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Versions: 00 01 02 03 04 05 06 rfc3655                                  
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-

   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

   The list of Internet-Draft Shadow Directories can be accessed at

   Comments should be sent to the authors or the DNSEXT WG mailing list

   This draft expires on July  10, 2002.

   Copyright Notice

   Copyright (C) The Internet Society (2002).  All rights reserved.


   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.

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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:

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   "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

   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

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   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

   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

   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:


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.


[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

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           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>

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