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Versions: 00 01 02 03 04 05 06 07 RFC 2535

DNS Security Working Group                        Donald E. Eastlake 3rd
INTERNET-DRAFT                                                 CyberCash
UPDATES RFC 1034, 1035
Expires: July 1998                                          January 1998

                 Domain Name System Security Extensions
                 ------ ---- ------ -------- ----------

Status of This Document

   This draft, file name draft-ietf-dnssec-secext2-03.txt, is intended
   to become a Proposed Standard RFC obsoleting Proposed Standard RFC
   2065.  Distribution of this document is unlimited. Comments should be
   sent to the DNS Security Working Group mailing list <dns-
   security@tis.com> or to the author.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months.  Internet-Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (East USA), ftp.isi.edu (West USA),
   nic.nordu.net (North Europe), ftp.nis.garr.it (South Europe),
   munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa).

Donald E. Eastlake 3rd                                          [Page 1]

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   Extensions to the Domain Name System (DNS) are described that provide
   data integrity and authentication to security aware resolvers or
   applications through the use of cryptographic digital signatures.
   These digital signatures are included in secured zones as resource
   records.  Security can also be provided through non-security aware
   DNS servers in some cases.

   The extensions provide for the storage of authenticated public keys
   in the DNS.  This storage of keys can support general public key
   distribution services as well as DNS security.  The stored keys
   enable security aware resolvers to learn the authenticating key of
   zones in addition to those for which they are initially configured.
   Keys associated with DNS names can be retrieved to support other
   protocols.  Provision is made for a variety of key types and

   In addition, the security extensions provide for the optional
   authentication of DNS protocol transactions and requests.

   This document incorporates feedback on RFC 2065 from early
   implementers and potential users.


   The significant contributions and suggestions of the following
   persons (in alphabetic order) to DNS security are gratefully

        James M. Galvin
        John Gilmore
        Olafur Gudmundsson
        Charlie Kaufman
        Edward Lewis
        Radia J. Perlman
        Jeffrey I. Schiller
        Steven (Xunhua) Wang
        Brian Wellington

Donald E. Eastlake 3rd                                          [Page 2]

INTERNET-DRAFT      DNS Protocol Security Extensions        January 1998

Table of Contents

      Status of This Document....................................1


      Table of Contents..........................................3

      1. Overview of Contents....................................5
      2. Overview of the DNS Extensions..........................6
      2.1 Services Not Provided..................................6
      2.2 Key Distribution.......................................6
      2.3 Data Origin Authentication and Integrity...............7
      2.3.1 The SIG Resource Record..............................8
      2.3.2 Authenticating Name and Type Non-existence...........8
      2.3.3 Special Considerations With Time-to-Live.............8
      2.3.4 Special Considerations at Delegation Points..........9
      2.3.5 Special Considerations with CNAME....................9
      2.3.6 Signers Other Than The Zone.........................10
      2.4 DNS Transaction and Request Authentication............10

      3. The KEY Resource Record................................12
      3.1 KEY RDATA format......................................12
      3.1.1 Object Types, DNS Names, and Keys...................12
      3.1.2 The KEY RR Flag Field...............................13
      3.1.3 The Protocol Octet..................................14
      3.2 The KEY Algorithm Number Specification................15
      3.3 Interaction of Flags, Algorithm, and Protocol Bytes...16
      3.4 Determination of Zone Secure/Unsecured Status.........17
      3.5 KEY RRs in the Construction of Responses..............18

      4. The SIG Resource Record................................19
      4.1 SIG RDATA Format......................................19
      4.1.1 ....................................................19
      4.1.2 Algorithm Number Field..............................20
      4.1.3 Labels Field........................................20
      4.1.4 Original TTL Field..................................20
      4.1.5 Signature Expiration and Inception Fields...........21
      4.1.6 Key Tag Field.......................................21
      4.1.7 Signer's Name Field.................................21
      4.1.8 Signature Field.....................................22 Calculating Transaction and Request SIGs..........22
      4.2 SIG RRs in the Construction of Responses..............23
      4.3 Processing Responses and SIG RRs......................24
      4.4 Signature Lifetime, Expiration, TTLs, and Validity....25
      4.5 SIG Under The Meta-Root Key and The Root Zone.........25

      5. Non-existent Names and Types...........................27
      5.1 The NXT Resource Record...............................27

Donald E. Eastlake 3rd                                          [Page 3]

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      5.2 NXT RDATA Format......................................28
      5.3 Additional Complexity Due to Wildcards................28
      5.4 Example...............................................29
      5.5 Special Considerations at Delegation Points...........30
      5.6 Zone Transfers........................................30
      5.6.1 Incremental Zone Transfers..........................30

      6. How to Resolve Securely and the AD and CD Bits.........32
      6.1 The AD and CD Header Bits.............................32
      6.2 Staticly Configured Keys..............................33
      6.3 Chaining Through The DNS..............................34
      6.3.1 Chaining Through KEYs...............................34
      6.3.2 Conflicting Data....................................36
      6.4 Secure Time...........................................36

      7. ASCII Representation of Security RRs...................37
      7.1 Presentation of KEY RRs...............................37
      7.2 Presentation of SIG RRs...............................38
      7.3 Presentation of NXT RRs...............................39

      8. Canonical Form and Order of Resource Records...........40
      8.1 Canonical RR Form.....................................40
      8.2 Canonical DNS Name Order..............................40
      8.3 Canonical RR Ordering Within An RRset.................41
      8.4 Canonical Ordering of RR Types........................41

      9. Conformance............................................42
      9.1 Server Conformance....................................42
      9.2 Resolver Conformance..................................42

      10. Security Considerations...............................43


      Author's Address..........................................46
      Expiration and File Name..................................46

      Appendix A: Base 64 Encoding..............................47

      Appendix B: Changes from RFC 2065.........................49

      Appendix C: Key Tag Calculation...........................51

Donald E. Eastlake 3rd                                          [Page 4]

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1. Overview of Contents

   This document standardizes extensions of the Domain Name System (DNS)
   protocol to support DNS security and public key distribution.  It
   assumes that the reader is familiar with the Domain Name System,
   particularly as described in RFCs 1033, 1034, 1035 and later RFCs.
   An earlier version of these extensions appears in RFC 2065.  This
   replacement for that RFC incorporates early implementation experience
   and requests from  potential users.

   Section 2 provides an overview of the extensions and the key
   distribution, data origin authentication, and transaction and request
   security they provide.

   Section 3 discusses the KEY resource record, its structure, and use
   in DNS responses.  These resource records represent the public keys
   of entities named in the DNS and are used for key distribution.

   Section 4 discusses the SIG digital signature resource record, its
   structure, and use in DNS responses.  These resource records are used
   to authenticate other resource records in the DNS and optionally to
   authenticate DNS transactions and requests.

   Section 5 discusses the NXT resource record (RR) and its use in DNS
   responses including full and incremental zone transfers.  The NXT RR
   permits authenticated denial of the existence of a name or of an RR
   type for an existing name.

   Section 6 discusses how a resolver can be configured with a starting
   key or keys and proceed to securely resolve DNS requests.
   Interactions between resolvers and servers are discussed for various
   combinations of security aware and security non-aware.  Two
   additional DNS header bits are defined for signaling between
   resolvers and servers.

   Section 7 describes the ASCII representation of the security resource
   records for use in master files and elsewhere.

   Section 8 defines the canonical form and order of RRs for DNS
   security purposes.

   Section 9 defines levels of conformance for resolvers and servers.

   Section 10 provides a few paragraphs on overall security

   Appendix A gives details of base 64 encoding which is used in the
   file representation of some RRs defined in this document.

   Appendix B summarizes changes between this draft and RFC 2065.

Donald E. Eastlake 3rd                                          [Page 5]

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   Appendix C specified how to calculate the simple checksum used as a
   key tag in the SIG RR.

2. Overview of the DNS Extensions

   The Domain Name System (DNS) protocol security extensions provide
   three distinct services: key distribution as described in Section 2.2
   below, data origin authentication as described in Section 2.3 below,
   and transaction and request authentication, described in Section 2.4

   Special considerations related to "time to live", CNAMEs, and
   delegation points are also discussed in Section 2.3.

2.1 Services Not Provided

   It is part of the design philosophy of the DNS that the data in it is
   public and that the DNS gives the same answers to all inquirers.
   Following this philosophy, no attempt has been made to include any
   sort of access control lists or other means to differentiate

   No effort has been made to provide for any confidentiality for
   queries or responses.  (This service may be available via IPSEC [RFC
   1825], TLS [draft-ietf-tls-*], or other security protocols.)

   Protection is not provided against denial of service.

2.2 Key Distribution

   A resource record format is defined to associate keys with DNS names.
   This permits the DNS to be used as a public key distribution
   mechanism in support of DNS security itself and other protocols.

   The syntax of a KEY resource record (RR) is described in Section 3.
   It includes an algorithm identifier, the actual public key
   parameter(s), and a variety of flags including those indicating the
   type of entity the key is associated with and/or asserting that there
   is no key associated with that entity.

   Under conditions described in Section 3.5, security aware DNS servers
   will automatically attempt to return KEY resources as additional
   information, along with those resource records actually requested, to
   minimize the number of queries needed.

Donald E. Eastlake 3rd                                          [Page 6]

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2.3 Data Origin Authentication and Integrity

   Authentication is provided by associating with resource record sets
   (RRsets) in the DNS cryptographically generated digital signatures.
   Commonly, there will be a single private key that authenticates an
   entire zone but there might be multiple keys for different
   algorithms, signers, etc. If a security aware resolver reliably
   learns a public key of the zone, it can authenticate, for signed data
   read from that zone, that it was properly authorized and is current.
   The most secure implementation is for the zone private key(s) to be
   kept off-line and used to re-sign all of the records in the zone
   periodically.  However, there are cases, for example dynamic update
   [RFCs 2136, 2137], where DNS private keys need to be on-line.

   This data origin authentication key(s) are associated with the zone
   and not to the servers that store copies of the data.  That means
   compromise of a secondary server or, if the key(s) are kept off line,
   even the primary server for a zone, will not necessarily affect the
   degree of assurance that a resolver has that it can determine whether
   data is genuine.

   A resolver could learn a public key of a zone either by reading it
   from the DNS or by having it or a key which authenticates it staticly
   configured.  To reliably learn a public key by reading it from the
   DNS, the key itself must be signed with a key the resolver trusts.
   The resolver must be configured with at least a public key which
   authenticates one zone as a starting point. From there, it can
   securely read public keys of other zones, if the intervening zones in
   the DNS tree are secure and their signed keys accessible.

   Adding data origin authentication and integrity requires no change to
   the "on-the-wire" DNS protocol beyond the addition of the signature
   resource type and the key resource type needed for key distribution.
   (Data non-existence authentication also requires the NXT RR as
   described in 2.3.2.)  This service can be supported by existing
   resolver and caching server implementations so long as they can
   support the additional resource types (see Section 9). The one
   exception is that CNAME referrals in a secure zone can not be
   authenticated if they are from non-security aware servers (see
   Section 2.3.5).

   If signatures are separately retrieved and verified when retrieving
   the information they authenticate, there will be more trips to the
   server and performance will suffer.  Security aware servers mitigate
   that degradation by attempting to send the signature(s) needed (see
   Section 4.2).

Donald E. Eastlake 3rd                                          [Page 7]

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2.3.1 The SIG Resource Record

   The syntax of a SIG resource record (signature) is described in
   Section 4.  It cryptographically binds the RRset being signed to the
   signer and a validity interval.

   Every name in a secured zone will have associated with it at least
   one SIG resource record for each resource type under that name except
   for glue address RRs and delegation point NS RRs.  A security aware
   server will attempt to return, with RRs retrieved, the corresponding
   SIGs.  If a server is not security aware, the resolver must retrieve
   all the SIG records for a name and select the one or ones that sign
   the resource record set(s) that resolver is interested in.

2.3.2 Authenticating Name and Type Non-existence

   The above security mechanism only provides a way to sign existing
   RRsets in a zone.  "Data origin" authentication is not obviously
   provided for the non-existence of a domain name in a zone or the
   non-existence of a type for an existing name.  This gap is filled by
   the NXT RR which authenticatably asserts a range of non-existent
   names in a zone and the non-existence of types for the existing name
   just before that range.

   Section 5 below covers the NXT RR.

2.3.3 Special Considerations With Time-to-Live

   A digital signature will fail to verify if any change has occurred to
   the data between the time it was originally signed and the time the
   signature is verified.  This conflicts with our desire to have the
   time-to-live (TTL) field of resource records tick down while they are

   This could be avoided by leaving the time-to-live out of the digital
   signature, but that would allow unscrupulous servers to set
   arbitrarily long TTL values undetected.  Instead, we include the
   "original" TTL in the signature and communicate that data along with
   the current TTL. Unscrupulous servers under this scheme can
   manipulate the TTL but a security aware resolver will bound the TTL
   value it uses at the original signed value.  Separately, signatures
   include a signature inception time and a signature expiration time.
   A resolver that knows the absolute time can determine securely
   whether a signature is in effect.  It is not possible to rely solely
   on the signature expiration as a substitute for the TTL, however,
   since the TTL is primarily a database consistency mechanism and non-

Donald E. Eastlake 3rd                                          [Page 8]

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   security aware servers that depend on TTL must still be supported.

2.3.4 Special Considerations at Delegation Points

   DNS security would like to view each zone as a unit of data
   completely under the control of the zone owner with each entry
   (RRset) signed by a special private key held by the zone.  But the
   DNS protocol views the leaf nodes in a zone, which are also the apex
   nodes of a subzone (i.e., delegation points), as "really" belonging
   to the subzone.  These nodes occur in two master files and might have
   RRs signed by both the upper and lower zone's keys.  A retrieval
   could get a mixture of these RRs and SIGs, especially since one
   server could be serving both the zone above and below a delegation
   point. [RFC 2181]

   There MUST be a zone KEY RR, signed by its superzone, for every
   subzone if the superzone is secure.  In the case of an unsecured
   subzone which can not or will not be modified to add any security
   RRs, a KEY declaring the subzone to be unsecured MUST appear in and
   be signed by the superzone, if the superzone is secure.  For all but
   one other RR type the data from the subzone is more authoritative so
   only the KEY RR in the superzone should be signed.  The NS and any
   glue address RRs should only be signed in the subzone. The SOA and
   any other RRs that have the zone name as owner should appear only in
   the subzone and thus are signed only there. The NXT RR type is the
   exceptional case that will always appear differently and
   authoritatively in both the superzone and subzone, if both are
   secure, as described in Section 5.

2.3.5 Special Considerations with CNAME

   There is a problem when security related RRs with the same owner name
   as a CNAME RR are retrieved from a non-security-aware server.  In
   particular, an initial retrieval for the CNAME or any other type may
   not retrieve any associated signature, KEY, or NXT RR. For retrieved
   types other than CNAME, it will retrieve that type at the target name
   of the CNAME (or chain of CNAMEs) and will also return the CNAME.  In
   particular, a specific retrieval for type SIG will not get the SIG,
   if any, at the original CNAME domain name but rather a SIG at the
   target name.

   Security aware servers must be used to securely CNAME in DNS.
   Security aware servers MUST (1) allow KEY, SIG, and NXT RRs along
   with CNAME RRs, (2) suppress CNAME processing on retrieval of these
   types as well as on retrieval of the type CNAME, and (3)
   automatically return SIG RRs authenticating the CNAME or CNAMEs

Donald E. Eastlake 3rd                                          [Page 9]

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   encountered in resolving a query.  This is a change from the previous
   DNS standard [RFCs 1034/1035] which prohibited any other RR type at a
   node where a CNAME RR was present.

2.3.6 Signers Other Than The Zone

   There are cases where a SIG resource record is signed by other than a
   private key used to authenticate a zone.

   One is for support of dynamic update [RFC 2136] (or future requests
   which require secure authentication) where an entity is permitted to
   authenticate/update its records [RFC 2137].  The public key of the
   entity must be present in the DNS and be appropriately signed but the
   other RR(s) may be signed with the entity's key.

   The second case is support of transaction and request authentication
   as described in Section 2.4.

2.4 DNS Transaction and Request Authentication

   The data origin authentication service described above protects
   retrieved resource records but provides no protection for DNS
   requests or for message headers.

   If header bits are falsely set by a bad server, there is little that
   can be done.  However, it is possible to add transaction
   authentication.  Such authentication means that a resolver can be
   sure it is at least getting messages from the server it thinks it
   queried and that the response is from the query it sent (i.e., that
   these messages have not been diddled in transit).  This is
   accomplished by optionally adding a special SIG resource record at
   the end of the reply which digitally signs the concatenation of the
   server's response and the resolver's query.

   Requests can also be authenticated by including a special SIG RR at
   the end of the request.  Authenticating requests serves no function
   in older DNS servers and requests with a non-empty additional
   information section are ignored by many DNS servers.  However, this
   syntax for signing requests is defined in connection with
   authenticating secure dynamic update requests [RFC 2137] or future
   requests requiring authentication.

   The private keys used in transaction and request security belong to
   the host composing the request or reply, not to the zone involved.
   The corresponding public key is normally stored in and retrieved from
   the DNS for verification.

Donald E. Eastlake 3rd                                         [Page 10]

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   Because requests and replies are highly variable, message
   authentication SIGs can not be pre-calculated.  Thus it will be
   necessary to keep the private key on-line, for example in software or
   in a directly connected piece of hardware.

Donald E. Eastlake 3rd                                         [Page 11]

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3. The KEY Resource Record

   The KEY resource record (RR) is used to store a public key that is
   associated with a Domain Name System (DNS) name.  This can be the
   public key of a zone, a user, or a host or other end entity.  A KEY
   RR is, like any other RR, authenticated by a SIG RR. Security aware
   DNS implementations MUST be designed to handle at least two
   simultaneously valid keys of the same type associated with the same

   The type number for the KEY RR is 25.

3.1 KEY RDATA format

   The RDATA for a KEY RR consists of flags, a protocol octet, the
   algorithm number, and the public key itself.  The format is as

                        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
   |             flags             |    protocol   |   algorithm   |
   |                                                               /
   /                          public key                           /
   /                                                               |

   The KEY RR is not intended for storage of certificates and a separate
   certificate RR is being developed, to be defined in a separate

   The meaning of the KEY RR owner name, flags, and protocol octet are
   described in Sections 3.1.1 through 3.1.5 below.  The flags and
   algorithm must be examined before any data following the algorithm
   octet as they control the existence and format of any following data.
   The algorithm and public key fields are described in Section 3.2.
   The format of the public key is algorithm dependent.

   KEY RRs do not specify their validity period but their authenticating
   SIG RR does as described in Section 4 below.

3.1.1 Object Types, DNS Names, and Keys

   The public key in a KEY RR is for the object named in the owner name.

Donald E. Eastlake 3rd                                         [Page 12]

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   This DNS name may refer to up to three different categories of
   things.  For example, foo.host.example could be (1) a zone, (2) a
   host or other end entity , or (3) the mapping into a DNS name of the
   user or account foo@host.example.  Thus, there are flag bits, as
   described below, in the KEY RR to indicate with which of these roles
   the owner name and public key are associated.  Note that an
   appropriate zone KEY RR MUST occur at the apex node of a secure zone
   and zone KEY RRs occur only at delegation points.

3.1.2 The KEY RR Flag Field

   In the "flags" field:

                                             1   1   1   1   1   1
     0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
   |  A/C  | Z | XT| Z | Z | NAMTYP| Z | Z | Z | Z |      SIG      |

   Bit 0 and 1 are the key "type" bits.
           10: Bit 0 a one indicates that use of the key is prohibited
      for authentication.
           01: Bit 1 a one indicates that use of the key is prohibited
      for confidentiality.
           00: If this field is zero, then use of the key for
      authentication and/or confidentiality is permitted. Note that DNS
      security makes use of keys for authentication only.
      Confidentiality use flagging is provided for use of keys in other
      protocols.  Implementations not intended to support key
      distribution for confidentiality MAY require that the
      confidentiality use prohibited bit be on for keys they serve.
           11: If both bits are one, the "no key" value, there is no key
      information and the RR stops after the algorithm octet.  By the
      use of this "no key" value, a signed KEY RR can authenticatably
      assert that, for example, a zone is not secured.  See section 3.4

   Bits 2 is reserved and must be zero.

   Bits 3 is reserved as a flag extension bit.  If it is a one, a second
      16 bit flag field is added after the algorithm octet and before
      the key data.  This bit MUST NOT be set unless one or more such
      additional bits have been defined and are non-zero.

   Bits 4-5 are reserved and must be zero.

   Bits 6 and 7 form a field that encodes the name type.
           0 - indicates that this is a key associated with a "user" or

Donald E. Eastlake 3rd                                         [Page 13]

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      "account" at an end entity, usually a host.  The coding of the
      owner name is that used for the responsible individual mailbox in
      the SOA and RP RRs: The owner name is the user name as the name of
      a node under the entity name.  For example, "j_random_user" on
      host.subdomain.example could have a public key associated through
      a KEY RR with name j_random_user.host.subdomain.example.  It could
      be used in a security protocol where authentication of a user was
      desired.  This key might be useful in IP or other security for a
      user level service such a telnet, ftp, rlogin, etc.
           1 - indicates that this is a zone key for the zone whose name
      is the KEY RR owner name.  This is the public key used for the
      primary DNS security feature of data origin authentication.  Zone
      KEY RRs occur only at delegation points.
           2 - indicates that this is a key associated with the non-zone
      "entity" whose name is the RR owner name.  This will commonly be a
      host but could, in some parts of the DNS tree, be some other type
      of entity such as a telephone number [RFC 1530] or numeric IP
      address.  This is the public key used in connection with DNS
      request and transaction authentication services if the owner name
      designates a DNS resolver or server host.  It could also be used
      in an IP-security protocol where authentication at the host,
      rather than user, level was desired, such as routing, NTP, etc.
           3 - reserved.

   Bits 8-11 are reserved and must be zero.

   Bits 12-15 are the "signatory" field.  If non-zero, they indicate
      that the key can validly sign RRs or updates of the same name in
      connection with DNS dynamic update [RFC 2137].  If the owner name
      is a wildcard, then RRs or updates with any name which is in the
      wildcard's scope can, in some cases, be signed.  Fifteen different
      non-zero values are possible for this field and any differences in
      their meaning are reserved for definition with DNS dynamic update.
      Note that zone keys (see bits 6 and 7 above) always have authority
      to sign any RRs in the zone regardless of the value of the
      signatory field.  The signatory field, like all other aspects of
      the KEY RR, is only effective if the KEY RR is appropriately
      signed by a SIG RR.

3.1.3 The Protocol Octet

   It is anticipated that keys stored in DNS will be used in conjunction
   with a variety of Internet protocols.  It is intended that the
   protocol octet and possibly some of the currently unused (must be
   zero) bits in the KEY RR flags as specified in the future will be
   used to indicate a key's validity for different protocols.

   The following values of the Protocol Octet are reserved as indicated:

Donald E. Eastlake 3rd                                         [Page 14]

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        VALUE   Protocol

          0      -reserved
          1     TLS
          2     email
          3     dnssec
          4     IPSEC
         5-254   -available for assignment by IANA
        255     All

   In more detail:
        1 is reserved to refer to the TLS standard being developed by
   the tls working group.  The presence of a KEY resource with this
   protocol value is an assertion that the host speaks TLS.
        2 is reserved for use in connection with email.
        3 is used for DNS security.  The protocol field should be set to
   this value for zone keys and other keys used in DNS security.
   Implementations that can determine that a key is a DNS security key
   by the fact that flags label it a zone key or the signatory flag
   field is non-zero are not required to check the protocol field.
        4 is reserved to refer to the Oakley/IPSEC [RFC 1825] protocol
   and indicates that this key is valid for use in conjunction with that
   security standard.  This key could be used in connection with secured
   communication on behalf of an end entity or user whose name is the
   owner name of the KEY RR if the entity or user bits are on.  The
   presence of a KEY resource with this protocol value is an assertion
   that the host speaks Oakley/IPSEC.
        255 indicates that the key can be used in connection with any
   protocol for which KEY RR protocol octet values have been defined.
   The use of this value is discouraged and the use of different keys
   for different protocols is encouraged.

3.2 The KEY Algorithm Number Specification

   This octet is the key algorithm parallel to the same field for the
   SIG resource as described in Section 4.1.  The following values are

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      VALUE   Protocol

        0      - reserved
        1     RSA/MD5 [RFC xxx1] - recommended
        2     Diffie-Hellman [RFC xxx2] - key only
        3     DSA [RFC xxx3] - MANDATORY
        4     reserved for elliptic curve
      5-251   - available (see below)
      252     indirect keys [RFC xxx4]
      253     - available (but was "null" [RFC 2065])
      254     private (see below)
      255      - reserved

   Algorithm specific formats and procedures are given in separate
   documents.  The mandatory to implement for interoperability algorithm
   is number 3, DSA.  It is recommended that the RSA/MD5 algorithm,
   number 1, also be implemented.  Algorithm 2 is used to indicate
   Diffie-Hellman keys and algorithm 4 is reserved for elliptic curve.

   Numbers 5 through 251 and 253 are available for assignment should
   sufficient reason arise.  However, the designation of a new algorithm
   could have a major impact on interoperability and requires an IETF
   standards action.

   Number 254 is reserved for private use and will never be assigned a
   specific algorithm.  For number 254, the public key area for the KEY
   RR and the signature will actually begin with a length byte followed
   by an Object Identifier (ISO OID) of that length.  The OID indicates
   the private algorithm in use and the remainder of the area is
   whatever is required by that algorithm.

   Values 0 and 255 are reserved but the value 0 is used in the
   algorithm field when that field is not used.  An example is in a KEY
   RR with the top two flag bits on, the "no-key" value, where no key is
   present.  Such a KEY RR should have an algorithm field of zero.

3.3 Interaction of Flags, Algorithm, and Protocol Bytes

   Various combinations of the no-key type flags, algorithm byte,
   protocol byte, and any future assigned protocol indicating flags are
   possible.  The meaning of these combinations is indicated below:

   NK = no key type flags (bits 0 and 1 on)
   AL = algorithm byte
   PR = protocols indicated by protocol byte or future assigned flags

   x represents any valid non-zero value(s).

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    AL  PR   NK  Meaning
     0   0   0   Illegal, claims key but has bad algorithm field.
     0   0   1   Specifies total lack of security for owner zone.
     0   x   0   Illegal, claims key but has bad algorithm field.
     0   x   1   Specified protocols unsecured, others may be secure.
     x   0   0   Useless.  Gives key but no protocols to use it.
     x   0   1   Useless.  Denies key but for no protocols.
     x   x   0   Specifies key for protocols.
     x   x   1   Algorithm not understood for protocol.

3.4 Determination of Zone Secure/Unsecured Status

   A zone KEY RR with the "no-key" type field value (both bits 0 and 1
   on) indicates that the zone named is unsecured while a zone KEY RR
   with a key present indicates that the zone named is secure.  It is
   possible for conflicting zone KEY RRs to be present.

   Zone KEY RRs, like all RRs, are only trusted if they are
   authenticated by a SIG RR whose signer field is a signer for which
   the resolver has a public key they trust and where resolver policy
   permits that signer to sign for the KEY owner name.  Untrusted zone
   KEY RRs MUST be ignored in determining the security status of the
   zone.  However, there can be multiple sets of trusted zone KEY RRs
   for a zone with different algorithms, signers, etc.

   Zones can be (1) secure, indicating that any retrieved RR must be
   authenticated by a SIG RR or it will be discarded as bogus, (2)
   unsecured, indicating that SIG RRs are not expected or required for
   RRs retrieved from the zone, or (3) experimentally secure, which
   indicates that SIG RRs might or might not be present but must be
   checked if found.  The status of a zone is determined as follows:

   1. If, for a zone, every trusted zone KEY RR for the zone says there
      is no key for that zone, it is unsecured.

   2. If, there is at least one trusted no-key zone KEY RR and one
      trusted key specifying zone KEY RR, then that zone is only
      experimentally secure.  Both authenticated and non-authenticated
      RRs for it should be accepted by the resolver.

   3. If every trusted zone KEY RR for the zone has is key specifying,
      then it is secure and only authenticated RRs from it will be


   (1)  A resolver only trusts signatures by the superzone within the
   DNS hierarchy so it will look only at the KEY RRs that are signed by

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   the superzone.  If it finds only no-key KEY RRs, it will assume the
   zone is not secure.  If it finds only key specifying KEY RRs, it will
   assume the zone is secure and reject any unsigned responses.  If it
   finds both, it will assume the zone is experimentally secure

   (2)  A resolver trusts the superzone of zone Z (to which it got
   securely from its local zone) and a third party, cert-auth.xy.  When
   considering data from zone Z, it may be signed by the superzone of Z,
   by cert-auth.xy, by both, or by neither.  The following table
   indicates whether zone Z will be considered secure, experimentally
   secure, or unsecured, depending on the signed zone KEY RRs for Z;

                         c e r t - a u t h . x y

              |   None    |  NoKeys   |  Mixed   |   Keys   |
    S       --+-----------+-----------+----------+----------+
    u  None   | illegal   | unsecured | experim. | secure   |
    p         +-----------+-----------+----------+----------+
    e  NoKeys | unsecured | unsecured | experim. | secure   |
    r         +-----------+-----------+----------+----------+
    Z  Mixed  | experim.  | experim.  | experim. | secure   |
    o         +-----------+-----------+----------+----------+
    n  Keys   | secure    | secure    | secure   | secure   |
    e         +-----------+-----------+----------+----------+

3.5 KEY RRs in the Construction of Responses

   An explicit request for KEY RRs does not cause any special additional
   information processing except, of course, for the corresponding SIG
   RR from a security aware server (see Section 4.2).

   Security aware DNS servers include KEY RRs as additional information
   in responses, where a KEY is available, in the following cases:

   (1) On the retrieval of SOA or NS RRs, the KEY RRset with the same
   name (usually just a zone key) SHOULD be included as additional
   information if space is available.  There will always be at least one
   such KEY RR in a secure zone in connection with a subzone delegation
   point, even if it has the no-key type value to indicate that the
   subzone is unsecured.  If not all additional information will fit,
   type A and AAAA glue RRs have higher priority than KEY RR(s).

   (2) On retrieval of type A or AAAA RRs, the KEY RRset with the same
   name (usually just a host RR and NOT the zone key which usually would
   have a different name) SHOULD be included if space is available.  On
   inclusion of A or AAAA RRs as additional information, the KEY RRset
   with the same name should also be included but with lower priority
   than the A or AAAA RRs.

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4. The SIG Resource Record

   The SIG or "signature" resource record (RR) is the fundamental way
   that data is authenticated in the secure Domain Name System (DNS). As
   such it is the heart of the security provided.

   The SIG RR unforgably authenticates an RRset of a particular type,
   class, and name and binds it to a time interval and the signer's
   domain name.  This is done using cryptographic techniques and the
   signer's private key.  The signer is frequently the owner of the zone
   from which the RR originated.  The SIG RR is only intended to be
   meaningful to DNS security.

   The type number for the SIG RR type is 24.

4.1 SIG RDATA Format

   The RDATA portion of a SIG RR is as shown below.  The integrity of
   the RDATA information is protected by the signature field.

                           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
      |        type covered           |  algorithm    |     labels    |
      |                         original TTL                          |
      |                      signature expiration                     |
      |                      signature inception                      |
      |            key  tag           |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         signer's name         +
      |                                                               /
      /                                                               /
      /                            signature                          /
      /                                                               /


   The "type covered" is the type of the other RRs covered by this SIG.

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4.1.2 Algorithm Number Field

   This octet is as described in section 3.2.

4.1.3 Labels Field

   The "labels" octet is an unsigned count of how many labels there are
   in the original SIG RR owner name not counting the null label for
   root and not counting any initial "*" for a wildcard.  If a secured
   retrieval is the result of wild card substitution, it is necessary
   for the resolver to use the original form of the name in verifying
   the digital signature.  This field makes it easy to determine the
   original form.

   If, on retrieval, the RR appears to have a longer name than indicated
   by "labels", the resolver can tell it is the result of wildcard
   substitution.  If the RR owner name appears to be shorter than the
   labels count, the SIG RR must be considered corrupt and ignored.  The
   maximum number of labels allowed in the current DNS is 127 but the
   entire octet is reserved and would be required should DNS names ever
   be expanded to 255 labels.  The following table gives some examples.
   The value of "labels" is at the top, the retrieved owner name on the
   left, and the table entry is the name to use in signature
   verification except that "bad" means the RR is corrupt.

   labels= |  0  |   1  |    2   |      3   |      4   |
          .|   . | bad  |  bad   |    bad   |    bad   |
         d.|  *. |   d. |  bad   |    bad   |    bad   |
       c.d.|  *. | *.d. |   c.d. |    bad   |    bad   |
     b.c.d.|  *. | *.d. | *.c.d. |   b.c.d. |    bad   |
   a.b.c.d.|  *. | *.d. | *.c.d. | *.b.c.d. | a.b.c.d. |

4.1.4 Original TTL Field

   The "original TTL" field is included in the RDATA portion to avoid
   (1) authentication problems that caching servers would otherwise
   cause by decrementing the real TTL field and (2) security problems
   that unscrupulous servers could otherwise cause by manipulating the
   real TTL field.  This original TTL is protected by the signature
   while the current TTL field is not.

   NOTE:  The "original TTL" must be restored into the covered RRs when
   the signature is verified (see Section 8).  This implies that all RRs
   for a particular type, name, and class must have the same TTL to
   start with.

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4.1.5 Signature Expiration and Inception Fields

   The SIG is valid from the "signature inception" time until the
   "signature expiration" time.  Both are unsigned numbers of seconds
   since the start of 1 January 1970, GMT, ignoring leap seconds.  (See
   also Section 4.4.)  Ring arithmetic is used as for DNS SOA serial
   numbers [RFC 1982] which means that these times can never be more
   than about 136.09 years in the future.

   (To prevent misordering of network requests to update a zone
   dynamically, monotonically increasing "signature inception" times may
   be necessary.  [RFC 2137])

   SOA serial numbers for secure zones MUST not only be advanced when
   their data is updated but also when new SIG RRs are inserted (ie, the
   zone or any part of it is re-signed).

   A SIG RR may have an expiration time numerically less than the time
   signed if time is near the 32 bit wrap around point and/or the
   signature is long lived.

4.1.6 Key Tag Field

   The "key Tag" is a two octet quantity that is used to efficiently
   select between multiple keys which may be applicable and thus check
   that a public key about to be used for the computationally expensive
   effort to check the signature is possibly valid.  For algorithm 1
   (MD5/RSA) as defined in [RFC xxx1], it is the next to the bottom two
   octets of the public key modulus needed to decode the signature
   field.  That is to say, the most significant 16 of the lest
   significant 24 bits of the modulus in network (big endian) order.
   For all other algorithms, including private algorithms, it is
   calculated as a simple checksum of the KEY RR as described in
   Appendix C.

4.1.7 Signer's Name Field

   The "signer's name" field is the domain name of the signer generating
   the SIG RR.  This is the owner of the public KEY RR that can be used
   to verify the signature.  It is frequently the zone which contained
   the RRset being authenticated.  Which signers should be authorized to
   sign what is a significant resolver policy question as discussed in
   Section 6. The signer's name may be compressed with standard DNS name
   compression when being transmitted over the network.

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4.1.8 Signature Field

   The structure of the "signature" field is described below.

   The actual signature portion of the SIG RR binds the other RDATA
   fields to the RRset of the "type covered" RRs with that owner name
   and class.  This covered RRset is thereby authenticated.  To
   accomplish this, a data sequence is constructed as follows:

              data = RDATA | RR(s)...

   where "|" is concatenation, RDATA is wire format of all the RDATA
   fields in the SIG RR itself including the canonical form of the
   signers name before but not including the signature, and RR(s) is the
   RRset of the RR(s) of the type covered with the same owner name and
   class as the SIG RR in canonical form and order as defined in Section
   8.  How this data sequence is processed into the signature is
   algorithm dependent.  These algorithm dependent formats and
   procedures are described in separate documents (Section 3.2).

   SIGs SHOULD NOT be included in a zone for any "meta-type" such as
   ANY, AXFR, etc. Calculating Transaction and Request SIGs

   A response message from a security aware server may optionally
   contain a special SIG at the end of the additional information
   section to authenticate the transaction.

   This SIG has a "type covered" field of zero, which is not a valid RR
   type.  It is calculated by using a "data" (see Section 4.1.8) of the
   entire preceding DNS reply message, including DNS header but not the
   IP header and before the reply RR counts have been adjusted for the
   inclusion of any transaction SIG, concatenated with the entire DNS
   query message that produced this response, including the query's DNS
   header and any request SIGs but not its IP header.  That is

     data = full response (less transaction SIG) | full query

   Verification of the transaction SIG (which is signed by the server
   host key, not the zone key) by the requesting resolver shows that the
   query and response were not tampered with in transit, that the
   response corresponds to the intended query, and that the response
   comes from the queried server.

   A DNS request may be optionally signed by including one or more SIGs
   at the end of the query. Such SIGs are identified by having a "type
   covered" field of zero. They sign the preceding DNS request message

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   including DNS header but not including the IP header or any request
   SIGs at the end and before the request RR counts have been adjusted
   for the inclusions of any request SIG(s).

   WARNING: Request SIGs are unnecessary for any currently defined
   request other than update [RFC 2136, 2137] and will cause many
   existing DNS servers to ignore a query.  However, such SIGs may in
   the future been needed for other requests.

   Except where needed to authenticate an update or similar privileged
   request, servers are not required to check request SIGs.

4.2 SIG RRs in the Construction of Responses

   Security aware DNS servers SHOULD, for every authenticated RR the
   query will return, attempt to send the available SIG RRs which
   authenticate the requested RR.  The following rules apply to the
   inclusion of SIG RRs in responses:

     1. when an RRset is placed in a response, its SIG RR has a higher
        priority for inclusion than additional RRs that may need to be
        included.  If space does not permit its inclusion, the response
        MUST be considered truncated except as provided in 2 below.

     2. when a SIG RR is present in the zone for an additional
        information section RR, the response MUST NOT be considered
        truncated merely because space does not permit the inclusion of
        its SIG RR.

     3. SIGs to authenticate non-authoritative data (glue records and NS
        RRs for subzones) are unnecessary and MUST NOT be sent.  (Note
        that KEYs given for a subzone in that subzone's superzone are
        controlling so the superzone's signature on the KEY MUST be
        included (unless the KEY was additional information and the SIG
        did not fit).)

     4. If a SIG covers any RR that would be in the answer section of
        the response, its automatic inclusion MUST be in the answer
        section.  If it covers an RR that would appear in the authority
        section, its automatic inclusion MUST be in the authority
        section.  If it covers an RR that would appear in the additional
        information section it MUST appear in the additional information
        section.  This is a change in the existing standard [RFCs 1034,
        1035] which contemplates only NS and SOA RRs in the authority

     5. Optionally, DNS transactions may be authenticated by a SIG RR at
        the end of the response in the additional information section

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        (Section  Such SIG RRs are signed by the DNS server
        originating the response.  Although the signer field MUST be the
        name of the originating server host, the owner name, class, TTL,
        and original TTL, are meaningless.  The class and TTL fields
        SHOULD be zero.  To conserve space, the owner name SHOULD be
        root (a single zero octet).  If transaction authentication is
        desired, that SIG RR must be considered the highest priority for

4.3 Processing Responses and SIG RRs

   The following rules apply to the processing of SIG RRs included in a

     1. a security aware resolver that receives a response from a
        security aware server via a secure communication with the AD bit
        (see Section 6.1) set, MAY choose to accept the RRs as received
        without verifying the zone SIG RRs.

     2. in other cases, a security aware resolver SHOULD verify the SIG
        RRs for the RRs of interest.  This may involve initiating
        additional queries for SIG or KEY RRs, especially in the case of
        getting a response from an server that does not implement
        security.  (As explained in 2.3.5 above, it will not be possible
        to secure CNAMEs being served up by non-secure resolvers.)

        NOTE: Implementers might expect the above SHOULD to be a MUST.
        However, local policy or the calling application may not require
        the security services.

     3. If SIG RRs are received in response to a user query explicitly
        specifying the SIG type, no special processing is required.

   If the message does not pass integrity checks or the SIG does not
   check against the signed RRs, the SIG RR is invalid and should be
   ignored.  If all of the SIG RR(s) purporting to authenticate an RRset
   are invalid, then the RRset is not authenticated.

   If the SIG RR is the last RR in a response in the additional
   information section and has a type covered of zero, it is a
   transaction signature of the response and the query that produced the
   response.  It MAY be optionally checked and the message rejected if
   the checks fail.  But even if the checks succeed, such a transaction
   authentication SIG does NOT authenticate any RRs in the message.
   Only a proper SIG RR signed by the zone or a key tracing its
   authority to the zone or to static resolver configuration can
   authenticate RRs depending on resolver policy (see Section 6).  If a
   resolver does not implement transaction and/or request SIGs, it MUST

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   ignore them without error.

   If all checks indicate that the SIG RR is valid then RRs verified by
   it should be considered authenticated.

4.4 Signature Lifetime, Expiration, TTLs, and Validity

   Security aware servers MUST NOT consider SIG RRs to authenticate
   anything before their signature inception or after its expiration
   time.  (Actually after the authentication chain expiration time, see
   Section 6.) Security aware servers MUST NOT consider any RR to be
   authenticated after all its signatures have expired.  When a secure
   server caches authenticated data, if the TTL would expire at a time
   further in the future than the authentication expiration time, the
   server SHOULD trim the TTL in the cache entry not to extent beyond
   the authentication expiration time.  Within these constraint, servers
   should continue to follow DNS TTL aging.  Thus authoritative servers
   should continue to follow the zone refresh and expire parameters and
   a non-authoritative server should count down the TTL and discard RRs
   when the TTL is zero (even for a SIG that has not yet reached its
   authentication expiration time).  In addition, when RRs are
   transmitted in a query response, the TTL should be trimmed so that
   current time plus the TTL does not extend beyond the authentication
   expiration time.  Thus, in general, the TTL on a transmitted RR would


   When signatures are generated, signature expiration times should be
   set far enough in the future that it is quite certain that new
   signatures can be generated before the old ones expire.  However,
   setting expiration too far into the future could, if bad data or
   signatures were ever generated, mean a long time to flush such

   It is recommended that signature lifetime be a small multiple of the
   TTL (ie, 4 to 16 times the TTL) but not less than a reasonable
   maximum re-signing interval and not less than the zone expiry time.

4.5 SIG Under The Meta-Root Key and The Root Zone

   To minimize exposure of the ultimate key of the DNS tree, there will
   be a "meta-root" key used rarely and then only to sign a sequence of
   regular root key RRsets with overlapping time validity periods that
   are to be rolled out.  The root zone contains the meta-root and
   current regular root KEY RR(s) signed by SIG RRs under both the

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   meta-root and other root key(s) themselves.

   For example, assume that the regular root key is to be changed once a
   month.  If the meta-root key were to be exposed only once a year,
   then for each exposure you might use the meta-key to sign twenty four
   key RRsets as follows:
        one with a date signed of the middle of January and expiring the
   middle of February with the January and Jan/Feb root keys,
        one with a date signed of the beginning of February and expiring
   the end of February with the Jan/Feb and February root keys,
        one with a date signed of the middle of February and expiring
   the middle of March with the February and Feb/Mar root keys,
        one with the data signed of the beginning of March and expiring
   the end of March with with Feb/Mar and March root keys,

   During the first half of January, the data in the root zone with the
   above hypothetical key policy would be signed with the Dec/Jan and
   January keys.  During the second half of January, it would be signed
   with the January and Jan/Feb keys.  During the first half of
   February, it would be signed with the Jan/Feb and February keys. Etc.

   Security in the storage and use of the meta-root key should be
   maximized.  The particular techniques are precautions to be used are
   an operational matter beyond the scope of this document.

   It should also be noted that in DNS the root is a single level zone
   unto itself. Thus the root zone key should only be seen signing
   itself or signing RRs with names one level below root, such as .aq,
   .com, or signature of records with a name more than one level below

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5. Non-existent Names and Types

   The SIG RR mechanism described in Section 4 above provides strong
   authentication of RRs that exist in a zone.  But is it not clear
   above how to verifiably deny the existence of a name in a zone or a
   type for an existent name.

   The nonexistence of a name in a zone is indicated by the NXT ("next")
   RR for a name interval containing the nonexistent name. An NXT RR or
   RRs and its or their SIG(s) are returned in the authority section,
   along with the error, if the server is security aware.  The same is
   true for a non-existent type under an existing name except that there
   is no error indication other than an empty answer section
   accompanying the NXT(s). This is a change in the existing standard
   [RFCs 1034/1035] which contemplates only NS and SOA RRs in the
   authority section. NXT RRs will also be returned if an explicit query
   is made for the NXT type.

   The existence of a complete set of NXT records in a zone means that
   any query for any name and any type to a security aware server
   serving the zone will result in an reply containing at least one
   signed RR unless it is a query for delegation point NS or glue A or
   AAAA RRs.

5.1 The NXT Resource Record

   The NXT resource record is used to securely indicate that RRs with an
   owner name in a certain name interval do not exist in a zone and to
   indicate what RR types are present for an existing name.

   The owner name of the NXT RR is an existing name in the zone.  It's
   RDATA is a "next" name and a type bit map. The presence of the NXT RR
   means that no name between its owner name and the name in its RDATA
   area exists and that no other types exist under its owner name.  This
   implies a canonical ordering of all domain names in a zone as
   described in Section 8.

   There is a potential problem with the last NXT in a zone as it wants
   to have an owner name which is the last existing name in canonical
   order, which is easy, but it is not obvious what name to put in its
   RDATA to indicate the entire remainder of the name space.  This is
   handled by treating the name space as circular and putting the zone
   name in the RDATA of the last NXT in a zone.

   The NXT RRs for a zone SHOULD be automatically calculated and added
   to the zone when SIGs are added.  The NXT RR's TTL SHOULD NOT exceed
   the zone minimum TTL.

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   The type number for the NXT RR is 30.

5.2 NXT RDATA Format

   The RDATA for an NXT RR consists simply of a domain name followed by
   a bit map, as shown below.

                        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
   |                  next domain name                             /
   |                    type bit map                               /

   The NXT RR type bit map format currently defined is one bit per RR
   type present for the owner name similar to the WKS RR socket bit map.
   A one bit indicates that at least one RR of that type is present for
   the owner name.  A zero indicates that no such RR is present.  All
   bits not specified because they are beyond the end of the bit map are
   assumed to be zero.  Note that bit 30, for NXT, will always be on so
   the minimum bit map length is actually four octets.  Trailing zero
   octets are prohibited in this format.  The first bit represents RR
   type zero (an illegal type which can not be present) and so will be
   zero in this format.  This format must be used unless there are RRs
   with a type number greater than 127.  If the zero bit of the type bit
   map is a one, it indicates that there exists at least on RR with a
   type number greater than 127 and a different format is in use which
   is to be defined.

   The NXT bit map should be printed as a list of RR type mnemonics or
   decimal numbers similar to the WKS RR.

   The domain name may be compressed with standard DNS name compression
   when being transmitted over the network.  The size of the bit map can
   be inferred from the RDLENGTH and the length of the next domain name.

5.3 Additional Complexity Due to Wildcards

   Proving that a non-existent name response is correct or that a
   wildcard expansion response is correct makes things a little more

   In particular, when a non-existent name response is returned, an NXT
   must be returned showing that the exact name queried did not exist
   and, in general, one or more additional NXT's need to be returned to

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   also prove that there wasn't a wildcard whose expansion should have
   been returned. All the NXT's are returned in the authority section of
   the response.

   Furthermore, if a wildcard expansion is returned in a response, in
   general one or more NXTs needs to also be returned in the authority
   section to prove that no more specific name (including possibly more
   specific wildcards in the zone) existed on which the response should
   have been based.

5.4 Example

   Assume zone foo.nil has entries for


   Then a query to a security aware server for huge.foo.nil would
   produce an error reply with an RCODE of NXDOMAIN and the authority
   section data including something like the following:

      foo.nil    NXT big.foo.nil NS KEY SOA NXT ;prove no *.foo.nil
      foo.nil    SIG NXT 1 2 ( ;type-cov=NXT, alg=1, labels=2
                       19970102030405 ;signature expiration
                       19961211100908 ;time signed
                       2143           ;key identifier
                       foo.nil.       ;signer
      fiYy2X+8XpFjwICHc398kzWsTMKlxovpz2FnCTM= ;signature (640 bits)
      big.foo.nil. NXT medium.foo.nil. A MX SIG NXT ;prove no huge.foo.nil
      big.foo.nil. SIG NXT 1 3 ( ;type-cov=NXT, alg=1, labels=3
                       19970102030405 ;signature expiration
                       19961211100908 ;time signed
                       2143           ;key identifier
                       foo.nil.       ;signer
       1tVfSCSqQYn6//11U6Nld80jEeC8aTrO+KKmCaY= ;signature (640 bits)

   Note that this response implies that big.foo.nil is an existing name
   in the zone and thus has other RR types associated with it than NXT.
   However, only the NXT (and its SIG) RR appear in the response to this
   query for huge.foo.nil, which is a non-existent name.

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5.5 Special Considerations at Delegation Points

   A name (other than root) which is the head of a zone also appears as
   the leaf in a superzone.  If both are secure, there will always be
   two different NXT RRs with the same name.  They can be distinguished
   by their signers, the next domain name fields, the presence of the
   SOA type bit, etc.  Security aware servers should return the correct
   NXT automatically when required to authenticate the non-existence of
   a name and both NXTs, if available, on explicit query for type NXT.

   Non-security aware servers will never automatically return an NXT and
   some old implementations may only return the NXT from the subzone on
   explicit queries.

5.6 Zone Transfers

   The sections below describe how full and incremental zone transfers
   are secured.

   SIG RRs secure all authoritative RRs transferred for both full and
   incremental [RFC 1995] zone transfers.  NXT RRs are an essential
   elements in secure zone transfers and assure that every authoritative
   name and type will be present; however, if there are multiple SIGs
   with the same name and type covered a subset of the SIGs could be
   sent as long as at least one is present and, in the case of unsigned
   delegation point NS or glue A or AAAA RRs a subset of these RRs could
   be sent as long as at least one of each type is included.

   To provide server authentication that a complete transfer has
   occurred, transaction authentication SHOULD be used on all full zone
   transfers.  This provides strong server based protection for the
   entire zone in transit.

   When an incremental or full zone transfer request is received with
   the same or newer version number than that of the server's copy of
   the zone, it is replied to with just the SOA RR of the server's
   current version and the SIG RRset verifying that SOA RR.

5.6.1 Incremental Zone Transfers

   Individual RRs in an incremental (IXFR) transfer [RFC 1995] can be
   verified in the same way as for a full zone transfer and the
   integrity of the NXT name chain and correctness of the NXT type bits
   for the zone after the incremental RR deletes and adds can check each
   disjoint area of the zone updated.  But the completeness of an

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   incremental transfer can not be confirmed because usually neither the
   deleted RR section nor the added RR section has a compete NXT chain.
   As a result, a server which securely supports IXFR must handle IXFR
   SIG RRs for each incremental transfer set that it maintains.

   The IXFR SIG is calculated over the incremental zone update
   collection of RRs in the order in which it is transmitted: old SOA,
   then deleted RRs, then new SOA and added RRs.  Within each section,
   RRs must be ordered as specified in Section 8.  If condensation of
   adjacent incremental update sets is done by the zone owner, the
   original IXFR SIG for each set included in the condensation must be
   discarded and a new on IXFR SIG calculated to cover the resulting
   condensed set.

   The IXFR SIG really belongs to the zone as a whole, not to the zone
   name.  Although it should be correct for the zone name, the labels
   field of an IXFR SIG is otherwise meaningless.  The IXFR SIG is only
   sent as part of an incremental zone transfer.  After validation of
   the IXFR SIG, the transferred RRs MAY be considered valid without
   verification of the internal SIGs.

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6. How to Resolve Securely and the AD and CD Bits

   Retrieving or resolving secure data from the Domain Name System (DNS)
   involves starting with one or more trusted public keys that have been
   staticly configured at the resolver.  With starting trusted keys, a
   resolver willing to perform cryptography can progress securely
   through the secure DNS structure to the zone of interest as described
   in Section 6.3. Such trusted public keys would normally be configured
   in a manner similar to that described in Section 6.2.  However, as a
   practical matter, a security aware resolver would still gain some
   confidence in the results it returns even if it was not configured
   with any keys but trusted what it got from a local well known server
   as if it were staticly configured.

   Data stored at a security aware server needs to be internally
   categorized as Authenticated, Pending, or Insecure. There is also a
   fourth transient state of Bad which indicates that all SIG checks
   have explicitly failed on the data. Such Bad data is not retained at
   a security aware server. Authenticated means that the data has a
   valid SIG under a KEY traceable via a chain of zero or more SIG and
   KEY RRs allowed by the resolvers policies to a KEY staticly
   configured at the resolver.  Pending data has no authenticated SIGs
   and at least one additional SIG the resolver is still trying to
   authenticate.  Insecure data is data which it is known can never be
   either Authenticated or found Bad in the zone where it was found
   because it is in or has been reached via a unsecured zone or because
   it is unsigned glue address or delegation point NS data. Behavior in
   terms of control of and flagging based on such data labels is
   described in Section 6.1.

   The proper validation of signatures requires a reasonably secure
   shared opinion of the absolute time between resolvers and servers as
   described in Section 6.4.

6.1 The AD and CD Header Bits

   Two previously unused bits are allocated out of the DNS
   query/response format header. The AD (authentic data) bit indicates
   in a response that the data included in the answer and authority
   sections has been authenticated by the server according to the
   policies of that server or is accompanying glue address or delegation
   point NS data.  The CD (checking disabled) bit indicates in a query
   that Pending (non-authenticated) data is acceptable to the resolver
   sending the query.

   These bits are allocated from the previously must-be-zero Z field as

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                                             1  1  1  1  1  1
               0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
             |                      ID                       |
             |QR|   Opcode  |AA|TC|RD|RA| Z|AD|CD|   RCODE   |
             |                    QDCOUNT                    |
             |                    ANCOUNT                    |
             |                    NSCOUNT                    |
             |                    ARCOUNT                    |

   These bits are zero in old servers and resolvers.  Thus the responses
   of old servers are not flagged as authenticated to security aware
   resolvers and queries from non-security aware resolvers do not assert
   the checking disabled bit and thus will be answered by security aware
   servers only with Authenticated or Insecure data.  Security aware
   resolvers MUST NOT trust the AD bit unless they trust the server they
   are talking to and either have a secure path to it or use DNS
   transaction security.

   Any security aware resolver willing to do cryptography SHOULD assert
   the CD bit on all queries to permit it to impose its own policies and
   to reduce DNS latency time by allowing security aware servers to
   answer with Pending data.

   Security aware servers MUST NOT return Bad data.  For non-security
   aware resolvers or security aware resolvers requesting service by
   having the CD bit clear, security aware servers MUST return only
   Authenticated or Insecure data in the answer and authority sections
   with the AD bit set in the response.  Security aware servers SHOULD
   return Pending data, with the AD bit clear in the response, to
   security aware resolvers requesting the service by asserting the CD
   bit in their request.  The AD bit MUST NOT be set on a response
   unless all of the RRs in the answer and authority sections of the
   response are either Authenticated or Insecure.

6.2 Staticly Configured Keys

   The public key to authenticate a zone SHOULD be defined in local
   configuration files before that zone is loaded at the primary server
   so the zone can be authenticated.

   While it might seem logical for everyone to start with the meta-root

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   public key and staticly configure this in every resolver, this has
   problems.  The logistics of updating every DNS resolver in the world
   should this key ever change would be severe.  Furthermore, many
   organizations will explicitly wish their "interior" DNS
   implementations to completely trust only their own DNS servers.
   Interior resolvers of such organizations can then go through the
   organization's zone servers to access data outsize the organization's
   domain and should not be configured with keys above the
   organization's DNS apex.

   Host resolvers that are not part of a larger organization may be
   configured with a key for the domain of their local ISP whose
   recursive secure DNS caching server they use.

6.3 Chaining Through The DNS

   Starting with one or more trusted keys for any zone, it should be
   possible to retrieve signed keys for that zone's subzones which have
   a key and, if the zone is not root, for its superzone. If an
   authoritative secure zone server will have its public key (or that of
   any subzone) staticly configured, then it MUST also include the KEY
   RR for one or more super-zones (possibly including root) signed by
   the secure zone via static configuration. It is safest to always
   include this upward key.  This makes it possible to climb the tree of
   zones if one starts below root.  A secure sub-zone is indicated by a
   KEY RR with non-null key information appearing with the NS RRs for
   the sub-zone.  These make it possible to descend within the tree of

6.3.1 Chaining Through KEYs

   In general, some RRset that you wish to validate in the secure DNS
   will be signed by one or more SIG RRs.  Each of these SIG RRs has a
   signer under whose name is stored the public KEY to use in
   authenticating the SIG.  Each of those KEYs will, generally, also be
   signed with a SIG.  And those SIGs will have signer names also
   referring to KEYs.  And so on.  As a result, authentication leads to
   chains of alternating SIG and KEY RRs with the first SIG signing the
   original data whose authenticity is to be shown and the final KEY
   being some key staticly configured at the resolver performing the

   In testing such a chain, the validity periods of the SIGs encountered
   must be intersected to determine the validity period of the
   authentication of the data, a purely algorithm process. In addition,
   the validation of each SIG over the data with reference to a KEY must

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   meet the objective cryptographic test implied by the cryptographic
   algorithm used, although even here the resolver may have policies as
   to trusted algorithms and key lengths.  In addition,   The judgement
   that a SIG with a particular signer name can authenticate data
   (possibly a KEY RRset) with a particular owner name, however, is
   primarily a policy question.  Ultimately, this is a policy local to
   the resolver and any clients that depend on that resolver's
   decisions.  It is, however, strongly recommended, that the following
   policy be adopted:

        Let A < B mean that A is a shorter domain name than B formed by
        dropping one or more whole labels from the left end of B, i.e.,
        A is a direct or indirect superdomain of B.  Let A = B mean that
        A and B are the same domain name (i.e., are identical after
        letter case canonicalization).  Let A > B mean that A is a
        longer domain name than B formed by adding one or more whole
        labels on the left end of B, i.e., A is a direct or indirect
        subdomain of B

        Let Static be the owner names of the set of staticly configured
        trusted keys at a resolver.

        Then Signer is a valid signer name for a SIG authenticating data
        (possibly a KEY RRset) with owner name Owner at a resolver if
        any of the following three rules apply:

        (1) Owner > or = Signer (except that if Signer is root, Owner
        must be root or a top level domain name).

        (2) ( Owner < or = Signer ) and ( Signer > some Static ).

        (3) Signer = some Static.

   Rule 1 is the rule for descending the DNS tree and includes a special
   prohibition on the root zone key due to the restriction that the root
   zone be only one label deep.  This is the most fundamental rule.

   Rule 2 is the rule for ascending the DNS tree from one or more
   staticly configured keys.  Rule 2 has no effect if only root keys or
   the meta-root key are staticly configured.

   Rule 3 is a rule permitting direct cross certification.  Rule 3 has
   no effect if only root keys or the meta-root key are staticly

   Great care should be taken that the consequences have been fully
   considered before making any local policy adjustments to these rules
   (other than dispensing with rules 2 and 3 if only root keys or the
   meta-root key are staticly configured).

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6.3.2 Conflicting Data

   It is possible that there will be multiple SIG-KEY chains that appear
   to authenticate conflicting RRset answers to the same query.  A
   resolver should choose only the most reliable answer to return and
   discard other data.  This choice of most reliable is a matter of
   local policy which could take into account differing trust in
   algorithms, key sizes, staticly configured keys, zones traversed,
   etc.  The technique given below is recommended for taking into
   account SIG-KEY chain length.

   A resolver should keep track of the number of successive secure zones
   traversed from a staticly configured key starting point to any secure
   zone it can reach.  In general, the lower such a distance number is,
   the greater the confidence in the data.  Staticly configured data
   should be given a distance number of zero.  If a query encounters
   different Authenticated data for the same query with different
   distance values, that with a larger value should be ignored unless
   some other local policy covers the case.

   A security conscious resolver should completely refuse to step from a
   secure zone into a unsecured zone unless the unsecured zone is
   certified to be non-secure by the presence of an authenticated KEY RR
   for the unsecured zone with the no-key type value.  Otherwise the
   resolver is getting bogus or spoofed data.

   If legitimate unsecured zones are encountered in traversing the DNS
   tree, then no zone can be trusted as secure that can be reached only
   via information from such non-secure zones. Since the unsecured zone
   data could have been spoofed, the "secure" zone reach via it could be
   counterfeit.  The "distance" to data in such zones or zones reached
   via such zones could be set to 256 or more as this exceeds the
   largest possible distance through secure zones in the DNS.

6.4 Secure Time

   Coordinated interpretation of the time fields in SIG RRs requires
   that reasonably consistent time be available to the hosts
   implementing the DNS security extensions.

   A variety of time synchronization protocols exist including the
   Network Time Protocol (NTP [RFC 1305]).  If such protocols are used,
   they MUST be used securely so that time can not be spoofed.
   Otherwise, for example, a host could get its clock turned back and
   might then believe old SIG RRs, and the data they authenticate, which
   were valid but are no longer.

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7. ASCII Representation of Security RRs

   This section discusses the format for master file and other ASCII
   presentation of the three DNS security resource records.

   The algorithm field in KEY and SIG RRs can be represented as either
   an unsigned integer or symbolicly.  The following initial symbols are
   defined as indicated:

        Value  Symbol

        001    RSAMD5
        002    DH
        003    DSA
        004    ECC
        252    INDIRECT
        253    NULL (obsolete, see RFC 2065)
        254    PRIVATE

7.1 Presentation of KEY RRs

   KEY RRs may appear as single logical lines in a zone data master file
   [RFC 1033].

   The flag field is represented as an unsigned integer or a sequence of
   mnemonics as follows:

     BIT  Mnemonic  Explanation
    0-1           key type
        NOCONF    =1 confidentiality use prohibited
        NOAUTH    =2 authentication use prohibited
        NOKEY     =3 no key present
    2   FLAG2     - reserved
    3   EXTEND    flags extension
    4   FLAG4     - reserved
    5   FLAG5     -reserved
    6-7           name type
        USER      =0 (default, may be omitted)
        ZONE      =1
        HOST      =2 (host or other end entity)
        NTYP3     - reserved
    8   FLAG8     - reserved
    9   FLAG9     - reserved
   10   FLAG10    - reserved
   11   FLAG11    - reserved
   12-15          signatory field, values 0 to 15
            can be represented by SIG0, SIG1, ... SIG15

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   The protocol octet can be represented as either an unsigned integer
   or symbolicly.  The following initial symbols are defined:

        000    NONE
        001    TLS
        002    EMAIL
        003    DNSSEC
        004    IPSEC
        255    ALL

   Note that if the type field has the NOKEY value, nothing appears
   after the algorithm octet.

   The remaining public key portion is represented in base 64 (see
   Appendix A) and may be divided up into any number of white space
   separated substrings, down to single base 64 digits, which are
   concatenated to obtain the full signature.  These substrings can span
   lines using the standard parenthesis.

   Note that the public key may have internal sub-fields but these do
   not appear in the master file representation.  For example, with
   algorithm 1 there is a public exponent size, then a public exponent,
   and then a modulus.  With algorithm 254, there will be an OID size,
   an OID, and algorithm dependent information. But in both cases only a
   single logical base 64 string will appear in the master file.

7.2 Presentation of SIG RRs

   A SIG RR may be represented as a single logical line in a zone data
   file [RFC 1033] but there are some special considerations as
   described below.  (It does not make sense to include a transaction or
   request authenticating SIG RR in a file as they are a transient
   authentication that covers data including an ephemeral transaction
   number and so must be calculated in real time.)

   There is no particular problem with the signer, covered type, and
   times.  The time fields appears in the form YYYYMMDDHHMMSS where YYYY
   is the year, the first MM is the month number (01-12), DD is the day
   of the month (01-31), HH is the hour in 24 hours notation (00-23),
   the second MM is the minute (00-59), and SS is the second (00-59).

   The original TTL field appears as an unsigned integer.

   If the original TTL, which applies to the type signed, is the same as
   the TTL of the SIG RR itself, it may be omitted.  The date field
   which follows it is larger than the maximum possible TTL so there is
   no ambiguity.

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   The "labels" field appears as an unsigned integer.

   The key tag appears as an unsigned number.

   However, the signature itself can be very long.  It is the last data
   field and is represented in base 64 (see Appendix A) and may be
   divided up into any number of white space separated substrings, down
   to single base 64 digits, which are concatenated to obtain the full
   signature.  These substrings can be split between lines using the
   standard parenthesis.

7.3 Presentation of NXT RRs

   NXT RRs do not appear in original unsigned zone master files since
   they should be derived from the zone as it is being signed.  If a
   signed file with NXTs added is printed or NXTs are printed by
   debugging code, they appear as the next domain name followed by the
   RR type present bits in the same format as the WKS RR.

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8. Canonical Form and Order of Resource Records

   This section describes the canonical form of resource records (RRs),
   their default name order, and their order, for purposes of domain
   name system (DNS) security.  A canonical name order is necessary to
   construct the NXT name chain.  A canonical form and ordering within
   an RRset is necessary in constructing SIG RRs.  A canonical ordering
   of types within a name is required in connection with incremental
   transfer (Section 5.6.1).

8.1 Canonical RR Form

   For purposes of DNS security, the canonical form for an RR is the
   wire format of the RR with domain names (1) fully expanded (no name
   compression via pointers), (2) all domain name letters set to lower
   case, (3) owner name wild cards in master file form (no substitution
   made for *), and (4) the original TTL substituted for the current

8.2 Canonical DNS Name Order

   For purposes of DNS security, the canonical ordering of owner names
   is to sort individual labels as unsigned left justified octet strings
   where the absence of a octet sorts before a zero value octet and
   upper case letters are treated as lower case letters.  Names are then
   sorted by sorting on the highest level label and then, within those
   names with the same highest level label by the next lower label, etc.
   down to leaf node labels.  Within a zone, the zone name itself always
   exists and all other names are the zone name with some prefix of
   lower level labels.  Thus the zone name itself always sorts first.


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8.3 Canonical RR Ordering Within An RRset

   Within any particular owner name and type, RRs are sorted by RDATA as
   a left justified unsigned octet sequence where the absence of an
   octet sorts before the zero octet.

8.4 Canonical Ordering of RR Types

   When RRs of the same name but different types must be ordered, they
   are ordered by type, considering the type to be an unsigned integer,
   except that SIG RRs are placed immediately after the type they cover.
   Thus, for example, an A record would be put before an MX record but
   if both were signed, the order would be A < SIG(A) < MX < SIG(MX).

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9. Conformance

   Levels of server and resolver conformance are defined below.

9.1 Server Conformance

   Two levels of server conformance for DNS security are defined as

   BASIC:  Basic server compliance is the ability to store and retrieve
   (including zone transfer) SIG, KEY, and NXT RRs.  Any secondary or
   caching server for a secure zone MUST have at least basic compliance
   and even then some things, such as secure CNAMEs, will not work
   without full compliance.

   FULL:  Full server compliance adds the following to basic compliance:
   (1) ability to read SIG, KEY, and NXT RRs in zone files and (2)
   ability, given a zone file and private key, to add appropriate SIG
   and NXT RRs, possibly via a separate application, (3) proper
   automatic inclusion of SIG, KEY, and NXT RRs in responses, (4)
   suppression of CNAME following on retrieval of the security type RRs,
   (5) recognize the CD query header bit and set the AD query header
   bit, as appropriate, and (6) proper handling of the two NXT RRs at
   delegation points.  Primary servers for secure zones MUST be fully
   compliant and for complete secure operation, all secondary, caching,
   and other servers handling the zone SHOULD be fully compliant as

9.2 Resolver Conformance

   Two levels of resolver compliance (including the resolver portion of
   a server) are defined for DNS Security:

   BASIC: A basic compliance resolver can handle SIG, KEY, and NXT RRs
   when they are explicitly requested.

   FULL: A fully compliant resolver (1) understands KEY, SIG, and NXT
   RRs including verification of SIGs, (2) maintains appropriate
   information in its local caches and database to indicate which RRs
   have been authenticated and to what extent they have been
   authenticated, (3) performs additional queries as necessary to
   attempt to obtain KEY, SIG, or NXT RRs from non-security aware
   servers, (4) normally sets the CD query header bit on its queries.

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10. Security Considerations

   This document specifies extensions to the Domain Name System (DNS)
   protocol to provide data integrity and origin authentication, public
   key distribution, and optional transaction and request security.

   It should be noted that, at most, these extensions guarantee the
   validity of resource records, including KEY resource records,
   retrieved from the DNS.  They do not magically solve other security
   problems.  For example, using secure DNS you can have high confidence
   in the IP address you retrieve for a host name; however, this does
   not stop someone for substituting an unauthorized host at that
   address or capturing packets sent to that address and falsely
   responding with packets apparently from that address.  Any reasonably
   complete security system will require the protection of many
   additional facets of the Internet.

   A number of precautions in DNS implementation have evolved over the
   years to provide maximum resisitence of the insecure DNS against
   spoofing.  These precautions should not be abandoned but should be
   considered to provide minor additional protection in case of key
   compromise in secure DNS.

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   [RFC 1032] - M. Stahl, "Domain Administrators Guide", November 1987.

   [RFC 1033] - M. Lottor, "Domain Administrators Operations Guide",
   November 1987.

   [RFC 1034] - P. Mockapetris, "Domain Names - Concepts and
   Facilities", STD 13, November 1987.

   [RFC 1035] - P. Mockapetris, "Domain Names - Implementation and
   Specifications", STD 13, November 1987.

   [RFC 1305] - D. Mills, "Network Time Protocol (v3)", March 1992.

   [RFC 1530] - C. Malamud, and M. Rose, "Principles of Operation for
   the TPC.INT Subdomain: General Principles and Policy", October 1993.

   [RFC 1750] - D. Eastlake, S. Crocker, and J. Schiller, "Randomness
   Requirements for Security", December 1994.

   [RFC 1825] - Ran Atkinson, "Security Architecture for the Internet
   Protocol", August 1995.

   [RFC 1982] - Robert Elz, Rrandy Bush, "Serial Number Arithmetic",

   [RFC 1995] - Masatka Ohta, "Incremental Zone Transfer in DNS", August

   [RFC 2045] - N. Freed & N. Borenstein, "Multipurpose Internet Mail
   Extensions (MIME) Part One: Format of Internet Message Bodies",
   November 1996.

   [RFC 2065] - Donald Eastlake, Charles Kaufman, "Domain Name System
   Security Extensions", 01/03/1997.

   [RFC 2136] - P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
   Updates in the Domain Name System (DNS UPDATE)", 04/21/1997.

   [RFC 2137] - Donald Eastlake, "Secure Domain Name System Dynamic
   Update", 04/21/1997.

   [RFC 2181] - Robert Elz, Randy Bush, "Clarifications to the DNS
   Specification", July 1997.

   [RFC xxx1] - draft-ietf-dnssec-rsa-*, "RSA/MD5 KEYs and SIGs in the
   Domain Name System (DNS)".

   [RFC xxx2] - draft-ietf-dnssec-dhk-*, "Storage of Diffie-Hellman Keys

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   in the Domain Name System (DNS)".

   [RFC xxx3] - draft-ietf-dnssec-dss-*, "DSA KEYs and SIGs in the
   Domain Name System (DNS)".

   [RFC xxx4] - draft-ietf-dnssec-indirect-key-*, "Indirect KEY RRs in
   the Domain Name System (DNS)".

   [RSA FAQ] - RSADSI Frequently Asked Questions periodic posting.


Donald E. Eastlake 3rd                                         [Page 45]

INTERNET-DRAFT      DNS Protocol Security Extensions        January 1998

Author's Address

   Donald E. Eastlake 3rd
   CyberCash, Inc.
   318 Acton Street
   Carlisle, MA 01741 USA

   Telephone:   +1 978-287-4877
                +1 978-371-7148 (fax)
                +1 703-620-4200 (main office, Reston, Virginia, USA)
   email:       dee@cybercash.com

Expiration and File Name

   This draft expires July 1998.

   Its file name is draft-ietf-dnssec-secext2-03.txt.

Donald E. Eastlake 3rd                                         [Page 46]

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Appendix A: Base 64 Encoding

   The following encoding technique is taken from [RFC 2045] by N.
   Borenstein and N. Freed.  It is reproduced here in an edited form for

   A 65-character subset of US-ASCII is used, enabling 6 bits to be
   represented per printable character. (The extra 65th character, "=",
   is used to signify a special processing function.)

   The encoding process represents 24-bit groups of input bits as output
   strings of 4 encoded characters. Proceeding from left to right, a
   24-bit input group is formed by concatenating 3 8-bit input groups.
   These 24 bits are then treated as 4 concatenated 6-bit groups, each
   of which is translated into a single digit in the base 64 alphabet.

   Each 6-bit group is used as an index into an array of 64 printable
   characters. The character referenced by the index is placed in the
   output string.

                         Table 1: The Base 64 Alphabet

      Value Encoding  Value Encoding  Value Encoding  Value Encoding
          0 A            17 R            34 i            51 z
          1 B            18 S            35 j            52 0
          2 C            19 T            36 k            53 1
          3 D            20 U            37 l            54 2
          4 E            21 V            38 m            55 3
          5 F            22 W            39 n            56 4
          6 G            23 X            40 o            57 5
          7 H            24 Y            41 p            58 6
          8 I            25 Z            42 q            59 7
          9 J            26 a            43 r            60 8
         10 K            27 b            44 s            61 9
         11 L            28 c            45 t            62 +
         12 M            29 d            46 u            63 /
         13 N            30 e            47 v
         14 O            31 f            48 w         (pad) =
         15 P            32 g            49 x
         16 Q            33 h            50 y

   Special processing is performed if fewer than 24 bits are available
   at the end of the data being encoded.  A full encoding quantum is
   always completed at the end of a quantity.  When fewer than 24 input
   bits are available in an input group, zero bits are added (on the
   right) to form an integral number of 6-bit groups.  Padding at the
   end of the data is performed using the '=' character.  Since all base
   64 input is an integral number of octets, only the following cases
   can arise: (1) the final quantum of encoding input is an integral
   multiple of 24 bits; here, the final unit of encoded output will be

Donald E. Eastlake 3rd                                         [Page 47]

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   an integral multiple of 4 characters with no "=" padding, (2) the
   final quantum of encoding input is exactly 8 bits; here, the final
   unit of encoded output will be two characters followed by two "="
   padding characters, or (3) the final quantum of encoding input is
   exactly 16 bits; here, the final unit of encoded output will be three
   characters followed by one "=" padding character.

Donald E. Eastlake 3rd                                         [Page 48]

INTERNET-DRAFT      DNS Protocol Security Extensions        January 1998

Appendix B: Changes from RFC 2065

   This section summarizes the most important changes that have been
   made since RFC 2065.

   1. Most of Section 7 of [RFC 2065] called "Operational
      Considerations", has been removed and may be made into a separate

   2. The KEY RR has been changed by (2a) eliminating the "experimental"
      flag as unnecessary, (2b) reserving a flag  bit for flags
      expansion, (2c) more compactly encoding a number of bit fields in
      such a way as to leave unchanged bits actually used by the limited
      code currently deployed, (2d) eliminating the IPSEC and email flag
      bits which are replaced by values of the protocol field and adding
      a protocol field value for dnssec, (2e) adding material to
      indicate that zone KEY RRs occur only at delegation points, and
      (2f) removing the description of the RSA/MD5 algorithm to a
      separate document.  Section 3.4 describing the meaning of various
      combinations of "no-key" and key present KEY RRs has been added.

   3. The SIG RR has been changed by (3a) renaming the "time signed"
      field to be the "signature inception" field, (3b) clarifying that
      signature expiration and inception used serial number ring
      arithmetic, (3c) changing the definition of the key footprint/tag
      for algorithms other than 1 (i.e., algorithms to be defined in the
      future) and adding Appendix C to document its calculation.  In
      addition, the SIG covering type AXFR has been eliminated while one
      covering IXFR has been added.

   4. Algorithm 3, the DSA algorithm, is designated as the mandatory to
      implement algorithm.  Algorithm 1, the RSA/MD5 algorithm, is now a
      recommended option.  Both the KEY and SIG RR definitions have been
      simplified by eliminating the "null" algorithm 253 as defined in
      [RFC 2065].  That algorithm had been included because at the time
      it was thought it might be useful in DNS dynamic update [RFC
      2136].  It was in fact not so used and it is dropped to simplify
      DNS security.

   5. The NXT RR has been changed so that (5a) the NXT RRs in a zone
      cover all names, including wildcards as literal names without
      expansion, except for glue address records whose names would not
      otherwise appear, (5b) all NXT bit map areas whose first octet has
      bit zero set have been reserved for future definition, and (5c)
      extending the number of and circumstances under which an NXT must
      be returned in connection with wildcard names.

   6. Information on the canonical form and ordering of RRs has been
      moved into a separate Section 8.

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   7. A subsection covering incremental and full zone transfer has been
      added in Section 5.

   8. Concerning DNS chaining: Further specification and policy
      recommendations on secure resolution have been added, primarily in
      Section 6.3.1.  That authenticated data has a validity period of
      the intersection of the validity periods of the SIG RRs in its
      authentication chain was clarified.  The requirement to staticly
      configure a superzone's key signed by a zone in all of the zone's
      authoritative servers has been relaxed in cases where the public
      key for that zone and all of its direct and indirect subzones will
      never be staticly configured.  The recommendation was dropped to
      continue DNS security checks in a secure island of DNS data that
      is separated from other parts of the DNS tree by insecure zones
      and does not contain a zone for which a key has been staticly

   9. The concept of a meta-root key has been added in Section 4.5.

   10.  That the presence of the AD bit in a response does not apply to
      the additional information section or to glue address or
      delegation point NS RRs was clarified.

Donald E. Eastlake 3rd                                         [Page 50]

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Appendix C: Key Tag Calculation

   The key tag field in the SIG RR is just a means of more efficiently
   selecting the correct KEY RR to use in verifying the signature when
   there is more than one KEY RR candidate.  It is possible for more
   than one candidate key to have the same tag, in which case each must
   be tried in verifying the signature until one works or all fail.  The
   following reference implementation is in ANSI C.  It is coded for
   clarity, not efficiency.

   /* assumes int is at least 16 bits
      first byte of key tag is the most significant byte of return value
      second byte of key tag is the least significant byte of return value */

   int keytag (
           unsigned char key[],  /* the RDATA part of the KEY RR */
           unsigned int keysize, /* the RDLENGTH */
   long int    ac;    /* assumed to be 32 bits or larger */

   for ( ac = 0, i = 0; i < keysize; ++i )
       ac += (i&1) ? key[i] : key[i]<<8;
   ac += (ac>>16) & 0xFFFF;
   return ac & 0xFFFF;

Donald E. Eastlake 3rd                                         [Page 51]

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