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Versions: (RFC 4641) 00 01 02 03 04 05 06 07
08 09 10 11 12 13 RFC 6781
DNSOP O. Kolkman
Internet-Draft W. Mekking
Obsoletes: 2541 (if approved) NLnet Labs
Intended status: Informational Oct 31, 2011
Expires: May 3, 2012
DNSSEC Operational Practices, Version 2
draft-ietf-dnsop-rfc4641bis-08
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 Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on May 3, 2012.
Copyright Notice
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Copyright (c) 2011 IETF Trust and the persons identified as the
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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. Keys Generation and Storage . . . . . . . . . . . . . . . . . 7
3.1. Operational Motivation for Zone Signing 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 . . . . . . . . . . . . . . . 13
3.4.1. Key Algorithm . . . . . . . . . . . . . . . . . . . . 13
3.4.2. Key Sizes . . . . . . . . . . . . . . . . . . . . . . 14
3.4.3. Private Key Storage . . . . . . . . . . . . . . . . . 15
3.4.4. Key Generation . . . . . . . . . . . . . . . . . . . . 16
3.4.5. Differentiation for 'High-Level' Zones? . . . . . . . 17
4. Signature Generation, Key Rollover, and Related Policies . . . 17
4.1. Key Rollovers . . . . . . . . . . . . . . . . . . . . . . 17
4.1.1. Zone Signing Key Rollovers . . . . . . . . . . . . . . 17
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4.1.1.1. Pre-Publish Zone Signing Key Rollover . . . . . . 18
4.1.1.2. Double Signature Zone Signing Key Rollover . . . . 20
4.1.1.3. Pros and Cons of the Schemes . . . . . . . . . . . 22
4.1.2. Key Signing Key Rollovers . . . . . . . . . . . . . . 22
4.1.2.1. Special Considerations for RFC5011 KSK rollover . 24
4.1.3. Difference Between ZSK and KSK Rollovers . . . . . . . 24
4.1.4. Rollover for a Single Type Signing Key rollover . . . 25
4.1.5. Algorithm rollovers . . . . . . . . . . . . . . . . . 27
4.1.5.1. Single Type Signing Scheme Algorithm Rollover . . 31
4.1.5.2. Algorithm rollover, RFC5011 style . . . . . . . . 31
4.1.5.3. Single Signing Type Algorithm Rollover,
RFC5011 style . . . . . . . . . . . . . . . . . . 32
4.1.5.4. NSEC to NSEC3 algorithm rollover . . . . . . . . . 33
4.1.6. Considerations for Automated Key Rollovers . . . . . . 33
4.2. Planning for Emergency Key Rollover . . . . . . . . . . . 34
4.2.1. KSK Compromise . . . . . . . . . . . . . . . . . . . . 34
4.2.1.1. Keeping the Chain of Trust Intact . . . . . . . . 35
4.2.1.2. Breaking the Chain of Trust . . . . . . . . . . . 36
4.2.2. ZSK Compromise . . . . . . . . . . . . . . . . . . . . 36
4.2.3. Compromises of Keys Anchored in Resolvers . . . . . . 36
4.2.4. Stand-by keys . . . . . . . . . . . . . . . . . . . . 37
4.3. Parent Policies . . . . . . . . . . . . . . . . . . . . . 37
4.3.1. Initial Key Exchanges and Parental Policies
Considerations . . . . . . . . . . . . . . . . . . . . 37
4.3.2. Storing Keys or Hashes? . . . . . . . . . . . . . . . 38
4.3.3. Security Lameness . . . . . . . . . . . . . . . . . . 39
4.3.4. DS Signature Validity Period . . . . . . . . . . . . . 39
4.3.5. Changing DNS Operators . . . . . . . . . . . . . . . . 40
4.3.5.1. Cooperating DNS operators . . . . . . . . . . . . 40
4.3.5.2. Non Cooperating DNS Operators . . . . . . . . . . 42
4.4. Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . 43
4.4.1. Time Considerations . . . . . . . . . . . . . . . . . 44
4.4.2. Signature Validation Periods . . . . . . . . . . . . . 46
4.4.2.1. Maximum Value . . . . . . . . . . . . . . . . . . 46
4.4.2.2. Minimum Value . . . . . . . . . . . . . . . . . . 46
4.4.2.3. Differentiation between RR sets . . . . . . . . . 48
5. Next Record type . . . . . . . . . . . . . . . . . . . . . . . 49
5.1. Differences between NSEC and NSEC3 . . . . . . . . . . . 49
5.2. NSEC or NSEC3 . . . . . . . . . . . . . . . . . . . . . . 50
5.3. NSEC3 parameters . . . . . . . . . . . . . . . . . . . . . 51
5.3.1. NSEC3 Algorithm . . . . . . . . . . . . . . . . . . . 51
5.3.2. NSEC3 Iterations . . . . . . . . . . . . . . . . . . . 51
5.3.3. NSEC3 Salt . . . . . . . . . . . . . . . . . . . . . . 51
5.3.4. Opt-out . . . . . . . . . . . . . . . . . . . . . . . 52
6. Security Considerations . . . . . . . . . . . . . . . . . . . 52
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 52
8. Contributors and Acknowledgments . . . . . . . . . . . . . . . 52
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
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9.1. Normative References . . . . . . . . . . . . . . . . . . . 53
9.2. Informative References . . . . . . . . . . . . . . . . . . 54
Appendix A. Terminology . . . . . . . . . . . . . . . . . . . . . 55
Appendix B. Typographic Conventions . . . . . . . . . . . . . . . 57
Appendix C. Transition Figures for Special Case Algorithm
Rollovers . . . . . . . . . . . . . . . . . . . . . . 59
Appendix D. Document Editing History . . . . . . . . . . . . . . 63
D.1. draft-ietf-dnsop-rfc4641-00 . . . . . . . . . . . . . . . 63
D.2. version 0->1 . . . . . . . . . . . . . . . . . . . . . . . 64
D.3. version 1->2 . . . . . . . . . . . . . . . . . . . . . . . 64
D.4. version 2->3 . . . . . . . . . . . . . . . . . . . . . . . 65
D.5. version 3->4 . . . . . . . . . . . . . . . . . . . . . . . 66
D.6. version 4->5 . . . . . . . . . . . . . . . . . . . . . . . 66
D.7. version 5->6 . . . . . . . . . . . . . . . . . . . . . . . 66
D.8. version 6->7 . . . . . . . . . . . . . . . . . . . . . . . 66
D.9. version 7->8 . . . . . . . . . . . . . . . . . . . . . . . 67
D.10. Subversion information . . . . . . . . . . . . . . . . . . 67
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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 [1] and RFC 1035 [2]) and want to deploy DNSSEC
(RFC 4033 [3], RFC 4034 [4], and RFC 4035 [5]). 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 relation 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. At the
time of writing -the root has just been signed and the first secure
delegations are provisioned- there exists relatively little
experience with DNSSEC in production environments below the 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 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. Section 4.1 and Section 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 keys publication. Section 5 covers
the considerations regarding selecting and using NSEC and NSEC3.
The typographic conventions used in this document are explained in
Appendix B.
Since this is a document with operational suggestions and there are
no protocol specifications, the RFC 2119 [6] language does not apply.
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This document obsoletes RFC 4641 [14].
1.1. The Use of the Term 'key'
It is assumed that the reader is familiar with the concept of
asymmetric keys on which DNSSEC is based (public key cryptography
RFC4949 [15]). 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 and that it is the
public part that is used in key exchanges.
1.2. Time Definitions
In this document, we will be using a number of time-related terms.
The following definitions apply:
o "Signature validity period" The period that a signature is valid.
It starts at the time specified in the signature inception field
of the RRSIG RR and ends at the time specified in the expiration
field of the RRSIG RR.
o "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.
o "Key effectivity period" The period during which a key pair is
expected to be effective. It is defined as the time between the
first 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.
o "Maximum/Minimum Zone Time to Live (TTL)" The maximum or minimum
value of the TTLs from the complete set of RRs in a zone. Note
that the minimum TTL is not the same as the MINIMUM field in the
SOA RR. See RFC2308 [9] for more information.
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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
RFC4033 [3] 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 nameservers and
clients may be fetching data from caching non-authoritative servers.
In this light, note that the time for a zone transfer from master to
slave can be negligible when using NOTIFY [8] and incremental
transfer (IXFR) [7]. It increases when full zone transfers (AXFR)
are used in combination with NOTIFY. It increases even more if you
rely on full zone transfers based on only 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
nameserver, 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 trade-off 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. Keys Generation and Storage
This section describes a number of considerations with respect to the
use of keys. For the design of a operational procedure for key
generation and storage then a number of decisions need to be made:
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o Does one differentiate between Zone Signing 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?
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 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
case the differentiation between KSK and ZSK is not made, keys have
both the role of 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 key set is signed with the KSK, all the keys in the key
set can be used as ZSKs. If there has been an event that increases
the risk that a ZSK is compromised it can be simply dropped from the
key set. The new key set is then re-signed with the KSK.
Changing a key that is a secure entry point (SEP) for a zone can be
relatively expensive as it involves interaction with 3rd parties:
When a key is only pointed to by a DS 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
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RR: If the flag field is an odd number the RR is a KSK; otherwise it
is a ZSK.
There is also a risk that keys are compromised through theft or loss.
For keys that are installed on file-systems of nameservers 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 limitation 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 costs involved. For example, a KSK can be
stored off-line or with more limitation 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 filesystem
accessible by operators for daily routine may provide a 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 the
operational flexibility. In those cases, a KSK-ZSK split is not more
beneficial than the Single-Type signing scheme.
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 somewhere of
the order of 10 to 100, the associated key sizes will vary only about
a byte in size for symmetric keys. When translated to asymmetric
keys, is still too insignificant a size difference 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
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zone in emergency situations -- is predictable.
If the above holds 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 and that the SEP flag is exclusively set on KSKs.
3.2. Practical Consequences of KSK and ZSK Separation
A key that acts only as a Zone Signing Key can be used to sign all
the data but 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 signature validity periods on the order of days.
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 (such
as e.g. RFC5011 [16]) 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 3 schools of thought on rolling a KSK that is not a trust
anchor:
o It should be done frequently and regularly (possibly every few
months) so that a key rollover remains an operational routine.
o It should be done frequently but irregularly. Frequently meaning
every few months, again based on the argument that a rollover is a
practiced and common operational routine, and irregular meaning
with a large jitter, so that 3rd parties do not start to rely on
the key and will not be tempted to configure it as a trust-anchor.
o It should only be done when it is known or strongly suspected that
the key can be or has been compromised.
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
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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 owner 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
owner there may be stability costs for the validator administrator
that (wrongfully) configured the trust-anchor when the zone owner
roles 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 owner publishes a "DNSSEC Signing Policy and Practice Statement"
[25] that should be explicit about the fact whether the existence of
trust anchors will be taken into account in any way or not. There
may be cases where local policies enforce the configuration of trust-
anchors on zones which 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 owners 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 is smaller. In fact, the costs for those users
can be minimized by automating the rollover RFC5011 [16] and by
rolling the key regularly (and advertising such) so that the
operators of recursive nameservers will put the appropriate mechanism
in place to deal with these stability costs, or, in other words,
budget for these costs instead of incurring them unexpectedly.
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It is therefore recommended, to roll KSKs that are likely to be used
as trust-anchors, on a regular basis if and only if those rollovers
can be tracked using standardized (e.g. RFC5011) mechanisms.
3.2.3. The use of the SEP flag
The so-called Secure Entry Point (SEP) [5] 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 the failure it is used
in practice for operational purposes such as for the rollover
mechanism described in RFC5011 [16]. 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 recommended that the SEP flag is 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 KSK and ZSK is not made (i.e. for a
Single Type signing scheme) it is recommended that the SEP flag is
set on all keys.
Note that 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 of the order
of 2 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 chooses 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.
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As argued above, this annual rollover gives 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 on on-line systems 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 from Section 4.1 and Section 4.4. Key
replacement endures for a couple of Zone TTLs, depending on the
rollover scenario. Therefore, a multiple of Zone TTL is a reasonable
lower limit on the key effectivity period. Forcing a smaller key
effectivity period will result your zone to have a ever-growing
keyset.
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. If ZSK's are
maintained on cryptographic Hardware Security Modules (HSM) than the
motivation to have different key effectivity periods is weakened.
In fact, if the risk of loss, theft or other compromise is the same
for a zone and key signing key 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 (e.g. where the the costs and risk of
compromise, and the costs and risks involved with having to perform
an emergency roll are also low) that the use of a single type signing
scheme with a long key effectivity period is a good choice.
3.4. Cryptographic Considerations
3.4.1. Key Algorithm
At the time of writing, there are three types of signature algorithms
that can be used in DNSSEC: RSA, 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, DSA in context of DNSSEC is limited to the
maximum of 1024 bit keys.
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We suggest the use of RSA/SHA-256 as the preferred signature
algorithms 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 have an effective attack.
At the time of publication, it is known that the SHA-1 hash has
cryptanalysis issues and work is in progress on addressing them. We
recommend the use of public key algorithms based on hashes stronger
than SHA-1 (e.g., SHA-256) as soon as these algorithms are available
in implementations (see RFC5702 [23] and RFC4509 [20]).
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), you may want 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
(e.g. RFC3766 [12]). Signing and verifiying with a 2048-bit key
takes of course 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) slow down about eight times.
Another way to decide on the size of 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 TLS 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
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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 build 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 recommended 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.
When relying on dynamic update [10], 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 the security of the host. Although
not mandatory, one could administer a zone using a "hidden master"
scheme that minimize the risk. In this arrangement the master that
processes the updates is unavailable from general hosts on the
Internet; it is not listed in the NS RRSet. The nameservers 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.
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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 unauthorized access to the master
copy in order to prevent modification of DNS data before its 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 users).
o The costs of rolling the (compromised) keys. (The costs of
rolling a ZSK is lowest and the costs 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 [10], 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 is an
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 [13] and NIST SP 800-90 [19]. 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 strongly recommended
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.
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3.4.5. Differentiation for 'High-Level' Zones?
In an earlier version of this document (RFC4641 [14]) we made a
differentiation between key lengths for KSKs used for zones that are
high in the DNS hierarchy and those for KSKs used low down.
This distinction is now considered not relevant. 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. Designing a long-term successful attack is difficult for
keys at any level.
4. Signature Generation, Key Rollover, and Related Policies
4.1. Key Rollovers
Regardless of whether a zone uses periodic key rollovers in order to
practice for emergencies, or only rolls over keys in an emergency,
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 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".
4.1.1. Zone Signing Key Rollovers
If the choice for splitting zone and key signing keys has been made
than those two types of keys can be rolled separately and zone
signing keys can be rolled without taking into account DS records
from the parent or the configuration of such a key as trust-anchor.
For "Zone Signing Key rollovers", there are two ways to make sure
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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 schema, described in Section 4.1.1.2, uses
double signatures; the other uses key pre-publication
(Section 4.1.1.1). The pros, cons, and recommendations 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 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
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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, 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 the 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 TTL value of the
key set.
new RRSIGs: At the "new RRSIGs" stage (SOA serial 2), 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 version 1 of the zone can still be
verified with key sets fetched from version 2 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
the DNSKEY_K_1 and DNSKEY_Z_11.
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)":
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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 key 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 Maximum Zone TTL.
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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, the
Zone Signing Key.
new DNSKEY: At the "New DNSKEY" stage (SOA serial 1) 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 of the zone has expired
from remote caches. This will take at least the Maximum Zone TTL
of version 0 of the zone.
DNSKEY removal: DNSKEY_Z_10 is removed from the zone. All the
signatures from DNSKEY_Z_10 are removed from the zone. 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 the
other way around. The data from the current version can be verified
with the data from the previous version of the zone. The duration of
the "new DNSKEY" phase and the period between rollovers should be at
least the Maximum Zone TTL.
Making sure that the "new DNSKEY" phase lasts until the signature
expiration time of the data in the initial version of the zone is
recommended. This way all caches are cleared of the old signatures.
However, this duration could be considerably longer than the Maximum
Zone TTL, making the rollover a lengthy procedure.
Note that in this example we assumed that the zone was not modified
during the rollover. New data can be introduced in the zone as long
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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 steps. Also the Pre-Publish scheme involves more
parental work when used for KSK rollovers as explained in
Section 4.1.3.
Double Signature ZSK rollover: The drawback of this signing scheme
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 steps.
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.
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---------------------------------------------------------------------
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 relation, 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
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RRs at the parent.
4.1.2.1. Special Considerations for RFC5011 KSK rollover
The scenario sketched above assumes that the KSK is not in use as a
trust-anchor too but that validating nameservers exclusively depend
on the parental DS record to establish the zone's security. If it is
known that validating nameservers have configured trust-anchors then
such needs to be taken into account. Here we assume that operators
of zones will deploy RFC5011 [16] style rollovers.
RFC5011 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. Difference Between ZSK and KSK Rollovers
Note that KSK rollovers and ZSK rollovers are different in the sense
that a KSK rollover requires interaction with the parent (and
possibly replacing of trust anchors) and the ensuing delay while
waiting for it.
A ZSK rollover can be handled in two different ways, meaningful: Pre-
Publish (Section 4.1.1.1) and Double Signature (Section 4.1.1.2).
As the KSK is used to validate the key set and because the KSK is not
changed during a ZSK rollover, a cache is able to validate the new
key set of the zone. A Pre-Publish method is also possible for KSKs,
known as the Double-DS rollover. The name being a give away, the
record that needs to be pre-published is the DS RR at the parent.
The Pre-Publish method has some drawbacks for KSKs. We first
describe the rollover scheme and then indicate these drawbacks.
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--------------------------------------------------------------------
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_S_1 and DS_S_2, pointing to
DNSKEY_S_1 and DNSKEY_S_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_S_1 with
DNSKEY_S_2. Immediately after that ("DS/DNSKEY removal" phase), it
can notify the parent 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_S_2 RR and DNSKEY_S_2
using the DNS -- as DNSKEY_S_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.
4.1.4. Rollover for a Single Type Signing Key rollover
The rollover of a DNSKEY 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
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Single Type Signing Scheme is motivated by operational simplicity we
first 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 RR sets in the zone
are signed with DNSKEY_S_1.
new DNSKEY: A new key (DNSKEY_S_2) is introduced and all the RR sets
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 .
There is a second variety of this rollover during which one
introduces a new DNSKEY into the key set and signs the keyset with
both keys while signing the zone data with only the original
DNSKEY_S_1. One replaces the DNSKEY_S_1 signatures with signatures
made with DNSKEY_S_2 at the moment of DNSKEY_S_1 removal.
The second variety of this rollover can be considered when zone size
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considerations prevent the introduction of double signatures over all
of the zone data although in that case choosing for a KSK/ZSK split
may be a better option.
A Double-DS rollover scheme is compatible with a rollover using a
Single Type Signing Scheme although in order to maintain a valid
chain of trust, the zone data would need to be published with double
signatures or a double keyset. Since this leads to increase in zone
and packet size at both child and parent there are little benefits to
a Double-DS rollover with a Single Type Signing Scheme.
There is also a second variety of the Double-DS rollover during which
one introduces a new DNSKEY into the key set and submit the new DS to
the parent. The new key is not yet used to sign RRsets. One
replaces the DNSKEY_S_1 signatures with signatures made with
DNSKEY_S_2 at the moment that DNSKEY_S_2 and DS_S_2 have been
propagated.
Again, this second variety of this rollover can be considered when
zone size considerations prevent the introduction of double
signatures over all of the zone data although also in this case,
choosing for a KSK/ZSK split may be a better option.
4.1.5. Algorithm rollovers
A special class of key rollover is the one needed for a change of key
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 below.
There exist a conservative and a liberal approach for algorithm
rollover. This has to do with section 2.2 in RFC4035 [5]:
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 strict,
meaning that it expects that all RRset has a valid signature for
every algorithm signalled by the zone apex DNSKEY RRset, no matter
where this RRset is kept. Important to know is that this also
includes the resolvers cache. The liberal approach uses a more loose
interpretation of the section and limits the rule to RRsets in the
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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. in RFC4035: Zone Signing.
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 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 that 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 in this
section will explain the stages of deployment assuming the
conservative approach.
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 Key Rollover. You cannot use Pre-
Publish key rollover method when you do key algorithm rollover.
When removing an old algorithm, the DNSKEY should be removed first,
but only after the DS for the old algorithm was removed from the
parent zone.
Figure 7 describes the steps. The underscored number indicates the
algorithm and ZSK and KSK indicate the obvious difference in key use.
For example DNSKEY_K_1 is a the DNSKEY RR representing the public
part of the old key signing key of algorithm type 1 while
RRSIG_Z_2(SOA) is the RRSIG RR made with the private part of the new
zone signing key of algorithm type 2 over a SOA RR. It is assumed
that the key that signs the SOA RR also signes all other non-DNSKEY
RRset data.
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----------------------------------------------------------------
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 DNSKEY removal RRSIGs removal
----------------------------------------------------------------
Parent:
SOA_0 ------------------------------------------------------->
RRSIG_par(SOA) ---------------------------------------------->
DS_K_2 ------------------------------------------------------>
RRSIG_par(DS_K_2) ------------------------------------------->
Child:
-------------------> SOA_3 SOA_4
-------------------> RRSIG_Z_1(SOA)
-------------------> RRSIG_Z_2(SOA) RRSIG_Z_2(SOA)
------------------->
-------------------> DNSKEY_K_2 DNSKEY_K_2
------------------->
-------------------> DNSKEY_Z_2 DNSKEY_Z_2
------------------->
-------------------> RRSIG_K_2(DNSKEY) RRSIG_K_2(DNSKEY)
----------------------------------------------------------------
Figure 7: Stages of Deployment during an Algorithm Rollover
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initial: Describes state of the zone before any transition is done.
Number of the keys may vary, but the algorithm of keys in the zone
is same for all DNSKEY records.
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 you do not do that, it might happen that the
resolver picks up the new DNSKEY RRset (with the new algorithm
included), but does not yet have the corresponding new signatures,
because it has a previous version of the RRset plus signatures in
the cache.
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 cache data has expired, the new key can be
added to the zone.
new DS: After the cache data for the DNSKEY 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 DS has expired, the old
algorithm can be removed. This time the key needs to be removed
first, before removing the signatures.
RRSIGs removal: After the cache data for the DNSKEY has expired, the
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 you have
chosen not to differentiate between Zone and Key signing keys
(Section 4.1.5.1)
2: RFC5011 Algorithm Rollover : when trust-anchors can track the
roll via RFC5011 style rollover (Section 4.1.5.2)
3: 1 and 2 combined : when a Single Type Signing Scheme Algorithm
rollover is RFC5011-enabled (Section 4.1.5.3)
In addition to the narrative below these special cases are
represented in Figure 11, Figure 12 and Figure 13 in Appendix C.
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4.1.5.1. Single Type Signing Scheme Algorithm Rollover
If one key is used that acts both as ZSK and KSK, the same scheme and
figure as above applies whereby all DNSKEY_Z_* records from the table
are removed and all RRSIG_Z_* are replaced with RRSIG_K_*. 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.
Also see Figure 11 in Appendix C.
4.1.5.2. Algorithm rollover, RFC5011 style
Trust anchor algorithm rollover is almost as simple as a regular
RFC5011 based rollover. However, the old trust anchor must be
revoked before it is removed from the zone.
Take a look at the Figure 7 above. After the "new DS" step, we need
an additional step where the DNSKEY is revoked (revoke DNSKEY):
---------------------------------
revoke DNSKEY
---------------------------------
Parent:
----------------------------->
----------------------------->
----------------------------->
----------------------------->
Child:
SOA_3
RRSIG_Z_1(SOA)
RRSIG_Z_2(SOA)
DNSKEY_K_1_REVOKED
DNSKEY_K_2
DNSKEY_Z_1
DNSKEY_Z_2
RRSIG_K_1(DNSKEY)
RRSIG_K_2(DNSKEY)
--------------------------------
Figure 8: The Revoke DNSKEY state that is added to an algorithm
rollover when RFC5011 is in use.
There is one exception to the rule above. While all zone data must
be signed with an unrevoked key, it is permissible to sign the keyset
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with a revoked key. The somewhat esoteric argument follows.
Resolvers that do not understand the RFC5011 Revoke flag will handle
DNSKEY_K_1_REVOKED the same as if it was 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 RFC5011 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 means, we only need RRSIG_K_2(DNSKEY) to
authenticate the DNSKEY RRset, and we still are compliant with
section 2.2 from RFC 4035: There must be a RRSIG for each RRset using
at least one DNSKEY of each algorithm in the zone apex DNSKEY RRset.
Also see Figure 12 in Appendix C.
4.1.5.3. Single Signing Type Algorithm Rollover, RFC5011 style
Combining the Single Signing Type Scheme Algorithm Rollover and
RFC5011 style rollovers is not trivial.
Should you choose to perform an RFC5011 style rollover with a Single
Signing Type key then remember that section 2.1, 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 if you revoke DNSKEY_S_1, it cannot be used to
validate its signatures over non-DNSKEY RRsets. Thus, those RRsets
should be signed with a shadow key, DNSKEY_Z_1, during the algorithm
rollover. This shadow key can be introduced at the same time the
signatures are pre-published, in step 2 (new RRSIGs). The shadow key
must be removed at the same time the revoked DNSKEY_S_1 is removed
from the zone. De-facto you temporarily falling back to a KSK/ZSK
split model.
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. This can be
the ZSK. 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 candidate for
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revocation. That key can than temporarily act as ZSK during the
algorithm rollover.
Just like with algorithm rollover RFC5011 style, while all zone data
must be signed with an unrevoked key, it is permissible to sign the
keyset with a revoked key, for the same esoteric argument described
in Section 4.1.5.2.
The lesson of all of this is that a Single Type Signing scheme
algorithm rollover using RFC5011 is as complicated as the name of the
rollover implies, one is better off explicitly using a split key
temporarily.
Also see Figure 12 in Appendix C.
4.1.5.4. 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 RR set has
populated distant caches(at least one TTL into stage 4, or at any
time during stage 5). 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 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 when changing from
zone serial number, realizing that that involves a resigning of the
zone and the introduction of the NSECPARAM record in order to signal
authoritative servers to start serving NSEC3 authenticated denial of
existence.
Summarizing, 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.
4.1.6. 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 parent and child. Data
exchange is needed to provide the new keys to the parent;
consequently, this data must be authenticated and integrity must
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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.
Our advice is to have a documented procedure ready for when a key
compromise is suspected or confirmed.
When the private material of one of your keys is compromised it can
be used 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,
o as long as the DS RR in the parent zone points to the compromised
key,
o as long as the 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 your compromised key exists, your namespace is
vulnerable to abuse by anyone who has obtained illegitimate
possession of the key. Zone operators have to make a trade-off if
the abuse of the compromised key is worse than having data in caches
that cannot be validated. If the zone operator 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 zone containing a DNSKEY RRSet with a compromised KSK is vulnerable
as long as the compromised KSK is configured as trust anchor or a DS
record in the parent zone points to it.
A compromised KSK can be used to sign the key set of an attacker's
zone. That zone could be used to poison the DNS.
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
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is only one DS record at the parent. If there are multiple DS
records this does not apply -- however the chain of trust of this
particular key is broken).
Note that an attacker's 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 zone to appear as valid and the child's
zone as Bogus. Therefore, we advise not to remove the KSK before the
parent has a DS record for the new KSK in place.
4.2.1.1. Keeping the Chain of Trust Intact
If we follow this advice, 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. We therefore have to
make sure 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 it will expire
faster from caches.
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 DSes 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 in 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 interval.
An additional danger of a key compromise is that the compromised key
could be used to facilitate a legitimate DNSKEY/DS rollover and/or
nameserver 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.
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Note that this is only a problem when the DNSKEY and or DS records
are used for authentication at the parent.
4.2.1.2. Breaking the Chain of Trust
There are two methods to break 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 possesion 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.
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 be
still 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 validate the new key. New keys should be authenticated out-
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of-band, for example, through the use of an announcement website that
is secured using secure sockets (TLS) [22].
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 DNS operation at location A, to prepare
stand-by keys you need to:
o Generate a stand-by ZSK and KSK. Store them safely in a different
location (B) than the currently used ZSK and KSK (that are at
location A).
o Pre-publish DNSKEY RR of the stand-by ZSK in the zone.
o Pre-publish DS of the stand-by KSK in the parent zone.
Now suppose a disaster occurs at location A, that disables the access
to your currently used keys. To recover from that situation, follow
these procedures:
o Set up your DNS operations and import the stand-by keys from
location B.
o Post-publish the old ZSK and sign the zone with the stand-by keys.
o After some time, when the new signatures have been propagated, the
old ZSK and DS can be removed.
o Generate a new stand-by keyset 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 model
where the key material is to be passed from the DNS operator, to the
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(parent) registry via a registrar, where both DNS operator and
registrar are selected by the registrant and might be different
organisations. 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 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 [5] to select the proper key
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 the
child is secure.
Note: the out-of-band verification is still needed when the key
material is fetched via the DNS. The parent can never be sure
whether or not the DNSKEY RRs have been spoofed.
With some type 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 DSes. 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 recommend that
registry systems at least support storing DS records (also see
draft-ietf-dnsop-dnssec-trust-anchor [26]).
The storage considerations also relate to the design of the customer
interface and the method by which data is transferred between
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) [17], 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 for moving the
data between the customer and the registrar. Different registrars
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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 non-existing DNSKEY RR. Security lameness may occur
temporarily during a Double-DS rollover scheme. However care should
be taken that not all DS RRs are security lame which may 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 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 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 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 operator unavailability
during weekends shows the need for DS signature validity periods
longer than two days. We recommend an absolute minimum for a 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 interval
increases the operational risk for the parent. Therefore, the parent
may have policy to use a signature validity interval that is
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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 months.
In addition to the signature validity period, which sets a lower
bound on the number of times the zone owner will need to sign the
zone data and which sets an upper bound to the time 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 relation 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. (See
[11] for a comprehensive definition). Registrants may out-source the
maintenance of their DNS system, including the maintenance of DNSSEC
key material, to the registrar or to another third party, which we
will call the DNS operator. The DNS operator that has control over
the DNS zone and its keys may prevent the registrant to make a timely
move to a different 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 they have 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 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 one could proceed 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 keysets that they combine and both publish in
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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)
------------------------------------------------------------
------------------------------------------------------------
Redelegation | post migration |
------------------------------------------------------------
Parent:
NS_B NS_B
DS_B DS_B
------------------------------------------------------------
Child at A: Child at B: Child at B:
SOA_A2 SOA_B1 SOA_B2
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
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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 9: 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, 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 requirement to exchange signatures has a couple of drawbacks. It
requires more operational overhead, because not only the operators
have to exchange public keys, they also have to exchange the
signatures of the new DNSKEY RRset. Also, it disallows the children
to refresh the signatures when they expire for a certain period.
Both drawbacks do not exist if you replace the Double Signature KSK
rollover with a Double-DS KSK rollover.
Thus, if the registry and registrars allow for DS records to be
published, that do not point to a published DNSKEY in the child zone,
the Double-DS KSK rollover is preferred (also known as Pre-
Publication KSK Rollover, see Figure 5), in combination with the Pre-
Publish ZSK rollover. This does not require to share the KSK
signatures between the operators. Both the losing and the gaining
operator still need to publish the public ZSK of each other.
4.3.5.2. Non Cooperating DNS Operators
In the non-cooperative case matters are more complicated. The losing
operator may not cooperate and leave the data in the DNS as is. In
the extreme case the losing operator may become obstructive and
publish a DNSKEY RR with a high TTL and corresponding signature
validity so that registrar A's DNSKEY could end up in caches for (in
theory at least) tens of years.
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 redelegation
from the losing operator to the gaining operator has taken place.
One could imagine a rollover scenario where the gaining operator
pulls all RRSIGs created by the losing operator and publishes those
in conjunction with its own signatures, but that would not allow any
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changes in the zone content. Since a redelegation took place the NS
RRset has - by definition - changed so such 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 A's
RRSIGs are transferred.
The only viable option for the registrant is to publish its zone
unsigned and ask the registry to remove the DS RR pointing to the
losing operator's DNSKEY.
Note that some behavior of resolver implementations may aid in the
process of changing DNS operators:
o TTL sanity checking, as described in RFC2308 [9], will limit the
impact 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 to keep using a nameserver that might be undelegated by the
parent.
o Limiting the time 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 reduces the
effects of the problem of non-cooperating registars.
However, there is no operational methodology to work around this
business issue, and proper contractual relationships between all
involved parties seems 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 that zone references also
change operator, possibly involving some downtime.
In any case, to minimise such problems, the classic recommendation is
to have relative short TTL 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
elapsed after a slave server synchronized with a master server. The
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Time to Live (TTL) value and the SOA RR minimum TTL parameter [9] are
used to determine how long a forwarder should cache data 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
draft-ietf-dnsop-dnssec-key-timing [24]
4.4.1. Time Considerations
Because of the expiration of signatures, one should consider the
following:
o We suggest the Maximum Zone TTL of your zone data to be a fraction
of your signature validity period.
If the TTL was of similar order as the signature validity
period, then all RRSets fetched during the validity period
would be cached until the signature expiration time. Section
8.1 of RFC4033 [3] 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,
query load on authoritative servers would peak at signature
expiration time, as this is also the time at which records
simultaneously expire from caches.
To avoid query load peaks, we suggest the TTL on all the RRs in
your zone to be at least a few times smaller than your
signature validity period.
o We suggest the signature publication period to end at least one
Maximum Zone TTL duration (but preferably 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 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 resigning
operation and those signatures that are within a so called
refresh interval from signature expiration are recreated. Also
see Section 4.4.2 below.
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In 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 the signatures will survive a weekend in case of such
operational havoc. This is called the Refresh period (see
Section 4.4.2).
o We suggest the Minimum Zone TTL to be long enough to both fetch
and verify all the RRs in the trust chain. In workshop
environments, it has been demonstrated [18] 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 nameservers.
Data at delegation points, DS, DNSKEY, and RRSIG RRs benefit
from caching. The TTL on those should be relatively long.
Data at the leafs in the DNS tree has less impact on recursive
nameservers.
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 zone, then it may well 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.
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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 the SOA expiration timer being 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 the actual signature times out.
We also suggest that operators of nameservers 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 my secondaries expire the zone? How quickly can I reach an
administrator of secondary servers to load a valid zone? These
questions are not DNSSEC specific but may influence the choice
of your signature validity intervals.
4.4.2. Signature Validation 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 in
Section 3.2.2 play a role: when a zone is re-signed with some
regularity then operators remain conscious about 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: what is the time that a failure will be noticed, what
is the time that action is expected to be taken? By answering these
questions availability of operators during (long) weekends or time
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taken to access to backup media can be taken into account. The
result could easily suggest a minimum Signature Validity period of a
few days.
Note however, 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 close to expiring: the signer will only
refresh signatures if they are within the Refresh Period from the
signature expiration time. The Re-Sign Period must be smaller than
the Refresh Period in order for zone data to be signed in timely
fashion.
If an operational problem occurs during resigning 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.
In other words, the minimum Signature Validity interval 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
Resign period sets the time in which operational havoc can be
resolved.
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. The jitter should
not influence your calculation as long as it is smaller than the
refresh period and the resign period is at least half the refresh
period.
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Inception Signing Expiration
time time time
| | | | |
|---------------------|---------------------------|.......|.......|
| | | | |
+/- jitter
| Inception offset | |
|<------------------->| Validity Period |
| |<------------------------------------>|
Inception Signing reuse reuse reuse new Expiration
time time signature time
| | | | | | |
|------------------|-----------------------------------------|
| | | | | | |
<----> <----> <----> <---->
Resign Period
| |
|<-Refresh Period->|
| |
Figure 10: Signature Timing Parameters
Note that in the figure the validity of the signature starts shortly
before the inception time. That is done to deal with validators that
might have some clock skew. The inception offset should be chosen so
that you minimize the false negatives to a reasonable level.
4.4.2.3. Differentiation between RR sets
It is possible to vary signature validity periods between signatures
over different RR sets 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) is what should be leading in
determining the signature validity period since the TTL on the data
itself still are 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.
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When a zone contains secure delegations then a relatively short
signature validity interval protects the child against replay
attacks, in the case 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 interval 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 type
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 non-existent names that lie between the names that do
exist.
There are two mechanisms to provide authenticated proof of non-
existence 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 non-existence 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
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NSEC RRs via the "Next Domain Name" field.
Though all agree DNS data is accessible through query mechanisms, a
side effect of NSEC is that it allows the contents of a zone file to
be enumerated in full by sequential queries. Whilst for some
operators this behavior is acceptable or even desirable, for others
it is undesirable for policy, regulatory or other reasons. This is
the first difference between NSEC and NSEC3.
The second difference between NSEC and NSEC3 is that NSEC requires a
signature over every RR in the zonefile, thereby ensuring that any
denial of existence is cryptographically signed. However, in a large
zonefile 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 update (a typical
example might be a TLD operator with few registrants using secure
delegations). NSEC3 allows intervals between two such delegations to
"Opt-out" in which case they may contain one more 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 RRlabel. To
increase the workload required to guess entries in the zone, the
number of hashing iteration's 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.
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 the in-
addr.arpa, ip6.arpa and e164.arpa can be trivially enumerated using
ordinary DNS properties while for small zones that only contain
contain records in the APEX and a few common RRlabels such as "www"
or "mail" guessing zone content and proving completeness is also
trivial when using NSEC3.
In those cases the use of NSEC is recommended to ease the work
required by signers and validating resolvers.
For large zones where there is an implication of "not readily
available" RRlabels, such as those where one has to sign a non-
disclosure agreement before obtaining it, NSEC3 is recommended.
The considerations for the second reason to deploy NSEC3 are
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discussed below (Section 5.3.4).
5.3. NSEC3 parameters
The NSEC3 hashing algorithm is performed on the Fully Qualified
Domain Name (FQDN) in its uncompressed form. This ensures brute
force work done by an attacker for one (FQDN) RRlabel cannot be re-
used for another (FQDN) RRlabel attack, as these entries are, by
definition unique.
5.3.1. NSEC3 Algorithm
At the moment of writing there is only one NSEC3 Hashing algorithm
defined. [21] specifically calls out that when a new hash algorithm
for use with NSEC3 is specified, 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 a pre-calculated dictionary attack
could be made in order to assess if certain domain names exist within
the zones or not. Two mechanisms are introduced in the NSEC3
specification to increase the costs of such dictionary attacks:
Iterations and Salt.
RFC5155 Section 10.3 [21] considers the trade-offs between incurring
cost during the signing process and imposing costs to the validating
nameserver, 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 your nameservers would be halved, regardless of key
size [27].
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
owner would cause an attacker to lose all precalculated work for that
zone.
The FQDN RRlabel, which is part of the value that is hashed, already
ensures that brute force work for one RRlabel can not be re-used to
attack other RRlabel (e.g. in other domains) due to their uniqueness.
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The salt of all NSEC3 records in a zone needs to be the same. Since
changing the salt requires all the NSEC3 records to be regenerated,
and thus requires generating new RRSIG's over these NSEC3 records, it
is recommended to align the change of the salt with a change of the
Zone Signing Key, as that process in itself already usually requires
all RRSIG's to be regenerated (you can have a smooth ZSK rollover by
honoring the Refresh period). 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. However, unlike Zone Signing
Key changes, NSEC3 salt changes do not need special rollover
procedures. It is possible to change the salt each time the zone is
updated.
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 a 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 zones and
where the number of secure delegations is small. This consideration
typically holds for large top-level-domains and similar zones; in
most other circumstances Opt-Out should not be deployed. Further
considerations can be found in RFC5155 section 12.2 [21].
6. Security Considerations
DNSSEC adds data integrity to the DNS. This document tries to assess
the operational considerations to maintain a stable and secure DNSSEC
service. Not taking into account the 'data propagation' properties
in the DNS will cause validation failures and may make secured zones
unavailable to security-aware resolvers.
7. IANA considerations
There are no IANA considerations with respect to this document
8. Contributors and Acknowledgments
Significant parts of the text of this document is copied from RFC4641
[14]. That document was edited by Olaf Kolkman and Miek Gieben.
Other people that contributed or where otherwise involved in that
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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, and Lindy
Foster, and O. Courtay.
For this version of the document we would like to acknowledge a few
people for significant contributions:
Paul Hoffman for his contribution on the choice of cryptographic
parameters and addressing some of the trust anchor issues;
Jelte Jansen who provided the initial text in Section 4.1.5;
Paul Wouters who provided the initial text for Section 5 and Alex
Bligh who improved it;
Erik Rescorla whose blogpost on "the Security of ZSK rollovers"
inspired text in Section 3.1;
Stephen Morris who made a pass on English style and grammar;
Matthijs Mekking thorougly reviewed and provided concrete
improvements on the specific types of keyrollovers (e.g. he
provided the tables in Appendix C); and
Olafur Gudmundsson and Ondrej Sury who provided input on
Section 4.1.5 based on actual operational experience.
Rickard Bellgrim reviewed the document extensively.
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 where provided by Mark Andrews, Patrik Faltstrom, Tony Finch,
Alfred Hines, Bill Manning, Scott Rose, and Wouter Wijngaards.
9. References
9.1. Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
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[3] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[4] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
[5] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions",
RFC 4035, March 2005.
9.2. Informative References
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[7] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
August 1996.
[8] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
(DNS NOTIFY)", RFC 1996, August 1996.
[9] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)",
RFC 2308, March 1998.
[10] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[11] Hollenbeck, S., "Generic Registry-Registrar Protocol
Requirements", RFC 3375, September 2002.
[12] Orman, H. and P. Hoffman, "Determining Strengths For Public
Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
April 2004.
[13] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[14] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
RFC 4641, September 2006.
[15] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949,
August 2007.
[16] StJohns, M., "Automated Updates of DNS Security (DNSSEC) Trust
Anchors", RFC 5011, September 2007.
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[17] Gould, J. and S. Hollenbeck, "Domain Name System (DNS) Security
Extensions Mapping for the Extensible Provisioning Protocol
(EPP)", RFC 5910, May 2010.
[18] Rose, S., "NIST DNSSEC workshop notes", , June 2001.
[19] Barker, E. and J. Kelsey, "Recommendation for Random Number
Generation Using Deterministic Random Bit Generators
(Revised)", NIST Special Publication 800-90, March 2007.
[20] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS)
Resource Records (RRs)", RFC 4509, May 2006.
[21] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of Existence",
RFC 5155, March 2008.
[22] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246, August 2008.
[23] Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY and
RRSIG Resource Records for DNSSEC", RFC 5702, October 2009.
[24] Morris, S., Ihren, J., and J. Dickinson, "DNSSEC Key Timing
Considerations", draft-ietf-dnsop-dnssec-key-timing-01 (work in
progress), October 2010.
[25] Ljunggren, F., Eklund-Lowinder, A., and T. Okubo, "DNSSEC
Policy & Practice Statement Framework",
draft-ietf-dnsop-dnssec-dps-framework-03 (work in progress),
November 2010.
[26] Larson, M. and O. Gudmundsson, "DNSSEC Trust Anchor
Configuration and Maintenance",
draft-ietf-dnsop-dnssec-trust-anchor-04 (work in progress),
October 2010.
[27] Schaeffer, Y., "NSEC3 Hash Performance", NLnet Labs
document 2010-02, 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.
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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 RFC4033 [3]. An RRSet in DNSSEC is
marked "Bogus" when a signature of an RRSet does not validate
against a DNSKEY.
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 use of
the DNSKEY Resource Record (DNSKEY RR). Private keys should
remain private.
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.
Refresh Period: The period before the expiration time of the
signature, during which the signature is refreshed by the signer.
Re-Signing frequency: Frequency with which a signing pass on the
zone is performed. Alternatively expressed as "Re-Signing
Period". It defines when the zone is exposed to the signer.
During a signing pass not all signatures in the zone may be
refreshed, that depend refresh frequency/interval.
Secure Entry Point (SEP) key: A KSK that has a DS record in the
parent zone pointing to it or is configured as a trust anchor.
Although not required by the protocol, we recommend that the SEP
flag [5] is 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.
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Signing Jitter: Jitter applied to the signature validty interval.
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.
Single Type Signing Scheme: A signing scheme whereby the distinction
between Zone Signing Keys and Key Signing Keys is not made.
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.
Singing the zone file: The term used for the event where an
administrator joyfully signs its zone file while producing melodic
sound patterns.
Zone administrator: The 'role' that is responsible for signing a
zone and publishing it on the primary authoritative server.
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 Keys Signing Key, Z for Zone
Signing Key and S when there is no distinction made between KSK
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.
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. A 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(RRSet),
which means that RRSet is signed with DNSKEY_x_y.
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"
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SOA representation: SOAs are represented as SOA_x, where x is the
serial number.
RRsets ignored: If the signature of non DNSKEY RRsets have the same
parameters as the SOA than 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.
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
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) ; 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 14.
Appendix C. Transition Figures for Special Case Algorithm Rollovers
The figures appendix complement and illustrate the special cases of
algorithm rollovers as described in Section 4.1.5
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----------------------------------------------------------------
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)
----------------------------------------------------------------
Also see Section 4.1.5.1.
Figure 11: Single Type Signing Scheme Algorithm Roll
----------------------------------------------------------------
initial new RRSIGs new DNSKEY
----------------------------------------------------------------
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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_0 ------------------------------------------------------->
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:
------------------------------------->
------------------------------------->
------------------------------------->
------------------------------------->
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Child:
SOA_5
RRSIG_Z_2(SOA)
DNSKEY_K_2
DNSKEY_Z_2
RRSIG_K_2(DNSKEY)
----------------------------------------------------------------
Also see Section 4.1.5.2.
Figure 12: RFC5011 Style algorithm roll
----------------------------------------------------------------
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_1(SOA) RRSIG_Z_1(SOA) RRSIG_Z_1(SOA)
RRSIG_S_2(SOA) RRSIG_S_2(SOA)
DNSKEY_S_1 DNSKEY_S_1 DNSKEY_S_1
DNSKEY_Z_1 DNSKEY_Z_1 DNSKEY_Z_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 revoke DNSKEY DNSKEY removal
----------------------------------------------------------------
Parent:
SOA_0 ------------------------------------------------------->
RRSIG_par(SOA) ---------------------------------------------->
DS_S_2 ------------------------------------------------------>
RRSIG_par(DS_S_2) ------------------------------------------->
Child:
-------------------> SOA_3 SOA_4
-------------------> RRSIG_Z_1(SOA) RRSIG_Z_1(SOA)
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-------------------> RRSIG_S_2(SOA) RRSIG_S_2(SOA)
-------------------> DNSKEY_S_1_REVOKED
-------------------> DNSKEY_S_2 DNSKEY_S_2
-------------------> DNSKEY_Z_1
-------------------> 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)
----------------------------------------------------------------
Also see Section 4.1.5.3.
Figure 13: RFC5011 algorithm roll in a Single Type Signing Scheme
Environment
Appendix D. Document Editing History
[To be removed prior to publication as an RFC]
D.1. draft-ietf-dnsop-rfc4641-00
Version 0 was differs from RFC4641 in the following ways.
o Status of this memo appropriate for I-D
o TOC formatting differs.
o Whitespaces, linebreaks, and pagebreaks may be slightly different
because of xml2rfc generation.
o References slightly reordered.
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o Applied the errata from
http://www.rfc-editor.org/errata_search.php?rfc=4641
o Inserted trivial "IANA considerations" section.
In other words it should not contain substantive changes in content
as intended by the working group for the original RFC4641.
D.2. version 0->1
Cryptography details rewritten. (See http://www.nlnetlabs.nl/svn/
rfc4641bis/trunk/open-issues/cryptography_flawed)
o Reference to NIST 800-90 added
o RSA/SHA256 is being recommended in addition to RSA/SHA1.
o Complete rewrite of Section 3.4.2 removing the table and
suggesting a keysize of 1024 for keys in use for less than 8
years, issued up to at least 2015.
o Replaced the reference to Schneiers' applied cryptography with a
reference to RFC4949.
o Removed the KSK for high level zones consideration
Applied some differentiation with respect of the use of a KSK for
parent or trust-anchor relation http://www.nlnetlabs.nl/svn/
rfc4641bis/trunk/open-issues/differentiation_trustanchor_parent
http://www.nlnetlabs.nl/svn/rfc4641bis/trunk/open-issues/
rollover_assumptions
Added Section 4.1.5 as suggested by Jelte Jansen in http://
www.nlnetlabs.nl/svn/rfc4641bis/trunk/open-issues/Key_algorithm_roll
Added Section 4.3.5.1 Issue identified by Antoin Verschuren http://
www.nlnetlabs.nl/svn/rfc4641bis/trunk/open-issues/
non-cooperative-registrars
In Appendix A: ZSK does not necessarily sign the DNSKEY RRset.
D.3. version 1->2
o Significant rewrite of Section 3 whereby the argument is made that
the timescales for rollovers are made purely on operational
arguments hopefully resolving http://www.nlnetlabs.nl/svn/
rfc4641bis/trunk/open-issues/discussion_of_timescales
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o Added Section 5 based on http://www.nlnetlabs.nl/svn/rfc4641bis/
trunk/open-issues/NSEC-NSEC3
o Added a reference to draft-morris-dnsop-dnssec-key-timing [24] for
the quantitative analysis on keyrolls
o Updated Section 4.3.5 to reflect that the problem occurs when
changing DNS operators, and not DNS registrars, also added the
table indicating the redelegation procedure. Added text about the
fact that implementations will dismiss keys that fail to validate
at some point.
o Updated a number of references.
D.4. version 2->3
o Added bulleted list to serve as an introduction on the decision
tree in Section 3.
o In section Section 3.1:
* tried to motivate that key length is not a strong motivation
for KSK ZSK split (based on http://www.educatedguesswork.org/
2009/10/on_the_security_of_zsk_rollove.html)
* Introduced Common Signing Key terminology and made the
arguments for the choice of a Common Signing Key more explicit.
* Moved the SEP flag considerations to its own paragraph
o In a few places in the document, but section Section 4 in
particular the comments from Patrik Faltstrom (On Mar 24, 2010) on
the clarity on the roles of the registrant, dns operator,
registrar and registry was addressed.
o Added some terms based on http://www.nlnetlabs.nl/svn/rfc4641bis/
trunk/open-issues/timing_terminology
o Added paragraph 2 and clarified the second but last paragraph of
Section 3.2.2.
o Clarified the table and some text in Section 4.1.5. Also added
some text on what happens when the algorithm rollover also
involves a roll from NSEC to NSEC3.
o Added a paragraph about rolling KSKs that are also configured as
trust-anchors in Section 4.1.2
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o Added Section 4.1.4.
o Added Section 4.4.2 to address issue "Signature_validity"
D.5. version 3->4
o Stephen Morris submitted a large number of language, style and
editorial nits.
o Section 4.1.5 improved based on comments from Olafur Gudmundsson
and Ondrej Sury.
o Tried to improve consistency of notation in the various rollover
figures
D.6. version 4->5
o Improved consistency of notation
o Matthijs Mekking provided substantive feedback on algorithm
rollover and suggested the content of the subsections of
Section 4.1.5 and the content of the figures in Appendix C
D.7. version 5->6
o More improved consistency of notation and some other nits
o Review of Rickard Bellgrim
o Review of Sebastian Castro
o Added a section about Stand-by keys
o Algorithm rollover: Conservative or Liberal Approach
o Added a reference to NSEC3 hash performance report
o More clarifications on the topic of non cooperating operators
D.8. version 6->7
o Fixed minor nits.
o Clarified the Double DS Rollover in Changing DNS Operator
sections.
o Adjusted STSS Rollover Figures.
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o Remove the ZSK RRSIGs over DNSKEY RRset in Figures.
o Added text: second variety on STSS Double DS Rollover.
o Reviewed by Antoin Verschuren, Marc Lampo, George Barwood.
D.9. version 7->8
o Signatures over DNSKEY RRset does not need to be propagated in the
new RRSIGS step.
D.10. Subversion information
www.nlnetlabs.nl/svn/rfc4641bis/
$Id: draft-ietf-dnsop-rfc4641bis.xml 110 2011-10-28 14:15:25Z matje $
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
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