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DNSEXT Working Group Brian Wellington
INTERNET-DRAFT Olafur Gudmundsson
<draft-ietf-dnsext-ad-is-secure-05.txt> March 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 September 25, 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.
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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 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 authoritative servers
Primary server for a secure zone the data, 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 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 authoritative. 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 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
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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
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.
Authoritative 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
authoritative 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
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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>
Full Copyright Statement
Copyright (C) The Internet Society (2002>. 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."
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DNSEXT Working Group Brian Wellington
INTERNET-DRAFT Olafur Gudmundsson
<draft-ietf-dnsext-ad-is-secure-06.txt> June 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 December 25, 2002.
Copyright Notice
Copyright (C) The Internet Society (2002). All rights reserved.
Abstract
Based on implementation experience, the RFC2535 definition of the
Authenticated Data (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 (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 draft 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, 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 a 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 - 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 full resolver. The remote resolver 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 remote resolver is not trusted.
An alternate solution would be to embed a full DNSSEC resolver into
every application. 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.
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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 RFC2119.
1.3 - 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 cryptographically verified or
complies with local policy. The AD bit MUST only be set if DNSSEC
records have been requested via the OK bit [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:
"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.
Expires December 2002 [Page 3]
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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 authoritative. 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 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 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 the full function resolver over a secure transport mechanism or
using message authentication such as TSIG [RFC2845] or SIG(0)
[RFC2931] and is explicitly configured to trust this resolver.
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 out of band mechanism such as:
- Fiat: An organization can dictate that it is OK to trust certain DNS
servers.
- Personal: Because of a personal relationship or the reputation of a
resolver operator, a DNS consumer can decide to trust that
resolver.
- Knowledge: If a resolver operator posts the configured policy of a
resolver a consumer can decide that resolver is trustworthy.
In the absence of one or more of these factors AD bit from a resolver
SHOULD NOT be trusted. For example, home users frequently depend on
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their ISP to provide recursive DNS service; it is not advisable to
trust these resolvers. A roaming/traveling host SHOULD not use DNS
resolvers offered by DHCP when looking up information where security
status matters.
When faced with a situation where there are no satisfactory recursive
resolvers 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.
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 responder is
using the updated definition, which is any DNS server/resolver
supporting the OK 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 servers
and recursive resolver.
Resolvers (full or stub) that trust the AD bit on answers from a
configured set of resolvers are DNSSEC security compliant.
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: Robert
Elz, Andreas Gustafsson, Bob Halley, Steven Jacob, Erik Nordmark,
Edward Lewis, Jakob Schlyter, Roy Arends, Ted Lindgreen.
Normative References:
[RFC1035] P. Mockapetris, ``Domain Names - Implementation and
Specification'', STD 13, RFC 1035, November 1987.
Expires December 2002 [Page 5]
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[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
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 Road 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 (2002>. 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
Expires December 2002 [Page 6]
INTERNET-DRAFT AD bit set on secure answers June 2002
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Expires December 2002 [Page 7]