DNSEXT Working Group Brian Wellington INTERNET-DRAFT Olafur Gudmundsson <draft-ietf-dnsext-ad-is-secure-04.txt> February 2002 Updates: RFC 2535 Redefinition of DNS AD bit Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 Comments should be sent to the authors or the DNSEXT WG mailing list namedroppers@ops.ietf.org This draft expires on July 10, 2002. Copyright Notice Copyright (C) The Internet Society (2002). All rights reserved. Abstract Based on implementation experience, the current definition of the AD bit in the DNS header is not useful. This draft changes the specification so that the AD bit is only set on answers where signatures have been cryptographically verified or the server is authoritative for the data and is allowed to set the bit by policy. Expires August 2002 [Page 1]
INTERNET-DRAFT AD bit set on secure answers February 2002 1 - Introduction Familiarity with the DNS system [RFC1035] and DNS security extensions [RFC2535] is helpful but not necessary. As specified in RFC 2535 (section 6.1), the AD 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 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 draft proposes to redefine 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, a response containing properly delegated insecure data will not have AD set, nor will a response from a server configured without DNSSEC keys. As before, data which failed to verify will not be returned. An application running on a host that has trust relationship with the server performing the recursive query can now use the value of the AD bit to determine if the data is secure or not. 1.1 - Requirements The key words "MAY", "MAY NOT" "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", in this document are to be interpreted as described in RFC2119. 1.2 - Updated documents and sections The definition of the AD bit in RFC2535, 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 checked or complies with local policy. The AD bit MUST only be set if DNSSEC records have been requested [RFC3225] and relevant SIG records are returned. 2.1 - Setting of AD bit by recursive servers Section 6.1 of RFC2535 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: Expires August 2002 [Page 2]
INTERNET-DRAFT AD bit set on secure answers February 2002 "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 authorative servers A primary server for a secure zone the data MAY have a 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 transfered 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. 2.2.1 - Justification for setting AD bit w/o verifying data The setting of the AD bit by authoritative servers affects only a small set of resolvers that are configured to directly query and trust authoritative servers. This only affects servers that function as both recursive and authorative. All recursive 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 answer begin 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 that require that all DNS responses contain cryptographically verified data must separate the functions of authoritative and recursive servers, as authoritative servers are not required to validate local secure data. 3 - Interpretation of the AD bit A response containing data marked Insecure in the answer or authority section will 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 the server over a secure transport mechanism or using message authentication such as TSIG [RFC2845] or SIG(0) [RFC2931] and is Expires August 2002 [Page 3]
INTERNET-DRAFT AD bit set on secure answers February 2002 configured to trust the server. 4 - 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 server is using the updated definition, which is any server supporting the OK bit. Authoritative servers that set the AD bit on answers without doing cryptographic checks must only do so if explicitly configured to. This only affects resolvers that directly query and trust the authoritative server, and this functionality should only be used on servers that act both as authoritative servers and recursive resolver. Authorative servers that set the AD bit on answers without doing cryptographic checks must only do so on explicit zone by zone enablement. This only affects resolvers that trust the server and this functionality should only be used on servers that act both as authorative servers and recursive resolver. 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. 5 - IANA Considerations: None 6 - Internationalization Considerations: None, this document does not change any textual data in any protocol. 7 - Acknowledgments: The following people have provided input on this document: Andreas Gustafsson, Bob Halley, Steven Jacob, Edward Lewis, Jakob Schlyter, Roy Arends, Ted Lindgreen. References: [RFC1035] P. Mockapetris, ``Domain Names - Implementation and Specification'', STD 13, RFC 1035, November 1987. [RFC2535] D. Eastlake, ``Domain Name System Security Extensions'', RFC 2535, March 1999. [RFC2845] P. Vixie, O. Gudmundsson, D. Eastlake, B. Wellington, ``Secret Key Transaction Authentication for DNS (TSIG)'', RFC Expires August 2002 [Page 4]
INTERNET-DRAFT AD bit set on secure answers February 2002 2845, May 2000. [RFC2931] D. Eastlake, ``DNS Request and Transaction Signatures (SIG(0))'', RFC 2931, September 2000. [RFC3225] D. Conrad, ``Indicating Resolver Support of DNSSEC'', RFC 3225, December 2001. Authors Addresses Brian Wellington Olafur Gudmundsson Nominum Inc. 2385 Bay Street 3826 Legation Street, NW Redwood City, CA, 94063 Washington, DC, 20015 USA USA <Brian.Wellington@nominum.com> <ogud@ogud.com> 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 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." Expires August 2002 [Page 5]
