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rfc:rfc2065

Network Working Group D. Eastlake, 3rd Request for Comments: 2065 CyberCash Updates: 1034, 1035 C. Kaufman Category: Standards Track Iris

                                                          January 1997
               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.

Abstract

 The Domain Name System (DNS) has become a critical operational part
 of the Internet infrastructure yet it has no strong security
 mechanisms to assure data integrity or authentication.  Extensions to
 the DNS are described that provide these services to security aware
 resolvers or applications through the use of cryptographic digital
 signatures.  These digital signatures are included in secured zones
 as resource records.  Security can still be provided even through
 non-security aware DNS servers in many cases.
 The extensions also provide for the storage of authenticated public
 keys in the DNS.  This storage of keys can support general public key
 distribution service as well as DNS security.  The stored keys enable
 security aware resolvers to learn the authenticating key of zones in
 addition to 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.

Eastlake & Kaufman Standards Track [Page 1] RFC 2065 DNS Security Extensions January 1997

Acknowledgments

 The significant contributions of the following persons (in alphabetic
 order) to this document are gratefully acknowledged:
         Harald T. Alvestrand
         Madelyn Badger
         Scott Bradner
         Matt Crawford
         James M. Galvin
         Olafur Gudmundsson
         Edie Gunter
         Sandy Murphy
         Masataka Ohta
         Michael A. Patton
         Jeffrey I. Schiller

Table of Contents

 1. Overview of Contents....................................3
 2.  Overview of the DNS Extensions.........................4
 2.1 Services Not Provided..................................4
 2.2 Key Distribution.......................................5
 2.3 Data Origin Authentication and Integrity...............5
 2.3.1 The SIG Resource Record..............................6
 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..........7
 2.3.5 Special Considerations with CNAME RRs................8
 2.3.6 Signers Other Than The Zone..........................8
 2.4 DNS Transaction and Request Authentication.............8
 3. The KEY Resource Record.................................9
 3.1 KEY RDATA format......................................10
 3.2 Object Types, DNS Names, and Keys.....................10
 3.3 The KEY RR Flag Field.................................11
 3.4 The Protocol Octet....................................13
 3.5 The KEY Algorithm Number and the MD5/RSA Algorithm....13
 3.6 Interaction of Flags, Algorithm, and Protocol Bytes...14
 3.7 KEY RRs in the Construction of Responses..............15
 3.8 File Representation of KEY RRs........................16
 4. The SIG Resource Record................................16
 4.1 SIG RDATA Format......................................17
 4.1.1 Signature Data......................................19
 4.1.2 MD5/RSA Algorithm Signature Calculation.............20
 4.1.3 Zone Transfer (AXFR) SIG............................21
 4.1.4 Transaction and Request SIGs........................22
 4.2 SIG RRs in the Construction of Responses..............23
 4.3 Processing Responses and SIG RRs......................24

Eastlake & Kaufman Standards Track [Page 2] RFC 2065 DNS Security Extensions January 1997

 4.4 Signature Expiration, TTLs, and Validity..............24
 4.5 File Representation of SIG RRs........................25
 5. Non-existent Names and Types...........................26
 5.1 The NXT Resource Record...............................26
 5.2 NXT RDATA Format......................................27
 5.3 Example...............................................28
 5.4 Interaction of NXT RRs and Wildcard RRs...............28
 5.5 Blocking NXT Pseudo-Zone Transfers....................29
 5.6 Special Considerations at Delegation Points...........29
 6. The AD and CD Bits and How to Resolve Securely.........30
 6.1 The AD and CD Header Bits.............................30
 6.2 Boot File Format......................................32
 6.3 Chaining Through Zones................................32
 6.4 Secure Time...........................................33
 7. Operational Considerations.............................33
 7.1 Key Size Considerations...............................34
 7.2 Key Storage...........................................34
 7.3 Key Generation........................................35
 7.4 Key Lifetimes.........................................35
 7.5 Signature Lifetime....................................36
 7.6 Root..................................................36
 8. Conformance............................................36
 8.1 Server Conformance....................................36
 8.2 Resolver Conformance..................................37
 9. Security Considerations................................37
 References................................................38
 Authors' Addresses........................................39
 Appendix: Base 64 Encoding................................40

1. Overview of Contents

 This document describes extensions of the Domain Name System (DNS)
 protocol to support DNS security and public key distribution.  It
 assumes that the reader is familiar with the Domain Name System,
 particularly as described in RFCs 1033, 1034, and 1035.
 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, use in
 DNS responses, and file representation.  These resource records
 represent the public keys of entities named in the DNS and are used
 for key distribution.

Eastlake & Kaufman Standards Track [Page 3] RFC 2065 DNS Security Extensions January 1997

 Section 4 discusses the SIG digital signature resource record, its
 structure, use in DNS responses, and file representation.  These
 resource records are used to authenticate other resource records in
 the DNS and optionally to authenticate DNS transactions and requests.
 Section 5 discusses the NXT resource record and its use in DNS
 responses.  The NXT RR permits authenticated denial in the DNS of the
 existence of a name or of a particular type for an existing name.
 Section 6 discusses how a resolver can be configured with a starting
 key or keys and proceed to securely resolve DNS requests.
 Interactions between resolvers and servers are discussed for all
 combinations of security aware and security non-aware.  Two
 additional query header bits are defined for signaling between
 resolvers and servers.
 Section 7 reviews a variety of operational considerations including
 key generation, lifetime, and storage.
 Section 8 defines levels of conformance for resolvers and servers.
 Section 9 provides a few paragraphs on overall security
 considerations.
 An Appendix is provided that gives details of base 64 encoding which
 is used in the file representation of some RR's defined in this
 document.

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.

Eastlake & Kaufman Standards Track [Page 4] RFC 2065 DNS Security Extensions January 1997

 In addition, no effort has been made to provide for any
 confidentiality for queries or responses.  (This service may be
 available via IPSEC [RFC 1825].)

2.2 Key Distribution

 Resource records (RRs) are defined to associate keys with DNS names.
 This permits the DNS to be used as a public key distribution
 mechanism in support of the DNS data origin authentication and other
 security services.
 The syntax of a KEY resource record (RR) is described in Section 3.
 It includes an algorithm identifier, the actual public key
 parameters, and a variety of flags including those indicating the
 type of entity the key is associated with and/or asserting that there
 is no key associated with that entity.
 Under conditions described in Section 3.7, 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 records in
 the DNS cryptographically generated digital signatures.  Commonly,
 there will be a single private key that signs for an entire zone. If
 a security aware resolver reliably learns the public key of the zone,
 it can verify, for signed data read from that zone, that it was
 properly authorized and is reasonably current.  The expected
 implementation is for the zone private key to be kept off-line and
 used to re-sign all of the records in the zone periodically.
 This data origin authentication key belongs to the zone and not to
 the servers that store copies of the data.  That means compromise of
 a server or even all servers for a zone will not necessarily affect
 the degree of assurance that a resolver has that it can determine
 whether data is genuine.
 A resolver can learn the public key of a zone either by reading it
 from DNS or by having it staticly configured.  To reliably learn the
 public key by reading it from DNS, the key itself must be signed.
 Thus, to provide a reasonable degree of security, the resolver must
 be configured with at least the public key of one zone that it can
 use to authenticate signatures.  From there, it can securely read the
 public keys of other zones, if the intervening zones in the DNS tree
 are secure and their signed keys accessible.  (It is in principle
 more secure to have the resolver manually configured with the public

Eastlake & Kaufman Standards Track [Page 5] RFC 2065 DNS Security Extensions January 1997

 keys of multiple zones, since then the compromise of a single zone
 would not permit the faking of information from other zones.  It is
 also more administratively cumbersome, however, particularly when
 public keys change.)
 Adding data origin authentication and integrity requires no change to
 the "on-the-wire" DNS protocol beyond the addition of the signature
 resource type and, as a practical matter, the key resource type
 needed for key distribution. This service can be supported by
 existing resolver and server implementations so long as they can
 support the additional resource types (see Section 8). The one
 exception is that CNAME referrals from a secure zone can not be
 authenticated if they are from non-security aware servers (see
 Section 2.3.5).
 If signatures are always separately retrieved and verified when
 retrieving the information they authenticate, there will be more
 trips to the server and performance will suffer.  To avoid this,
 security aware servers mitigate that degradation by always attempting
 to send the signature(s) needed.

2.3.1 The SIG Resource Record

 The syntax of a SIG resource record (signature) is described in
 Section 4.  It includes the type of the RR(s) being signed, the name
 of the signer, the time at which the signature was created, the time
 it expires (when it is no longer to be believed), its original time
 to live (which may be longer than its current time to live but cannot
 be shorter), the cryptographic algorithm in use, and the actual
 signature.
 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 RRs and delgation point NS RRs.  A security aware server
 supporting the performance enhanced version of the DNS protocol
 security extensions will attempt to return, with RRs retrieved, the
 corresponding SIGs.  If a server does not support the protocol, the
 resolver must retrieve all the SIG records for a name and select the
 one or ones that sign the resource record(s) that resolver is
 interested in.

Eastlake & Kaufman Standards Track [Page 6] RFC 2065 DNS Security Extensions January 1997

2.3.2 Authenticating Name and Type Non-existence

 The above security mechanism provides only a way to sign existing RRs
 in a zone.  "Data origin" authentication is not obviously provided
 for the non-existence of a domain name in a zone or the non-existence
 of a type for an existing name.  This gap is filled by the NXT RR
 which authenticatably asserts a range of non-existent names in a zone
 and the non-existence of types for the 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 field tick down when resource records are cached.
 This could be avoided by leaving the time-to-live out of the digital
 signature, but that would allow unscrupulous servers to set
 arbitrarily long time to live values undetected.  Instead, we include
 the "original" time-to-live in the signature and communicate that
 data in addition to the current time-to-live. Unscrupulous servers
 under this scheme can manipulate the time to live but a security
 aware resolver will bound the TTL value it uses at the original
 signed value.  Separately, signatures include a time signed and an
 expiration time.  A resolver that knows the absolute time can
 determine securely whether a signature has expired.  It is not
 possible to rely solely on the signature expiration as a substitute
 for the TTL, however, since the TTL is primarily a database
 consistency mechanism and, in any case, 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 and signed by the
 zone's key.  But the operational DNS views the leaf nodes in a zone,
 which are also the apex nodes of a subzone (i.e., delegation points),
 as "really" belonging to the subzone.  These nodes occur in two
 master files and may have RRs signed by both the upper and lower
 zone's keys.  A retrieval could get a mixture of these RRs and SIGs,
 especially since one server could be serving both the zone above and
 below a delegation point.
 In general, there must be a zone KEY RR for the subzone in the
 superzone and the copy signed in the superzone is controlling.  For
 all but one other RR type that should appearing in both the superzone

Eastlake & Kaufman Standards Track [Page 7] RFC 2065 DNS Security Extensions January 1997

 and subzone, the data from the subzone is more authoritative.  To
 avoid conflicts, only the KEY RR in the superzone should be signed
 and the NS and any A (glue) RRs should only be signed in the subzone.
 The SOA and any other RRs that have the zone name as owner should
 appear only in the subzone and thus are signed there. The NXT RR type
 is an 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 RRs

 There is a significant 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 will not retrieve any associated signature, key, or NXT
 RR. For types other than CNAME, it will retrieve that type at the
 target name of the CNAME (or chain of CNAMEs) and will return the
 CNAME as additional information.  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.
 In general, security aware servers MUST be used to securely CNAME in
 DNS.  Security aware servers must (1) allow KEY, SIG, and NXT RRs
 along with CNAME RRs, (2) suppress CNAME processing on retrieval of
 these types as well as on retrieval of the type CNAME, and (3)
 automatically return SIG RRs authenticating the CNAME or CNAMEs
 encountered in resolving a query.  This is a change from the previous
 DNS standard 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 two cases where a SIG resource record is signed by other
 than the zone private key.  One is for support of dynamic update
 where an entity is permitted to authenticate/update its own records.
 The public key of the entity must be present in the DNS and be
 appropriately signed but the other RR(s) may be signed with the
 entity's key.  The other is for support of transaction and request
 authentication as described in Section 2.4 immediately below.

2.4 DNS Transaction and Request Authentication

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

Eastlake & Kaufman Standards Track [Page 8] RFC 2065 DNS Security Extensions January 1997

 If header bits are falsely set by a server, there is little that can
 be done.  However, it is possible to add transaction authentication.
 Such authentication means that a resolver can be sure it is at least
 getting messages from the server it thinks it queried, that the
 response is from the query it sent, and that these messages have not
 been diddled in transit.  This is accomplished by optionally adding a
 special SIG resource record at the end of the reply which digitally
 signs the concatenation of the server's response and the resolver's
 query.
 Requests can also be authenticated by including a special SIG RR at
 the end of the request.  Authenticating requests serves no function
 in the current DNS and requests with a non-empty additional
 information section are ignored by almost all current DNS servers.
 However, this syntax for signing requests is defined in connection
 with authenticating future secure dynamic update requests or the
 like.
 The private keys used in transaction and request security belongs to
 the host composing the request or reply message, not to the zone
 involved.  The corresponding public key is normally stored in and
 retrieved from the DNS.
 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 document a key that is
 associated with a Domain Name System (DNS) name.  It will be a public
 key as only public keys are stored in the DNS.  This can be the
 public key of a zone, a host or other end entity, or a user.  A KEY
 RR is, like any other RR, authenticated by a SIG RR. Security aware
 DNS implementations MUST be designed to handle at least two
 simultaneously valid keys of the same type associated with a name.
 The type number for the KEY RR is 25.

Eastlake & Kaufman Standards Track [Page 9] RFC 2065 DNS Security Extensions January 1997

3.1 KEY RDATA format

 The RDATA for a KEY RR consists of flags, a protocol octet, the
 algorithm number, and the public key itself.  The format is as
 follows:
                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             flags             |    protocol   |   algorithm   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               /
 /                          public key                           /
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 The meaning of the KEY RR owner name, flags, and protocol octet are
 described in Sections 3.2, 3.3 and 3.4 below respectively.  The flags
 and algorithm must be examined before any data following the
 algorithm octet as they control the format and even whether there is
 any following data.  The algorithm and public key fields are
 described in Section 3.5.  The format of the public key is algorithm
 dependent.

3.2 Object Types, DNS Names, and Keys

 The public key in a KEY RR belongs to the object named in the owner
 name.
 This DNS name may refer to up to three different categories of
 things.  For example, dee.cybercash.com could be (1) a zone, (2) a
 host or other end entity , and (3) the mapping into a DNS name of the
 user or account dee@cybercash.com.  Thus, there are flags, 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 at every leaf node which is a delegation point (and thus the same
 owner name as the apex of a subzone) within a secure zone.
 Although the same name can be used for up to all three of these
 categories, such overloading of a name is discouraged.  It is also
 possible to use the same key for different things with the same name
 or even different names, but this is strongly discouraged.  In
 particular, the use of a zone key as a non-zone key will usually
 require that the corresponding private key be kept on line and
 thereby become more vulnerable.

Eastlake & Kaufman Standards Track [Page 10] RFC 2065 DNS Security Extensions January 1997

 In addition to the name type bits, there are additional flag bits
 including the "type" field, "experimental" bit, "signatory" field,
 etc., as described below.

3.3 The KEY RR Flag Field

 In the "flags" field:
      Bit 0 and 1 are the key "type" field.  Bit 0 a one indicates
 that use of the key is prohibited for authentication.  Bit 1 a one
 indicates that use of the key is prohibited for confidentiality. If
 this field is zero, then use of the key for authentication and/or
 confidentiality is permitted. Note that DNS security makes use of
 keys for authentication only. Confidentiality use flagging is
 provided for use of keys in other protocols.  Implementations not
 intended to support key distribution for confidentiality MAY require
 that the confidentiality use prohibited bit be on for keys they
 serve.  If both bits of this field 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.
      Bit 2 is the "experimental" bit.  It is ignored if the type
 field indicates "no key" and the following description assumes that
 type field to be non-zero.  Keys may be associated with zones,
 entities, or users for experimental, trial, or optional use, in which
 case this bit will be one.  If this bit is a zero, it means that the
 use or availability of security based on the key is "mandatory".
 Thus, if this bit is off for a zone key, the zone should be assumed
 secured by SIG RRs and any responses indicating the zone is not
 secured should be considered bogus.  If this bit is a one for a host
 or end entity, it might sometimes operate in a secure mode and at
 other times operate without security.  The experimental bit, like all
 other aspects of the KEY RR, is only effective if the KEY RR is
 appropriately signed by a SIG RR.  The experimental bit must be zero
 for safe secure operation and should only be a one for a minimal
 transition period.
      Bits 3-4 are reserved and must be zero.
      Bit 5 on indicates that this is a key associated with a "user"
 or "account" at an end entity, usually a host.  The coding of the
 owner name is that used for the responsible individual mailbox in the
 SOA 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.domain could have a public key associated through a
 KEY RR with name j\.random_user.host.subdomain.domain and the user
 bit a one.  It could be used in an security protocol where

Eastlake & Kaufman Standards Track [Page 11] RFC 2065 DNS Security Extensions January 1997

 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.
      Bit 6 on indicates that this is a key associated with the non-
 zone "entity" whose name is the RR owner name.  This will commonly be
 a host but could, in some parts of the DNS tree, be some other type
 of entity such as a telephone number [RFC 1530].  This is the public
 key used in connection with the optional DNS transaction
 authentication service if the owner name is a DNS server host.  It
 could also be used in an IP-security protocol where authentication of
 at the host, rather than user, level was desired, such as routing,
 NTP, etc.
      Bit 7 is the "zone" bit and indicates that this is a zone key
 for the zone whose name is the KEY RR owner name.  This is the public
 key used for DNS data origin authentication.
      Bit 8 is reserved to be the IPSEC [RFC 1825] bit and indicates
 that this key is valid for use in conjunction with that security
 standard.  This key could be used in connection with secured
 communication on behalf of an end entity or user whose name is the
 owner name of the KEY RR if the entity or user bits are on.  The
 presence of a KEY resource with the IPSEC and entity bits on and
 experimental and no-key bits off is an assertion that the host speaks
 IPSEC.
      Bit 9 is reserved to be the "email" bit and indicate that this
 key is valid for use in conjunction with MIME security multiparts.
 This key could be used in connection with secured communication on
 behalf of an end entity or user whose name is the owner name of the
 KEY RR if the entity or user bits are on.
      Bits 10-11 are reserved and must be zero.
      Bits 12-15 are the "signatory" field.  If non-zero, they
 indicate that the key can validly sign RRs or updates of the same
 name.  If the owner name is a wildcard, then RRs or updates with any
 name which is in the wildcard's scope can be signed.  Fifteen
 different non-zero values are possible for this field and any
 differences in their meaning are reserved for definition in
 connection with DNS dynamic update or other new DNS commands.  Zone
 keys always have authority to sign any RRs in the zone regardless of
 the value of this field.  The signatory field, like all other aspects
 of the KEY RR, is only effective if the KEY RR is appropriately
 signed by a SIG RR.

Eastlake & Kaufman Standards Track [Page 12] RFC 2065 DNS Security Extensions January 1997

3.4 The Protocol Octet

 It is anticipated that some keys stored in DNS will be used in
 conjunction with Internet protocols other than DNS (keys with zone
 bit or signatory field non-zero) and IPSEC/email (keys with IPSEC
 and/or email bit set).  The protocol octet is provided to indicate
 that a key is valid for such use and, for end entity keys or the host
 part of user keys, that the secure version of that protocol is
 implemented on that entity or host.
 Values between 1 and 191 decimal inclusive are available for
 assignment by IANA for such protocols.  The 63 values between 192 and
 254 inclusive will not be assigned to a specific protocol and are
 available for experimental use under bilateral agreement. Value 0
 indicates, for a particular key, that it is not valid for any
 particular additional protocol beyond those indicated in the flag
 field. And value 255 indicates that the key is valid for all assigned
 protocols (those in the 1 to 191 range).
 It is intended that new uses of DNS stored keys would initially be
 implemented, and operational experience gained, using the
 experimental range of the protocol octet.  If demand for widespread
 deployment for the indefinite future warrants, a value in the
 assigned range would then be designated for the protocol.  Finally,
 (1) should the protocol become so widespread in conjunction with
 other protocols and with which it shares key values that duplicate
 RRs are a serious burden and (2) should the protocol provide
 substantial facilities not available in any protocol for which a
 flags field bit has been allocated, then one of the remaining flag
 field bits may be allocated to the protocol. When such a bit has been
 allocated, a key can be simultaneously indicated as valid for that
 protocol and the entity or host can be simultaneously flagged as
 implementing the secure version of that protocol, along with other
 protocols for which flag field bits have been assigned.

3.5 The KEY Algorithm Number and the MD5/RSA Algorithm

 This octet is the key algorithm parallel to the same field for the
 SIG resource.  The MD5/RSA algorithm described in this document is
 number 1. Numbers 2 through 252 are available for assignment should
 sufficient reason arise.  However, the designation of a new algorithm
 could have a major impact on interoperability and requires an IETF
 standards action.  Number 254 is reserved for private use and will
 never be assigned a specific algorithm.  For number 254, the public
 key area shown in the packet diagram above will actually begin with a
 length byte followed by an Object Identifier (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. Number 253 is

Eastlake & Kaufman Standards Track [Page 13] RFC 2065 DNS Security Extensions January 1997

 reserved as the "expiration date algorithm" for use where the
 expiration date or other labeling fields of SIGs are desired without
 any actual security. It is anticipated that this algorithm will only
 be used in connection with some modes of DNS dynamic update.  For
 number 253, the public key area is null. Values 0 and 255 are
 reserved.
 If the type field does not have the "no key" value and the algorithm
 field is 1, indicating the MD5/RSA algorithm, the public key field is
 structured as follows:
                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | pub exp length|        public key exponent                    /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               /
 +-                           modulus                            /
 |                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-/
 To promote interoperability, the exponent and modulus are each
 limited to 2552 bits in length.  The public key exponent is a
 variable length unsigned integer.  Its length in octets is
 represented as one octet if it is in the range of 1 to 255 and by a
 zero octet followed by a two octet unsigned length if it is longer
 than 255 bytes.  The public key modulus field is a multiprecision
 unsigned integer.  The length of the modulus can be determined from
 the RDLENGTH and the preceding RDATA fields including the exponent.
 Leading zero bytes are prohibited in the exponent and modulus.

3.6 Interaction of Flags, Algorithm, and Protocol Bytes

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

Eastlake & Kaufman Standards Track [Page 14] RFC 2065 DNS Security Extensions January 1997

    NK = no key type value
    AL = algorithm byte
    PR = protocols indicated by protocol byte or protocol 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.
      0   x   0   Illegal, claims key but has bad algorithm field.
      0   x   1   Specified protocols insecure, others may be secure.
      x   0   0   Useless.  Gives key but no protocols to use it.
      x   0   1   Useless.  Denies key but for no protocols.
      x   x   0   Specifies key for protocols and asserts that
                    those protocols are implemented with security.
      x   x   1   Algorithm not understood for protocol.
    (remember, in reference to the above table, that a protocol
     byte of 255 means all protocols with protocol byte values
     assigned)

3.7 KEY RRs in the Construction of Responses

 An explicit request for KEY RRs does not cause any special additional
 information processing except, of course, for the corresponding SIG
 RR from a security aware server.
 Security aware DNS servers MUST include KEY RRs as additional
 information in responses where appropriate including the following:
 (1) On the retrieval of NS RRs, the zone key KEY RR(s) for the zone
 served by these name servers MUST be included as additional
 information if space is avilable.  There will always be at least one
 such KEY RR in a secure zone, even if it has the no-key type value to
 indicate that the subzone is insecure.  If not all additional
 information will fit, the KEY RR(s) have higher priority than type A
 or AAAA glue RRs.  If such a KEY RR does not fit on a retrieval, the
 retrieval must be considered truncated.
 (2) On retrieval of type A or AAAA RRs, the end entity KEY RR(s) MUST
 be included if space is available.  On inclusion of A or AAAA RRs as
 additional information, their KEY RRs will also be included but with
 lower priority than the relevant A or AAAA RRs.

Eastlake & Kaufman Standards Track [Page 15] RFC 2065 DNS Security Extensions January 1997

3.8 File Representation of KEY RRs

 KEY RRs may appear as lines in a zone data master file.
 The flag field, protocol, and algorithm number octets are then
 represented as unsigned integers.  Note that if the type field has
 the "no key" value or the algorithm specified is 253, nothing appears
 after the algorithm octet.
 The remaining public key portion is represented in base 64 (see
 Appendix) and may be divided up into any number of white space
 separated substrings, down to single base 64 digits, which are
 concatenated to obtain the full signature.  These substrings can span
 lines using the standard parenthesis.
 Note that the public key may have internal sub-fields but these do
 not appear in the master file representation.  For example, with
 algorithm 1 there is a public exponent 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.

4. The SIG Resource Record

 The SIG or "signature" resource record (RR) is the fundamental way
 that data is authenticated in the secure Domain Name System (DNS). As
 such it is the heart of the security provided.
 The SIG RR unforgably authenticates other RRs of a particular type,
 class, and name and binds them to a time interval and the signer's
 domain name.  This is done using cryptographic techniques and the
 signer's private key.  The signer is frequently the owner of the zone
 from which the RR originated.

Eastlake & Kaufman Standards Track [Page 16] RFC 2065 DNS Security Extensions January 1997

4.1 SIG RDATA Format

 The RDATA portion of a SIG RR is as shown below.  The integrity of
 the RDATA information is protected by the signature field.
                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        type covered           |  algorithm    |     labels    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         original TTL                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      signature expiration                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         time signed                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         key footprint         |                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         signer's name         /
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               /
 +-                          signature                           /
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The value of the SIG RR type is 24.
 The "type covered" is the type of the other RRs covered by this SIG.
 The algorithm number is an octet specifying the digital signature
 algorithm used parallel to the algorithm octet for the KEY RR.  The
 MD5/RSA algorithm described in this document is number 1.  Numbers 2
 through 252 are available for assignment should sufficient reason
 arise to allocate them.  However, the designation of a new algorithm
 could have a major impact on the interoperability of the global DNS
 system and requires an IETF standards action.  Number 254 is reserved
 for private use and will not be assigned a specific algorithm.  For
 number 254, the "signature" area shown above will actually begin with
 a length byte followed by an Object Identifier (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.  Number 253,
 known as the "expiration date algorithm", is used when the expiration
 date or other non-signature fields of the SIG are desired without any
 actual security.  It is anticipated that this algorithm will only be
 used in connection with some modes of DNS dynamic update.  For number
 253, the signature field will be null.  Values 0 and 255 are
 reserved.

Eastlake & Kaufman Standards Track [Page 17] RFC 2065 DNS Security Extensions January 1997

 The "labels" octet is an unsigned count of how many labels there are
 in the original SIG RR owner name not counting the null label for
 root and not counting any initial "*" for a wildcard.  If a secured
 retrieval is the result of wild card substitution, it is necessary
 for the resolver to use the original form of the name in verifying
 the digital signature.  This field helps optimize the determination
 of the original form thus reducing the effort in authenticating
 signed data.
 If, on retrieval, the RR appears to have a longer name than indicated
 by "labels", the resolver can tell it is the result of wildcard
 substitution.  If the RR owner name appears to be shorter than the
 labels count, the SIG RR must be considered corrupt and ignored.  The
 maximum number of labels allowed in the current DNS is 127 but the
 entire octet is reserved and would be required should DNS names ever
 be expanded to 255 labels.  The following table gives some examples.
 The value of "labels" is at the top, the retrieved owner name on the
 left, and the table entry is the name to use in signature
 verification except that "bad" means the RR is corrupt.
      labels= |  0  |   1  |    2   |      3   |      4   |
      --------+-----+------+--------+----------+----------+
             .|   . | bad  |  bad   |    bad   |    bad   |
            d.|  *. |   d. |  bad   |    bad   |    bad   |
          c.d.|  *. | *.d. |   c.d. |    bad   |    bad   |
        b.c.d.|  *. | *.d. | *.c.d. |   b.c.d. |    bad   |
      a.b.c.d.|  *. | *.d. | *.c.d. | *.b.c.d. | a.b.c.d. |
 The "original TTL" field is included in the RDATA portion to avoid
 (1) authentication problems that caching servers would otherwise
 cause by decrementing the real TTL field and (2) security problems
 that unscrupulous servers could otherwise cause by manipulating the
 real TTL field.  This original TTL is protected by the signature
 while the current TTL field is not.
 NOTE:  The "original TTL" must be restored into the covered RRs when
 the signature is verified.  This implies that all RRs for a
 particular type, name, and class must have the same TTL to start
 with.
 The SIG is valid until the "signature expiration" time which is an
 unsigned number of seconds since the start of 1 January 1970, GMT,
 ignoring leap seconds.  (See also Section 4.4.)  SIG RRs should not
 have a date signed significantly in the future.  To prevent
 misordering of network requests to update a zone dynamically,
 monotonically increasing "time signed" dates may be necessary.

Eastlake & Kaufman Standards Track [Page 18] RFC 2065 DNS Security Extensions January 1997

 The "time signed" field is an unsigned number of seconds since the
 start of 1 January 1970, GMT, ignoring leap seconds.
 A SIG RR with an expiration date before the time signed must be
 considered corrupt and ignored.
 The "key footprint" is a 16 bit quantity that is used to help
 efficiently select between multiple keys which may be applicable and
 as a quick check that a public key about to be used for the
 computationally expensive effort to check the signature is possibly
 valid.  Its exact meaning is algorithm dependent.  For the MD5/RSA
 algorithm, it is the next to the bottom two octets of the public key
 modulus needed to decode the signature field.  That is to say, the
 most significant 16 of the lest significant 24 bits of the modulus in
 network order.
 The "signer's name" field is the domain name of the signer generating
 the SIG RR.  This is the owner of the public KEY RR that can be used
 to verify the signature.  It is frequently the zone which contained
 the RR(s) being authenticated.  The signer's name may be compressed
 with standard DNS name compression when being transmitted over the
 network.
 The structure of the "signature" field is described below.

4.1.1 Signature Data

 Except for algorithm number 253 where it is null, the actual
 signature portion of the SIG RR binds the other RDATA fields to all
 of the "type covered" RRs with that owner name and class.  These
 covered RRs are thereby authenticated.  To accomplish this, a data
 sequence is constructed as follows:
         data = RDATA | RR(s)...
 where "|" is concatenation, RDATA is all the RDATA fields in the SIG
 RR itself before and not including the signature, and RR(s) are all
 the RR(s) of the type covered with the same owner name and class as
 the SIG RR in canonical form and order.  How this data sequence is
 processed into the signature is algorithm dependent.
 For purposes of DNS security, the canonical form for an RR is the RR
 with domain names (1) fully expanded (no name compression via
 pointers), (2) all domain name letters set to lower case, and (3) the
 original TTL substituted for the current TTL.

Eastlake & Kaufman Standards Track [Page 19] RFC 2065 DNS Security Extensions January 1997

 For purposes of DNS security, the canonical order for RRs is to sort
 them in ascending order by name, considering labels as a left
 justified unsigned octet sequence in network (transmission) order
 where a missing octet sorts before a zero octet.  (See also ordering
 discussion in Section 5.1.)  Within any particular name they are
 similarly sorted by type and then RDATA as a left justified unsigned
 octet sequence. EXCEPT that the type SIG RR(s) covering any
 particular type appear immediately after the other RRs of that type.
 (This special consideration for SIG RR(s) in ordering really only
 applies to calculating the AXFR SIG RR as explained in section 4.1.3
 below.)  Thus if at name a.b there are two A RRs and one KEY RR,
 their order with SIGs for concatenating the "data" to be signed would
 be as follows:
         a.b.  A ....
         a.b.  A ....
         a.b.  SIG A ...
         a.b.  KEY ...
         a.b.  SIG KEY ...
 SIGs covering type ANY should not be included in a zone.

4.1.2 MD5/RSA Algorithm Signature Calculation

 For the MD5/RSA algorithm, the signature is as follows
    hash = MD5 ( data )
    signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n)
 where MD5 is the message digest algorithm documented in RFC 1321, "|"
 is concatenation, "e" is the private key exponent of the signer, and
 "n" is the modulus of the signer's public key.  01, FF, and 00 are
 fixed octets of the corresponding hexadecimal value. "prefix" is the
 ASN.1 BER MD5 algorithm designator prefix specified in PKCS1, that
 is,
         hex 3020300c06082a864886f70d020505000410 [NETSEC].
 This prefix is included to make it easier to use RSAREF or similar
 packages.  The FF octet is repeated the maximum number of times such
 that the value of the quantity being exponentiated is one octet
 shorter than the value of n.
 (The above specifications are identical to the corresponding part of
 Public Key Cryptographic Standard #1 [PKCS1].)

Eastlake & Kaufman Standards Track [Page 20] RFC 2065 DNS Security Extensions January 1997

 The size of n, including most and least significant bits (which will
 be 1) SHALL be not less than 512 bits and not more than 2552 bits.  n
 and e SHOULD be chosen such that the public exponent is small.
 Leading zeros bytes are not permitted in the MD5/RSA algorithm
 signature.
 A public exponent of 3 minimizes the effort needed to decode a
 signature.  Use of 3 as the public exponent may be weak for
 confidentiality uses since, if the same data can be collected
 encrypted under three different keys with an exponent of 3 then,
 using the Chinese Remainder Theorem, the original plain text can be
 easily recovered.  This weakness is not significant for DNS because
 we seek only authentication, not confidentiality.

4.1.3 Zone Transfer (AXFR) SIG

 The above SIG mechanisms assure the authentication of all zone signed
 RRs of a particular name, class and type.  However, to efficiently
 assure the completeness and security of zone transfers, a SIG RR
 owned by the zone name must be created with a type covered of AXFR
 that covers all zone signed RRs in the zone and their zone SIGs but
 not the SIG AXFR itself.  The RRs are ordered and concatenated for
 hashing as described in Section 4.1.1.  (See also ordering discussion
 in Section 5.1.)
 The AXFR SIG must be calculated last of all zone key signed SIGs in
 the zone.  In effect, when signing the zone, you order, as described
 above, all RRs to be signed by the zone, and all associated glue RRs
 and delegation point NS RRs.  You can then make one pass inserting
 all the zone SIGs.  As you proceed you hash RRs to be signed into
 both an RRset hash and the zone hash.  When the name or type changes
 you calculate and insert the RRset zone SIG, clear the RRset hash,
 and hash that SIG into the zone hash (note that glue RRs and
 delegation point NSs are not zone signed but zone apex NSs are).
 When you have finished processing all the starting RRs as described
 above, you can then use the cumulative zone hash RR to calculate and
 insert an AXFR SIG covering the zone.  Of course any computational
 technique producing the same results as above is permitted.
 The AXFR 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 AXFR SIG is otherwise meaningless. The AXFR SIG is only
 retrieved as part of a zone transfer.  After validation of the AXFR
 SIG, the zone MAY be considered valid without verification of the
 internal zone signed SIGs in the zone; however, any RRs authenticated
 by SIGs signed by entity keys or the like MUST still be validated.
 The AXFR SIG SHOULD be transmitted first in a zone transfer so the

Eastlake & Kaufman Standards Track [Page 21] RFC 2065 DNS Security Extensions January 1997

 receiver can tell immediately that they may be able to avoid
 verifying other zone signed SIGs.
 RRs which are authenticated by a dynamic update key and not by the
 zone key (see Section 3.2) are not included in the AXFR SIG. They may
 originate in the network and might not, in general, be migrated to
 the recommended off line zone signing procedure (see Section 7.2).
 Thus, such RRs are not directly signed by the zone, are not included
 in the AXFR SIG, and are protected against omission from zone
 transfers only to the extent that the server and communication can be
 trusted.

4.1.4 Transaction and Request SIGs

 A response message from a security aware server may optionally
 contain a special SIG as the last item in the additional information
 section to authenticate the transaction.
 This SIG has a "type covered" field of zero, which is not a valid RR
 type.  It is calculated by using a "data" (see Section 4.1.2) of the
 entire preceding DNS reply message, including DNS header but not the
 IP header, concatenated with the entire DNS query message that
 produced this response, including the query's DNS header but not its
 IP header.  That is
      data = full response (less final transaction SIG) | full query
 Verification of the transaction SIG (which is signed by the server
 host key, not the zone key) by the requesting resolver shows that the
 query and response were not tampered with in transit, that the
 response corresponds to the intended query, and that the response
 comes from the queried server.
 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 at the
 begining or any preceding request SIGs at the end. Such request SIGs
 are included in the "data" used to form any optional response
 transaction SIG.
 WARNING: Request SIGs are unnecessary for currently defined queries
 and will cause almost all existing DNS servers to completely ignore a
 query.  However, such SIGs may be needed to authenticate future DNS
 secure dynamic update or other requests.

Eastlake & Kaufman Standards Track [Page 22] RFC 2065 DNS Security Extensions January 1997

4.2 SIG RRs in the Construction of Responses

 Security aware DNS servers MUST, for every authoritative RR the query
 will return, attempt to send the available SIG RRs which authenticate
 the requested RR.  The following rules apply to the inclusion of SIG
 RRs in responses:
 1. when an RR set is placed in a response, its SIG RR has a higher
    priority for inclusion than other additional RRs that may need to
    be included.  If space does not permit its inclusion, the response
    MUST be considered truncated except as provided in 2 below.
 2. when a SIG RR is present in the zone for an additional information
    section RR, the response MUST NOT be considered truncated merely
    because space does not permit the inclusion of its SIG RR.
 3. SIGs to authenticate non-authoritative data (glue records and NS
    RRs for subzones) are unnecessary and MUST NOT be sent.  (Note
    that KEYs for subzones are controlling in a superzone so the
    superzone's signature on the KEY MUST be included (unless the KEY
    was additional information and the SIG did not fit).)
 4. If a SIG covers any RR that would be in the answer section of the
    response, its automatic inclusion MUST be 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 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 4.1.4).  Such SIG RRs are signed by the DNS server
    originating the response.  Although the signer field MUST be the
    name of the originating server host, the owner name, class, TTL,
    and original TTL, are meaningless.  The class and TTL fields
    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 higher priority for inclusion than
    any other RR in the response.

Eastlake & Kaufman Standards Track [Page 23] RFC 2065 DNS Security Extensions January 1997

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 what it
    believes to be a security aware server via a secure communication
    with the AD bit (see Section 6.1) set, MAY choose to accept the
    RRs as received without verifying the zone SIG RRs.
 2. in other cases, a security aware resolver SHOULD verify the SIG
    RRs for the RRs of interest.  This may involve initiating
    additional queries for SIG or KEY RRs, especially in the case of
    getting a response from an insecure server.  (As explained in 4.2
    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 reasonable checks or the SIG does not
 check against the signed RRs, the SIG RR is invalid and should be
 ignored.  If all of the SIG RR(s) purporting to authenticate a set of
 RRs are invalid, then the set of RR(s) is not authenticated.
 If the SIG RR is the last RR in a response in the additional
 information section and has a type covered of zero, it is a
 transaction signature of the response and the query that produced the
 response.  It MAY be optionally checked and the message rejected if
 the checks fail.  But even if the checks succeed, such a transaction
 authentication SIG does NOT authenticate any RRs in the message.
 Only a proper SIG RR signed by the zone or a key tracing its
 authority to the zone or to static resolver configuration can
 authenticate RRs.  If a resolver does not implement transaction
 and/or request SIGs, it MUST ignore them without error.
 If all reasonable checks indicate that the SIG RR is valid then RRs
 verified by it should be considered authenticated.

4.4 Signature Expiration, TTLs, and Validity

 Security aware servers must not consider SIG RRs to authenticate
 anything after their expiration time and not consider any RR to be
 authenticated after its signatures have expired.  Within that

Eastlake & Kaufman Standards Track [Page 24] RFC 2065 DNS Security Extensions January 1997

 constraint, servers should continue to follow DNS TTL aging.  Thus
 authoritative servers should continue to follow the zone refresh and
 expire parameters and a non-authoritative server should count down
 the TTL and discard RRs when the TTL is zero.  In addition, when RRs
 are transmitted in a query response, the TTL should be trimmed so
 that current time plus the TTL does not extend beyond the signature
 expiration time.  Thus, in general, the TTL on an transmitted RR
 would be
       min(sigExpTim,max(zoneMinTTL,min(originalTTL,currentTTL)))

4.5 File Representation of SIG RRs

 A SIG RR can be represented as a single logical line in a zone data
 file [RFC1033] but there are some special considerations as described
 below.  (It does not make sense to include a transaction or request
 authenticating SIG RR in a file as they are a transient
 authentication that covers data including an ephemeral transaction
 number and so must be calculated in real time.)
 There is no particular problem with the signer, covered type, and
 times.  The time fields appears in the form YYYYMMDDHHMMSS where YYYY
 is the year, the first MM is the month number (01-12), DD is the day
 of the month (01-31), HH is the hour in 24 hours notation (00-23),
 the second MM is the minute (00-59), and SS is the second (00-59).
 The original TTL and algorithm fields appear as unsigned integers.
 If the original TTL, which applies to the type signed, is the same as
 the TTL of the SIG RR itself, it may be omitted.  The date field
 which follows it is larger than the maximum possible TTL so there is
 no ambiguity.
 The "labels" field does not appear in the file representation as it
 can be calculated from the owner name.
 The key footprint appears as an unsigned decimal number.
 However, the signature itself can be very long.  It is the last data
 field and is represented in base 64 (see Appendix) and may be divided
 up into any number of white space separated substrings, down to
 single base 64 digits, which are concatenated to obtain the full
 signature.  These substrings can be split between lines using the
 standard parenthesis.

Eastlake & Kaufman Standards Track [Page 25] RFC 2065 DNS Security Extensions January 1997

5. Non-existent Names and Types

 The SIG RR mechanism described in Section 4 above provides strong
 authentication of RRs that exist in a zone.  But is it not clear
 above how to authenticatably deny the existence of a name in a zone
 or a type for an existent name.
 The nonexistence of a name in a zone is indicated by the NXT ("next")
 RR for a name interval containing the nonexistent name. A NXT RR and
 its SIG are returned in the authority section, along with the error,
 if the server is security aware.  The same is true for a non-existent
 type under an existing name.  This is a change in the existing
 standard 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 always result in an reply containing at least
 one signed RR.
 NXT RRs do not appear in zone master files since they can be derived
 from the rest of the zone.

5.1 The NXT Resource Record

 The NXT resource record is used to securely indicate that RRs with an
 owner name in a certain name interval do not exist in a zone and to
 indicate what zone signed RR types are present for an existing name.
 The owner name of the NXT RR is an existing name in the zone.  It's
 RDATA is a "next" name and a type bit map. The presence of the NXT RR
 means that generally no name between its owner name and the name in
 its RDATA area exists and that no other zone signed types exist under
 its owner name.  This implies a canonical ordering of all domain
 names in a zone.
 The ordering is to sort labels as unsigned left justified octet
 strings where the absence of a octet sorts before a zero value octet
 and upper case letters are treated as lower case letters.  Names are
 then sorted by sorting on the highest level label and then, within
 those names with the same highest level label by the next lower
 label, etc. down to leaf node labels.  Since we are talking about a
 zone, the zone name itself always exists and all other names are the
 zone name with some prefix of lower level labels.  Thus the zone name
 itself always sorts first.

Eastlake & Kaufman Standards Track [Page 26] RFC 2065 DNS Security Extensions January 1997

 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.
 There are special considerations due to interaction with wildcards as
 explained below.
 The NXT RRs for a zone SHOULD be automatically calculated and added
 to the zone by the same recommended off-line process that signs the
 zone (see Section 7.2).  The NXT RR's TTL SHOULD not exceed the zone
 minimum TTL.

5.2 NXT RDATA Format

 The RDATA for an NXT RR consists simply of a domain name followed by
 a bit map.
 The type number for the NXT RR is 30.
                         1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         next domain name                                      /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    type bit map                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The NXT RR type bit map is one bit per RR type present for the owner
 name similar to the WKS socket bit map.  The first bit represents RR
 type zero (an illegal type which should not be present.) A one bit
 indicates that at least one RR of that type is present for the owner
 name.  A zero indicates that no such RR is present.  All bits not
 specified because they are beyond the end of the bit map are assumed
 to be zero.  Note that bit 30, for NXT, will always be on so the
 minimum bit map length is actually four octets.  The NXT bit map
 should be printed as a list of RR type mnemonics or decimal numbers
 similar to the WKS RR.
 The domain name may be compressed with standard DNS name compression
 when being transmitted over the network.  The size of the bit map can
 be inferred from the RDLENGTH and the length of the next domain name.

Eastlake & Kaufman Standards Track [Page 27] RFC 2065 DNS Security Extensions January 1997

5.3 Example

 Assume zone foo.tld has entries for
             big.foo.tld,
             medium.foo.tld.
             small.foo.tld.
             tiny.foo.tld.
 Then a query to a security aware server for huge.foo.tld would
 produce an error reply with the authority section data including
 something like the following:
 big.foo.tld. NXT medium.foo.tld. A MX SIG NXT
 big.foo.tld. SIG NXT 1 3 ( ;type-cov=NXT, alg=1, labels=3
                  19960102030405 ;signature expiration
                  19951211100908 ;time signed
                  21435          ;key footprint
                  foo.tld.       ;signer
  MxFcby9k/yvedMfQgKzhH5er0Mu/vILz45IkskceFGgiWCn/GxHhai6VAuHAoNUz4YoU
  1tVfSCSqQYn6//11U6Nld80jEeC8aTrO+KKmCaY= ;signature (640 bits)
                        )
 Note that this response implies that big.foo.tld is an existing name
 in the zone and thus has other RR types associated with it than NXT.
 However, only the NXT (and its SIG) RR appear in the response to this
 query for huge.foo.tld, which is a non-existent name.

5.4 Interaction of NXT RRs and Wildcard RRs

 Since, in some sense, a wildcard RR causes all possible names in an
 interval to exist, there should not be an NXT RR that would cover any
 part of this interval.  Thus if *.X.ZONE exists you would expect an
 NXT RR that ends at X.ZONE and one that starts with the last name
 covered by *.X.ZONE.  However, this "last name covered" is something
 very ugly and long like \255\255\255....X.zone.  So the NXT for the
 interval following is simply given the owner name *.X.ZONE and an
 RDATA of the next name after the wildcard.  This "*" type owner name
 is not expanded when the NXT is returned as authority information in
 connection with a query for a non-existent name.
 If there could be any wildcard RRs in a zone and thus wildcard NXTs,
 care must be taken in interpreting the results of explicit NXT
 retrievals as the owner name may be a wildcard expansion.
 The existence of one or more wildcard RRs covering a name interval
 makes it possible for a malicious server to hide any more
 specifically named RRs in the internal.  The server can just falsely

Eastlake & Kaufman Standards Track [Page 28] RFC 2065 DNS Security Extensions January 1997

 return the wildcard match NXT instead of the more specifically named
 RRs.  If there is a zone wide wildcard, there will be an NXT RR whose
 owner name is the wild card and whose RDATA is the zone name. In this
 case a server could conceal the existence of any more specific RRs in
 the zone.  It would be possible to design a more strict NXT feature
 which would eliminate this possibility.  But it would be more complex
 and might be so constraining as to make any dynamic update feature
 very difficult.

5.5 Blocking NXT Pseudo-Zone Transfers

 In a secure zone, a resolver can query for the initial NXT associated
 with the zone name.  Using the next domain name RDATA field from that
 RR, it can query for the next NXT RR.  By repeating this, it can walk
 through all the NXTs in the zone.  If there are no wildcards, it can
 use this technique to find all names in a zone. If it does type ANY
 queries, it can incrementally get all information in the zone and
 thus defeat attempts to administratively block zone transfers.
 If there are any wildcards, this NXT walking technique will not find
 any more specific RR names in the part of the name space the wildcard
 covers.  By doing explicit retrievals for wildcard names, a resolver
 could determine what intervals are covered by wildcards but still
 could not, with these techniques, find any names inside such
 intervals except by trying every name.
 If it is desired to block NXT walking, the recommended method is to
 add a zone wide wildcard of the KEY type with the no-key type value
 and with no type (zone, entity, or user) bit on.  This will cause
 there to be one zone covering NXT RR and leak no information about
 what real names exist in the zone.  This protection from pseudo-zone
 transfers is bought at the expense of eliminating the data origin
 authentication of the non-existence of names that NXT RRs can
 provide.  If an entire zone is covered by a wildcard, a malicious
 server can return an RR produced by matching the resulting wildcard
 NXT and can thus hide all the real data and delegations in the zone
 that have more specific names.

5.6 Special Considerations at Delegation Points

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

Eastlake & Kaufman Standards Track [Page 29] RFC 2065 DNS Security Extensions January 1997

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

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

 Retrieving or resolving authentic data from the Domain Name System
 (DNS) involves starting with one or more trusted public keys for one
 or more zones. With trusted keys, a resolver willing to perform
 cryptography can progress securely through the secure DNS zone
 structure to the zone of interest as described in Section 6.3. Such
 trusted public keys would normally be configured in a manner similar
 to that described in Section 6.2.  However, as a practical matter, a
 security aware resolver would still gain some confidence in the
 results it returns even if it was not configured with any keys but
 trusted what it got from a local well known server as a starting
 point.
 Data stored at a 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 to a KEY configured at the resolver via its boot file.
 Pending data has no authenticated SIGs and at least one additional
 SIG the resolver is still trying to authenticate.  Insecure data is
 data which it is known can never be either Authenticated or found Bad
 because it is in or has been reached via a non-secured zone. Behavior
 in terms of control of and flagging based on such data labels is
 described in Section 6.1.
 The proper validation of signatures requires a reasonably secure
 shared opinion of the absolute time between resolvers and servers as
 described in Section 6.4.

6.1 The AD and CD Header Bits

 Two previously unused bits are allocated out of the DNS
 query/response format header. The AD (authentic data) bit indicates
 in a response that the data included has been verified by the server
 providing it.  The CD (checking disabled) bit indicates in a query
 that non-verified data is acceptable to the resolver sending the
 query.

Eastlake & Kaufman Standards Track [Page 30] RFC 2065 DNS Security Extensions January 1997

 These bits are allocated from the 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 data. 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 reduce DNS latency time by allowing
 security aware servers to answer before they have resolved the
 validity of data.
 Security aware servers NEVER return Bad data.  For non-security aware
 resolvers or security aware resolvers requesting service by having
 the CD bit clear, security aware servers MUST return only
 Authenticated or Insecure data with the AD bit set in the response.
 Security aware resolvers will know that if data is Insecure versus
 Authentic by the absence of SIG RRs.  Security aware servers MAY
 return Pending data to security aware resolvers requesting the
 service by clearing the AD bit in the response.  The AD bit MUST NOT
 be set on a response unless all of the RRs in the response are either
 Authenticated or Insecure.

Eastlake & Kaufman Standards Track [Page 31] RFC 2065 DNS Security Extensions January 1997

6.2 Boot File Format

 Two boot file directives are added as described in this section.
 The format for a boot file directive to configure a starting zone key
 is as follows:
      pubkey name flags protocol algorithm key-data
 for a public key.  "name" is the owner name (if the line is
 translated into a KEY RR).  Flags indicates the type of key and is
 the same as the flag octet in the KEY RR.  Protocol and algorithm
 also have the same meaning as they do in the KEY RR.  The material
 after the algorithm is algorithm dependent and, for private
 algorithms (algorithm 254), starts with the algorithm's identifying
 OID and its length.  If the "no key" type value is set in flags or
 the algorithm is specified as 253, then the key-data after algorithm
 is null.  When present the key-data is treated as an octet stream and
 encoded in base 64 (see Appendix).
 A file of keys for cross certification or other purposes can be
 configured though the keyfile directive as follows:
      keyfile filename
 The file looks like a master file except that it can only contain KEY
 and SIG RRs with the SIGs signed under a key configured with the
 pubkey directive.
 While it might seem logical for everyone to start with the key for
 the root zone, this has problems.  The logistics of updating every
 DNS resolver in the world when the root key changes would be
 excessive.  It may be some time before there even is a root key.
 Furthermore, many organizations will explicitly wish their "interior"
 DNS implementations to completely trust only their own zone.  Such
 interior resolvers can then go through the organization's zone
 servers to access data outsize the organization's domain and should
 only be configured with the key forthe organization's DNS apex.

6.3 Chaining Through Zones

 Starting with one or more trusted keys for a zone, it should be
 possible to retrieve signed keys for its subzones which have a key
 and, if the zone is not root, for its superzone. Every authoritative
 secure zone server MUST also include the KEY RR for a super-zone
 signed by the secure zone via a keyfile directive. This makes it
 possible to climb the tree of zones if one starts below root.  A
 secure sub-zone is indicated by a KEY RR with non-null key

Eastlake & Kaufman Standards Track [Page 32] RFC 2065 DNS Security Extensions January 1997

 information appearing with the NS RRs for the sub-zone.  These make
 it possible to descend within the tree of zones.
 A resolver should keep track of the number of successive secure zones
 traversed from a starting point to any secure zone it can reach.  In
 general, the lower such a distance number is, the greater the
 confidence in the data.  Data configured via a boot file directive
 should be given a distance number of zero.  If a query encounters
 different data for the same query with different distance values,
 that with a larger value should be ignored.
 A security conscious resolver should completely refuse to step from a
 secure zone into a non-secure zone unless the non-secure zone is
 certified to be non-secure, or only experimentally secure, by the
 presence of an authenticated KEY RR for the non-secure zone with the
 no-key type value or the presence of a KEY RR with the experimental
 bit set.  Otherwise the resolver is getting bogus or spoofed data.
 If legitimate non-secure zones are encountered in traversing the DNS
 tree, then no zone can be trusted as secure that can be reached only
 via information from such non-secure zones. Since the non-secure zone
 data could have been spoofed, the "secure" zone reach via it could be
 counterfeit.  The "distance" to data in such zones or zones reached
 via such zones could be set to 512 or more as this exceeds the
 largest possible distance through secure zones in the DNS.
 Nevertheless, continuing to apply secure checks within "secure" zones
 reached via non-secure zones is a good practice and will, as a
 practical matter, provide some small increase in security.

6.4 Secure Time

 Coordinated interpretation of the time fields in SIG RRs requires
 that reasonably consistent time be available to the hosts
 implementing the DNS security extensions.
 A variety of time synchronization protocols exist including the
 Network Time Protocol (NTP, RFC1305).  If such protocols are used,
 they MUST be used securely so that time can not be spoofed.
 Otherwise, for example, a host could get its clock turned back and
 might then believe old SIG and KEY RRs which were valid but no longer
 are.

7. Operational Considerations

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

Eastlake & Kaufman Standards Track [Page 33] RFC 2065 DNS Security Extensions January 1997

7.1 Key Size Considerations

 There are a number of factors that effect public key size choice for
 use in the DNS security extension.  Unfortunately, these factors
 usually do not all point in the same direction.  Choice of zone key
 size should generally be made by the zone administrator depending on
 their local conditions.
 For most schemes, larger keys are more secure but slower.  Given a
 small public exponent, verification (the most common operation) for
 the MD5/RSA algorithm will vary roughly with the square of the
 modulus length, signing will vary with the cube of the modulus
 length, and key generation (the least common operation) will vary
 with the fourth power of the modulus length.  The current best
 algorithms for factoring a modulus and breaking RSA security vary
 roughly with the 1.6 power of the modulus itself.  Thus going from a
 640 bit modulus to a 1280 bit modulus only increases the verification
 time by a factor of 4 but increases the work factor of breaking the
 key by over 2^900.  An upper bound of 2552 bits has been established
 for the MD5/RSA DNS security algorithm for interoperability purposes.
 However, larger keys increase the size of the KEY and SIG RRs.  This
 increases the chance of DNS UDP packet overflow and the possible
 necessity for using higher overhead TCP in responses.
 The recommended minimum RSA algorithm modulus size, 640 bits, is
 believed by the authors to be secure at this time but high level
 zones in the DNS tree may wish to set a higher minimum, perhaps 1000
 bits, for security reasons.  (Since the United States National
 Security Agency generally permits export of encryption systems using
 an RSA modulus of up to 512 bits, use of that small a modulus, i.e.
 n, must be considered weak.)
 For a key used only to secure data and not to secure other keys, 640
 bits should be adequate at this time.

7.2 Key Storage

 It is recommended that zone private keys and the zone file master
 copy be kept and used in off-line non-network connected physically
 secure machines only.  Periodically an application can be run to add
 authentication to a zone by adding SIG and NXT RRs and adding no-key
 type KEY RRs for subzones where a real KEY RR is not provided. Then
 the augmented file can be transferred, perhaps by sneaker-net, to the
 networked zone primary server machine.
 The idea is to have a one way information flow to the network to
 avoid the possibility of tampering from the network.  Keeping the

Eastlake & Kaufman Standards Track [Page 34] RFC 2065 DNS Security Extensions January 1997

 zone master file on-line on the network and simply cycling it through
 an off-line signer does not do this.  The on-line version could still
 be tampered with if the host it resides on is compromised.  For
 maximum security, the master copy of the zone file should be off net
 and should not be updated based on an unsecured network mediated
 communication.
 Note, however, that secure resolvers must be configured with some
 trusted on-line public key information (or a secure path to such a
 resolver) or they will be unable to authenticate.
 Non-zone private keys, such as host or user keys, generally have to
 be kept on line to be used for real-time purposes such as DNS
 transaction security, IPSEC session set-up, or secure mail.

7.3 Key Generation

 Careful key generation is a sometimes overlooked but absolutely
 essential element in any cryptographically secure system.  The
 strongest algorithms used with the longest keys are still of no use
 if an adversary can guess enough to lower the size of the likely key
 space so that it can be exhaustively searched.  Suggestions will be
 found in RFC 1750.
 It is strongly recommended that key generation also occur off-line,
 perhaps on the machine used to sign zones (see Section 7.2).

7.4 Key Lifetimes

 No key should be used forever.  The longer a key is in use, the
 greater the probability that it will have been compromised through
 carelessness, accident, espionage, or cryptanalysis.  Furthermore, if
 key rollover is a rare event, there is an increased risk that, when
 the time does come up change the key, no one at the site will
 remember how to do it or other problems will have developed in the
 procedures.
 While key lifetime is a matter of local policy, these considerations
 suggest that no zone key should have a lifetime significantly over
 four years.  A reasonable maximum lifetime for zone keys that are
 kept off-line and carefully guarded is 13 months with the intent that
 they be replaced every year.  A reasonable maximum lifetime for end
 entity and useer keys that are used for IP-security or the like and
 are kept on line is 36 days with the intent that they be replaced
 monthly or more often.  In some cases, an entity key lifetime of
 somewhat over a day may be reasonable.

Eastlake & Kaufman Standards Track [Page 35] RFC 2065 DNS Security Extensions January 1997

7.5 Signature Lifetime

 Signature expiration times must be set far enough in the future that
 it is quite certain that new signatures can be generated before the
 old ones expire.  However, setting expiration too far into the future
 could, if bad data or signatures were ever generated, mean a long
 time to flush such badness.
 It is recommended that signature lifetime be a small multiple of the
 TTL but not less than a reasonable re-signing interval.

7.6 Root

 It should be noted that in DNS the root is a zone unto itself.  Thus
 the root zone key should only be seen signing itself or signing RRs
 with names one level below root, such as .aq, .edu, or .arpa.
 Implementations MAY reject as bogus any purported root signature of
 records with a name more than one level below root.  The root zone
 contains the root KEY RR signed by a SIG RR under the root key
 itself.

8. Conformance

 Levels of server and resolver conformance are defined.

8.1 Server Conformance

 Two levels of server conformance are defined as follows:
    Minimal server compliance is the ability to store and retrieve
    (including zone transfer) SIG, KEY, and NXT RRs.  Any secondary,
    caching, or other server for a secure zone MUST be at least
    minimally compliant and even then some things, such as secure
    CNAMEs, will not work without full compliance.
 Full server compliance adds the following to basic compliance:
    (1) ability to read SIG, KEY, and NXT RRs in zone files and (2)
    ability, given a zone file and private key, to add appropriate SIG
    and NXT RRs, possibly via a separate application, (3) proper
    automatic inclusion of SIG, KEY, and NXT RRs in responses, (4)
    suppression of CNAME following on retrieval of the security type
    RRs, (5) recognize the CD query header bit and set the AD query
    header bit, as appropriate, and (6) proper handling of the two NXT
    RRs at delegation points.  Primary servers for secure zones MUST
    be fully compliant and for completely successful secure operation,
    all secondary, caching, and other servers handling the zone SHOULD
    be fully compliant as well.

Eastlake & Kaufman Standards Track [Page 36] RFC 2065 DNS Security Extensions January 1997

8.2 Resolver Conformance

 Two levels of resolver compliance are defined:
    A basic compliance resolver can handle SIG, KEY, and NXT RRs when
    they are explicitly requested.
    A fully compliant resolver (1) understands KEY, SIG, and NXT RRs,
    (2) maintains appropriate information in its local caches and
    database to indicate which RRs have been authenticated and to what
    extent they have been authenticated, (3) performs additional
    queries as necessary to attempt to obtain KEY, SIG, or NXT RRs
    from non-security aware servers, (4) normally sets the CD query
    header bit on its queries.

9. Security Considerations

 This document describes technical details of extensions to the Domain
 Name System (DNS) protocol to provide data integrity and origin
 authentication, public key distribution, and optional transaction and
 request security.
 It should be noted that, at most, these extensions guarantee the
 validity of resource records, including KEY resource records,
 retrieved from the DNS.  They do not magically solve other security
 problems.  For example, using secure DNS you can have high confidence
 in the IP address you retrieve for a host name; however, this does
 not stop someone for substituting an unauthorized host at that
 address or capturing packets sent to that address and falsely
 responding with packets apparently from that address.  Any reasonably
 complete security system will require the protection of many
 additional facets of the Internet.

Eastlake & Kaufman Standards Track [Page 37] RFC 2065 DNS Security Extensions January 1997

References

 [NETSEC] -  Network Security: PRIVATE Communications in a PUBLIC
             World, Charlie Kaufman, Radia Perlman, & Mike Speciner,
             Prentice Hall Series in Computer Networking and
             Distributed Communications 1995.
 [PKCS1] -   PKCS #1: RSA Encryption Standard, RSA Data Security,
             Inc., 3 June 1991, Version 1.4.
 [RFC1032] - Stahl, M., "Domain Administrators Guide", RFC 1032,
             November 1987.
 [RFC1033] - Lottor, M., "Domain Administrators Operations Guide",
             RRFC 1033, November 1987.
 [RFC1034] - Mockapetris, P., "Domain Names - Concepts and
             Facilities", STD 13, RFC 1034, November 1987.
 [RFC1035] - Mockapetris, P., "Domain Names - Implementation and
             Specifications", STD 13, RFC 1035, November 1987.
 [RFC1305] - Mills, D., "Network Time Protocol (v3)", RFC 1305, March
             1992.
 [RFC1321] - Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
             April 1992.
 [RFC1530] - Malamud, C., and M. Rose, "Principles of Operation for
             the TPC.INT Subdomain: General Principles and Policy",
             RFC 1530, October 1993.
 [RFC1750] - Eastlake, D., Crocker, S., and J, Schiller, "Randomness
             Requirements for Security", RFC 1750, December 1994.
 [RFC1825] - Atkinson, R., "Security Architecture for the Internet
             Protocol", RFC 1825, August 1995.
 [RSA FAQ] - RSADSI Frequently Asked Questions periodic posting.

Eastlake & Kaufman Standards Track [Page 38] RFC 2065 DNS Security Extensions January 1997

Authors' Addresses

 Donald E. Eastlake 3rd
 CyberCash, Inc.
 318 Acton Street
 Carlisle, MA 01741 USA
 Telephone:   +1 508-287-4877
              +1 508-371-7148(fax)
              +1 703-620-4200(main office, Reston, Virginia, USA)
 EMail:       dee@cybercash.com
 Charles W. Kaufman
 Iris Associates
 1 Technology Park Drive
 Westford, MA 01886 USA
 Telephone:   +1 508-392-5276
 EMail:       charlie_kaufman@iris.com

Eastlake & Kaufman Standards Track [Page 39] RFC 2065 DNS Security Extensions January 1997

Appendix: Base 64 Encoding

 The following encoding technique is taken from RFC 1521 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 & Kaufman Standards Track [Page 40] RFC 2065 DNS Security Extensions January 1997

 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 & Kaufman Standards Track [Page 41]

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