path: root/doc/rfc/rfc3445.txt

Network Working Group                                          D. Massey
Request for Comments: 3445                                       USC/ISI
Updates: 2535                                                    S. Rose
Category: Standards Track                                           NIST
                                                           December 2002


           Limiting the Scope of the KEY Resource Record (RR)

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document limits the Domain Name System (DNS) KEY Resource Record
   (RR) to only keys used by the Domain Name System Security Extensions
   (DNSSEC).  The original KEY RR used sub-typing to store both DNSSEC
   keys and arbitrary application keys.  Storing both DNSSEC and
   application keys with the same record type is a mistake.  This
   document removes application keys from the KEY record by redefining
   the Protocol Octet field in the KEY RR Data.  As a result of removing
   application keys, all but one of the flags in the KEY record become
   unnecessary and are redefined.  Three existing application key sub-
   types are changed to reserved, but the format of the KEY record is
   not changed.  This document updates RFC 2535.

1. Introduction

   This document limits the scope of the KEY Resource Record (RR).  The
   KEY RR was defined in [3] and used resource record sub-typing to hold
   arbitrary public keys such as Email, IPSEC, DNSSEC, and TLS keys.
   This document eliminates the existing Email, IPSEC, and TLS sub-types
   and prohibits the introduction of new sub-types.  DNSSEC will be the
   only allowable sub-type for the KEY RR (hence sub-typing is
   essentially eliminated) and all but one of the KEY RR flags are also
   eliminated.






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   Section 2 presents the motivation for restricting the KEY record and
   Section 3 defines the revised KEY RR.  Sections 4 and 5 summarize the
   changes from RFC 2535 and discuss backwards compatibility.  It is
   important to note that this document restricts the use of the KEY RR
   and simplifies the flags, but does not change the definition or use
   of DNSSEC keys.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

2. Motivation for Restricting the KEY RR

   The KEY RR RDATA [3] consists of Flags, a Protocol Octet, an
   Algorithm type, and a Public Key.  The Protocol Octet identifies the
   KEY RR sub-type.  DNSSEC public keys are stored in the KEY RR using a
   Protocol Octet value of 3.  Email, IPSEC, and TLS keys were also
   stored in the KEY RR and used Protocol Octet values of 1,2, and 4
   (respectively).  Protocol Octet values 5-254 were available for
   assignment by IANA and values were requested (but not assigned) for
   applications such as SSH.

   Any use of sub-typing has inherent limitations.  A resolver can not
   specify the desired sub-type in a DNS query and most DNS operations
   apply only to resource records sets.  For example, a resolver can not
   directly request the DNSSEC subtype KEY RRs.  Instead, the resolver
   has to request all KEY RRs associated with a DNS name and then search
   the set for the desired DNSSEC sub-type.  DNSSEC signatures also
   apply to the set of all KEY RRs associated with the DNS name,
   regardless of sub-type.

   In the case of the KEY RR, the inherent sub-type limitations are
   exacerbated since the sub-type is used to distinguish between DNSSEC
   keys and application keys.  DNSSEC keys and application keys differ
   in virtually every respect and Section 2.1 discusses these
   differences in more detail.  Combining these very different types of
   keys into a single sub-typed resource record adds unnecessary
   complexity and increases the potential for implementation and
   deployment errors.  Limited experimental deployment has shown that
   application keys stored in KEY RRs are problematic.

   This document addresses these issues by removing all application keys
   from the KEY RR.  Note that the scope of this document is strictly
   limited to the KEY RR and this document does not endorse or restrict
   the storage of application keys in other, yet undefined, resource
   records.





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2.1 Differences Between DNSSEC and Application Keys

   DNSSEC keys are an essential part of the DNSSEC protocol and are used
   by both name servers and resolvers in order to perform DNS tasks.  A
   DNS zone key, used to sign and authenticate RR sets, is the most
   common example of a DNSSEC key.  SIG(0) [4] and TKEY [3]  also use
   DNSSEC keys.

   Application keys such as Email keys, IPSEC keys, and TLS keys are
   simply another type of data.  These keys have no special meaning to a
   name server or resolver.

   The following table summarizes some of the differences between DNSSEC
   keys and application keys:

      1.  They serve different purposes.

      2.  They are managed by different administrators.

      3.  They are authenticated according to different rules.

      4.  Nameservers use different rules when including them in
          responses.

      5.  Resolvers process them in different ways.

      6.  Faults/key compromises have different consequences.

   1.  The purpose of a DNSSEC key is to sign resource records
   associated with a DNS zone (or generate DNS transaction signatures in
   the case of SIG(0)/TKEY).  But the purpose of an application key is
   specific to the application.  Application keys, such as PGP/email,
   IPSEC, TLS, and SSH keys, are not a mandatory part of any zone and
   the purpose and proper use of application keys is outside the scope
   of DNS.

   2.  DNSSEC keys are managed by DNS administrators, but application
   keys are managed by application administrators.  The DNS zone
   administrator determines the key lifetime, handles any suspected key
   compromises, and manages any DNSSEC key changes.  Likewise, the
   application administrator is responsible for the same functions for
   the application keys related to the application.  For example, a user
   typically manages her own PGP key and a server manages its own TLS
   key.  Application key management tasks are outside the scope of DNS
   administration.






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   3.  DNSSEC zone keys are used to authenticate application keys, but
   by definition, application keys are not allowed to authenticate DNS
   zone keys.  A DNS zone key is either configured as a trusted key or
   authenticated by constructing a chain of trust in the DNS hierarchy.
   To participate in the chain of trust, a DNS zone needs to exchange
   zone key information with its parent zone [3].  Application keys are
   not configured as trusted keys in the DNS and are never part of any
   DNS chain of trust.  Application key data is not needed by the parent
   and does not need to be exchanged with the parent zone for secure DNS
   resolution to work.  A resolver considers an application key RRset as
   authenticated DNS information if it has a valid signature from the
   local DNS zone keys, but applications could impose additional
   security requirements before the application key is accepted as
   authentic for use with the application.

   4.  It may be useful for nameservers to include DNS zone keys in the
   additional section of a response, but application keys are typically
   not useful unless they have been specifically requested.  For
   example, it could be useful to include the example.com zone key along
   with a response that contains the www.example.com A record and SIG
   record.  A secure resolver will need the example.com zone key in
   order to check the SIG and authenticate the www.example.com A record.
   It is typically not useful to include the IPSEC, email, and TLS keys
   along with the A record.  Note that by placing application keys in
   the KEY record, a resolver would need the IPSEC, email, TLS, and
   other key associated with example.com if the resolver intends to
   authenticate the example.com zone key (since signatures only apply to
   the entire KEY RR set).  Depending on the number of protocols
   involved, the KEY RR set could grow unwieldy for resolvers, and DNS
   administrators to manage.

   5.  DNS zone keys require special handling by resolvers, but
   application keys are treated the same as any other type of DNS data.
   The DNSSEC keys are of no value to end applications, unless the
   applications plan to do their own DNS authentication.  By definition,
   secure resolvers are not allowed to use application keys as part of
   the authentication process.  Application keys have no unique meaning
   to resolvers and are only useful to the application requesting the
   key.  Note that if sub-types are used to identify the application
   key, then either the interface to the resolver needs to specify the
   sub-type or the application needs to be able to accept all KEY RRs
   and pick out the desired sub-type.

   6.  A fault or compromise of a DNS zone key can lead to invalid or
   forged DNS data, but a fault or compromise of an application key
   should have no impact on other DNS data.  Incorrectly adding or
   changing a DNS zone key can invalidate all of the DNS data in the
   zone and in all of its subzones.  By using a compromised key, an



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   attacker can forge data from the effected zone and for any of its
   sub-zones.  A fault or compromise of an application key has
   implications for that application, but it should not have an impact
   on the DNS.  Note that application key faults and key compromises can
   have an impact on the entire DNS if the application key and DNS zone
   keys are both stored in the KEY RR.

   In summary, DNSSEC keys and application keys differ in most every
   respect.  DNSSEC keys are an essential part of the DNS infrastructure
   and require special handling by DNS administrators and DNS resolvers.
   Application keys are simply another type of data and have no special
   meaning to DNS administrators or resolvers.  These two different
   types of data do not belong in the same resource record.

3. Definition of the KEY RR

   The KEY RR uses type 25 and is used as resource record for storing
   DNSSEC keys.  The RDATA for a KEY RR consists of flags, a protocol
   octet, the algorithm number octet, and the public key itself.  The
   format is as follows:

   ---------------------------------------------------------------------


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

                             KEY RR Format

   ---------------------------------------------------------------------

   In the flags field, all bits except bit 7 are reserved and MUST be
   zero.  If Bit 7 (Zone bit) is set to 1, then the KEY is a DNS Zone
   key.  If Bit 7 is set to 0, the KEY is not a zone key.  SIG(0)/TKEY
   are examples of DNSSEC keys that are not zone keys.

   The protocol field MUST be set to 3.

   The algorithm and public key fields are not changed.





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4. Changes from RFC 2535 KEY RR

   The KEY RDATA format is not changed.

   All flags except for the zone key flag are eliminated:

      The A/C bits (bits 0 and 1) are eliminated.  They MUST be set to 0
      and MUST be ignored by the receiver.

      The extended flags bit (bit 3) is eliminated.  It MUST be set to 0
      and MUST be ignored by the receiver.

      The host/user bit (bit 6) is eliminated.  It MUST be set to 0 and
      MUST be ignored by the receiver.

      The zone bit (bit 7) remains unchanged.

      The signatory field (bits 12-15) are eliminated by [5].  They MUST
      be set to 0 and MUST be ignored by the receiver.

      Bits 2,4,5,8,9,10,11 remain unchanged.  They are reserved, MUST be
      set to zero and MUST be ignored by the receiver.

   Assignment of any future KEY RR Flag values requires a standards
   action.

   All Protocol Octet values except DNSSEC (3) are eliminated:

      Value 1 (Email) is renamed to RESERVED.

      Value 2 (IPSEC) is renamed to RESERVED.

      Value 3 (DNSSEC) is unchanged.

      Value 4 (TLS) is renamed to RESERVED.

      Value 5-254 remains unchanged (reserved).

      Value 255 (ANY) is renamed to RESERVED.

   The authoritative data for a zone MUST NOT include any KEY records
   with a protocol octet other than 3.  The registry maintained by IANA
   for protocol values is closed for new assignments.

   Name servers and resolvers SHOULD accept KEY RR sets that contain KEY
   RRs with a value other than 3.  If out of date DNS zones contain
   deprecated KEY RRs with a protocol octet value other than 3, then
   simply dropping the deprecated KEY RRs from the KEY RR set would



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   invalidate any associated SIG record(s) and could create caching
   consistency problems.  Note that KEY RRs with a protocol octet value
   other than 3 MUST NOT be used to authenticate DNS data.

   The algorithm and public key fields are not changed.

5. Backward Compatibility

   DNSSEC zone KEY RRs are not changed and remain backwards compatible.
   A properly formatted RFC 2535 zone KEY would have all flag bits,
   other than the Zone Bit (Bit 7), set to 0 and would have the Protocol
   Octet set to 3.  This remains true under the restricted KEY.

   DNSSEC non-zone KEY RRs (SIG(0)/TKEY keys) are backwards compatible,
   but the distinction between host and user keys (flag bit 6) is lost.

   No backwards compatibility is provided for application keys.  Any
   Email, IPSEC, or TLS keys are now deprecated.  Storing application
   keys in the KEY RR created problems such as keys at the apex and
   large RR sets and some change in the definition and/or usage of the
   KEY RR would have been required even if the approach described here
   were not adopted.

   Overall, existing nameservers and resolvers will continue to
   correctly process KEY RRs with a sub-type of DNSSEC keys.

6. Storing Application Keys in the DNS

   The scope of this document is strictly limited to the KEY record.
   This document prohibits storing application keys in the KEY record,
   but it does not endorse or restrict the storing application keys in
   other record types.  Other documents can describe how DNS handles
   application keys.

7. IANA Considerations

   RFC 2535 created an IANA registry for DNS KEY RR Protocol Octet
   values.  Values 1, 2, 3, 4, and 255 were assigned by RFC 2535 and
   values 5-254 were made available for assignment by IANA.  This
   document makes two sets of changes to this registry.

   First, this document re-assigns DNS KEY RR Protocol Octet values 1,
   2, 4, and 255 to "reserved".  DNS Key RR Protocol Octet Value 3
   remains unchanged as "DNSSEC".







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   Second, new values are no longer available for assignment by IANA and
   this document closes the IANA registry for DNS KEY RR Protocol Octet
   Values.  Assignment of any future KEY RR Protocol Octet values
   requires a standards action.

8. Security Considerations

   This document eliminates potential security problems that could arise
   due to the coupling of DNS zone keys and application keys.  Prior to
   the change described in this document, a correctly authenticated KEY
   set could include both application keys and DNSSEC keys.  This
   document restricts the KEY RR to DNS security usage only.  This is an
   attempt to simplify the security model and make it less user-error
   prone.  If one of the application keys is compromised, it could be
   used as a false zone key to create false DNS signatures (SIG
   records).  Resolvers that do not carefully check the KEY sub-type
   could believe these false signatures and incorrectly authenticate DNS
   data.  With this change, application keys cannot appear in an
   authenticated KEY set and this vulnerability is eliminated.

   The format and correct usage of DNSSEC keys is not changed by this
   document and no new security considerations are introduced.

9. Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

   [3]  Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC
        2930, September 2000.

   [4]  Eastlake, D., "DNS Request and Transaction Signatures
        (SIG(0)s)", RFC 2931, September 2000.

   [5]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
        Update", RFC 3007, November 2000.












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10. Authors' Addresses

   Dan Massey
   USC Information Sciences Institute
   3811 N. Fairfax Drive
   Arlington, VA  22203
   USA

   EMail: masseyd@isi.edu


   Scott Rose
   National Institute for Standards and Technology
   100 Bureau Drive
   Gaithersburg, MD  20899-3460
   USA

   EMail: scott.rose@nist.gov

































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11.  Full Copyright Statement

   Copyright (C) The Internet Society (2002).  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.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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