DNS Extensions D. Massey
Internet-Draft USC/ISI
Expires: December 27, 2002 S. Rose
NIST
June 28, 2002
Limiting the Scope of the KEY Resource Record
draft-ietf-dnsext-restrict-key-for-dnssec-03
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document limits the Domain Name System KEY resource record to
only keys used by the Domain Name System Security Extensions
(DNSSEC). The original KEY resource record used sub-typing to store
both DNSSEC keys and arbitrary application keys. Storing both DNSSEC
and application keys in one record was a mistake. This document
removes application keys from the KEY record by redefining the
Protocol Octet field in the KEY Resource Record Data. As a result of
removing application keys, all but one of the flags in the KEY record
become unnecessary and are removed. Three existing application key
sub-types are changed to reserved, but the format of the KEY record
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is not changed. This document updates RFC 2535.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation for Restricting the KEY Record . . . . . . . . . . 4
2.1 Differences Between DNSSEC and Application Keys . . . . . . . 4
3. Definition of the KEY Resource Record . . . . . . . . . . . . 7
4. Changes from RFC 2535 KEY Record . . . . . . . . . . . . . . . 8
5. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 10
6. Storing Application Keys in the DNS . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
This document limits the scope the KEY resource record. The KEY
resource record was defined in [2] and used resource record sub-
typing to hold arbitrary public keys such as Email, IPSEC, DNSSEC,
and TLS keys. This document eliminates the existing Email, IPSEC,
and TLS sub-types and prohibits the introduction of new sub-types.
DNSSEC will be the only allowable sub-type for the KEY record (hence
sub-typing is essentially eliminated) and all but one of the KEY
record flags are also eliminated.
Section 2 presents the motivation for restricting the KEY record and
Section 3 defines the revised KEY record. Sections 4 and 5 summarize
the changes from RFC 2535 and discuss backwards compatibility. It is
important to note that this document restricts the use of the KEY
record and simplifies the flags, but does not change the definition
or use of DNSSEC keys.
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2. Motivation for Restricting the KEY Record
The KEY record RDATA [2] consists of Flags, a Protocol Octet, an
Algorithm type, and a Public Key. The Protocol Octet identifies the
KEY record sub-type. DNSSEC public keys are stored in the KEY record
using a Protocol Octet value of 3. Email, IPSEC, and TLS keys were
also stored in the KEY record and used Protocol Octet values of 1,2,
and 4 (respectively). Protocol Octet values 5-254 were available for
assignment by IANA and values were requested (but not assigned) for
applications such as SSH.
Any use of sub-typing has inherent limitations. A resolver can not
specify the desired sub-type in a DNS query and most DNS operations
apply only to resource records sets. For example, a resolver can not
directly request the DNSSEC subtype KEY records. Instead, the
resolver has to request all KEY records associated with a DNS name
and then search the set for the desired DNSSEC sub-type. DNSSEC
signatures also 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 the sub-type is used to distinguish between DNSSEC
keys and application keys. DNSSEC keys and application keys differ
in virtually every respect and Section 2.1 discusses these
differences in more detail. Combining these very different types of
keys into a single sub-typed resource record adds unnecessary
complexity and increases the potential for implementation and
deployment errors. Limited experimental deployment has shown that
application keys stored in KEY records are problematic.
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.
2.1 Differences Between DNSSEC and Application Keys
DNSSEC keys are an essential part of the DNSSEC protocol and are used
by both name servers and resolvers in order to perform DNS tasks. A
DNS zone key, used to sign and authenticate RR sets, is the most
common example of a DNSSEC key. SIG(0) [3] and TKEY [2] also use
DNSSEC keys.
Application keys such as Email keys, IPSEC keys, and TLS keys are
simply another type data. These keys have no special meaning to a
name server or resolver.
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The following table summarizes some of the differences between DNSSEC
keys and Application keys:
1. They serve different purposes.
2. They are managed by different administrators.
3. They are authenticated according to different rules.
4. Nameservers use different rules when including them in responses.
5. Resolvers process them in different ways.
6. Faults/key compromises have different consequences.
1. The purpose of a DNSSEC key is to sign resource records
associated with a DNS zone (or generate DNS transaction signatures
in the case of SIG(0)/TKEY). But the purpose of an application key
is specific to the application. 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.
2. DNSSEC keys are managed by DNS administrators, but application
keys are managed by application administrators. The DNS zone
administrator determines the key lifetime, handles any suspected key
compromises, and manages any DNSSEC key changes. Likewise, the
application administrator is responsible for the same functions for
the application 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.
3. DNSSEC zone keys are used to authenticate application keys, but
application keys MUST NOT be used to authenticate DNS zone keys. A
DNS zone key is either configured as trusted key or authenticated by
constructing a chain of trust in the DNS hierarchy. To participate
in the chain of trust, a DNS zone needs to exchange zone key
information with its parent zone [2]. 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 could impose additional requirements before
the application key is accepted as authentic.
4. It MAY be useful for nameservers to include DNS zone keys in the
additional section of a response, but application keys are typically
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not useful unless they have been specifically requested. For
example, it could 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
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 RR set).
5. DNS zone keys require special handling by resolvers, but
application keys are treated the same as any other type of DNS data.
The DNSSEC keys are of no value to end applications, unless the
applications 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 useful 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 needs to specify the sub-type or the
application needs to be able to accept all KEY records and pick out
the desired the sub-type.
6. A fault or compromise of a 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 application, but it SHOULD not have an impact on the DNS.
Note that application 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, DNSSEC keys and application keys differ in most every
respect. DNSSEC keys are an essential part of the DNS infrastructure
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.
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3. Definition of the KEY Resource Record
The KEY record uses type 25 and is used as resource record for
storing DNSSEC keys. The RDATA for a KEY RR consists of flags, a
protocol octet, the algorithm number octet, and the public key
itself. The format is as follows:
---------------------------------------------------------------------
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | protocol | algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ public key /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
KEY RR Format
---------------------------------------------------------------------
In the flags field, all bits except bit 7 are reserved and SHOULD be
zero. If Bit 7 (Zone bit) is set to 1, then the KEY is a DNS Zone
key. If Bit 7 is set to 0, the KEY is not a zone key. SIG(0)/TKEY
are examples of DNSSEC keys that are not zone keys.
The protocol field MUST be set to 3.
The algorithm and public key fields are not changed.
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4. Changes from RFC 2535 KEY Record
The KEY RDATA format is not changed.
All flags except for the zone key flag are eliminated:
The A/C bits (bits 0 and 1) are eliminated. They MUST be set to 0
and MUST be ignored by the receiver.
The extended flags bit (bit 3) is eliminated. It MUST be set to 0
and MUST be ignored by the receiver.
The host/user bit (bit 6) is eliminated. It MUST be set to 0 and
MUST be ignored by the receiver.
The zone bit (bit 7) remains unchanged.
The signatory field (bits 12-15) are eliminated by [4]. They MUST
be set to 0 and MUST be ignored by the receiver.
Bits 2,4,5,8,9,10,11 remain unchanged. They are reserved, MUST be
set to zero and MUST be ignored by the receiver.
Assignment of any future KEY record Flag values requires a standards
action.
All Protocol Octet values except DNSSEC (3) are eliminated:
Value 1 (Email) is renamed to reserved.
Value 2 (IPSEC) is renamed to reserved.
Value 3 (DNSSEC) is unchanged.
Value 4 (TLS) is renamed to reserved.
Value 5-254 remains unchanged (reserved).
Value 255 (ANY) is renamed to reserved.
The authoritative data for a zone MUST NOT include any KEY records
with a protocol octet other than 3. Any future KEY record Protocol
Octet values requires a standards action.
Name servers and resolvers SHOULD accept KEY RR sets that contain KEY
records with a value other than 3. If out of date DNS zones contain
deprecated KEY records with a protocol octet value other than 3, then
simply dropping the deprecated KEY records from the KEY RR set would
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invalidate any associated SIG record(s) and could create caching
consistency problems. Note that KEY records with a protocol octet
value other than 3 MUST NOT be used to authenticate DNS data.
The algorithm and public key fields are not changed.
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5. Backward Compatibility
DNSSEC zone key records are not changed and remain backwards
compatible. A properly formatted RFC 2535 zone KEY would have all
flag bits, other than the Zone Bit (Bit 7), set to 0 and would have
the Protocol Octet set to 3. This remains true under the restricted
KEY.
DNSSEC non-zone key records (SIG(0)/TKEY keys) are backwards
compatible, but the distinction between host and user keys (flag bit
6) is lost.
No backwards compatibility is provided for application keys. Any
Email, IPSEC, or TLS keys are now deprecated. Storing application
keys in the KEY record created problems such as keys at the apex and
large RR sets and some change in the definition and/or usage of the
KEY record would have been required even if the approach described
here were not adopted.
Overall, existing nameservers and resolvers will continue to
correctly process KEY records with a sub-type of DNSSEC keys.
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6. 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 can describe how DNS handles
application keys.
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7. IANA Considerations
RFC 2535 created an IANA registry for DNS KEY Resource Record
Protocol Octet values. Values to 1,2,3, 4, and 255 were assigned by
RFC 2535 and values 5-254 were made available for assignment by IANA.
This document makes two sets of changes to this registry.
First, this document re-assigns DNS KEY Resource Record Protocol
Octet values 1, 2, 4, and 255 to ``reserved''. DNS Key Resource
Record Protocol Octet Value 3 remains unchanged as ``DNSSEC''.
Second, new values are no longer available for assignment by IANA and
this document closes the IANA registry for DNS KEY Resource Record
Protocol Octet Values. Assignment of any future KEY Resource Record
Protocol Octet values requires a standards action.
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8. Security Considerations
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 this document, a correctly authenticated 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 false DNS signatures (SIG records). Resolvers that do
not carefully check the KEY sub-type could believe these false
signatures and incorrectly authenticate DNS data. With this change,
application keys cannot appear in an authenticated KEY set and this
vulnerability is eliminated.
The format and correct usage of DNSSEC keys is not changed by this
document and no new security considerations are introduced.
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References (Normative)
[1] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[2] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC
2930, September 2000.
[3] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000.
[4] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
Authors' Addresses
Dan Massey
USC Information Sciences Institute
3811 N. Fairfax Drive
Arlington, VA 22203
USA
EMail: masseyd@isi.edu
Scott Rose
National Institute for Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899-3460
USA
EMail: scott.rose@nist.gov
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Full Copyright Statement
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