DNSEXT Working Group                                Olafur Gudmundsson
  INTERNET-DRAFT                                               June 2002

  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
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   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
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   The list of current Internet-Drafts can be accessed at

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   Comments should be sent to the authors or the DNSEXT WG mailing list

   This draft expires on December 30, 2002.

   Copyright Notice

   Copyright (C) The Internet Society (2002).  All rights reserved.


   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

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   operational considerations. The intent is to use this resource record
   as an explicit statement about the delegation, rather than relying on

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

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
   registrar 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 simplifies 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.

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   Another complication of the DNSSEC key model is that the KEY record
   can be used to store public keys for other protocols in addition to
   DNSSEC keys.  There are number of potential problems with this,
   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
   2. The KEY RRset may require frequent updates.
   3. The probability of compromised or lost keys, which trigger
      emergency key rollover procedures, increases.
   4. The parent may refuse sign KEY RRsets with non-DNSSEC zone keys.
   5. The parent may not meet the child's expectations in turnaround
      time for resigning the KEY RRset.

   Given these and other reasons, there is good reason to explore
   alternatives to using only KEY records to create a chain of trust.

   Some of these problems can be reduced or eliminated by operational
   rules or protocol changes. To reduce the number of keys at the zone
   apex, a rule to require applications to store their KEY records at
   the SRV name for that application is one possibility. Another is to
   restrict the KEY record to only DNSSEC keys and create a new record
   type for all non-DNSSEC keys. A third possible solution is to
   prohibit the storage of non-DNSSEC keys at the zone apex. There are
   other possible solutions, but they are outside the scope of this

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 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.

   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.

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   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 and other
   applications and protocols 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 other keys 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."
   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 also be used 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 required
   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 required by RFC 2535.  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:
        parent NS RRset
        DS and SIG(DS)          (if DS is present)
        parent NXT and SIG(NXT) (If no DS)

   This increases the size of referral messages and may cause 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.

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

   The following section 2.2.1 replaces RFC2535 sections 2.3.4 and 3.4,
   section 2.2.2 replaces RFC3008 section 2.7, and RFC3090 updates are
   in section 2.2.3.

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 one of the
   parent zone's private key. 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.

2.2.2 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

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   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.

2.2.4 Changes to RFC3090

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

2.3 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

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   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 interoperability 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 may even refuse to pass on the DS or NXT records.

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  |
      |                SHA-1 digest                                   |
      |                (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 interoperability
   reasons, as few digest algorithms as possible should be reserved. The

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   only reason to reserve additional digest types is to increase

   DS records MUST point to zone KEY records that are allowed to
   authenticate DNS data.  The indicated KEY record's protocol field
   MUST be set to 3; flag field bits 0 and 6 MUST be set to 0; bit 7
   MUST be set to 1.  The value of other 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.

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 that a KEY RR
   must match.

   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

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   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
   RFC1035 delegation (in parent) has:

   RFC1035           NS

   RFC2535 adds the following two cases:

   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 a flag day is required.

2.7 KEY and corresponding DS record example

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

   dskey.example. KEY  256 3 1 (
                  ) ; key id = 28668
             DS   28668 1  1  49FD46E6C4B45C55D4AC69CBD3CD34AC1AFE51DE

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3 Resolver

3.1 DS Example

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

   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.   SIG(KEY) <key-tag=12345 alg=3>
   secure.example.   SIG(SOA) <key-tag=12345 alg=3>
   secure.example.   SIG(NS)  <key-tag=12345 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.  It is strongly
   recommended that the DS RRset be kept small: two or three DS records
   SHOULD be sufficient in all cases.

   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.

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3.1 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 may require an extra query to get the DS record and this
   may add to the overall cost of the query, 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.

   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.

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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, 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, Harald Alvestrand,
   and others have provided useful comments.

Normative References:

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

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

[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.

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

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Author Address

      Olafur Gudmundsson
      3826 Legation Street, NW
      Washington, DC,  20015

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