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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.

Massey & Rose Standards Track [Page 1] RFC 3445 Limiting the KEY Resource Record (RR) December 2002

 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.

Massey & Rose Standards Track [Page 2] RFC 3445 Limiting the KEY Resource Record (RR) December 2002

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.

Massey & Rose Standards Track [Page 3] RFC 3445 Limiting the KEY Resource Record (RR) December 2002

 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

Massey & Rose Standards Track [Page 4] RFC 3445 Limiting the KEY Resource Record (RR) December 2002

 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".

Massey & Rose Standards Track [Page 7] RFC 3445 Limiting the KEY Resource Record (RR) December 2002

 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.

Massey & Rose Standards Track [Page 8] RFC 3445 Limiting the KEY Resource Record (RR) December 2002

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

Massey & Rose Standards Track [Page 9] RFC 3445 Limiting the KEY Resource Record (RR) December 2002

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.

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 "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.

Massey & Rose Standards Track [Page 10]