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rfc:rfc2535

Network Working Group D. Eastlake Request for Comments: 2535 IBM Obsoletes: 2065 March 1999 Updates: 2181, 1035, 1034 Category: Standards Track

               Domain Name System Security Extensions

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

 Extensions to the Domain Name System (DNS) are described that provide
 data integrity and authentication to security aware resolvers and
 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
 algorithms.
 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.

Eastlake Standards Track [Page 1] RFC 2535 DNS Security Extensions March 1999

Acknowledgments

 The significant contributions and suggestions of the following
 persons (in alphabetic order) to DNS security are gratefully
 acknowledged:
    James M. Galvin
    John Gilmore
    Olafur Gudmundsson
    Charlie Kaufman
    Edward Lewis
    Thomas Narten
    Radia J. Perlman
    Jeffrey I. Schiller
    Steven (Xunhua) Wang
    Brian Wellington

Table of Contents

 Abstract...................................................1
 Acknowledgments............................................2
 1. Overview of Contents....................................4
 2. Overview of the DNS Extensions..........................5
 2.1 Services Not Provided..................................5
 2.2 Key Distribution.......................................5
 2.3 Data Origin Authentication and Integrity...............6
 2.3.1 The SIG Resource Record..............................7
 2.3.2 Authenticating Name and Type Non-existence...........7
 2.3.3 Special Considerations With Time-to-Live.............7
 2.3.4 Special Considerations at Delegation Points..........8
 2.3.5 Special Considerations with CNAME....................8
 2.3.6 Signers Other Than The Zone..........................9
 2.4 DNS Transaction and Request Authentication.............9
 3. The KEY Resource Record................................10
 3.1 KEY RDATA format......................................10
 3.1.1 Object Types, DNS Names, and Keys...................11
 3.1.2 The KEY RR Flag Field...............................11
 3.1.3 The Protocol Octet..................................13
 3.2 The KEY Algorithm Number Specification................14
 3.3 Interaction of Flags, Algorithm, and Protocol Bytes...15
 3.4 Determination of Zone Secure/Unsecured Status.........15
 3.5 KEY RRs in the Construction of Responses..............17
 4. The SIG Resource Record................................17
 4.1 SIG RDATA Format......................................17
 4.1.1 Type Covered Field..................................18
 4.1.2 Algorithm Number Field..............................18
 4.1.3 Labels Field........................................18
 4.1.4 Original TTL Field..................................19

Eastlake Standards Track [Page 2] RFC 2535 DNS Security Extensions March 1999

 4.1.5 Signature Expiration and Inception Fields...........19
 4.1.6 Key Tag Field.......................................20
 4.1.7 Signer's Name Field.................................20
 4.1.8 Signature Field.....................................20
 4.1.8.1 Calculating Transaction and Request SIGs..........21
 4.2 SIG RRs in the Construction of Responses..............21
 4.3 Processing Responses and SIG RRs......................22
 4.4 Signature Lifetime, Expiration, TTLs, and Validity....23
 5. Non-existent Names and Types...........................24
 5.1 The NXT Resource Record...............................24
 5.2 NXT RDATA Format......................................25
 5.3 Additional Complexity Due to Wildcards................26
 5.4 Example...............................................26
 5.5 Special Considerations at Delegation Points...........27
 5.6 Zone Transfers........................................27
 5.6.1 Full Zone Transfers.................................28
 5.6.2 Incremental Zone Transfers..........................28
 6. How to Resolve Securely and the AD and CD Bits.........29
 6.1 The AD and CD Header Bits.............................29
 6.2 Staticly Configured Keys..............................31
 6.3 Chaining Through The DNS..............................31
 6.3.1 Chaining Through KEYs...............................31
 6.3.2 Conflicting Data....................................33
 6.4 Secure Time...........................................33
 7. ASCII Representation of Security RRs...................34
 7.1 Presentation of KEY RRs...............................34
 7.2 Presentation of SIG RRs...............................35
 7.3 Presentation of NXT RRs...............................36
 8. Canonical Form and Order of Resource Records...........36
 8.1 Canonical RR Form.....................................36
 8.2 Canonical DNS Name Order..............................37
 8.3 Canonical RR Ordering Within An RRset.................37
 8.4 Canonical Ordering of RR Types........................37
 9. Conformance............................................37
 9.1 Server Conformance....................................37
 9.2 Resolver Conformance..................................38
 10. Security Considerations...............................38
 11. IANA Considerations...................................39
 References................................................39
 Author's Address..........................................41
 Appendix A: Base 64 Encoding..............................42
 Appendix B: Changes from RFC 2065.........................44
 Appendix C: Key Tag Calculation...........................46
 Full Copyright Statement..................................47

Eastlake Standards Track [Page 3] RFC 2535 DNS Security Extensions March 1999

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
 considerations.
 Section 11 specified IANA considerations for allocation of additional
 values of paramters defined in this document.

Eastlake Standards Track [Page 4] RFC 2535 DNS Security Extensions March 1999

 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 memo and RFC 2065.
 Appendix C specified how to calculate the simple checksum used as a
 key tag in most SIG RRs.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

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
 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
 inquirers.
 No effort has been made to provide for any confidentiality for
 queries or responses.  (This service may be available via IPSEC [RFC
 2401], 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.

Eastlake Standards Track [Page 5] RFC 2535 DNS Security Extensions March 1999

 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.

2.3 Data Origin Authentication and Integrity

 Authentication is provided by associating with resource record sets
 (RRsets [RFC 2181]) 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 is properly authorized.  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 [RFC
 2541].
 The data origin authentication key(s) are associated with the zone
 and not with 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 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).

Eastlake Standards Track [Page 6] RFC 2535 DNS Security Extensions March 1999

 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).

2.3.1 The SIG Resource Record

 The syntax of a SIG resource record (signature) is described in
 Section 4.  It cryptographicly 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
 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 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

Eastlake Standards Track [Page 7] RFC 2535 DNS Security Extensions March 1999

 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-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 manager.
 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. This will normally appear in the
 subzone and may also be included in the superzone.  But, 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 with the superzone signature in 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 subzone KEY RR should be
 signed in the superzone if it appears there. 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 SIG, 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.

Eastlake Standards Track [Page 8] RFC 2535 DNS Security Extensions March 1999

 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.  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 the signer in a SIG resource record is other
 than one of the private key(s) 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] and the zone is operating
 in a mode where the zone key is not on line. The public key of the
 entity must be present in the DNS and be signed by a zone level key
 but the other RR(s) may be signed with the entity's key.
 A second case is support of transaction and request authentication as
 described in Section 2.4.
 In additions, signatures can be included on resource records within
 the DNS for use by applications other than DNS. DNS related
 signatures authenticate that data originated with the authority of a
 zone owner or that a request or transaction originated with the
 relevant entity. Other signatures can provide other types of
 assurances.

2.4 DNS Transaction and Request Authentication

 The data origin authentication service described above protects
 retrieved resource records and the non-existence of 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.

Eastlake Standards Track [Page 9] RFC 2535 DNS Security Extensions March 1999

 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 produce error returns or may even be ignored by
 many of them. However, this syntax for signing requests is defined as
 a way of authenticating secure dynamic update requests [RFC 2137] or
 future requests requiring authentication.
 The private keys used in transaction security belong to the entity
 composing the reply, not to the zone involved.  Request
 authentication may also involve the private key of the host or other
 entity composing the request or other private keys depending on the
 request authority it is sought to establish. The corresponding public
 key(s) are normally stored in and retrieved from the DNS for
 verification.
 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.

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. Security
 aware DNS implementations MUST be designed to handle at least two
 simultaneously valid keys of the same type associated with the same
 name.
 The type number for the KEY RR is 25.
 A KEY RR is, like any other RR, authenticated by a SIG RR.  KEY RRs
 must be signed by a zone level key.

3.1 KEY RDATA format

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

Eastlake Standards Track [Page 10] RFC 2535 DNS Security Extensions March 1999

                      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 has been developed for that purpose, defined in [RFC
 2538].
 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(s) do 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.
 A DNS name may refer 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:
   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 whose values have the following
 meanings:

Eastlake Standards Track [Page 11] RFC 2535 DNS Security Extensions March 1999

         10: Use of the key is prohibited for authentication.
         01: Use of the key is prohibited for confidentiality.
         00: 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 below.
 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. Field values
 have the following meanings:
         00: 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 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.
         01: 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.
         10: 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

Eastlake Standards Track [Page 12] RFC 2535 DNS Security Extensions March 1999

             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.  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.
         11: 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 things as specified in DNS
            dynamic update [RFC 2137].  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.

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:
      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 for use in connection with 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 2401] protocol
        and indicates that this key is valid for use in conjunction

Eastlake Standards Track [Page 13] RFC 2535 DNS Security Extensions March 1999

        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 flag bits are set.  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
 assigned:
 VALUE   Algorithm
   0      - reserved, see Section 11
   1     RSA/MD5 [RFC 2537] - recommended
   2     Diffie-Hellman [RFC 2539] - optional, key only
   3     DSA [RFC 2536] - MANDATORY
   4     reserved for elliptic curve crypto
 5-251    - available, see Section 11
 252     reserved for indirect keys
 253     private - domain name (see below)
 254     private - OID (see below)
 255      - reserved, see Section 11
 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.
 Algorithm number 252 indicates an indirect key format where the
 actual key material is elsewhere.  This format is to be defined in a
 separate document.
 Algorithm numbers 253 and 254 are reserved for private use and will
 never be assigned a specific algorithm.  For number 253, the public
 key area and the signature begin with a wire encoded domain name.
 Only local domain name compression is permitted.  The domain name
 indicates the private algorithm to use and the remainder of the
 public key area is whatever is required by that algorithm.  For
 number 254, the public key area for the KEY RR and the signature
 begin with an unsigned length byte followed by a BER encoded Object

Eastlake Standards Track [Page 14] RFC 2535 DNS Security Extensions March 1999

 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.  Entities should only use domain names
 and OIDs they control to designate their private algorithms.
 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.

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).
  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   Gives key but no protocols to use it.
   x   0   1   Denies key for specific algorithm.
   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 key type flag
 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.  The secured versus unsecured status of a zone may vary with
 different cryptographic algorithms.  Even for the same algorithm,
 conflicting zone KEY RRs may 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.

Eastlake Standards Track [Page 15] RFC 2535 DNS Security Extensions March 1999

 For any particular algorithm, 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 and algorithm, every trusted zone KEY RR for the
    zone says there is no key for that zone, it is unsecured for that
    algorithm.
 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 for the algorithm.  Both authenticated and
    non-authenticated RRs for it should be accepted by the resolver.
 3. If every trusted zone KEY RR that the zone and algorithm has is
    key specifying, then it is secure for that algorithm and only
    authenticated RRs from it will be accepted.
 Examples:
 (1)  A resolver initially trusts only signatures by the superzone of
 zone Z within the DNS hierarchy.  Thus it will look only at the KEY
 RRs that are signed by 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.example.
 When considering data from zone Z, it may be signed by the superzone
 of Z, by cert-auth.example, 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 . e x a m p l e
      KEY RRs|   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   |

Eastlake Standards Track [Page 16] RFC 2535 DNS Security Extensions March 1999

   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 (perhaps just a zone key) SHOULD be included as additional
 information if space is available. 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.

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 [RFC 2181] 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 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.

Eastlake Standards Track [Page 17] RFC 2535 DNS Security Extensions March 1999

                         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                          /
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.1.1 Type Covered Field

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

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.

Eastlake Standards Track [Page 18] RFC 2535 DNS Security Extensions March 1999

 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 generaly implies
 that all RRs for a particular type, name, and class, that is, all the
 RRs in any particular RRset, must have the same TTL to start with.

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 68 years in the past or the future.  This means that these
 times are ambiguous modulo ~136.09 years.  However there is no
 security flaw because keys are required to be changed to new random
 keys by [RFC 2541] at least every five years.  This means that the
 probability that the same key is in use N*136.09 years later should
 be the same as the probability that a random guess will work.
 A SIG RR may have an expiration time numerically less than the
 inception time if the expiration time is near the 32 bit wrap around
 point and/or the signature is long lived.
 (To prevent misordering of network requests to update a zone
 dynamically, monotonically increasing "signature inception" times may
 be necessary.)
 A secure zone must be considered changed for SOA serial number
 purposes not only when its data is updated but also when new SIG RRs
 are inserted (ie, the zone or any part of it is re-signed).

Eastlake Standards Track [Page 19] RFC 2535 DNS Security Extensions March 1999

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 2537], 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 least
 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 name 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.

4.1.8 Signature Field

 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 the wire format of all the RDATA fields in the SIG RR itself
 (including the canonical form of the signer's 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).

Eastlake Standards Track [Page 20] RFC 2535 DNS Security Extensions March 1999

 SIGs SHOULD NOT be included in a zone for any "meta-type" such as
 ANY, AXFR, etc. (but see section 5.6.2 with regard to IXFR).

4.1.8.1 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
 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 some old
 DNS servers to give an error return or ignore a query.  However, such
 SIGs may in the future be 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 RRset the
 query will return, attempt to send the available SIG RRs which
 authenticate the requested RRset.  The following rules apply to the
 inclusion of SIG RRs in responses:

Eastlake Standards Track [Page 21] RFC 2535 DNS Security Extensions March 1999

   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
      the SIG RR with the additional information.
   3. SIGs to authenticate glue records and NS RRs for subzones at a
      delegation point are unnecessary and MUST NOT be sent.
   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
      section.
   5. Optionally, DNS transactions may be authenticated by a SIG RR at
      the end of the response in the additional information section
      (Section 4.1.8.1).  Such SIG RRs are signed by the DNS server
      originating the response.  Although the signer field MUST be a
      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
      inclusion.

4.3 Processing Responses and SIG RRs

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

Eastlake Standards Track [Page 22] RFC 2535 DNS Security Extensions March 1999

      getting a response from a 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 directly 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
 directly authenticate RRs, depending on resolver policy (see Section
 6).  If a resolver does not implement transaction and/or request
 SIGs, it MUST 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 (see also 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 constraints, 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

Eastlake Standards Track [Page 23] RFC 2535 DNS Security Extensions March 1999

 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 be
    min(authExpTim,max(zoneMinTTL,min(originalTTL,currentTTL)))
 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 mean a long time to
 flush any bad data or signatures that may have been generated.
 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.

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 it is 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.

Eastlake Standards Track [Page 24] RFC 2535 DNS Security Extensions March 1999

 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. Thus the NXT RRs in a zone
 create a chain of all of the literal owner names in that zone,
 including unexpanded wildcards but omitting the owner name of glue
 address records unless they would otherwise be included. This implies
 a canonical ordering of all domain names in a zone as described in
 Section 8. 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.
 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.
 The type number for the NXT RR is 30.
 NXT RRs are only signed by zone level keys.

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.  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
 is not used if there exists an RR with a type number greater than

Eastlake Standards Track [Page 25] RFC 2535 DNS Security Extensions March 1999

 127.  If the zero bit of the type bit map is a one, it indicates that
 a different format is being used which will always be the case if a
 type number greater than 127 is present.
 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
 complex.
 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
 also prove that there wasn't a wildcard whose expansion should have
 been returned. (There is no need to return multiple copies of the
 same NXT.) These NXTs, if any, 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
        big.foo.nil,
        medium.foo.nil.
        small.foo.nil.
        tiny.foo.nil.
 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:

Eastlake Standards Track [Page 26] RFC 2535 DNS Security Extensions March 1999

 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 ;signature inception
                  2143           ;key identifier
                  foo.nil.       ;signer
 AIYADP8d3zYNyQwW2EM4wXVFdslEJcUx/fxkfBeH1El4ixPFhpfHFElxbvKoWmvjDTCm
 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 ;signature inception
                  2143           ;key identifier
                  foo.nil.       ;signer
  MxFcby9k/yvedMfQgKzhH5er0Mu/vILz45IkskceFGgiWCn/GxHhai6VAuHAoNUz4YoU
  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.

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 easily
 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 subsections 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
 element 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

Eastlake Standards Track [Page 27] RFC 2535 DNS Security Extensions March 1999

 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 or
 simply a modified set could be sent as long as at least one of each
 type is included.
 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.
 The complete NXT chains specified in this document enable a resolver
 to obtain, by successive queries chaining through NXTs, all of the
 names in a zone even if zone transfers are prohibited.  Different
 format NXTs may be specified in the future to avoid this.

5.6.1 Full Zone Transfers

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

5.6.2 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
 incremental transfer can not be confirmed because usually neither the
 deleted RR section nor the added RR section has a compete zone 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

Eastlake Standards Track [Page 28] RFC 2535 DNS Security Extensions March 1999

 the IXFR SIG, the transferred RRs MAY be considered valid without
 verification of the internal SIGs if such trust in the server
 conforms to local policy.

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 all the data included in the answer and authority
 portion of the response has been authenticated by the server
 according to the policies of that server. The CD (checking disabled)
 bit indicates in a query that Pending (non-authenticated) data is
 acceptable to the resolver sending the query.

Eastlake Standards Track [Page 29] RFC 2535 DNS Security Extensions March 1999

 These bits are allocated from the previously must-be-zero Z field as
 follows:
                                         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 this 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.  The AD bit does not
 cover the additional information section.

Eastlake Standards Track [Page 30] RFC 2535 DNS Security Extensions March 1999

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 a public key
 associated with the root zone 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 outside the organization's
 domain and need 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. A secure sub-zone is indicated by a KEY RR with non-null key
 information appearing with the NS RRs in the sub-zone and which may
 also be present in the parent.  These make it possible to descend
 within the tree of zones.

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 trusted key staticly configured at the resolver performing
 the authentication.
 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 algorithmic process. In
 addition, the validation of each SIG over the data with reference to
 a KEY must meet the objective cryptographic test implied by the

Eastlake Standards Track [Page 31] RFC 2535 DNS Security Extensions March 1999

 cryptographic algorithm used (although even here the resolver may
 have policies as to trusted algorithms and key lengths).  Finally,
 the judgement that a SIG with a particular signer name can
 authenticate data (possibly a KEY RRset) with a particular owner
 name, 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, recommended, that the policy below 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 an
      RRset (possibly a KEY RRset) with owner name Owner at the
      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).  That is, Owner is the
      same as or a subdomain of Signer.
      (2) ( Owner < Signer ) and ( Signer > or = some Static ).  That
      is, Owner is a superdomain of Signer and Signer is staticly
      configured or a subdomain of a staticly configured key.
      (3) Signer = some Static.  That is, the signer is exactly some
      staticly configured key.
 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 zone
 keys are staticly configured.
 Rule 3 is a rule permitting direct cross certification.  Rule 3 has
 no effect if only root zone keys are staticly configured.

Eastlake Standards Track [Page 32] RFC 2535 DNS Security Extensions March 1999

 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 zone keys are
 staticly configured).

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 reached 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, 2030]).  If such protocols are
 used, they MUST be used securely so that time can not be spoofed.

Eastlake Standards Track [Page 33] RFC 2535 DNS Security Extensions March 1999

 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.

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    PRIVATEDNS
      254    PRIVATEOID

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 separated by instances of the verticle bar ("|")
 character:
   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

Eastlake Standards Track [Page 34] RFC 2535 DNS Security Extensions March 1999

 10   FLAG10    - reserved
 11   FLAG11    - reserved
 12-15          signatory field, values 0 to 15
          can be represented by SIG0, SIG1, ... SIG15
 No flag mnemonic need be present if the bit or field it represents is
 zero.
 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 flags 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 data 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).

Eastlake Standards Track [Page 35] RFC 2535 DNS Security Extensions March 1999

 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.
 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 as an unsigned interger or sequence of RR
 mnemonics.

8. Canonical Form and Order of Resource Records

 This section specifies, for purposes of domain name system (DNS)
 security, the canonical form of resource records (RRs), their name
 order, and their overall order.  A canonical name order is necessary
 to construct the NXT name chain.  A canonical form and ordering
 within an RRset is necessary in consistently constructing and
 verifying SIG RRs.  A canonical ordering of types within a name is
 required in connection with incremental transfer (Section 5.6.2).

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
 TTL.

Eastlake Standards Track [Page 36] RFC 2535 DNS Security Extensions March 1999

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 in a
 zone are 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.
 Example:
        foo.example
        a.foo.example
        yljkjljk.a.foo.example
        Z.a.foo.example
        zABC.a.FOO.EXAMPLE
        z.foo.example
        *.z.foo.example
        \200.z.foo.example

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
 because A is type 1 and MX is type 15 but if both were signed, the
 order would be A < SIG(A) < MX < SIG(MX).

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
 follows:

Eastlake Standards Track [Page 37] RFC 2535 DNS Security Extensions March 1999

 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
 well.

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 at least for the mandatory
 algorithm, (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 when
 needed, (4) normally sets the CD query header bit on its queries.

10. Security Considerations

 This document specifies extensions to the Domain Name System (DNS)
 protocol to provide data integrity and data 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

Eastlake Standards Track [Page 38] RFC 2535 DNS Security Extensions March 1999

 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 beyond DNS.
 The implementation of NXT RRs as described herein enables a resolver
 to determine all the names in a zone even if zone transfers are
 prohibited (section 5.6).  This is an active area of work and may
 change.
 A number of precautions in DNS implementation have evolved over the
 years to harden the insecure DNS against spoofing.  These precautions
 should not be abandoned but should be considered to provide
 additional protection in case of key compromise in secure DNS.

11. IANA Considerations

 KEY RR flag bits 2 and 8-11 and all flag extension field bits can be
 assigned by IETF consensus as defined in RFC 2434.  The remaining
 values of the NAMTYP flag field and flag bits 4 and 5 (which could
 conceivably become an extension of the NAMTYP field) can only be
 assigned by an IETF Standards Action [RFC 2434].
 Algorithm numbers 5 through 251 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 [RFC 2434].  The existence of the private algorithm
 types 253 and 254 should satify most needs for private or proprietary
 algorithms.
 Additional values of the Protocol Octet (5-254) can be assigned by
 IETF Consensus [RFC 2434].
 The meaning of the first bit of the NXT RR "type bit map" being a one
 can only be assigned by a standards action.

References

 [RFC 1033]  Lottor, M., "Domain Administrators Operations Guide", RFC
             1033, November 1987.
 [RFC 1034]  Mockapetris, P., "Domain Names - Concepts and
             Facilities", STD 13, RFC 1034, November 1987.
 [RFC 1035]  Mockapetris, P., "Domain Names - Implementation and
             Specifications", STD 13, RFC 1035, November 1987.

Eastlake Standards Track [Page 39] RFC 2535 DNS Security Extensions March 1999

 [RFC 1305]  Mills, D., "Network Time Protocol (v3)", RFC 1305, March
             1992.
 [RFC 1530]  Malamud, C. and M. Rose, "Principles of Operation for the
             TPC.INT Subdomain: General Principles and Policy", RFC
             1530, October 1993.
 [RFC 2401]  Kent, S. and R. Atkinson, "Security Architecture for the
             Internet Protocol", RFC 2401, November 1998.
 [RFC 1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC
             1982, September 1996.
 [RFC 1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
             August 1996.
 [RFC 2030]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4
             for IPv4, IPv6 and OSI", RFC 2030, October 1996.
 [RFC 2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
             Extensions (MIME) Part One: Format of Internet Message
             Bodies", RFC 2045, November 1996.
 [RFC 2065]  Eastlake, D. and C. Kaufman, "Domain Name System Security
             Extensions", RFC 2065, January 1997.
 [RFC 2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC 2136]  Vixie, P., Thomson, S., Rekhter, Y. and J. Bound,
             "Dynamic Updates in the Domain Name System (DNS UPDATE)",
             RFC 2136, April 1997.
 [RFC 2137]  Eastlake, D., "Secure Domain Name System Dynamic Update",
             RFC 2137, April 1997.
 [RFC 2181]  Elz, R. and R. Bush, "Clarifications to the DNS
             Specification", RFC 2181, July 1997.
 [RFC 2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.
 [RFC 2537]  Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name
             System (DNS)", RFC 2537, March 1999.
 [RFC 2539]  Eastlake, D., "Storage of Diffie-Hellman Keys in the
             Domain Name System (DNS)", RFC 2539, March 1999.

Eastlake Standards Track [Page 40] RFC 2535 DNS Security Extensions March 1999

 [RFC 2536]  Eastlake, D., "DSA KEYs and SIGs in the Domain Name
             System (DNS)", RFC 2536, March 1999.
 [RFC 2538]  Eastlake, D. and O. Gudmundsson, "Storing Certificates in
             the Domain Name System", RFC 2538, March 1999.
 [RFC 2541]  Eastlake, D., "DNS Operational Security Considerations",
             RFC 2541, March 1999.
 [RSA FAQ] - RSADSI Frequently Asked Questions periodic posting.

Author's Address

 Donald E. Eastlake 3rd
 IBM
 65 Shindegan Hill Road
 RR #1
 Carmel, NY 10512
 Phone:   +1-914-784-7913 (w)
          +1-914-276-2668 (h)
 Fax:     +1-914-784-3833 (w-fax)
 EMail:   dee3@us.ibm.com

Eastlake Standards Track [Page 41] RFC 2535 DNS Security Extensions March 1999

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
 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 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

Eastlake Standards Track [Page 42] RFC 2535 DNS Security Extensions March 1999

 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) 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 Standards Track [Page 43] RFC 2535 DNS Security Extensions March 1999

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
    document [RFC 2541].
 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 DNS security itself, (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 [RFC 2537].  Section 3.4 describing the
    meaning of various combinations of "no-key" and key present KEY
    RRs has been added and the secure / unsecure status of a zone has
    been clarified as being per algorithm.
 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 use serial number ring
    arithmetic, (3c) changing the definition of the key footprint/tag
    for algorithms other than 1 and adding Appendix C to specify its
    calculation.  In addition, the SIG covering type AXFR has been
    eliminated while one covering IXFR [RFC 1995] has been added (see
    section 5.6).
 4. Algorithm 3, the DSA algorithm, is now designated as the mandatory
    to implement algorithm.  Algorithm 1, the RSA/MD5 algorithm, is
    now a recommended option.  Algorithm 2 and 4 are designated as the
    Diffie-Hellman key and elliptic cryptography algorithms
    respectively, all to be defined in separate documents. Algorithm
    code point 252 is designated to indicate "indirect" keys, to be
    defined in a separate document, where the actual key is elsewhere.
    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.  Howver, that algorithm number has been re-used to
    indicate private algorithms where a domain name specifies the
    algorithm.

Eastlake Standards Track [Page 44] RFC 2535 DNS Security Extensions March 1999

 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, (5c) the
    number of and circumstances under which an NXT must be returned in
    connection with wildcard names has been extended, and (5d) in
    connection with the bit map, references to the WKS RR have been
    removed and verticle bars ("|") have been added between the RR
    type mnemonics in the ASCII representation.
 6. Information on the canonical form and ordering of RRs has been
    moved into a separate Section 8.
 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.  It is now clearly stated that authenticated data
    has a validity period of the intersection of the validity periods
    of the SIG RRs in its authentication chain.  The requirement to
    staticly configure a superzone's key signed by a zone in all of
    the zone's authoritative servers has been removed.  The
    recommendation 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 configured was dropped.
 9. It was clarified 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.  The AD bit only indicates
    that the answer and authority sections of the response are
    authoritative.
 10. It is now required that KEY RRs and NXT RRs be signed only with
     zone-level keys.
 11.  Add IANA Considerations section and references to RFC 2434.

Eastlake Standards Track [Page 45] RFC 2535 DNS Security Extensions March 1999

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 when there is more than one KEY
 RR candidate available, for example, in verifying a signature.  It is
 possible for more than one candidate key to have the same tag, in
 which case each must be tried until one works or all fail.  The
 following reference implementation of how to calculate the Key Tag,
 for all algorithms other than algorithm 1, is in ANSI C.  It is coded
 for clarity, not efficiency.  (See section 4.1.6 for how to determine
 the Key Tag of an algorithm 1 key.)
 /* assumes int is at least 16 bits
    first byte of the key tag is the most significant byte of return
    value
    second byte of the 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;
 }

Eastlake Standards Track [Page 46] RFC 2535 DNS Security Extensions March 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Eastlake Standards Track [Page 47]

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