Domain Name System Operations W. Mekking
Internet-Draft NLnet Labs
Intended status: Standards Track February 25, 2011
Expires: August 29, 2011
DNSSEC Key Timing Considerations Follow-Up
draft-mekking-dnsop-dnssec-key-timing-bis-00
Abstract
This document describes issues surrounding the timing of events in
enforcing key policy within DNSSEC. It presents timelines for
various key rollovers and changes into the policy with respect to the
key signing scheme. It explicitly identifies the relationships
between the various parameters affecting the process.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 29, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Mekking Expires August 29, 2011 [Page 1]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Key Rollover Considerations . . . . . . . . . . . . . . . 3
1.1.1. Key Goals . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Key Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Key Types . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Key States Unraveled . . . . . . . . . . . . . . . . . . . 5
2.3. Delay Timings . . . . . . . . . . . . . . . . . . . . . . 6
3. Key Rollovers . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. ZSK Rollovers . . . . . . . . . . . . . . . . . . . . . . 8
3.1.1. Double-Signature . . . . . . . . . . . . . . . . . . . 8
3.1.2. Pre-Publication . . . . . . . . . . . . . . . . . . . 10
3.1.3. Double-RRSIG . . . . . . . . . . . . . . . . . . . . . 13
3.2. KSK Rollovers . . . . . . . . . . . . . . . . . . . . . . 15
3.2.1. Double-RRset . . . . . . . . . . . . . . . . . . . . . 16
3.2.2. Double-Signature . . . . . . . . . . . . . . . . . . . 18
3.2.3. Double-DS . . . . . . . . . . . . . . . . . . . . . . 20
3.2.4. Interaction with Configured Trust Anchors . . . . . . 22
3.2.4.1. Adding a KSK . . . . . . . . . . . . . . . . . . . 22
3.2.4.2. Removing a KSK . . . . . . . . . . . . . . . . . . 22
3.3. Rollovers in a Single Type Signing Scheme . . . . . . . . 23
3.3.1. Double-RRset . . . . . . . . . . . . . . . . . . . . . 23
3.3.2. Double-Signature . . . . . . . . . . . . . . . . . . . 24
3.3.3. Pre-Publication . . . . . . . . . . . . . . . . . . . 25
3.3.4. Double-DS . . . . . . . . . . . . . . . . . . . . . . 28
3.4. Stand-by Keys . . . . . . . . . . . . . . . . . . . . . . 31
4. Policy rollover . . . . . . . . . . . . . . . . . . . . . . . 31
4.1. Enabling DNSSEC . . . . . . . . . . . . . . . . . . . . . 32
4.2. Disabling DNSSEC . . . . . . . . . . . . . . . . . . . . . 34
4.3. Algorithm Rollover . . . . . . . . . . . . . . . . . . . . 35
4.4. KSK-ZSK Split or Single Type Signing Scheme . . . . . . . 35
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
6. Security Considerations . . . . . . . . . . . . . . . . . . . 36
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
8. Changes with key-timing draft . . . . . . . . . . . . . . . . 36
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.1. Informative References . . . . . . . . . . . . . . . . . . 37
9.2. Normative References . . . . . . . . . . . . . . . . . . . 37
Mekking Expires August 29, 2011 [Page 2]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
1. Introduction
A zone is managed according to a given security policy. Such a
policy may enforce DNSSEC keys to be used and for how long. When
enforcing a lifetime on DNSSEC keys, key rollovers must to take
place. In addition, changes in the policy may trigger certain key
rollover events. Key rollovers are time critical, multiple steps
processes. This document describes issues surrounding the timing of
events in the rolling of DNSSEC keys.
[MM: Editorarial comments are indicated by square brackets and editor
initials]
1.1. Key Rollover Considerations
A key is used with a purpose: An operational decision has been made
to secure the zone with DNSSEC. That decision leads to a key being
created, published in the zone and used for signing. Policy may
enforce a lifetime on keys. As a result, current active keys need to
be replaced with new keys. The new key becomes active, while the
current key is retired. The keys need to be introcuded into and
removed from the zone at the appropriate times. Considerations that
must be taken into account are:
o Speed: A rollover should occur as fast and simple as possible.
However, DNSSEC records are not only held at the authoritative
nameserver, they are also cached at client resolvers. The data on
these systems can be interlinked, meaning a validating may try to
validate a signature retrieved from a cache with a key obtained
separately. The rollover process needs to happen in such a way
that at all times through the rollover the information is
consistent.
o Size of the zone and the DNS response: A rollover can be speed up
by introducing the DNSSEC records prematurely. However, adding
arbitrary signatures increases the size of your zone and DNS
responses significantly. To keep the sizes of the zone and
responses as small as possible, you might want to consider to
introduce the DNSSEC records only when they are required, For the
same reason, dead keys and signatures must be removed
periodically.
o Size of the DNSKEY RRset and the priming response: You can choose
to keep the size of the DNSKEY RRset to a minimum, to make priming
responses smaller in size. The larger the packet, the more
resolvers may have problems retrieving the response. Other
responses may have more signatures, since the initial size is
relatively small. The DNSKEY RRset is usually already quite large
Mekking Expires August 29, 2011 [Page 3]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
and should not grow too much anymore.
o Interactions with the Parent: A KSK sometimes needs its
corresponding DS record to be published at the parent zone, while
its predecessor needs to remove its DS record from the parent
zone. Such a request requires additional operational work and can
be a sufficient delay. Ideally, the interactions with the parent
is kept to a minimum.
1.1.1. Key Goals
We have identified three different goals for a key:
o Activate key: Make validating resolvers use the key's associated
information to perform authentication.
o Remove key: Make validating resolvers forget about the key's
associated information.
o Stand-by key: Pre-publish information for this key to speed up a
future (unscheduled) rollover.
Each key rollover and change in key signing scheme can now be
described by one or more goals that are put on a key.
1.2. Terminology
The terminology used in this document is as defined in [RFC4033] and
[RFC5011].
2. Key Definitions
2.1. Key Types
Keys can be used to authenticate information within the zone. Such
keys are said to be ZSKs. In addition, keys can be used to
authenticate the DNSKEY RRset in the zone. These keys are said to be
KSKs. Keys can be marked to be ZSK and KSK at the same time, for
example in a Single Type Signing Scheme (STSS).
Despite that ZSK and KSK only describe the usage of a key, the terms
are often used for identifiying a key. Thus when we talk about a
ZSK, we actually mean that the key is used as ZSK. In the same
spirt, a KSK is a key that is used as KSK.
DNSSEC recognises the classification of keys with its SEP bit set and
not set. Usually if a key is used as KSK, the SEP bit is set.
However, draft-ietf-dnsext-dnssec-bis-updates [dnssec-bis] says that
Mekking Expires August 29, 2011 [Page 4]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
a SEP bit setting has no effect on how a DNSKEY may be used. Policy
determines whether the bit should be set, depending on the key's
usage.
2.2. Key States Unraveled
We use unraveled key states to seperately represent the key and its
associated information. There can be up to three pieces of key
associated information: the public key (in DNSKEY format), its
created signatures (the RRSIG records) and the secured delegation
(the corresponding DS record). The state of the piece of information
is defined by 'RRtype State'.
Key conditions are essentially what are called key states in
draft-ietf-dnsop-dnssec-key-timing [key-timing]. A key can have
multiple conditions at the same time.
A piece of information may exist in up to two places: it can be
present in the corresponding zone and it may live in resolver caches.
This is true for every piece of associated information. Therefore,
all of the three pieces of information follow the same state diagram:
Generated --> Introduced --> Propagated --> Withdrawn --> Dead.
Generated: The information has been generated, but is not available
in the zone. In this state, no resolvers are able to fetch this
information.
- The key condition is said to be Generated, if no information has
passed the Introduced state yet.
Introduced: The information is introduced and, as a result, may be
available in the zone. In this state, there may be resolvers that
fetch this information.
- The key condition is said to be Published if it has its DNSKEY
state in Introduced.
- The key condition is said to be Active if it has its RRSIG state
in Introduced (for ZSKs).
- The key condition is said to be Submitted, or ActiveDS, if it
has its DS state in Introduced (for KSKs).
Propagated: The information is available in the zone and enough
time has passed to have it propagated into all resolver caches.
As a result, all resolvers fetch this information from cache of
from the authoritative name server.
- The key condition is said to be Known if it has its DNSKEY state
in Propagated.
- The key condition is said to be Safe if it has its RRSIG state
in Propagated (for ZSKs).
Mekking Expires August 29, 2011 [Page 5]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
- The key condition is said to be SafeDS if it has its DS state in
Propagated (for KSKs).
Withdrawn: The information is being withdrawn from the zone, but
may still be available in the zone. In this state, the
information can still live in resolver caches.
- The key condition is said to be Removed if it has its DNSKEY
state in Withdrawn.
- The key condition is said to be Retired if it has its RRSIG
state in Withdrawn (for ZSKs).
- The key condition is said to be RetiredDS if it has its DS state
in Withdrawn (for KSKs).
Dead: The information is not available in the zone anymore and
enough time has passed to have it expire from all resolver caches.
- The key condition is said to be Forgotten if it has its DNSKEY
state in Dead.
- The key condition is said to be Expired if it has its RRSIG
state in Dead (for ZSKs).
- The key condition is said to be ExpiredDS if it has its DS state
in Dead (for KSKs).
A key state can now be represented as the triplet (DNSKEY State,
RRSIG State, DS State). For example:
S(Kc) = (DNSKEY Propagated, RRSIG Introduced, DS Generated)
tells us that key Kc is published in the zone and all the resolvers
that have a copy of the DNSKEY RRset, have one that includes Kc. In
other words, Kc is said to be Known. In addition, the key is Active
as it is being used for signing RRsets: RRSIG records made with Kc
have been introduced in the zone. However, there may still be some
resolver caches that are unaware of these signatures. Finally, the
corresponding DS record is said to be Generated and has thus not yet
been submitted to the parent.
For convenience, we can represent a ZSK as a tuple (DNSKEY State,
RRSIG State), because the DS record is only used with KSKs. And we
can represent a KSK as a tuple (DNSKEY State, DS State), because the
RRSIG state only refers to ZSKs. The RRSIG record over the DNSKEY
RRset should be published at the same time when the corresponding
DNSKEY record is published. Therefore, both records will propagate
and expire at the same time from resolver caches.
2.3. Delay Timings
For every change we make in the zone, we have to take into account
several delays.
Mekking Expires August 29, 2011 [Page 6]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Software Delay (Dsfw): The time it takes for the software to
introduce the new information in the zone. This delay can vary
alot depending on the information that needs to be introduced.
One can imagine that the software needs more time to sign a
complete zone than when it pre-publishes a DNSKEY record. [MM:
Dsfw maps to Dsgn from the key-timing draft]
Propagation Delay (Dprp): The time it takes for any change
introduced at the master to replicate to all slave servers.
TTL Delay (Dttl): The time it takes to expire the previous
information from the resolver caches. This delay depends on what
RRsets need to expire from the caches. If not explicitly
mentioned otherwise, Dttl is considered the maximum TTL of the
information that needs to expire from caches. Otherwise,
Dttl(RRtype) shows which specific RRsets need to expire. [MM: TTL
terminology in key-timing draft: TTLds, TTLkey, TTLkeyC, TTLsoa,
TTLsoaC, TTLsoaP, TTLsig)]
Registration Delay to the Parent (Dreg): The time it takes to get
the DS record to be placed into the parent zone, after it is
submitted.
Propagation Delay of the Parent (DprpP): The time it takes for any
change introduced at the parent master to replicate to all parent
slave servers.
Despite these delays may vary for the different rollover methods, we
can identify the propagation delay to the caches as:
DcacheZ = Dsfw + Dprp + Dttl
DcacheK = Dsfw + Dprp + Dttl(DNSKEY)
DcacheP = Dreg + DprpP + Dttl(DS)
where DcacheZ is the propagation delay to the caches for information
published in our zone, DcacheK is the propagation delay to the caches
for our DNSKEY RRset and DcacheP is the propagation delay for
information published in our parent zone.
3. Key Rollovers
There are many different key rollover methods. In Section 1.1, we
have seen that there are several properties to prefer one method over
the other. Though there are many different type of key rollovers,
all methods share the same goal. There is a current key (Kc) that
needs to become Forgotten-Retired and a successor key (Ks) that needs
to become Known-Safe.
Mekking Expires August 29, 2011 [Page 7]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
3.1. ZSK Rollovers
The two most common rollover methods for ZSKs are Double-Signature
and Pre-Publication. Both are described in RFC4641 [RFC4641].
draft-ietf-dnsop-dnssec-key-timing [key-timing] also introduces ZSK
Double-RRSIG rollover. Double-Signature is the fastest way to
rollover a ZSK. Pre-Publication minimizes the number of signatures
over the RRsets in the zone and responses. Double-RRSIG keeps the
size of the DNSKEY RRset to a minimum.
3.1.1. Double-Signature
This involves introducing the new key into the zone and using it to
create additional RRSIG records; the old key and existing RRSIG
records are retained. During the period in which the zone is being
signed, client resolvers are always able to validate RRSIGs: any
combination of old and new DNSKEY RRset and RRSIG allows at least one
signature to be validated.
Once the signing process is complete and enough time has elapsed to
allow all old information to expire from caches, the old key and
signatures can be removed from the zone. As before, during this
period any combination of DNSKEY RRset and RRSIG will allow
validation of at least one signature.
Double-Signature is the fastest way to rollover to a new key, since
all new information is published right away. The drawback of this
method is a noticeable increase in the size of the DNSSEC data,
affecting both the overall size of the zone and the size of the
responses.
Only when Ks is said to be Known, e.g. the DNSKEY record of Ks is
known to all validating resolvers, we can remove the signatures made
with Kc. And only when we can ensure that all validators only use
the information of Ks for authentication, we can remove the DNSKEY
record for Kc. In other words, Ks needs to be Known and Safe, before
we can remove Kc. Thus, we first have to introduce all new
information into the zone. Once all has been propagated, we can
withdraw all information of Kc from the zone.
Mekking Expires August 29, 2011 [Page 8]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
The timeline diagram is shown below:
|1| |2| |3| |4| |5|
Kc | | | | |
- RRSIG |----|-------|----|DcacheZ|
- DNSKEY |----|-------|----|DcacheK|
| | | | |
Trem Tfor
Tret Texp
Ks | | | | |
- RRSIG | |DcacheZ|----|-------|---
- DNSKEY | |DcacheK|----|-------|---
| | | | |
Tgen Tpub Tkno
Tact Tsaf
Figure: ZSK Double-Signature Rollover.
Event 1: Key Ks is generated at the generate time (Tgen).
S(Ks) = (DNSKEY Generated, RRSIG Generated)
C(Ks) = Generated
Event 2: Key Ks is added to the DNSKEY RRset and is immediately used
to sign the zone; existing signatures in the zone are not removed.
This is Ks' publish time (Tpub) and Ks is said to be Published. It
is also Ks' active time (Tact), the time when Ks is said to be
Active. Because the Double-Signature rollover is in place, we now
temporarily have two active keys.
Tpub(Ks) >= Tgen(Ks), Tact(Ks) == Tpub(Ks)
S(Ks) = (DNSKEY Introduced, RRSIG Introduced)
C(ks) = Published Active
Event 3: The information for Ks must be published long enough to
ensure that the information have reached all validating resolvers
that may have RRsets from this zone cached. At the point in time
that the DNSKEY RRset including Ks has been propagated and Ks is said
to be Known (Tkno). At the point in time that the other RRsets
including a signature of Ks have been propagated (Tsaf), Ks is said
to be Safe.
Tkno(Ks) >= Tpub(Ks) + DcacheK
Tsaf(Ks) >= Tact(Ks) + DcacheZ
S(Ks) = (DNSKEY Propagated, RRSIG Propagated)
Mekking Expires August 29, 2011 [Page 9]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
C(ks) = Known Safe
Note that we could already retire Kc, i.e. stop signing with Kc,
after DcacheK. It does not matter if not all signatures of Ks have
been Propagated, since the resolver can validate RRsets with both Kc
and Ks. If the validator fetches a RRset from the cache, it uses the
DNSKEY of Kc for validation. Otherwise, it can use the DNSKEY of Ks.
Event 4: Once we have a successor key that is said to be Propagated,
we can retire Kc. This is Kc' retire time (Tret) and Kc is said to
be Retired. And once we have a successor key that is said to be
Safe, we can remove Kc. Therefore, it is also Kc' removal time
(Trem), the time that Kc is said to be Removed.
Tret(Kc) >= Tkno(Ks)
Trem(Kc) >= MAX(Tsaf(Ks), TsafDS(Ks))
S(Kc) = (DNSKEY Withdrawn, RRSIG Withdrawn)
C(kc) = Removed Retired
Event 5: From the perspective of the authoritative server, the
rollover is complete. After some delay, Kc and its signatures have
expired from the caches. This delay is the maximum of DcacheZ,
DcacheK. This is Tfor, the time that the key is said to be Forgotten
and Texp, the time that the key is said to be Expired.
Tfor(Kc) >= Trem(Kc) + DcacheK
Texp(Kc) >= Tret(Kc) + DcacheZ
S(Kc) = (DNSKEY Dead, RRSIG Dead)
C(Kc) = Forgotten Expired
3.1.2. Pre-Publication
With Pre-Publication, the new key is introduced into the DNSKEY
RRset, leaving the existing keys and signatures in place. This state
of affairs remains in place for long enough to ensure that any DNSKEY
RRsets cached in client validating resolvers contain both keys. At
that point signatures created with the old key can be replaced by
those created with the new key, and the old signatures can be
removed. During the re-signing process it doesn't matter which key
an RRSIG record retrieved by a client was created with; clients with
a cached copy of the DNSKEY RRset will have a copy containing both
the old and new keys.
Once the zone contains only signatures created with the new key,
there is an interval during which RRSIG records created with the old
key expire from client caches. After this, there will be no
Mekking Expires August 29, 2011 [Page 10]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
signatures anywhere that were created using the old key, and it can
can be removed from the DNSKEY RRset.
Pre-Publication is more complex than Double-Signature - introduce the
new key, approximately one TTL later sign the records, and
approximately one TTL after that remove the old key. It also takes
more time than the Double-Signature method. The delay is because we
don't want to publish signatures of both keys at the same time. As
an advantage, the amount of DNSSEC data is kept to a minimum which
reduces the impact on performance.
The timeline diagram looks like this:
|1| |2| |3| |4| |5| |6| |7|
Kc | | | | | | |
- RRSIG |----|-------|----|DcacheZ| | |
- DNSKEY |----|-------|----|-------|----|DcacheK|
| | | | | | |
Tret Texp Trem Tfor
Ks | | | | | | |
- RRSIG | | | |DcacheZ|----|-------|---
- DNSKEY | |DcacheK|----|-------|----|-------|---
| | | | | | |
Tgen Tpub Tkno Tact Tsaf
Figure: ZSK Pre-Publication Rollover.
Event 1: Key Ks is generated at the generate time (Tgen).
S(Ks) = (DNSKEY Generated, RRSIG Generated)
C(Ks) = Generated
Event 2: The DNSKEY record of Ks is put into the zone, i.e. it is
added to the DNSKEY RRset which is then re-signed with the current
KSK. The time at which this occurs is Ks' publication time (Tpub),
and the key is now said to be Published. Note that the key is not
yet used to sign records.
Tpub(Ks) >= Tgen(Ks)
S(Ks) = (DNSKEY Introduced, RRSIG Generated)
C(Ks) = Published
Event 3: Before Ks can be used, the DNSKEY record for Ks must be
published for long enough (DcacheK) to guarantee that any resolver
that has a copy of the DNSKEY RRset also includes this key. In other
words, that any prior cached information about the DNSKEY RRset has
Mekking Expires August 29, 2011 [Page 11]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
expired. After this delay, the key is said to be Known and could be
used to sign records. The time at which this event occurs is Tkno,
which is given by:
Tkno(Ks) >= Tpub(Ks) + DcacheK
S(Ks) = (DNSKEY Propagated, RRSIG Generated)
C(Ks) = Known
Event 4: At some point in time, the decision is made to actually
start signing the zone using the successor key. This will be when
the current key has been in use for an interval equal to the ZSK
lifetime. This point in time is Ks' active time (Tact), the time
that Ks is said to be Active. It is also Kc' retire time (Tret), the
time that Kc is said to be Retired.
Tact(Ks) >= Tkno(Ks), Tret(Kc) == Tact(Ks)
S(Kc) = (DNSKEY Propagated, RRSIG Withdrawn)
C(Kc) = Known Retired
S(Ks) = (DNSKEY Propagated, RRSIG Introduced)
C(Ks) = Known Active
Event 5: Kc needs to be retained in the zone whilst any RRSIG records
created by the retired key are still published in the zone or held in
resolver caches. In other words, Kc should be retained in the zone
until all RRSIG records created by Ks have been propagated. This
time is Ks' safe time (Tsaf), the time that Ks is considered to be
Safe. Consequently, at the same time Kc is considered to be Expired.
Tsaf(Ks) >= Tact(Ks) + DcacheZ
S(Kc) = (DNSKEY Propagated, RRSIG Dead)
C(Kc) = Known Expired
S(Ks) = (DNSKEY Propagated, RRSIG Propagated)
C(Ks) = Known Safe
Event 6: When all new signatures have been propagated, Kc can be
removed from the zone and the DNSKEY RRset re-signed with the current
KSK. This time is Kc' removal time (Trem), the time that Kc is
considered to be Removed.
Trem(Kc) >= Tsaf(Ks)
S(Kc) = (DNSKEY Withdrawn, RRSIG Dead)
C(Kc) = Removed Expired
Event 7: From the perspective of the authoritative server, the
Mekking Expires August 29, 2011 [Page 12]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
rollover is complete. After some delay, The DNSKEY record for Kc has
expired from the caches. This is Tfor, and the key is said to be
Forgotten.
Tfor(Kc) >= Trem(Kc) + DcacheK
S(Kc) = (DNSKEY Dead, RRSIG Dead)
C(Kc) = Forgotten Expired
3.1.3. Double-RRSIG
This involves introducing the new signatures first, while existing
signatures are being retained. This state of affairs remains in
place for long enough to ensure that all RRsets cached in client
validating resolvers contain two signatures. The DNSKEY RR can now
be switched. For the period of time before the predecessor key has
been expired from all caches, it does not matter if the validator
uses the cached key or the successor key that is in the zone. Both
corresponding signatures can be retrieved from the cache or from the
name server.
Once the signing process is complete and enough time has elapsed to
allow all old information to expire from caches, the old signatures
can be removed from the zone. As before, during this period any
combination of DNSKEY RRset and RRSIG will allow validation of at
least one signature.
Double-RRSIG is also more complex than Double-Signature - first
introducing the signatures, then switch the key and finally remove
the olds signatures. It also takes more time than the Double-
Signature method. The delay is because we cannot publish the public
data of both keys at the same time. As an advantage, the DNSKEY
RRset is kept to a minimum which reduces the impact on priming
performance.
Mekking Expires August 29, 2011 [Page 13]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
The timeline diagram is shown below:
|1| |2| |3| |4| |5| |6| |7|
Kc | | | | | | |
- RRSIG |----|-------|----|DcacheK| | |
- DNSKEY |----|-------|----|-------|----|DcacheZ|
| | | | | | |
Trem Tfor Tret Texp
Ks | | | | | | |
- RRSIG | |DcacheZ|----|-------|----|-------|---
- DNSKEY | | | |DcacheK|----|-------|---
| | | | | | |
Tgen Tact Tsaf Tpub Tkno
Figure: ZSK Double-RRSIG Rollover.
Event 1: Key Ks is generated at the generate time (Tgen).
S(Ks) = (DNSKEY Generated, RRSIG Generated)
C(Ks) = Generated
Event 2: The zone is signed with Ks but existing signatures are
retained. The DNSKEY RR for Ks remains unpublished. The time at
which this occurs is Ks' active time (Tact), and the key is now said
to be Active.
Tact(Ks) >= Tgen(Ks)
S(Ks) = (DNSKEY Generated, RRSIG Introduced)
C(Ks) = Active
Event 3: Before we can switch the DNSKEY from Kc to Ks, the
signatures of Ks must be published for long enough (DcacheZ) to
guarantee that any resolver that has a copy of any RRset, also has
both signatures. In other words, that any cached information is
double signed. After this delay, the key is said to be Safe. The
time at which this event occurs is Tsaf, which is given by:
Tsaf(Ks) >= Tact(Ks) + DcacheZ
S(Ks) = (DNSKEY Generated, RRSIG Propagated)
C(Ks) = Safe
Event 4: At some point in time, the decision is made to publish Ks.
This point in time is Ks' publish time (Tpub), the time that Ks is
said to be Published. At the same time, the DNSKEY RR for Kc is
removed from the zone, and Kc is said to be Removed.
Mekking Expires August 29, 2011 [Page 14]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Tpub(Ks) >= Tsaf(Ks), Trem(Kc) == Tpub(Ks)
S(Kc) = (DNSKEY Removed, RRSIG Propagated)
C(Kc) = Removed Safe
S(Ks) = (DNSKEY Introduced, RRSIG Propagated)
C(Ks) = Published Safe
Event 5: The signatures of Kc need to be retained in the zone until
the DNSKEY RR has expired from all resolver caches. When this
happens, Ks is said to be Known (Tkno) and Kc is said to be Forgotten
(Tfor).
Tfor(Kc) >= Trem(Kc) + DcacheK
Tkno(Ks) >= Tpub(Ks) + DcacheK
S(Kc) = (DNSKEY Dead, RRSIG Propagated)
C(Kc) = Forgotten Safe
S(Ks) = (DNSKEY Propagated, RRSIG Propagated)
C(Ks) = Known Safe
Event 6: The signatures of Kc can be removed when the DNSKEY RR for
Ks has been propagated. This time is Kc' retire time (Tret), the
time that Kc is considered to be Retired.
Tret(Kc) >= Tsaf(Ks)
S(Kc) = (DNSKEY Dead, RRSIG Withdrawn)
C(Kc) = Forgotten Retired
Event 7: From the perspective of the authoritative server, the
rollover is complete. After some delay, all signatures of Kc have
expired from the caches. This is Texp, and the key is said to be
Expired.
Texp(Kc) >= Tret(Kc) + DcacheZ
S(Kc) = (DNSKEY Dead, RRSIG Dead)
C(Kc) = Forgotten Expired
3.2. KSK Rollovers
The most common rollover method for KSKs is Double-Signature,
described in RFC4641 [RFC4641]. Two more methods are identified in
draft-ietf-dnsop-dnssec-key-timing [key-timing]: Double-DS and
Double-RRset. Double-RRset is the fastest way to rollover a KSK,
while Double-Signature minimizes the number of required interactions
to the parent, and Double-DS keeps your DNSKEY RRset as small as
possible.
Mekking Expires August 29, 2011 [Page 15]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Note that with these type of rollovers, we do not have to worry
whether the information within the zone is authentic. We assume that
there exists one or more ZSKs in the DNSKEY RRset that takes care of
this during the rollover.
3.2.1. Double-RRset
With Double-RRset, the new KSK is added to the DNSKEY RRset which is
then signed with both the old and new key, and the new DS record
added to the parent zone. After waiting a suitable interval for the
old DS and DNSKEY RRsets to expire from validating resolver caches,
the old DNSKEY and DS record are removed.
Only when Ks is said to be Known, e.g. the DNSKEY record of Ks is
known to all validating resolvers, we can remove the DS record of Kc.
And only when can ensure that all validators can use the DS record
for Ks to build the secure chain of trust, we can remove the DNSKEY
record of Kc. In other words, Ks needs to be Known and SafeDS.
Thus, we first have to introduce all new information into the zone.
Once all has been propagated, we can withdraw all information of Kc
from the zone.
The timeline diagram looks like this:
|1| |2| |3| |4| |5|
Kc | | | | |
- DNSKEY |----|-------|-----|DcacheK|
- DS |----|-------|-----|DcacheP|
| | | | |
Trem Tfor
TretDS TexpDS
Ks | | | | |
- DNSKEY | |DcacheK|-----|-------|---
- DS | |DcacheP|-----|-------|---
| | | | |
Tgen Tpub Tkno
TactDS TsafDS
Figure: KSK Double-RRset Rollover.
Event 1: Ks is generated at time Tgen.
S(Ks) = (DNSKEY Generated, DS Generated)
C(Ks) = Generated
Event 2: Ks is introduced into the zone; it is added to the DNSKEY
RRset, which is then signed by all currently active KSKs (including
Mekking Expires August 29, 2011 [Page 16]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Kc and Ks). In addition, the DS record is submitted to the parent.
This is Ks' publish time (Tpub), the time that Ks is said to be
Published. It is also Ks' submit time (TactDS), the time that the DS
record for Ks is Submitted (ActiveDS).
Tpub(ks) >= Tgen(Ks), TactDS(Ks) == Tpub(Ks)
S(Ks) = (DNSKEY Introduced, DS Introduced)
C(ks) = Published ActiveDS
After the registration delay, the DS is published in the parent.
Event 3: The information for Ks must be published long enough to
ensure that the information have reached all validating resolvers
that may have the DNSKEY or DS RRset from this zone cached. At the
point in time that the DNSKEY RRset including Ks has been propagated
(Tkno), Ks is said to be Known. At the point in time that the DS
RRset of Ks has been propagated (Tsaf), Ks is said to be SafeDS.
Tkno(Ks) >= Tpub(Ks) + DcacheK, TsafDS(Ks) >= TactDS(Ks) + DcacheP
S(Ks) = (DNSKEY Propagated, DS Propagated)
C(Ks) = Known SafeDS
Note that we could already send the request to the parent to withdraw
the DS record of Kc after DcacheK. It does not matter if the DS
record for Ks has not yet been propagated, since the resolver can
authenticate the DNSKEY RRset with both Kc and Ks. If the validator
fetches a DS RRset from the cache, it uses Kc. Otherwise, it can use
Ks.
Event 4: Once we have a successor key that is said to be Known, we
can withdraw the DS record for Kc. This is Kc' retire time (Tret),
the time that Kc is said to be RetiredDS. If Ks is also said to be
SafeDS, we no longer need to retain Kc in the zone. It is also Kc'
removal time (Trem), the time that Kc is said to be Removed.
TretDS(Kc) >= Tkno(ks)
Trem(Kc) >= MAX(TsafDS(Ks), Tkno(Ks))
S(Kc) = (DNSKEY Withdrawn, DS Withdrawn)
C(Kc) = Removed RetiredDS
Event 5: From the perspective of the authoritative server, the
rollover is complete. After some delay, Kc and its DS have also
expired from the caches.
Tfor(Kc) >= Trem(Kc) + DcachK
Mekking Expires August 29, 2011 [Page 17]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
TexpDS(Kc) >= TretDS(Kc) + DcacheP
S(Kc) = (DNSKEY Dead, DS Dead)
C(Kc) = Forgotten Expired
3.2.2. Double-Signature
With Double-Signature, the new KSK is added to the DNSKEY RRset which
is then signed with both the old and new key. After waiting for the
old RRset to expire from caches, the DS record in the parent zone is
changed. After waiting a further interval for this change to be
reflected in caches, the old key is removed from the RRset.
If you want to minimize the number of interactions to the parent,
this rollover method is preferred over the Double-RRset method. As a
consequence, you have to wait with submitting the DS record of Ks,
until it is safe to withdraw the DS record of Kc.
The timing diagram for such a rollover is:
|1| |2| |3| |4| |5| |6| |7|
Kc | | | | | | |
- DNSKEY |----|-------|-----|-------|------|DcacheK|
- DS |----|-------|-----|DcacheP| | |
| | | | | | |
TretDS TexpDS Trem Tfor
Ks | | | | | | |
- DNSKEY | |DcacheK|-----|-------|------|-------|---
- DS | | | |DcacheP|------|-------|---
| | | | | | |
Tgen Tpub Tkno TactDS TsafDS
Figure: KSK Double-Signature Rollover.
Event 1: Ks is generated at time Tgen.
S(Ks) = (DNSKEY Generated, DS Generated)
C(Ks) = Generated
Event 2: Ks is introduced into the zone; it is added to the DNSKEY
RRset, which is then signed by Ks and all currently actice KSKs
(including Kc). This is the publication time (Tpub), the time that
Ks is said to be Published.
Tpub(Ks) >= Tgen(Ks)
S(Ks) = (DNSKEY Introduced, DS Generated)
Mekking Expires August 29, 2011 [Page 18]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
C(Ks) = Published
Event 3: Before we can submit the corresponding DS, the DNSKEY record
for Ks must be published for long enough (DcacheK) to guarantee that
any resolver that has a copy of the DNSKEY RRset also includes this
key. In other words, that any prior cached information about the
DNSKEY RRset has expired. This time is Tkno and Ks is said to be
Known.
Tkno(Ks) >= Tpub(Ks) + DcacheK
S(Ks) = (DNSKEY Propagated, DS Generated)
C(Ks) = Known
Event 4: At some later time, the DS RR corresponding to Ks is
submitted to the parent zone for publication. In addition, the
request has been made to remove the DS RR corresponding to Kc from
the parent zone. This time is Ks' submit time (TactDS), the time
that Ks is considered to be Submitted. It is also Kc' retire time
(TretDS), the time that Kc is considered to be RetiredDS.
TactDS(Ks) >= Tkno(Ks)
TretDS(kc) == TactDS(Kc)
S(Kc) = (DNSKEY Propagated, DS Withdrawn)
C(Ks) = Known RetiredDS
S(Ks) = (DNSKEY Propagated, DS Introduced)
C(Ks) = Known ActiveDS
After the registration delay, the DS is published in the parent.
Event 5: At some time later, all validating resolvers that have the
DS RRset cached will have a a copy that includes the new DS record.
This is Ks' safe time (TsafDS), the time that the new KSK is said to
be SafeDS. Consequently, Kc is said to be ExpiredDS (TexpDS).
TsafDS(Ks) >= TactDS(Ks) + DcacheP
TexpDS(Kc) >= TretDS(Kc) + DcacheP
S(Kc) = (DNSKEY Propagated, DS Dead)
C(kc) = Known ExpiredDS
S(Ks) = (DNSKEY Propagated, DS Propagated)
C(Ks) = Known SafeDS
Event 6: When the new DS record has been propagated, the DNSKEY
record of Kc can be removed from the zone. This is Kc' removal time
(Trem), the time that Kc is said to be Removed.
Mekking Expires August 29, 2011 [Page 19]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Trem(Kc) >= TsafDS(Ks)
S(Kc) = (DNSKEY Withdrawn, DS Dead)
C(Kc) = Removed ExpiredDS
Event 7: From the perspective of the authoritative server, the
rollover is complete. After some delay, The DNSKEY record for Kc has
also expired from the caches.
Tfor(Kc) >= Trem(Kc) + DcacheK
S(Kc) = (DNSKEY Dead, DS Dead)
C(Kc) = Forgotten ExpiredDS
3.2.3. Double-DS
In this case, first the new DS record is published. After waiting
for this change to propagate into the caches of all validating
resolvers, the KSK is changed. After waiting another interval,
during which the old DNSKEY RRset expires from caches, the old DS
record is removed.
If you want to keep the size of the DNSKEY RRset to a minimum, this
rollover method is preferred over Double-RRset. It does require the
additional administrative overhead of two interactions with the
parent to roll a KSK.
The timeline diagram looks like this:
|1| |2| |3| |4| |5| |6| |7|
Kc | | | | | | |
- DNSKEY |----|-------|-------|DcacheK| | |
- DS |----|-------|-------|-------|-----|DcacheP|
| | | | | | |
Trem Tfor TretDS TexpDS
Ks | | | | | | |
- DNSKEY | | | |DcacheK|-----|-------|---
- DS | |DcacheP|-------|-------|-----|-------|---
| | | | | | |
Tgen TactDS TsafDS Tpub Tkno
Figure: KSK Double-DS Rollover.
Event 1: Ks is generated at time Tgen.
S(Ks) = (DNSKEY Generated, DS Generated)
C(Ks) = Generated
Mekking Expires August 29, 2011 [Page 20]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Event 2: Before we introduce the new key Ks into the zone, we are
going to submit the new DS. We can do that, because there exists a
valid chain of trust for the same algorithm (Kc). This time is Ks'
submit time (TactDS), the time that the DS record for Ks was
submitted and is said to be ActiveDS.
TactDS(Ks) >= Tgen(Ks)
S(Ks) = (DNSKEY Generated, DS Introduced)
C(Kc) = ActiveDS
After some delay, the DS becomes available in the parent zone.
Event 3: Some time later, the new DS RRset has been propagated. This
is Ks' safe time (TsafDS), the time that Ks is said to be SafeDS.
TsafDS(Ks) >= TactDS(Ks) + DcacheP
S(Ks) = (DNSKEY Generated, DS Propagated)
C(Ks) = SafeDS
Event 4: Because there are now two trust anchors a resolver can use,
we can switch the KSK in the DNSKEY RRset. We stop signing with Kc
and sign the DNSKEY RRset with Ks. This time is Ks' publish time
(Tpub), the time that Ks is said to be Published. It is also Kc'
removal time (Trem), the time that Kc is said to be Removed.
Tpub(Ks) >= TsafDS(Ks)
Trem(Kc) == Tpub(Ks)
S(Kc) = (DNSKEY Withdrawn, DS Propagated)
C(Kc) = Removed SafeDS
S(Ks) = (DNSKEY Introduced, DS Propagated)
C(Ks) = Published SafeDS
Event 5: We have to wait before Kc has been expired from the caches,
before we can withdraw the DS record of Kc. When the DNSKEY RRset
that includes Kc has been expired, Kc is said to be forgotten and Ks
is said to be Known. This happens at Ks' known time, given by:
Tkno(Ks) >= Tpub(Ks) + DcacheK
Tfor(Kc) == Tkno(Ks)
S(Kc) = (DNSKEY Dead, DS Propagated)
C(Kc) = Forgotten SafeDS
S(Ks) = (DNSKEY Propagated, DS Propagated)
C(Ks) = Known SafeDS
Mekking Expires August 29, 2011 [Page 21]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Event 6: Now that we have a key Ks that is said to be Propagated and
SafeDS, we are ready to withdraw the DS for Kc. We call this Kc'
retire time (TretDS), the time that we don't need a secure delegation
for Kc anymore.
TretDS(Kc) >= Tkno(Ks)
S(Kc) = (DNSKEY Dead, DS Withdrawn)
C(Kc) = Forgotten RetiredDS
Event 7: From the perspective of the authoritative server, the
rollover is complete. After some delay, The DS record for Kc has
expired from the caches. This is Texp, given by:
Texp(Kc) >= Tret(Kc) + DcacheP
S(Kc) = (DNSKEY Dead, DS Dead)
C(Kc) = Forgotten ExpiredDS
3.2.4. Interaction with Configured Trust Anchors
Zone managers may want to take into account the possibility that some
validating resolvers may have their KSK configured as a trust anchor
directly, as described in [RFC5011]. This influences the value of
DcacheK, the time to guarantee that any resolver that has a copy of
the newest DNSKEY RRset.
3.2.4.1. Adding a KSK
When the new key is introduced, the delay DcacheK between Tpub and
Tkno is also subject to the condition:
DcacheK' = MAX(DcacheK, 2 * (queryInterval + x * retryTime) + c)
The right hand side of this expression is two times the Active
Refresh time defined in section 2.3 in [RFC5011]. This ensures that
the successor key is at least seen twice by 5011-enabled validators.
The parameter x is the maximum number of retries that is taken as a
safety margin, in case an Active Refresh fails. The parameter c is a
constant that can be taken as an additional safety margin.
Most probably, this delays the time when a key is said to be Known.
3.2.4.2. Removing a KSK
When the current key is ready to be removed from the zone, it is
instead said to be Revoked. The REVOKE bit is said and the key is
published for DcacheK' time:
Mekking Expires August 29, 2011 [Page 22]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
DcacheK' = MAX(DcacheK, (queryInterval + x * retryTime) + c)
The right hand side of this expression is the Active Refresh time
defined in section 2.3 in RFC5011 [RFC5011]. This ensures that the
revoked key is at least seen once by 5011-enabled validators.
After that delay, we can guarantee that every 5011-enabled resolver
has seen the revoked key and it may be removed from the zone.
Another DcacheK delay, the key has fully expired from all the
resolver caches.
3.3. Rollovers in a Single Type Signing Scheme
In situations where you use a Single Type Signing Scheme, you can
combine one of the ZSK rollover methods with one of the KSK rollover
methods. However, not all combinations are possible. The KSK
Double-DS rollover is only suitable for combining with the ZSK
Double-RRSIG rollover, because both keep the DNSKEY RRset to a
minimum size. The other ZSK rollovers require a period where both
the current key and its successor are being served at the same time.
The KSK Double-RRset method is suitable with both the other ZSK
rollover methods, but does not gain any advantages when combined with
the ZSK Pre-Publication method. Therefore, we can leave that
combination out. The KSK Double-Signature method is suitable with
both the ZSK Double-Signature and the ZSK Pre-Publication method.
To conclude, we can identify four different rollover methods for the
Single Type Signing Scheme.
3.3.1. Double-RRset
This is a combination of the ZSK Double-Signature rollover and the
KSK Double-RRset rollover. The new KSK is added to the DNSKEY RRset,
and all RRsets are then signed with both the old and new key, and the
new DS record added to the parent zone. After waiting a suitable
interval for the old DS and all zone RRsets to expire from validating
resolver caches, the old DNSKEY and DS record are removed.
Double-RRset is the fastest way to replace keys in a Single Type
Signing Scheme. However, it does have a lot of disadvantages of - it
requires two signatures and two keys during the period of the
rollover, as well as two interactions with the parent.
Mekking Expires August 29, 2011 [Page 23]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
The timeline diagram looks like this:
|1| |2| |3| |4| |5|
Kc | | | | |
- RRSIG |----|-------|-----|DcacheZ|
- DNSKEY |----|-------|-----|DcacheK|
- DS |----|-------|-----|DcacheP|
| | | | |
Trem Tfor
Tret Texp
TretDS TexpDS
Ks | | | | |
- RRSIG | |DcacheZ|-----|-------|---
- DNSKEY | |DcacheK|-----|-------|---
- DS | |DcacheP|-----|-------|---
| | | | |
Tgen Tpub Tkno
Tact Tsaf
TactDS TsafDS
Figure: STSS Double-RRset Rollover.
The rollover method is almost the same as that of the KSK Double-
RRset rollover, except we now have to take DcacheZ into account.
3.3.2. Double-Signature
This is a combination of the ZSK Double-Signature rollover and the
KSK Double-Signature rollover. The new key is added to the DNSKEY
RRset and all RRsets are then signed with both the old and new key.
After waiting for the old RRsets to expire from caches, the DS record
in the parent zone is changed. After waiting a further interval for
this change to be reflected in caches, the old key is removed from
the DNSKEY RRset, and all RRsets are signed with the new key only.
This rollover minimizes the number of interactions with the parent
zone. However, for the period of the rollover all RRsets are still
signed with two keys, so increasing the size of the zone and the size
of the response.
Mekking Expires August 29, 2011 [Page 24]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
The timing diagram for such a rollover is:
|1| |2| |3| |4| |5| |6| |7|
Kc | | | | | | |
- RRSIG |----|-------|-----|-------|------|DcacheZ|
- DNSKEY |----|-------|-----|-------|------|DcacheK|
- DS |----|-------|-----|DcacheP| | |
| | | | | | |
TretDS TexpDS Trem Tfor
Tret Texp
Ks | | | | | | |
- RRSIG | |DcacheZ|-----|-------|------|-------|---
- DNSKEY | |DcacheK|-----|-------|------|-------|---
- DS | | | |DcacheP|------|-------|---
| | | | | | |
Tgen Tpub Tkno TactDS TsafDS
Tact Tsaf
Figure: STSS Double-Signature Rollover.
The rollover diagram is almost the same as that of the KSK Double-
Signature rollover, except we now have to take DcacheZ into account.
3.3.3. Pre-Publication
This is a combination of the ZSK Pre-Publication rollover and the KSK
Double-Signature rollover and requires only one interaction with the
parent. In addition, your non-DNSKEY RRsets require only one
signature during the rollover. If speed is not an issue, this
rollover method is considered to be the best practice in a Single
Type Signing Scheme environment.
The new key is added to the DNSKEY RRset and the DNSKEY RRset is then
signed with both the old and new key. Other RRsets will only be
signed with the old key. Only after the DS has been switched, the
signatures of other RRsets are replaced with that of the new key.
After waiting a further interval for this change to be reflected in
caches, the old key is removed from the RRset, and is signed with the
new key only.
Mekking Expires August 29, 2011 [Page 25]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
The timeline diagram looks like this:
|1| |2| |3| |4| |5| |6| |7| |8| |9|
Kc | | | | | | | | |
- RRSIG |----|-------|-----|-------|-------|DcacheZ| | |
- DNSKEY |----|-------|-----|-------|-------|-------|----|DcacheK|
- DS |----|-------|-----|DcacheP| | | | |
| | | | | | | | |
TretDS TexpDS Tret Texp Trem Tfor
Ks | | | | | | | | |
- RRSIG | | | | | |DcacheZ|----|-------|
- DNSKEY | |DcacheK|-----|-------|-------|-------|----|-------|
- DS | | | |DcacheP|-------|-------|----|-------|
| | | | | | | | |
Tgen Tpub Tkno TactDS TsafDS Tact Tsaf
Figure: STSS Pre-Publication Rollover.
Event 1: Key Ks is generated at the generate time (Tgen).
S(Ks) = (DNSKEY Generated, RRSIG Generated, DS Generated)
C(Ks) = Generated
Event 2: The DNSKEY record of Ks is put into the zone, i.e. it is
added to the DNSKEY RRset which is then re-signed with the Ks and all
other current KSKs (including Kc). The time at which this occurs is
Ks' publication time (Tpub), and the key is now said to be Published.
Note that the key is not yet used to sign other RRsets.
Tpub(Ks) >= Tgen(Ks)
S(Ks) = (DNSKEY Introduced, RRSIG Generated, DS Generated)
C(Ks) = Published
Event 3: Before we can switch the DS, the DNSKEY record for Ks must
be published for long enough (DcacheK) to guarantee that any resolver
that has a copy of the DNSKEY RRset also includes this key. After
this delay, the key is said to be Known and the DS record may be
submitted. The time at which this event occurs is Ks' known time
(Tkno), which is given by:
Tkno(Ks) >= Tpub(Ks) + DcacheK
S(Ks) = (DNSKEY Propagated, RRSIG Generated, DS Generated)
C(ks) = Known
Event 4: The time that the DS record of Ks is submitted is at Ks'
Mekking Expires August 29, 2011 [Page 26]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
submit time (TactDS). Ks is said to be ActiveDS. At the same time,
the DS record of Kc is withdrawn (TretDS) and Kc is said to be
RetiredDS.
TactDS(Ks) >= Tkno(Ks) TretDS(Kc) == TactDS(Ks)
S(Kc) = (DNSKEY Propagated, RRSIG Propagated, DS Withdrawn)
C(Kc) = Known Safe RetiredDS
S(Ks) = (DNSKEY Propagated, RRSIG Generated, DS Introduced)
C(Ks) = Known ActiveDS
Some time later, the new DS RRset is published at the parent.
Event 5: Some time later, we can guarantee that all validating
resolvers use the DS RRset that includes a copy of the DS record of
DS. At this time, Ks' safe time (TsafDS), Ks is said to be SafeDS.
But we still use Kc as ZSK.
TsafDS(Ks) >= TactDS(Ks) + DcacheP
TexpDS(Kc) >= TretDS(Kc) + DcacheP
S(Kc) = (DNSKEY Propagated, RRSIG Propagated, DS Dead)
C(kc) = Known Safe ExpiredDS
S(Ks) = (DNSKEY Propagated, RRSIG Generated, DS Propagated)
C(Ks) = Known SafeDS
Event 6: At some point in time, the decision is made to actually
start signing the zone using the successor key. This will be when
the current key has been in use for an interval equal to the ZSK
lifetime. This point in time is Ks' active time (Tact), the time
that Ks is said to be Active. It is also Kc' retire time (Tret), the
time that Kc is said to be Retired.
Tact(Ks) >= TsafDS(Ks)
Tret(Kc) == Tact(Ks)
S(Kc) = (DNSKEY Propagated, RRSIG Withdrawn, DS Dead))
C(Kc) = Known Retired ExpiredDS
S(Ks) = (DNSKEY Propagated, RRSIG Introduced, DS Propagated))
C(Ks) = Known Active SafeDS
Event 7: Kc needs to be retained in the zone whilst any RRSIG records
created by the retired key are still published in the zone or held in
resolver caches. In other words, Kc should be retained in the zone
until all RRSIG records created by Ks have been propagated. This
time is Ks' safe time (Tsaf), the time that Ks is considered to be
Safe, and Kc' expiration time (Texp), the time that Kc is considered
to be Expired.
Mekking Expires August 29, 2011 [Page 27]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Tsaf(Ks) >= Tact(Ks) + DcacheZ
Texp(Kc) == Tsaf(Ks)
S(Kc) = (DNSKEY Propagated, RRSIG Dead, DS Dead)
C(Kc) = Known Expired ExpiredDS
S(Ks) = (DNSKEY Propagated, RRSIG Propagated, DS Propagated)
C(Ks) = Known Safe SafeDS
Event 8: When all new signatures have been propagated, Kc can be
removed from the zone and the DNSKEY RRset re-signed with the current
KSK. This time is Kc' removal time (Trem), the time that Kc is
considered to be Removed.
Trem(Kc) >= Tsaf(Ks)
S(Kc) = (DNSKEY Withdrawn, RRSIG Dead, DS Dead)
C(Kc) = Removed Expired ExpiredDS
Event 9: From the perspective of the authoritative server, the
rollover is complete. After some delay, The DNSKEY record for Kc has
expired from the caches. This is Tfor, the time that the key is said
to be Forgotten.
Tfor(Kc) >= Trem(Kc) + DcacheK
S(Kc) = (DNSKEY Dead, RRSIG Dead, DS Dead)
C(Kc) = Forgotten Expired ExpiredDS
3.3.4. Double-DS
This is a combination of the ZSK Double-RRSIG rollover and the KSK
Double-DS rollover. This keeps your DNSKEY RRset to a minimum size,
but at the cost of double signatures in your zone and double DS at
the parent.
The new signatures are added to the zone and the new DS is submitted.
Once all signatures and the DS record have been propagated, the
DNSKEY is switched. After waiting a further interval for this switch
to be reflected in caches, the old signatures are removed and the old
DS is withdrawn from the parent zone.
Mekking Expires August 29, 2011 [Page 28]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
The timeline diagram looks like this:
|1| |2| |3| |4| |5| |6| |7| |8| |9|
Kc | | | | | | | | |
- RRSIG |----|-------|-----|-------|-------|DcacheZ| | |
- DNSKEY |----|-------|-----|-------|-------|-------|----|DcacheK|
- DS |----|-------|-----|DcacheP| | | | |
| | | | | | | | |
TretDS TexpDS Tret Texp Trem Tfor
Ks | | | | | | | | |
- RRSIG | | | | | |DcacheZ|----|-------|
- DNSKEY | |DcacheK|-----|-------|-------|-------|----|-------|
- DS | | | |DcacheP|-------|-------|----|-------|
| | | | | | | | |
Tgen Tpub Tkno TactDS TsafDS Tact Tsaf
Figure: STSS Double-DS Rollover.
Event 1: Key Ks is generated at the generate time (Tgen).
S(Ks) = (DNSKEY Generated, RRSIG Generated, DS Generated)
C(Ks) = Generated
Event 2: Before we introduce the new key Ks into the zone, we are
going to add the new signatures and submit the new DS. This time is
Ks' active time (Tact), the time that Ks is said to be Active. It is
also Ks' submit time (TactDS), the time that the DS record for Ks was
submitted and is said to be ActiveDS.
Tact(Ks) >= Tgen(Ks)
TactDS(Ks) >= Tgen(Ks)
S(Ks) = (DNSKEY Generated, RRSIG Introduced, DS Introduced)
C(Kc) = Active ActiveDS
After some delay, the DS becomes available in the parent zone.
Event 3: Some time later, the new signatures and the new DS RRset
have been propagated. This is Ks' safe time (Tsaf, TsafDS), the time
that Ks is said to be Safe and SafeDS.
Tsaf(Ks) >= Tact(Ks) + DcacheZ
TsafDS(Ks) >= TactDS(Ks) + DcacheP
S(Ks) = (DNSKEY Generated, RRSIG Propagated, DS Propagated)
C(Ks) = Safe SafeDS
Mekking Expires August 29, 2011 [Page 29]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Event 4: Because there are now two trust anchors a resolver can use,
we can switch the KSK in the DNSKEY RRset. This time is Ks' publish
time (Tpub), the time that Ks is said to be Published. It is also
Kc' removal time (Trem), the time that Kc is removed from the zone.
Tpub(Ks) >= MAX(TsafDS(Ks), Tsaf(Ks))
Trem(Kc) == Tpub(Ks)
S(Kc) = (DNSKEY Withdrawn, RRSIG Propagated, DS Propagated)
C(Kc) = Removed Safe SafeDS
S(Ks) = (DNSKEY Introduced, RRSIG Propagated, DS Propagated)
C(Ks) = Published Safe SafeDS
Event 5: We have to wait before the signatures of Kc and its
corresponding DS record have been expired from the caches, before we
can withdraw the DNSKEY record of Kc. When the DNSKEY RRset that
includes Kc has been expired, Ks is said to be Known and Kc is said
to be Removed. This happens at Ks' known time, given by:
Tkno(Ks) >= Tpub(Ks) + DcacheK, Trem(Kc) == Tkno(Ks)
S(Kc) = (DNSKEY Dead, RRSIG Propagated, DS Propagated)
C(Kc) = Forgotten Safe SafeDS
S(Ks) = (DNSKEY Propagated, RRSIG Propagated, DS Propagated)
C(Ks) = Known Safe SafeDS
Event 6: Now that we have a key Ks that is said to be Propagated and
SafeDS, we are ready to withdraw the signatures and DS for Kc. We
call this Kc' retire time (Tret, TretDS), the time Kc is said to be
Retired and RetiredDS.
Tret(Kc) >= Tkno(Ks)
TretDS(Kc) >= Tkno(Ks)
S(Kc) = (DNSKEY Dead, RRSIG Withdrawn, DS Withdrawn)
C(Kc) = Forgotten Retired RetiredDS
Event 7: From the perspective of the authoritative server, the
rollover is complete. After some delay, The signatures of Kc and its
corresponding DS record have expired from the caches.
Texp(Kc) >= Tret(Kc) + DcacheZ
TexpDS(Kc) >= TretDS(Kc) + DcacheP
S(Kc) = (DNSKEY Dead, RRSIG Dead, DS Dead)
C(Kc) = Forgotten Expired ExpiredDS
Mekking Expires August 29, 2011 [Page 30]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
3.4. Stand-by Keys
Although keys will usually be rolled according to some regular
schedule, there may be occasions when an emergency rollover is
required, e.g. if the active key is suspected of being compromised.
The aim of the emergency rollover is to allow the zone to be re-
signed with a new key as soon as possible. As a key must be ready to
sign the zone, having at least one additional key (a stand-by key) in
this state at all times will minimise delay.
In the case of a ZSK, a stand-by key only makes sense with the Pre-
Publication method, since with the Double-Signature and Double-RRSIG
methods, the stand-by key would be used for signing. The goal is to
make the stand-by key Known. This goal is reached at Tkno, step 3 in
the Pre-Publication method timeline diagram.
A successor key must always be published soon enough so that the key
lifetime of the predecessor key does not exceed. That means that the
successor ZSK Ks must at latest be published DcacheK delay before the
lifetime of the predecessor ZSK kc has reached:
Tpub(Ks) <= Tact(Kc) + Lzsk - DcacheK
Here, Lzsk is the lifetime of ZSKs according to policy.
In the case of a KSK, a stand-by key only makes sense with the
Double-DS method, since in the other cases, the key would be needed
to sign the DNSKEY RRset. The goal is to get the stand-by key in the
SafeDS condition. This goal is reached at TsafDS, step 3 in the
Double-DS method timeline diagram.
The DS record for the successor KSK Ks should be propagated to the
caches before the key lifetime of the predecessor KSK Kc exceeds:
TactDS(Ks) <= Tact(Kc) + Lksk - DcacheP
Here, Lksk is the lifetime of KSKs according to policy.
Because a stand-by KSK only makes sense with the Double-DS method,
stand-by keys in a STSS is not applicable. This is because the
Double-DS method is not easy integratable with one of the ZSK
rollover methods.
4. Policy rollover
Besides your scheduled and unscheduled key rollovers, changes in
policy may occur. The initial transition is enabling DNSSEC. The
counterpart, disabling DNSSEC, is also possible. Two other policy
Mekking Expires August 29, 2011 [Page 31]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
changes we have encountered are are algorithm rollover and changing
signing schemes.
4.1. Enabling DNSSEC
When a zone makes the transition from going insecure to secure, the
initial set of keys safely need to be introduced into the zone. The
goals of this event is to make a ZSK (Kz) and a KSK (Kk) both Known
and Safe.
One must take into account that resolver caches may hold unsigned
RRsets. Therefore, validating resolvers should not know about the
initial DNSKEY RRset before all unsigned RRsets have been expired
from the caches. This means that the zone must be fully signed,
before the DS associated with the initial KSK is published. Only if
you are afraid that a key scraper fetches your DNSKEY RRset too soon,
you should wait with publishing your DNSKEY RRset until enough time
has elapsed for all unsigned RRsets to expire from all resolver
caches. The ZSK and KSK can be the same key, for example in a Single
Type Signing Scheme.
The timeline diagram is shown below:
|1| |2| |3| |4| |5|
Kz | | | | |
- RRSIG | |DcacheZ|------|-------|---
- DNSKEY | |DcacheK|------|-------|---
| | | | |
Tgen Tpub Tkno
Tact Tsaf
Kk | | | | |
- DNSKEY | |DcacheK|------|-------|---
- DS | | | |DcacheP|---
| | | | |
Tgen Tpub Tkno TactDS TsafDS
Figure: Enabling DNSSEC.
Event 1: Kk and Kz are generated. We call this Tgen, the time that
the keys were Generated (note that Tgen for Kk could be different
that Tgen for Kz).
S(Kk) = (DNSKEY Generated, DS Generated)
C(Kk) = Generated
S(Kz) = (DNSKEY Generated, RRSIG Generated)
C(Kk) = Generated
Mekking Expires August 29, 2011 [Page 32]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Event 2: The keys are put into the zone and are immediately used for
signing. Because there exists no pointer to the fact that our zone
is DNSSEC enabled, the DNSKEY and RRSIG records may be introduced at
the same time. This is the publish time (Tpub), the time that the
keys are Published. It is also Kz' active time (Tact), the time that
Kz is said to be Active.
Tpub(Kk) >= Tgen(Kk)
Tpub(Kz) >= Tgen(Kz)
Tact(Kz) == Tpub(Kz)
S(Kk) = (DNSKEY Introduced, DS Generated)
C(Kk) = Published
S(Kz) = (DNSKEY Introduced, RRSIG Introduced)
C(Kz) = Published Active
Event 3: Before we can submit the DS record, Kz must be considered
Known and Safe. Once that has happened, we are done for the ZSK.
This time is Kz' known time (Tkno).
Tkno(Kk) >= Tpub(Kk) + DcacheP
Tkno(Kk) == Tkno(Kz)
Tsaf(Kz) >= Tact(Kz) + DcacheZ
S(Kk) = (DNSKEY Propagated, DS Generated)
C(Kk) = Known
S(Kz) = (DNSKEY Propagated, RRSIG Propagated)
C(Kz) = Known Safe
Because this is the first DNSKEY for this zone, the Dttl for the
DNSKEY RRset is Ingc, the negative cache interval from the zone's SOA
record, calculated according to RFC2308 [RFC2308] as the minimum of
the TTL of the SOA record itself and the MINIMUM field in the
record's parameters:
Ingc = min(TTL(SOA), MINIMUM)
Event 4: Once we are sure of the fact that the DNSKEY RRset and all
RRSIG records have reached the caches, we may submit the DS to the
parent. We call this TactDS, the time that the DS has been submitted
to the parent.
TactDS(Kk) >= Tkno(Kk)
S(Kk) = (DNSKEY Propagated, DS Introduced)
C(Kk) = Known ActiveDS
Event 5: Some time later, the DS has been published in the parent
Mekking Expires August 29, 2011 [Page 33]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
zone. Some more time later, all resolvers that have a copy of the DS
RRset have one that includes the DS record of Kk.
TsafDS(Kk) >= TactDS(Kk) + DcacheP
S(Kk) = (DNSKEY Propagated, DS Propagated)
C(Kk) = Known SafeDS
Because this is the first DS for this zone, the Dttl for the DS RRset
is Ingc, for the same reason as in step 3 for the DNSKEY RRset.
4.2. Disabling DNSSEC
When a zone decides for whatever reason to go back to the Insecure
status, the set of keys safely need to be removed from the zone. We
assume that there is a KSK (Kk) and a ZSK (Kz) that are Known and
Safe. The goals of this event is to make Kk and Kz both Forgotten
and Expired.
The timeline diagram is shown below:
|0| |1| |3| |4| |5|
Kz | | | | |
- RRSIG |----|-------|------|DcacheZ|
- DNSKEY |----|-------|------|DcacheK|
| | | | |
Trem Tfor
Tret Texp
Kk | | | | |
- DNSKEY |----|-------|------|DcacheK|
- DS |----|DcacheP| | |
| | | | |
TretDS TexpDS Trem Tfor
Figure: Disabling DNSSEC.
Event 1: The DS record of Kk needs to be withdrawn. This time is Kk'
retire time (TretDS), the time that Kk is said to be RetiredDS.
S(Kk) = (DNSKEY Propagated, DS Withdrawn)
C(Kk) = Known RetiredDS
Event 2: We have to wait until the DS record of Kk has expired from
all resolver caches. This time is Kk' expire time (TexpDS), the time
that Kk is said to be ExpiredDS.
Mekking Expires August 29, 2011 [Page 34]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
TexpDS(Kk) >= TretDS(Kk) + DcacheP
S(Kk) = (DNSKEY Propagated, DS Dead)
C(Kk) = Known ExpiredDS
Event 3: Now that we can guarantee that no secure chain of trust to
Kk exist anymore, we can retire the ZSK and withdraw both keys. This
time is Trem, the time that the keys are removed from the zone.
Trem(Kk) >= TexpDS(Kk)
Trem(Kz) == Trem(Kk)
Tret(Kz) == Trem(kz)
S(Kk) = (DNSKEY Withdrawn, DS Dead)
C(Kk) = Removed ExpiredDS
S(Kz) = (DNSKEY Withdrawn, RRSIG Withdrawn)
C(Kz) = Removed Retired
Event 4: After some delay, all information about the keys have
expired from the caches.
Tfor(Kk) >= Trem(Kk) + DcacheK
Tfor(Kz) == Tfor(Kk)
Texp(Kz) >= Tret(Kz) + DcacheZ
S(Kk) = (DNSKEY Dead, DS Dead)
C(Kk) = Forgotten ExpiredDS
S(Kz) = (DNSKEY Dead, RRSIG Dead)
C(Kz) = Forgotten Expired
4.3. Algorithm Rollover
When changing algorithms, you can either add, remove or replace an
algorithm. Adding and removing an algorithm follow the same timings
as enabling and disabling DNSSEC. Replacing an algorithm can be done
with a STSS Double-Signature rollover or a KSK and ZSK Double-
Signature Rollover at the same time. [MM: This needs more text, but
I am awaiting the discussion about algorithm rollover and how to
interpret section 2.2 of RFC 4035]
4.4. KSK-ZSK Split or Single Type Signing Scheme
When changing signing schemes, you should follow the timelines of the
most restricting signing scheme. The STSS signing scheme makes some
rollover combinations unsuitable, thus it can be considered the most
restricted signing scheme. In the case of moving to a KSK-ZSK Split,
Ks is used as the successor key in the STSS rollover methods, and it
now reflects both the successor ZSK and KSK. In the case of moving
Mekking Expires August 29, 2011 [Page 35]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
away from a KSK-ZSK Split, Kc is used as the predecessor key in the
STSS rollover methods, and it now reflects both the predecessor ZSK
and KSK. [MM: This could perhaps also use more explanation.]
5. IANA Considerations
This memo includes no request to IANA.
6. Security Considerations
This document does not introduce any new security issues beyond those
already discussed in RFC4033 [RFC4033], RFC4034 [RFC4034]. RFC4035
[RFC4035] and RFC5011 [RFC5011].
7. Acknowledgements
Special acknowledgments and gratitude go out to Stephen Morris, Johan
Ihren and John Dickinson, the authors of the key-timing draft
[key-timing]. Significant parts of the text is taken from that
document. Especially Section 3.1 and Section 3.2 are largely copied
and adjusted to the new introduced terminology from this document.
I also want to acknowledge Yuri Schaeffer, who brought to my
attention the idea of key goals (Section 1.1.1) and whose discussions
helped to shape this document.
8. Changes with key-timing draft
This document builds further on draft-ietf-dnsop-dnssec-key-timing
[key-timing]. The most important changes with respect to that
document are:
- Introduced the concept of Rollover Considerations (Speed vs Size vs
Interactions), that causes the existence of different key rollover
scenarios.
- Introduced the concept of Key Goals.
- Key States are unraveled to represent the status of each piece of
information seperately. Provides more flexibility. Used for
combining rollover methods in a Single Type Singing Scheme.
- What were Key States in the key-timing draft, are now called Key
Conditions. A key can have more than one condition.
- Four new Key Conditions are introduced: Known, Safe, Forgotten and
Expired, to represent whether information about the key exist in
resolver caches. The key conditions Ready and Dead are deprecated.
Mekking Expires August 29, 2011 [Page 36]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
- Timelines for STSS Rollovers.
- Timelines for enabling and disabling DNSSEC.
- Text about policy rollover, such as algorithm rollover and changing
signing schemes.
9. References
9.1. Informative References
[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational
Practices", RFC 4641, September 2006.
9.2. Normative References
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security
Extensions", RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC5011] StJohns, M., "Automated Updates of DNS Security
(DNSSEC) Trust Anchors", RFC 5011, September 2007.
[dnssec-bis] Weiler, S. and D. Blacka, "Clarifications and
Implementation Notes for DNSSECbis", November 2010.
[key-timing] Morris, S., Ihren, J., and J. Dickinson, "DNSSEC Key
Timing Considerations", July 2010.
Mekking Expires August 29, 2011 [Page 37]
Internet-Draft DNSSEC Key Timing Considerations Bis February 2011
Author's Address
Matthijs Mekking
NLnet Labs
Science Park 140
Amsterdam 1098 XG
The Netherlands
EMail: matthijs@nlnetlabs.nl
Mekking Expires August 29, 2011 [Page 38]