DNSEXT Working Group D. Massey INTERNET-DRAFT USC/ISI S. Rose Expires: April 2002 NIST Updates: RFC 2535 November 2001 Limiting the Scope of the KEY Resource Record ------------------------------ Status of this Document This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Distribution of this document is unlimited. Comments regarding this document should be sent to the author. 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 http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document limits the KEY resource record to only DNS zone keys. The original KEY resource record used sub-typing to store both DNS zone keys and arbitrary application keys. DNS security keys and application keys differ in almost every respect and should not be combined in a single sub-typed resource record. This document removes application keys from the KEY record by redefining the Protocol Octet field in the KEY RDATA. Three existing application key sub-types are changed to historic, but the format of the KEY record is not changed. This document updates RFC 2535. Massey, Rose [Page 1] INTERNET-DRAFT Limiting the Scope of KEY RRs November 2001 1. Introduction This document limits the scope the KEY resource record. The KEY resource record, originally defined in [DNSSEC], uses resource record sub-typing to hold any public key associated with "a zone, a user, or a host or other end entity". The KEY resource record is assigned type value of 25 and the Protocol Octet in the KEY RDATA identifies the sub-type. DNSSEC Zone, User and Host keys are stored in the KEY resource record and are identified by a Protocol Octet value of 3. Email, IPSEC, and TLS keys are also stored in the KEY resource record and are identified by Protocol Octet values of 1,2, and 4 (respectively). Protocol Octet values 5-254 are available for assignment by IANA and values have been requested (but not assigned) for applications such as SSH. Closer examination and limited experimental deployment has shown that application keys stored in KEY records are problematic. Any use of sub-typing has inherent limitations. A resolver can not specify the desired sub-type in a DNS query and many DNS operations group resource records into sets, based on the DNS name and type. For a example, a resolver can not directly request the DNSSEC key sub-type. Instead, the resolver must request all KEY records associated with a DNS name. DNSSEC signatures apply to the set of all KEY resource records associated with the DNS name, regardless of sub-type. In the case of the KEY record, the inherent sub-type limitations are exacerbated since DNS zone keys and application keys differ in virtually every respect. Combining two very different types of keys into a single sub-typed resource record adds unnecessary complexity and increases the potential for implementation and deployment errors. This document addresses these issues by removing all application keys from the KEY resource record. Note that the scope of this document is strictly limited to the KEY record and this document does not endorse or restrict the storage of application keys in other resource records. 2. DNS Zone Key and Application Key Differences In the original specification, all public keys were stored in KEY records, regardless of protocol or type. This proved to be a mistake as DNS security keys (zone, host and user) and application keys differ in the following ways: o They serve different purposes. o They are managed by different administrators. Massey, Rose [Page 2] INTERNET-DRAFT Limiting the Scope of KEY RRs November 2001 o They are authenticated according to different rules. o Nameservers use different rules when including them in responses. o Resolvers process them in different ways. o Faults/key compromises have different consequences. The purpose of a DNS zone key is to sign resource records associated with a DNS zone but the purpose of an application key is specific to the application. DNSSEC host and user KEY RRs are used to generate SIG(0) transaction signatures. 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. DNSSEC keys are managed by DNS administrators, but application keys are managed by application administrators. The DNS zone administra- tor determines the key lifetime, handles any suspected key comprom- ises, and manages any DNSSEC key changes. Likewise, the application administrator is responsible for the same functions for the applica- tion 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. DNS zone keys are used to authenticate application keys, but applica- tion keys MUST NOT be used to authenticate DNS zone keys. A DNS zone key is either configured as trusted key or authenticated by con- structing a chain of trust in the DNS hierarchy. To participate in the chain of trust, a DNS zone must exchange zone key information with its parent zone [DNSSEC]. Application keys are not configured as trusted keys in the DNS and are never part of any DNS chain of trust. Application key data should not be exchanged with the parent zone. A resolver considers an application key authenticated if it has a valid signature from the local DNS zone keys, but applications may impose additional requirements before the application key is accepted as authentic. 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 exam- ple, it may be useful to include the isi.edu zone key along with a response that contain the www.isi.edu A record and SIG record. A secure resolver will need the isi.edu zone key in order to check the SIG and authenticate the www.isi.edu A record. It is typical not useful to include the IPSEC, email, and TLS keys along with the A Massey, Rose [Page 3] INTERNET-DRAFT Limiting the Scope of KEY RRs November 2001 record. Note that by placing application keys in the KEY record, a resolver will need the IPSEC, email, TLS, and other key associated with isi.edu if the resolver intends to authenticate the isi.edu zone key (since signatures only apply to the entire KEY set). DNS zone keys require special handling by resolvers, but application keys should be treated the same as any other type of DNS data. The DNSSEC keys are of no value to end applications, unless the applica- tions plan to do their own DNS authentication. Secure resolvers MUST NOT use application keys as part of the authentication process. Application keys have no unique value to resolvers and are only use- ful 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 must specify the sub-type or the application must be able to accept all KEY records and pick out the desired the sub-type. A fault or compromise of 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 zone and in all of its subzones. By using a compromised key, an attacker can forge data from the effected zone and any for any of its sub-zones. A fault or compromise of an application key has implications for that applica- tion, but it should not have an impact on the DNS. Note that applica- tion 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 record. In summary, DNS zone keys and application keys differ in most every respect. DNS zone keys are an essential part of the DNS infrastruc- ture 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. Redefinition of the KEY Resource Record The KEY record is redefined as resource record for storing DNSSEC keys. The KEY RDATA format, as defined in [DNSSEC], is not changed, but the Protocol Octet is redefined as follows: VALUE Protocol 0 - reserved 1 HISTORIC 2 HISTORIC 3 dnssec 4 HISTORIC Massey, Rose [Page 4] INTERNET-DRAFT Limiting the Scope of KEY RRs November 2001 5-254 - reserved 255 HISTORIC All valid KEY records MUST have a Protocol Octet value of 3. KEY records with a Protocol Octet value other than 3 SHOULD NOT be stored in the DNS and SHOULD be ignored by nameservers and resolvers that receive them in a response. 4. Backward Compatibility Protocol Octet values of 1,2, 4, and 255 were previously defined in RFC 2535. These values are now deprecated. To insure backward compatibility, the Protocol Octet values 1,2, and 4 will be desig- nated as HISTORIC. Protocol values 5-254 are reserved and are no longer available for assignment by IANA. KEY records with a Protocol Value of 1,2, or 4 were never widely deployed in the DNS and some limited test deployment revealed prob- lems. Most notably, placing application keys in the KEY record can create very large key sets and application keys that appear in the zone apex can create zone management problems. Some change in the definition and/or usage of the KEY record would be required even if the approach described here were not required. KEY records with a Protocol Octet value of 1,2, or 4 SHOULD NOT be place in a DNS zone. Likewise, resolvers that receive KEY records in a response with HISTORIC or invalid protocol field values SHOULD be ignored and SHOULD NOT be stored in a resolver's/server's cache. No changes are made to the format of the KEY record or to the use of DNSSEC zone, host and user keys. Existing nameservers and resolvers will continue to correctly process KEY records that contain DNSSEC keys. 5. 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 should describe how DNS handles application keys. 6. IANA Consideration Protocol Octet values 1,2,4, and 255 are changed to HISTORIC. Massey, Rose [Page 5] INTERNET-DRAFT Limiting the Scope of KEY RRs November 2001 Protocol Octet values 5-255 are reserved and are no longer available for assignment by IANA. 7. Security Consideration 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 the document, a correctly authenti- cated KEY set could include both application keys and DNSSEC keys. If one of the application keys is compromised, it could be used as a false zone key to create phony DNS signatures (SIG records). Resolvers that do not carefully check the KEY sub-type may believe these false signatures and incorrectly authenticate DNS data. With this change, application keys cannot appear in an authenticated KEY set. Applications that accept keys based solely on DNSSEC rely on the DNS administrator to correctly enter the application key data and are only as secure as the weakest zone in the DNS chain of trust. Compromises or errors caused by DNS administrators when entering DNSSEC data could results in an application key failing to verify, or verified incorrectly. The format and correct usage of DNS zone keys is not changed by this document and no new security considerations are introduced. 8. Intellectual Property The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to per- tain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards- related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specifica- tion can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary Massey, Rose [Page 6] INTERNET-DRAFT Limiting the Scope of KEY RRs November 2001 rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. 9. References [DNSSEC] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. 10. Author Information Daniel Massey USC Information Sciences Institute 3811 North Fairfax Drive, Suite 200 Arlington, VA 22203 Scott Rose National Institute for Standards and Technology Gaithersburg, MD Expiration and File Name: This draft, titled expires April 2001 Full Copyright Statement Copyright (C) The Internet Society (2001). 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 copy- right 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. Massey, Rose [Page 7] INTERNET-DRAFT Limiting the Scope of KEY RRs November 2001 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." Massey, Rose [Page 8]