Internet DRAFT - draft-ietf-dnsext-dnssec-algo-signal
draft-ietf-dnsext-dnssec-algo-signal
DNS Extensions Working Group S. Crocker
Internet-Draft Shinkuro Inc.
Intended status: Standards Track S. Rose
Expires: October 10, 2013 NIST
April 08, 2013
Signaling Cryptographic Algorithm Understanding in DNSSEC
draft-ietf-dnsext-dnssec-algo-signal-10
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 digital
signature and hash algorithms they support. The proposed extensions
allow the signaling of 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.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "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
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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 10, 2013.
Copyright Notice
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Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Signaling DNSSEC Algorithm Understood (DAU), DS Hash
Understood (DHU) and NSEC3 Hash Understood (N3U) Using EDNS . 3
3. Client Considerations . . . . . . . . . . . . . . . . . . . . 4
3.1. Stub Resolvers . . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Validating Stub Resolvers . . . . . . . . . . . . . . 5
3.1.2. Non-Validating Stub Resolvers . . . . . . . . . . . . 5
3.2. Recursive Resolvers . . . . . . . . . . . . . . . . . . . 5
3.2.1. Validating Recursive Resolvers . . . . . . . . . . . 5
3.2.2. Non-validating Recursive Resolvers . . . . . . . . . 6
4. Intermediate System Considerations . . . . . . . . . . . . . 6
5. Server Considerations . . . . . . . . . . . . . . . . . . . . 6
6. Traffic Analysis Considerations . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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 tell validators which cryptographic algorithm
was used to generate the digital signature.
Likewise, Delegation Signer (DS) RRs and NSEC3 RRs use a hashed value
as part of their RDATA and, like digital signature algorithms, these
hash algorithms have code numbers. All three algorithm codes (RRSIG/
DNSKEY, DS and NSEC3) are maintained in unique IANA registries.
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This draft sets out to specify a way for validating end-system
resolvers to tell a server in a DNS query which digital signature and
/or hash algorithms they support. This is done using the new EDNS
options specified below in Section 2 for use in the OPT meta-RR
[I-D.ietf-dnsext-rfc2671bis-edns0]. These three new EDNS option
codes are all OPTIONAL to implement and use.
These proposed EDNS options serve to measure the acceptance and use
of new digital signing algorithms. These signaling options can be
used by zone administrators as a gauge to measure the successful
deployment of code that implements newly deployed digital signature
algorithm, DS hash and NSEC3 hash algorithm used with DNSSEC. A zone
administrator is able to determine when to stop signing with a
superseded 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 introduce 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 the set of algorithms and hashes which it
implements.
2. Signaling DNSSEC Algorithm Understood (DAU), DS Hash Understood
(DHU) and NSEC3 Hash Understood (N3U) Using EDNS
The EDNS0 specification outlined in
[I-D.ietf-dnsext-rfc2671bis-edns0] 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 three new
EDNS options for a client to signal which digital signature and/or
hash algorithms the client supports. These options can be used
independently of each other and MAY appear in any order in the OPT
RR. Each option code can appear only once in an OPT RR.
The figure below shows how each option is defined in the RDATA of the
OPT RR specified in [I-D.ietf-dnsext-rfc2671bis-edns0]:
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0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE (TBD) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| LIST-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ALG-CODE | ... \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
OPTION-CODE is the code for the given signaling option. They are:
o DNSSEC Algorithm Understood (DAU) option for DNSSEC digital
signing algorithms. Its value is fixed at TBD1.
o DS Hash Understood (DHU) option for DS RR hash algorithms. Its
value is fixed at TBD2.
o NSEC3 Hash Understood (N3U) option for NSEC3 hash algorithms. Its
value is fixed at TBD3.
LIST-LENGTH is the length of the list of digital signature or hash
algorithm codes in octets. Each algorithm code occupies a single
octet.
ALG-CODE is the list of assigned values of DNSSEC zone signing
algorithms, DS hash algorithms, or NSEC3 hash algorithms (depending
on the OPTION-CODE in use) that the client declares to be supported.
The order of the code values can be arbitrary and MUST NOT be used to
infer preference.
If all three options are included in the OPT RR, there is a potential
for the OPT RR to take up considerable size in the DNS message.
However, in practical terms, including all three options is likely to
take up 22-32 octets (average of 6-10 digital signature algorithms,
3-5 DS hash algorithms and 1-5 NSEC3 hash algorithms) including the
EDNS option codes and option lengths in potential future examples.
3. Client Considerations
A validating end-system resolver sets the DAU, DHU and/or N3U option,
or combination thereof in the OPT meta-RR when sending a query. The
validating end-system resolver MUST also set the DNSSEC-OK bit
[RFC4035] to indicate that it wishes to receive DNSSEC RRs in the
response.
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Note that the PRIVATEDNS (253) and/or the PRIVATEOID (254) digital
signature codes both 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 the DAU. Likewise,
clients MUST NOT include RESERVED codes in any of the options.
Likewise, a client is under no obligation to list every algorithm it
implements and MAY choose to only list algorithms the client wishes
to signal as understood.
Since the DAU, DHU and/or N3U options are 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.
3.1. Stub Resolvers
Typically, stub resolvers rely on an upstream recursive server (or
cache) to provide a response. So optimal setting of the DAU, DSU and
N3U options depends on whether the stub resolver elects to perform
its own validation.
3.1.1. Validating Stub Resolvers
A validating stub resolver already (usually) sets the DO bit
[RFC4035] to indicate that it wishes to receive additional DNSSEC RRs
(i.e. RRSIG RRs) in the response. Such validating resolvers SHOULD
include the DAU, DHU and/or the N3U option(s) in the OPT RR when
sending a query.
3.1.2. Non-Validating Stub Resolvers
The DAU, DHU and N3U EDNS options MUST NOT be included by non-
validating stub resolvers.
3.2. Recursive Resolvers
3.2.1. Validating Recursive Resolvers
A validating recursive resolver sets the DAU, DHU and/or N3U
option(s) when performing recursion based on its list of algorithms
and any DAU, DHU and/or N3U option lists in the stub client query.
When the recursive server receives a query with one or more of the
options set, the recursive server MUST set the algorithm list for any
outgoing iterative queries for that resolution chain to a union of
the stub client's list and the validating recursive resolver's list.
For example, if the recursive resolver's algorithm list for the DAU
option is (3, 5, 7) and the stub's algorithm list is (7, 8), the
final DAU algorithm list would be (3, 5, 7, 8).
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If the client did include the DO and CD bits, but did not include the
DAU, DHU and/or N3U option(s) in the query, the validating recursive
resolver MAY include the option(s) with its own list in full. If one
or more of the options are missing, the validating recursive resolver
MAY include the missing options with its own list in full.
Validating recursive resolvers MUST NOT set the DAU, DHU and/or N3U
option(s) in the final response to the stub client.
3.2.2. Non-validating Recursive Resolvers
Recursive resolvers that do not do validation MUST copy the DAU, DHU
and/or N3U option(s) seen in received queries as they represent the
wishes of the validating downstream resolver that issued the original
query.
4. Intermediate System Considerations
Intermediate proxies [RFC5625] (Section 4.4.2) that understand DNS
are RECOMMENDED to behave like a comparable recursive resolver when
dealing with the DAU, DHU and N3U options.
5. Server Considerations
When an authoritative server sees the DAU, DHU and/or N3U option(s)
in the OPT meta-RR in a request the normal algorithm for servicing
requests is followed. The options MUST NOT trigger any special
processing (e.g. RRSIG filtering in responses) on the server side.
If the options are present but the DNSSEC-OK (OK) bit is not set, the
server does not do any DNSSEC processing, including any recording of
the option(s).
If the server sees one (or more) of the options set with RESERVED
values, the server MAY ignore recoding of those values.
Authoritative servers MUST NOT set the DAU, DHU and/or N3U option(s)
on any responses. These values are only set in queries.
6. Traffic Analysis Considerations
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Zone administrators that are planning or are in the process of a
cryptographic algorithm rollover operation should monitor DNS query
traffic and record the number of queries, the presence of the OPT RR
in queries and the values of the DAU/DHU/N3U option(s) (if present).
This monitoring can be used to measure the deployment of client code
that implements (and signals) specific algorithms. Description of
the techniques used to capture DNS traffic and measure new algorithm
adoption is beyond the scope of this document.
Zone administrators that need to comply with changes to their
organization's security policy (with regards to cryptographic
algorithm use) can use this data to set milestone dates for
performing an algorithm rollover. For example, zone administrators
can use the data to 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 recognize the new algorithm. This may be in the
order of years rather than months.
Note that clients that do not implement these options are likely to
be older implementations which would also not implement any newly
deployed algorithm.
7. IANA Considerations
The algorithm codes used to identify DNSSEC algorithms, DS RR hash
algorithms and NSEC3 hash algorithms have already been established by
IANA. This document does not seek to alter that registry in any way.
This draft seeks to update the "DNS EDNS Options" registry by adding
the DAU, DHU and N3U options and referencing this document. The code
for these options are TBD1, TBD2 and TBD3 respectively.
8. Security Considerations
This document specifies a way for a client to signal its digital
signature and hash algorithm knowledge to a cache or server. It is
not meant to be a discussion on algorithm superiority. The signals
are optional codes contained in the OPT meta-RR used with EDNS. The
goal of these options are 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.
There is a possibility that an eavesdropper or server could infer the
validator in use by a client by the presence of the AU options and/or
algorithm code list. This information leakage in itself is not very
useful to a potential attacker but it could be used to identify the
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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.
9. Normative References
[I-D.ietf-dnsext-rfc2671bis-edns0]
Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", draft-ietf-dnsext-rfc2671bis-edns0-10
(work in progress), December 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
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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