Internet Engineering Task Force D. Wessels
Internet-Draft Verisign Labs
Intended status: Standards Track July 29, 2015
Expires: January 30, 2016
The EDNS Key Tag Option
draft-wessels-edns-key-tag-00
Abstract
The DNS Security Extensions (DNSSEC) were developed to provide origin
authentication and integrity protection for DNS data by using digital
signatures. These digital signatures can be verified by building a
chain-of-trust starting from a trust anchor and proceeding down to a
particular node in the DNS. This document specifies a way for
validating end-system resolvers to signal to a server which keys are
referenced in their chain-of-trust. The extensions allow zone
administrators to monitor the progress of rollovers in a DNSSEC-
signed zone.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 30, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Option Format . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Use By Queriers . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Stub Resolvers . . . . . . . . . . . . . . . . . . . . . . 5
5.1.1. Validating Stub Resolvers . . . . . . . . . . . . . . . 5
5.1.2. Non-validating Stub Resolvers . . . . . . . . . . . . . 6
5.2. Recursive Resolvers . . . . . . . . . . . . . . . . . . . . 6
5.2.1. Validating Recursive Resolvers . . . . . . . . . . . . 6
5.2.2. Non-validating Recursive Resolvers . . . . . . . . . . 6
6. Use By Responders . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and
[RFC4035] were developed to provide origin authentication and
integrity protection for DNS data by using digital signatures.
DNSSEC uses Key Tags to efficiently match signatures to the keys from
which they are generated. The Key Tag is a 16-bit value computed
from the RDATA portion of a DNSKEY RR using a formula not unlike a
ones-complement checksum. RRSIG RRs contain a Key Tag field whose
value is equal to the Key Tag of the DNSKEY RR that validates the
signature.
Likewise, Delegation Signer (DS) RRs also contain a Key Tag field
whose value is equal to the Key Tag of the DNSKEY RR to which it
refers.
This draft sets out to specify a way for validating end-system
resolvers to tell a server in a DNS query which DNSSEC key(s) they
would use to validate the expected response. This is done using the
new EDNS option specified below in Section 4 for use in the OPT
meta-RR [RFC6891]. This new EDNS option code is OPTIONAL to
implement and use.
This proposed EDNS option serves to measure the acceptance and use of
new trust anchors and key signing keys (KSKs). This signaling option
can be used by zone administrators as a gauge to measure the
successful deployment of new keys. This is of particular interest
for the DNS root zone in the event of key and/or algorithm rollovers
which relies on [RFC5011] to automatically update a validating end-
system's trust anchor.
This draft does not seek to introduce another process for rolling
keys or updating trust anchors. Rather, this document specifies a
means by which a client query can signal the set of keys that a
client uses for DNSSEC validation.
2. Requirements Terminology
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].
3. Terminology
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Trust Anchor: A configured DNSKEY RR or DS RR hash of a DNSKEY RR.
A validating security-aware resolver uses this public key or hash
as a starting point for building the authentication chain to a
signed DNS response. In general, a validating resolver will have
to obtain the initial values of its trust anchors via some secure
or trusted means outside the DNS protocol. Presence of a trust
anchor also implies that the resolver should expect the zone to
which the trust anchor points to be signed. (quoted from [RFC4033]
Section 2)
Key Tag: A 16-bit integer that identifies and enables efficient
selection of DNSSEC public keys. A Key Tag value can be computed
over the RDATA of a DNSKEY RR. The Key Tag field in the RRSIG and
DS records can be used to help select the corresponding DNSKEY RR
efficiently when more than one candidate DNSKEY RR is available.
For most algorithms the Key Tag is a simple 16-bit modular sum of
the DNSKEY RDATA. See [RFC4034] Appendix B.
4. Option Format
The edns-key-tag option is encoded as follows:
0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| KEY-TAG |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ... /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
OPTION-CODE: The EDNS0 option code assigned to edns-key-tag, [TBD].
OPTION-LENGTH: The value 2 x number of key-tag values present.
KEY-TAG: One or more 16-bit Key Tag values ([RFC4034], Appendix B).
5. Use By Queriers
A validating end-system resolver sets the edns-key-tag option in the
OPT meta-RR when sending a DNSKEY query. The validating end-system
resolver SHOULD also set the DNSSEC OK bit [RFC4034] to indicate that
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it wishes to receive DNSSEC RRs in the response.
A DNS client MUST NOT include the edns-key-tag option for non-DNSKEY
queries.
The KEY-TAG value(s) included in the edns-key-tag option represent
the Key Tag of the Trust Anchor or DNSKEY RR that will be used to
validate the expected response. When the client sends a DNSKEY
query, the edns-key-tag option represents the Key Tag(s) of the
KSK(s) of the zone for which the server is authoritative. A
validating end-system resolver learns the Key Tag(s) of the KSK(s)
from the zone's DS record(s) (found in the parent), or from a
configured trust anchor.
A DNS client SHOULD include the edns-key-tag option when issuing a
DNSKEY query for a zone corresponding to a configured Trust Anchor.
A DNS client MAY include the edns-key-tag option when issuing a
DNSKEY query for a non-Trust Anchor zone (i.e., Key Tags learned via
DS records). Since some DNSSEC validators implement bottom-up
validation, non-Trust Anchor Key Tags zone might not be known at the
time of the query. Such a validator can include the edns-key-tag
option based on previously cached data.
A DNS client MUST NOT include Key Tag(s) for keys which are not
learned via either configured Trust Anchor or DS records.
Since the edns-key-tag option is only set in the query, if a client
sees these options in the response, no action needs to be taken and
the client MUST ignore the option values.
5.1. Stub Resolvers
Typically, stub resolvers rely on an upstream recursive server (or
cache) to provide a response. Optimal setting of the edns-key-tag
option depends on whether the stub resolver elects to perform its own
validation.
5.1.1. Validating Stub Resolvers
A validating stub resolver sets the DNSSEC OK (DO) bit [RFC4034] to
indicate that it wishes to receive additional DNSSEC RRs (i.e., RRSIG
RRs) in the response. Such validating resolvers SHOULD include the
edns-key-tag option in the OPT RR when sending a DNSKEY query.
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5.1.2. Non-validating Stub Resolvers
The edns-key-tag option MUST NOT be included by non-validating stub
resolvers.
5.2. Recursive Resolvers
5.2.1. Validating Recursive Resolvers
A validating recursive resolver sets the edns-key-tag option when
performing recursion based on relevant keys it knows and any edns-
key-tag values in the stub client query. When the recursive server
receives a query with the option set, the recursive server SHOULD set
the edns-key-tag list for any outgoing iterative queries for that
resolution chain to a union of the stub client's Key Tag(s) and the
validating recursive resolver's Key Tag(s). For example, if the
recursive resolver's Key Tag list is (19036, 12345) and the stub's
list is (19036, 34567), the final edns-key-tag list would be (19036,
12345, 34567).
If the client included the DO and Checking Disabled (CD) bits, but
did not include the edns-key-tag option in the query, the validating
recursive resolver MAY include the option with its own Key Tag values
in full.
Validating recursive resolvers MUST NOT set the edns-key-tag option
in the final response to the stub client.
5.2.2. Non-validating Recursive Resolvers
Recursive resolvers that do not validate responses SHOULD copy the
edns-key-tag option seen in received queries, as they represent the
wishes of the validating downstream resolver that issued the original
query.
6. Use By Responders
An authoritative name server receiving queries with the edns-key-tag
option MAY log or otherwise collect the Key Tag values to provide
information to the zone operator.
A responder MUST NOT include the edns-key-tag option in any DNS
response.
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7. IANA Considerations
The IANA is directed to assign an EDNS0 option code for the edns-key-
tag option from the DNS EDNS0 Option Codes (OPT) registry as follows:
+-------+--------------+----------+-----------------+
| Value | Name | Status | Reference |
+-------+--------------+----------+-----------------+
| [TBA] | edns-key-tag | Optional | [This document] |
+-------+--------------+----------+-----------------+
8. Security Considerations
This document specifies a way for a client to signal its trust anchor
knowledge to a cache or server. The signals are optional codes
contained in the OPT meta-RR used with EDNS. The goal of these
options is to signal new trust anchor uptake in client code to allow
zone administrators to know when it is possible to complete a key
rollover in a DNSSEC-signed zone.
There is a possibility that an eavesdropper or server could infer the
validator in use by a client by the Key Tag list seen in queries.
This may allow an attacker to find validators using old, possibly
broken, keys. It could also be used to identify the validator or
narrow down the possible validator implementations in use by a
client, which could have a known vulnerability that could be
exploited by the attacker.
Consumers of data collected from the edns-key-tag option are advised
that provided Key Tag values might be "made up" by some DNS clients
with malicious or at least mischievous intentions.
DNSSEC does not require keys in a zone to have unique Key Tags.
During a rollover there is a small possibility that an old key and a
new key will have identical Key Tag values. Zone operators relying
on the edns-key-tag mechanism SHOULD take care to ensure that new
keys have unique Key Tag values.
9. Privacy Considerations
This proposal adds additional "signaling" to DNS queries in the form
of Key Tag values. While Key Tag values themselves are not
considered private information, it may be possible for an
eavesdropper to use Key Tag values as a fingerprinting technique to
identify particular DNS validating clients. This may be especially
true if the validator is configured with trust anchor for zones in
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addition to the root zone.
A validating end-system resolver need not transmit the edns-key-tag
option in every applicable query. Due to privacy concerns, such a
resolver MAY choose to transmit the edns-key-tag option for a subset
of queries (e.g., every 25th time), or by random chance with a
certain probability (e.g., 5%).
Implementations of this specification MAY be administratively
configured to only transmit the edns-key-tag option for certain
zones. For example, the software's configuration file may specify a
list of zones for which use of the option is allowed or denied.
Since the primary motivation for this specification is to provide
operational measurement data for root zone key rollovers, it is
RECOMMENDED that implementations at least include the edns-key-tag
option for root zone DNSKEY queries.
10. Acknowledgments
This document was inspired by and borrows heavily from [RFC6975] by
Scott Rose and Steve Crocker. The author would like to thank to
Casey Deccio and Burt Kalisky for early feedback.
11. References
11.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
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Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<http://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/
RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
11.2. Informative References
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
September 2007, <http://www.rfc-editor.org/info/rfc5011>.
[RFC6975] Crocker, S. and S. Rose, "Signaling Cryptographic
Algorithm Understanding in DNS Security Extensions
(DNSSEC)", RFC 6975, DOI 10.17487/RFC6975, July 2013,
<http://www.rfc-editor.org/info/rfc6975>.
Author's Address
Duane Wessels
Verisign Labs
12061 Bluemont Way
Reston, VA 20190
Phone: +1 703 948-3200
Email: dwessels@verisign.com
URI: http://verisigninc.com
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