Network Working Group D. Massey
Request for Comments: 3445 USC/ISI
Updates: 2535 S. Rose
Category: Standards Track NIST
December 2002
Limiting the Scope of the KEY Resource Record (RR)
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 Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document limits the Domain Name System (DNS) KEY Resource Record
(RR) to only keys used by the Domain Name System Security Extensions
(DNSSEC). The original KEY RR used sub-typing to store both DNSSEC
keys and arbitrary application keys. Storing both DNSSEC and
application keys with the same record type is a mistake. This
document removes application keys from the KEY record by redefining
the Protocol Octet field in the KEY RR Data. As a result of removing
application keys, all but one of the flags in the KEY record become
unnecessary and are redefined. Three existing application key sub-
types are changed to reserved, but the format of the KEY record is
not changed. This document updates RFC2535.
1. IntrodUCtion
This document limits the scope of the KEY Resource Record (RR). The
KEY RR was defined in [3] 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 RR (hence sub-typing is
essentially eliminated) and all but one of the KEY RR flags are also
eliminated.
Section 2 presents the motivation for restricting the KEY record and
Section 3 defines the revised KEY RR. Sections 4 and 5 summarize the
changes from RFC2535 and discuss backwards compatibility. It is
important to note that this document restricts the use of the KEY RR
and simplifies the flags, but does not change the definition or use
of DNSSEC keys.
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 RFC2119 [1].
2. Motivation for Restricting the KEY RR
The KEY RR RDATA [3] consists of Flags, a Protocol Octet, an
Algorithm type, and a Public Key. The Protocol Octet identifies the
KEY RR sub-type. DNSSEC public keys are stored in the KEY RR using a
Protocol Octet value of 3. Email, IPSEC, and TLS keys were also
stored in the KEY RR 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 RRs. Instead, the resolver
has to request all KEY RRs 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 RRs associated with the DNS name,
regardless of sub-type.
In the case of the KEY RR, 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 RRs are problematic.
This document addresses these issues by removing all application keys
from the KEY RR. Note that the scope of this document is strictly
limited to the KEY RR and this document does not endorse or restrict
the storage of application keys in other, yet undefined, 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) [4] and TKEY [3] also use
DNSSEC keys.
Application keys such as Email keys, IPSEC keys, and TLS keys are
simply another type of data. These keys have no special meaning to a
name server or resolver.
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
by definition, application keys are not allowed to authenticate DNS
zone keys. A DNS zone key is either configured as a 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 [3]. Application keys are
not configured as trusted keys in the DNS and are never part of any
DNS chain of trust. Application key data is not needed by the parent
and does not need to be exchanged with the parent zone for secure DNS
resolution to work. A resolver considers an application key RRset as
authenticated DNS information if it has a valid signature from the
local DNS zone keys, but applications could impose additional
security requirements before the application key is accepted as
authentic for use with the application.
4. 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
example, it could be useful to include the example.com zone key along
with a response that contains the www.example.com A record and SIG
record. A secure resolver will need the example.com zone key in
order to check the SIG and authenticate the www.example.com A record.
It is typically 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 would need the IPSEC, email, TLS, and
other key associated with example.com if the resolver intends to
authenticate the example.com zone key (since signatures only apply to
the entire KEY RR set). Depending on the number of protocols
involved, the KEY RR set could grow unwieldy for resolvers, and DNS
administrators to manage.
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. By definition,
secure resolvers are not allowed to use application keys as part of
the authentication process. Application keys have no unique meaning
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 RRs
and pick out the desired 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 the
zone and in all of its subzones. By using a compromised key, an
attacker can forge data from the effected zone and 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 RR.
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.
3. Definition of the KEY RR
The KEY RR 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 MUST 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.
4. Changes from RFC2535 KEY RR
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 [5]. 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 RR 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. The registry maintained by IANA
for protocol values is closed for new assignments.
Name servers and resolvers SHOULD accept KEY RR sets that contain KEY
RRs with a value other than 3. If out of date DNS zones contain
deprecated KEY RRs with a protocol octet value other than 3, then
simply dropping the deprecated KEY RRs from the KEY RR set would
invalidate any associated SIG record(s) and could create caching
consistency problems. Note that KEY RRs 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.
5. Backward Compatibility
DNSSEC zone KEY RRs are not changed and remain backwards compatible.
A properly formatted RFC2535 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 RRs (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 RR created problems such as keys at the apex and
large RR sets and some change in the definition and/or usage of the
KEY RR would have been required even if the approach described here
were not adopted.
Overall, existing nameservers and resolvers will continue to
correctly process KEY RRs with a sub-type of DNSSEC keys.
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.
7. IANA Considerations
RFC2535 created an IANA registry for DNS KEY RR Protocol Octet
values. Values 1, 2, 3, 4, and 255 were assigned by RFC2535 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 RR Protocol Octet values 1,
2, 4, and 255 to "reserved". DNS Key RR 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 RR Protocol Octet
Values. Assignment of any future KEY RR Protocol Octet values
requires a standards action.
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. This
document restricts the KEY RR to DNS security usage only. This is an
attempt to simplify the security model and make it less user-error
prone. 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.
9. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[2] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[3] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC
2930, September 2000.
[4] Eastlake, D., "DNS Request and Transaction Signatures
(SIG(0)s)", RFC2931, September 2000.
[5] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC3007, November 2000.
10. 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
11. Full Copyright Statement
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