DNSEXT Working Group D. Massey
INTERNET-DRAFT USC/ISI
S. Rose
Expires: April 2002 NIST
Updates: RFC 2535 November 2001
Limiting the Scope of the KEY Resource Record
------------------------------
<draft-ietf-dnsext-restrict-key-for-dnssec-00.txt>
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Abstract
This document limits the KEY resource record to only DNS zone keys.
The original KEY resource record used sub-typing to store both DNS
zone keys and arbitrary application keys. DNS security keys and
application keys differ in almost every respect and should not be
combined in a single sub-typed resource record. This document
removes application keys from the KEY record by redefining the
Protocol Octet field in the KEY RDATA. Three existing application key
sub-types are changed to historic, but the format of the KEY record
is not changed. This document updates RFC 2535.
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1. Introduction
This document limits the scope the KEY resource record. The KEY
resource record, originally defined in [DNSSEC], uses resource record
sub-typing to hold any public key associated with "a zone, a user, or
a host or other end entity". The KEY resource record is assigned
type value of 25 and the Protocol Octet in the KEY RDATA identifies
the sub-type. DNSSEC Zone, User and Host keys are stored in the KEY
resource record and are identified by a Protocol Octet value of 3.
Email, IPSEC, and TLS keys are also stored in the KEY resource record
and are identified by Protocol Octet values of 1,2, and 4
(respectively). Protocol Octet values 5-254 are available for
assignment by IANA and values have been requested (but not assigned)
for applications such as SSH.
Closer examination and limited experimental deployment has shown that
application keys stored in KEY records are problematic. Any use of
sub-typing has inherent limitations. A resolver can not specify the
desired sub-type in a DNS query and many DNS operations group
resource records into sets, based on the DNS name and type. For a
example, a resolver can not directly request the DNSSEC key sub-type.
Instead, the resolver must request all KEY records associated with a
DNS name. DNSSEC signatures apply to the set of all KEY resource
records associated with the DNS name, regardless of sub-type.
In the case of the KEY record, the inherent sub-type limitations are
exacerbated since DNS zone keys and application keys differ in
virtually every respect. Combining two very different types of keys
into a single sub-typed resource record adds unnecessary complexity
and increases the potential for implementation and deployment errors.
This document addresses these issues by removing all application keys
from the KEY resource record. Note that the scope of this document
is strictly limited to the KEY record and this document does not
endorse or restrict the storage of application keys in other resource
records.
2. DNS Zone Key and Application Key Differences
In the original specification, all public keys were stored in KEY
records, regardless of protocol or type. This proved to be a mistake
as DNS security keys (zone, host and user) and application keys
differ in the following ways:
o They serve different purposes.
o They are managed by different administrators.
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o They are authenticated according to different rules.
o Nameservers use different rules when including them in
responses.
o Resolvers process them in different ways.
o Faults/key compromises have different consequences.
The purpose of a DNS zone key is to sign resource records associated
with a DNS zone but the purpose of an application key is specific to
the application. DNSSEC host and user KEY RRs are used to generate
SIG(0) transaction signatures. Application keys, such as PGP/email,
IPSEC, TLS, and SSH keys, are not a mandatory part of any zone and
the purpose and proper use of application keys is outside the scope
of DNS.
DNSSEC keys are managed by DNS administrators, but application keys
are managed by application administrators. The DNS zone administra-
tor determines the key lifetime, handles any suspected key comprom-
ises, and manages any DNSSEC key changes. Likewise, the application
administrator is responsible for the same functions for the applica-
tion keys related to the application. For example, a user typically
manages her own PGP key and a server manages its own TLS key.
Application key management tasks are outside the scope of DNS
administration.
DNS zone keys are used to authenticate application keys, but applica-
tion keys MUST NOT be used to authenticate DNS zone keys. A DNS
zone key is either configured as trusted key or authenticated by con-
structing a chain of trust in the DNS hierarchy. To participate in
the chain of trust, a DNS zone must exchange zone key information
with its parent zone [DNSSEC]. Application keys are not configured
as trusted keys in the DNS and are never part of any DNS chain of
trust. Application key data should not be exchanged with the parent
zone. A resolver considers an application key authenticated if it
has a valid signature from the local DNS zone keys, but applications
may impose additional requirements before the application key is
accepted as authentic.
It MAY be useful for nameservers to include DNS zone keys in the
additional section of a response, but application keys are typically
not useful unless they have been specifically requested. For exam-
ple, it may be useful to include the isi.edu zone key along with a
response that contain the www.isi.edu A record and SIG record. A
secure resolver will need the isi.edu zone key in order to check the
SIG and authenticate the www.isi.edu A record. It is typical not
useful to include the IPSEC, email, and TLS keys along with the A
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record. Note that by placing application keys in the KEY record, a
resolver will need the IPSEC, email, TLS, and other key associated
with isi.edu if the resolver intends to authenticate the isi.edu zone
key (since signatures only apply to the entire KEY set).
DNS zone keys require special handling by resolvers, but application
keys should be treated the same as any other type of DNS data. The
DNSSEC keys are of no value to end applications, unless the applica-
tions plan to do their own DNS authentication. Secure resolvers
MUST NOT use application keys as part of the authentication process.
Application keys have no unique value to resolvers and are only use-
ful to the application requesting the key. Note that if sub-types
are used to identify the application key, then either the interface
to the resolver must specify the sub-type or the application must be
able to accept all KEY records and pick out the desired the sub-type.
A fault or compromise of DNS zone key can lead to invalid or forged
DNS data, but a fault or compromise of an application key should have
no impact on other DNS data. Incorrectly adding or changing a DNS
zone key can invalidate all of the DNS data in zone and in all of its
subzones. By using a compromised key, an attacker can forge data
from the effected zone and any for any of its sub-zones. A fault or
compromise of an application key has implications for that applica-
tion, but it should not have an impact on the DNS. Note that applica-
tion key faults and key compromises can have an impact on the entire
DNS if the application key and DNS zone keys are both stored in the
KEY record.
In summary, DNS zone keys and application keys differ in most every
respect. DNS zone keys are an essential part of the DNS infrastruc-
ture and require special handling by DNS administrators and DNS
resolvers. Application keys are simply another type of data and have
no special meaning to DNS administrators or resolvers. These two
different types of data do not belong in the same resource record.
3. Redefinition of the KEY Resource Record
The KEY record is redefined as resource record for storing DNSSEC
keys. The KEY RDATA format, as defined in [DNSSEC], is not changed,
but the Protocol Octet is redefined as follows:
VALUE Protocol
0 - reserved
1 HISTORIC
2 HISTORIC
3 dnssec
4 HISTORIC
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5-254 - reserved
255 HISTORIC
All valid KEY records MUST have a Protocol Octet value of 3. KEY
records with a Protocol Octet value other than 3 SHOULD NOT be stored
in the DNS and SHOULD be ignored by nameservers and resolvers that
receive them in a response.
4. Backward Compatibility
Protocol Octet values of 1,2, 4, and 255 were previously defined in
RFC 2535. These values are now deprecated. To insure backward
compatibility, the Protocol Octet values 1,2, and 4 will be desig-
nated as HISTORIC. Protocol values 5-254 are reserved and are no
longer available for assignment by IANA.
KEY records with a Protocol Value of 1,2, or 4 were never widely
deployed in the DNS and some limited test deployment revealed prob-
lems. Most notably, placing application keys in the KEY record can
create very large key sets and application keys that appear in the
zone apex can create zone management problems. Some change in the
definition and/or usage of the KEY record would be required even if
the approach described here were not required.
KEY records with a Protocol Octet value of 1,2, or 4 SHOULD NOT be
place in a DNS zone. Likewise, resolvers that receive KEY records
in a response with HISTORIC or invalid protocol field values SHOULD
be ignored and SHOULD NOT be stored in a resolver's/server's cache.
No changes are made to the format of the KEY record or to the use of
DNSSEC zone, host and user keys. Existing nameservers and resolvers
will continue to correctly process KEY records that contain DNSSEC
keys.
5. Storing Application Keys in the DNS
The scope of this document is strictly limited to the KEY record.
This document prohibits storing application keys in the KEY record,
but it does not endorse or restrict the storing application keys in
other record types. Other documents should describe how DNS handles
application keys.
6. IANA Consideration
Protocol Octet values 1,2,4, and 255 are changed to HISTORIC.
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Protocol Octet values 5-255 are reserved and are no longer available
for assignment by IANA.
7. Security Consideration
This document eliminates potential security problems that could arise
due to the coupling of DNS zone keys and application keys.
Prior to the change described in the document, a correctly authenti-
cated KEY set could include both application keys and DNSSEC keys.
If one of the application keys is compromised, it could be used as a
false zone key to create phony DNS signatures (SIG records).
Resolvers that do not carefully check the KEY sub-type may believe
these false signatures and incorrectly authenticate DNS data. With
this change, application keys cannot appear in an authenticated KEY
set.
Applications that accept keys based solely on DNSSEC rely on the DNS
administrator to correctly enter the application key data and are
only as secure as the weakest zone in the DNS chain of trust.
Compromises or errors caused by DNS administrators when entering
DNSSEC data could results in an application key failing to verify, or
verified incorrectly.
The format and correct usage of DNS zone keys is not changed by this
document and no new security considerations are introduced.
8. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to per-
tain 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 specifica-
tion 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
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rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
9. References
[DNSSEC] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
10. Author Information
Daniel Massey <masseyd@isi.edu>
USC Information Sciences Institute
3811 North Fairfax Drive, Suite 200
Arlington, VA 22203
Scott Rose <scott.rose@nist.gov>
National Institute for Standards and Technology
Gaithersburg, MD
Expiration and File Name:
This draft, titled <draft-ietf-dnsext-restrict-key-for-dnssec-00.txt> expires April 2001
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