Internet Engineering Task Force (IETF) O. Kolkman
Request for Comments: 6781 W. Mekking
Obsoletes: 4641 NLnet Labs
Category: Informational R. Gieben
ISSN: 2070-1721 SIDN Labs
October
November 2012
DNSSEC Operational Practices, Version 2
Abstract
This document describes a set of practices for operating the DNS with
security extensions (DNSSEC). The target audience is zone
administrators deploying DNSSEC.
The document discusses operational aspects of using keys and
signatures in the DNS. It discusses issues of key generation, key
storage, signature generation, key rollover, and related policies.
This document obsoletes RFC 4641, as it covers more operational
ground and gives more up-to-date requirements with respect to key
sizes and the DNSSEC operations.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6781.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. The Use of the Term 'key' . . . . . . . . . . . . . . . . 6
1.2. Time Definitions . . . . . . . . . . . . . . . . . . . . . 6
2. Keeping the Chain of Trust Intact . . . . . . . . . . . . . . 7
3. Key Generation and Storage . . . . . . . . . . . . . . . . . . 8
3.1. Operational Motivation for Zone Signing Keys and Key
Signing Keys . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Practical Consequences of KSK and ZSK Separation . . . . . 10
3.2.1. Rolling a KSK That Is Not a Trust Anchor . . . . . . . 10
3.2.2. Rolling a KSK That Is a Trust Anchor . . . . . . . . . 11
3.2.3. The Use of the SEP Flag . . . . . . . . . . . . . . . 12
3.3. Key Effectivity Period . . . . . . . . . . . . . . . . . . 12
3.4. Cryptographic Considerations . . . . . . . . . . . . . . . 14
3.4.1. Signature Algorithm . . . . . . . . . . . . . . . . . 14
3.4.2. Key Sizes . . . . . . . . . . . . . . . . . . . . . . 15
3.4.3. Private Key Storage . . . . . . . . . . . . . . . . . 16
3.4.4. Key Generation . . . . . . . . . . . . . . . . . . . . 17
3.4.5. Differentiation for 'High-Level' Zones? . . . . . . . 17
4. Signature Generation, Key Rollover, and Related Policies . . . 18
4.1. Key Rollovers . . . . . . . . . . . . . . . . . . . . . . 18
4.1.1. Zone Signing Key Rollovers . . . . . . . . . . . . . . 18
4.1.1.1. Pre-Publish Zone Signing Key Rollover . . . . . . 19
4.1.1.2. Double-Signature Zone Signing Key Rollover . . . . 21
4.1.1.3. Pros and Cons of the Schemes . . . . . . . . . . . 23
4.1.2. Key Signing Key Rollovers . . . . . . . . . . . . . . 23
4.1.2.1. Special Considerations for RFC 5011 KSK
Rollover . . . . . . . . . . . . . . . . . . . . . 25
4.1.3. Single-Type Signing Scheme Key Rollover . . . . . . . 26
4.1.4. Algorithm Rollovers . . . . . . . . . . . . . . . . . 28
4.1.4.1. Single-Type Signing Scheme Algorithm Rollover . . 31
4.1.4.2. Algorithm Rollover, RFC 5011 Style . . . . . . . . 32
4.1.4.3. Single Signing Type Algorithm Rollover, RFC
5011 Style . . . . . . . . . . . . . . . . . . . . 33
4.1.4.4. NSEC-to-NSEC3 Algorithm Rollover . . . . . . . . . 34
4.1.5. Considerations for Automated Key Rollovers . . . . . . 34
4.2. Planning for Emergency Key Rollover . . . . . . . . . . . 34
4.2.1. KSK Compromise . . . . . . . . . . . . . . . . . . . . 35
4.2.1.1. Emergency Key Rollover Keeping the Chain of
Trust Intact . . . . . . . . . . . . . . . . . . . 36
4.2.1.2. Emergency Key Rollover Breaking the Chain of
Trust . . . . . . . . . . . . . . . . . . . . . . 36
4.2.2. ZSK Compromise . . . . . . . . . . . . . . . . . . . . 37
4.2.3. Compromises of Keys Anchored in Resolvers . . . . . . 37
4.2.4. Stand-By Keys . . . . . . . . . . . . . . . . . . . . 38
4.3. Parent Policies . . . . . . . . . . . . . . . . . . . . . 38
4.3.1. Initial Key Exchanges and Parental Policies
Considerations . . . . . . . . . . . . . . . . . . . . 38
4.3.2. Storing Keys or Hashes? . . . . . . . . . . . . . . . 39
4.3.3. Security Lameness . . . . . . . . . . . . . . . . . . 40
4.3.4. DS Signature Validity Period . . . . . . . . . . . . . 40
4.3.5. Changing DNS Operators . . . . . . . . . . . . . . . . 41
4.3.5.1. Cooperating DNS Operators . . . . . . . . . . . . 41
4.3.5.2. Non-Cooperating DNS Operators . . . . . . . . . . 44
4.4. Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . 46
4.4.1. Time Considerations . . . . . . . . . . . . . . . . . 46
4.4.2. Signature Validity Periods . . . . . . . . . . . . . . 48
4.4.2.1. Maximum Value . . . . . . . . . . . . . . . . . . 48
4.4.2.2. Minimum Value . . . . . . . . . . . . . . . . . . 49
4.4.2.3. Differentiation between RRsets . . . . . . . . . . 50
5. "Next Record" Types . . . . . . . . . . . . . . . . . . . . . 51
5.1. Differences between NSEC and NSEC3 . . . . . . . . . . . . 51
5.2. NSEC or NSEC3 . . . . . . . . . . . . . . . . . . . . . . 52
5.3. NSEC3 Parameters . . . . . . . . . . . . . . . . . . . . . 52
5.3.1. NSEC3 Algorithm . . . . . . . . . . . . . . . . . . . 53
5.3.2. NSEC3 Iterations . . . . . . . . . . . . . . . . . . . 53
5.3.3. NSEC3 Salt . . . . . . . . . . . . . . . . . . . . . . 53
5.3.4. Opt-Out . . . . . . . . . . . . . . . . . . . . . . . 54
6. Security Considerations . . . . . . . . . . . . . . . . . . . 54
7. Contributors and Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 55
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 55
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 56
8.1. 55
9.1. Normative References . . . . . . . . . . . . . . . . . . . 56
8.2. 55
9.2. Informative References . . . . . . . . . . . . . . . . . . 56
Appendix A. Terminology . . . . . . . . . . . . . . . . . . . . . 58
Appendix B. Typographic Conventions . . . . . . . . . . . . . . . 60 59
Appendix C. Transition Figures for Special Cases of Algorithm
Rollovers . . . . . . . . . . . . . . . . . . . . . . 62
Appendix D. Transition Figure for Changing DNS Operators . . . . 66
Appendix E. Summary of Changes from RFC 4641 . . . . . . . . . . 68
1. Introduction
This document describes how to run a DNS Security (DNSSEC)-enabled
environment. It is intended for operators who have knowledge of the
DNS (see RFC 1034 [RFC1034] and RFC 1035 [RFC1035]) and want to
deploy DNSSEC (RFC 4033 [RFC4033], RFC 4034 [RFC4034], RFC 4035
[RFC4035], and RFC 5155 [RFC5155]). The focus of the document is on
serving authoritative DNS information and is aimed at zone owners,
name server operators, registries, registrars, and registrants. It
assumes that there is no direct relationship between those entities
and the operators of validating recursive name servers (validators).
During workshops and early operational deployment, operators and
system administrators have gained experience about operating the DNS
with security extensions (DNSSEC). This document translates these
experiences into a set of practices for zone administrators.
Although the DNS Root has been signed since July 15, 2010 and now
more than 80 secure delegations are provisioned in the root, at the
time of this writing there still exists relatively little experience
with DNSSEC in production environments below the Top-Level Domain
(TLD) level; this document should therefore explicitly not be seen as
representing 'Best Current Practices'. Instead, it describes the
decisions that should be made when deploying DNSSEC, gives the
choices available for each one, and provides some operational
guidelines. The document does not give strong recommendations. That
may be the subject for a future version of this document.
The procedures herein are focused on the maintenance of signed zones
(i.e., signing and publishing zones on authoritative servers). It is
intended that maintenance of zones, such as re-signing or key
rollovers, be transparent to any verifying clients.
The structure of this document is as follows. In Section 2, we
discuss the importance of keeping the "chain of trust" intact.
Aspects of key generation and storage of keys are discussed in
Section 3; the focus in this section is mainly on the security of the
private part of the key(s). Section 4 describes considerations
concerning the public part of the keys. Sections 4.1 and 4.2 deal
with the rollover, or replacement, of keys. Section 4.3 discusses
considerations on how parents deal with their children's public keys
in order to maintain chains of trust. Section 4.4 covers all kinds
of timing issues around key publication. Section 5 covers the
considerations regarding selecting and using the NSEC or NSEC3
[RFC5155] Resource Record.
The typographic conventions used in this document are explained in
Appendix B.
Since we describe operational suggestions and there are no protocol
specifications, the RFC 2119 [RFC2119] language does not apply to
this document, though we do use quotes from other documents that do
include the RFC 2119 language.
This document obsoletes RFC 4641 [RFC4641].
1.1. The Use of the Term 'key'
It is assumed that the reader is familiar with the concept of
asymmetric cryptography, or public-key cryptography, on which DNSSEC
is based (see the definition of 'asymmetric cryptography' in RFC 4949
[RFC4949]). Therefore, this document will use the term 'key' rather
loosely. Where it is written that 'a key is used to sign data', it
is assumed that the reader understands that it is the private part of
the key pair that is used for signing. It is also assumed that the
reader understands that the public part of the key pair is published
in the DNSKEY Resource Record (DNSKEY RR) and that it is the public
part that is used in signature verification.
1.2. Time Definitions
In this document, we will be using a number of time-related terms.
The following definitions apply:
Signature validity period: The period that a signature is valid. It
starts at the (absolute) time specified in the signature inception
field of the RRSIG RR and ends at the (absolute) time specified in
the expiration field of the RRSIG RR. The document sometimes also
uses the term 'validity period', which means the same.
Signature publication period: The period that a signature is
published. It starts at the time the signature is introduced in
the zone for the first time and ends at the time when the
signature is removed or replaced with a new signature. After one
stops publishing an RRSIG in a zone, it may take a while before
the RRSIG has expired from caches and has actually been removed
from the DNS.
Key effectivity period: The period during which a key pair is
expected to be effective. It is defined as the time between the
earliest inception time stamp and the last expiration date of any
signature made with this key, regardless of any discontinuity in
the use of the key. The key effectivity period can span multiple
signature validity periods.
Maximum/Minimum Zone Time to Live (TTL): The maximum or minimum
value of the TTLs from the complete set of RRs in a zone, that are
used by validators or resolvers. Note that the minimum TTL is not
the same as the MINIMUM field in the SOA RR. See RFC 2308
[RFC2308] for more information.
2. Keeping the Chain of Trust Intact
Maintaining a valid chain of trust is important because broken chains
of trust will result in data being marked as Bogus (as defined in
RFC 4033 [RFC4033] Section 5), which may cause entire (sub)domains to
become invisible to verifying clients. The administrators of secured
zones need to realize that, to verifying clients, their zone is part
of a chain of trust.
As mentioned in the introduction, the procedures herein are intended
to ensure that maintenance of zones, such as re-signing or key
rollovers, will be transparent to the verifying clients on the
Internet.
Administrators of secured zones will need to keep in mind that data
published on an authoritative primary server will not be immediately
seen by verifying clients; it may take some time for the data to be
transferred to other (secondary) authoritative name servers and
clients may be fetching data from caching non-authoritative servers.
In this light, note that the time until the data is available on the
slave can be negligible when using NOTIFY [RFC1996] and Incremental
Zone Transfer (IXFR) [RFC1995]. It increases when Authoritative
(full) Zone Transfers (AXFRs) are used in combination with NOTIFY.
It increases even more if you rely on the full zone transfers being
based only on the SOA timing parameters for refresh.
For the verifying clients, it is important that data from secured
zones can be used to build chains of trust, regardless of whether the
data came directly from an authoritative server, a caching name
server, or some middle box. Only by carefully using the available
timing parameters can a zone administrator ensure that the data
necessary for verification can be obtained.
The responsibility for maintaining the chain of trust is shared by
administrators of secured zones in the chain of trust. This is most
obvious in the case of a 'key compromise' when a tradeoff must be
made between maintaining a valid chain of trust and replacing the
compromised keys as soon as possible. Then zone administrators will
have to decide between keeping the chain of trust intact -- thereby
allowing for attacks with the compromised key -- or deliberately
breaking the chain of trust and making secured subdomains invisible
to security-aware resolvers (also see Section 4.2).
3. Key Generation and Storage
This section describes a number of considerations with respect to the
use of keys. For the design of an operational procedure for key
generation and storage, a number of decisions need to be made:
o Does one differentiate between Zone Signing Keys and Key Signing
Keys or is the use of one type of key sufficient?
o Are Key Signing Keys (likely to be) in use as trust anchors
[RFC4033]?
o What are the timing parameters that are allowed by the operational
requirements?
o What are the cryptographic parameters that fit the operational
need?
The following section discusses the considerations that need to be
taken into account when making those choices.
3.1. Operational Motivation for Zone Signing Keys and Key Signing Keys
The DNSSEC validation protocol does not distinguish between different
types of DNSKEYs. The motivations to differentiate between keys are
purely operational; validators will not make a distinction.
For operational reasons, described below, it is possible to designate
one or more keys to have the role of Key Signing Keys (KSKs). These
keys will only sign the apex DNSKEY RRset in a zone. Other keys can
be used to sign all the other RRsets in a zone that require
signatures. They are referred to as Zone Signing Keys (ZSKs). In
cases where the differentiation between the KSK and ZSK is not made,
i.e., where keys have the role of both KSK and ZSK, we talk about a
Single-Type Signing Scheme.
If the two functions are separated, then for almost any method of key
management and zone signing, the KSK is used less frequently than the
ZSK. Once a DNSKEY RRset is signed with the KSK, all the keys in the
RRset can be used as ZSKs. If there has been an event that increases
the risk that a ZSK is compromised, it can be simply replaced with a
ZSK rollover. The new RRset is then re-signed with the KSK.
Changing a key that is a Secure Entry Point (SEP) [RFC4034] for a
zone can be relatively expensive, as it involves interaction with
third parties: When a key is only pointed to by a Delegation Signer
(DS) [RFC4034] record in the parent zone, one needs to complete the
interaction with the parent and wait for the updated DS record to
appear in the DNS. In the case where a key is configured as a trust
anchor, one has to wait until one has sufficient confidence that all
trust anchors have been replaced. In fact, it may be that one is not
able to reach the complete user-base with information about the key
rollover.
Given the assumption that for KSKs the SEP flag is set, the KSK can
be distinguished from a ZSK by examining the flag field in the DNSKEY
RR: If the flag field is an odd number, it is a KSK; otherwise, it is
a ZSK.
There is also a risk that keys can be compromised through theft or
loss. For keys that are installed on file-systems of name servers
that are connected to the network (e.g., for dynamic updates), that
risk is relatively high. Where keys are stored on Hardware Security
Modules (HSMs) or stored off-line, such risk is relatively low.
However, storing keys off-line or with more limitations on access
control has a negative effect on the operational flexibility. By
separating the KSK and ZSK functionality, these risks can be managed
while making the tradeoff against the involved costs. For example, a
KSK can be stored off-line or with more limitations on access control
than ZSKs, which need to be readily available for operational
purposes such as the addition or deletion of zone data. A KSK stored
on a smartcard that is kept in a safe, combined with a ZSK stored on
a file-system accessible by operators for daily routine use, may
provide better protection against key compromise without losing much
operational flexibility. It must be said that some HSMs give the
option to have your keys online, giving more protection and hardly
affecting the operational flexibility. In those cases, a KSK-ZSK
split is not more beneficial than the Single-Type Signing Scheme.
It is worth mentioning that there's not much point in obsessively
protecting the key if you don't protect the zone files, which also
live on the file-systems.
Finally, there is a risk of cryptanalysis of the key material. The
costs of such analysis are correlated to the length of the key.
However, cryptanalysis arguments provide no strong motivation for a
KSK/ZSK split. Suppose one differentiates between a KSK and a ZSK,
whereby the KSK effectivity period is X times the ZSK effectivity
period. Then, in order for the resistance to cryptanalysis to be the
same for the KSK and the ZSK, the KSK needs to be X times stronger
than the ZSK. Since for all practical purposes X will be somewhere
on the order of 10 to 100, the associated key sizes will vary only by
about a byte in size for symmetric keys. When translated to
asymmetric keys, the size difference is still too insignificant to
warrant a key-split; it only marginally affects the packet size and
signing speed.
The arguments for differentiation between the ZSK and KSK are weakest
when:
o the exposure to risk is low (e.g., when keys are stored on HSMs);
o one can be certain that a key is not used as a trust anchor;
o maintenance of the various keys cannot be performed through tools
(is prone to human error); and
o the interaction through the child-parent provisioning chain -- in
particular, the timely appearance of a new DS record in the parent
zone in emergency situations -- is predictable.
If the above arguments hold, then the costs of the operational
complexity of a KSK-ZSK split may outweigh the costs of operational
flexibility, and choosing a Single-Type Signing Scheme is a
reasonable option. In other cases, we advise that the separation
between KSKs and ZSKs is made.
3.2. Practical Consequences of KSK and ZSK Separation
A key that acts only as a Zone Signing Key is used to sign all the
data except the DNSKEY RRset in a zone on a regular basis. When a
ZSK is to be rolled, no interaction with the parent is needed. This
allows for a relatively short key effectivity period.
A key with only the Key Signing Key role is to be used to sign the
DNSKEY RRs in a zone. If a KSK is to be rolled, there may be
interactions with other parties. These can include the
administrators of the parent zone or administrators of verifying
resolvers that have the particular key configured as secure entry
points. In the latter case, everyone relying on the trust anchor
needs to roll over to the new key, a process that may be subject to
stability costs if automated trust anchor rollover mechanisms (e.g.,
RFC 5011 [RFC5011]) are not in place. Hence, the key effectivity
period of these keys can and should be made much longer.
3.2.1. Rolling a KSK That Is Not a Trust Anchor
There are three schools of thought on rolling a KSK that is not a
trust anchor:
1. It should be done frequently and regularly (possibly every few
months), so that a key rollover remains an operational routine.
2. It should be done frequently but irregularly. "Frequently" means
every few months, again based on the argument that a rollover is
a practiced and common operational routine; "irregular" means
with a large jitter, so that third parties do not start to rely
on the key and will not be tempted to configure it as a trust
anchor.
3. It should only be done when it is known or strongly suspected
that the key can be or has been compromised, or in conjunction
with operator change policies and procedures, like when a new
algorithm or key storage is required.
There is no widespread agreement on which of these three schools of
thought is better for different deployments of DNSSEC. There is a
stability cost every time a non-anchor KSK is rolled over, but it is
possibly low if the communication between the child and the parent is
good. On the other hand, the only completely effective way to tell
if the communication is good is to test it periodically. Thus,
rolling a KSK with a parent is only done for two reasons: to test and
verify the rolling system to prepare for an emergency, and in the
case of (preventing) an actual emergency.
Finally, in most cases a zone administrator cannot be fully certain
that the zone's KSK is not in use as a trust anchor somewhere. While
the configuration of trust anchors is not the responsibility of the
zone administrator, there may be stability costs for the validator
administrator that (wrongfully) configured the trust anchor when the
zone administrator rolls a KSK.
3.2.2. Rolling a KSK That Is a Trust Anchor
The same operational concerns apply to the rollover of KSKs that are
used as trust anchors: If a trust anchor replacement is done
incorrectly, the entire domain that the trust anchor covers will
become Bogus until the trust anchor is corrected.
In a large number of cases, it will be safe to work from the
assumption that one's keys are not in use as trust anchors. If a
zone administrator publishes a DNSSEC signing policy and/or a DNSSEC
practice statement [DNSSEC-DPS], that policy or statement should be
explicit regarding whether or not the existence of trust anchors will
be taken into account. There may be cases where local policies
enforce the configuration of trust anchors on zones that are mission
critical (e.g., in enterprises where the trust anchor for the
enterprise domain is configured in the enterprise's validator). It
is expected that the zone administrators are aware of such
circumstances.
One can argue that because of the difficulty of getting all users of
a trust anchor to replace an old trust anchor with a new one, a KSK
that is a trust anchor should never be rolled unless it is known or
strongly suspected that the key has been compromised. In other
words, the costs of a KSK rollover are prohibitively high because
some users cannot be reached.
However, the "operational habit" argument also applies to trust
anchor reconfiguration at the clients' validators. If a short key
effectivity period is used and the trust anchor configuration has to
be revisited on a regular basis, the odds that the configuration
tends to be forgotten are smaller. In fact, the costs for those
users can be minimized by automating the rollover with RFC 5011
[RFC5011] and by rolling the key regularly (and advertising such) so
that the operators of validating resolvers will put the appropriate
mechanism in place to deal with these stability costs: In other
words, budget for these costs instead of incurring them unexpectedly.
It is therefore preferable to roll KSKs that are expected to be used
as trust anchors on a regular basis if and only if those rollovers
can be tracked using standardized (e.g., RFC 5011 [RFC5011])
mechanisms.
3.2.3. The Use of the SEP Flag
The so-called SEP [RFC4035] flag can be used to distinguish between
keys that are intended to be used as the secure entry point into the
zone when building chains of trust, i.e., they are (to be) pointed to
by parental DS RRs or configured as a trust anchor.
While the SEP flag does not play any role in validation, it is used
in practice for operational purposes such as for the rollover
mechanism described in RFC 5011 [RFC5011]. The common convention is
to set the SEP flag on any key that is used for key exchanges with
the parent and/or potentially used for configuration as a trust
anchor. Therefore, it is suggested that the SEP flag be set on keys
that are used as KSKs and not on keys that are used as ZSKs, while in
those cases where a distinction between a KSK and ZSK is not made
(i.e., for a Single-Type Signing Scheme), it is suggested that the
SEP flag be set on all keys.
Note: Some signing tools may assume a KSK/ZSK split and use the
(non-)presence of the SEP flag to determine which key is to be used
for signing zone data; these tools may get confused when a Single-
Type Signing Scheme is used.
3.3. Key Effectivity Period
In general, the available key length sets an upper limit on the key
effectivity period. For all practical purposes, it is sufficient to
define the key effectivity period based on purely operational
requirements and match the key length to that value. Ignoring the
operational perspective, a reasonable effectivity period for KSKs
that have corresponding DS records in the parent zone is on the order
of two decades or longer. That is, if one does not plan to test the
rollover procedure, the key should be effective essentially forever
and only rolled over in case of emergency.
When one opts for a regular key rollover, a reasonable key
effectivity period for KSKs that have a parent zone is one year,
meaning you have the intent to replace them after 12 months. The key
effectivity period is merely a policy parameter and should not be
considered a constant value. For example, the real key effectivity
period may be a little bit longer than 12 months, because not all
actions needed to complete the rollover could be finished in time.
As argued above, this annual rollover gives an operational practice
of rollovers for both the zone and validator administrators.
Besides, in most environments a year is a time span that is easily
planned and communicated.
Where keys are stored online and the exposure to various threats of
compromise is fairly high, an intended key effectivity period of a
month is reasonable for Zone Signing Keys.
Although very short key effectivity periods are theoretically
possible, when replacing keys one has to take into account the
rollover considerations discussed in Sections 4.1 and 4.4. Key
replacement endures for a couple of Maximum Zone TTLs, depending on
the rollover scenario. Therefore, a multiple of Maximum Zone TTL
durations is a reasonable lower limit on the key effectivity period.
Forcing a shorter key effectivity period will result in an
unnecessary and inconveniently large DNSKEY RRset published in the
zone.
The motivation for having the ZSK's effectivity period shorter than
the KSK's effectivity period is rooted in the operational
consideration that it is more likely that operators have more
frequent read access to the ZSK than to the KSK. Thus, in cases
where the ZSK cannot be afforded the same level of protection as the
KSK (such as when zone keys are kept online), and where the risk of
unauthorized disclosure of the ZSK's private key is not negligible
(e.g., when HSMs are not in use), the ZSK's effectivity period should
be kept shorter than the KSK's effectivity period.
In fact, if the risk of loss, theft, or other compromise is the same
for a ZSK and a KSK, there is little reason to choose different
effectivity periods for ZSKs and KSKs. And when the split between
ZSKs and KSKs is not made, the argument is redundant.
There are certainly cases in which the use of a Single-Type Signing
Scheme with a long key effectivity period is a good choice, for
example, where the costs and risks of compromise, and the costs and
risks involved with having to perform an emergency roll, are low.
3.4. Cryptographic Considerations
3.4.1. Signature Algorithm
At the time of this writing, there are three types of signature
algorithms that can be used in DNSSEC: RSA, Digital Signature
Algorithm (DSA), and GOST. Proposals for other algorithms are in the
making. All three are fully specified in many freely available
documents and are widely considered to be patent-free. The creation
of signatures with RSA and DSA takes roughly the same time, but DSA
is about ten times slower for signature verification. Also note
that, in the context of DNSSEC, DSA is limited to a maximum of
1024-bit keys.
We suggest the use of RSA/SHA-256 as the preferred signature
algorithm and RSA/SHA-1 as an alternative. Both have advantages and
disadvantages. RSA/SHA-1 has been deployed for many years, while
RSA/SHA-256 has only begun to be deployed. On the other hand, it is
expected that if effective attacks on either algorithm appear, they
will appear for RSA/SHA-1 first. RSA/MD5 should not be considered
for use because RSA/MD5 will very likely be the first common-use
signature algorithm to be targeted for an effective attack.
At the time of publication, it is known that the SHA-1 hash has
cryptanalysis issues, and work is in progress to address them. The
use of public-key algorithms based on hashes stronger than SHA-1
(e.g., SHA-256) is recommended, if these algorithms are available in
implementations (see RFC 5702 [RFC5702] and RFC 4509 [RFC4509]).
Also, at the time of publication, digital signature algorithms based
on Elliptic Curve (EC) Cryptography with DNSSEC (GOST [RFC5933],
Elliptic Curve Digital Signature Algorithm (ECDSA) [RFC6605]) are
being standardized and implemented. The use of EC has benefits in
terms of size. On the other hand, one has to balance that against
the amount of validating resolver implementations that will not
recognize EC signatures and thus treat the zone as insecure. Beyond
the observation of this tradeoff, we will not discuss this further.
3.4.2. Key Sizes
This section assumes RSA keys, as suggested in the previous section.
DNSSEC signing keys should be large enough to avoid all known
cryptographic attacks during the effectivity period of the key. To
date, despite huge efforts, no one has broken a regular 1024-bit key;
in fact, the best completed attack is estimated to be the equivalent
of a 700-bit key. An attacker breaking a 1024-bit signing key would
need to expend phenomenal amounts of networked computing power in a
way that would not be detected in order to break a single key.
Because of this, it is estimated that most zones can safely use
1024-bit keys for at least the next ten years. (A 1024-bit
asymmetric key has an approximate equivalent strength of a symmetric
80-bit key.)
Depending on local policy (e.g., owners of keys that are used as
extremely high value trust anchors, or non-anchor keys that may be
difficult to roll over), it may be advisable to use lengths longer
than 1024 bits. Typically, the next larger key size used is
2048 bits, which has the approximate equivalent strength of a
symmetric 112-bit key (RFC 3766 [RFC3766]). Signing and verifying
with a 2048-bit key takes longer than with a 1024-bit key. The
increase depends on software and hardware implementations, but public
operations (such as verification) are about four times slower, while
private operations (such as signing) are about eight times slower.
Another way to decide on the size of a key to use is to remember that
the effort it takes for an attacker to break a 1024-bit key is the
same, regardless of how the key is used. If an attacker has the
capability of breaking a 1024-bit DNSSEC key, he also has the
capability of breaking one of the many 1024-bit Transport Layer
Security (TLS) [RFC5246] trust anchor keys that are currently
installed in web browsers. If the value of a DNSSEC key is lower to
the attacker than the value of a TLS trust anchor, the attacker will
use the resources to attack the latter.
It is possible that there will be an unexpected improvement in the
ability for attackers to break keys and that such an attack would
make it feasible to break 1024-bit keys but not 2048-bit keys. If
such an improvement happens, it is likely that there will be a huge
amount of publicity, particularly because of the large number of
1024-bit TLS trust anchors built into popular web browsers. At that
time, all 1024-bit keys (both ones with parent zones and ones that
are trust anchors) can be rolled over and replaced with larger keys.
Earlier documents (including the previous version of this document)
urged the use of longer keys in situations where a particular key was
"heavily used". That advice may have been true 15 years ago, but it
is not true today when using RSA algorithms and keys of 1024 bits or
higher.
3.4.3. Private Key Storage
It is preferred that, where possible, zone private keys and the zone
file master copy that is to be signed be kept and used in off-line,
non-network-connected, physically secure machines only.
Periodically, an application can be run to add authentication to a
zone by adding RRSIG and NSEC/NSEC3 RRs. Then the augmented file can
be transferred to the master.
When relying on dynamic update [RFC3007] or any other update
mechanism that runs at a regular interval to manage a signed zone, be
aware that at least one private key of the zone will have to reside
on the master server or reside on an HSM to which the server has
access. This key is only as secure as the amount of exposure the
server receives to unknown clients and on the level of security of
the host. Although not mandatory, one could administer a zone using
a "hidden master" scheme to minimize the risk. In this arrangement,
the master name server that processes the updates is unavailable to
general hosts on the Internet; it is not listed in the NS RRset. The
name servers in the NS RRset are able to receive zone updates through
IXFR, AXFR, or an out-of-band distribution mechanism, possibly in
combination with NOTIFY or another mechanism to trigger zone
replication.
The ideal situation is to have a one-way information flow to the
network to avoid the possibility of tampering from the network.
Keeping the zone master on-line on the network and simply cycling it
through an off-line signer does not do this. The on-line version
could still be tampered with if the host it resides on is
compromised. For maximum security, the master copy of the zone file
should be off-net and should not be updated based on an unsecured
network-mediated communication.
The ideal situation may not be achievable because of economic
tradeoffs between risks and costs. For instance, keeping a zone file
off-line is not practical and will increase the costs of operating a
DNS zone. So, in practice, the machines on which zone files are
maintained will be connected to a network. Operators are advised to
take security measures to shield the master copy against unauthorized
access in order to prevent modification of DNS data before it is
signed.
Similarly, the choice for storing a private key in an HSM will be
influenced by a tradeoff between various concerns:
o The risks that an unauthorized person has unnoticed read access to
the private key.
o The remaining window of opportunity for the attacker.
o The economic impact of the possible attacks (for a TLD, that
impact will typically be higher than for an individual user).
o The costs of rolling the (compromised) keys. (The cost of rolling
a ZSK is lowest, and the cost of rolling a KSK that is in wide use
as a trust anchor is highest.)
o The costs of buying and maintaining an HSM.
For dynamically updated secured zones [RFC3007], both the master copy
and the private key that is used to update signatures on updated RRs
will need to be on-line.
3.4.4. Key Generation
Careful generation of all keys is a sometimes overlooked but
absolutely essential element in any cryptographically secure system.
The strongest algorithms used with the longest keys are still of no
use if an adversary can guess enough to lower the size of the likely
key space so that it can be exhaustively searched. Technical
suggestions for the generation of random keys will be found in
RFC 4086 [RFC4086] and NIST SP 800-90A [NIST-SP-800-90A]. In
particular, one should carefully assess whether the random number
generator used during key generation adheres to these suggestions.
Typically, HSMs tend to provide a good facility for key generation.
Keys with a long effectivity period are particularly sensitive, as
they will represent a more valuable target and be subject to attack
for a longer time than short-period keys. It is preferred that long-
term key generation occur off-line in a manner isolated from the
network via an air gap or, at a minimum, high-level secure hardware.
3.4.5. Differentiation for 'High-Level' Zones?
An earlier version of this document (RFC 4641 [RFC4641]) made a
differentiation between key lengths for KSKs used for zones that are
high in the DNS hierarchy and those for KSKs used lower down in the
hierarchy.
This distinction is now considered irrelevant. Longer key lengths
for keys higher in the hierarchy are not useful because the
cryptographic guidance is that everyone should use keys that no one
can break. Also, it is impossible to judge which zones are more or
less valuable to an attacker. An attack can only take place if the
key compromise goes unnoticed and the attacker can act as a man-in-
the-middle (MITM). For example, if example.com is compromised, and
the attacker forges answers for somebank.example.com. and sends them
out during an MITM, when the attack is discovered it will be simple
to prove that example.com has been compromised, and the KSK will be
rolled.
4. Signature Generation, Key Rollover, and Related Policies
4.1. Key Rollovers
Regardless of whether a zone uses periodic key rollovers or only
rolls keys in case of an irregular event, key rollovers are a fact of
life when using DNSSEC. Zone administrators who are in the process
of rolling their keys have to take into account the fact that data
published in previous versions of their zone still lives in caches.
When deploying DNSSEC, this becomes an important consideration;
ignoring data that may be in caches may lead to loss of service for
clients.
The most pressing example of this occurs when zone material signed
with an old key is being validated by a resolver that does not have
the old zone key cached. If the old key is no longer present in the
current zone, this validation fails, marking the data Bogus.
Alternatively, an attempt could be made to validate data that is
signed with a new key against an old key that lives in a local cache,
also resulting in data being marked Bogus.
The typographic conventions used in the diagrams below are explained
in Appendix B.
4.1.1. Zone Signing Key Rollovers
If the choice for splitting ZSKs and KSKs has been made, then those
two types of keys can be rolled separately, and ZSKs can be rolled
without taking into account DS records from the parent or the
configuration of such a key as the trust anchor.
For "Zone Signing Key rollovers", there are two ways to make sure
that during the rollover data still cached can be verified with the
new key sets or newly generated signatures can be verified with the
keys still in caches. One scheme, described in Section 4.1.1.1, uses
key pre-publication; the other uses double signatures, as described
in Section 4.1.1.2. The pros and cons are described in
Section 4.1.1.3.
4.1.1.1. Pre-Publish Zone Signing Key Rollover
This section shows how to perform a ZSK rollover without the need to
sign all the data in a zone twice -- the "Pre-Publish key rollover".
This method has advantages in the case of a key compromise. If the
old key is compromised, the new key has already been distributed in
the DNS. The zone administrator is then able to quickly switch to
the new key and remove the compromised key from the zone. Another
major advantage is that the zone size does not double, as is the case
with the Double-Signature ZSK rollover.
Pre-Publish key rollover from DNSKEY_Z_10 to DNSKEY_Z_11 involves
four stages as follows:
------------------------------------------------------------
initial new DNSKEY new RRSIGs
------------------------------------------------------------
SOA_0 SOA_1 SOA_2
RRSIG_Z_10(SOA) RRSIG_Z_10(SOA) RRSIG_Z_11(SOA)
DNSKEY_K_1 DNSKEY_K_1 DNSKEY_K_1
DNSKEY_Z_10 DNSKEY_Z_10 DNSKEY_Z_10
DNSKEY_Z_11 DNSKEY_Z_11
RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY)
------------------------------------------------------------
------------------------------------------------------------
DNSKEY removal
------------------------------------------------------------
SOA_3
RRSIG_Z_11(SOA)
DNSKEY_K_1
DNSKEY_Z_11
RRSIG_K_1(DNSKEY)
------------------------------------------------------------
Figure 1: Pre-Publish Key Rollover
initial: Initial version of the zone: DNSKEY_K_1 is the Key Signing
Key. DNSKEY_Z_10 is used to sign all the data of the zone, i.e.,
it is the Zone Signing Key.
new DNSKEY: DNSKEY_Z_11 is introduced into the key set (note that no
signatures are generated with this key yet, but this does not
secure against brute force attacks on its public key). The
minimum duration of this pre-roll phase is the time it takes for
the data to propagate to the authoritative servers, plus the TTL
value of the key set.
new RRSIGs: At the "new RRSIGs" stage, DNSKEY_Z_11 is used to sign
the data in the zone exclusively (i.e., all the signatures from
DNSKEY_Z_10 are removed from the zone). DNSKEY_Z_10 remains
published in the key set. This way, data that was loaded into
caches from the zone in the "new DNSKEY" step can still be
verified with key sets fetched from this version of the zone. The
minimum time that the key set including DNSKEY_Z_10 is to be
published is the time that it takes for zone data from the
previous version of the zone to expire from old caches, i.e., the
time it takes for this zone to propagate to all authoritative
servers, plus the Maximum Zone TTL value of any of the data in the
previous version of the zone.
DNSKEY removal: DNSKEY_Z_10 is removed from the zone. The key set,
now only containing DNSKEY_K_1 and DNSKEY_Z_11, is re-signed with
DNSKEY_K_1.
The above scheme can be simplified by always publishing the "future"
key immediately after the rollover. The scheme would look as follows
(we show two rollovers); the future key is introduced in "new DNSKEY"
as DNSKEY_Z_12 and again a newer one, numbered 13, in "new DNSKEY
(II)":
initial new RRSIGs new DNSKEY
-----------------------------------------------------------------
SOA_0 SOA_1 SOA_2
RRSIG_Z_10(SOA) RRSIG_Z_11(SOA) RRSIG_Z_11(SOA)
DNSKEY_K_1 DNSKEY_K_1 DNSKEY_K_1
DNSKEY_Z_10 DNSKEY_Z_10 DNSKEY_Z_11
DNSKEY_Z_11 DNSKEY_Z_11 DNSKEY_Z_12
RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY)
----------------------------------------------------------------
----------------------------------------------------------------
new RRSIGs (II) new DNSKEY (II)
----------------------------------------------------------------
SOA_3 SOA_4
RRSIG_Z_12(SOA) RRSIG_Z_12(SOA)
DNSKEY_K_1 DNSKEY_K_1
DNSKEY_Z_11 DNSKEY_Z_12
DNSKEY_Z_12 DNSKEY_Z_13
RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY)
----------------------------------------------------------------
Figure 2: Pre-Publish Zone Signing Key Rollover, Showing Two
Rollovers
Note that the key introduced in the "new DNSKEY" phase is not used
for production yet; the private key can thus be stored in a
physically secure manner and does not need to be 'fetched' every time
a zone needs to be signed.
4.1.1.2. Double-Signature Zone Signing Key Rollover
This section shows how to perform a ZSK rollover using the double
zone data signature scheme, aptly named "Double-Signature rollover".
During the "new DNSKEY" stage, the new version of the zone file will
need to propagate to all authoritative servers and the data that
exists in (distant) caches will need to expire, requiring at least
the propagation delay plus the Maximum Zone TTL of previous versions
of the zone.
Double-Signature ZSK rollover involves three stages as follows:
----------------------------------------------------------------
initial new DNSKEY DNSKEY removal
----------------------------------------------------------------
SOA_0 SOA_1 SOA_2
RRSIG_Z_10(SOA) RRSIG_Z_10(SOA)
RRSIG_Z_11(SOA) RRSIG_Z_11(SOA)
DNSKEY_K_1 DNSKEY_K_1 DNSKEY_K_1
DNSKEY_Z_10 DNSKEY_Z_10
DNSKEY_Z_11 DNSKEY_Z_11
RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY)
----------------------------------------------------------------
Figure 3: Double-Signature Zone Signing Key Rollover
initial: Initial version of the zone: DNSKEY_K_1 is the Key Signing
Key. DNSKEY_Z_10 is used to sign all the data of the zone, i.e.,
it is the Zone Signing Key.
new DNSKEY: At the "new DNSKEY" stage, DNSKEY_Z_11 is introduced
into the key set and all the data in the zone is signed with
DNSKEY_Z_10 and DNSKEY_Z_11. The rollover period will need to
continue until all data from version 0 (i.e., the version of the
zone data containing SOA_0) of the zone has been replaced in all
secondary servers and then has expired from remote caches. This
will take at least the propagation delay plus the Maximum Zone TTL
of version 0 of the zone.
DNSKEY removal: DNSKEY_Z_10 is removed from the zone, as are all
signatures created with it. The key set, now only containing
DNSKEY_Z_11, is re-signed with DNSKEY_K_1 and DNSKEY_Z_11.
At every instance, RRSIGs from the previous version of the zone can
be verified with the DNSKEY RRset from the current version and vice
versa. The duration of the "new DNSKEY" phase and the period between
rollovers should be at least the propagation delay to secondary
servers plus the Maximum Zone TTL of the previous version of the
zone.
Note that in this example we assumed for simplicity that the zone was
not modified during the rollover. In fact, new data can be
introduced at any time during this period, as long as it is signed
with both keys.
4.1.1.3. Pros and Cons of the Schemes
Pre-Publish key rollover: This rollover does not involve signing the
zone data twice. Instead, before the actual rollover, the new key
is published in the key set and thus is available for
cryptanalysis attacks. A small disadvantage is that this process
requires four stages. Also, the Pre-Publish scheme involves more
parental work when used for KSK rollovers, as explained in
Section 4.1.2.
Double-Signature ZSK rollover: The drawback of this approach is that
during the rollover the number of signatures in your zone doubles;
this may be prohibitive if you have very big zones. An advantage
is that it only requires three stages.
4.1.2. Key Signing Key Rollovers
For the rollover of a Key Signing Key, the same considerations as for
the rollover of a Zone Signing Key apply. However, we can use a
Double-Signature scheme to guarantee that old data (only the apex key
set) in caches can be verified with a new key set and vice versa.
Since only the key set is signed with a KSK, zone size considerations
do not apply.
Note that KSK rollovers and ZSK rollovers are different in the sense
that a KSK rollover requires interaction with the parent (and
possibly replacing trust anchors) and the ensuing delay while waiting
for it.
---------------------------------------------------------------------
initial new DNSKEY DS change DNSKEY removal
---------------------------------------------------------------------
Parent:
SOA_0 -----------------------------> SOA_1 ------------------------>
RRSIG_par(SOA) --------------------> RRSIG_par(SOA) --------------->
DS_K_1 ----------------------------> DS_K_2 ----------------------->
RRSIG_par(DS) ---------------------> RRSIG_par(DS) ---------------->
Child:
SOA_0 SOA_1 -----------------------> SOA_2
RRSIG_Z_10(SOA) RRSIG_Z_10(SOA) -------------> RRSIG_Z_10(SOA)
DNSKEY_K_1 DNSKEY_K_1 ------------------>
DNSKEY_K_2 ------------------> DNSKEY_K_2
DNSKEY_Z_10 DNSKEY_Z_10 -----------------> DNSKEY_Z_10
RRSIG_K_1(DNSKEY) RRSIG_K_1 (DNSKEY) ---------->
RRSIG_K_2 (DNSKEY) ----------> RRSIG_K_2(DNSKEY)
---------------------------------------------------------------------
Figure 4: Stages of Deployment for a Double-Signature Key Signing Key
Rollover
initial: Initial version of the zone. The parental DS points to
DNSKEY_K_1. Before the rollover starts, the child will have to
verify what the TTL is of the DS RR that points to DNSKEY_K_1 --
it is needed during the rollover, and we refer to the value as
TTL_DS.
new DNSKEY: During the "new DNSKEY" phase, the zone administrator
generates a second KSK, DNSKEY_K_2. The key is provided to the
parent, and the child will have to wait until a new DS RR has been
generated that points to DNSKEY_K_2. After that DS RR has been
published on all servers authoritative for the parent's zone, the
zone administrator has to wait at least TTL_DS to make sure that
the old DS RR has expired from caches.
DS change: The parent replaces DS_K_1 with DS_K_2.
DNSKEY removal: DNSKEY_K_1 has been removed.
The scenario above puts the responsibility for maintaining a valid
chain of trust with the child. It also is based on the premise that
the parent only has one DS RR (per algorithm) per zone. An
alternative mechanism has been considered. Using an established
trust relationship, the interaction can be performed in-band, and the
removal of the keys by the child can possibly be signaled by the
parent. In this mechanism, there are periods where there are two DS
RRs at the parent. This is known as a KSK Double-DS rollover and is
shown in Figure 5. This method has some drawbacks for KSKs. We
first describe the rollover scheme and then indicate these drawbacks.
--------------------------------------------------------------------
initial new DS new DNSKEY DS removal
--------------------------------------------------------------------
Parent:
SOA_0 SOA_1 ------------------------> SOA_2
RRSIG_par(SOA) RRSIG_par(SOA) ---------------> RRSIG_par(SOA)
DS_K_1 DS_K_1 ----------------------->
DS_K_2 -----------------------> DS_K_2
RRSIG_par(DS) RRSIG_par(DS) ----------------> RRSIG_par(DS)
Child:
SOA_0 -----------------------> SOA_1 ---------------------------->
RRSIG_Z_10(SOA) -------------> RRSIG_Z_10(SOA) ------------------>
DNSKEY_K_1 ------------------> DNSKEY_K_2 ----------------------->
DNSKEY_Z_10 -----------------> DNSKEY_Z_10 ---------------------->
RRSIG_K_1 (DNSKEY) ----------> RRSIG_K_2 (DNSKEY) --------------->
--------------------------------------------------------------------
Figure 5: Stages of Deployment for a Double-DS Key Signing Key
Rollover
When the child zone wants to roll, it notifies the parent during the
"new DS" phase and submits the new key (or the corresponding DS) to
the parent. The parent publishes DS_K_1 and DS_K_2, pointing to
DNSKEY_K_1 and DNSKEY_K_2, respectively. During the rollover ("new
DNSKEY" phase), which can take place as soon as the new DS set
propagated through the DNS, the child replaces DNSKEY_K_1 with
DNSKEY_K_2. If the old key has expired from caches, at the "DS
removal" phase the parent can be notified that the old DS record can
be deleted.
The drawbacks of this scheme are that during the "new DS" phase, the
parent cannot verify the match between the DS_K_2 RR and DNSKEY_K_2
using the DNS, as DNSKEY_K_2 is not yet published. Besides, we
introduce a "security lame" key (see Section 4.3.3). Finally, the
child-parent interaction consists of two steps. The "Double
Signature" method only needs one interaction.
4.1.2.1. Special Considerations for RFC 5011 KSK Rollover
The scenario sketched above assumes that the KSK is not in use as a
trust anchor but that validating name servers exclusively depend on
the parental DS record to establish the zone's security. If it is
known that validating name servers have configured trust anchors,
then that needs to be taken into account. Here, we assume that zone
administrators will deploy RFC 5011 [RFC5011] style rollovers.
RFC 5011 style rollovers increase the duration of key rollovers: The
key to be removed must first be revoked. Thus, before the DNSKEY_K_1
removal phase, DNSKEY_K_1 must be published for one more Maximum Zone
TTL with the REVOKE bit set. The revoked key must be self-signed, so
in this phase the DNSKEY RRset must also be signed with DNSKEY_K_1.
4.1.3. Single-Type Signing Scheme Key Rollover
The rollover of a key when a Single-Type Signing Scheme is used is
subject to the same requirement as the rollover of a KSK or ZSK:
During any stage of the rollover, the chain of trust needs to
continue to validate for any combination of data in the zone as well
as data that may still live in distant caches.
There are two variants for this rollover. Since the choice for a
Single-Type Signing Scheme is motivated by operational simplicity, we
describe the most straightforward rollover scheme first.
-------------------------------------------------------------------
initial new DNSKEY DS change DNSKEY removal
-------------------------------------------------------------------
Parent:
SOA_0 --------------------------> SOA_1 ---------------------->
RRSIG_par(SOA) -----------------> RRSIG_par(SOA) ------------->
DS_S_1 -------------------------> DS_S_2 --------------------->
RRSIG_par(DS_S_1) --------------> RRSIG_par(DS_S_2) ---------->
Child:
SOA_0 SOA_1 ----------------------> SOA_2
RRSIG_S_1(SOA) RRSIG_S_1(SOA) ------------->
RRSIG_S_2(SOA) -------------> RRSIG_S_2(SOA)
DNSKEY_S_1 DNSKEY_S_1 ----------------->
DNSKEY_S_2 -----------------> DNSKEY_S_2
RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY) ---------->
RRSIG_S_2(DNSKEY) ----------> RRSIG_S_2(DNSKEY)
-------------------------------------------------------------------
Figure 6: Stages of the Straightforward Rollover in a Single-Type
Signing Scheme
initial: Parental DS points to DNSKEY_S_1. All RRsets in the zone
are signed with DNSKEY_S_1.
new DNSKEY: A new key (DNSKEY_S_2) is introduced, and all the RRsets
are signed with both DNSKEY_S_1 and DNSKEY_S_2.
DS change: After the DNSKEY RRset with the two keys had time to
propagate into distant caches (that is, the key set exclusively
containing DNSKEY_S_1 has been expired), the parental DS record
can be changed.
DNSKEY removal: After the DS RRset containing DS_S_1 has expired
from distant caches, DNSKEY_S_1 can be removed from the DNSKEY
RRset.
In this first variant, the new signatures and new public key are
added to the zone. Once they are propagated, the DS at the parent is
switched. If the old DS has expired from the caches, the old
signatures and old public key can be removed from the zone.
This rollover has the drawback that it introduces double signatures
over all data of the zone. Taking these zone size considerations
into account, it is possible to not introduce the signatures made
with DNSKEY_S_2 at the "new DNSKEY" step. Instead, signatures of
DNSKEY_S_1 are replaced with signatures of DNSKEY_S_2 in an
additional stage between the "DS change" and "DNSKEY removal" step:
After the DS RRset containing DS_S_1 has expired from distant caches,
the signatures can be swapped. Only after the new signatures made
with DNSKEY_S_2 have been propagated can the old public key
DNSKEY_S_1 be removed from the DNSKEY RRset.
The second variant of the Single-Type Signing Scheme Key rollover is
the Double-DS rollover. In this variant, one introduces a new DNSKEY
into the key set and submits the new DS to the parent. The new key
is not yet used to sign RRsets. The signatures made with DNSKEY_S_1
are replaced with signatures made with DNSKEY_S_2 at the moment that
DNSKEY_S_2 and DS_S_2 have been propagated.
-----------------------------------------------------------------------
initial new DS new RRSIG DS removal
-----------------------------------------------------------------------
Parent:
SOA_0 SOA_1 -------------------------> SOA_2
RRSIG_par(SOA) RRSIG_par(SOA) ----------------> RRSIG_par(SOA)
DS_S_1 DS_S_1 ------------------------>
DS_S_2 ------------------------> DS_S_2
RRSIG_par(DS) RRSIG_par(DS) -----------------> RRSIG_par(DS)
Child:
SOA_0 SOA_1 SOA_2 SOA_3
RRSIG_S_1(SOA) RRSIG_S_1(SOA) RRSIG_S_2(SOA) RRSIG_S_2(SOA)
DNSKEY_S_1 DNSKEY_S_1 DNSKEY_S_1
DNSKEY_S_2 DNSKEY_S_2 DNSKEY_S_2
RRSIG_S_1 (DNSKEY) RRSIG_S_2(DNSKEY) RRSIG_S_2(DNSKEY)
-----------------------------------------------------------------------
Figure 7: Stages of Deployment for a Double-DS Rollover in a
Single-Type Signing Scheme
4.1.4. Algorithm Rollovers
A special class of key rollovers is the one needed for a change of
signature algorithms (either adding a new algorithm, removing an old
algorithm, or both). Additional steps are needed to retain integrity
during this rollover. We first describe the generic case; special
considerations for rollovers that involve trust anchors and single-
type keys are discussed later.
There exist both a conservative and a liberal approach for algorithm
rollover. This has to do with Section 2.2 of RFC 4035 [RFC4035]:
There MUST be an RRSIG for each RRset using at least one DNSKEY
of each algorithm in the zone apex DNSKEY RRset. The apex
DNSKEY RRset itself MUST be signed by each algorithm appearing
in the DS RRset located at the delegating parent (if any).
The conservative approach interprets this section very strictly,
meaning that it expects that every RRset has a valid signature for
every algorithm signaled by the zone apex DNSKEY RRset, including
RRsets in caches. The liberal approach uses a more loose
interpretation of the section and limits the rule to RRsets in the
zone at the authoritative name servers. There is a reasonable
argument for saying that this is valid, because the specific section
is a subsection of Section 2 ("Zone Signing") of RFC 4035.
When following the more liberal approach, algorithm rollover is just
as easy as a regular Double-Signature KSK rollover (Section 4.1.2).
Note that the Double-DS KSK rollover method cannot be used, since
that would introduce a parental DS of which the apex DNSKEY RRset has
not been signed with the introduced algorithm.
However, there are implementations of validators known to follow the
more conservative approach. Performing a Double-Signature KSK
algorithm rollover will temporarily make your zone appear as Bogus by
such validators during the rollover. Therefore, the rollover
described in this section will explain the stages of deployment and
will assume that the conservative approach is used.
When adding a new algorithm, the signatures should be added first.
After the TTL of RRSIGs has expired and caches have dropped the old
data covered by those signatures, the DNSKEY with the new algorithm
can be added.
After the new algorithm has been added, the DS record can be
exchanged using Double-Signature KSK rollover.
When removing an old algorithm, the DS for the algorithm should be
removed from the parent zone first, followed by the DNSKEY and the
signatures (in the child zone).
Figure 8 describes the steps.
----------------------------------------------------------------
initial new RRSIGs new DNSKEY
----------------------------------------------------------------
Parent:
SOA_0 -------------------------------------------------------->
RRSIG_par(SOA) ----------------------------------------------->
DS_K_1 ------------------------------------------------------->
RRSIG_par(DS_K_1) -------------------------------------------->
Child:
SOA_0 SOA_1 SOA_2
RRSIG_Z_10(SOA) RRSIG_Z_10(SOA) RRSIG_Z_10(SOA)
RRSIG_Z_11(SOA) RRSIG_Z_11(SOA)
DNSKEY_K_1 DNSKEY_K_1 DNSKEY_K_1
DNSKEY_K_2
DNSKEY_Z_10 DNSKEY_Z_10 DNSKEY_Z_10
DNSKEY_Z_11
RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY)
RRSIG_K_2(DNSKEY)
----------------------------------------------------------------
new DS DNSKEY removal RRSIGs removal
----------------------------------------------------------------
Parent:
SOA_1 ------------------------------------------------------->
RRSIG_par(SOA) ---------------------------------------------->
DS_K_2 ------------------------------------------------------>
RRSIG_par(DS_K_2) ------------------------------------------->
Child:
-------------------> SOA_3 SOA_4
-------------------> RRSIG_Z_10(SOA)
-------------------> RRSIG_Z_11(SOA) RRSIG_Z_11(SOA)
------------------->
-------------------> DNSKEY_K_2 DNSKEY_K_2
------------------->
-------------------> DNSKEY_Z_11 DNSKEY_Z_11
------------------->
-------------------> RRSIG_K_2(DNSKEY) RRSIG_K_2(DNSKEY)
----------------------------------------------------------------
Figure 8: Stages of Deployment during an Algorithm Rollover
initial: Describes the state of the zone before any transition is
done. The number of the keys may vary, but all keys (in DNSKEY
records) for the zone use the same algorithm.
new RRSIGs: The signatures made with the new key over all records in
the zone are added, but the key itself is not. This step is
needed to propagate the signatures created with the new algorithm
to the caches. If this is not done, it is possible for a resolver
to retrieve the new DNSKEY RRset (containing the new algorithm)
but to have RRsets in its cache with signatures created by the old
DNSKEY RRset (i.e., without the new algorithm).
The RRSIG for the DNSKEY RRset does not need to be pre-published
(since these records will travel together) and does not need
special processing in order to keep them synchronized.
new DNSKEY: After the old data has expired from caches, the new key
can be added to the zone.
new DS: After the cache data for the old DNSKEY RRset has expired,
the DS record for the new key can be added to the parent zone and
the DS record for the old key can be removed in the same step.
DNSKEY removal: After the cache data for the old DS RRset has
expired, the old algorithm can be removed. This time, the old key
needs to be removed first, before removing the old signatures.
RRSIGs removal: After the cache data for the old DNSKEY RRset has
expired, the old signatures can also be removed during this step.
Below, we deal with a few special cases of algorithm rollovers:
1: Single-Type Signing Scheme Algorithm rollover: when there is no
differentiation between ZSKs and KSKs (Section 4.1.4.1).
2: RFC 5011 Algorithm rollover: when trust anchors can track the
roll via RFC 5011 style rollover (Section 4.1.4.2).
3: 1 and 2 combined: when a Single-Type Signing Scheme Algorithm
rollover is performed RFC 5011 style (Section 4.1.4.3).
In addition to the narrative below, these special cases are
represented in Figures 12, 13, and 14 in Appendix C.
4.1.4.1. Single-Type Signing Scheme Algorithm Rollover
If one key is used that acts as both ZSK and KSK, the same scheme and
figure as above (Figure 8 in Section 4.1.4) applies, whereby all
DNSKEY_Z_* records from the table are removed and all RRSIG_Z_* are
replaced with RRSIG_S_*. All DNSKEY_K_* records are replaced with
DNSKEY_S_*, and all RRSIG_K_* records are replaced with RRSIG_S_*.
The requirement to sign with both algorithms and make sure that old
RRSIGs have the opportunity to expire from distant caches before
introducing the new algorithm in the DNSKEY RRset is still valid.
This is shown in Figure 12 in Appendix C.
4.1.4.2. Algorithm Rollover, RFC 5011 Style
Trust anchor algorithm rollover is almost as simple as a regular
RFC 5011-based rollover. However, the old trust anchor must be
revoked before it is removed from the zone.
The timeline (see Figure 13 in Appendix C) is similar to that of
Figure 8 above, but after the "new DS" step, an additional step is
required where the DNSKEY is revoked. The details of this step
("revoke DNSKEY") are shown in Figure 9 below.
---------------------------------
revoke DNSKEY
---------------------------------
Parent:
----------------------------->
----------------------------->
----------------------------->
----------------------------->
Child:
SOA_3
RRSIG_Z_10(SOA)
RRSIG_Z_11(SOA)
DNSKEY_K_1_REVOKED
DNSKEY_K_2
DNSKEY_Z_11
RRSIG_K_1(DNSKEY)
RRSIG_K_2(DNSKEY)
---------------------------------
Figure 9: The Revoke DNSKEY State That Is Added to an Algorithm
Rollover when RFC 5011 Is in Use
There is one exception to the requirement from RFC 4035 quoted in
Section 4.1.4 above: While all zone data must be signed with an
unrevoked key, it is permissible to sign the key set with a revoked
key. The somewhat esoteric argument is as follows:
Resolvers that do not understand the RFC 5011 REVOKE flag will handle
DNSKEY_K_1_REVOKED the same as if it were DNSKEY_K_1. In other
words, they will handle the revoked key as a normal key, and thus
RRsets signed with this key will validate. As a result, the
signature matches the algorithm listed in the DNSKEY RRset.
Resolvers that do implement RFC 5011 will remove DNSKEY_K_1 from the
set of trust anchors. That is okay, since they have already added
DNSKEY_K_2 as the new trust anchor. Thus, algorithm 2 is the only
signaled algorithm by now. That is, we only need RRSIG_K_2(DNSKEY)
to authenticate the DNSKEY RRset, and we are still compliant with
Section 2.2 of RFC 4035: There must be an RRSIG for each RRset using
at least one DNSKEY of each algorithm in the zone apex DNSKEY RRset.
4.1.4.3. Single Signing Type Algorithm Rollover, RFC 5011 Style
If a decision is made to perform an RFC 5011 style rollover with a
Single Signing Scheme key, it should be noted that Section 2.1 of
RFC 5011 states:
Once the resolver sees the REVOKE bit, it MUST NOT use this key
as a trust anchor or for any other purpose except to validate
the RRSIG it signed over the DNSKEY RRset specifically for the
purpose of validating the revocation.
This means that once DNSKEY_S_1 is revoked, it cannot be used to
validate its signatures over non-DNSKEY RRsets. Thus, those RRsets
should be signed with a shadow key, DNSKEY_Z_10, during the algorithm
rollover. The shadow key can be removed at the same time the revoked
DNSKEY_S_1 is removed from the zone. In other words, the zone must
temporarily fall back to a KSK/ZSK split model during the rollover.
In other words, the rule that at every RRset there must be at least
one signature for each algorithm used in the DNSKEY RRset still
applies. This means that a different key with the same algorithm,
other than the revoked key, must sign the entire zone. Thus, more
operations are needed if the Single-Type Signing Scheme is used.
Before rolling the algorithm, a new key must be introduced with the
same algorithm as the key that is a candidate for revocation. That
key can than temporarily act as a ZSK during the algorithm rollover.
As with algorithm rollover RFC 5011 style, while all zone data must
be signed with an unrevoked key, it is permissible to sign the key
set with a revoked key using the same esoteric argument given in
Section 4.1.4.2.
The lesson of all of this is that a Single-Type Signing Scheme
algorithm rollover using RFC 5011 is as complicated as the name of
the rollover implies: Reverting to a split-key scheme for the
duration of the rollover may be preferable.
4.1.4.4. NSEC-to-NSEC3 Algorithm Rollover
A special case is the rollover from an NSEC signed zone to an NSEC3
signed zone. In this case, algorithm numbers are used to signal
support for NSEC3 but they do not mandate the use of NSEC3.
Therefore, NSEC records should remain in the zone until the rollover
to a new algorithm has completed and the new DNSKEY RRset has
populated distant caches, at the end of the "new DNSKEY" stage. At
that point, the validators that have not implemented NSEC3 will treat
the zone as unsecured as soon as they follow the chain of trust to
the DS that points to a DNSKEY of the new algorithm, while validators
that support NSEC3 will happily validate using NSEC. Turning on
NSEC3 can then be done during the "new DS" step: increasing the
serial number, introducing the NSEC3PARAM record to signal that
NSEC3-authenticated data related to denial of existence should be
served, and re-signing the zone.
In summary, an NSEC-to-NSEC3 rollover is an ordinary algorithm
rollover whereby NSEC is used all the time and only after that
rollover finished NSEC3 needs to be deployed. The procedures are
also listed in Sections 10.4 and 10.5 of RFC 5155 [RFC5155].
4.1.5. Considerations for Automated Key Rollovers
As keys must be renewed periodically, there is some motivation to
automate the rollover process. Consider the following:
o ZSK rollovers are easy to automate, as only the child zone is
involved.
o A KSK rollover needs interaction between the parent and child.
Data exchange is needed to provide the new keys to the parent;
consequently, this data must be authenticated, and integrity must
be guaranteed in order to avoid attacks on the rollover.
4.2. Planning for Emergency Key Rollover
This section deals with preparation for a possible key compromise.
It is advisable to have a documented procedure ready for those times
when a key compromise is suspected or confirmed.
When the private material of one of a zone's keys is compromised, it
can be used by an attacker for as long as a valid trust chain exists.
A trust chain remains intact for
o as long as a signature over the compromised key in the trust chain
is valid, and
o as long as the DS RR in the parent zone points to the
(compromised) key signing the DNSKEY RRset, and
o as long as the (compromised) key is anchored in a resolver and is
used as a starting point for validation (this is generally the
hardest to update).
While a trust chain to a zone's compromised key exists, your
namespace is vulnerable to abuse by anyone who has obtained
illegitimate possession of the key. Zone administrators have to make
a decision as to whether the abuse of the compromised key is worse
than having data in caches that cannot be validated. If the zone
administrator chooses to break the trust chain to the compromised
key, data in caches signed with this key cannot be validated.
However, if the zone administrator chooses to take the path of a
regular rollover, during the rollover the malicious key holder can
continue to spoof data so that it appears to be valid.
4.2.1. KSK Compromise
A compromised KSK can be used to sign the key set of an attacker's
version of the zone. That zone could be used to poison the DNS.
A zone containing a DNSKEY RRset with a compromised KSK is vulnerable
as long as the compromised KSK is configured as the trust anchor or a
DS record in the parent zone points to it.
Therefore, when the KSK has been compromised, the trust anchor or the
parent DS record should be replaced as soon as possible. It is local
policy whether to break the trust chain during the emergency
rollover. The trust chain would be broken when the compromised KSK
is removed from the child's zone while the parent still has a DS
record pointing to the compromised KSK. The assumption is that there
is only one DS record at the parent. If there are multiple DS
records, this does not apply, although the chain of trust of this
particular key is broken.
Note that an attacker's version of the zone still uses the
compromised KSK, and the presence of the corresponding DS record in
the parent would cause the data in this zone to appear as valid.
Removing the compromised key would cause the attacker's version of
the zone to appear as valid and the original zone as Bogus.
Therefore, we advise administrators not to remove the KSK before the
parent has a DS record for the new KSK in place.
4.2.1.1. Emergency Key Rollover Keeping the Chain of Trust Intact
If it is desired to perform an emergency key rollover in a manner
that keeps the chain of trust intact, the timing of the replacement
of the KSK is somewhat critical. The goal is to remove the
compromised KSK as soon as the new DS RR is available at the parent.
This means ensuring that the signature made with a new KSK over the
key set that contains the compromised KSK expires just after the new
DS appears at the parent. Expiration of that signature will cause
expiration of that key set from the caches.
The procedure is as follows:
1. Introduce a new KSK into the key set; keep the compromised KSK in
the key set. Lower the TTL for DNSKEYs so that the DNSKEY RRset
will expire from caches sooner.
2. Sign the key set, with a short validity period. The validity
period should expire shortly after the DS is expected to appear
in the parent and the old DSs have expired from caches. This
provides an upper limit on how long the compromised KSK can be
used in a replay attack.
3. Upload the DS for this new key to the parent.
4. Follow the procedure of the regular KSK rollover: Wait for the DS
to appear at the authoritative servers, and then wait as long as
the TTL of the old DS RRs. If necessary, re-sign the DNSKEY
RRset and modify/extend the expiration time.
5. Remove the compromised DNSKEY RR from the zone, and re-sign the
key set using your "normal" TTL and signature validity period.
An additional danger of a key compromise is that the compromised key
could be used to facilitate a legitimate-looking DNSKEY/DS rollover
and/or name server changes at the parent. When that happens, the
domain may be in dispute. An authenticated out-of-band and secure
notify mechanism to contact a parent is needed in this case.
Note that this is only a problem when the DNSKEY and/or DS records
are used to authenticate communication with the parent.
4.2.1.2. Emergency Key Rollover Breaking the Chain of Trust
There are two methods to perform an emergency key rollover in a
manner that breaks the chain of trust. The first method causes the
child zone to appear Bogus to validating resolvers. The other causes
the child zone to appear Insecure. These are described below.
In the method that causes the child zone to appear Bogus to
validating resolvers, the child zone replaces the current KSK with a
new one and re-signs the key set. Next, it sends the DS of the new
key to the parent. Only after the parent has placed the new DS in
the zone is the child's chain of trust repaired. Note that until
that time, the child zone is still vulnerable to spoofing: The
attacker is still in possession of the compromised key that the DS
points to.
An alternative method of breaking the chain of trust is by removing
the DS RRs from the parent zone altogether. As a result, the child
zone would become Insecure. After the DS has expired from distant
caches, the keys and signatures are removed from the child zone, new
keys and signatures are introduced, and finally, a new DS is
submitted to the parent.
4.2.2. ZSK Compromise
Primarily because there is no interaction with the parent required
when a ZSK is compromised, the situation is less severe than with a
KSK compromise. The zone must still be re-signed with a new ZSK as
soon as possible. As this is a local operation and requires no
communication between the parent and child, this can be achieved
fairly quickly. However, one has to take into account that -- just
as with a normal rollover -- the immediate disappearance of the old
compromised key may lead to verification problems. Also note that
until the RRSIG over the compromised ZSK has expired, the zone may
still be at risk.
4.2.3. Compromises of Keys Anchored in Resolvers
A key can also be pre-configured in resolvers as a trust anchor. If
trust anchor keys are compromised, the administrators of resolvers
using these keys should be notified of this fact. Zone
administrators may consider setting up a mailing list to communicate
the fact that a SEP key is about to be rolled over. This
communication will of course need to be authenticated by some means,
e.g., by using digital signatures.
End-users faced with the task of updating an anchored key should
always verify the new key. New keys should be authenticated out-of-
band, for example, through the use of an announcement website that is
secured using Transport Layer Security (TLS) [RFC5246].
4.2.4. Stand-By Keys
Stand-by keys are keys that are published in your zone but are not
used to sign RRsets. There are two reasons why someone would want to
use stand-by keys. One is to speed up the emergency key rollover.
The other is to recover from a disaster that leaves your production
private keys inaccessible.
The way to deal with stand-by keys differs for ZSKs and KSKs. To
make a stand-by ZSK, you need to publish its DNSKEY RR. To make a
stand-by KSK, you need to get its DS RR published at the parent.
Assuming you have your normal DNS operation, to prepare stand-by keys
you need to:
o Generate a stand-by ZSK and KSK. Store them safely in a location
different than the place where the currently used ZSK and KSK are
held.
o Pre-publish the DNSKEY RR of the stand-by ZSK in the zone.
o Pre-publish the DS of the stand-by KSK in the parent zone.
Now suppose a disaster occurs and disables access to the currently
used keys. To recover from that situation, follow these procedures:
o Set up your DNS operations and introduce the stand-by KSK into the
zone.
o Post-publish the disabled ZSK and sign the zone with the stand-by
keys.
o After some time, when the new signatures have been propagated, the
old keys, old signatures, and the old DS can be removed.
o Generate a new stand-by key set at a different location and
continue "normal" operation.
4.3. Parent Policies
4.3.1. Initial Key Exchanges and Parental Policies Considerations
The initial key exchange is always subject to the policies set by the
parent. It is specifically important in a registry-registrar-
registrant model where a registry maintains the parent zone, and the
registrant (the user of the child-domain name) deals with the
registry through an intermediary called a registrar (see [RFC3375]
for a comprehensive definition). The key material is to be passed
from the DNS operator to the parent via a registrar, where both the
DNS operator and registrar are selected by the registrant and might
be different organizations. When designing a key exchange policy,
one should take into account that the authentication and
authorization mechanisms used during a key exchange should be as
strong as the authentication and authorization mechanisms used for
the exchange of delegation information between the parent and child.
That is, there is no implicit need in DNSSEC to make the
authentication process stronger than it is for regular DNS.
Using the DNS itself as the source for the actual DNSKEY material has
the benefit that it reduces the chances of user error. A DNSKEY
query tool can make use of the SEP bit [RFC4035] to select the proper
key(s) from a DNSSEC key set, thereby reducing the chance that the
wrong DNSKEY is sent. It can validate the self-signature over a key,
thereby verifying the ownership of the private key material.
Fetching the DNSKEY from the DNS ensures that the chain of trust
remains intact once the parent publishes the DS RR indicating that
the child is secure.
Note: Out-of-band verification is still needed when the key material
is fetched for the first time, even via DNS. The parent can never be
sure whether or not the DNSKEY RRs have been spoofed.
With some types of key rollovers, the DNSKEY is not pre-published,
and a DNSKEY query tool is not able to retrieve the successor key.
In this case, the out-of-band method is required. This also allows
the child to determine the digest algorithm of the DS record.
4.3.2. Storing Keys or Hashes?
When designing a registry system, one should consider whether to
store the DNSKEYs and/or the corresponding DSs. Since a child zone
might wish to have a DS published using a message digest algorithm
not yet understood by the registry, the registry can't count on being
able to generate the DS record from a raw DNSKEY. Thus, we suggest
that registry systems should be able to store DS RRs, even if they
also store DNSKEYs (see also "DNSSEC Trust Anchor Configuration and
Maintenance" [DNSSEC-TRUST-ANCHOR]).
The storage considerations also relate to the design of the customer
interface and the method by which data is transferred between the
registrant and registry: Will the child-zone administrator be able to
upload DS RRs with unknown hash algorithms, or does the interface
only allow DNSKEYs? When registries support the Extensible
Provisioning Protocol (EPP) [RFC5910], that can be used for
registrar-registry interactions, since that protocol allows the
transfer of both DS and, optionally, DNSKEY RRs. There is no
standardized way to move the data between the customer and the
registrar. Different registrars have different mechanisms, ranging
from simple web interfaces to various APIs. In some cases, the use
of the DNSSEC extensions to EPP may be applicable.
Having an out-of-band mechanism such as a registry directory (e.g.,
Whois) to find out which keys are used to generate DS Resource
Records for specific owners and/or zones may also help with
troubleshooting.
4.3.3. Security Lameness
Security lameness is defined as the state whereby the parent has a DS
RR pointing to a nonexistent DNSKEY RR. Security lameness may occur
temporarily during a Double-DS rollover scheme. However, care should
be taken that not all DS RRs are pointing to a nonexistent DNSKEY RR,
which will cause the child's zone to be marked Bogus by verifying DNS
clients.
As part of a comprehensive delegation check, the parent could, at key
exchange time, verify that the child's key is actually configured in
the DNS. However, if a parent does not understand the hashing
algorithm used by the child, the parental checks are limited to only
comparing the key id.
Child zones should be very careful in removing DNSKEY material --
specifically, SEP keys -- for which a DS RR exists.
Once a zone is "security lame", a fix (e.g., removing a DS RR) will
take time to propagate through the DNS.
4.3.4. DS Signature Validity Period
Since the DS can be replayed as long as it has a valid signature, a
short signature validity period for the DS RRSIG minimizes the time
that a child is vulnerable in the case of a compromise of the child's
KSK(s). A signature validity period that is too short introduces the
possibility that a zone is marked Bogus in the case of a
configuration error in the signer. There may not be enough time to
fix the problems before signatures expire (this is a generic
argument; also see Section 4.4.2). Something as mundane as zone
administrator unavailability during weekends shows the need for DS
signature validity periods longer than two days. Just like any
signature validity period, we suggest an absolute minimum for the DS
signature validity period of a few days.
The maximum signature validity period of the DS record depends on how
long child zones are willing to be vulnerable after a key compromise.
On the other hand, shortening the DS signature validity period
increases the operational risk for the parent. Therefore, the parent
may have a policy to use a signature validity period that is
considerably longer than the child would hope for.
A compromise between the policy/operational constraints of the parent
and minimizing damage for the child may result in a DS signature
validity period somewhere between a week and several months.
In addition to the signature validity period, which sets a lower
bound on the number of times the zone administrator will need to sign
the zone data and an upper bound on the time that a child is
vulnerable after key compromise, there is the TTL value on the DS
RRs. Shortening the TTL reduces the damage of a successful replay
attack. It does mean that the authoritative servers will see more
queries. But on the other hand, a short TTL lowers the persistence
of DS RRsets in caches, thereby increasing the speed with which
updated DS RRsets propagate through the DNS.
4.3.5. Changing DNS Operators
The parent-child relationship is often described in terms of a
registry-registrar-registrant model, where a registry maintains the
parent zone and the registrant (the user of the child-domain name)
deals with the registry through an intermediary called a registrar
[RFC3375]. Registrants may outsource the maintenance of their DNS
system, including the maintenance of DNSSEC key material, to the
registrar or to another third party, referred to here as the DNS
operator.
For various reasons, a registrant may want to move between DNS
operators. How easy this move will be depends principally on the DNS
operator from which the registrant is moving (the losing operator),
as the losing operator has control over the DNS zone and its keys.
The following sections describe the two cases: where the losing
operator cooperates with the new operator (the gaining operator), and
where the two do not cooperate.
4.3.5.1. Cooperating DNS Operators
In this scenario, it is assumed that the losing operator will not
pass any private key material to the gaining operator (that would
constitute a trivial case) but is otherwise fully cooperative.
In this environment, the change could be made with a Pre-Publish ZSK
rollover, whereby the losing operator pre-publishes the ZSK of the
gaining operator, combined with a Double-Signature KSK rollover where
the two registrars exchange public keys and independently generate a
signature over those key sets that they combine and both publish in
their copy of the zone. Once that is done, they can use their own
private keys to sign any of their zone content during the transfer.
------------------------------------------------------------
initial | pre-publish |
------------------------------------------------------------
Parent:
NS_A NS_A
DS_A DS_A
------------------------------------------------------------
Child at A: Child at A: Child at B:
SOA_A0 SOA_A1 SOA_B0
RRSIG_Z_A(SOA) RRSIG_Z_A(SOA) RRSIG_Z_B(SOA)
NS_A NS_A NS_B
RRSIG_Z_A(NS) NS_B RRSIG_Z_B(NS)
RRSIG_Z_A(NS)
DNSKEY_Z_A DNSKEY_Z_A DNSKEY_Z_A
DNSKEY_Z_B DNSKEY_Z_B
DNSKEY_K_A DNSKEY_K_A DNSKEY_K_A
DNSKEY_K_B DNSKEY_K_B
RRSIG_K_A(DNSKEY) RRSIG_K_A(DNSKEY) RRSIG_K_A(DNSKEY)
RRSIG_K_B(DNSKEY) RRSIG_K_B(DNSKEY)
------------------------------------------------------------
------------------------------------------------------------
re-delegation | post-migration |
------------------------------------------------------------
Parent:
NS_B NS_B
DS_B DS_B
------------------------------------------------------------
Child at A: Child at B: Child at B:
SOA_A1 SOA_B0 SOA_B1
RRSIG_Z_A(SOA) RRSIG_Z_B(SOA) RRSIG_Z_B(SOA)
NS_A NS_B NS_B
NS_B RRSIG_Z_B(NS) RRSIG_Z_B(NS)
RRSIG_Z_A(NS)
DNSKEY_Z_A DNSKEY_Z_A
DNSKEY_Z_B DNSKEY_Z_B DNSKEY_Z_B
DNSKEY_K_A DNSKEY_K_A
DNSKEY_K_B DNSKEY_K_B DNSKEY_K_B
RRSIG_K_A(DNSKEY) RRSIG_K_A(DNSKEY)
RRSIG_K_B(DNSKEY) RRSIG_K_B(DNSKEY) RRSIG_K_B(DNSKEY)
------------------------------------------------------------
Figure 10: Rollover for Cooperating Operators
In this figure, A denotes the losing operator and B the gaining
operator. RRSIG_Z is the RRSIG produced by a ZSK, RRSIG_K is
produced with a KSK, and the appended A or B indicates the producers
of the key pair. "Child at A" is how the zone content is represented
by the losing DNS operator, and "Child at B" is how the zone content
is represented by the gaining DNS operator.
The zone is initially delegated from the parent to the name servers
of operator A. Operator A uses his own ZSK and KSK to sign the zone.
The cooperating operator A will pre-publish the new NS record and the
ZSK and KSK of operator B, including the RRSIG over the DNSKEY RRset
generated by the KSK of operator B. Operator B needs to publish the
same DNSKEY RRset. When that DNSKEY RRset has populated the caches,
the re-delegation can be made, which involves adjusting the NS and DS
records in the parent zone to point to operator B. And after all
DNSSEC records related to operator A have expired from the caches,
operator B can stop publishing the keys and signatures belonging to
operator A, and vice versa.
The requirement to exchange signatures has a couple of drawbacks. It
requires more operational overhead, because not only do the operators
have to exchange public keys but they also have to exchange the
signatures of the new DNSKEY RRset. This drawback does not exist if
the Double-Signature KSK rollover is replaced with a Double-DS KSK
rollover. See Figure 15 in Appendix D for the diagram.
Thus, if the registry and registrars allow DS records to be published
that do not point to a published DNSKEY in the child zone, the
Double-DS KSK rollover is preferred (see Figure 5), in combination
with the Pre-Publish ZSK rollover. This does not require sharing the
KSK signatures between the operators, but both operators still have
to publish each other's ZSKs.
4.3.5.2. Non-Cooperating DNS Operators
In the non-cooperating case, matters are more complicated. The
losing operator may not cooperate and leave the data in the DNS as
is. In extreme cases, the losing operator may become obstructive and
publish a DNSKEY RR with a high TTL and corresponding signature
validity period so that registrar A's DNSKEY could end up in caches
for (in theory at least) decades.
The problem arises when a validator tries to validate with the losing
operator's key and there is no signature material produced with the
losing operator available in the delegation path after re-delegation
from the losing operator to the gaining operator has taken place.
One could imagine a rollover scenario where the gaining operator
takes a copy of all RRSIGs created by the losing operator and
publishes those in conjunction with its own signatures, but that
would not allow any changes in the zone content. Since a re-
delegation took place, the NS RRset has by definition changed, so
such a rollover scenario will not work. Besides, if zone transfers
are not allowed by the losing operator and NSEC3 is deployed in the
losing operator's zone, then the gaining operator's zone will not
have certainty that all of the losing operator's RRSIGs have been
copied.
The only viable operation for the registrant is to have his zone go
Insecure for the duration of the change. The registry should be
asked to remove the DS RR pointing to the losing operator's DNSKEY
and to change the NS RRset to point to the gaining operator. Once
this has propagated through the DNS, the registry should be asked to
insert the DS record pointing to the (newly signed) zone at
operator B.
Note that some behaviors of resolver implementations may aid in the
process of changing DNS operators:
o TTL sanity checking, as described in RFC 2308 [RFC2308], will
limit the impact of the actions of an obstructive losing operator.
Resolvers that implement TTL sanity checking will use an upper
limit for TTLs on RRsets in responses.
o If RRsets at the zone cut (are about to) expire, the resolver
restarts its search above the zone cut. Otherwise, the resolver
risks continuing to use a name server that might be un-delegated
by the parent.
o Limiting the time that DNSKEYs that seem to be unable to validate
signatures are cached and/or trying to recover from cases where
DNSKEYs do not seem to be able to validate data also reduce the
effects of the problem of non-cooperating registrars.
However, there is no operational methodology to work around this
business issue, and proper contractual relationships between all
involved parties seem to be the only solution to cope with these
problems. It should be noted that in many cases, the problem with
temporary broken delegations already exists when a zone changes from
one DNS operator to another. Besides, it is often the case that when
operators are changed, the services that are referenced by that zone
also change operators, possibly involving some downtime.
In any case, to minimize such problems, the classic configuration is
to have relatively short TTLs on all involved Resource Records. That
will solve many of the problems regarding changes to a zone,
regardless of whether DNSSEC is used.
4.4. Time in DNSSEC
Without DNSSEC, all times in the DNS are relative. The SOA fields
REFRESH, RETRY, and EXPIRATION are timers used to determine the time
that has elapsed after a slave server synchronized with a master
server. The TTL value and the SOA RR minimum TTL parameter [RFC2308]
are used to determine how long a forwarder should cache data (or
negative responses) after it has been fetched from an authoritative
server. By using a signature validity period, DNSSEC introduces the
notion of an absolute time in the DNS. Signatures in DNSSEC have an
expiration date after which the signature is marked as invalid and
the signed data is to be considered Bogus.
The considerations in this section are all qualitative and focused on
the operational and managerial issues. A more thorough quantitative
analysis of rollover timing parameters can be found in "DNSSEC Key
Timing Considerations" [DNSSEC-KEY-TIMING].
4.4.1. Time Considerations
Because of the expiration of signatures, one should consider the
following:
o We suggest that the Maximum Zone TTL value of your zone data be
smaller than your signature validity period.
If the TTL duration was similar to that of the signature
validity period, then all RRsets fetched during the validity
period would be cached until the signature expiration time.
Section 8.1 of RFC 4033 [RFC4033] suggests that "the resolver
may use the time remaining before expiration of the signature
validity period of a signed RRset as an upper bound for the
TTL". As a result, the query load on authoritative servers
would peak at the signature expiration time, as this is also
the time at which records simultaneously expire from caches.
Having a TTL that is at least a few times smaller than your
signature validity period avoids query load peaks.
o We suggest that the signature publication period end at least one
Maximum Zone TTL duration (but preferably a minimum of a few days)
before the end of the signature validity period.
Re-signing a zone shortly before the end of the signature
validity period may cause the simultaneous expiration of data
from caches. This in turn may lead to peaks in the load on
authoritative servers. To avoid this, schemes are deployed
whereby the zone is periodically visited for a re-signing
operation, and those signatures that are within a so-called
Refresh Period from signature expiration are recreated. Also
see Section 4.4.2 below.
In the case of an operational error, you would have one Maximum
Zone TTL duration to resolve the problem. Re-signing a zone a
few days before the end of the signature validity period
ensures that the signatures will survive at least a (long)
weekend in case of such operational havoc. This is called the
Refresh Period (see Section 4.4.2).
o We suggest that the Minimum Zone TTL be long enough to both fetch
and verify all the RRs in the trust chain. In workshop
environments, it has been demonstrated [NIST-Workshop] that a low
TTL (under 5 to 10 minutes) caused disruptions because of the
following two problems:
1. During validation, some data may expire before the validation
is complete. The validator should be able to keep all data
until it is completed. This applies to all RRs needed to
complete the chain of trust: DS, DNSKEY, RRSIG, and the final
answers, i.e., the RRset that is returned for the initial
query.
2. Frequent verification causes load on recursive name servers.
Data at delegation points, DS, DNSKEY, and RRSIG RRs benefits
from caching. The TTL on those should be relatively long.
Data at the leaves in the DNS tree has less impact on
recursive name servers.
o Slave servers will need to be able to fetch newly signed zones
well before the RRSIGs in the zone served by the slave server pass
their signature expiration time.
When a slave server is out of synchronization with its master
and data in a zone is signed by expired signatures, it may be
better for the slave server not to give out any answer.
Normally, a slave server that is not able to contact a master
server for an extended period will expire a zone. When that
happens, the server will respond differently to queries for
that zone. Some servers issue SERVFAIL, whereas others turn
off the AA bit in the answers. The time of expiration is set
in the SOA record and is relative to the last successful
refresh between the master and the slave servers. There exists
no coupling between the signature expiration of RRSIGs in the
zone and the expire parameter in the SOA.
If the server serves a DNSSEC-secured zone, then it may happen
that the signatures expire well before the SOA expiration timer
counts down to zero. It is not possible to completely prevent
this by modifying the SOA parameters.
However, the effects can be minimized where the SOA expiration
time is equal to or shorter than the Refresh Period (see
Section 4.4.2).
The consequence of an authoritative server not being able to
update a zone for an extended period of time is that signatures
may expire. In this case, non-secure resolvers will continue
to be able to resolve data served by the particular slave
servers, while security-aware resolvers will experience
problems because of answers being marked as Bogus.
We suggest that the SOA expiration timer be approximately one
third or a quarter of the signature validity period. It will
allow problems with transfers from the master server to be
noticed before signatures time out.
We also suggest that operators of name servers that supply
secondary services develop systems to identify upcoming
signature expirations in zones they slave and take appropriate
action where such an event is detected.
When determining the value for the expiration parameter, one
has to take the following into account: What are the chances
that all secondaries expire the zone? How quickly can the
administrators of the secondary servers be reached to load a
valid zone? These questions are not DNSSEC-specific but may
influence the choice of your signature validity periods.
4.4.2. Signature Validity Periods
4.4.2.1. Maximum Value
The first consideration for choosing a maximum signature validity
period is the risk of a replay attack. For low-value, long-term
stable resources, the risks may be minimal, and the signature
validity period may be several months. Although signature validity
periods of many years are allowed, the same "operational habit"
arguments as those given in Section 3.2.2 play a role: When a zone is
re-signed with some regularity, then zone administrators remain
conscious of the operational necessity of re-signing.
4.4.2.2. Minimum Value
The minimum value of the signature validity period is set for the
time by which one would like to survive operational failure in
provisioning: At what time will a failure be noticed, and at what
time is action expected to be taken? By answering these questions,
availability of zone administrators during (long) weekends or time
taken to access backup media can be taken into account. The result
could easily suggest a minimum signature validity period of a few
days.
Note, however, that the argument above is assuming that zone data has
just been signed and published when the problem occurred. In
practice, it may be that a zone is signed according to a frequency
set by the Re-Sign Period, whereby the signer visits the zone content
and only refreshes signatures that are within a given amount of time
(the Refresh Period) of expiration. The Re-Sign Period must be
smaller than the Refresh Period in order for zone data to be signed
in a timely fashion.
If an operational problem occurs during re-signing, then the
signatures in the zone to expire first are the ones that have been
generated longest ago. In the worst case, these signatures are the
Refresh Period minus the Re-Sign Period away from signature
expiration.
To make matters slightly more complicated, some signers vary the
signature validity period over a small range (the jitter interval) so
that not all signatures expire at the same time.
In other words, the minimum signature validity period is set by first
choosing the Refresh Period (usually a few days), then defining the
Re-Sign Period in such a way that the Refresh Period minus the Re-
Sign Period, minus the maximum jitter sets the time in which
operational havoc can be resolved.
The relationship between signature times is illustrated in Figure 11.
Inception Signing Expiration
time time time
| | | | |
|------------------|---------------------------------|.....|.....|
| | | | |
+/-jitter
| Inception offset | |
|<---------------->| Validity Period |
| |<---------------------------------------->|
Inception Signing Reuse Reuse Reuse New Expiration
time time RRSIG time
| | | | | | |
|------------------|-------------------------------|-------|
| | | | | | |
<-----> <-----> <-----> <----->
Re-Sign Period
| Refresh |
|<----------->|
| Period |
Figure 11: Signature Timing Parameters
Note that in the figure the validity of the signature starts shortly
before the signing time. That is done to deal with validators that
might have some clock skew. This is called the inception offset, and
it should be chosen so that false negatives are minimized to a
reasonable level.
4.4.2.3. Differentiation between RRsets
It is possible to vary signature validity periods between signatures
over different RRsets in the zone. In practice, this could be done
when zones contain highly volatile data (which may be the case in
dynamic-update environments). Note, however, that the risk of replay
(e.g., by stale secondary servers) should be the leading factor in
determining the signature validity period, since the TTLs on the data
itself are still the primary parameter for cache expiry.
In some cases, the risk of replaying existing data might be different
from the risk of replaying the denial of data. In those cases, the
signature validity period on NSEC or NSEC3 records may be tweaked
accordingly.
When a zone contains secure delegations, then a relatively short
signature validity period protects the child against replay attacks
in the case where the child's key is compromised (see Section 4.3.4).
Since there is a higher operational risk for the parent registry when
choosing a short validity period and a higher operational risk for
the child when choosing a long validity period, some (price)
differentiation may occur for validity periods between individual DS
RRs in a single zone.
There seem to be no other arguments for differentiation in validity
periods.
5. "Next Record" Types
One of the design tradeoffs made during the development of DNSSEC was
to separate the signing and serving operations instead of performing
cryptographic operations as DNS requests are being serviced. It is
therefore necessary to create records that cover the very large
number of nonexistent names that lie between the names that do exist.
There are two mechanisms to provide authenticated proof of
nonexistence of domain names in DNSSEC: a clear-text one and an
obfuscated-data one. Each mechanism:
o includes a list of all the RRTYPEs present, which can be used to
prove the nonexistence of RRTYPEs at a certain name;
o stores only the name for which the zone is authoritative (that is,
glue in the zone is omitted); and
o uses a specific RRTYPE to store information about the RRTYPEs
present at the name: The clear-text mechanism uses NSEC, and the
obfuscated-data mechanism uses NSEC3.
5.1. Differences between NSEC and NSEC3
The clear-text mechanism (NSEC) is implemented using a sorted linked
list of names in the zone. The obfuscated-data mechanism (NSEC3) is
similar but first hashes the names using a one-way hash function,
before creating a sorted linked list of the resulting (hashed)
strings.
The NSEC record requires no cryptographic operations aside from the
validation of its associated signature record. It is human readable
and can be used in manual queries to determine correct operation.
The disadvantage is that it allows for "zone walking", where one can
request all the entries of a zone by following the linked list of
NSEC RRs via the "Next Domain Name" field. Though all agree that DNS
data is accessible through query mechanisms, for some zone
administrators this behavior is undesirable for policy, regulatory,
or other reasons.
Furthermore, NSEC requires a signature over every RR in the zone
file, thereby ensuring that any denial of existence is
cryptographically signed. However, in a large zone file containing
many delegations, very few of which are to signed zones, this may
produce unacceptable additional overhead, especially where insecure
delegations are subject to frequent updates (a typical example might
be a TLD operator with few registrants using secure delegations).
NSEC3 allows intervals between two secure delegations to "opt out",
in which case they may contain one or more insecure delegations, thus
reducing the size and cryptographic complexity of the zone at the
expense of the ability to cryptographically deny the existence of
names in a specific span.
The NSEC3 record uses a hashing method of the requested name. To
increase the workload required to guess entries in the zone, the
number of hashing iterations can be specified in the NSEC3 record.
Additionally, a salt can be specified that also modifies the hashes.
Note that NSEC3 does not give full protection against information
leakage from the zone (you can still derive the size of the zone,
which RRTYPEs are in there, etc.).
5.2. NSEC or NSEC3
The first motivation to deploy NSEC3 -- prevention of zone
enumeration -- only makes sense when zone content is not highly
structured or trivially guessable. Highly structured zones, such as
in-addr.arpa., ip6.arpa., and e164.arpa., can be trivially enumerated
using ordinary DNS properties, while for small zones that only
contain records in the apex of the zone and a few common names such
as "www" or "mail", guessing zone content and proving completeness is
also trivial when using NSEC3. In these cases, the use of NSEC is
preferred to ease the work required by signers and validating
resolvers.
For large zones where there is an implication of "not readily
available" names, such as those where one has to sign a non-
disclosure agreement before obtaining it, NSEC3 is preferred. The
second reason to consider NSEC3 is "Opt-Out", which can reduce the
number of NSEC3 records required. This is discussed further below
(Section 5.3.4).
5.3. NSEC3 Parameters
NSEC3 is controlled by a number of parameters, some of which can be
varied: This section discusses the choice of those parameters.
5.3.1. NSEC3 Algorithm
The NSEC3 hashing algorithm is performed on the Fully Qualified
Domain Name (FQDN) in its uncompressed form. This ensures that brute
force work done by an attacker for one FQDN cannot be reused for
another FQDN attack, as these entries are by definition unique.
At the time of this writing, there is only one NSEC3 hash algorithm
defined. [RFC5155] specifically states: "When specifying a new hash
algorithm for use with NSEC3, a transition mechanism MUST also be
defined". Therefore, this document does not consider NSEC3 hash
algorithm transition.
5.3.2. NSEC3 Iterations
One of the concerns with NSEC3 is that a pre-calculated dictionary
attack could be performed in order to assess whether or not certain
domain names exist within a zone. Two mechanisms are introduced in
the NSEC3 specification to increase the costs of such dictionary
attacks: iterations and salt.
The iterations parameter defines the number of additional times the
hash function has been performed. A higher value results in greater
resiliency against dictionary attacks, at a higher computational cost
for both the server and resolver.
RFC 5155 Section 10.3 [RFC5155] considers the tradeoffs between
incurring cost during the signing process and imposing costs to the
validating name server, while still providing a reasonable barrier
against dictionary attacks. It provides useful limits of iterations
for a given RSA key size. These are 150 iterations for 1024-bit
keys, 500 iterations for 2048-bit keys, and 2,500 iterations for
4096-bit keys. Choosing a value of 100 iterations is deemed to be a
sufficiently costly, yet not excessive, value: In the worst-case
scenario, the performance of name servers would be halved, regardless
of key size [NSEC3-HASH-PERF].
5.3.3. NSEC3 Salt
While the NSEC3 iterations parameter increases the cost of hashing a
dictionary word, the NSEC3 salt reduces the lifetime for which that
calculated hash can be used. A change of the salt value by the zone
administrator would cause an attacker to lose all pre-calculated work
for that zone.
There must be a complete NSEC3 chain using the same salt value, that
matches the salt value in the NSEC3PARAM record. NSEC3 salt changes
do not need special rollover procedures. Since changing the salt
requires that all the NSEC3 records be regenerated and thus requires
generating new RRSIGs over these NSEC3 records, it makes sense to
align the change of the salt with a change of the Zone Signing Key,
as that process in itself already usually requires that all RRSIGs be
regenerated. If there is no critical dependency on incremental
signing and the zone can be signed with little effort, there is no
need for such alignment.
5.3.4. Opt-Out
The Opt-Out mechanism was introduced to allow for a gradual
introduction of signed records in zones that contain mostly
delegation records. The use of the Opt-Out flag changes the meaning
of the NSEC3 span from authoritative denial of the existence of names
within the span to proof that DNSSEC is not available for the
delegations within the span. This allows for the addition or removal
of the delegations covered by the span without recalculating or re-
signing RRs in the NSEC3 RR chain.
Opt-Out is specified to be used only over delegation points and will
therefore only bring relief to zones with a large number of insecure
delegations. This consideration typically holds for large TLDs and
similar zones; in most other circumstances, Opt-Out should not be
deployed. Further considerations can be found in Section 12.2 of RFC
5155 [RFC5155].
6. Security Considerations
DNSSEC adds data origin authentication and data integrity to the DNS,
using digital signatures over Resource Record sets. DNSSEC does not
protect against denial-of-service attacks, nor does it provide
confidentiality. For more general security considerations related to
DNSSEC, please see RFC 4033 [RFC4033], RFC 4034 [RFC4034], and
RFC 4035 [RFC4035].
This document tries to assess the operational considerations to
maintain a stable and secure DNSSEC service. When performing key
rollovers, it is important to keep in mind that it takes time for the
data to be propagated to the verifying clients. It is also important
to note that this data may be cached. Not taking into account the
'data propagation' properties in the DNS may cause validation
failures, because cached data may mismatch data fetched from the
authoritative servers; this will make secured zones unavailable to
security-aware resolvers.
7. Contributors and Acknowledgments
Significant parts of the text of this document are copied from
RFC 4641 [RFC4641]. That document was edited by Olaf Kolkman and
Miek Gieben. Other people that contributed or were otherwise
involved in that work were, in random order: Rip Loomis, Olafur
Gudmundsson, Wesley Griffin, Michael Richardson, Scott Rose, Rick van
Rein, Tim McGinnis, Gilles Guette, Olivier Courtay, Sam Weiler, Jelte
Jansen, Niall O'Reilly, Holger Zuleger, Ed Lewis, Hilarie Orman,
Marcos Sanz, Peter Koch, Mike StJohns, Emma Bretherick, Adrian
Bedford, Lindy Foster, and O. Courtay.
For this version of the document, we would like to acknowledge a few people for significant contributions:
who were actively involved in the compilation of the document. In
random order: Mark Andrews, Patrik Faltstrom, Tony Finch, Alfred
Hoenes, Bill Manning, Scott Rose, Wouter Wijngaards, Antoin
Verschuren, Marc Lampo, George Barwood, Sebastian Castro, Suresh
Krishnaswamy, Eric Rescorla, Stephen Morris, Olafur Gudmundsson,
Ondrej Sury, and Rickard Bellgrim.
8. Contributors
Significant contributions to this document were from:
Paul Hoffman, for his contribution who contributed on the choice of cryptographic
parameters and addressing some of the trust anchor issues;
Jelte Jansen, who provided the initial text in Section 4.1.4;
Paul Wouters, who provided the initial text for Section 5, and
Alex Bligh, who improved it;
Eric Rescorla, whose blog post on "the Security of ZSK rollovers"
inspired text in Section 3.1;
Stephen Morris, who made a pass on English style and grammar;
Olafur Gudmundsson and Ondrej Sury, who provided input on
Section 4.1.4 based on actual operational experience; and
Rickard Bellgrim, who reviewed the document extensively. it.
The figure in Section 4.4.2 was adapted from the OpenDNSSEC user
documentation.
In addition, valuable contributions in the form of text, comments, or
review were provided by Mark Andrews, Patrik Faltstrom, Tony Finch,
Alfred Hoenes, Bill Manning, Scott Rose, Wouter Wijngaards, Antoin
Verschuren, Marc Lampo, George Barwood, Sebastian Castro, and Suresh
Krishnaswamy.
8.
9. References
8.1.
9.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[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.
[RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
(DS) Resource Records (RRs)", RFC 4509, May 2006.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
[RFC5702] Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
and RRSIG Resource Records for DNSSEC", RFC 5702,
October 2009.
8.2.
9.2. Informative References
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
August 1996.
[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[RFC3375] Hollenbeck, S., "Generic Registry-Registrar Protocol
Requirements", RFC 3375, September 2002.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
RFC 4641, September 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", RFC 5011, September 2007.
[RFC5910] Gould, J. and S. Hollenbeck, "Domain Name System (DNS)
Security Extensions Mapping for the Extensible
Provisioning Protocol (EPP)", RFC 5910, May 2010.
[RFC5933] Dolmatov, V., Chuprina, A., and I. Ustinov, "Use of GOST
Signature Algorithms in DNSKEY and RRSIG Resource Records
for DNSSEC", RFC 5933, July 2010.
[RFC6605] Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital
Signature Algorithm (DSA) for DNSSEC", RFC 6605,
April 2012.
[NIST-Workshop]
Rose, S., "NIST DNSSEC workshop notes", July 2001, <http:/
/www.ietf.org/mail-archive/web/dnsop/current/
msg01020.html>.
[NIST-SP-800-90A]
Barker, E. and J. Kelsey, "Recommendation for Random
Number Generation Using Deterministic Random Bit
Generators", NIST Special Publication 800-90A,
January 2012.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[DNSSEC-KEY-TIMING]
Morris, S., Ihren, J., and J. Dickinson, "DNSSEC Key
Timing Considerations", Work in Progress, July 2012.
[DNSSEC-DPS]
Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
Framework for DNSSEC Policies and DNSSEC Practice
Statements", Work in Progress, September 2012.
[DNSSEC-TRUST-ANCHOR]
Larson, M. and O. Gudmundsson, "DNSSEC Trust Anchor
Configuration and Maintenance", Work in Progress,
October 2010.
[NSEC3-HASH-PERF]
Schaeffer, Y., "NSEC3 Hash Performance", NLnet Labs
document 2010-002, March 2010.
Appendix A. Terminology
In this document, there is some jargon used that is defined in other
documents. In most cases, we have not copied the text from the
documents defining the terms but have given a more elaborate
explanation of the meaning. Note that these explanations should not
be seen as authoritative.
Anchored key: A DNSKEY configured in resolvers around the globe.
This key is hard to update, hence the term 'anchored'.
Bogus: Also see Section 5 of RFC 4033 [RFC4033]. An RRset in DNSSEC
is marked "Bogus" when a signature of an RRset does not validate
against a DNSKEY.
Key rollover: A key rollover (also called key supercession in some
environments) is the act of replacing one key pair with another at
the end of a key effectivity period.
Key Signing Key or KSK: A Key Signing Key (KSK) is a key that is
used exclusively for signing the apex key set. The fact that a
key is a KSK is only relevant to the signing tool.
Key size: The term 'key size' can be substituted by 'modulus size'
throughout the document for RSA keys. It is mathematically more
correct to use modulus size for RSA keys, but as this is a
document directed at operators we feel more at ease with the term
'key size'.
Private and public keys: DNSSEC secures the DNS through the use of
public-key cryptography. Public-key cryptography is based on the
existence of two (mathematically related) keys, a public key and a
private key. The public keys are published in the DNS by the use
of the DNSKEY Resource Record (DNSKEY RR). Private keys should
remain private.
Refresh Period: The period before the expiration time of the
signature, during which the signature is refreshed by the signer.
Re-Sign Period: This refers to the frequency with which a signing
pass on the zone is performed. The Re-Sign Period defines when
the zone is exposed to the signer. And on the signer, not all
signatures in the zone have to be regenerated: That depends on the
Refresh Period.
Secure Entry Point (SEP) key: A KSK that has a DS record in the
parent zone pointing to it or that is configured as a trust
anchor. Although not required by the protocol, we suggest that
the SEP flag [RFC4034] be set on these keys.
Self-signature: This only applies to signatures over DNSKEYs; a
signature made with DNSKEY x over DNSKEY x is called a self-
signature. Note: Without further information, self-signatures
convey no trust. They are useful to check the authenticity of the
DNSKEY, i.e., they can be used as a hash.
Signing jitter: A random variation in the signature validity period
of RRSIGs in a zone to prevent all of them from expiring at the
same time.
Signer: The system that has access to the private key material and
signs the Resource Record sets in a zone. A signer may be
configured to sign only parts of the zone, e.g., only those RRsets
for which existing signatures are about to expire.
Singing the zone file: The term used for the event where an
administrator joyfully signs its zone file while producing melodic
sound patterns.
Single-Type Signing Scheme: A signing scheme whereby the distinction
between Zone Signing Keys and Key Signing Keys is not made.
Zone administrator: The 'role' that is responsible for signing a
zone and publishing it on the primary authoritative server.
Zone Signing Key (ZSK): A key that is used for signing all data in a
zone (except, perhaps, the DNSKEY RRset). The fact that a key is
a ZSK is only relevant to the signing tool.
Appendix B. Typographic Conventions
The following typographic conventions are used in this document:
Key notation: A key is denoted by DNSKEY_x_y, where x is an
identifier for the type of key: K for Key Signing Key, Z for Zone
Signing Key, and S when there is no distinction made between KSKs
and ZSKs but the key is used as a secure entry point. The 'y'
denotes a number or an identifier; y could be thought of as the
key id.
RRsets ignored: If the signatures of non-DNSKEY RRsets have the same
parameters as the SOA, then those are not mentioned; e.g., in the
example below, the SOA is signed with the same parameters as the
foo.example.com A RRset and the latter is therefore ignored in the
abbreviated notation.
RRset notations: RRs are only denoted by the type. All other
information -- owner, class, rdata, and TTL -- is left out. Thus:
"example.com 3600 IN A 192.0.2.1" is reduced to "A". RRsets are a
list of RRs. An example of this would be "A1, A2", specifying the
RRset containing two "A" records. This could again be abbreviated
to just "A".
Signature notation: Signatures are denoted as RRSIG_x_y(type), which
means that the RRset with the specific RRTYPE 'type' is signed
with DNSKEY_x_y. Signatures in the parent zone are denoted as
RRSIG_par(type).
SOA representation: SOAs are represented as SOA_x, where x is the
serial number.
DS representation: DSs are represented as DS_x_y, where x and y are
identifiers similar to the key notation: x is an identifier for
the type of key the DS record refers to; y is the 'key id' of the
key it refers to.
Zone representation: Using the above notation we have simplified the
representation of a signed zone by leaving out all unnecessary
details, such as the names, and by representing all data by
"SOA_x".
Using this notation, the following signed zone:
example.com. 3600 IN SOA ns1.example.com. olaf.example.net. (
2005092303 ; serial
450 ; refresh (7 minutes 30 seconds)
600 ; retry (10 minutes)
345600 ; expire (4 days)
300 ; minimum (5 minutes)
)
3600 RRSIG SOA 5 2 3600 20120824013000 (
20100424013000 14 example.com.
NMafnzmmZ8wevpCOI+/JxqWBzPxrnzPnSXfo
...
OMY3rTMA2qorupQXjQ== )
3600 NS ns1.example.com.
3600 NS ns2.example.com.
3600 NS ns3.example.com.
3600 RRSIG NS 5 2 3600 20120824013000 (
20100424013000 14 example.com.
p0Cj3wzGoPFftFZjj3jeKGK6wGWLwY6mCBEz
...
+SqZIoVHpvE7YBeH46wuyF8w4XknA4Oeimc4
zAgaJM/MeG08KpeHhg== )
3600 TXT "Net::DNS domain"
3600 RRSIG TXT 5 2 3600 20120824013000 (
20100424013000 14 example.com.
o7eP8LISK2TEutFQRvK/+U3wq7t4X+PQaQkp
...
BcQ1o99vwn+IS4+J1g== )
300 NSEC foo.example.com. NS SOA TXT RRSIG NSEC DNSKEY
300 RRSIG NSEC 5 2 300 20120824013000 (
20100424013000 14 example.com.
JtHm8ta0diCWYGu/TdrE1O1sYSHblN2i/IX+
...
PkXNI/Vgf4t3xZaIyw== )
3600 DNSKEY 256 3 5 (
AQPaoHW/nC0fj9HuCW3hACSGiP0AkPS3dQFX
...
sAuryjQ/HFa5r4mrbhkJ
) ; key id = 14
3600 DNSKEY 257 3 5 (
AQPUiszMMAi36agx/V+7Tw95l8PYmoVjHWvO
...
oy88Nh+u2c9HF1tw0naH
) ; key id = 15
3600 RRSIG DNSKEY 5 2 3600 20120824013000 (
20100424013000 14 example.com.
HWj/VEr6p/FiUUiL70QQWtk+NBIlsJ9mdj5U
...
QhhmMwV3tIxJk2eDRQ== )
3600 RRSIG DNSKEY 5 2 3600 20120824013000 (
20100424013000 15 example.com.
P47CUy/xPV8qIEuua4tMKG6ei3LQ8RYv3TwE
...
JWL70YiUnUG3m9OL9w== )
foo.example.com. 3600 IN A 192.0.2.2
3600 RRSIG A 5 3 3600 20120824013000 (
20100424013000 14 example.com.
xHr023P79YrSHHMtSL0a1nlfUt4ywn/vWqsO
...
JPV/SA4BkoFxIcPrDQ== )
300 NSEC example.com. A RRSIG NSEC
300 RRSIG NSEC 5 3 300 20120824013000 (
20100424013000 14 example.com.
Aaa4kgKhqY7Lzjq3rlPlFidymOeBEK1T6vUF
...
Qe000JyzObxx27pY8A== )
is reduced to the following representation:
SOA_2005092303
RRSIG_Z_14(SOA_2005092303)
DNSKEY_K_14
DNSKEY_Z_15
RRSIG_K_14(DNSKEY)
RRSIG_Z_15(DNSKEY)
The rest of the zone data has the same signature as the SOA record,
i.e., an RRSIG created with DNSKEY_K_14.
Appendix C. Transition Figures for Special Cases of Algorithm Rollovers
The figures in this appendix complement and illustrate the special
cases of algorithm rollovers as described in Section 4.1.4.
----------------------------------------------------------------
initial new RRSIGs new DNSKEY
----------------------------------------------------------------
Parent:
SOA_0 -------------------------------------------------------->
RRSIG_par(SOA) ----------------------------------------------->
DS_S_1 ------------------------------------------------------->
RRSIG_par(DS_S_1) -------------------------------------------->
Child:
SOA_0 SOA_1 SOA_2
RRSIG_S_1(SOA) RRSIG_S_1(SOA) RRSIG_S_1(SOA)
RRSIG_S_2(SOA) RRSIG_S_2(SOA)
DNSKEY_S_1 DNSKEY_S_1 DNSKEY_S_1
DNSKEY_S_2
RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY)
RRSIG_S_2(DNSKEY) RRSIG_S_2(DNSKEY)
----------------------------------------------------------------
new DS DNSKEY removal RRSIGs removal
----------------------------------------------------------------
Parent:
SOA_1 ------------------------------------------------------->
RRSIG_par(SOA) ---------------------------------------------->
DS_S_2 ------------------------------------------------------>
RRSIG_par(DS_S_2) ------------------------------------------->
Child:
-------------------> SOA_3 SOA_4
-------------------> RRSIG_S_1(SOA)
-------------------> RRSIG_S_2(SOA) RRSIG_S_2(SOA)
------------------->
-------------------> DNSKEY_S_2 DNSKEY_S_2
-------------------> RRSIG_S_1(DNSKEY)
-------------------> RRSIG_S_2(DNSKEY) RRSIG_S_2(DNSKEY)
----------------------------------------------------------------
Figure 12: Single-Type Signing Scheme Algorithm Roll
Also see Section 4.1.4.1.
----------------------------------------------------------------
initial new RRSIGs new DNSKEY
----------------------------------------------------------------
Parent:
SOA_0 -------------------------------------------------------->
RRSIG_par(SOA) ----------------------------------------------->
DS_K_1 ------------------------------------------------------->
RRSIG_par(DS_K_1) -------------------------------------------->
Child:
SOA_0 SOA_1 SOA_2
RRSIG_Z_1(SOA) RRSIG_Z_1(SOA) RRSIG_Z_1(SOA)
RRSIG_Z_2(SOA) RRSIG_Z_2(SOA)
DNSKEY_K_1 DNSKEY_K_1 DNSKEY_K_1
DNSKEY_K_2
DNSKEY_Z_1 DNSKEY_Z_1 DNSKEY_Z_1
DNSKEY_Z_2
RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY) RRSIG_K_1(DNSKEY)
RRSIG_K_2(DNSKEY)
----------------------------------------------------------------
new DS revoke DNSKEY DNSKEY removal
----------------------------------------------------------------
Parent:
SOA_1 ------------------------------------------------------->
RRSIG_par(SOA) ---------------------------------------------->
DS_K_2 ------------------------------------------------------>
RRSIG_par(DS_K_2) ------------------------------------------->
Child:
-------------------> SOA_3 SOA_4
-------------------> RRSIG_Z_1(SOA) RRSIG_Z_1(SOA)
-------------------> RRSIG_Z_2(SOA) RRSIG_Z_2(SOA)
-------------------> DNSKEY_K_1_REVOKED
-------------------> DNSKEY_K_2 DNSKEY_K_2
------------------->
-------------------> DNSKEY_Z_2 DNSKEY_Z_2
-------------------> RRSIG_K_1(DNSKEY)
-------------------> RRSIG_K_2(DNSKEY) RRSIG_K_2(DNSKEY)
----------------------------------------------------------------
RRSIGs removal
----------------------------------------------------------------
Parent:
------------------------------------->
------------------------------------->
------------------------------------->
------------------------------------->
Child:
SOA_5
RRSIG_Z_2(SOA)
DNSKEY_K_2
DNSKEY_Z_2
RRSIG_K_2(DNSKEY)
----------------------------------------------------------------
Figure 13: RFC 5011 Style Algorithm Roll
Also see Section 4.1.4.2.
----------------------------------------------------------------
initial new RRSIGs new DNSKEY
----------------------------------------------------------------
Parent:
SOA_0 -------------------------------------------------------->
RRSIG_par(SOA) ----------------------------------------------->
DS_S_1 ------------------------------------------------------->
RRSIG_par(DS_S_1) -------------------------------------------->
Child:
SOA_0 SOA_1 SOA_2
RRSIG_S_1(SOA)
RRSIG_Z_10(SOA) RRSIG_Z_10(SOA) RRSIG_Z_10(SOA)
RRSIG_S_2(SOA) RRSIG_S_2(SOA)
DNSKEY_S_1 DNSKEY_S_1 DNSKEY_S_1
DNSKEY_Z_10 DNSKEY_Z_10 DNSKEY_Z_10
DNSKEY_S_2
RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY)
RRSIG_S_2(DNSKEY) RRSIG_S_2(DNSKEY)
----------------------------------------------------------------
new DS revoke DNSKEY DNSKEY removal
----------------------------------------------------------------
Parent:
SOA_1 ------------------------------------------------------->
RRSIG_par(SOA) ---------------------------------------------->
DS_S_2 ------------------------------------------------------>
RRSIG_par(DS_S_2) ------------------------------------------->
Child:
-------------------> SOA_3 SOA_4
-------------------> RRSIG_Z_10(SOA)
-------------------> RRSIG_S_2(SOA) RRSIG_S_2(SOA)
-------------------> DNSKEY_S_1_REVOKED
-------------------> DNSKEY_Z_10
-------------------> DNSKEY_S_2 DNSKEY_S_2
-------------------> RRSIG_S_1(DNSKEY) RRSIG_S_1(DNSKEY)
-------------------> RRSIG_S_2(DNSKEY) RRSIG_S_2(DNSKEY)
----------------------------------------------------------------
RRSIGs removal
----------------------------------------------------------------
Parent:
------------------------------------->
------------------------------------->
------------------------------------->
------------------------------------->
Child:
SOA_5
RRSIG_S_2(SOA)
DNSKEY_S_2
RRSIG_S_2(DNSKEY)
----------------------------------------------------------------
Figure 14: RFC 5011 Algorithm Roll in a Single-Type Signing Scheme
Environment
Also see Section 4.1.4.3.
Appendix D. Transition Figure for Changing DNS Operators
The figure in this Appendix complements and illustrates the special
case of changing DNS operators as described in Section 4.3.5.1.
------------------------------------------------------------
new DS | pre-publish |
------------------------------------------------------------
Parent:
NS_A NS_A
DS_A DS_B DS_A DS_B
------------------------------------------------------------
Child at A: Child at A: Child at B:
SOA_A0 SOA_A1 SOA_B0
RRSIG_Z_A(SOA) RRSIG_Z_A(SOA) RRSIG_Z_B(SOA)
NS_A NS_A NS_B
RRSIG_Z_A(NS) NS_B RRSIG_Z_B(NS)
RRSIG_Z_A(NS)
DNSKEY_Z_A DNSKEY_Z_A DNSKEY_Z_A
DNSKEY_Z_B DNSKEY_Z_B
DNSKEY_K_A DNSKEY_K_A DNSKEY_K_B
RRSIG_K_A(DNSKEY) RRSIG_K_A(DNSKEY) RRSIG_K_A(DNSKEY)
RRSIG_K_B(DNSKEY) RRSIG_K_B(DNSKEY)
------------------------------------------------------------
------------------------------------------------------------
re-delegation | post-migration |
------------------------------------------------------------
Parent:
NS_B NS_B
DS_A DS_B DS_B
------------------------------------------------------------
Child at A: Child at B: Child at B:
SOA_A1 SOA_B0 SOA_B1
RRSIG_Z_A(SOA) RRSIG_Z_B(SOA) RRSIG_Z_B(SOA)
NS_A NS_B NS_B
NS_B RRSIG_Z_B(NS) RRSIG_Z_B(NS)
RRSIG_Z_A(NS)
DNSKEY_Z_A DNSKEY_Z_A
DNSKEY_Z_B DNSKEY_Z_B DNSKEY_Z_B
DNSKEY_K_A DNSKEY_K_B DNSKEY_K_B
RRSIG_K_A(DNSKEY) RRSIG_K_B(DNSKEY) RRSIG_K_B(DNSKEY)
------------------------------------------------------------
Figure 15: An Alternative Rollover Approach for Cooperating Operators
Appendix E. Summary of Changes from RFC 4641
This document differs from RFC 4641 [RFC4641] in the following ways:
o Addressed the errata listed on
<http://www.rfc-editor.org/errata_search.php?rfc=4641>.
o Recommended RSA/SHA-256 in addition to RSA/SHA-1.
o Did a complete rewrite of Section 3.5 of RFC 4641 (Section 3.4.2
of this document), removing the table and suggesting a key size of
1024 for keys in use for less than 8 years, issued up to at least
2015.
o Removed the KSK for high-level zones consideration.
o Added text on algorithm rollover.
o Added text on changing (non-cooperating) DNS registrars.
o Did a significant rewrite of Section 3, whereby the argument is
made that the timescales for rollovers are made purely on
operational arguments.
o Added Section 5.
o Introduced Single-Type Signing Scheme terminology and made the
arguments for the choice of a Single-Type Signing Scheme more
explicit.
o Added a section about stand-by keys.
Authors' Addresses
Olaf M. Kolkman
NLnet Labs
Science Park 400
Amsterdam 1098 XH
The Netherlands
EMail: olaf@nlnetlabs.nl
URI: http://www.nlnetlabs.nl
W. (Matthijs) Mekking
NLnet Labs
Science Park 400
Amsterdam 1098 XH
The Netherlands
EMail: matthijs@nlnetlabs.nl
URI: http://www.nlnetlabs.nl
R. (Miek) Gieben
SIDN Labs
Meander 501
Arnhem 6825 MD
The Netherlands
EMail: miek.gieben@sidn.nl
URI: http://www.sidn.nl