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Keywords: dnssec
Network Working Group O. Kolkman
Request for Comments: 3757 RIPE NCC
Updates: 3755, 2535 J. Schlyter
Category: Standards Track NIC-SE
E. Lewis
ARIN
April 2004
Domain Name System KEY (DNSKEY) Resource Record (RR)
Secure Entry Point (SEP) Flag
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 (2004). All Rights Reserved.
Abstract
With the Delegation Signer (DS) resource record (RR), the concept of
a public key acting as a secure entry point (SEP) has been
introduced. During exchanges of public keys with the parent there is
a need to differentiate SEP keys from other public keys in the Domain
Name System KEY (DNSKEY) resource record set. A flag bit in the
DNSKEY RR is defined to indicate that DNSKEY is to be used as a SEP.
The flag bit is intended to assist in operational procedures to
correctly generate DS resource records, or to indicate what DNSKEYs
are intended for static configuration. The flag bit is not to be
used in the DNS verification protocol. This document updates RFC
2535 and RFC 3755.
Kolkman, et al. Standard Track [Page 1]
RFC 3757 DNSKEY RR SEP Flag April 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Secure Entry Point (SEP) Flag. . . . . . . . . . . . . . . 4
3. DNSSEC Protocol Changes. . . . . . . . . . . . . . . . . . . . 4
4. Operational Guidelines . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations. . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 6
7. Internationalization Considerations. . . . . . . . . . . . . . 6
8. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 7
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 8
1. Introduction
"All keys are equal but some keys are more equal than others" [6].
With the definition of the Delegation Signer Resource Record (DS RR)
[5], it has become important to differentiate between the keys in the
DNSKEY RR set that are (to be) pointed to by parental DS RRs and the
other keys in the DNSKEY RR set. We refer to these public keys as
Secure Entry Point (SEP) keys. A SEP key either used to generate a
DS RR or is distributed to resolvers that use the key as the root of
a trusted subtree [3].
In early deployment tests, the use of two (kinds of) key pairs for
each zone has been prevalent. For one kind of key pair the private
key is used to sign just the zone's DNSKEY resource record (RR) set.
Its public key is intended to be referenced by a DS RR at the parent
or configured statically in a resolver. The private key of the other
kind of key pair is used to sign the rest of the zone's data sets.
The former key pair is called a key-signing key (KSK) and the latter
is called a zone-signing key (ZSK). In practice there have been
usually one of each kind of key pair, but there will be multiples of
each at times.
It should be noted that division of keys pairs into KSK's and ZSK's
is not mandatory in any definition of DNSSEC, not even with the
introduction of the DS RR. But, in testing, this distinction has
been helpful when designing key roll over (key super-cession)
schemes. Given that the distinction has proven helpful, the labels
KSK and ZSK have begun to stick.
Kolkman, et al. Standard Track [Page 2]
RFC 3757 DNSKEY RR SEP Flag April 2004
There is a need to differentiate the public keys for the key pairs
that are used for key signing from keys that are not used key signing
(KSKs vs ZSKs). This need is driven by knowing which DNSKEYs are to
be sent for generating DS RRs, which DNSKEYs are to be distributed to
resolvers, and which keys are fed to the signer application at the
appropriate time.
In other words, the SEP bit provides an in-band method to communicate
a DNSKEY RR's intended use to third parties. As an example we
present 3 use cases in which the bit is useful:
The parent is a registry, the parent and the child use secured DNS
queries and responses, with a preexisting trust-relation, or plain
DNS over a secured channel to exchange the child's DNSKEY RR sets.
Since a DNSKEY RR set will contain a complete DNSKEY RRset the SEP
bit can be used to isolate the DNSKEYs for which a DS RR needs to
be created.
An administrator has configured a DNSKEY as root for a trusted
subtree into security aware resolver. Using a special purpose
tool that queries for the KEY RRs from that domain's apex, the
administrator will be able to notice the roll over of the trusted
anchor by a change of the subset of KEY RRs with the DS flag set.
A signer might use the SEP bit on the public key to determine
which private key to use to exclusively sign the DNSKEY RRset and
which private key to use to sign the other RRsets in the zone.
As demonstrated in the above examples it is important to be able to
differentiate the SEP keys from the other keys in a DNSKEY RR set in
the flow between signer and (parental) key-collector and in the flow
between the signer and the resolver configuration. The SEP flag is
to be of no interest to the flow between the verifier and the
authoritative data store.
The reason for the term "SEP" is a result of the observation that the
distinction between KSK and ZSK key pairs is made by the signer, a
key pair could be used as both a KSK and a ZSK at the same time. To
be clear, the term SEP was coined to lessen the confusion caused by
the overlap. (Once this label was applied, it had the side effect of
removing the temptation to have both a KSK flag bit and a ZSK flag
bit.)
The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED",
"RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be
interpreted as described in BCP 14, RFC 2119 [1].
Kolkman, et al. Standard Track [Page 3]
RFC 3757 DNSKEY RR SEP Flag April 2004
2. The Secure Entry Point (SEP) Flag
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 |S| protocol | algorithm |
| |E| | |
| |P| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ public key /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DNSKEY RR Format
This document assigns the 15th bit in the flags field as the secure
entry point (SEP) bit. If the bit is set to 1 the key is intended to
be used as secure entry point key. One SHOULD NOT assign special
meaning to the key if the bit is set to 0. Operators can recognize
the secure entry point key by the even or odd-ness of the decimal
representation of the flag field.
3. DNSSEC Protocol Changes
The bit MUST NOT be used during the resolving and verification
process. The SEP flag is only used to provide a hint about the
different administrative properties of the key and therefore the use
of the SEP flag does not change the DNS resolution protocol or the
resolution process.
4. Operational Guidelines
The SEP bit is set by the key-pair-generator and MAY be used by the
zone signer to decide whether the public part of the key pair is to
be prepared for input to a DS RR generation function. The SEP bit is
recommended to be set (to 1) whenever the public key of the key pair
will be distributed to the parent zone to build the authentication
chain or if the public key is to be distributed for static
configuration in verifiers.
When a key pair is created, the operator needs to indicate whether
the SEP bit is to be set in the DNSKEY RR. As the SEP bit is within
the data that is used to compute the 'key tag field' in the SIG RR,
changing the SEP bit will change the identity of the key within DNS.
In other words, once a key is used to generate signatures, the
setting of the SEP bit is to remain constant. If not, a verifier
will not be able to find the relevant KEY RR.
Kolkman, et al. Standard Track [Page 4]
RFC 3757 DNSKEY RR SEP Flag April 2004
When signing a zone, it is intended that the key(s) with the SEP bit
set (if such keys exist) are used to sign the KEY RR set of the zone.
The same key can be used to sign the rest of the zone data too. It
is conceivable that not all keys with a SEP bit set will sign the
DNSKEY RR set, such keys might be pending retirement or not yet in
use.
When verifying a RR set, the SEP bit is not intended to play a role.
How the key is used by the verifier is not intended to be a
consideration at key creation time.
Although the SEP flag provides a hint on which public key is to be
used as trusted root, administrators can choose to ignore the fact
that a DNSKEY has its SEP bit set or not when configuring a trusted
root for their resolvers.
Using the SEP flag a key roll over can be automated. The parent can
use an existing trust relation to verify DNSKEY RR sets in which a
new DNSKEY RR with the SEP flag appears.
5. Security Considerations
As stated in Section 3 the flag is not to be used in the resolution
protocol or to determine the security status of a key. The flag is
to be used for administrative purposes only.
No trust in a key should be inferred from this flag - trust MUST be
inferred from an existing chain of trust or an out-of-band exchange.
Since this flag might be used for automating public key exchanges, we
think the following consideration is in place.
Automated mechanisms for roll over of the DS RR might be vulnerable
to a class of replay attacks. This might happen after a public key
exchange where a DNSKEY RR set, containing two DNSKEY RRs with the
SEP flag set, is sent to the parent. The parent verifies the DNSKEY
RR set with the existing trust relation and creates the new DS RR
from the DNSKEY RR that the current DS RR is not pointing to. This
key exchange might be replayed. Parents are encouraged to implement
a replay defense. A simple defense can be based on a registry of
keys that have been used to generate DS RRs during the most recent
roll over. These same considerations apply to entities that
configure keys in resolvers.
Kolkman, et al. Standard Track [Page 5]
RFC 3757 DNSKEY RR SEP Flag April 2004
6. IANA Considerations
IANA has assigned the 15th bit in the DNSKEY Flags Registry (see
Section 4.3 of [4]) as the Secure Entry Point (SEP) bit.
7. Internationalization Considerations
Although SEP is a popular acronym in many different languages, there
are no internationalization considerations.
8. Acknowledgments
The ideas documented in this document are inspired by communications
we had with numerous people and ideas published by other folk. Among
others Mark Andrews, Rob Austein, Miek Gieben, Olafur Gudmundsson,
Daniel Karrenberg, Dan Massey, Scott Rose, Marcos Sanz and Sam Weiler
have contributed ideas and provided feedback.
This document saw the light during a workshop on DNSSEC operations
hosted by USC/ISI in August 2002.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[3] Lewis, E., "DNS Security Extension Clarification on Zone
Status", RFC 3090, March 2001.
[4] Weiler, S., "Legacy Resolver Compatibility for Delegation Signer
(DS)", RFC 3755, April 2004.
9.2. Informative References
[5] Gudmundsson, O., "Delegation Signer (DS) Resource Record (RR)",
RFC 3658, December 2003.
[6] Orwell, G. and R. Steadman (illustrator), "Animal Farm; a Fairy
Story", ISBN 0151002177 (50th anniversary edition), April 1996.
Kolkman, et al. Standard Track [Page 6]
RFC 3757 DNSKEY RR SEP Flag April 2004
10. Authors' Addresses
Olaf M. Kolkman
RIPE NCC
Singel 256
Amsterdam 1016 AB
NL
Phone: +31 20 535 4444
EMail: olaf@ripe.net
URI: http://www.ripe.net/
Jakob Schlyter
NIC-SE
Box 5774
SE-114 87 Stockholm
Sweden
EMail: jakob@nic.se
URI: http://www.nic.se/
Edward P. Lewis
ARIN
3635 Concorde Parkway Suite 200
Chantilly, VA 20151
US
Phone: +1 703 227 9854
EMail: edlewis@arin.net
URI: http://www.arin.net/
Kolkman, et al. Standard Track [Page 7]
RFC 3757 DNSKEY RR SEP Flag April 2004
11. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM 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
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Kolkman, et al. Standard Track [Page 8]