DNS Operations M. Larson
Internet-Draft VeriSign
Expires: August 14, 2008 O. Gudmundsson
OGUD Consulting LLC
February 11, 2008
DNSSEC Trust Anchor Configuration and Maintenance
draft-ietf-dnsop-dnssec-trust-anchor-01
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Abstract
This document recommends a preferred format for specifying trust
anchors in DNSSEC validating security-aware resolvers and describes
how such a resolver should initialize trust anchors for use. This
document also describes different mechanisms for keeping trust
anchors up to date over time.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Trust Anchor Format . . . . . . . . . . . . . . . . . . . . . 4
3. Trust Anchor Priming . . . . . . . . . . . . . . . . . . . . . 5
4. Trust Anchor Maintenance . . . . . . . . . . . . . . . . . . . 7
5. Security considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
The DNSSEC standards documents ([2], [3] and [4]) describe the need
for trust anchors and how they are used. A validating security-aware
resolver (subsequently referred to as a "validating resolver") needs
to be configured with one or more trust anchors, which specify the
public keys of signed zones. To authenticate DNS data, a validating
resolver builds a chain of trust from a configured trust anchor to
that data.
In a widespread public DNSSEC deployment, the DNS root zone would be
signed and a validating resolver would need to be configured with at
least the root zone's trust anchor. A validating resolver might need
additional trust anchors configured to accommodate islands of
security. (An island of security is a signed, delegated zone that
does not have an authentication chain from its delegating parent.)
For example, consider the situation where the root zone is signed but
a given top-level domain (TLD) zone is not. Various second-level
zones under this unsigned TLD might be signed and resolver operators
might want to validate responses from those zones, requiring a
validating resolver to be configured with those zones' trust anchors.
Because many validating resolvers would be configured with trust
anchors in a widespread DNSSEC deployment, there is a benefit to
creating a common trust anchor format. A similar situation has
occurred with the "root hints", the list of root name server names
and IP addresses: this information is distributed in standard master
file format and many resolver implementations support this common
format.
To simplify this trust anchor configuration process that will occur
on a large number of resolvers, this document offers guidance to
validating resolver implementers by specifying a standardized format
for describing trust anchors. The document also describes how a
validating resolver should initialize or "prime" trust anchors before
first use. Finally, the document lists options for keeping trust
anchor information current over time.
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 [1].
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2. Trust Anchor Format
A trust anchor is a DNSSEC public key configured in a validating
resolver. A validating resolver's configuration MUST allow one or
more trust anchors to be specified. According to the definition in
Section 2 of RFC 4033 [2], a trust anchor can be specified as either
a public key from a DNSKEY resource record (RR) or the hash of a
public key as found in a DS RR. (DS records are defined in Section 5
of RFC 4034 [3].)
This document RECOMMENDS that a trust anchor be specified using the
hash of a public key rather than the key itself, i.e., the fields
from a DS record rather than from a DNSKEY record. A trust anchor
specified in this manner will use all the fields from the
corresponding key's DS record, including the owner name to indicate
which zone the trust anchor corresponds to as well as the various
fields from the DS RDATA. The digest algorithm SHOULD be SHA-256
[5], which is DS digest type 2. DS records using SHA-1 (DS digest
type 1) to specify trust anchors are NOT RECOMMENDED: RFC 4509
encourages the use of DS RRs using SHA-256 over those using SHA-1.
Specifying a trust anchor using a DS format instead of a DNSKEY
format offers a slight advantage because it forces the resolver to
make a DNS query to obtain the trust anchor's complete DNSKEY RRSet
during a priming operation (described below). If only a DNSKEY
record were specified, resolver implementers could conceivably avoid
priming the trust anchor. But priming is desirable because it causes
the resolver to retrieve an up-to-date version of a zone's DNSKEY
RRSet from one of the zone's authoritative servers. It should be
noted that in practice, priming is almost always required because
data in the trust anchor zone will usually be signed with a different
key than the one configured as the trust anchor, thus requiring the
validating resolver to obtain all keys in the DNSKEY RRSet.
Using a DS format is also recommended because it is smaller than the
DNSKEY format and is easier to enter manually, either by typing or
cutting and pasting.
Another advantage of configuring a trust anchor using a DS record is
that the entire hash of the public key in the DS RDATA need not
necessarily be specified. A validating resolver MAY support
configuration using a truncated DS hash value as a human-factors
convenience: shorter strings are easier to type and less prone to
error when entered manually. Even with a truncated hash configured,
a validating resolver can still verify that the corresponding DNSKEY
is present in the trust anchor zone's apex DNSKEY RRSet.
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3. Trust Anchor Priming
A validating resolver needs to obtain and validate the DNSKEY RRSet
corresponding to a configured DS for that trust anchor to be usable
in DNSSEC validation. This process is called "priming" the trust
anchor. Priming can occur when the validating resolver starts, but a
validating resolver SHOULD defer priming of individual trust anchors
until each is first needed for verification. This priming on demand
is especially important when a validating resolver is configured with
a large number of trust anchors to avoid sending a large number of
DNS queries on start-up. This section adds additional details to the
discussion of trust anchors in Section 5 of RFC 4035 [4].
Following are the steps a validating resolver SHOULD take to prime a
configured trust anchor:
1. Read the trust anchor's information (corresponding to the fields
in a DS record) from the validating resolver's configuration
(e.g., a text file).
2. Look up the DNSKEY RRSet corresponding to the owner name of the
trust anchor. (The validating resolver can either perform
iterative resolution or request recursive service from a
recursive name server, depending on its capabilities.)
3. Verify that the DNSKEY RR corresponding to the configured trust
anchor (i.e., the DNSKEY whose hash is configured) appears in the
DNSKEY RRSet and that this DNSKEY RR has the Zone Key Flag
(DNSKEY RDATA bit 7) set. (This bit only indicates that the
DNSKEY is allowed to sign the zone. This DNSKEY may or not be a
zone signing key.)
4. Verify that the DNSKEY RRSet is signed by one of the DNSKEYs
found in the previous step, i.e., that there exists a valid RRSIG
(cryptographically and temporally) for the DNSKEY RRSet generated
with the private key corresponding to the DNSKEY found in the
previous step.
If the validating resolver can successfully complete the steps above,
all DNSKEY RRs in the RRSet ought to be considered authenticated and
can be used authenticate RRSets at or below the trust anchor.
If any of the steps above result in an error, the validating resolver
SHOULD log them.
If there are multiple trust anchors configured for a zone, any one of
them is sufficient to validate data in the zone. For this reason,
old trust anchors SHOULD be removed from a validating resolver's
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trust anchor list soon after the corresponding keys are no longer
used by the zone. A validating resolver should remove a trust anchor
that has been revoked as indicated by the REVOKE bit in the
corresponding DNSKEY record as described in RFC 5011 [6].
If a validating resolver is unable to retrieve a signed DNSKEY RRSet
corresponding to a trust anchor (i.e., prime the trust anchor), it
SHOULD log this condition as an error. Inability to prime a zone's
trust anchor results in the validating resolver's inability to
validate data from the corresponding zone. The validating resolver
SHOULD treat this zone as bogus.
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4. Trust Anchor Maintenance
Trust anchors correspond to zones' key signing keys and these keys do
change in the course of normal operation. Validating resolver
operators MUST ensure that configured trust anchor information
remains current and does not go stale: each configured trust anchor
SHOULD correspond to a DNSKEY RR in the trust anchor zone's apex
DNSKEY RRSet. This process is called trust anchor maintenance.
(Initial trust anchor configuration requires human intervention to
verify the trust anchor's authenticity using out-of-band means and is
outside the scope of this document.)
This section provides a brief overview of some possible mechanisms to
keep trust anchor information current:
Manual configuration: The validating resolver operator MAY choose to
maintain trust anchor information completely manually. In this
case, the operator assumes responsibility for noticing stale trust
anchor information (i.e., DS records that no longer point to a
corresponding DNSKEY RR in the trust anchor zone's apex DNSKEY
RRSet) and updating that information. This process MAY require
the operator to use the same out-of-band verification mechanism as
used for initial configuration to ensure that the new trust anchor
DS record is trustworthy. Because manual maintenance is
burdensome and prone to error, and because other automated trust
anchor maintenance processes either exist or are in development,
manual trust anchor maintenance is NOT RECOMMENDED.
DNSSEC In-band Update: The IETF DNS Extensions Working Group has
developed a protocol to automatically update DNSSEC trust anchors,
which is described in RFC 5011 [6]. This protocol relies on a
small DNSSEC protocol change (an additional flag in the DNSKEY
record) and can be implemented either in a validating resolver
itself or in an external program with access to the validating
resolver's trust anchor configuration data.
Trusted update mechanism: Updated trust anchor information MAY be
obtained via a trusted non-DNS update mechanism. One possibility
is the operating system update mechanism provided by most software
vendors. Operators already place considerable trust in this
mechanism, so it is reasonable to extend this trust to allow
distribution and update of DNSSEC public key material. Another
possibility is to obtain trust anchor configuration directly from
the validating resolver software vendor. This mechanism is
realistically only feasible for updating a small number of trust
anchors, such as for the top-level domains. In a public DNSSEC
deployment, the root zone would be signed and only the root's
trust anchor would need updating.
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Combination of update mechanisms: It is possible that for a given
validating resolver, different trust anchors will be maintained by
different mechanisms. For example, some trust anchors might be
kept up to date by a trusted update mechanism and others
maintained by some site-specific mechanism. In this case, it is
important that the mechanisms maintain a mutually exclusive set of
trust anchors.
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5. Security considerations
This document proposes a standard format for documenting DNSSEC trust
anchors. Configuration of trust anchors, especially those obtained
from third parties as part of an automated process, is a critical
security operation. The procedures listed above describe the minimal
checks that should be performed and reporting that should be done
when configuring trust anchors.
In a widespread DNSSEC deployment, the root zone and many TLD zones
would be signed, thus greatly reducing the number trust anchors that
validating resolvers would need to store and keep track of.
If multiple mechanisms are updating the trust anchor list then there
is the possibility of conflict, such as one mechanism reinserting an
expired trust anchor.
Trust anchors are configuration information. A validating resolver
ought to treat this information differently than DNS data obtained
over the network and never use the configured trust anchors as part
of an answer.
A signed zone that plans to transition to an unsigned state must
first give a warning that it is going insecure. Failure to do so
will cause all validating resolvers that keep a trust anchor for the
zone configured to treat responses from the zone as bogus, causing
resolution failures.
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6. IANA considerations
This document does not have any IANA actions.
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7. Acknowledgments
This work was undertaken at the suggestion of the DNSSEC Deployment
working group (www.dnssec-deployment.org).
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8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[3] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
[4] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions",
RFC 4035, March 2005.
[5] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS)
Resource Records (RRs)", RFC 4509, May 2006.
[6] StJohns, M., "Automated Updates of DNS Security (DNSSEC) Trust
Anchors", RFC 5011, September 2007.
8.2. Informative References
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Authors' Addresses
Matt Larson
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
Email: mlarson@verisign.com
Olafur Gudmundsson
OGUD Consulting LLC
3821 Village Park Drive
Chevy Chase, MD 20815
USA
Email: ogud@ogud.com
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