|Title||Redefinition of DNS Authenticated Data (AD) bit
|Author||B. Wellington, O.
|Obsoleted by||RFC4033, RFC4034, RFC4035
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 (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.
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
recursive query can now use the value of the AD bit to determine
whether the data is secure.
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.
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
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
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:
- 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; it
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
8. Intellectual Property Rights Notice
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pertain to the implementation or use of the technology described in
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proprietary rights by implementors or users of this specification can
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The IETF invites any interested party to bring to its attention any
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this standard. Please address the information to the IETF Executive
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.
11. Authors' Addresses
2385 Bay Road
Redwood City, CA, 94063
3821 Village Park Drive
Chevy Chase, MD, 20815
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