Internet DRAFT - draft-ietf-dnsop-edns-key-tag
draft-ietf-dnsop-edns-key-tag
Internet Engineering Task Force D. Wessels
Internet-Draft Verisign
Intended status: Standards Track W. Kumari
Expires: August 20, 2017 Google
P. Hoffman
ICANN
February 16, 2017
Signaling Trust Anchor Knowledge in DNS Security Extensions (DNSSEC)
draft-ietf-dnsop-edns-key-tag-05
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 two different
ways for validating resolvers to signal to a server which keys are
referenced in their chain-of-trust. The data from such signaling
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 20, 2017.
Copyright Notice
Copyright (c) 2017 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
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Design Evolution . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Terminology . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Using the edns-key-tag Option . . . . . . . . . . . . . . . . 5
4.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Use By Queriers . . . . . . . . . . . . . . . . . . . . . 5
4.2.1. Stub Resolvers . . . . . . . . . . . . . . . . . . . 6
4.2.1.1. Validating Stub Resolvers . . . . . . . . . . . . 6
4.2.1.2. Non-validating Stub Resolvers . . . . . . . . . . 6
4.2.2. Recursive Resolvers . . . . . . . . . . . . . . . . . 6
4.2.2.1. Validating Recursive Resolvers . . . . . . . . . 6
4.2.2.2. Non-validating Recursive Resolvers . . . . . . . 7
4.3. Use By Responders . . . . . . . . . . . . . . . . . . . . 7
5. Using the Key Tag Query . . . . . . . . . . . . . . . . . . . 7
5.1. Query Format . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Use By Queriers . . . . . . . . . . . . . . . . . . . . . 8
5.3. Use By Responders . . . . . . . . . . . . . . . . . . . . 8
5.3.1. Interaction With Aggressive Negative Caching . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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
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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 document specifies how validating resolvers can tell a server,
in a DNS query, which DNSSEC key(s) they would use to validate the
server's responses. It describes two independent methods for
conveying Key Tag information bewteen clients and servers: placing an
EDNS option in the OPT meta-RR [RFC6891] that contains the key tags
(described in Section 4), and by periodically sending special "key
tag queries" to a server authoritative for the zone (described in
Section 5).
Each of these new signaling mechanisms is OPTIONAL to implement and
use. These mechanisms serve to measure the acceptance and use of new
DNSSEC trust anchors and key signing keys (KSKs). This signaling
data 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
that rely on [RFC5011] to automatically update a validating DNS
resolver's trust anchor.
This document does not introduce new processes for rolling keys or
updating trust anchors. Rather, it specifies a means by which a DNS
query can signal the set of keys that a client uses for DNSSEC
validation.
1.1. Design Evolution
Initially, when the work on this document started, it proposed
including Key Tag values in a new EDNS(0) option code. It was
modeled after [RFC6975], which provides DNSSEC algorithm signaling.
The authors received feedback from dnsop Working Group participants
that it might be better to convey Key Tags in QNAME of a separate DNS
query, rather than as an EDNS(0) option. Mostly this is because
forwarding (e.g. from stub to recursive to authoritative) could be
problematic. Reasons include:
1. EDNS(0) is a hop-by-hop protocol. Unknown option codes would not
be forwarded by default, as per [RFC6891].
2. Middleboxes might block entire queries containing unknown EDNS(0)
option codes.
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3. A recursive might need to remember Key Tag values (i.e., keep
state) received from its stub clients and then forward them at a
later opportunity.
One advantage of the EDNS(0) option code is that it is possible to
see that a stub client has a different Key Tag list than its
forwarder. In the QNAME-based approach, this is not possible because
queries originated by a stub and a forwarder are indistinguishable.
The authors feel this advantage is not sufficient to justify the
EDNS(0) approach.
One downside to the QNAME approach is that it uses a separate query,
whereas with EDNS(0) the Key Tag values are "piggy-backed" on to an
existing DNSKEY query. For this reason, this document recommends
only sending QNAME-based key tag queries for configured trust
anchors, although EDNS-based key tags can be sent with any DNSKEY
query.
Another downside to the QNAME-based approach is that since the trust
anchor zone might not contain labels matching the QNAME, these
queries could be subject to aggressive negative caching features now
in development by the Working Group. This could affect the amount of
signaling sent by some clients compared to others.
A probably minor downside to the QNAME-based approach is that it
cannot be used with extremely long query names if the addition of the
prefix would cause the name to be longer than 255 octets.
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
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
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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. Using the edns-key-tag Option
4.1. 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).
4.2. Use By Queriers
A validating resolver sets the edns-key-tag option in the OPT meta-RR
when sending a DNSKEY query. The validating resolver SHOULD also set
the DNSSEC OK bit [RFC4034] to indicate that 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 resolver learns the Key Tag(s) of the KSK(s) from the
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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.
4.2.1. 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.
4.2.1.1. 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.
4.2.1.2. Non-validating Stub Resolvers
The edns-key-tag option MUST NOT be included by non-validating stub
resolvers.
4.2.2. Recursive Resolvers
4.2.2.1. Validating Recursive Resolvers
A validating recursive resolver is, by definition, configured with at
least one trust anchor. Thus, a recursive resolver SHOULD include
the edns-key-tag option in its DNSKEY queries as described above.
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In addition, the clients of a validating recursive resolver might be
configured to do their own validation, with their own trust
anchor(s). When a validating recursive resolver receives a query
that includes the edns-key-tag option with a Key Tag list that
differs from its own, it SHOULD forward both the client's Key Tag
list as well as its own. When doing so, the recursive resolver
SHOULD transmit the two Key Tag lists using separate instances of the
edns-key-tag option code in the OPT meta-RR. For example, if the
recursive resolver's Key Tag list is (19036, 12345) and the stub/
client's list is (19036, 34567), the recursive would include the
edns-key-tag option twice: Once with values (19036, 12345) and once
with values (19036, 34567).
A validating recursive resolver MAY combine stub/client Key Tag
values from multiple incoming queries into a single outgoing query.
It is RECOMMENDED that implementations place reasonable limits on the
number of Key Tags to include in the outgoing edns-key-tag option.
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.
4.2.2.2. 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.
4.3. 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.
5. Using the Key Tag Query
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5.1. Query Format
A key tag query consists of a standard DNS query of type NULL and of
class IN [RFC1035].
The first component of the query name is the string "_ta-" followed
by a sorted, hyphen-separated list of hexadecimal-encoded Key Tag
values. The zone name corresponding to the trust anchor is appended
to this first component.
For example, a validating DNS resolver that has a single root zone
trust anchor with key tag 17476 (decimal) would originate a query of
the form QTYPE=NULL, QCLASS=IN, QNAME=_ta-4444.
Hexadecimal values MUST be zero-padded. For example, if the key tag
is 999 (decimal), it is represented in hexadecimal as 03e7.
When representing multiple key tag values, they MUST be sorted in
order from smallest to largest. For example, A validating DNS
resolver that has a three trust anchors for the example.com zone with
key tags 1589, 43547, 31406 (decimal) would originate a query of the
form QTYPE=NULL, QCLASS=IN, QNAME=_ta-0635-7aae-aa1b.example.com.
5.2. Use By Queriers
A validating DNS resolver (stub or recursive) SHOULD originate a key
tag query whenever it also originates a DNSKEY query for a configured
Trust Anchor zone. In other words, the need to issue a DNSKEY query
is also the trigger to issue a key tag query.
The value(s) included in the key tag query represent the Key Tag(s)
of the Trust Anchor that will be used to validate the expected DNSKEY
response.
A DNS validating resolver SHOULD NOT originate key tag queries when
also originating DNSKEY queries for non-Trust Anchor zones.
A non-validating DNS resolver MUST NOT originate key tag queries.
DNS resolvers with caches SHOULD cache and reuse the response to a
key tag query just as it would any other response.
5.3. Use By Responders
An authoritative name server receiving key tag queries MAY log or
otherwise collect the Key Tag values to provide information to the
zone operator.
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An authoritative name server MUST generate an appropriate response to
the key tag query. A server does not need to have built-in logic
that determines the response to key tag queries: the response code is
determined by whether the data is in the zone file or covered by
wildcard. The zone operator might want to add specific key tag
records to its zone, perhaps with specific TTLs, to affect the
frequency of key tag queries from clients.
5.3.1. Interaction With Aggressive Negative Caching
Aggressive NSEC/NSEC3 negative caching
[draft-ietf-dnsop-nsec-aggressiveuse] may also affect the quality of
key tag signaling. When the response code for a key tag query is
NXDOMAIN, DNS resolvers that implement aggressive negative caching
will send fewer key tag queries than resolvers that do not implement
it.
For this reason, zone operators might choose to create records
corresponding to expected key tag queries. During a rollover from
key tag 1111 (hex) to key tag 2222 (hex), the zone could include the
following records:
_ta-1111 IN NULL \# 0
_ta-2222 IN NULL \# 0
_ta-1111-2222 IN NULL \# 0
Recall that when multiple key tags are present, the originating
client MUST sort them from smallest to largest in the query name.
6. 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] |
+-------+--------------+----------+-----------------+
7. Security Considerations
This document specifies a way for a client to signal its trust anchor
knowledge to a cache or server. The goal of these optional
mechanisms is to signal new trust anchor uptake in clients to allow
zone administrators to know when it is possible to complete a key
rollover in a DNSSEC-signed zone.
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There is a possibility that an eavesdropper or server could infer the
validator in use by a client by the Key Tag list seen. 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 mechanisms are advised that
provided Key Tag values might be "made up" by some DNS clients with
malicious or at least mischievous intentions. For example, an
attacker with sufficient resources might try to generate large
numbers of queries including only old Key Tag values, with the
intention of delaying the completion of a key rollover.
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.
8. Privacy Considerations
This proposal adds additional, optional "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
addition to the root zone.
A validating resolver need not transmit the key tags in every
applicable query. Due to privacy concerns, such a resolver MAY
choose to transmit the key tags 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 key tags for certain zones. For
example, the software's configuration file may specify a list of
zones for which use of the mechanisms described here 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.
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9. Acknowledgments
This document was inspired by and borrows heavily from [RFC6975] by
Scott Rose and Steve Crocker. The authors would like to thank Mark
Andrews, Casey Deccio, Burt Kalisky, Bob Harold, Edward Lewis, Tim
Wicinski, Suzanne Woolf, and other members of the dnsop working group
for their input.
10. References
10.1. Normative References
[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.
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>.
10.2. Informative References
[draft-ietf-dnsop-nsec-aggressiveuse]
Fujiwara, K., "Aggressive use of NSEC/NSEC3", 2016.
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[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>.
Authors' Addresses
Duane Wessels
Verisign
12061 Bluemont Way
Reston, VA 20190
United States
Phone: +1 703 948-3200
Email: dwessels@verisign.com
URI: http://verisigninc.com
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States
Email: warren@kumari.net
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
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