DNS Extensions Working Group S. Crocker
Internet-Draft Shinkuro Inc.
Intended status: Standards Track S. Rose
Expires: October 1, 2011 NIST
March 30, 2011
Signaling Cryptographic Algorithm Understanding in DNSSEC
draft-ietf-dnsext-dnssec-algo-signal-01
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 generated using
different algorithms. This draft sets out to specify a way for
validating end-system resolvers to signal to a server which
cryptographic algorithms they support.
Requirements Language
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 [RFC2119].
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 October 1, 2011.
Copyright Notice
Copyright (c) 2011 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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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Signaling DNSSEC Algorithm Understood (DAU) Using EDNS . . . . 3
3. Client Considerations . . . . . . . . . . . . . . . . . . . . . 4
3.1. Stub Resolvers . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Validating Stub Resolvers . . . . . . . . . . . . . . . . . 5
3.3. Non-Validating Stub Resolvers . . . . . . . . . . . . . . . 5
3.4. Recursive Resolvers . . . . . . . . . . . . . . . . . . . . 5
3.4.1. Validating Recursive Resolvers . . . . . . . . . . . . 5
3.4.2. Non-validating Recursive Resolvers . . . . . . . . . . 6
4. Intermediate Middlebox Considerations . . . . . . . . . . . . . 6
5. Server Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Traffic Analysis Considerations . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . . 7
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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. Each
digital signature RR (RRSIG) contains an algorithm code number.
These algorithm codes tells validators which cryptographic algorithm
was used to generate the digital signature. Authentication across
delegation boundaries is maintained by storing a hash of a subzone's
key in the parent zone stored in a Delegation Signer (DS) RR. These
DS RR's contain a second code number to identify the hash algorithm
used to construct the DS RR.
This draft sets out to specify a way for validating end-system
resolvers to tell a server which cryptographic and/or hash algorithms
they support in a DNS query. This is done using the EDNS attribute
values in the OPT meta-RR [RFC2671].
This proposed EDNS option serves to measure the acceptance and use of
new digital signing and hash algorithms. This algorithm signaling
option can be used by zone administrators as a gauge to measure the
successful deployment of code that implements a newly deployed
digital signature or hash algorithm used with DNSSEC. A zone
administrator may be able to determine when to stop serving the old
algorithm when the server sees that a significant number of its
clients signal that they are able to accept the new algorithm. Note
that this survey may be conducted over the period of years before a
tipping point is seen.
This draft does not seek to include another process for including new
algorithms for use with DNSSEC. It also does not address the
question of which algorithms are to be included in any official list
of mandatory or recommended cryptographic algorithms for use with
DNSSEC. Rather, this document specifies a means by which a client
query can signal a set of algorithms it implements.
2. Signaling DNSSEC Algorithm Understood (DAU) Using EDNS
The ENDS0 specification outlined in [RFC2671] defines a way to
include new options using a standardized mechanism. These options
are contained in the RDATA of the OPT meta-RR. This document defines
a new EDNS0 option for a client to signal which algorithms the client
supports.
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The figure below shows how the signaling attribute is defined in the
RDATA of the OPT RR specified in [RFC2671]:
0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE (TBD) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DIGITAL-SIG-LIST-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ALG-CODE | ... \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DS-HASH-LIST-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| HASH-CODE | ... \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
OPTION-CODE is the code for the DNSSEC Algorithm Understood (DAU)
option. Its value is fixed at TBD.
DIGITAL-SIG-LIST-LENGTH is the length of the list of digital
signature algorithms in octets. DNSSEC algorithm codes are 1 octet
long so this value is the number of octets.
ALG-CODE is the list of assigned values of DNSSEC zone signing
algorithms that the client indicates it understands. The values
SHOULD be in descending order of preference, with the most preferred
algorithm first. For example, if a validating client implements RSA/
SHA-1, RSA/SHA-256 and prefers the latter, the value of ALG-CODE
would be: 8 (RSA/SHA-256), 5 (RSA/SHA-1).
DS-HASH-LIST-LENGTH is the length of the list of hash algorithms in
octets. DNSSEC DS hash codes are 1 octet long so this value is the
number of octets.
HASH-CODE is the list of assigned values of DNSSEC DS hash algorithms
that the client indicates as understood. Like the ALG-CODE above,
the values SHOULD be in descending order of preference, with the most
preferred algorithm first.
3. Client Considerations
A validating end-system resolver sets the DAU option in the OPT
meta-RR when sending a query. The validating end-system resolver
sets the value(s) in the order of preference, with the most preferred
algorithm(s) first as described in section 2. The end-system
resolver SHOULD also set the DNSSEC-OK bit [RFC4035] to indicate that
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it wishes to receive DNSSEC RRs in the response.
Note that the PRIVATEDNS (253) and/or the PRIVATEOID (254) codes
cover a potentially wide range of algorithms and are likely not
useful to a server. There is no compelling reason for a client to
include these codes in its list of understood algorithms.
3.1. Stub Resolvers
Typically, stub resolvers rely on an upstream recursive server (or
cache) to provide a response; any algorithm support on the stub
resolver's side SHOULD be honored (if possible) by an upstream
recursive cache.
3.2. Validating Stub Resolvers
A validating stub resolver MUST set the DO bit [RFC4035] to indicate
that it wishes to receive DNSSEC RRs in the response. Such
validating resolvers SHOULD include the DAU option in the OPT RR when
sending a query. This way the validating stub resolver indicates
which cryptographic algorithm(s) it supports by setting the values(s)
in the order of preference, with the most preferred algorithm(s)
first as described in Section 2.
3.3. Non-Validating Stub Resolvers
The DAU EDNS option is NOT RECOMMENDED for non-validating stub
resolvers.
3.4. Recursive Resolvers
3.4.1. Validating Recursive Resolvers
A validating recursive resolver MUST set the DO bit [RFC4035] to
indicate that it wishes to receive DNSSEC RRs in the response. If
the client of the recursive resolver did not include the DO bit in
the query the recursive resolver SHOULD include the DAU option
according to its own local policy.
If the client did include the DO and CD bits, but did not include the
DAU option in the query, the validating recursive resolver SHOULD NOT
include the DAU option to avoid conflicts.
If the client did set the DO bit and the DAU option in the query, the
validating recursive resolver SHOULD include the DAU option based on
the setting of the CD bit. If the CD bit is set, the validating
recursive resolver SHOULD include the DAU option based on the client
query or a superset of the client DAU option list and the validator's
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own list (if different). If the CD bit is not set, the validating
recursive resolver MAY copy the client DAU option or substitute its
own DAU option list.
3.4.2. Non-validating Recursive Resolvers
Recursive resolvers that do not do validation or caching SHOULD copy
the DAU option seen in received queries as they represent the wishes
of the validating downstream resolver that issued the original query.
4. Intermediate Middlebox Considerations
Intermediate middleboxes SHOULD behave like a comparable recursive
resolver when dealing with the DAU option [RFC5625].
5. Server Considerations
When an authoritative server sees the DAU option in the OPT meta-RR
in a request the normal algorithm for servicing requests is followed.
The DAU option does not trigger any special processing on the server
side.
If the DAU option is present but the DNSSEC-OK (OK) bit is not set,
the server does not do any DNSSEC processing, including any recording
of the DAU option.
6. Traffic Analysis Considerations
Zone administrators that are planning or are in the process of a
cryptographic algorithm rollover operation should monitor DNS query
traffic and record the values of the DAU option in queries. This
monitoring can measure the deployment of client code that implements
(and signals) certain algorithms. Exactly how to capture DNS traffic
and measure new algorithm adoption is beyond the scope of this
document.
Zone administrators can use this data to set plans for starting an
algorithm rollover and determine when older algorithms can be phased
out without disrupting a significant number of clients. In order to
keep this disruption to a minimum, zone administrators should wait to
complete an algorithm rollover until a large majority of clients
signal that they understand the new algorithm. This may be in the
order of years rather than months. Note that clients that do not
implement the DAU option are likely to be older implementations which
would also not implement any newly deployed algorithm.
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7. IANA Considerations
The algorithm codes used to identify DNSSEC algorithms has already
been established by IANA. This document does not seek to alter that
registry in any way.
This draft seeks to update the "DNS EDNS0 Options" registry by adding
the DAU option and referencing this document. The code for the
option should be TBD.
8. Security Considerations
This document specifies a way for a client to signal its digital
signature algorithm preference to a cache or server. It is not meant
to be a discussion on algorithm superiority. The signal is an
optional code contained in the OPT meta-RR used with EDNS0. The goal
of this option is to signal new algorithm uptake in client code to
allow zone administrators to know when it is possible to complete an
algorithm rollover in a DNSSEC signed zone.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, August 2009.
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Authors' Addresses
Steve Crocker
Shinkuro Inc.
5110 Edgemoor Lane
Bethesda, MD 20814
USA
EMail: steve@shinkuro.com
Scott Rose
NIST
100 Bureau Dr.
Gaithersburg, MD 20899
USA
Phone: +1-301-975-8439
EMail: scottr.nist@gmail.com
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