DNSEXT Working Group                                Olafur Gudmundsson
  INTERNET-DRAFT                                               July 2001

  Updates: RFC 1035, RFC 2535, RFC 3008.

                  Delegation Signer record in parent.

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

  The list of Internet-Draft Shadow Directories can be accessed at

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

  This draft expires on January 15, 2002.

  Copyright Notice

  Copyright (C) The Internet Society (2001).  All rights reserved.


  One of the biggest problems in DNS occur when records of the same type
  can appear on both sides of an delegation. If the contents of these
  sets differs clients can get confused.  RFC2535 KEY records follows
  the same model as for NS records, parent is responsible for the
  records but the child is responsible for the contents. This document

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  proposes to store a different record in the parent that specifies
  which one of the child's keys are authorized to sign the child's KEY
  set. This change is not backwards compatible with RFC2535 but
  simplifies DNSSEC operation.

1 - Introduction

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

  When the same data can reside in two administratively different DNS
  zones sources it is common that the data gets out of sync. NS record
  in a zone indicates that there is a delegation at this name and the NS
  record lists the authorative servers for the real zone. Based on
  actual measurements 10-30% of all delegations in the Internet have
  differing NS sets at parent and child. There are number of reasons for
  this, including lack of communication between parent and child and
  bogus name-servers being listed to meet registrar requirements.

  DNSSEC [RFC2535,RFC3008,RFC3090] specifies that child must have its
  KEY set signed by the parent to create a verifiable chain of KEYs.
  There is some debate, where the signed KEY set should reside,
  parent[Parent] or child[RFC2535]. If the KEY set resides at the child,
  frequent communication is needed between the two parties, to transmit
  key sets up to parent and then the signed set or signatures down to
  child. If the KEY set resides at the parent[Parent] the communication
  is reduced having only child send updated key sets to parent.
  DNSSEC[RFC2535] requires that the parent store NULL key set for
  unsecure children, this complicates resolution process as in many
  cases as servers for both parent and child need to be queried for KEY
  set the [Parent] proposal simplifies this.

  Further complication of the DNSSEC KEY model is that KEY record is
  used to store DNS zone keys and public keys for other protocols. This
  can lead to large key sets at delegation points. There are number of
  potential problems with this including:
  1. KEY set may become quite large if many applications/protocols store
  their keys at the zone apex. Example of protocols are IPSEC, HTTP,
  SMTP, SSH etc.
  2. Key set may require frequent updates.
  3. Probability of compromised/lost keys increases and triggers
  emergency key rollover.
  4. Parent may refuse sign key sets with NON DNS zone keys.
  5. Parent may not have QoS on key changes that meets child's

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  Given these and other reasons there is good reason to explore
  alternatives to using only KEY records to create 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 apex, rule
  to require applications to store their KEY records at the SRV name for
  that application is one possibility. Another is to restrict KEY record
  to DNS keys only and create a new type for all non DNS keys. Third
  possible solution is to ban the storage of non DNS related keys at
  zone apex. There are other possible solutions but they are outside the
  scope of this document.

1.1 - Delegation Signer Record model

  This document proposes an alternative to the KEY record chain of
  trust, that uses a special record that can only reside at the parent.
  This record will identify the key(s) that child will use to self sign
  its own KEY set.

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

  Communication between the parent and child is reduced as the parent
  only needs to know of changes in DNS zone KEY(s) used to sign the apex
  KEY set. If other KEY records are stored at the zone apex, the parent
  does not need to be aware of them.

  This approach has the advantage that it minimizes the communication
  between the parent and child and the child is the authority for the
  KEY set with full control over the contents. This enables each to
  operate and maintain each zone independent of each other. Thus if
  child wants to have frequent key rollover for its DNS keys parent does
  not need to be aware of it as the child can use one key to only sign
  its apex KEY set and other keys to sign the other record sets in the
  zone. The child can just as well use the same key to sign all records
  in its zone.

  Another advantage is that this model fits well with slow rollout of
  DNSSEC and islands of security model. In the islands of security model
  someone that trusts "good.example." preconfigures a key from
  "good.example." as a trusted keys and from then on trusts any data
  that is signed by that key or 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. If DS records can
  also be used to identify trusted keys instead of KEY records.

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  The main disadvantage of this approach is double the number of
  signatures that need to be verified for the each delegation KEY set.
  There is no impact on verifying other record sets.

1.2 - Reserved words

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

2 - DS (Delegation KEY signer) record:

2.1 Protocol change

  DS record MUST only appear at secure delegations in the parent zone.
  The record lists the child's keys that SHOULD sign the child's key
  set.  Insecure delegation MUST NOT have a DS record, the presence of
  DS record SHOULD be considered a hint that the child might be secure.
  Resolver MUST only trust KEY records that match a DS record.
  NOTE: It has been suggested that NULL DS record for insecure children
  is better than no record. The advantage is to have authenticated
  denial of child's security status, the drawback is for large
  delegating zones there will be many NULL DS records. If parent uses
  NXT records adding NXT record to the authority section in the cases
  when no DS record exists at delegation will give the same result as
  NULL DS record.
  WG please comment on which approach is better.

  Updates RFC2535 sections 2.3.4 and 3.4, as well as RFC3008 section 
  2.7: Delegating zones MUST NOT store KEY records for delegations. The
  only records that can appear at delegation in parent are NS, SIG, NXT
  and DS.

  Zone MUST self sign its apex KEY set, it SHOULD sign it with a key
  that corresponds to a DS record in the parent. The KEY used to sign
  the apex KEY RRset CAN sign other RRsets in the zone.

  If child apex KEY RRset is not signed with one of the keys specified
  in the DS record the child is locally secure[RFC3090] and SHOULD only
  be considered secure the resolver has been instructed to trust the key
  used, via preconfiguration.

  Authorative server answering a query, that has the OK bit set[OKbit],
  MUST include the DS set in the additional section if the answer is a
  referral and there is space. Caching servers SHOULD return the DS
  record in the additional section under the same condition.

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2.1.1 - Comments on protocol change

  Over the years there has been various discussions on that the
  delegation model in DNS is broken as there is no real good way to
  assert if delegation exists. In RFC2535 version of DNSSEC the
  authentication of a delegation is the NS bit in the NXT bitmap at the
  delegation point. Something more explicit is needed and the DS record
  addresses this for secure delegations.

  DS record is the first DNS record to be only stored at the upper side
  of a delegation. NS records appear at both sides as do SIG and NXT.
  All other records can only appear at the lower side. This will cause
  some problems as servers authorative for parent may reject DS record
  even if the server understands unknown types, or not hand them out
  unless explicitly asked. Similarly a nameserver acting as a
  authorative for child and as a caching recursive server may never
  return the DS record.  A caching server does not care from which side
  DS record comes from and thus should not have to be changed if it
  supports unknown types.  Different TTL values on the childs NS set and
  parents DS set may cause the DS set to expire before the NS set. In
  this case an non-DS aware server would ask the child server for the DS
  set and get NXDOMAIN answer. DS aware server will know to ask the
  parent for the DS record.

  Secure resolvers need to know about the DS record and how to interpret
  it.  In the worst case, introducing the DS record, doubles the
  signatures that need to be checked to validate a KEY set.
  Note: The working group must determine if the tradeoff between more
  work in resolvers is justified by the operational simplification of
  this model. The author think this is a small price to pay to have a
  cleaner delegations structure. One argument put forward is that DNS
  should be optimized for read when ever possible, and on the face of it
  adding the DS record makes reading data from DNS more expensive. The
  operational complexities and legal hurdles that KEY records in parents
  or children make prevent DNSSEC to ever get deployed.

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2.2 Wire format of DS record

  The DS record consists of algorithm, size, key tag and SHA-1 digest of
  the public key KEY record allowed to sign the child's delegation.

                          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             |               size            |
     |  algorithm    |              SHA-1 digest                     |
     |                (20 bytes)                                     |
     |                                                               |
     |                                                               |
     |                                                               |
     |               |

  The key tag is calculated as specified in RFC2535, the size is the
  size of the public key in bits as specified in the document specifying
  the algorithm. Algorithm MUST be an algorithm number assigned in the
  range 1..251.  The SHA-1 digest is calculated over the canonical name
  of the delegation followed by the RDATA of the KEY record.
  The size of the DS RDATA is 25 bytes, regardless of the key size.
  NOTE: if 160 bits is to large the SHA-1 digest can be shortened but
  that weakens the overall security of the system.

2.2.1 Justifications for fields

  The algorithm and size fields are here to allow resolvers to quickly
  identify the candidate KEY records to examine. Key Tag is to allow
  quick check if this is a good candidate.  The key tag is redundant but
  provides some greater assurance than SHA-1 digest on its own. SHA-1 is
  a strong cryptographic checksum, it is hard for attacker to generate a
  KEY record that has the same SHA-1 digest. Making sure that the KEY
  record is a valid public key is much harder. Combining the name of the
  key and the key data as input to the digest provides stronger
  assurance of the binding.  Combining the SHA-1 with the other fields
  makes the task of the attacker is as hard breaking the public key.
  Even if someone creates a database of all SHA-1 key hashes seen so
  far, the addition of the name renders that database useless for
  attacks against random zones.

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2.3 Presentation format of DS record

  The presentation format of DS record consists of 2 numbers, followed
  by either the name of the signature algorithm or the algorithm number.
  The digest is to be presented in hex.

2.4 Justifications for compact format

  This format allows concise representation of the keys that child will
  use, thus keeping down the size of the answer for the delegation,
  reducing the probability of packet overflow. The SHA-1 hash is strong
  enough to uniquely identify the key. This is similar to the PGP

  Each DS record has RDATA size of 25, regardless of the size of the
  keys, keeping the answers from the parent smaller than if public key
  was used. The smallest currently defined KEY record RDATA is 70 bytes.

  Compact DS format is also better suited to list trusted keys for
  islands of security in configuration files.

2.5 Transition issues for installed base

  RFC2535 compliant resolver will assume that all DS secured delegations
  are locally secure. This is a bad thing, thus it might be necessary
  for a transition period to support both DS and SIG@Child. The cost is
  one more signatures in the answer and that early adopters have to
  cumbersome communications that DS is supposed to solve.

  Resolvers will not get confused as they will select signatures with
  the KEY they trust and ignore the other one.

3 Resolver Example

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

  Assume the key for domain example. is trusted.  In zone "example." we
  example.        KEY     <stuff>
  secure.example. DS      tag=12345 size=1024 alg=dsa <foofoo>
  secure.example. NS      ns1.secure.example.
                  NS      ns1.secure.example. s
  secure.example. NXT     NS SIG NXT DS tail.example.
  secure.example. SIG(NXT)
  secure.example. SIG(DS)

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  In zone "secure.example." we have
  secure.example. SOA      <soa stuff>
  secure.example. NS       ns1.secure.example.
                  NS       ns1.secure.example.
  secure.example. KEY      <tag=12345 size=1024 alg=dsa>
                  KEY      <tag=54321 size=512 alg=rsa/sha1>
                  KEY      <tag=32145 size=1024 alg=dsa>
  secure.example. SIG(KEY) <key-tag=12345 size=1024 alg=dsa>
  secure.example. SIG(SOA) <key-tag=54321 size=512 alg=rsa/sha1>
  secure.example. SIG(NS)  <key-tag=54321 size=512 alg=rsa/sha1>

  In this example the trusted key for example signs the DS record for
  "secure.example.", making that a trusted record. The DS record states
  what key is supposed to sign the KEY record at secure.example.  In
  this example "secure.example." has three different KEY records and the
  one corresponding to the KEY identified in the DS record signs the KEY
  set, thus the key set is validated and trusted.  Note that one of the
  other keys in the keyset actually signs the zone data, and resolvers
  will trust the signatures as the key appears in the KEY set.

  This example has only one DS record for the child but there no reason
  to outlaw multiple DS records. More than one DS record is needed
  during signing key rollover. It is strongly recommended that the DS
  set be kept small.

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 record has to be verified and possibly
  some other records based on policy, for example the contents of the NS
  set. Once the resolver gets to the appropriate delegation validating
  the answer may require verifying one or more signatures. For a simple
  A record lookup requires at least N delegations to be verified and 1
  RRset. For a DS enabled resolver the cost is 2N+1.  For MX record the
  cost where the target of the MX record is in the same zone as the MX
  record the costs are N+2 and 2N+2. In the case of negative answer the
  same holds ratios hold true.
  Resolver may require an extra query to get the DS record but 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, increases the size of
  delegation answers. DS requires much less storage in large delegation
  zones than SIG@Parent.

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4 Acknowledgments

  Number of people have over the last few years contributed number of
  ideas that are captured in this document. The core idea of using one
  key to that has only the role of signing a key set, comes from
  discussions with Bill Manning and Perry Metzger on how to put in a
  single root key in all resolver that lives for a long time.  Brian
  Wellington, Dan Massey, Edward Lewis, Havard Eidnes, Jakob Schlyter,
  Mark Kosters, Miek Gieben, Roy Arens, Scott Rosen have provided
  usefull comments.

4 - Security Considerations:

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

  In the representation of DS record, there is a possibility that an
  attacker can generate an valid KEY that matches all the checks thus
  starting to forge data from the child. This is considered impractical
  as on average more than 2**80 keys must be generated before one is
  found that will match.

  DS record is a change to DNSSEC protocol and there is some installed
  base of implementations, as well as text books on how to set up
  secured delegations. Implementations that do not understand DS record
  will not be able to follow the KEY to DS to KEY chain and consider all
  zone secured that way insecure.

5 - IANA Considerations:

  IANA needs to allocate RR type code for DS from the standard RR type

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[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.

[OKbit]    D. Conrad, ``Indicating Resolver Support of DNSSEC'', work in
           progress <draft-ietf-dnsext-dnssec-okbit-02.txt>, April 2001.

[Parent]   R. Gieben, T. Lindgreen, ``Parent stores the child's zone
           KEYs'', work in progress <draft-ietf-dnsext-parent-stores-
           zones-keys-01.txt>, May 2001.

Author Address

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

Appendix A: Changes from Prior versions

Changes from version 00
  Changed name from DK to DS based on working group comments.
  Dropped verbose format based on WG comments.
  Added text about TTL issue/problem in caching servers.
  Added text about islands of security and clarified the cost impact.
  Major editing of arguments and some reordering of text for clarity.
  Added section on transition issues.

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