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Network Working Group                                         O. KolkmanRequest for Comments: 3757                                      RIPE NCCUpdates:3755,2535                                          J. SchlyterCategory: Standards Track                                         NIC-SE                                                                E. Lewis                                                                    ARIN                                                              April 2004Domain Name System KEY (DNSKEY) Resource Record (RR)Secure Entry Point (SEP) FlagStatus 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 (2004).  All Rights Reserved.Abstract   With the Delegation Signer (DS) resource record (RR), the concept of   a public key acting as a secure entry point (SEP) has been   introduced.  During exchanges of public keys with the parent there is   a need to differentiate SEP keys from other public keys in the Domain   Name System KEY (DNSKEY) resource record set.  A flag bit in the   DNSKEY RR is defined to indicate that DNSKEY is to be used as a SEP.   The flag bit is intended to assist in operational procedures to   correctly generate DS resource records, or to indicate what DNSKEYs   are intended for static configuration.  The flag bit is not to be   used in the DNS verification protocol.  This document updatesRFC2535 andRFC 3755.Kolkman, et al.              Standard Track                     [Page 1]

RFC 3757                   DNSKEY RR SEP Flag                 April 2004Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  The Secure Entry Point (SEP) Flag. . . . . . . . . . . . . . .43.  DNSSEC Protocol Changes. . . . . . . . . . . . . . . . . . . .44.  Operational Guidelines . . . . . . . . . . . . . . . . . . . .45.  Security Considerations. . . . . . . . . . . . . . . . . . . .56.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . .67.  Internationalization Considerations. . . . . . . . . . . . . .68.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .69.  References . . . . . . . . . . . . . . . . . . . . . . . . . .69.1.  Normative References . . . . . . . . . . . . . . . . . .69.2.  Informative References . . . . . . . . . . . . . . . . .610. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .711. Full Copyright Statement . . . . . . . . . . . . . . . . . . .81.  Introduction   "All keys are equal but some keys are more equal than others" [6].   With the definition of the Delegation Signer Resource Record (DS RR)   [5], it has become important to differentiate between the keys in the   DNSKEY RR set that are (to be) pointed to by parental DS RRs and the   other keys in the DNSKEY RR set.  We refer to these public keys as   Secure Entry Point (SEP) keys.  A SEP key either used to generate a   DS RR or is distributed to resolvers that use the key as the root of   a trusted subtree [3].   In early deployment tests, the use of two (kinds of) key pairs for   each zone has been prevalent.  For one kind of key pair the private   key is used to sign just the zone's DNSKEY resource record (RR) set.   Its public key is intended to be referenced by a DS RR at the parent   or configured statically in a resolver.  The private key of the other   kind of key pair is used to sign the rest of the zone's data sets.   The former key pair is called a key-signing key (KSK) and the latter   is called a zone-signing key (ZSK).  In practice there have been   usually one of each kind of key pair, but there will be multiples of   each at times.   It should be noted that division of keys pairs into KSK's and ZSK's   is not mandatory in any definition of DNSSEC, not even with the   introduction of the DS RR.  But, in testing, this distinction has   been helpful when designing key roll over (key super-cession)   schemes.  Given that the distinction has proven helpful, the labels   KSK and ZSK have begun to stick.Kolkman, et al.              Standard Track                     [Page 2]

RFC 3757                   DNSKEY RR SEP Flag                 April 2004   There is a need to differentiate the public keys for the key pairs   that are used for key signing from keys that are not used key signing   (KSKs vs ZSKs).  This need is driven by knowing which DNSKEYs are to   be sent for generating DS RRs, which DNSKEYs are to be distributed to   resolvers, and which keys are fed to the signer application at the   appropriate time.   In other words, the SEP bit provides an in-band method to communicate   a DNSKEY RR's intended use to third parties.  As an example we   present 3 use cases in which the bit is useful:      The parent is a registry, the parent and the child use secured DNS      queries and responses, with a preexisting trust-relation, or plain      DNS over a secured channel to exchange the child's DNSKEY RR sets.      Since a DNSKEY RR set will contain a complete DNSKEY RRset the SEP      bit can be used to isolate the DNSKEYs for which a DS RR needs to      be created.      An administrator has configured a DNSKEY as root for a trusted      subtree into security aware resolver.  Using a special purpose      tool that queries for the KEY RRs from that domain's apex, the      administrator will be able to notice the roll over of the trusted      anchor by a change of the subset of KEY RRs with the DS flag set.      A signer might use the SEP bit on the public key to determine      which private key to use to exclusively sign the DNSKEY RRset and      which private key to use to sign the other RRsets in the zone.   As demonstrated in the above examples it is important to be able to   differentiate the SEP keys from the other keys in a DNSKEY RR set in   the flow between signer and (parental) key-collector and in the flow   between the signer and the resolver configuration.  The SEP flag is   to be of no interest to the flow between the verifier and the   authoritative data store.   The reason for the term "SEP" is a result of the observation that the   distinction between KSK and ZSK key pairs is made by the signer, a   key pair could be used as both a KSK and a ZSK at the same time.  To   be clear, the term SEP was coined to lessen the confusion caused by   the overlap.  (Once this label was applied, it had the side effect of   removing the temptation to have both a KSK flag bit and a ZSK flag   bit.)   The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED",   "RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be   interpreted as described inBCP 14,RFC 2119 [1].Kolkman, et al.              Standard Track                     [Page 3]

RFC 3757                   DNSKEY RR SEP Flag                 April 20042.  The Secure Entry Point (SEP) Flag                        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          |S|   protocol    |   algorithm   |   |                             |E|               |               |   |                             |P|               |               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               /   /                        public key                             /   /                                                               /   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                          DNSKEY RR Format   This document assigns the 15th bit in the flags field as the secure   entry point (SEP) bit.  If the bit is set to 1 the key is intended to   be used as secure entry point key.  One SHOULD NOT assign special   meaning to the key if the bit is set to 0.  Operators can recognize   the secure entry point key by the even or odd-ness of the decimal   representation of the flag field.3.  DNSSEC Protocol Changes   The bit MUST NOT be used during the resolving and verification   process.  The SEP flag is only used to provide a hint about the   different administrative properties of the key and therefore the use   of the SEP flag does not change the DNS resolution protocol or the   resolution process.4.  Operational Guidelines   The SEP bit is set by the key-pair-generator and MAY be used by the   zone signer to decide whether the public part of the key pair is to   be prepared for input to a DS RR generation function.  The SEP bit is   recommended to be set (to 1) whenever the public key of the key pair   will be distributed to the parent zone to build the authentication   chain or if the public key is to be distributed for static   configuration in verifiers.   When a key pair is created, the operator needs to indicate whether   the SEP bit is to be set in the DNSKEY RR.  As the SEP bit is within   the data that is used to compute the 'key tag field' in the SIG RR,   changing the SEP bit will change the identity of the key within DNS.   In other words, once a key is used to generate signatures, the   setting of the SEP bit is to remain constant.  If not, a verifier   will not be able to find the relevant KEY RR.Kolkman, et al.              Standard Track                     [Page 4]

RFC 3757                   DNSKEY RR SEP Flag                 April 2004   When signing a zone, it is intended that the key(s) with the SEP bit   set (if such keys exist) are used to sign the KEY RR set of the zone.   The same key can be used to sign the rest of the zone data too.  It   is conceivable that not all keys with a SEP bit set will sign the   DNSKEY RR set, such keys might be pending retirement or not yet in   use.   When verifying a RR set, the SEP bit is not intended to play a role.   How the key is used by the verifier is not intended to be a   consideration at key creation time.   Although the SEP flag provides a hint on which public key is to be   used as trusted root, administrators can choose to ignore the fact   that a DNSKEY has its SEP bit set or not when configuring a trusted   root for their resolvers.   Using the SEP flag a key roll over can be automated.  The parent can   use an existing trust relation to verify DNSKEY RR sets in which a   new DNSKEY RR with the SEP flag appears.5.  Security Considerations   As stated inSection 3 the flag is not to be used in the resolution   protocol or to determine the security status of a key.  The flag is   to be used for administrative purposes only.   No trust in a key should be inferred from this flag - trust MUST be   inferred from an existing chain of trust or an out-of-band exchange.   Since this flag might be used for automating public key exchanges, we   think the following consideration is in place.   Automated mechanisms for roll over of the DS RR might be vulnerable   to a class of replay attacks.  This might happen after a public key   exchange where a DNSKEY RR set, containing two DNSKEY RRs with the   SEP flag set, is sent to the parent.  The parent verifies the DNSKEY   RR set with the existing trust relation and creates the new DS RR   from the DNSKEY RR that the current DS RR is not pointing to.  This   key exchange might be replayed.  Parents are encouraged to implement   a replay defense.  A simple defense can be based on a registry of   keys that have been used to generate DS RRs during the most recent   roll over.  These same considerations apply to entities that   configure keys in resolvers.Kolkman, et al.              Standard Track                     [Page 5]

RFC 3757                   DNSKEY RR SEP Flag                 April 20046.  IANA Considerations   IANA has assigned the 15th bit in the DNSKEY Flags Registry (see   Section 4.3 of [4]) as the Secure Entry Point (SEP) bit.7.  Internationalization Considerations   Although SEP is a popular acronym in many different languages, there   are no internationalization considerations.8.  Acknowledgments   The ideas documented in this document are inspired by communications   we had with numerous people and ideas published by other folk.  Among   others Mark Andrews, Rob Austein, Miek Gieben, Olafur Gudmundsson,   Daniel Karrenberg, Dan Massey, Scott Rose, Marcos Sanz and Sam Weiler   have contributed ideas and provided feedback.   This document saw the light during a workshop on DNSSEC operations   hosted by USC/ISI in August 2002.9.  References9.1.  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]  Lewis, E., "DNS Security Extension Clarification on Zone        Status",RFC 3090, March 2001.   [4]  Weiler, S., "Legacy Resolver Compatibility for Delegation Signer        (DS)",RFC 3755, April 2004.9.2.  Informative References   [5]  Gudmundsson, O., "Delegation Signer (DS) Resource Record (RR)",RFC 3658, December 2003.   [6]  Orwell, G. and R. Steadman (illustrator), "Animal Farm; a Fairy        Story", ISBN 0151002177 (50th anniversary edition), April 1996.Kolkman, et al.              Standard Track                     [Page 6]

RFC 3757                   DNSKEY RR SEP Flag                 April 200410.  Authors' Addresses   Olaf M. Kolkman   RIPE NCC   Singel 256   Amsterdam  1016 AB   NL   Phone: +31 20 535 4444   EMail: olaf@ripe.net   URI:http://www.ripe.net/   Jakob Schlyter   NIC-SE   Box 5774   SE-114 87 Stockholm   Sweden   EMail: jakob@nic.se   URI:http://www.nic.se/   Edward P. Lewis   ARIN   3635 Concorde Parkway Suite 200   Chantilly, VA  20151   US   Phone: +1 703 227 9854   EMail: edlewis@arin.net   URI:http://www.arin.net/Kolkman, et al.              Standard Track                     [Page 7]

RFC 3757                   DNSKEY RR SEP Flag                 April 200411.  Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 78 and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE   INTERNET ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed   to pertain 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; nor does it   represent that it has made any independent effort to identify any   such rights.  Information on the procedures with respect to   rights in RFC documents can be found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this   specification can be obtained from the IETF on-line IPR repository   athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention   any copyrights, patents or patent applications, or other   proprietary rights that may cover technology that may be required   to implement this standard.  Please address the information to the   IETF at ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Kolkman, et al.              Standard Track                     [Page 8]