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Network Working Group                                      B. WellingtonRequest for Comments: 3655                                O. GudmundssonUpdates:2535                                              November 2003Category: Standards TrackRedefinition of DNS Authenticated Data (AD) bitStatus 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 (2003).  All Rights Reserved.Abstract   This document alters the specification defined inRFC 2535.  Based on   implementation experience, the Authenticated Data (AD) bit in the DNS   header is not useful.  This document redefines the AD bit such that   it is only set if all answers or records proving that no answers   exist in the response has been cryptographically verified or   otherwise meets the server's local security policy.1.  Introduction   Familiarity with the DNS system [RFC1035] and DNS security extensions   [RFC2535] is helpful but not necessary.   As specified inRFC 2535 (section 6.1), the AD (Authenticated Data)   bit indicates in a response that all data included in the answer and   authority sections of the response have been authenticated by the   server according to the policies of that server.  This is not   especially useful in practice, since a conformant server SHOULD never   reply with data that failed its security policy.   This document redefines the AD bit such that it is only set if all   data in the response has been cryptographically verified or otherwise   meets the server's local security policy.  Thus, neither a response   containing properly delegated insecure data, nor a server configured   without DNSSEC keys, will have the AD set.  As before, data that   failed to verify will not be returned.  An application running on a   host that has a trust relationship with the server performing theWellington & Gudmundsson    Standards Track                     [Page 1]

RFC 3655               Redefinition of DNS AD bit          November 2003   recursive query can now use the value of the AD bit to determine   whether the data is secure.1.1.  Motivation   A full DNSSEC capable resolver called directly from an application   can return to the application the security status of the RRsets in   the answer.  However, most applications use a limited stub resolver   that relies on an external recursive name server which incorporates a   full resolver.  The recursive nameserver can use the AD bit in a   response to indicate the security status of the data in the answer,   and the local resolver can pass this information to the application.   The application in this context can be either a human using a DNS   tool or a software application.   The AD bit SHOULD be used by the local resolver if and only if it has   been explicitly configured to trust the remote resolver.  The AD bit   SHOULD be ignored when the recursive name server is not trusted.   An alternate solution would be to embed a full DNSSEC resolver into   every application, but this has several disadvantages.   -  DNSSEC validation is both CPU and network intensive, and caching      SHOULD be used whenever possible.   -  DNSSEC requires non-trivial configuration - the root key must be      configured, as well as keys for any "islands of security" that      will exist until DNSSEC is fully deployed.  The number of      configuration points should be minimized.1.2.  Requirements   The key words "MAY", "MAY NOT" "MUST", "MUST NOT", "SHOULD", "SHOULD   NOT", "RECOMMENDED", in this document are to be interpreted as   described inBCP 14,RFC 2119 [RFC2119].1.3.  Updated documents and sections   The definition of the AD bit inRFC 2535, Section 6.1, is changed.2.  Setting of AD bit   The presence of the CD (Checking Disabled) bit in a query does not   affect the setting of the AD bit in the response.  If the CD bit is   set, the server will not perform checking, but SHOULD still set the   AD bit if the data has already been cryptographically verified orWellington & Gudmundsson    Standards Track                     [Page 2]

RFC 3655               Redefinition of DNS AD bit          November 2003   complies with local policy.  The AD bit MUST only be set if DNSSEC   records have been requested via the DO bit [RFC3225] and relevant SIG   records are returned.2.1.  Setting of AD bit by recursive serversSection 6.1 of RFC 2535 says:   "The AD bit MUST NOT be set on a response unless all of the RRs in   the answer and authority sections of the response are either   Authenticated or Insecure."   The replacement text reads:   "The AD bit MUST NOT be set on a response unless all of the RRsets in   the answer and authority sections of the response are Authenticated."   "The AD bit SHOULD be set if and only if all RRs in the answer   section and any relevant negative response RRs in the authority   section are Authenticated."   A recursive DNS server following this modified specification will   only set the AD bit when it has cryptographically verified the data   in the answer.2.2.  Setting of AD bit by authoritative servers   A primary server for a secure zone MAY have the policy of treating   authoritative secure zones as Authenticated.  Secondary servers MAY   have the same policy, but SHOULD NOT consider zone data Authenticated   unless the zone was transferred securely and/or the data was   verified.  An authoritative server MUST only set the AD bit for   authoritative answers from a secure zone if it has been explicitly   configured to do so.  The default for this behavior SHOULD be off.   Note that having the AD bit clear on an authoritative answer is   normal and expected behavior.2.2.1.  Justification for setting AD bit w/o verifying data   The setting of the AD bit by authoritative servers affects only the   small set of resolvers that are configured to directly query and   trust authoritative servers.  This only affects servers that function   as both recursive and authoritative.  Iterative resolvers SHOULD   ignore the AD bit.   The cost of verifying all signatures on load by an authoritative   server can be high and increases the delay before it can beginWellington & Gudmundsson    Standards Track                     [Page 3]

RFC 3655               Redefinition of DNS AD bit          November 2003   answering queries.  Verifying signatures at query time is also   expensive and could lead to resolvers timing out on many queries   after the server reloads zones.   Organizations requiring that all DNS responses contain   cryptographically verified data will need to separate the   authoritative name server and signature verification functions, since   name servers are not required to validate signatures of data for   which they are authoritative.3.  Interpretation of the AD bit   A response containing data marked Insecure in the answer or authority   section MUST never have the AD bit set.  In this case, the resolver   SHOULD treat the data as Insecure whether or not SIG records are   present.   A resolver MUST NOT blindly trust the AD bit unless it communicates   with a recursive nameserver over a secure transport mechanism or   using a message authentication such as TSIG [RFC2845] or SIG(0)   [RFC2931] and is explicitly configured to trust this recursive name   server.4.  Applicability statement   The AD bit is intended to allow the transmission of the indication   that a resolver has verified the DNSSEC signatures accompanying the   records in the Answer and Authority section.  The AD bit MUST only be   trusted when the end consumer of the DNS data has confidence that the   intermediary resolver setting the AD bit is trustworthy.  This can   only be accomplished via an out of band mechanism such as:   -  Fiat: An organization that can dictate whether it is OK to trust      certain DNS servers.   -  Personal: Because of a personal relationship or the reputation of      a recursive nameserver operator, a DNS consumer can decide to      trust that recursive nameserver.   -  Knowledge: If a recursive nameserver operator posts the configured      policy of a recursive nameserver, a consumer can decide that      recursive nameserver is trustworthy.   In the absence of one or more of these factors AD bit from a   recursive name server SHOULD NOT be trusted.  For example, home users   frequently depend on their ISP to provide recursive DNS service; itWellington & Gudmundsson    Standards Track                     [Page 4]

RFC 3655               Redefinition of DNS AD bit          November 2003   is not advisable to trust these recursive nameservers.  A   roaming/traveling host SHOULD not use recursive DNS servers offered   by DHCP when looking up information where security status matters.   In the latter two cases, the end consumer must also completely trust   the path to the trusted recursive name servers, or a secure transport   must be employed to protect the traffic.   When faced with a situation where there are no satisfactory recursive   nameservers available, running one locally is RECOMMENDED.  This has   the advantage that it can be trusted, and the AD bit can still be   used to allow applications to use stub resolvers.5.  Security Considerations   This document redefines a bit in the DNS header.  If a resolver   trusts the value of the AD bit, it must be sure that the responder is   using the updated definition, which is any DNS server/resolver   supporting the DO bit [RFC3225].   Authoritative servers can be explicitly configured to set the AD bit   on answers without doing cryptographic checks.  This behavior MUST be   off by default.  The only affected resolvers are those that directly   query and trust the authoritative server, and this functionality   SHOULD only be used on servers that act both as authoritative and   recursive name servers.   Resolvers (full or stub) that blindly trust the AD bit without   knowing the security policy of the server generating the answer can   not be considered security aware.   A resolver MUST NOT blindly trust the AD bit unless it communicates   such as IPsec, or using message authentication such as TSIG [RFC2845]   or SIG(0) [RFC2931].  In addition, the resolver must have been   explicitly configured to trust this recursive name server.6.  IANA Considerations   None.7.  Internationalization Considerations   None.  This document does not change any textual data in any   protocol.Wellington & Gudmundsson    Standards Track                     [Page 5]

RFC 3655               Redefinition of DNS AD bit          November 20038.  Intellectual Property Rights Notice   The IETF takes no position regarding the validity or scope of any   intellectual property 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; 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 inBCP-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 specification 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   rights which may cover technology that may be required to practice   this standard.  Please address the information to the IETF Executive   Director.9.  Acknowledgments   The following people have provided input on this document: Robert   Elz, Andreas Gustafsson, Bob Halley, Steven Jacob, Erik Nordmark,   Edward Lewis, Jakob Schlyter, Roy Arends, Ted Lindgreen.10.  Normative References   [RFC1035] Mockapetris, P., "Domain Names - Implementation and             Specification", STD 13,RFC 1035, November 1987.   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2535] Eastlake, D., "Domain Name System Security Extensions",RFC2535, March 1999.   [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D. and B.             Wellington, "Secret Key Transaction Authentication for DNS             (TSIG)",RFC 2845, May 2000.   [RFC2931] Eastlake, D., "DNS Request and Transaction Signatures             (SIG(0))",RFC 2931, September 2000.   [RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",RFC3225, December 2001.Wellington & Gudmundsson    Standards Track                     [Page 6]

RFC 3655               Redefinition of DNS AD bit          November 200311.  Authors' Addresses   Brian Wellington   Nominum Inc.   2385 Bay Road   Redwood City, CA, 94063   USA   EMail: Brian.Wellington@nominum.com   Olafur Gudmundsson   3821 Village Park Drive   Chevy Chase, MD, 20815   USA   EMail: ogud@ogud.comWellington & Gudmundsson    Standards Track                     [Page 7]

RFC 3655               Redefinition of DNS AD bit          November 200312.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  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 assignees.   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.Wellington & Gudmundsson    Standards Track                     [Page 8]