Delegation Signer (DS) Resource Record (RR)
draft-ietf-dnsext-delegation-signer-15

DNSEXT Working Group                                Olafur Gudmundsson
  INTERNET-DRAFT                                               June 2003
  <draft-ietf-dnsext-delegation-signer-15.txt>

  Updates: RFC 1035, RFC 2535, RFC 3008, RFC 3090.

                   Delegation Signer Resource Record

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   This draft expires on January 19, 2004.

   Copyright Notice

   Copyright (C) The Internet Society (2003).  All rights reserved.

Abstract

   The delegation signer (DS) resource record is inserted at a zone cut
   (i.e., a delegation point) to indicate that the delegated zone is
   digitally signed and that the delegated zone recognizes the indicated
   key as a valid zone key for the delegated zone. The DS RR is a
   modification to the DNS Security Extensions definition, motivated by
   operational considerations. The intent is to use this resource record
   as an explicit statement about the delegation, rather than relying on
   inference.

   Gudmundsson               Expires January 2004                  [Page 1]
INTERNET-DRAFT          Delegation Signer Record               June 2003

      This document defines the DS RR, gives examples of how it is used and
      describes the implications on resolvers. This change is not backwards
      compatible with RFC 2535.
      This document updates RFC1035, RFC2535, RFC3008 and RFC3090.

   Table of contents

   Status of this Memo  . . . . . . . . . . . . . . . . . . . . . . . .   1
   Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   1
   Table of contents  . . . . . . . . . . . . . . . . . . . . . . . . .   2
   1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.2 Reserved Words"  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2 Specification of the Delegation key Signer"  . . . . . . . . . . .   4
   2.1 Delegation Signer Record Model"  . . . . . . . . . . . . . . . .   4
   2.2 Protocol Change" . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.2.1 RFC2535 2.3.4 and 3.4: Special Considerations at
   Delegation Points" . . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.2.1.1  Special processing for DS queries"  . . . . . . . . . . . .   6
   2.2.1.2 Special processing when child and an ancestor share
   server"  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   2.2.1.3 Modification on use of KEY RR in the construction of
   Responses" . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   2.2.2 Signer's Name (replaces RFC3008 section 2.7)"  . . . . . . . .   9
   2.2.3 Changes to RFC3090"  . . . . . . . . . . . . . . . . . . . . .   9
   2.2.3.1 RFC3090: Updates to section 1: Introduction" . . . . . . . .   9
   2.2.3.2 RFC3090 section 2.1: Globally Secured" . . . . . . . . . . .   9
   2.2.3.3 RFC3090 section 3: Experimental Status."   . . . . . . . . .  10
   2.2.4 NULL KEY elimination"  . . . . . . . . . . . . . . . . . . . .  10
   2.3 Comments on Protocol Changes"  . . . . . . . . . . . . . . . . .  10
   2.4 Wire Format of the DS record"  . . . . . . . . . . . . . . . . .  11
   2.4.1 Justifications for Fields" . . . . . . . . . . . . . . . . . .  12
   2.5 Presentation Format of the DS Record"  . . . . . . . . . . . . .  12
   2.6 Transition Issues for Installed Base"  . . . . . . . . . . . . .  12
   2.6.1 Backwards compatibility with RFC2535 and RFC1035"  . . . . . .  12
   2.7 KEY and corresponding DS record example" . . . . . . . . . . . .  13
   3 Resolver"  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   3.1 DS Example"  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   3.2 Resolver Cost Estimates for DS Records"  . . . . . . . . . . . .  15
   4 Security Considerations: " . . . . . . . . . . . . . . . . . . . .  15
   5 IANA Considerations: " . . . . . . . . . . . . . . . . . . . . . .  16
   6 Acknowledgments" . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Normative References: "  . . . . . . . . . . . . . . . . . . . . . .  16
   Informational References" "  . . . . . . . . . . . . . . . . . . . .  17
   Author Address"  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Full Copyright Statement"  . . . . . . . . . . . . . . . . . . . . .  17

   Gudmundsson               Expires January 2004                  [Page 2]
INTERNET-DRAFT          Delegation Signer Record               June 2003

   1 Introduction

      Familiarity with the DNS system [RFC1035], DNS security extensions
      [RFC2535] and DNSSEC terminology [RFC3090] is important.

      Experience shows that when the same data can reside in two
      administratively different DNS zones, the data frequently gets out of
      sync. The presence of an NS RRset in a zone anywhere other than at
      the apex indicates a zone cut or delegation.  The RDATA of the NS
      RRset specifies the authoritative servers for the delegated or
      "child" zone. Based on actual measurements, 10-30% of all delegations
      on the Internet have differing NS RRsets at parent and child. There
      are a number of reasons for this, including a lack of communication
      between parent and child and bogus name servers being listed to meet
      registry requirements.

      DNSSEC [RFC2535,RFC3008,RFC3090] specifies that a child zone needs to
      have its KEY RRset signed by its parent to create a verifiable chain
      of KEYs. There has been some debate on where the signed KEY RRset
      should reside, whether at the child [RFC2535] or at the parent. If
      the KEY RRset resides at the child, maintaining the signed KEY RRset
      in the child requires frequent two-way communication between the two
      parties. First the child transmits the KEY RRset to the parent and
      then the parent sends the signature(s) to the child. Storing the KEY
      RRset at the parent was thought to simplify the communication.

      DNSSEC [RFC2535] requires that the parent store a NULL KEY record for
      an unsecure child zone to indicate that the child is unsecure. A NULL
      KEY record is a waste: an entire signed RRset is used to communicate
      effectively one bit of information--that the child is unsecure.
      Chasing down NULL KEY RRsets complicates the resolution process in
      many cases, because servers for both parent and child need to be
      queried for the KEY RRset if the child server does not return it.
      Storing the KEY RRset only in the parent zone simplifies this and
      would allow the elimination of the NULL KEY RRsets entirely. For
      large delegation zones the cost of NULL keys is a significant barrier
      to deployment.

      Prior to the restrictions imposed by RFC3445[RFC3445], another
      implication of the DNSSEC key model is that the KEY record could be
      used to store public keys for other protocols in addition to DNSSEC
      keys.  There are number of potential problems with this, including:
      1. The KEY RRset can become quite large if many applications and
         protocols store their keys at the zone apex. Possible protocols
         are IPSEC, HTTP, SMTP, SSH and others that use public key
         cryptography.
      2. The KEY RRset may require frequent updates.
      3. The probability of compromised or lost keys, which trigger
         emergency key rollover procedures, increases.

   Gudmundsson               Expires January 2004                  [Page 3]
INTERNET-DRAFT          Delegation Signer Record               June 2003

      4. The parent may refuse to sign KEY RRsets with non-DNSSEC zone
      keys.
      5. The parent may not meet the child's expectations of turnaround
         time for resigning the KEY RRset.

      Given these reasons, SIG@parent isn't any better than SIG/KEY@Child.

   1.2 Reserved Words

      The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED",
      "RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be
      interpreted as described in RFC2119.

   2 Specification of the Delegation key Signer

      This section defines the Delegation Signer (DS) RR type (type code
      TBD) and the changes to DNS to accommodate it.

   2.1 Delegation Signer Record Model

      This document presents a replacement for the DNSSEC KEY record chain
      of trust [RFC2535] that uses a new RR that resides only at the
      parent.  This record identifies the key(s) that the child uses to
      self-sign its own KEY RRset.

      Even though DS identifies two roles for KEYs, Key Signing Key (KSK)
      and Zone Signing Key (ZSK), there is no requirement that zone use two
      different keys for these roles. It is expected that many small zones
      will only use one key, while larger zones will be more likely to use
      multiple keys.

      The chain of trust is now established by verifying the parent KEY
      RRset, the DS RRset from the parent and the KEY RRset at the child.
      This is cryptographically equivalent to using just KEY records.

      Communication between the parent and child is greatly reduced, since
      the child only needs to notify the parent about changes in keys that
      sign its apex KEY RRset.  The parent is ignorant of all other keys in
      the child's apex KEY RRset. Furthermore, the child maintains full
      control over the apex KEY RRset and its content.  The child can
      maintain any policies regarding its KEY usage for DNSSEC with minimal
      impact on the parent. Thus if the child wants to have frequent key
      rollover for its DNS zone keys, the parent does not need to be aware
      of it. The child can use one key to sign only its apex KEY RRset and
      a different key to sign the other RRsets in the zone.

      This model fits well with a slow roll out of DNSSEC and the islands
      of security model. In this model, someone who trusts "good.example."

   Gudmundsson               Expires January 2004                  [Page 4]
INTERNET-DRAFT          Delegation Signer Record               June 2003

      can preconfigure a key from "good.example." as a trusted key, and
      from then on trusts any data signed by that key or that has a chain
      of trust to that key.  If "example." starts advertising DS records,
      "good.example." does not have to change operations by suspending
      self-signing. DS records can be used in configuration files to
      identify trusted keys instead of KEY records.  Another significant
      advantage is that the amount of information stored in large
      delegation zones is reduced: rather than the NULL KEY record at every
      unsecure delegation demanded by RFC 2535, only secure delegations
      require additional information in the form of a signed DS RRset.

      The main disadvantage of this approach is that verifying a zone's KEY
      RRset requires two signature verification operations instead of the
      one in RFC 2535 chain of trust.  There is no impact on the number of
      signatures verified for other types of RRsets.

   2.2 Protocol Change

      All DNS servers and resolvers that support DS MUST support the OK bit
      [RFC3225] and a larger message size [RFC3226].  In order for a
      delegation to be considered secure the delegation MUST contain a DS
      RRset.  If a query contains the OK bit, a server returning a referral
      for the delegation MUST include the following RRsets in the authority
      section in this order:
      If DS RRset is present:
           parent's copy of child's NS RRset
           DS and SIG(DS)
      If no DS RRset is present:
           parent's copy of child's NS RRset
           parent's zone NXT and SIG(NXT)

      This increases the size of referral messages, possibly causing some
      or all glue to be omitted. If the DS or NXT RRsets with signatures do
      not fit in the DNS message, the TC bit MUST be set.  Additional
      section processing is not changed.

      A DS RRset accompanying a NS RRset indicates that the child zone is
      secure. If a NS RRset exists without a DS RRset, the child zone is
      unsecure (from the parents point of view).  DS RRsets MUST NOT appear
      at non-delegation points or at a zone's apex.

      Section 2.2.1 defines special considerations related to authoritative
      servers responding to DS queries and replaces RFC2535 sections 2.3.4
      and 3.4. Section 2.2.2 replaces RFC3008 section 2.7, and section
      2.2.3 updates RFC3090.

   Gudmundsson               Expires January 2004                  [Page 5]
INTERNET-DRAFT          Delegation Signer Record               June 2003

   2.2.1 RFC2535 2.3.4 and 3.4: Special Considerations at Delegation Points

      DNS security views 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 that are also the apex nodes of a child zone
      (i.e., delegation points) as "really" belonging to the child zone.
      The corresponding domain names appear in two master files and might
      have RRsets signed by both the parent and child zones' keys. A
      retrieval could get a mixture of these RRsets and SIGs, especially
      since one server could be serving both the zone above and below a
      delegation point [RFC 2181].

      Each DS RRset stored in the parent zone MUST be signed by at least
      one of the parent zone's private keys. The parent zone MUST NOT
      contain a KEY RRset at any delegation point. Delegations in the
      parent MAY contain only the following RR types: NS, DS, NXT and SIG.
      The NS RRset MUST NOT be signed.  The NXT RRset is the exceptional
      case: it will always appear differently and authoritatively in both
      the parent and child zones if both are secure.

      A secure zone MUST contain a self-signed KEY RRset at its apex.  Upon
      verifying the DS RRset from the parent, a resolver MAY trust any KEY
      identified in the DS RRset as a valid signer of the child's apex KEY
      RRset. Resolvers configured to trust one of the keys signing the KEY
      RRset MAY now treat any data signed by the zone keys in the KEY RRset
      as secure.  In all other cases resolvers MUST consider the zone
      unsecure. A DS RRset MUST NOT appear at a zone's apex.

      An authoritative server queried for type DS MUST return the DS RRset
      in the answer section.

   2.2.1.1  Special processing for DS queries

      When a server is authoritative for the parent zone at a delegation
      point and receives a query for the DS record at that name, it MUST
      answer based on data in the parent zone, return DS or negative
      answer.  This is true whether or not it is also authoritative for the
      child zone.

      When the server is authoritative for the child zone at a delegation
      point but not the parent zone, there is no natural response, since
      the child zone is not authoritative for the DS record at the zone's
      apex.  As these queries are only expected to originate from recursive
      servers which are not DS-aware, the authoritative server MUST answer
      with:
           RCODE:             NOERROR
           AA bit:            set

   Gudmundsson               Expires January 2004                  [Page 6]
INTERNET-DRAFT          Delegation Signer Record               June 2003

           Answer Section:    Empty
           Authority Section: SOA [+ SIG(SOA) + NXT + SIG(NXT)]

      That is, it answers as if it is authoritative and the DS record does
      not exist.  DS-aware recursive servers will query the parent zone at
      delegation points, so will not be affected by this.

      A server authoritative for only the child zone, that is also a
      caching server MAY (if the RD bit is set in the query) perform
      recursion to find the DS record at the delegation point, or MAY
      return the DS record from its cache.  In this case, the AA bit MUST
      not be set in the response.

   2.2.1.2 Special processing when child and an ancestor share server

      Special rules are needed to permit DS RR aware servers to gracefully
      interact with older caches which otherwise might falsely label a
      server as lame because of the placement of the DS RR set.

      Such a situation might arise when a server is authoritative for both
      a zone and it's grandparent, but not the parent.  This sounds like an
      obscure example, but it is very real.  The root zone is currently
      served on 13 machines, and "root-servers.net." is served on 4 of the
      same 13, but "net." is served elsewhere.

      When a server receives a query for (<QNAME>, DS, <QCLASS>), the
      response MUST be determined from reading these rules in order:

      1) If the server is authoritative for the zone that holds the DS RR
      set (i.e., the zone that delegates <QNAME>, aka the "parent" zone),
      the response contains the DS RR set as an authoritative answer.

      2) If the server is offering recursive service and the RD bit is set
      in the query, the server performs the query itself (according to the
      rules for resolvers described below) and returns its findings.

      3) If the server is authoritative for the zone that holds the
      <QNAME>'s SOA RR set, the response is an authoritative negative
      answer as described in 2.2.1.1.

      4) If the server is authoritative for a zone or zones above the
      QNAME, a referral to the most enclosing zone's servers is made.

      5) If the server is not authoritative for any part of the QNAME, a
      response indicating a lame server for QNAME is given.

   Gudmundsson               Expires January 2004                  [Page 7]
INTERNET-DRAFT          Delegation Signer Record               June 2003

      Using these rules will require some special processing on the part of
      a DS RR aware resolver.  To illustrate this, an example is used.

      Assuming a server is authoritative for roots.example.net. and for the
      root zone but not the intervening two zones (or the intervening two
      label deep zone).  Assume that QNAME=roots.example.net., QTYPE=DS,
      and QCLASS=IN.

      The resolver will issue this request (assuming no cached data)
      expecting a referral to a net. server.  Instead, rule number 3 above
      applies and a negative answer is returned by the server.  The
      reaction by the resolver is not to accept this answer as final as it
      can determine from the SOA RR in the negative answer the context
      within which the server has answered.

      A solution to this is to instruct the resolver to hunt for the
      authoritative zone of the data in a brute force manner.

      This can be accomplished by taking the owner name of the returned SOA
      RR and striping off enough left-hand labels until a successful NS
      response is obtained.  A successful response here means that the
      answer has NS records in it.  (Entertaining the possibility that a
      cut point can be two labels down in a zone.)

      Returning to the example, the response will include a negative answer
      with either the SOA RR for "roots.example.net." or "example.net."
      depending on whether roots.example.net is a delegated domain.  In
      either case, removing the left most label of the SOA owner name will
      lead to the location of the desired data.

   2.2.1.3 Modification on use of KEY RR in the construction of Responses

      This section updates RFC2535 section 3.5 by replacing it with the
      following:

      A query for KEY RR MUST NOT trigger any additional section
      processing.  Security aware resolvers will include corresponding SIG
      records in the answer section.

      KEY records SHOULD NOT be added to the additional records section in
      response to any query.

      RFC2535 specified that KEY records be added to the additional section
      when SOA or NS records where included in an answer. This was done to
      reduce round trips (in the case of SOA) and to force out NULL KEYs
      (in the NS case).  As this document obsoletes NULL keys there is no
      need for the inclusion of KEYs with NSs. Furthermore as SOAs are
      included in the authority section of negative answers, including the

   Gudmundsson               Expires January 2004                  [Page 8]
INTERNET-DRAFT          Delegation Signer Record               June 2003

      KEYs each time will cause redundant transfers of KEYs.

      RFC2535 section 3.5 also included rule for adding the KEY RRset to
      the response for a query for A and AAAA types. As Restrict
      KEY[RFC3445] eliminated use of KEY RR by all applications this rule
      is no longer needed.

   2.2.2 Signer's Name (replaces RFC3008 section 2.7)

      The signer's name field of a SIG RR MUST contain the name of the zone
      to which the data and signature belong.  The combination of signer's
      name, key tag, and algorithm MUST identify a zone key if the SIG is
      to be considered material.  This document defines a standard policy
      for DNSSEC validation; local policy MAY override the standard policy.

      There are no restrictions on the signer field of a SIG(0) record.
      The combination of signer's name, key tag, and algorithm MUST
      identify a key if this SIG(0) is to be processed.

   2.2.3 Changes to RFC3090

      A number of sections of RFC3090 need to be updated to reflect the DS
      record.

   2.2.3.1 RFC3090: Updates to section 1: Introduction

      Most of the text is still relevant but the words ``NULL key'' are to
      be replaced with ``missing DS RRset''. In section 1.3 the last three
      paragraphs discuss the confusion in sections of RFC 2535 that are
      replaced in section 2.2.1 above. Therefore, these paragraphs are now
      obsolete.

   2.2.3.2 RFC3090 section 2.1: Globally Secured

      Rule 2.1.b is replaced by the following rule:

      2.1.b. The KEY RRset at a zone's apex MUST be self-signed by a
      private key whose public counterpart MUST appear in a zone signing
      KEY RR (2.a) owned by the zone's apex and specifying a mandatory-to-
      implement algorithm.  This KEY RR MUST be identified by a DS RR in a
      signed DS RRset in the parent zone.

      If a zone cannot get its parent to advertise a DS record for it, the
      child zone cannot be considered globally secured.  The only exception
      to this is the root zone, for which there is no parent zone.

   Gudmundsson               Expires January 2004                  [Page 9]
INTERNET-DRAFT          Delegation Signer Record               June 2003

   2.2.3.3 RFC3090 section 3: Experimental Status.

      The only difference between experimental status and globally secured
      is the missing DS RRset in the parent zone. All locally secured zones
      are experimental.

   2.2.4 NULL KEY elimination

      RFC3445 section 3 eliminates the top two bits in the flags field of
      KEY RR. These two bits were used to indicate NULL KEY or NO KEY.
      RFC3090 defines that zone is either secure or not, these rules
      eliminates the possible need to put NULL keys in the zone apex to
      indicate that the zone is not secured for a algorithm.  Along with
      this document these other two eliminate all uses for the NULL KEY,
      This document obsoletes NULL KEY.

   2.3 Comments on Protocol Changes

      Over the years there have been various discussions surrounding the
      DNS delegation model, declaring it to be broken because there is no
      good way to assert if a delegation exists. In the RFC2535 version of
      DNSSEC, the presence of the NS bit in the NXT bit map proves there is
      a delegation at this name.  Something more explicit is needed and the
      DS record addresses this need for secure delegations.

      The DS record is a major change to DNS: it is the first resource
      record that can appear only on the upper side of a delegation. Adding
      it will cause interoperabilty problems and requires a flag day for
      DNSSEC. Many old servers and resolvers MUST be upgraded to take
      advantage of DS.  Some old servers will be able to be authoritative
      for zones with DS records but will not add the NXT or DS records to
      the authority section.  The same is true for caching servers; in
      fact, some might even refuse to pass on the DS or NXT records.

   Gudmundsson               Expires January 2004                 [Page 10]
INTERNET-DRAFT          Delegation Signer Record               June 2003

   2.4 Wire Format of the DS record

      The DS (type=TDB) record contains these fields: key tag, algorithm,
      digest type, and the digest of a public key KEY record that is
      allowed and/or used to sign the child's apex KEY RRset. Other keys
      MAY sign the child's apex KEY RRset.

                              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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           key tag             |  algorithm    |  Digest type  |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                digest  (length depends on type)               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                (SHA-1 digest is 20 bytes)                     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The key tag is calculated as specified in RFC2535. Algorithm MUST be
      an algorithm number assigned in the range 1..251 and the algorithm
      MUST be allowed to sign DNS data.  The digest type is an identifier
      for the digest algorithm used. The digest is calculated over the
      canonical name of the delegated domain name followed by the whole
      RDATA of the KEY record (all four fields).

         digest = hash( canonical FQDN on KEY RR | KEY_RR_rdata)

         KEY_RR_rdata = Flags | Protocol | Algorithm | Public Key

      Digest type value 0 is reserved, value 1 is SHA-1, and reserving
      other types requires IETF standards action. For interoperabilty
      reasons, keeping number of digest algorithms low is strongly
      RECOMMENDED.  The only reason to reserve additional digest types is
      to increase security.

      DS records MUST point to zone KEY records that are allowed to
      authenticate DNS data.  The indicated KEY records protocol field MUST
      be set to 3; flag field bit 7 MUST be set to 1.  The value of other
      flag bits is not significant for the purposes of this document.

      The size of the DS RDATA for type 1 (SHA-1) is 24 bytes, regardless
      of key size.  New digest types probably will have larger digests.

   Gudmundsson               Expires January 2004                 [Page 11]
INTERNET-DRAFT          Delegation Signer Record               June 2003

   2.4.1 Justifications for Fields

      The algorithm and key tag fields are present to allow resolvers to
      quickly identify the candidate KEY records to examine.  SHA-1 is a
      strong cryptographic checksum: it is computationally infeasible for
      an attacker to generate a KEY record that has the same SHA-1 digest.
      Combining the name of the key and the key rdata as input to the
      digest provides stronger assurance of the binding.  Having the key
      tag in the DS record adds greater assurance than the SHA-1 digest
      alone, as there are now two different mapping functions.

      This format allows concise representation of the keys that the child
      will use, thus keeping down the size of the answer for the
      delegation, reducing the probability of DNS message overflow. The
      SHA-1 hash is strong enough to uniquely identify the key and is
      similar to the PGP key footprint. The digest type field is present
      for possible future expansion.

      The DS record is well suited to listing trusted keys for islands of
      security in configuration files.

   2.5 Presentation Format of the DS Record

      The presentation format of the DS record consists of three numbers
      (key tag, algorithm and digest type) followed by the digest itself
      presented in hex:
         example.   DS  12345 3 1 123456789abcdef67890123456789abcdef67890

   2.6 Transition Issues for Installed Base

      No backwards compatibility with RFC2535 is provided.

      RFC2535-compliant resolvers will assume that all DS-secured
      delegations are locally secure. This is bad, but the DNSEXT Working
      Group has determined that rather than dealing with both
      RFC2535-secured zones and DS-secured zones, a rapid adoption of DS is
      preferable.  Thus the only option for early adopters is to upgrade to
      DS as soon as possible.

   2.6.1 Backwards compatibility with RFC2535 and RFC1035

      This section documents how a resolver determines the type of
      delegation.
      RFC1035 delegation (in parent) has:

      RFC1035           NS

      RFC2535 adds the following two cases:

   Gudmundsson               Expires January 2004                 [Page 12]
INTERNET-DRAFT          Delegation Signer Record               June 2003

      Secure RFC2535:   NS + NXT + SIG(NXT)
                        NXT bit map contains: NS SIG NXT
      Unsecure RFC2535: NS + KEY + SIG(KEY) + NXT + SIG(NXT)
                        NXT bit map contains: NS SIG KEY NXT
                        KEY must be a NULL key.

      DNSSEC with DS has the following two states:

      Secure DS:        NS + DS + SIG(DS)
                        NXT bit map contains: NS SIG NXT DS
      Unsecure DS:      NS + NXT + SIG(NXT)
                        NXT bit map contains: NS SIG NXT

      It is difficult for a resolver to determine if a delegation is secure
      RFC 2535 or unsecure DS. This could be overcome by adding a flag to
      the NXT bit map, but only upgraded resolvers would understand this
      flag, anyway. Having both parent and child signatures for a KEY RRset
      might allow old resolvers to accept a zone as secure, but the cost of
      doing this for a long time is much higher than just prohibiting RFC
      2535-style signatures at child zone apexes and forcing rapid
      deployment of DS-enabled servers and resolvers.

      RFC 2535 and DS can in theory be deployed in parallel, but this would
      require resolvers to deal with RFC 2535 configurations forever.  This
      document obsoletes the NULL KEY in parent zones, which is a difficult
      enough change that to cause a flag day.

   2.7 KEY and corresponding DS record example

      This is an example of a KEY record and the corresponding DS record.

      dskey.example. KEY  256 3 1 (
                     AQPwHb4UL1U9RHaU8qP+Ts5bVOU1s7fYbj2b3CCbzNdj
                     4+/ECd18yKiyUQqKqQFWW5T3iVc8SJOKnueJHt/Jb/wt
                     ) ; key id = 28668
                DS   28668 1  1  49FD46E6C4B45C55D4AC69CBD3CD34AC1AFE51DE

   Gudmundsson               Expires January 2004                 [Page 13]
INTERNET-DRAFT          Delegation Signer Record               June 2003

   3 Resolver

   3.1 DS Example

      To create a chain of trust, a resolver goes from trusted KEY to DS to
      KEY.

      Assume the key for domain "example." is trusted.  Zone "example."
      contains at least the following records:
      example.          SOA     <soa stuff>
      example.          NS       ns.example.
      example.          KEY     <stuff>
      example.          NXT      NS SOA KEY SIG NXT secure.example.
      example.          SIG(SOA)
      example.          SIG(NS)
      example.          SIG(NXT)
      example.          SIG(KEY)
      secure.example.   NS      ns1.secure.example.
      secure.example.   DS      tag=12345 alg=3 digest_type=1 <foofoo>
      secure.example.   NXT     NS SIG NXT DS unsecure.example.
      secure.example.   SIG(NXT)
      secure.example.   SIG(DS)
      unsecure.example  NS      ns1.unsecure.example.
      unsecure.example. NXT     NS SIG NXT example.
      unsecure.example. SIG(NXT)

      In zone "secure.example." following records exist:
      secure.example.   SOA      <soa stuff>
      secure.example.   NS       ns1.secure.example.
      secure.example.   KEY      <tag=12345 alg=3>
      secure.example.   KEY      <tag=54321 alg=5>
      secure.example.   NXT      <nxt stuff>
      secure.example.   SIG(KEY) <key-tag=12345 alg=3>
      secure.example.   SIG(SOA) <key-tag=54321 alg=5>
      secure.example.   SIG(NS)  <key-tag=54321 alg=5>
      secure.example.   SIG(NXT) <key-tag=54321 alg=5>

      In this example the private key for "example." signs the DS record
      for "secure.example.", making that a secure delegation. The DS record
      states which key is expected to sign the KEY RRset at
      "secure.example.".  Here "secure.example." signs its KEY RRset with
      the KEY identified in the DS RRset, thus the KEY RRset is validated
      and trusted.

      This example has only one DS record for the child, but parents MUST
      allow multiple DS records to facilitate key rollover and multiple KEY
      algorithms.

   Gudmundsson               Expires January 2004                 [Page 14]
INTERNET-DRAFT          Delegation Signer Record               June 2003

      The resolver determines the security status of "unsecure.example." by
      examining the parent zone's NXT record for this name.  The absence of
      the DS bit indicates an unsecure delegation. Note the NXT record
      SHOULD only be examined after verifying the corresponding signature.

   3.2 Resolver Cost Estimates for DS Records

      From a RFC2535 resolver point of view, for each delegation followed
      to chase down an answer, one KEY RRset has to be verified.
      Additional RRsets might also need to be verified based on local
      policy (e.g., the contents of the NS RRset). Once the resolver gets
      to the appropriate delegation, validating the answer might require
      verifying one or more signatures.  A simple A record lookup requires
      at least N delegations to be verified and one RRset. For a DS-enabled
      resolver, the cost is 2N+1.  For an MX record, where the target of
      the MX record is in the same zone as the MX record, the costs are N+2
      and 2N+2, for RFC 2535 and DS, respectively. In the case of negatives
      answer the same ratios hold true.

      The resolver have to do an extra query to get the DS record and this
      increases the overall cost of resolving this question, but this is
      never worse than chasing down NULL KEY records from the parent in
      RFC2535 DNSSEC.

      DS adds processing overhead on resolvers and increases the size of
      delegation answers, but much less than storing signatures in the
      parent zone.

   4 Security Considerations:

      This document proposes a change to the validation chain of KEY
      records in DNSSEC. The change is not believed to reduce security in
      the overall system. In RFC2535 DNSSEC, the child zone has to
      communicate keys to its parent and prudent parents will require some
      authentication with that transaction. The modified protocol will
      require the same authentication, but allows the child to exert more
      local control over its own KEY RRset.

      There is a remote possibility that an attacker could generate a valid
      KEY that matches all the DS fields, of a specific DS set, and thus
      forge data from the child. This possibility is considered
      impractical, as on average more than
          2 ^ (160 - <Number of keys in DS set>)
      keys would have to be generated before a match would be found.

      An attacker that wants to match any DS record will have to generate
      on average at least 2^80 keys.

   Gudmundsson               Expires January 2004                 [Page 15]
INTERNET-DRAFT          Delegation Signer Record               June 2003

      The DS record represents a change to the DNSSEC protocol and there is
      an installed base of implementations, as well as textbooks on how to
      set up secure delegations. Implementations that do not understand the
      DS record will not be able to follow the KEY to DS to KEY chain and
      will consider all zones secured that way as unsecure.

   5 IANA Considerations:

      IANA needs to allocate an RR type code for DS from the standard RR
      type space (type 43 requested).

      IANA needs to open a new registry for the DS RR type for digest
      algorithms. Defined types are:
          0 is Reserved,
          1 is SHA-1.
      Adding new reservations requires IETF standards action.

   6 Acknowledgments

      Over the last few years a number of people have contributed ideas
      that are captured in this document. The core idea of using one key to
      sign only the KEY RRset comes from discussions with Bill Manning and
      Perry Metzger on how to put in a single root key in all resolvers.
      Alexis Yushin, Brian Wellington, Sam Weiler, Paul Vixie, Jakob
      Schlyter, Scott Rose, Edward Lewis, Lars-Johan Liman, Matt Larson,
      Mark Kosters, Dan Massey, Olaf Kolman, Phillip Hallam-Baker, Miek
      Gieben, Havard Eidnes, Donald Eastlake 3rd., Randy Bush, David
      Blacka, Steve Bellovin, Rob Austein, Derek Atkins, Roy Arends, Mark
      Andrews, Harald Alvestrand, and others have provided useful comments.

   Normative References:

   [RFC1035]  P. Mockapetris, ``Domain Names - Implementation and
              Specification'', STD 13, RFC 1035, November 1987.

   [RFC2535]  D. Eastlake, ``Domain Name System Security Extensions'', RFC
              2535, March 1999.

   [RFC3008]  B. Wellington, ``Domain Name System Security (DNSSEC) Signing
              Authority'', RFC 3008, November 2000.

   [RFC3090]  E. Lewis `` DNS Security Extension Clarification on Zone
              Status'', RFC 3090, March 2001.

   [RFC3225]  D. Conrad, ``Indicating Resolver Support of DNSSEC'', RFC
              3225, December 2001.

   [RFC3445]  D. Massey, S. Rose ``Limiting the scope of the KEY Resource
              Record (RR)``, RFC 3445, December 2002.

   Gudmundsson               Expires January 2004                 [Page 16]
INTERNET-DRAFT          Delegation Signer Record               June 2003

   Informational References

   [RFC2181]  R. Elz, R. Bush, ``Clarifications to the DNS Specification'',
              RFC 2181, July 1997.

   [RFC3226]  O. Gudmundsson, ``DNSSEC and IPv6 A6 aware server/resolver
              message size requirements'', RFC 3226, December 2001.

   Author Address

         Olafur Gudmundsson
         3821 Village Park Drive
         Chevy Chase, MD,  20815
         USA
         <ogud@ogud.com>

   Full Copyright Statement

      Copyright (C) The Internet Society (2003).  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."

   Gudmundsson               Expires January 2004                 [Page 17]
INTERNET-DRAFT          Delegation Signer Record               June 2003

   Gudmundsson               Expires January 2004                  [Page 1]