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Network Working Group                                          D. MasseyRequest for Comments: 3445                                       USC/ISIUpdates:2535                                                    S. RoseCategory: Standards Track                                           NIST                                                           December 2002Limiting 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 updatesRFC 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 2002Section 2 presents the motivation for restricting the KEY record andSection 3 defines the revised KEY RR.  Sections4 and5 summarize the   changes fromRFC 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 inRFC 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 andSection 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 20022.1 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, anMassey & 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.Massey & Rose               Standards Track                     [Page 5]

RFC 3445         Limiting the KEY Resource Record (RR)     December 20024. Changes fromRFC 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 wouldMassey & Rose               Standards Track                     [Page 6]

RFC 3445         Limiting the KEY Resource Record (RR)     December 2002   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 formattedRFC 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 ConsiderationsRFC 2535 created an IANA registry for DNS KEY RR Protocol Octet   values.  Values 1, 2, 3, 4, and 255 were assigned byRFC 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",RFC2535, March 1999.   [3]  Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)",RFC2930, 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 200210. 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.govMassey & Rose               Standards Track                     [Page 9]

RFC 3445         Limiting the KEY Resource Record (RR)     December 200211.  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.Massey & Rose               Standards Track                    [Page 10]