DNS Security Working Group                       Donald E. Eastlake, 3rd
INTERNET-DRAFT                                                 CyberCash
UPDATES RFC 1034, 1035                                Charles W. Kaufman
Expires: 10 April 1996                                   11 October 1995

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

Status of This Document

   This draft, file name draft-ietf-dnssec-secext-06.txt, is intended to
   be become a Proposed Standard RFC.  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 authors.

   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, ftp.isi.edu, nic.nordu.net,
   ftp.nis.garr.it, munnari.oz.au, or ftp.is.co.za.

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INTERNET-DRAFT      DNS Protocol Security Extensions     11 October 1995


   The Domain Name System (DNS) has become a critical operational part
   of the Internet infrastructure yet it has no strong security
   mechanisms to assure data integrity or authentication.  Extensions to
   the DNS are described that provide these services 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 still be provided even through
   non-security aware DNS servers in many cases.

   The extensions also provide for the storage of authenticated public
   keys in the DNS.  This storage of keys can support general public key
   distribution service as well as DNS security.  The stored keys enable
   security aware resolvers to learn the authenticating key of zones in
   addition to keys 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.


   The significant contributions of the following persons (in alphabetic
   order) to this draft are gratefully acknowledged:

        Madelyn Badger
        Matt Crawford
        James M. Galvin
        Olafur Gudmundsson
        Sandy Murphy
        Masataka Ohta
        Michael A. Patton
        Jeffrey I. Schiller
        Susan E. Thomson

Eastlake, Kaufman                                               [Page 2]

INTERNET-DRAFT      DNS Protocol Security Extensions     11 October 1995

Table of Contents

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


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

      1. Overview of Contents....................................5

      2.  Overview of the 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 RRs................9
      2.3.6 Signers Other Than The Zone.........................10
      2.4 DNS Transaction Authentication........................10

      3. The KEY Resource Record................................11
      3.1 KEY RDATA format......................................11
      3.2 Object Types, DNS Names, and Keys.....................11
      3.3 The KEY RR Flag Field.................................12
      3.4 The Protocol Octet....................................14
      3.5 The KEY Algorithm Number and the MD5/RSA Algorithm....14
      3.6 Interaction of Flags, Algorithm, and Protocol Bytes...15
      3.7 KEY RRs in the Construction of Responses..............16
      3.8 File Representation of KEY RRs........................16

      4. The SIG Resource Record................................18
      4.1 SIG RDATA Format......................................18
      4.1.1 Signature Data......................................20
      4.1.2 MD5/RSA Algorithm Signature Calculation.............21
      4.1.3 Zone Transfer (AXFR) SIG............................22
      4.1.4 Transaction SIGs....................................22
      4.2 SIG RRs in the Construction of Responses..............23
      4.3 Processing Responses and SIG RRs......................24
      4.4 Signature Expiration, TTLs, and Validity..............25
      4.5 File Representation of SIG RRs........................25

      5. Non-existent Names and Types...........................27
      5.1 The NXT Resource Record...............................27
      5.2 NXT RDATA Format......................................28
      5.3 Example...............................................28
      5.4 Interaction of NXT RRs and Wildcard RRs...............29
      5.5 Blocking NXT Pseudo-Zone Transfers....................30

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      5.6 Special Considerations at Delegation Points...........30

      6. The AD and CD Bits and How to Resolve Securely.........31
      6.1 The AD and CD Header Bits.............................31
      6.2 Boot File Format......................................32
      6.3 Chaining Through Zones................................33
      6.4 Secure Time...........................................34

      7. Operational Considerations.............................35
      7.1 Key Size Considerations...............................35
      7.2 Key Storage...........................................36
      7.3 Key Generation........................................36
      7.4 Key Lifetimes.........................................36
      7.5 Signature Lifetime....................................37
      7.6 Root..................................................37

      8. Conformance............................................38
      8.1 Server Conformance....................................38
      8.2 Resolver Conformance..................................38

      9. Security Considerations................................39

      Authors Addresses.........................................41
      Expiration and File Name..................................41

      Appendix: Base 64 Encoding................................42

Eastlake, Kaufman                                               [Page 4]

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1. Overview of Contents

   This draft describes 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 1034 and 1035.

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

   Section 3 discusses the KEY resource record, its structure, use in
   DNS responses, and file representation.  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, use in DNS responses, and file representation.  These
   resource records are used to authenticate other resource records in
   the DNS and optionally to authenticate DNS transactions.

   Section 5 discusses the NXT resource record and its use in DNS
   responses.  The NXT RR permits authenticated denial in the DNS of the
   existence of a name or of a particular 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 all
   combinations of security aware and security non-aware.  Two
   additional query header bits are defined for signaling between
   resolvers and servers.

   Section 7 reviews a variety of operational considerations including
   key generation, lifetime, and storage.

   Section 8 defines levels of conformance for resolvers and servers.

   Section 9 provides a few paragraphs on overall security

   An Appendix is provided that gives some details of base64 encoding
   which is used in the file representation of some RR's defined in this

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2.  Overview of the 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 authentication, described in Section 2.4 below.

   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

   In addition, no effort has been made to provide for any
   confidentiality for queries or responses.  (This service may be
   available via IPSEC. [put refs to IPSEC RFCs here if available])

2.2 Key Distribution

   Resource records (RRs) are defined to associate keys with DNS names.
   This permits the DNS to be used as a general public key distribution
   mechanism in support of the data origin authentication and
   transaction authentication DNS services as well as other security

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

   Under conditions described in Section 3, security aware DNS servers
   may automatically attempt to return KEY resources as additional
   information, along with those resource records actually requested, to
   minimize the number of queries needed.

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2.3 Data Origin Authentication and Integrity

   Authentication is provided by associating with resource records in
   the DNS cryptographically generated digital signatures.  Commonly,
   there will be a single private key that signs for an entire zone.  If
   a security aware resolver reliably learns the public key of the zone,
   it can verify, for any data read from that zone, that it was properly
   authorized and is reasonably current.  The expected implementation is
   for the zone private key to be kept off-line and used to re-sign all
   of the records in the zone periodically.

   This data origin authentication key belongs to the zone and not to
   the servers that store copies of the data.  That means compromise of
   a server or even all servers 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 can learn the public key of a zone either by reading it
   from DNS or by having it staticly configured.  To reliably learn the
   public key by reading it from DNS, the key itself must be signed.
   Thus, to provide a reasonable degree of security, the resolver must
   be configured with at least the public key of one zone that it can
   use to authenticate signatures.  From that, it can securely read the
   public keys of other zones if the intervening zones in the DNS tree
   are secure.  (It is in principle more secure to have the resolver
   manually configured with the public keys of multiple zones, since
   then the compromise of a single zone would not permit the faking of
   information from other zones.  It is also more administratively
   cumbersome, however, particularly when public keys change.)

   Adding data origin authentication and integrity requires no change to
   the "on-the-wire" DNS protocol beyond the addition of the signature
   resource types and, as a practical matter, the key resource type
   needed for key distribution.  This service can be supported by
   existing resolver and server implementations so long as they can
   support the additional resource types (see Section 8) with the one
   exception that CNAME referals can not be authenticated if they are
   from non-security aware servers (see Section 2.3.5).

   If signatures are always separately retrieved and verified when
   retrieving the information they authenticate, there will be more
   trips to the server and performance will suffer.  To avoid this,
   security aware servers mitigate that degradation by always sending
   the signature(s) needed.

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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 includes the type of the RR(s) being signed, the name
   of the signer, the time at which the signature was created, the time
   it expires (when it is no longer to be believed), its original time
   to live (which may be longer than its current time to live but cannot
   be shorter), the cryptographic algorithm in use, and the actual

   Every name in a secured zone will have associated with it at least
   one SIG resource record for each resource type under that name.  A
   security aware server supporting the performance enhanced version of
   the DNS protocol security extensions will attempt to return, with all
   records retrieved, the corresponding SIGs.  If a server does not
   support the protocol, the resolver must retrieve all the SIG records
   for a name and select the one or ones that sign the resource
   record(s) that resolver is interested in.

2.3.2 Authenticating Name and Type Non-existence

   The above security mechanism provides only a way to sign existing RRs
   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 types for the initial name in 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 field tick down when resource records are cached.

   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 time to live values undetected.  Instead, we include
   the "original" time-to-live in the signature and communicate that
   data in addition to the current time-to-live.  Unscrupulous servers
   under this scheme can manipulate the time to live but a security
   aware resolver will bound the TTL value it uses at the original
   signed value.  Separately, signatures include a time signed and an
   expiration time.  A resolver that knows the absolute time can

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   determine securely whether a signature has expired.  It is not
   possible to rely solely on the signature expiration as a substitute
   for the TTL, however, since non-security aware servers must still be

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 and signed by the
   zone's key.  But the operational DNS 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 will 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.

   In general, the KEY RR from the superzone is authoritative while for
   all other RRs, the data from the subzone is more authoritative.  To
   avoid conflicts, only the KEY RR in the superzone should be signed
   and the NS and any A (glue) RRs should only be signed in the subzone
   along with the SOA and any other RRs that have the zone name as
   owner.  The only exception is the NXT RR type 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 RRs

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

   In general, 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
   encountered in resolving a query.

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2.3.6 Signers Other Than The Zone

   There are two cases where a SIG resource record is signed by other
   than the zone private key.  One is for support of dynamic update
   where an entity is permitted to authenticate/update its own records.
   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 other is for support of transaction authentication
   as described in Section 2.4 below.

2.4 DNS Transaction Authentication

   The data origin authentication service described above protects
   resource records but provides no protection for DNS message headers.
   If header bits are falsely set by a 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, that the
   response is from the query it sent, and that these messages have not
   been diddled in transit.

   This is accomplished by optionally adding a special SIG resource
   record to the end of the reply which digitally signs the
   concatenation of the server's response and the resolver's query.  The
   private key used belongs to the host composing the reply, not to the
   zone being queried.  The corresponding public key is stored in and
   retrieved from the DNS.  Because 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.

   DNS level transaction authentication will be unnecessary when IPSEC
   end-to-end security protocol is generally available [refernce IPSEC
   RFCs when RFC numbers assigned].  However, there will be a
   significant time during which there will be systems on which it will
   be hard to add such a protocol but relatively easy to replace the DNS

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3. The KEY Resource Record

   The KEY resource record (RR) is used to document a key that is
   associated with a Domain Name System (DNS) name.  It will be a public
   key as only public keys are stored in the DNS.  This can be the
   public key of a zone, of a host or other end entity, or a user.  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 a name.

   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 meaning of the KEY RR owner name, flags, and protocol octet are
   described in Sections 3.2, 3.3 and 3.4 below respectively.  The flags
   and protocol must be examined before any following data as they
   control the format and even whether there is any following data.  The
   algorithm and public key fields are described in Section 3.5.  The
   format of the public key is algorithm dependent.

3.2 Object Types, DNS Names, and Keys

   The public key in a KEY RR belongs to the object named in the owner

   This DNS name may refer to up to three different things.  For
   example, dee.cybercash.com could be (1) a zone, (2) a host or other
   end entity , and (3) the mapping into a DNS name of the user or
   account dee@cybercash.com .  Thus, there are flags in the KEY RR to
   indicate with which of these roles the owner name and public key are
   associated as described below.

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   Although the same name can be used for up to all three of these
   contexts, such overloading of a name is discouraged.  It is also
   possible to use the same key for different things with the same name
   or even different names, but this is strongly discouraged.  In
   particular, the use of a zone key as a non-zone key will usually
   require that the private key be kept on line and thereby become much
   more vulnerable.

   It would be desirable for the growth of DNS to be managed so that
   additional possible simultaneous uses for names are NOT added.  New
   uses should be distinguished by exclusive domains.  For example, all
   IP autonomous system numbers have been mapped into the in-as.arpa
   domain [draft-ietf-dnssec-as-map-*.txt] and all telephone numbers in
   the world have been mapped into the tpc.int domain [RFC 1530].  This
   is much preferable to having the same name possibly be an autonomous
   system number, telephone number, and/or host as well as a zone and a

   In addition to the name type bits, there are additional control bits,
   the "no key" bit, the "experimental" bit, the "signatory" field,
   etc., as described below.

3.3 The KEY RR Flag Field

   In the "flags" field:

        Bit 0 is the "no key" bit.  If this bit is on, there is no key
   information and the RR stops after the algorithm octet.  By the use
   of this bit, a signed KEY RR can authenticatably assert that, for
   example, a zone is not secured.

        Bit 1 is the "experimental" bit.  It is ignored if the "no key"
   bit is on and the following description assumes the "no key" bit to
   be off.  Keys may be associated with zones, entities, or users for
   experimental, trial, or optional use, in which case this bit will be
   one.  If this bit is a zero, it means that the use or availability of
   security based on the key is "mandatory".  Thus, if this bit is off
   for a zone key, the zone should be assumed secured by SIG RRs and any
   responses indicating the zone is not secured should be considered
   bogus.  If this bit is a one for a host or end entity, it might
   sometimes operate in a secure mode and at other times operate without
   security.  The experimental bit, like all other aspects of the KEY
   RR, is only effective if the KEY RR is appropriately signed by a SIG
   RR.  The experimental bit must be zero for safe secure operation and
   should only be a one for a minimal transition period.

        Bits 2-4 are reserved and must be zero.

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        Bit 5 on indicates that this is a key associated with a "user"
   or "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 record: 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.domain could have a public key associated through a
   KEY RR with name j\.random_user.host.subdomain.domain.  It could be
   used in an security protocol where authentication of a user was
   desired.  This key would be useful in IP or other security for a user
   level service such a telnet, ftp, rlogin, etc.

        Bit 6 on 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].  This is the public
   key used in connection with the optional DNS transaction
   authentication service that can be used if the owner name is a DNS
   server host.  It could also be used in an IP-security protocol where
   authentication of a host was desired such as routing, NTP, etc.

        Bit 7 is the "zone" bit and indicates that this is a zone key
   for the zone whose name is the KEY RR owner name.  This is the
   fundamental type of DNS data origin authentication public key.

        Bit 8 is reserved to be the IPSEC bit and indicate that this key
   is valid for use in conjunction with the IP level 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 the IPSEC and entity bits on and experimental and no-
   key bits off is a guarantee that the host speaks IPSEC.

        Bit 9 is reserved to be the "email" bit and indicate that this
   key is valid for use in conjunction with MIME security multiparts.
   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.

        Bits 10-11 are reserved and must be zero.

        Bits 12-15 are the "signatory" field.  If non-zero, it indicates
   that the key can validly sign RRs of the same name.  If the owner
   name is a wildcard, then RRs with any name which is in the wildcard's
   scope can be signed.  Fifteen different non-zero values are possible
   for this field and any differences in their meaning are reserved for
   definition in connection with DNS dynamic update or other new DNS
   commands.  This field is meaningless for zone keys which always have
   authority to sign any RRs in the zone.  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.

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3.4 The Protocol Octet

   It is anticipated that keys stored in DNS will be used in conjunction
   with Internet protocols other than DNS (keys with zone bit or
   signatory field non-zero) and IPSEC (keys with IPSEC bit set).  The
   protocol octet is provided to indicate that a key is valid for such
   use and, for end entity keys or the host part of user keys, that the
   secure version of that protocol is implemented on that entity or

   Values between 1 and 191 decimal inclusive are available for
   assignment by IANA for such protocols.  The 63 values between 192 and
   254 inclusive will not be assigned to a specific protocol and are
   available for experimental use under bilateral agreement. Value 0
   indicates, for a particular key, that it is not valid for any
   particular additional protocol beyond those indicated in the flag
   field. And value 255 indicates that the key is valid for all assigned
   protocols (those in the 1 to 191 range).

   It is intended that new uses of DNS stored keys would initially be
   implemented, and operational experience gained, using the
   experimental range of the protocol octet.  If demand for widespread
   deployment for the indefinite future warrants, a value in the
   assigned range would then be designated for the protocol.  Finally,
   (1) should the protocol become so widespread in conjunction with
   other protocols which are indicated via the protocol octet and with
   which it shares key values that duplicate RRs are a serious burden
   and (2) should the protocol provide substantial facilities not
   available in any protocol for which a flags field bit has been
   allocated, then a flag field bit may be allocated to the protocol.
   Then a key can be simultaneously indicated as valid for that protocol
   and the entity or host can be simultaneously flagged as implementing
   the secure version of that protocol, along with other protocols for
   which flag field bits have been assigned.

   Note that the IPSEC protocol being developed may provide facilities
   adequate for all point to point IP communication meaning that
   additional flag field bits would only be assigned, when appropriate
   as indicated above, to protocols with a store-and-forward nature
   (other than DNS) or otherwise not subsumed into a point-to-point

3.5 The KEY Algorithm Number and the MD5/RSA Algorithm

   This octet is the key algorithm parallel to the same field for the
   SIG resource.  The MD5/RSA algorithm described in this draft is
   number 1. Numbers 2 through 252 are available for assignment should
   sufficient reason arise.  However, the designation of a new algorithm

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   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 shown in the packet diagram above will actually begin with
   an Object Identifier (OID) indicating the private algorithm in use
   and the remainder of the area is whatever is required by that
   algorithm. Number 253 is reserved for use where the date or other
   labeling fields of SIGs are desired withouth any actual security. For
   number 253, the public key area is null.  Values 0 and 255 are

   If the no key bit is zero and the algorithm field is 1, indicating
   the MD5/RSA algorithm, the public key field is structured as follows:

                        1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | pub exp length|        public key exponent                    /
   |                                                               /
   +-                           modulus                            /
   |                                                               /

   To promote interoperability, the exponent and modulus are each
   limited to 2552 bits in length.  The public key exponent is a
   variable length unsigned integer.  Its length in octets is
   represented as one octet if it is in the range of 1 to 255 and by a
   zero octet followed by a two octet unsigned length if it is longer
   than 255 bytes.  The public key modulus field is a multiprecision
   unsigned integer.  The length of the modulus can be determined from
   the RDLENGTH and the preceding RDATA fields including the exponent.
   Leading zero bytes are prohibited in the exponent and modulus.

3.6 Interaction of Flags, Algorithm, and Protocol Bytes

   Various combinations of the no-key bit, algorithm byte, protocol
   byte, and any protocol indicating flags (such as the reserved IPSEC
   flag) are possible.  (Note that the zone flag bit being on or the
   signatory field being non-zero is effectively a DNS protocol flag
   on.)  The meaning of these combinations is indicated below:

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      NK = no key flag
      AL = alogrithm byte
      PR = protocols indicated by protocol byte or protocol flags

      x represents any valid non-zero value.

       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.
        0   x   0   Illegal, claims key but has bad algorithm field.
        0   x   1   Specified protocols insecure, 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 and certifies that
                      those protocols are implemented with security.
        x   x   1   Algorithm not understood for protocol.

      (remember, in reference to the above table, that a protocol
       byte of 255 means all protocols with protocol bytes assigned)

3.7 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.

   Security aware DNS servers will include KEY RRs as additional
   information in responses where appropriate including the following:

   (1) On the retrieval of NS RRs, the zone key KEY RR(s) for the zone
   served by these name servers will be included as additional
   information.  If not all additional info will fit, the KEY RR(s) have
   higher priority than type A (or AAAA) glue RRs.

   (2) On retrieval of type A (or AAAA) RRs, the end entity KEY RR(s)
   will be included.  On inclusion of A (or AAAA) RRs as additional
   information, their KEY RRs will also be included but with lower
   priority than the relevant A (or AAAA) RRs.

3.8 File Representation of KEY RRs

   KEY RRs may appear as lines in a zone data file.

   The flag field, protocol,  and algorithm number octets are then
   represented as unsigned integers.  Note that if the "no key" flag is
   on in the flags or the algorithm specified is 253, nothing appears

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   after the algorithm octet.

   The remaining public key portion is represented in base 64 (see
   Appendix) 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 and then a modulus and with
   algorithm 254, there will be an OID followed by algorithm dependent

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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 other RRs of a particular type,
   class, and name and binds them 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.

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                     |
   |                         time signed                           |
   |         key footprint         |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         signer's name         /
   /                                                               /
   |                                                               /
   +-                          signature                           /
   /                                                               /

   The value of the SIG RR type is 24.

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

   The algorithm number is an octet specifying the digital signature
   algorithm used parallel to the algorithm octet for the KEY RR.  The
   MD5/RSA algorithm described in this draft is number 1.  Numbers 2
   through 252 are available for assignment should sufficient reason
   arise to allocate them.  However, the designation of a new algorithm
   could have a major impact on the interoperability of the global DNS
   systems and requires an IETF standards action.  Number 254 is

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   reserved for private use and will not be assigned a specific
   algorithm.  For number 254, the "signature" area shown above will
   actually begin with an Object Identifier (OID) indicating the private
   algorithm in use and the remainder of the signature area is whatever
   is required by that algorithm.  Number 253 is used when the time
   fields or other non-signature fields of the SIG are desired without
   any actual security.  For number 253, the signature field will be
   null.  Values 0 and 255 are reserved.

   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 helps optimize the determination
   of the original form reducing the effort in authenticating signed

   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 should be considered corrupt and ignored.
   The maximum number of labels possible 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. |

   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.  This implies that the RRs for a
   particular type need to all have the same TTL to start with.

   The SIG is valid until the "signature expiration" time which is an
   unsigned number of seconds since the start of 1 January 1970, GMT,

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   ignoring leap seconds.  (See also Section 4.4.)  SIG RRs should not
   have a date signed significantly in the future.  To prevent
   misordering of network request to update a zone dynamically,
   monotonically increasing "time signed" dates are necessary.

   The "time signed" field is an unsigned number of seconds since the
   start of 1 January 1970, GMT, ignoring leap seconds.

   A SIG RR with an expiration date before the date signed is

   The "key footprint" is a 16 bit quantity that is used to help
   efficiently select between multiple keys which may be applicable and
   as a quick check that a public key about to be used for the
   computationally expensive effort to check the signature is possibly
   valid.  Its exact meaning is algorithm dependent.  For the MD5/RSA
   algorithm, 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 this quantity
   in network order.

   The "signer's name" field is the domain name of the signer generating
   the SIG RR.  This is frequently the zone which contained the RR(s)
   being authenticated.  The signer's name may be compressed with
   standard DNS name compression when being transmitted over the

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

4.1.1 Signature Data

   Except for algorithm number 253 where it is null, the actual
   signature portion of the SIG RR binds the other RDATA fields to all
   of the "type covered" RRs with that owner name.  These covered RRs
   are thereby authenticated.  To accomplish this, a data sequence is
   constructed as follows:

        data = RDATA | RR(s)...

   where | is concatenation, RDATA is all the RDATA fields in the SIG RR
   itself before the signature, and RR(s) are all the RR(s) of the type
   covered with the same owner name and class as the SIG RR in canonical
   form and order.

   For purposes of DNS security, the canonical form for an RR is the RR
   with domain names (1) fully expanded (no name compression via
   pointers) and (2) all domain name letters set to lower case.

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   For purposes of DNS security, the canonical order for RRs is to sort
   them in ascending order by name, then by type, as left justified
   unsigned octet sequences in network (transmission) order where a
   missing octet sorts before a zero octet.  (See also ordering
   discussion in Section 5.1.)  Within any particular name and type they
   are similarly sorted by RDATA as a left justified unsigned octet
   sequence. EXCEPT that the type SIG RR(s) covering any particular type
   appear immediately after the other RRs of that type.  Thus if at name
   a.b there is one A RR and one KEY RR, their order with SIGs for
   concatenating the "data" to be signed would be as follows:

        a.b.  A ....
        a.b.  SIG A ...
        a.b.  KEY ...
        a.b.  SIG KEY ...

   (SIGs on type ANY should not be included in a zone.)

   How this data sequence is processed into the signature is algorithm

4.1.2 MD5/RSA Algorithm Signature Calculation

   For the MD5/RSA algorithm, the signature is as follows

      hash = MD5 ( data )

      signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n)

   where MD5 is the message digest algorithm documented in RFC 1321, "|"
   is concatenation, "e" is the secret key exponent of the signer, and
   "n" is the public modulus of the signer's public key.  01, FF, and 00
   are fixed octets of the corresponding hexadecimal value. "prefix" is
   the ASN.1 BER MD5 algorithm designator prefix specified in PKCS1,
   that is,
        hex 3020300c06082a864886f70d020505000410 [NETSEC].
   The FF octet is repeated the maximum number of times such that the
   value of the quantity being exponentiated is one octet shorter than
   the value of n.

   The above specifications are exactly Public Key Cryptographic
   Standard #1 [PKCS1].

   The size of n, including most and least significant bits (which will
   be 1) SHALL be not less than 512 bits and not more than 2552 bits.  n
   and e SHOULD be chosen such that the public exponent is small.

   Leading zeros bytes are not permitted in the MD5/RSA algorithm

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   A public exponent of 3 minimizes the effort needed to decode a
   signature.  Use of 3 as the public exponent may be weak for
   confidentiality uses since, if the same data can be collected
   encrypted under three different keys with an exponent of 3 then,
   using the Chinese Remainder Theorem, the original plain text can be
   easily recovered.  This weakness is not significant for DNS because
   we seek only authentication, not confidentiality.

4.1.3 Zone Transfer (AXFR) SIG

   The above SIG mechanisms assure the authentication of all zone signed
   RRs of a particular name, class and type.  However, to efficiently
   secure complete zone transfers, a SIG RR owned by the zone name must
   be created with a type covered of AXFR that covers all zone signed
   RRs other than the SIG AXFR itself.  It will be calculated by hashing
   together all other static zone RRs, including SIGs.  The RRs are
   ordered and concatenated for hashing as described in Section 4.1.1.
   (See also ordering discussion in Section 5.1.)

   The AXFR SIG must be calculated last of all zone key signed SIGs in
   the zone.  It really belongs to the zone as a whole, not to the zone
   name.  The AXFR SIG is only retrieved as part of a zone transfer.
   After validation of the AXFR SIG, the zone may be considered valid
   without verification of all the internal zone SIGs in the zone;
   however, any SIGs signed by entity keys or the like must still be
   validated.  The AXFR SIG is transmitted first in a zone transfer so
   the receiver can tell immediately that they may be able to avoid
   verifying other zone signed SIGs.

   Dynamic zone RRs which might be added by a dynamic zone update
   protocol and signed by an end entity or user key rather than a zone
   key (see Section 3.2) are not included in the AXFR SIG.  They
   originate in the network and will not, in general, be migrated to the
   recommended off line zone signing procedure (see Section 8.2).  Thus
   such dynamic RRs are not directly signed by the zone, are not
   included in the AXFR SIG, and are not generally protected against
   omission from zone transfers.

4.1.4 Transaction SIGs

   A response message from a security aware server may optionally
   contain a special SIG as the last item in the additional information
   section to authenticate the transaction.

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   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 4.1.2) of the
   entire preceding DNS reply message, including DNS header,
   concatenated with the entire DNS query message that produced this
   response, including the query's DNS header.  That is

        data = full response (less final 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.

4.2 SIG RRs in the Construction of Responses

   Security aware servers MUST, for every authoritative 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 RR is placed in a response, its SIG RR has a higher
      priority for inclusion than other RRs that may need to be
      included.  If space does not permit its inclusion, the response
      MUST be considered truncated.

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

   Sending SIGs to authenticate non-authoritative data (glue records and
   NS RRs for subzones) is unnecessary and must be avoided.  Note that
   KEYs for subzones are authoritative.

   If a SIG covers any RR that would be in the answer section of the
   response, its automatic inclusion MUST be 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.

   Optionally, DNS transactions may be authenticated by a SIG RR at the
   end of the response in the additional information section (Section
   4.1.4).  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 can be zero.  To

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   conserve space, the owner name SHOULD be root (a single zero octet).
   If transaction authentication is desired, that SIG RR must be
   considered higher priority for inclusion than any other RR in the

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 what it
      believes to be a security aware server via a communication path
      that it believes to be secure with the AD bit (see Section 6.1)
      set, MAY choose to accept the RRs as received without verifying
      the 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, at least in the case of
      getting a response from an insecure server.  (As explained in 4.2
      above, it will not be possible to secure CNAMEs being served up by
      non-secure resolvers.)

      NOTE: Implementors 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 reasonable 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 a set of
   RRs are invalid, then the set of RR(s) 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 can authenticate RRs.  If a
   resolver does not implement transaction SIGs, it MUST at least ignore
   them without error.

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

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4.4 Signature Expiration, TTLs, and Validity

   Security aware servers must not consider SIG RRs to be authentic
   after their expiration time and not consider any RR to be
   authenticated after its signatures have expired.  Within that
   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.  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 signature
   expiration time.  Thus, in general, the TTL on an transmitted RR
   would be


4.5 File Representation of SIG RRs

   A SIG RR can be represented as a single logical line in a zone data
   file [RFC1033] but there are some special considerations as described
   below.  (It does not make sense to include a transaction
   authenticating SIG RR in a file as it is a transient authentication
   that covers data including an ephemeral transaction number so it must
   be calculated in real time by the DNS server.)

   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 and algorithm fields appear as unsigned integers.

   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.

   The "labels" field does not appear in the file representation as it
   can be calculated from the owner name.

   The key footprint appears as an unsigned decimal number.

   However, the signature itself can be very long.  It is the last data
   field and is represented in base 64 (see Appendix) 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

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   standard parenthesis.

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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
   immediately clear how to authenticatably 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 and
   its SIG 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.  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 type to a security aware server serving
   the zone will result in an reply containing at least one signed RR.

   NXT RRs do not appear in zone master files since they can be derived
   from the rest of the zone.

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 zone signed type RRs 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 generally 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

   The ordering is to sort labels as unsigned left justified octet
   strings where the absence of a octet sorts before a zero 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.
   Since we are talking about 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.

   There is a problem with the last NXT in a zone as it wants to have an
   owner name which is the last existing name in sort 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

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   the last NXT.

   There are special considerations due to interaction with wildcards as
   explained below.

   The NXT RRs for a zone should be automatically calculated and added
   to the zone by the same recommended off-line process that signs the
   zone.  The NXT RR's TTL should not exceed the zone minimum TTL.

5.2 NXT RDATA Format

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

   The type number for the NXT RR is 30.

                        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 is one bit per RR type present for the owner
   name similar to the WKS socket bit map.  The first bit represents RR
   type zero (an illegal type which should not be present.) 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.  The NXT bit map
   should be printed as a list of type mnemonics or decimal numbers
   similar to the WKS RR.

   The size of the bit map can be inferred from the RDLENGTH and the
   length of the next domain name.

5.3 Example

   Assume zone foo.tld has entries for


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

        big.foo.tld. NXT medium.foo.tld. A MX SIG NXT
        big.foo.tld. SIG NXT 1 3 ( ;type-cov=NXT, alg=1, labels=3
                         19960102030405 ;signature expiration
                         19951211100908 ;time signed
                         2143658709     ;key footprint
                         foo.tld.       ;signer
         1tVfSCSqQYn6//11U6Nld80jEeC8aTrO+KKmCaY= ;signature (640 bits)

   Note that this response implies that big.foo.tld 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.tld, which is a non-existent name.

5.4 Interaction of NXT RRs and Wildcard RRs

   Since, in some sense, a wildcard RR causes all possible names in an
   interval to exist, there should not be an NXT RR that would cover any
   part of this interval.  Thus if *.X.ZONE exists you would expect an
   NXT RR that ends at X.ZONE and one that starts with the last name
   covered by *.X.ZONE.  However, this "last name covered" is something
   very ugly and long like \255\255\255....X.zone.  So the NXT for the
   interval following is simply given the owner name *.X.ZONE.  This "*"
   type name is not expanded when the NXT is returned as authority
   information in connection with a query for a non-existent name.

   If there could be any wildcard RRs in a zone and thus wildcard NXTs,
   care must be taken in interpreting the results of explicit NXT
   retrievals as the owner name may be a wildcard expansion.

   The existence of one or more wildcard RRs covering a name interval
   makes it possible for a malicious server to hide any more specificly
   named RRs in the internal.  The server can just falsely return the
   wildcard match NXT instead of the more specificly named RRs.  If
   there is a zone wide wildcard, there will be an NXT RR whose owner
   name is the wild card and whose RDATA is the zone name.  In this case
   a server could conceal the existence of any more specific RRs in the
   zone.  (It would be possible to design a more strict NXT feature
   which would eliminate this possibility.  But it would be more complex
   and might be so constraining as to make any dynamic update feature
   that could create new names very difficult (see Section 3.2).)

   What name should be put into the RDATA of an NXT RR with an owner

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   name that is within a wild card scope?  Since the "next" existing
   name will be one that matches the wild card, the wild card name
   should be used.  Inclusion of such NXTs within a wild card scope is

5.5 Blocking NXT Pseudo-Zone Transfers

   In a secure zone, a resolver can query for the initial NXT associated
   with the zone name.  Using the next domain name RDATA field from that
   RR, it can query for the next NXT RR.  By repeating this, it can walk
   through all the NXTs in the zone.  If there are no wildcards, it can
   use this technique to find all names in a zone. If it does type ANY
   queries, it can incrementally get all information in the zone and
   perhaps defeat attempts to administratively block zone transfers.

   If there are any wildcards, this NXT walking technique will not find
   any more specific RR names in the part of the name space the wildcard
   covers.  By doing explicit retrievals for wildcard names, a resolver
   could determine what intervals are covered by wildcards but still
   could not, with these techniques, find any names inside such
   intervals except by trying every name.

   If it is desired to block NXT walking, the recommended method is to
   add a zone wide wildcard of the KEY type with the no key bit on and
   with no type (zone, entity, or user) bit on.  This will cause there
   to be one zone covering NXT RR and leak no information about what
   real names exist in the zone.  This protection from pseudo-zone
   transfers is bought at the expense of eliminating the data origin
   authentication of the non-existence of names that NXT RRs can
   provide.  If an entire zone is covered by a wildcard, a malicious
   server can return an RR produced by matching the resulting wildcard
   NXT and can thus hide all the real data and delegations with more
   specific names in the zone.

5.6 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 and next domain name fields.  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.

   Insecure servers will never automatically return an NXT and may only
   return the NXT from the subzone on explicit queries.

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

   Retrieving or resolving authentic data from the Domain Name System
   (DNS) involves starting with one or more trusted public keys. With
   trusted keys, a resolver willing to perform cryptography can progress
   securely through the secure DNS zone 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 a starting point.

   Data stored at a server needs security labels of Authenticated,
   Pending, or Insecure. There is also a fourth transient state of Bad
   which indicates that 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 to a KEY
   configured at the resolver via its boot file.  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 because it is in
   a zone with no key or an experimental key.  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.

   In getting to the data of interest to respond to a query, a secure
   resolver may encounter genuine non-secure zones.  It may proceed
   through such zones but should report this as described in Section

6.1 The AD and CD Header Bits

   Two 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 has been verified by the server providing it.  The CD
   (checking disabled) bit indicates in a query that non-verified data
   is acceptable to the resolver sending the query.

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

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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 data.  Of course security aware
   resolvers can not trust the AD bit unless they trust the server they
   are talking to and have a secure path to it.

   Any security aware resolver willing to do cryptography should assert
   the CD bit on all queries to reduce DNS latency time by allowing
   security aware servers to answer before they have resolved the
   validity of data.

   Security aware servers never return Bad data.  For non-security aware
   resolvers or security aware resolvers requesting service by having
   the CD bit clear, security aware servers return only Authenticated or
   Insecure data with the AD bit set in the response.  Security aware
   resolvers will know that if data is Insecure versus Authentic by the
   absence of SIG RRs.  Security aware servers may return Pending data
   to security aware resolvers requesting the service by clearing the AD
   bit in the response.  The AD bit may only be set on a response if the
   RRs in the response are either Authenticated or Insecure.

6.2 Boot File Format

   The format for a boot file directive to configure a starting zone key
   is as follows:

        pubkey name flags protocol algorithm key-data

   for a public key.  "name" is the owner name (if the line is
   translated into a KEY RR).  Flags indicates the type of key and is

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   the same as the flag octet in the KEY RR.  Algorithm is the algorithm
   in use where one is the MD5/RSA algorithm and 254 indicates a private
   algorithm.  The material after the algorithm is algorithm dependent
   and, for private algorithms, starts with the algorithm's identifying
   OID.  If the "no key" bit is on in flags or the algorithm is
   specified as 253, then the key-data after algorithm  may be omitted.
   It is treated as an octet stream and encoded in base 64 (see

   A file of keys for cross certification or other purposes can be
   configured though the keyfile directive as follows:

        keyfile filename

   The file looks like a master file except that it can only contain KEY
   and SIG RRs with the SIGs signed under a key configured with the
   pubkey directive.

   While it might seem logical for everyone to start with the key for
   the root zone, this has problems.  The logistics of updating every
   DNS resolver in the world when the root key changes would be
   excessive.  It may be some time before there even is a root key.
   Furthermore, many organizations will explicitly wish their "interior"
   DNS implementations to completely trust only their own zone.  These
   interior resolvers can then go through the organization's zone server
   to access data outsize the organization's domain.

6.3 Chaining Through Zones

   Starting with one or more trusted keys for a zone, it should be
   possible to retrieve signed keys for its subzones which have a key
   and, if the zone is not root, for some superzone. Every authoritative
   secure zone server should also include the KEY RR for a super-zone
   signed by the secure zone via a keyfile directive. 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 appearing with the NS RRs
   for the sub-zone.  These make it possible to descend within the tree
   of zones.

   A resolver should keep track of the number of successive secure zones
   traversed from a 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.  Data configured via a boot file directive
   should be given a distance number of zero.  Should a query encounter
   different data for the same query with different distance values,
   that with a larger value should be ignored.

   A security conscious resolver should completely refuse to step from a

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   secure zone into a non-secure zone unless the non-secure zone is
   certified to be non-secure, or only experimentally secure, by the
   presence of an authenticated KEY RR for the non-secure zone with a no
   key flag or the presence of a KEY RR with the experimental bit set.
   Otherwise the resolver is probably getting completely bogus or
   spoofed data.

   If legitimate non-secure 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 non-secure 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 512 or more as this exceeds the
   largest possible distance through secure zones in the DNS.
   Nevertheless, continuing to apply secure checks within "secure" zones
   reached via non-secure zones will, as a practical matter, provide
   some increase in security.

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, RFC1305).  If such protocols are used,
   they should 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 and KEY RRs which were valid but no longer

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7. Operational Considerations

   This section discusses a variety of considerations in secure
   operation of the Domain Name System (DNS) using these proposed
   protocol extensions.

7.1 Key Size Considerations

   There are a number of factors that effect public key size choice for
   use in the DNS security extension.  Unfortunately, these factors
   usually do not all point in the same direction.  Choice of zone key
   size should generally be made by the zone administrator depending on
   their local conditions.

   For most schemes, larger keys are more secure but slower.  Given a
   small public exponent, verification (the most common operation) for
   the MD5/RSA algorithm will vary roughly with the square of the
   modulus length, signing will vary with the cube of the modulus
   length, and key generation (the least common operation) will vary
   with the fourth power of the modulus length.  The current best
   algorithms for factoring a modulus and breaking RSA security vary
   roughly with the square of the modulus itself.  Thus going from a 640
   bit modulus to a 1280 bit modulus only increases the verification
   time by a factor of 4 but increases the work factor of breaking the
   key by over 2^3000.  [RSA FAQ] An upper bound of 2552 bit has been
   established for the MD4/RSA DNS security algorithm for
   interoperability purposes.

   However, larger keys increase the size of the KEY and SIG RRs.  This
   increases the chance of DNS UDP packet overflow and the possible
   necessity for using higher overhead TCP in responses.

   The recommended minimum RSA algorithm modulus size, 640 bits, is
   believed by the authors to be secure at this time and for some years
   but high level zones in the DNS tree may wish to set a higher
   minimum, perhaps 1000 bits, for security reasons.  (Since the United
   States National Security Agency generally permits export of
   encryption systems using an RSA modulus of up to 512 bits, use of
   that small a modulus, i.e. n, must be considered weak.)

   For a key used only to secure data and not to secure other keys, 640
   bits should be entirely adequate.

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7.2 Key Storage

   It is strongly recommended that zone private keys and the zone file
   master copy be kept and used in off-line non-network connected
   physically secure machines only.  Periodically an application can be
   run to re-sign the RRs in a zone by adding NXT and SIG RRs.  Then the
   augmented file can be transferred, perhaps by sneaker-net, to the
   networked zone primary server machine.

   The idea is to have a one way information flow to the network to
   avoid the possibility of tampering from the network.  Keeping the
   zone master file on-line on the network and simply cycling it through
   an off-line signer does not do this.  The on-line version could still
   be tampered with if the host it resides on is compromised.  For
   maximum security, the master copy of the zone file should be off net
   and should not be updated based on an unsecured network mediated

   Note, however, that secure resolvers need to be configured with some
   trusted on-line public key information (or a secure path to such a

   Non-zone private keys, such as host or user keys, may have to be kept
   on line to be used for real-time purposes such as DNS transaction
   security, IPSEC session set-up, or secure mail.

7.3 Key Generation

   Careful key generation is a sometimes overlooked but absolutely
   essential element in any cryptographically secure system.  The
   strongest algorithms used with the longest keys are still of no use
   if an adversary can guess enough to lower the size of the likely key
   space so that it can be exhaustively searched.  Suggestions will be
   found in RFC 1750.

   It is strongly recommended that key generation also occur off-line,
   perhaps on the machine used to sign zones (see Section 7.2).

7.4 Key Lifetimes

   No key should be used forever.  The longer a key is in use, the
   greater the probability that it will have been compromised through
   carelessness, accident, espionage, or cryptanalysis.  Furthermore, if
   key rollover is a rare event, there is an increased risk that, when
   the time does come up change the key, no one at the site will
   remember how to do it or other problems will have developed in the

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   While key lifetime is a matter of local policy, these considerations
   suggest that no zone key should have a lifetime significantly over
   four years.  A reasonable maximum lifetime for zone keys that are
   kept off-line and carefully guarded is 13 months with the intent that
   they be replaced every year.  A reasonable maximum lifetime for end
   entity keys that are used for IP-security or the like and are kept on
   line is 36 days with the intent that they be replaced monthly or more
   often.  In some cases, an entity key lifetime of somewhat over a day
   may be reasonable.

7.5 Signature Lifetime

   Signature expiration times must 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 badness.

   It is recommended that signature lifetime be a small multiple of the
   TTL but not less than a reasonable re-signing interval.

7.6 Root

   It should be noted that in DNS the root is a 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, .edu, or .arpa.
   Implementations MAY reject as bogus any purported root signature of
   records with a name more than one level below root.

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8. Conformance

   Several levels of server and resolver conformance are defined.

8.1 Server Conformance

   Two levels of server conformance are defined as follows:

        Minimal server compliance is the ability to store and retrieve
   (including zone transfer) SIG, KEY, and NXT RRs.  Any secondary,
   caching, or other server for a secure zone must be at least minimally
   compliant and even then some things, such as secure CNAMEs, will not

        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 completely successful secure operation, all
   secondary, caching, and other servers handling the zone should be
   fully compliant as well.

8.2 Resolver Conformance

   Two levels of resolver compliance are defined:

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

        A fully compliant resolver (1) understands KEY, SIG, and NXT
   RRs, (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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9. Security Considerations

   This document concerns technical details of extensions to the Domain
   Name System (DNS) protocol to provide data integrity and origin
   authentication, public key distribution, and optional transaction

   If 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 other
   facets of the Internet.


   [NETSEC] - Network Security: PRIVATE Communications in a PUBLIC
   World, Charlie Kaufman, Radia Perlman, & Mike Speciner, Prentice Hall
   Series in Computer Networking and Distributed Communications 1995.

   [PKCS1] - PKCS #1: RSA Encryption Standard, RSA Data Security, Inc.,
   3 June 1991, Version 1.4.

   [RFC1032] - Domain Administrators Guide, M. Stahl, November 1987

   [RFC1033] - Domain Administrators Operations Guide, M. Lottor,
   November 1987

   [RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
   November 1987

   [RFC1035] - Domain Names - Implementation and Specifications

   [RFC1305] - Network Time Protocol (v3), D. Mills, April 9 1992.

   [RFC1321] - The MD5 Message-Digest Algorithm, R. Rivest, April 16

   [RFC1530] - Principles of Operation for the TPC.INT Subdomain:
   General Principles and Policy, C. Malamud, M. Rose, October 6 1993.

   [RFC1750] - Randomness Requirements for Security, D. Eastlake, S.
   Crocker, J. Schiller, December 1994.

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

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Authors Addresses

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

   Telephone:   +1 508 287 4877
   EMail:       dee@cybercash.com

   Charles W. Kaufman
   Iris Associates
   1 Technology Park Drive
   Westford, MA 01886 USA

   Telephone:   +1 508-392-5276
   EMail:       charlie_kaufman@iris.com

Expiration and File Name

   This draft expires 10 April 1995.

   Its file name is draft-ietf-dnssec-secext-06.txt.

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Appendix: Base 64 Encoding

   The following encoding technique is taken from RFC 1521 by Borenstein
   and Freed.  It is reproduced here in an edited form for convenience.

   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 base64 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 Base64 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
   base64 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 an integral multiple of 4 characters with no "=" padding, (2)

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   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.

Eastlake, Kaufman                                              [Page 43]