.  However, they do not guarantee
   that the server transmits the data as originally published and do not
   provide any methods to verify data that is read after transmission is
   complete.  For example, a name server loading saved zone data upon
   restart cannot guarantee that the on-disk data has not been modified.
   Such modification could be the result of an accidental corruption of
   the file or perhaps an incomplete saving of the file
   ([DISK-FULL-FAILURE]).  For these reasons, it is preferable to
   protect the integrity of the data itself.

   Why not simply rely on DNSSEC, which provides certain data security
   guarantees?  For zones that are signed, a recipient could validate
   all of the signed RRsets.  Additionally, denial-of-existence records
   prove that RRsets have not been added or removed.  However,
   delegations (non-apex NS records) are not signed by DNSSEC and
   neither are any glue records.  ZONEMD protects the integrity of
   delegation, glue, and other records that are not otherwise covered by
   DNSSEC.  Furthermore, zones that employ NSEC3 with Opt-Out
   ([RFC5155]) are susceptible to the removal or addition of names
   between the signed nodes.  Whereas DNSSEC primarily protects
   consumers of DNS response messages, this protocol protects consumers
   of zones.

   There are existing tools and protocols that provide data security,
   such as OpenPGP ([RFC4880]) and S/MIME ([RFC8551]).  In fact, the
   internic.net site publishes Pretty Good Privacy (PGP) signatures
   alongside the root zone and other files available there.  However,
   this is a detached signature with no strong association to the
   corresponding zone file other than its timestamp.  Attached
   signatures are of course possible, but these necessarily change the
   format of the file being distributed; a zone signed with OpenPGP or
   S/MIME no longer looks like a DNS zone and could not directly be
   loaded into a name server.  Once loaded, the signature data is lost,
   so it cannot be further propagated.

   It seems the desire for data security in DNS zones was envisioned as
   far back as 1997.  [RFC2065] is an obsoleted specification of the
   first generation DNSSEC Security Extensions.  It describes a zone
   transfer signature, identified as the AXFR SIG, which is similar to
   the technique proposed by this document.  That is, it proposes
   ordering all (signed) RRsets in a zone, hashing their contents, and
   then signing the zone hash.  The AXFR SIG is described only for use
   during zone transfers.  It did not postulate the need to validate
   zone data distributed outside of the DNS.  Furthermore, its
   successor, [RFC2535], omits the AXFR SIG while at the same time
   introducing an IXFR SIG.  (Note: RFC 2535 was obsoleted by [RFC4033],
   [RFC4034], and [RFC4035].)

1.3.  Design Overview

   This document specifies a new Resource Record type to convey a
   message digest of the content of a zone.  The digest is calculated at
   the time of zone publication.  If the zone is signed with DNSSEC, any
   modifications of the digest can be detected.  The procedures for
   digest calculation and DNSSEC signing are similar.  Both require data
   to be processed in a well-defined order and format.  It may be
   possible to perform DNSSEC signing and digest calculation in
   parallel.

   The zone digest is designed to be used on zones that have infrequent
   updates.  As specified herein, the digest is recalculated over the
   entire zone content each time the zone is updated.  This
   specification does not provide an efficient mechanism for updating
   the digest on incremental updates of zone data.  It is, however,
   extensible so that future schemes may be defined to support efficient
   incremental digest updates.

   It is expected that verification of a zone digest will be implemented
   in name server software.  That is, a name server can verify the zone
   data it was given and refuse to serve a zone that fails verification.
   For signed zones, the name server needs a trust anchor to perform
   DNSSEC validation.  For signed non-root zones, the name server may
   need to send queries to validate a chain of trust.  Digest
   verification could also be performed externally.

1.4.  Use Cases

1.4.1.  Root Zone

   The root zone ([InterNIC]) is one of the most widely distributed DNS
   zones on the Internet, served by more than 1000 separate instances
   ([ROOT-SERVERS]) at the time of this writing.  Additionally, many
   organizations configure their own name servers to serve the root zone
   locally.  Reasons for doing so include privacy and reduced access
   time.  [RFC8806] describes one way to do this.  As the root zone
   spreads beyond its traditional deployment boundaries, the
   verification of the completeness of the zone contents becomes more
   important.

1.4.2.  Providers, Secondaries, and Anycast

   Since its very early days, the developers of the DNS recognized the
   importance of secondary name servers and service diversity.  However,
   modern DNS service has complex provisioning that includes multiple
   third-party providers ([RFC8901]) and hundreds of anycast instances
   ([RFC3258]).  Instead of a simple primary-to-secondary zone
   distribution system, today it is possible to have multiple levels,
   multiple parties, and multiple protocols involved in the distribution
   of zone data.  This complexity introduces new places for problems to
   arise.  The zone digest protects the integrity of data that flows
   through such systems.

1.4.3.  Response Policy Zones

   A Response Policy Zone (RPZ) is "a mechanism to introduce a
   customized policy in Domain Name System servers, so that recursive
   resolvers return possibly modified results" ([RPZ]).  The policy
   information is carried inside specially constructed DNS zones.  A
   number of companies provide RPZ feeds, which are consumed by name
   server and firewall products.  While RPZs can be signed with DNSSEC,
   the data is not queried directly and would not be subject to DNSSEC
   validation.

1.4.4.  Centralized Zone Data Service

   ICANN operates the Centralized Zone Data Service ([CZDS]), which is a
   repository of top-level domain zone files.  Users that have been
   granted access are then able to download zone data.  Adding a zone
   digest to these would provide CZDS users with assurances that the
   data has not been modified between origination and retrieval.  Note
   that ZONEMD could be added to zone data supplied to CZDS without
   requiring it to be present in the zone data served by production name
   servers, since the digest is inherently attached to the specific copy
   of the zone.

1.4.5.  General Purpose Comparison Check

   Since the zone digest calculation does not depend on presentation
   format, it could be used to compare multiple copies of a zone
   received from different sources, or copies generated by different
   processes.  In this case, it serves as a checksum and can be useful
   even for unsigned zones.

1.5.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

   The terms Private Use, Reserved, Unassigned, and Specification
   Required are to be interpreted as defined in [RFC8126].

2.  The ZONEMD Resource Record

   This section describes the ZONEMD Resource Record, including its
   fields, wire format, and presentation format.  The Type value for the
   ZONEMD RR is 63.  The ZONEMD RR is class independent.  The RDATA of
   the resource record consists of four fields: Serial, Scheme, Hash
   Algorithm, and Digest.

2.1.  Non-apex ZONEMD Records

   This document specifies ZONEMD RRs located at the zone apex.  Non-
   apex ZONEMD RRs are not forbidden, but have no meaning in this
   specification.  Non-apex ZONEMD RRs MUST NOT be used for
   verification.

   During digest calculation, non-apex ZONEMD RRs are treated as
   ordinary RRs.  They are digested as is, and the RR is not replaced by
   a placeholder RR.

   Unless explicitly stated otherwise, "ZONEMD" always refers to apex
   records throughout this document.

2.2.  ZONEMD RDATA Wire Format

   The ZONEMD RDATA wire format is encoded as follows:

                        1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Serial                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Scheme     |Hash Algorithm |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                             Digest                            |
   /                                                               /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2.1.  The Serial Field

   The Serial field is a 32-bit unsigned integer in network byte order.
   It is the serial number from the zone's SOA record ([RFC1035],
   Section 3.3.13) for which the zone digest was generated.

   It is included here to clearly bind the ZONEMD RR to a particular
   version of the zone's content.  Without the serial number, a stand-
   alone ZONEMD digest has no obvious association to any particular
   instance of a zone.

2.2.2.  The Scheme Field

   The Scheme field is an 8-bit unsigned integer that identifies the
   methods by which data is collated and presented as input to the
   hashing function.

   Herein, SIMPLE, with Scheme value 1, is the only standardized Scheme
   defined for ZONEMD records and it MUST be supported by
   implementations.  The "ZONEMD Schemes" registry is further described
   in Section 5.

   Scheme values 240-254 are allocated for Private Use.

2.2.3.  The Hash Algorithm Field

   The Hash Algorithm field is an 8-bit unsigned integer that identifies
   the cryptographic hash algorithm used to construct the digest.

   Herein, SHA384 ([RFC6234]), with Hash Algorithm value 1, is the only
   standardized Hash Algorithm defined for ZONEMD records that MUST be
   supported by implementations.  When SHA384 is used, the size of the
   Digest field is 48 octets.  The result of the SHA384 digest algorithm
   MUST NOT be truncated, and the entire 48-octet digest is published in
   the ZONEMD record.

   SHA512 ([RFC6234]), with Hash Algorithm value 2, is also defined for
   ZONEMD records and SHOULD be supported by implementations.  When
   SHA512 is used, the size of the Digest field is 64 octets.  The
   result of the SHA512 digest algorithm MUST NOT be truncated, and the
   entire 64-octet digest is published in the ZONEMD record.

   Hash Algorithm values 240-254 are allocated for Private Use.

   The "ZONEMD Hash Algorithms" registry is further described in
   Section 5.

2.2.4.  The Digest Field

   The Digest field is a variable-length sequence of octets containing
   the output of the hash algorithm.  The length of the Digest field is
   determined by deducting the fixed size of the Serial, Scheme, and
   Hash Algorithm fields from the RDATA size in the ZONEMD RR header.

   The Digest field MUST NOT be shorter than 12 octets.  Digests for the
   SHA384 and SHA512 hash algorithms specified herein are never
   truncated.  Digests for future hash algorithms MAY be truncated but
   MUST NOT be truncated to a length that results in less than 96 bits
   (12 octets) of equivalent strength.

   Section 3 describes how to calculate the digest for a zone.
   Section 4 describes how to use the digest to verify the contents of a
   zone.

2.3.  ZONEMD Presentation Format

   The presentation format of the RDATA portion is as follows:

   *  The Serial field is represented as an unsigned decimal integer.

   *  The Scheme field is represented as an unsigned decimal integer.

   *  The Hash Algorithm field is represented as an unsigned decimal
      integer.

   *  The Digest is represented as a sequence of case-insensitive
      hexadecimal digits.  Whitespace is allowed within the hexadecimal
      text.

2.4.  ZONEMD Example

   The following example shows a ZONEMD RR in presentation format:

   example.com. 86400 IN ZONEMD 2018031500 1 1 (
       FEBE3D4CE2EC2FFA4BA99D46CD69D6D29711E55217057BEE
       7EB1A7B641A47BA7FED2DD5B97AE499FAFA4F22C6BD647DE )

2.5.  Including ZONEMD RRs in a Zone

   The zone operator chooses an appropriate hash algorithm and scheme
   and includes the calculated zone digest in the apex ZONEMD RRset.
   The zone operator MAY choose any of the defined hash algorithms and
   schemes, including the Private Use code points.

   The ZONEMD RRset MAY contain multiple records to support algorithm
   agility ([BCP201]).  When multiple ZONEMD RRs are present, each MUST
   specify a unique Scheme and Hash Algorithm tuple.  It is RECOMMENDED
   that a zone include only one ZONEMD RR, unless the zone operator is
   in the process of transitioning to a new scheme or hash algorithm.

3.  Calculating the Digest

   The algorithm described in this section is designed for the common
   case of offline DNSSEC signing.  Slight deviations may be permitted
   or necessary in other situations, such as with unsigned zones or
   online DNSSEC signing.  Implementations that deviate from the
   described algorithm are advised to ensure that it produces ZONEMD
   RRs, signatures, and denial-of-existence records that are identical
   to the ones generated by this procedure.

3.1.  Add ZONEMD Placeholder

   In preparation for calculating the zone digest(s), any existing
   ZONEMD records (and covering RRSIGs) at the zone apex are first
   deleted.

   Prior to calculation of the digest, and prior to signing with DNSSEC,
   one or more placeholder ZONEMD records are added to the zone apex.
   This ensures that denial-of-existence (NSEC, NSEC3) records are
   created correctly if the zone is signed with DNSSEC.  If placeholders
   were not added prior to signing, the later addition of ZONEMD records
   would also require updating the Type Bit Maps field of any apex NSEC/
   NSEC3 RRs, which then invalidates the calculated digest value.

   When multiple ZONEMD RRs are published in the zone, e.g., during an
   algorithm rollover, each MUST specify a unique Scheme and Hash
   Algorithm tuple.

   It is RECOMMENDED that the TTL of the ZONEMD record match the TTL of
   the Start of Authority (SOA).  However, the TTL of the ZONEMD record
   may be safely ignored during verification in all cases.

   In the placeholder record, the Serial field is set to the current SOA
   Serial.  The Scheme field is set to the value for the chosen
   collation scheme.  The Hash Algorithm field is set to the value for
   the chosen hash algorithm.  Since apex ZONEMD records are excluded
   from digest calculation, the value of the Digest field does not
   matter at this point in the process.

3.2.  Optionally, Sign the Zone

   Following the addition of placeholder records, the zone may be signed
   with DNSSEC.  When the digest calculation is complete, and the ZONEMD
   record is updated, the signature(s) for the ZONEMD RRset MUST be
   recalculated and updated as well.  Therefore, the signer is not
   required to calculate a signature over the placeholder record at this
   step in the process, but it is harmless to do so.

3.3.  Scheme-Specific Processing

   Herein, only the SIMPLE collation scheme is defined.  Additional
   schemes may be defined in future updates to this document.

3.3.1.  The SIMPLE Scheme

   For the SIMPLE scheme, the digest is calculated over the zone as a
   whole.  This means that a change to a single RR in the zone requires
   iterating over all RRs in the zone to recalculate the digest.  SIMPLE
   is a good choice for zones that are small and/or stable, but it is
   probably not good for zones that are large and/or dynamic.

   Calculation of a zone digest requires RRs to be processed in a
   consistent format and ordering.  This specification uses DNSSEC's
   canonical on-the-wire RR format (without name compression) and
   ordering as specified in Sections 6.1, 6.2, and 6.3 of [RFC4034] with
   the additional provision that RRsets having the same owner name MUST
   be numerically ordered, in ascending order, by their numeric RR TYPE.

3.3.1.1.  SIMPLE Scheme Inclusion/Exclusion Rules

   When iterating over records in the zone, the following inclusion/
   exclusion rules apply:

   *  All records in the zone, including glue records, MUST be included
      unless excluded by a subsequent rule.

   *  Occluded data ([RFC5936], Section 3.5) MUST be included.

   *  If there are duplicate RRs with equal owner, class, type, and
      RDATA, only one instance is included ([RFC4034], Section 6.3) and
      the duplicates MUST be omitted.

   *  The placeholder apex ZONEMD RR(s) MUST NOT be included.

   *  If the zone is signed, DNSSEC RRs MUST be included, except:

   *  The RRSIG covering the apex ZONEMD RRset MUST NOT be included
      because the RRSIG will be updated after all digests have been
      calculated.

3.3.1.2.  SIMPLE Scheme Digest Calculation

   A zone digest using the SIMPLE scheme is calculated by concatenating
   all RRs in the zone, in the format and order described in
   Section 3.3.1 subject to the inclusion/exclusion rules described in
   Section 3.3.1.1, and then applying the chosen hash algorithm:

   digest = hash( RR(1) | RR(2) | RR(3) | ... )

   where "|" denotes concatenation.

3.4.  Update ZONEMD RR

   The calculated zone digest is inserted into the placeholder ZONEMD
   RR.  Repeat for each digest if multiple digests are to be published.

   If the zone is signed with DNSSEC, the RRSIG record(s) covering the
   ZONEMD RRset MUST then be added or updated.  Because the ZONEMD
   placeholder was added prior to signing, the zone will already have
   the appropriate denial-of-existence (NSEC, NSEC3) records.

   Some DNSSEC implementations (especially "online signing") might
   update the SOA serial number whenever a new signature is made.  To
   preserve the calculated digest, generation of a ZONEMD signature MUST
   NOT also result in a change to the SOA serial number.  The ZONEMD RR
   and the matching SOA MUST be published at the same time.

4.  Verifying Zone Digest

   The recipient of a zone that has a ZONEMD RR verifies the zone by
   calculating the digest as follows:

      |  Note: If multiple ZONEMD RRs are present in the zone, e.g.,
      |  during an algorithm rollover, a match using any one of the
      |  recipient's supported Schemes and Hash Algorithms is sufficient
      |  to verify the zone.  The verifier MAY ignore a ZONEMD RR if its
      |  Scheme and Hash Algorithm violates local policy.

   1.  The verifier MUST first determine whether or not to expect DNSSEC
       records in the zone.  By examining locally configured trust
       anchors and, if necessary, querying for and validating Delegation
       Signer (DS) RRs in the parent zone, the verifier knows whether or
       not the zone to be verified should include DNSSEC keys and
       signatures.  For zones where signatures are not expected, or if
       DNSSEC validation is not performed, digest verification continues
       at step 4 below.

   2.  For zones where signatures are expected, the existence of the
       apex ZONEMD record MUST be validated.  If the DNSSEC data proves
       the ZONEMD RRset does not exist, digest verification cannot
       occur.  If the DNSSEC data proves the ZONEMD does exist, but is
       not found in the zone, digest verification MUST NOT be considered
       successful.

   3.  For zones where signatures are expected, the SOA and ZONEMD
       RRsets MUST have valid signatures, chaining up to a trust anchor.
       If DNSSEC validation of the SOA or ZONEMD RRsets fails, digest
       verification MUST NOT be considered successful.

   4.  When multiple ZONEMD RRs are present, each MUST specify a unique
       Scheme and Hash Algorithm tuple.  If the ZONEMD RRset contains
       more than one RR with the same Scheme and Hash Algorithm, digest
       verification for those ZONEMD RRs MUST NOT be considered
       successful.

   5.  Loop over all apex ZONEMD RRs and perform the following steps:

       a.  The SOA Serial field MUST exactly match the ZONEMD Serial
           field.  If the fields do not match, digest verification MUST
           NOT be considered successful with this ZONEMD RR.

       b.  The Scheme field MUST be checked.  If the verifier does not
           support the given scheme, verification MUST NOT be considered
           successful with this ZONEMD RR.

       c.  The Hash Algorithm field MUST be checked.  If the verifier
           does not support the given hash algorithm, verification MUST
           NOT be considered successful with this ZONEMD RR.

       d.  The Digest field size MUST be checked.  If the size of the
           given Digest field is smaller than 12 octets, or if the size
           is not equal to the size expected for the corresponding Hash
           Algorithm, verification MUST NOT be considered successful
           with this ZONEMD RR.

       e.  The zone digest is computed over the zone data as described
           in Section 3.3 using the Scheme and Hash Algorithm for the
           current ZONEMD RR.

       f.  The computed digest is compared to the received digest.  If
           the two digest values match, verification is considered
           successful.  Otherwise, verification MUST NOT be considered
           successful for this ZONEMD RR.

   Each time zone verification is performed, the verifier SHOULD report
   the status as either successful or unsuccessful.  When unsuccessful,
   the verifier SHOULD report the reason(s) that verification did not
   succeed.

5.  IANA Considerations

5.1.  ZONEMD RRtype

   This document defines a new DNS RR type, ZONEMD, whose value 63 has
   been allocated by IANA from the "Resource Record (RR) TYPEs"
   subregistry of the "Domain Name System (DNS) Parameters" registry:

   Type:  ZONEMD
   Value:  63
   Meaning:  Message Digest Over Zone Data
   Reference:  [RFC8976]

5.2.  ZONEMD Scheme

   IANA has created a new subregistry in the "Domain Name System (DNS)
   Parameters" registry as follows:

   Registry Name:  ZONEMD Schemes
   Registration Procedure:  Specification Required
   Reference:  [RFC8976]

       +=========+=========================+==========+===========+
       | Value   | Description             | Mnemonic | Reference |
       +=========+=========================+==========+===========+
       | 0       | Reserved                |          | [RFC8976] |
       +---------+-------------------------+----------+-----------+
       | 1       | Simple ZONEMD collation | SIMPLE   | [RFC8976] |
       +---------+-------------------------+----------+-----------+
       | 2-239   | Unassigned              |          |           |
       +---------+-------------------------+----------+-----------+
       | 240-254 | Private Use             | N/A      | [RFC8976] |
       +---------+-------------------------+----------+-----------+
       | 255     | Reserved                |          | [RFC8976] |
       +---------+-------------------------+----------+-----------+

                     Table 1: ZONEMD Scheme Registry

5.3.  ZONEMD Hash Algorithms

   IANA has created a new subregistry in the "Domain Name System (DNS)
   Parameters" registry as follows:

   Registry Name:  ZONEMD Hash Algorithms
   Registration Procedure:  Specification Required
   Reference:  [RFC8976]

             +=========+=============+==========+===========+
             | Value   | Description | Mnemonic | Reference |
             +=========+=============+==========+===========+
             | 0       | Reserved    |          | [RFC8976] |
             +---------+-------------+----------+-----------+
             | 1       | SHA-384     | SHA384   | [RFC8976] |
             +---------+-------------+----------+-----------+
             | 2       | SHA-512     | SHA512   | [RFC8976] |
             +---------+-------------+----------+-----------+
             | 3-239   | Unassigned  |          |           |
             +---------+-------------+----------+-----------+
             | 240-254 | Private Use | N/A      | [RFC8976] |
             +---------+-------------+----------+-----------+
             | 255     | Reserved    |          | [RFC8976] |
             +---------+-------------+----------+-----------+

                 Table 2: ZONEMD Hash Algorithms Registry

6.  Security Considerations

6.1.  Using Zone Digest without DNSSEC

   Users of ZONEMD with unsigned zones are advised that it provides no
   real protection against attacks.  While zone digests can be used in
   the absence of DNSSEC, this only provides protection against
   accidental zone corruption such as transmission errors and
   truncation.  When used in this manner, it effectively serves only as
   a checksum.  For zones not signed with DNSSEC, an attacker can make
   any zone modifications appear to be valid by recomputing the Digest
   field of a ZONEMD RR.

6.2.  Attacks against the Zone Digest

   An attacker, whose goal is to modify zone content before it is used
   by the victim, may consider a number of different approaches.

   The attacker might perform a downgrade attack to an unsigned zone.
   This is why Section 4 talks about determining whether or not to
   expect DNSSEC signatures for the zone in step 1.

   The attacker might perform a downgrade attack by removing one or more
   ZONEMD records.  Such a removal is detectable only with DNSSEC
   validation and is why Section 4 talks about checking denial-of-
   existence proofs in step 2 and signature validation in step 3.

   The attacker might alter the Scheme, Hash Algorithm, or Digest fields
   of the ZONEMD record.  Such modifications are detectable only with
   DNSSEC validation.

   As stated in [BCP201], cryptographic algorithms age and become weaker
   as cryptanalysis techniques and computing resources improve with
   time.  Implementors and publishers of zone digests should anticipate
   the need for algorithm agility on long timescales.

6.3.  Use of Multiple ZONEMD Hash Algorithms

   When a zone publishes multiple ZONEMD RRs, the overall security is
   only as good as the weakest hash algorithm in use.  For this reason,
   Section 2 recommends only publishing multiple ZONEMD RRs when
   transitioning to a new scheme or hash algorithm.  Once the transition
   is complete, the old scheme or hash algorithm should be removed from
   the ZONEMD RRset.

6.4.  DNSSEC Timing Considerations

   As with all DNSSEC signatures, the ability to perform signature
   validation of a ZONEMD record is limited in time.  If the DS
   record(s) or trust anchors for the zone to be verified are no longer
   available, the recipient cannot validate the ZONEMD RRset.  This
   could happen even if the ZONEMD signature is still current (not
   expired), since the zone's DS record(s) may have been withdrawn
   following a Key Signing Key (KSK) rollover.

   For zones where it may be important to validate a ZONEMD RRset
   through its entire signature validity period, the zone operator
   should ensure that KSK rollover timing takes this into consideration.

6.5.  Attacks Utilizing ZONEMD Queries

   Nothing in this specification prevents clients from making, and
   servers from responding to, ZONEMD queries.  Servers SHOULD NOT
   calculate zone digests dynamically (for each query) as this can be
   used as a CPU resource exhaustion attack.

   ZONEMD responses could be used in a distributed denial-of-service
   amplification attack.  The ZONEMD RR is moderately sized, much like
   the DS RR.  A single ZONEMD RR contributes approximately 65 to 95
   octets to a DNS response for digest types defined herein.  Other RR
   types, such as DNS Public Key (DNSKEY), can result in larger
   amplification effects.

6.6.  Resilience and Fragility

   ZONEMD is used to detect incomplete or corrupted zone data prior to
   its use, thereby increasing resilience by not using corrupt data, but
   also introduces some denial-of-service fragility by making good data
   in a zone unavailable if some other data is missing or corrupt.
   Publishers and consumers of zones containing ZONEMD records should be
   aware of these trade-offs.  While the intention is to secure the zone
   data, misconfigurations or implementation bugs are generally
   indistinguishable from intentional tampering and could lead to
   service failures when verification is performed automatically.

   Zone publishers may want to deploy ZONEMD gradually perhaps by
   utilizing one of the Private Use hash algorithm code points listed in
   Section 5.3.  Similarly, recipients may want to initially configure
   verification failures only as a warning, and later as an error after
   gaining experience and confidence with the feature.

7.  Performance Considerations

   This section is provided to make zone publishers aware of the
   performance requirements and implications of including ZONEMD RRs in
   a zone.

7.1.  SIMPLE SHA384

   As mentioned previously, the SIMPLE scheme may be impractical for use
   in zones that are either large or highly dynamic.  Zone publishers
   should carefully consider the use of ZONEMD in such zones since it
   might cause consumers of zone data (e.g., secondary name servers) to
   expend resources on digest calculation.  For such use cases, it is
   recommended that ZONEMD only be used when digest calculation time is
   significantly less than propagation times and update intervals.

   The authors' implementation (Appendix B.1) includes an option to
   record and report CPU usage of its operation.  The software was used
   to generate digests for more than 800 Top-Level Domain (TLD) zones
   available from [CZDS].  The table below summarizes the results for
   the SIMPLE scheme and SHA384 hash algorithm grouped by zone size.
   The Rate column is the mean amount of time per RR to calculate the
   digest, running on commodity hardware in early 2020.

                 +=====================+================+
                 |     Zone Size (RRs) | Rate (msec/RR) |
                 +=====================+================+
                 |             10 - 99 |        0.00683 |
                 +---------------------+----------------+
                 |           100 - 999 |        0.00551 |
                 +---------------------+----------------+
                 |         1000 - 9999 |        0.00505 |
                 +---------------------+----------------+
                 |       10000 - 99999 |        0.00602 |
                 +---------------------+----------------+
                 |     100000 - 999999 |        0.00845 |
                 +---------------------+----------------+
                 |   1000000 - 9999999 |         0.0108 |
                 +---------------------+----------------+
                 | 10000000 - 99999999 |         0.0148 |
                 +---------------------+----------------+

                                 Table 3

   For example, based on the above table, it takes approximately 0.13
   seconds to calculate a SIMPLE SHA384 digest for a zone with 22,000
   RRs, and about 2.5 seconds for a zone with 300,000 RRs.

   These benchmarks attempt to emulate a worst-case scenario and take
   into account the time required to canonicalize the zone for
   processing.  Each of the 800+ zones were measured three times and
   then averaged, with a different random sorting of the input data
   prior to each measurement.

8.  Privacy Considerations

   This specification has no impact on user privacy.

9.  References

9.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [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>.

9.2.  Informative References

   [BCP201]   Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, November 2015.

              <https://www.rfc-editor.org/info/bcp201>

   [CZDS]     Internet Corporation for Assigned Names and Numbers
              (ICANN), "Centralized Zone Data Service", October 2018,
              <https://czds.icann.org/>.

   [DISK-FULL-FAILURE]
              DENIC, "Background of the Partial Failure of the Name
              Service for .de Domains", May 2010,
              <https://web.archive.org/web/20100618032705/
              https://www.denic.de/en/denic-in-dialogue/news/2733.html>.

   [DNS-TOOLS]
              "DNS tools for zone signature (file, pkcs11-hsm) and
              validation, and zone digest (ZONEMD)", commit 489de21,
              December 2020, <https://github.com/niclabs/dns-tools>.

   [DPRIVE-XFR-OVER-TLS]
              Toorop, W., Dickinson, S., Sahib, S., Aras, P., and A.
              Mankin, "DNS Zone Transfer-over-TLS", Work in Progress,
              Internet-Draft, draft-ietf-dprive-xfr-over-tls-05, 20
              January 2021, <https://tools.ietf.org/html/draft-ietf-
              dprive-xfr-over-tls-05>.

   [InterNIC] InterNIC, "Index of ftp://rs.internic.net/", May 2018,
              <ftp://ftp.internic.net/domain/>.

   [LDNS-ZONE-DIGEST]
              "Implementation of Message Digests for DNS Zones using the
              ldns library", commit 71c0cd1, January 2021,
              <https://github.com/verisign/ldns-zone-digest>.

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

   [RFC2065]  Eastlake 3rd, D. and C. Kaufman, "Domain Name System
              Security Extensions", RFC 2065, DOI 10.17487/RFC2065,
              January 1997, <https://www.rfc-editor.org/info/rfc2065>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC2535]  Eastlake 3rd, D., "Domain Name System Security
              Extensions", RFC 2535, DOI 10.17487/RFC2535, March 1999,
              <https://www.rfc-editor.org/info/rfc2535>.

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <https://www.rfc-editor.org/info/rfc2931>.

   [RFC3258]  Hardie, T., "Distributing Authoritative Name Servers via
              Shared Unicast Addresses", RFC 3258, DOI 10.17487/RFC3258,
              April 2002, <https://www.rfc-editor.org/info/rfc3258>.

   [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,
              <https://www.rfc-editor.org/info/rfc4033>.

   [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,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,
              <https://www.rfc-editor.org/info/rfc4880>.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
              <https://www.rfc-editor.org/info/rfc5155>.

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", RFC 8551, DOI 10.17487/RFC8551,
              April 2019, <https://www.rfc-editor.org/info/rfc8551>.

   [RFC8806]  Kumari, W. and P. Hoffman, "Running a Root Server Local to
              a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,
              <https://www.rfc-editor.org/info/rfc8806>.

   [RFC8901]  Huque, S., Aras, P., Dickinson, J., Vcelak, J., and D.
              Blacka, "Multi-Signer DNSSEC Models", RFC 8901,
              DOI 10.17487/RFC8901, September 2020,
              <https://www.rfc-editor.org/info/rfc8901>.

   [RFC8945]  Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D.,
              Gudmundsson, O., and B. Wellington, "Secret Key
              Transaction Authentication for DNS (TSIG)", STD 93,
              RFC 8945, DOI 10.17487/RFC8945, November 2020,
              <https://www.rfc-editor.org/info/rfc8945>.

   [ROOT-SERVERS]
              Root Server Operators, "root-servers.org", July 2018,
              <https://www.root-servers.org/>.

   [RPZ]      Wikipedia, "Response policy zone", May 2020,
              <https://en.wikipedia.org/w/
              index.php?title=Response_policy_zone&oldid=960043728>.

   [ZONE-DIGEST-HACKATHON]
              "Prototype implementation of ZONEMD for the IETF 102
              hackathon", commit 76ad7a7, August 2019,
              <https://github.com/shane-kerr/ZoneDigestHackathon>.

   [ZONE-DIGEST-TESTS]
              IETF, "RFC 8976 ZONEMD Test Cases", January 2021,
              <https://trac.ietf.org/trac/dnsop/wiki/
              RFC8976ZONEMDTestCases>.

Appendix A.  Example Zones with Digests

   This appendix contains example zones with accurate ZONEMD records.
   These can be used to verify an implementation of the zone digest
   protocol.  Additional and more extensive test cases can be found via
   the ZONEMD Tests Wiki ([ZONE-DIGEST-TESTS]) maintained by the IETF
   DNSOP Working Group.

A.1.  Simple EXAMPLE Zone

   Here, the EXAMPLE zone contains an SOA record, NS and glue records,
   and a ZONEMD record.

   example.      86400  IN  SOA     ns1 admin 2018031900 (
                                    1800 900 604800 86400 )
                 86400  IN  NS      ns1
                 86400  IN  NS      ns2
                 86400  IN  ZONEMD  2018031900 1 1 (
                                    c68090d90a7aed71
                                    6bc459f9340e3d7c
                                    1370d4d24b7e2fc3
                                    a1ddc0b9a87153b9
                                    a9713b3c9ae5cc27
                                    777f98b8e730044c )
   ns1           3600   IN  A       203.0.113.63
   ns2           3600   IN  AAAA    2001:db8::63

A.2.  Complex EXAMPLE Zone

   Here, the EXAMPLE zone contains duplicate RRs, an occluded RR,
   uppercase names, a wildcard, a multi-record RRset, a non-apex ZONEMD
   RR, and one out-of-zone RR.

   example.      86400  IN  SOA     ns1 admin 2018031900 (
                                    1800 900 604800 86400 )
                 86400  IN  NS      ns1
                 86400  IN  NS      ns2
                 86400  IN  ZONEMD  2018031900 1 1 (
                                    a3b69bad980a3504
                                    e1cffcb0fd6397f9
                                    3848071c93151f55
                                    2ae2f6b1711d4bd2
                                    d8b39808226d7b9d
                                    b71e34b72077f8fe )
   ns1           3600   IN  A       203.0.113.63
   NS2           3600   IN  AAAA    2001:db8::63
   occluded.sub  7200   IN  TXT     "I'm occluded but must be digested"
   sub           7200   IN  NS      ns1
   duplicate     300    IN  TXT     "I must be digested just once"
   duplicate     300    IN  TXT     "I must be digested just once"
   foo.test.     555    IN  TXT     "out-of-zone data must be excluded"
   UPPERCASE     3600   IN  TXT     "canonicalize uppercase owner names"
   *             777    IN  PTR     dont-forget-about-wildcards
   mail          3600   IN  MX      20 MAIL1
   mail          3600   IN  MX      10 Mail2.Example.
   sortme        3600   IN  AAAA    2001:db8::5:61
   sortme        3600   IN  AAAA    2001:db8::3:62
   sortme        3600   IN  AAAA    2001:db8::4:63
   sortme        3600   IN  AAAA    2001:db8::1:65
   sortme        3600   IN  AAAA    2001:db8::2:64
   non-apex      900    IN  ZONEMD  2018031900 1 1 (
                                    616c6c6f77656420
                                    6275742069676e6f
                                    7265642e20616c6c
                                    6f77656420627574
                                    2069676e6f726564
                                    2e20616c6c6f7765 )

A.3.  EXAMPLE Zone with Multiple Digests

   Here, the EXAMPLE zone contains multiple ZONEMD records.  It has both
   SHA384 and SHA512 digests using the SIMPLE scheme.  It also includes
   ZONEMD records with Scheme and Hash Algorithm values in the private
   range (240-254).  These additional private-range digests are not
   verifiable.

   example.      86400  IN  SOA     ns1 admin 2018031900 (
                                    1800 900 604800 86400 )
   example.      86400  IN  NS      ns1.example.
   example.      86400  IN  NS      ns2.example.
   example.      86400  IN  ZONEMD  2018031900 1 1 (
                                    62e6cf51b02e54b9
                                    b5f967d547ce4313
                                    6792901f9f88e637
                                    493daaf401c92c27
                                    9dd10f0edb1c56f8
                                    080211f8480ee306 )
   example.      86400  IN  ZONEMD  2018031900 1 2 (
                                    08cfa1115c7b948c
                                    4163a901270395ea
                                    226a930cd2cbcf2f
                                    a9a5e6eb85f37c8a
                                    4e114d884e66f176
                                    eab121cb02db7d65
                                    2e0cc4827e7a3204
                                    f166b47e5613fd27 )
   example.      86400  IN  ZONEMD  2018031900 1 240 (
                                    e2d523f654b9422a
                                    96c5a8f44607bbee )
   example.      86400  IN  ZONEMD  2018031900 241 1 (
                                    e1846540e33a9e41
                                    89792d18d5d131f6
                                    05fc283e )
   ns1.example.  3600   IN  A       203.0.113.63
   ns2.example.  86400  IN  TXT     "This example has multiple digests"
   NS2.EXAMPLE.  3600   IN  AAAA    2001:db8::63

A.4.  The URI.ARPA Zone

   The following sample zone is the URI.ARPA zone retrieved 2021-01-21.
   Note this sample zone has been re-signed with unpublished keys, so
   that the added ZONEMD RR also has a signature.

   uri.arpa.       3600    IN      SOA     sns.dns.icann.org. (
      noc.dns.icann.org. 2018100702 10800 3600 1209600 3600 )
   uri.arpa.       3600    IN      RRSIG   SOA 8 2 3600 (
       20210217232440 20210120232440 37444 uri.arpa.
       GzQw+QzwLDJr13REPGVmpEChjD1D2XlX0ie1DnWHpgaEw1E/dhs3lCN3+B
       mHd4Kx3tffTRgiyq65HxR6feQ5v7VmAifjyXUYB1DZur1eP5q0Ms2ygCB3
       byoeMgCNsFS1oKZ2LdzNBRpy3oace8xQn1SpmHGfyrsgg+WbHKCT1dY= )
   uri.arpa.       86400   IN      NS      a.iana-servers.net.
   uri.arpa.       86400   IN      NS      b.iana-servers.net.
   uri.arpa.       86400   IN      NS      c.iana-servers.net.
   uri.arpa.       86400   IN      NS      ns2.lacnic.net.
   uri.arpa.       86400   IN      NS      sec3.apnic.net.
   uri.arpa.       86400   IN      RRSIG   NS 8 2 86400 (
       20210217232440 20210120232440 37444 uri.arpa.
       M+Iei2lcewWGaMtkPlrhM9FpUAHXFkCHTVpeyrjxjEONeNgKtHZor5e4V4
       qJBOzNqo8go/qJpWlFBm+T5Hn3asaBZVstFIYky38/C8UeRLPKq1hTTHAR
       YUlFrexr5fMtSUAVOgOQPSBfH3xBq/BgSccTdRb9clD+HE7djpqrLS4= )
   uri.arpa.       600     IN      MX      10 pechora.icann.org.
   uri.arpa.       600     IN      RRSIG   MX 8 2 600 (
       20210217232440 20210120232440 37444 uri.arpa.
       kQAJQivmv6A5hqYBK8h6Z13ESY69gmosXwKI6WE09I8RFetfrxr24ecdnY
       d0lpnDtgNNSoHkYRSOoB+C4+zuJsoyAAzGo9uoWMWj97/2xeGhf3PTC9me
       Q9Ohi6hul9By7OR76XYmGhdWX8PBi60RUmZ1guslFBfQ8izwPqzuphs= )
   uri.arpa.       3600    IN      DNSKEY  256 3 8 (
       AwEAAbMxuFuLeVDuOwIMzYOTD/bTREjLflo7wOi6ieIJhqltEzgjNzmWJf
       9kGwwDmzxU7kbthMEhBNBZNn84zmcyRSCMzuStWveL7xmqqUlE3swL8kLO
       vdZvc75XnmpHrk3ndTyEb6eZM7slh2C63Oh6K8VR5VkiZAkEGg0uZIT3Nj
       sF )
   uri.arpa.       3600    IN      DNSKEY  257 3 8 (
       AwEAAdkTaWkZtZuRh7/OobBUFxM+ytTst+bCu0r9w+rEwXD7GbDs0pIMhM
       enrZzoAvmv1fQxw2MGs6Ri6yPKfNULcFOSt9l8i6BVBLI+SKTY6XXeDUQp
       SEmSaxohHeRPMQFzpysfjxINp/L2rGtZ7yPmxY/XRiFPSO0myqwGJa9r06
       Zw9CHM5UDHKWV/E+zxPFq/I7CfPbrrzbUotBX7Z6Vh3Sarllbe8cGUB2UF
       NaTRgwB0TwDBPRD5ER3w2Dzbry9NhbElTr7vVfhaGWeOGuqAUXwlXEg6Cr
       NkmJXJ2F1Rzr9WHUzhp7uWxhAbmJREGfi2dEyPAbUAyCjBqhFaqglknvc= )
   uri.arpa.       3600    IN      DNSKEY  257 3 8 (
       AwEAAenQaBoFmDmvRT+/H5oNbm0Tr5FmNRNDEun0Jpj/ELkzeUrTWhNpQm
       ZeIMC8I0kZ185tEvOnRvn8OvV39B17QIdrvvKGIh2HlgeDRCLolhaojfn2
       QM0DStjF/WWHpxJOmE6CIuvhqYEU37yoJscGAPpPVPzNvnL1HhYTaao1VR
       YWQ/maMrJ+bfHg+YX1N6M/8MnRjIKBif1FWjbCKvsn6dnuGGL9oCWYUFJ3
       DwofXuhgPyZMkzPc88YkJj5EMvbMH4wtelbCwC+ivx732l0w/rXJn0ciQS
       OgoeVvDio8dIJmWQITWQAuP+q/ZHFEFHPlrP3gvQh5mcVS48eLX71Bq7c= )
   uri.arpa.       3600    IN      RRSIG   DNSKEY 8 2 3600 (
       20210217232440 20210120232440 12670 uri.arpa.
       DBE2gkKAoxJCfz47KKxzoImN/0AKArhIVHE7TyTwy0DdRPo44V5R+vL6th
       UxlQ1CJi2Rw0jwAXymx5Y3Q873pOEllH+4bJoIT4dmoBmPXfYWW7Clvw9U
       PKHRP0igKHmCVwIeBYDTU3gfLcMTbR4nEWPDN0GxlL1Mf7ITaC2Ioabo79
       Ip3M/MR8I3Vx/xZ4ZKKPHtLn3xUuJluPNanqJrED2gTslL2xWZ1tqjsAjJ
       v7JnJo2HJ8XVRB5zBto0IaJ2oBlqcjdcQ/0VlyoM8uOy1pDwHQ2BJl7322
       gNMHBP9HSiUPIOaIDNUCwW8eUcW6DIUk+s9u3GN1uTqwWzsYB/rA== )
   uri.arpa.       3600    IN      RRSIG   DNSKEY 8 2 3600 (
       20210217232440 20210120232440 30577 uri.arpa.
       Kx6HwP4UlkGc1UZ7SERXtQjPajOF4iUvkwDj7MEG1xbQFB1KoJiEb/eiW0
       qmSWdIhMDv8myhgauejRLyJxwxz8HDRV4xOeHWnRGfWBk4XGYwkejVzOHz
       oIArVdUVRbr2JKigcTOoyFN+uu52cNB7hRYu7dH5y1hlc6UbOnzRpMtGxc
       gVyKQ+/ARbIqGG3pegdEOvV49wTPWEiyY65P2urqhvnRg5ok/jzwAdMx4X
       Gshiib7Ojq0sRVl2ZIzj4rFgY/qsSO8SEXEhMo2VuSkoJNiofVzYoqpxEe
       GnANkIT7Tx2xJL1BWyJxyc7E8Wr2QSgCcc+rYL6IkHDtJGHy7TaQ== )
   uri.arpa.       3600    IN      ZONEMD  2018100702 1 1 (
       0dbc3c4dbfd75777c12ca19c337854b1577799901307c482e9d91d5d15
       cd934d16319d98e30c4201cf25a1d5a0254960 )
   uri.arpa.       3600    IN      RRSIG   ZONEMD 8 2 3600 (
       20210217232440 20210120232440 37444 uri.arpa.
       QDo4XZcL3HMyn8aAHyCUsu/Tqj4Gkth8xY1EqByOb8XOTwVtA4ZNQORE1s
       iqNqjtJUbeJPtJSbLNqCL7rCq0CzNNnBscv6IIf4gnqJZjlGtHO30ohXtK
       vEc4z7SU3IASsi6bB3nLmEAyERdYSeU6UBfx8vatQDIRhkgEnnWUTh4= )
   uri.arpa.       3600    IN      NSEC    ftp.uri.arpa. (
       NS SOA MX RRSIG NSEC DNSKEY ZONEMD )
   uri.arpa.       3600    IN      RRSIG   NSEC 8 2 3600 (
       20210217232440 20210120232440 37444 uri.arpa.
       dU/rXLM/naWd1+1PiWiYVaNJyCkiuyZJSccr91pJI673T8r3685B4ODMYF
       afZRboVgwnl3ZrXddY6xOhZL3n9V9nxXZwjLJ2HJUojFoKcXTlpnUyYUYv
       VQ2kj4GHAo6fcGCEp5QFJ2KbCpeJoS+PhKGRRx28icCiNT4/uXQvO2E= )
   ftp.uri.arpa.   604800  IN      NAPTR   0 0 "" "" (
       "!^ftp://([^:/?#]*).*$!\\1!i" . )
   ftp.uri.arpa.   604800  IN      RRSIG   NAPTR 8 3 604800 (
       20210217232440 20210120232440 37444 uri.arpa.
       EygekDgl+Lyyq4NMSEpPyOrOywYf9Y3FAB4v1DT44J3R5QGidaH8l7ZFjH
       oYFI8sY64iYOCV4sBnX/dh6C1L5NgpY+8l5065Xu3vvjyzbtuJ2k6YYwJr
       rCbvl5DDn53zAhhO2hL9uLgyLraZGi9i7TFGd0sm3zNyUF/EVL0CcxU= )
   ftp.uri.arpa.   3600    IN      NSEC    http.uri.arpa. (
       NAPTR RRSIG NSEC )
   ftp.uri.arpa.   3600    IN      RRSIG   NSEC 8 3 3600 (
       20210217232440 20210120232440 37444 uri.arpa.
       pbP4KxevPXCu/bDqcvXiuBppXyFEmtHyiy0eAN5gS7mi6mp9Z9bWFjx/Ld
       H9+6oFGYa5vGmJ5itu/4EDMe8iQeZbI8yrpM4TquB7RR/MGfBnTd8S+sjy
       QtlRYG7yqEu77Vd78Fme22BKPJ+MVqjS0JHMUE/YUGomPkAjLJJwwGw= )
   http.uri.arpa.  604800  IN      NAPTR   0 0 "" "" (
       "!^http://([^:/?#]*).*$!\\1!i" . )
   http.uri.arpa.  604800  IN      RRSIG   NAPTR 8 3 604800 (
       20210217232440 20210120232440 37444 uri.arpa.
       eTqbWvt1GvTeXozuvm4ebaAfkXFQKrtdu0cEiExto80sHIiCbO0WL8UDa/
       J3cDivtQca7LgUbOb6c17NESsrsVkc6zNPx5RK2tG7ZQYmhYmtqtfg1oU5
       BRdHZ5TyqIXcHlw9Blo2pir1Y9IQgshhD7UOGkbkEmvB1Lrd0aHhAAg= )
   http.uri.arpa.  3600    IN      NSEC    mailto.uri.arpa. (
       NAPTR RRSIG NSEC )
   http.uri.arpa.  3600    IN      RRSIG   NSEC 8 3 3600 (
       20210217232440 20210120232440 37444 uri.arpa.
       R9rlNzw1CVz2N08q6DhULzcsuUm0UKcPaGAWEU40tr81jEDHsFHNM+khCd
       OI8nDstzA42aee4rwCEgijxJpRCcY9hrO1Ysrrr2fdqNz60JikMdarvU5O
       0p0VXeaaJDfJQT44+o+YXaBwI7Qod3FTMx7aRib8i7istvPm1Rr7ixA= )
   mailto.uri.arpa.        604800  IN      NAPTR   0 0 "" "" (
       "!^mailto:(.*)@(.*)$!\\2!i" . )
   mailto.uri.arpa.        604800  IN      RRSIG   NAPTR 8 3 604800 (
       20210217232440 20210120232440 37444 uri.arpa.
       Ch2zTG2F1plEvQPyIH4Yd80XXLjXOPvMbiqDjpJBcnCJsV8QF7kr0wTLnU
       T3dB+asQudOjPyzaHGwFlMzmrrAsszN4XAMJ6htDtFJdsgTMP/NkHhYRSm
       Vv6rLeAhd+mVfObY12M//b/GGVTjeUI/gJaLW0fLVZxr1Fp5U5CRjyw= )
   mailto.uri.arpa.        3600    IN      NSEC    urn.uri.arpa. (
       NAPTR RRSIG NSEC )
   mailto.uri.arpa.        3600    IN      RRSIG   NSEC 8 3 3600 (
       20210217232440 20210120232440 37444 uri.arpa.
       fQUbSIE6E7JDi2rosah4SpCOTrKufeszFyj5YEavbQuYlQ5cNFvtm8KuE2
       xXMRgRI4RGvM2leVqcoDw5hS3m2pOJLxH8l2WE72YjYvWhvnwc5Rofe/8y
       B/vaSK9WCnqN8y2q6Vmy73AGP0fuiwmuBra7LlkOiqmyx3amSFizwms= )
   urn.uri.arpa.   604800  IN      NAPTR   0 0 "" "" (
       "/urn:([^:]+)/\\1/i" . )
   urn.uri.arpa.   604800  IN      RRSIG   NAPTR 8 3 604800 (
       20210217232440 20210120232440 37444 uri.arpa.
       CVt2Tgz0e5ZmaSXqRfNys/8OtVCk9nfP0zhezhN8Bo6MDt6yyKZ2kEEWJP
       jkN7PCYHjO8fGjnUn0AHZI2qBNv7PKHcpR42VY03q927q85a65weOO1YE0
       vPYMzACpua9TOtfNnynM2Ws0uN9URxUyvYkXBdqOC81N3sx1dVELcwc= )
   urn.uri.arpa.   3600    IN      NSEC    uri.arpa. NAPTR RRSIG NSEC
   urn.uri.arpa.   3600    IN      RRSIG   NSEC 8 3 3600 (
       20210217232440 20210120232440 37444 uri.arpa.
       JuKkMiC3/j9iM3V8/izcouXWAVGnSZjkOgEgFPhutMqoylQNRcSkbEZQzF
       K8B/PIVdzZF0Y5xkO6zaKQjOzz6OkSaNPIo1a7Vyyl3wDY/uLCRRAHRJfp
       knuY7O+AUNXvVVIEYJqZggd4kl/Rjh1GTzPYZTRrVi5eQidI1LqCOeg= )

A.5.  The ROOT-SERVERS.NET Zone

   The following sample zone is the ROOT-SERVERS.NET zone retrieved
   2018-10-21.

   root-servers.net.     3600000 IN  SOA     a.root-servers.net. (
       nstld.verisign-grs.com. 2018091100 14400 7200 1209600 3600000 )
   root-servers.net.     3600000 IN  NS      a.root-servers.net.
   root-servers.net.     3600000 IN  NS      b.root-servers.net.
   root-servers.net.     3600000 IN  NS      c.root-servers.net.
   root-servers.net.     3600000 IN  NS      d.root-servers.net.
   root-servers.net.     3600000 IN  NS      e.root-servers.net.
   root-servers.net.     3600000 IN  NS      f.root-servers.net.
   root-servers.net.     3600000 IN  NS      g.root-servers.net.
   root-servers.net.     3600000 IN  NS      h.root-servers.net.
   root-servers.net.     3600000 IN  NS      i.root-servers.net.
   root-servers.net.     3600000 IN  NS      j.root-servers.net.
   root-servers.net.     3600000 IN  NS      k.root-servers.net.
   root-servers.net.     3600000 IN  NS      l.root-servers.net.
   root-servers.net.     3600000 IN  NS      m.root-servers.net.
   a.root-servers.net.   3600000 IN  AAAA    2001:503:ba3e::2:30
   a.root-servers.net.   3600000 IN  A       198.41.0.4
   b.root-servers.net.   3600000 IN  MX      20 mail.isi.edu.
   b.root-servers.net.   3600000 IN  AAAA    2001:500:200::b
   b.root-servers.net.   3600000 IN  A       199.9.14.201
   c.root-servers.net.   3600000 IN  AAAA    2001:500:2::c
   c.root-servers.net.   3600000 IN  A       192.33.4.12
   d.root-servers.net.   3600000 IN  AAAA    2001:500:2d::d
   d.root-servers.net.   3600000 IN  A       199.7.91.13
   e.root-servers.net.   3600000 IN  AAAA    2001:500:a8::e
   e.root-servers.net.   3600000 IN  A       192.203.230.10
   f.root-servers.net.   3600000 IN  AAAA    2001:500:2f::f
   f.root-servers.net.   3600000 IN  A       192.5.5.241
   g.root-servers.net.   3600000 IN  AAAA    2001:500:12::d0d
   g.root-servers.net.   3600000 IN  A       192.112.36.4
   h.root-servers.net.   3600000 IN  AAAA    2001:500:1::53
   h.root-servers.net.   3600000 IN  A       198.97.190.53
   i.root-servers.net.   3600000 IN  MX      10 mx.i.root-servers.org.
   i.root-servers.net.   3600000 IN  AAAA    2001:7fe::53
   i.root-servers.net.   3600000 IN  A       192.36.148.17
   j.root-servers.net.   3600000 IN  AAAA    2001:503:c27::2:30
   j.root-servers.net.   3600000 IN  A       192.58.128.30
   k.root-servers.net.   3600000 IN  AAAA    2001:7fd::1
   k.root-servers.net.   3600000 IN  A       193.0.14.129
   l.root-servers.net.   3600000 IN  AAAA    2001:500:9f::42
   l.root-servers.net.   3600000 IN  A       199.7.83.42
   m.root-servers.net.   3600000 IN  AAAA    2001:dc3::35
   m.root-servers.net.   3600000 IN  A       202.12.27.33
   root-servers.net.     3600000 IN  SOA     a.root-servers.net. (
       nstld.verisign-grs.com. 2018091100 14400 7200 1209600 3600000 )
   root-servers.net.     3600000 IN  ZONEMD  2018091100 1 1 (
       f1ca0ccd91bd5573d9f431c00ee0101b2545c97602be0a97
       8a3b11dbfc1c776d5b3e86ae3d973d6b5349ba7f04340f79 )

Appendix B.  Implementation Status

   This section records the status of known implementations of the
   protocol defined by this specification at the time of publication,
   and is inspired by the concepts described in RFC 7942.

   Please note that the listing of any individual implementation here
   does not imply endorsement by the IETF.  Furthermore, no effort has
   been spent to verify the information presented here that was supplied
   by IETF contributors.  This is not intended as, and must not be
   construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

B.1.  Authors' Implementation

   The authors have an open-source implementation in C, using the ldns
   library ([LDNS-ZONE-DIGEST]).  This implementation is able to perform
   the following functions:

   *  Read an input zone and output a zone with the ZONEMD placeholder.

   *  Compute the zone digest over the signed zone and update the ZONEMD
      record.

   *  Recompute DNSSEC signatures over the ZONEMD record.

   *  Verify the zone digest from an input zone.

   This implementation does not:

   *  Perform DNSSEC validation of the ZONEMD record during
      verification.

B.2.  Shane Kerr's Implementation

   Shane Kerr wrote an implementation of this specification during the
   IETF 102 hackathon ([ZONE-DIGEST-HACKATHON]).  This implementation is
   in Python and is able to perform the following functions:

   *  Read an input zone and output a zone with ZONEMD record.

   *  Verify the zone digest from an input zone.

   *  Output the ZONEMD record in its defined presentation format.

   This implementation does not:

   *  Recompute DNSSEC signatures over the ZONEMD record.

   *  Perform DNSSEC validation of the ZONEMD record.

B.3.  NIC Chile Lab's Implementation

   NIC Chile Labs wrote an implementation of this specification as part
   of "dns-tools" suite ([DNS-TOOLS]), which besides digesting, can also
   sign and verify zones.  This implementation is in Go and is able to
   perform the following functions:

   *  Compute zone digest over signed zone and update the ZONEMD record.

   *  Verify the zone digest from an input zone.

   *  Perform DNSSEC validation of the ZONEMD record during
      verification.

   *  Recompute DNSSEC signatures over the ZONEMD record.

Acknowledgments

   The authors wish to thank David Blacka, Scott Hollenbeck, and Rick
   Wilhelm for providing feedback on early drafts of this document.
   Additionally, they thank Joe Abley, Mark Andrews, Ralph Dolmans,
   Donald Eastlake 3rd, Richard Gibson, Olafur Gudmundsson, Bob Harold,
   Paul Hoffman, Evan Hunt, Shumon Huque, Tatuya Jinmei, Mike St. Johns,
   Burt Kaliski, Shane Kerr, Matt Larson, Barry Leiba, John Levine, Ed
   Lewis, Matt Pounsett, Mukund Sivaraman, Petr Spacek, Ondrej Sury,
   Willem Toorop, Florian Weimer, Tim Wicinski, Wouter Wijngaards, Paul
   Wouters, and other members of the DNSOP Working Group for their
   input.

   The authors would again like to thank Tim Wicinski, who served as the
   Document Shepherd for this document.

Authors' Addresses

   Duane Wessels
   Verisign
   12061 Bluemont Way
   Reston, VA 20190
   United States of America

   Phone: +1 703 948-3200
   Email: dwessels@verisign.com
   URI:   https://verisign.com


   Piet Barber
   Verisign
   12061 Bluemont Way
   Reston, VA 20190
   United States of America

   Phone: +1 703 948-3200
   Email: pbarber@verisign.com
   URI:   https://verisign.com


   Matt Weinberg
   Amazon

   Email: matweinb@amazon.com
   URI:   https://amazon.com


   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America

   Email: warren@kumari.net


   Wes Hardaker
   USC/ISI
   P.O. Box 382
   Davis, CA 95617
   United States of America

   Email: ietf@hardakers.net
