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Network Working Group B. Wellington Request for Comments: 3655 O. Gudmundsson Updates: 2535 November 2003 Category: Standards Track

          Redefinition of DNS Authenticated Data (AD) bit

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 (2003).  All Rights Reserved.


 This document alters the specification defined in RFC 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 in RFC 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 the

Wellington & 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.
  1. DNSSEC validation is both CPU and network intensive, and caching

SHOULD be used whenever possible.

  1. 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 in BCP 14, RFC 2119 [RFC2119].

1.3. Updated documents and sections

 The definition of the AD bit in RFC 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 or

Wellington & 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 servers

 Section 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 begin

Wellington & 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
 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

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:
  1. Fiat: An organization that can dictate whether it is OK to trust

certain DNS servers.

  1. Personal: Because of a personal relationship or the reputation of

a recursive nameserver operator, a DNS consumer can decide to

    trust that recursive nameserver.
  1. 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; it

Wellington & 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


7. Internationalization Considerations

 None.  This document does not change any textual data in any

Wellington & Gudmundsson Standards Track [Page 5] RFC 3655 Redefinition of DNS AD bit November 2003

8. 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 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 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

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", RFC
           2535, 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", RFC
           3225, December 2001.

Wellington & Gudmundsson Standards Track [Page 6] RFC 3655 Redefinition of DNS AD bit November 2003

11. Authors' Addresses

 Brian Wellington
 Nominum Inc.
 2385 Bay Road
 Redwood City, CA, 94063
 Olafur Gudmundsson
 3821 Village Park Drive
 Chevy Chase, MD, 20815

Wellington & Gudmundsson Standards Track [Page 7] RFC 3655 Redefinition of DNS AD bit November 2003

12. 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
 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


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Wellington & Gudmundsson Standards Track [Page 8]

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