Internet Engineering Task Force S. Morris
Internet-Draft ISC
Intended status: Informational J. Ihren
Expires: January 2, 2011 Netnod
J. Dickinson
Sinodun
July 1, 2010
DNSSEC Key Timing Considerations
draft-ietf-dnsop-dnssec-key-timing-00.txt
Abstract
This document describes the issues surrounding the timing of events
in the rolling of a key in a DNSSEC-secured zone. It presents
timelines for the key rollover and explicitly identifies the
relationships between the various parameters affecting the process.
Status of this Memo
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Key Rolling Considerations . . . . . . . . . . . . . . . . 3
1.2. Types of Keys . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Rollover Methods . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. ZSK Rollovers . . . . . . . . . . . . . . . . . . . . . . 4
2.2. KSK Rollovers . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Key Rollover Timelines . . . . . . . . . . . . . . . . . . . . 8
3.1. Key States . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Zone-Signing Key Timelines . . . . . . . . . . . . . . . . 9
3.2.1. Pre-Publication Method . . . . . . . . . . . . . . . . 9
3.2.2. Double-Signature Method . . . . . . . . . . . . . . . 13
3.2.3. Double-RRSIG Method . . . . . . . . . . . . . . . . . 14
3.3. Key-Signing Key Rollover Timelines . . . . . . . . . . . . 17
3.3.1. Double-Signature Method . . . . . . . . . . . . . . . 17
3.3.2. Double-DS Method . . . . . . . . . . . . . . . . . . . 20
3.3.3. Double-RRset Method . . . . . . . . . . . . . . . . . 22
3.3.4. Interaction with Configured Trust Anchors . . . . . . 25
3.3.4.1. Addition of KSK . . . . . . . . . . . . . . . . . 25
3.3.4.2. Removal of KSK . . . . . . . . . . . . . . . . . . 25
3.3.5. Introduction of First KSK . . . . . . . . . . . . . . 26
4. Standby Keys . . . . . . . . . . . . . . . . . . . . . . . . . 27
5. Algorithm Considerations . . . . . . . . . . . . . . . . . . . 28
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
8. Security Considerations . . . . . . . . . . . . . . . . . . . 28
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
10. Change History . . . . . . . . . . . . . . . . . . . . . . . . 29
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Normative References . . . . . . . . . . . . . . . . . . . 30
11.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. List of Symbols . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
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1. Introduction
1.1. Key Rolling Considerations
When a zone is secured with DNSSEC, the zone manager must be prepared
to replace ("roll") the keys used in the signing process. The
rolling of keys may be caused by compromise of one or more of the
existing keys, or it may be due to a management policy that demands
periodic key replacement for security or operational reasons. In
order to implement a key rollover, the keys need to be introduced
into and removed from the zone at the appropriate times.
Considerations that must be taken into account are:
o DNSKEY records and associated information (such as RRSIG records
created with the key or the associated DS records) are not only
held at the authoritative nameserver, they are also cached at
client resolvers. The data on these systems can be interlinked,
e.g. a validating resolver may try to validate a signature
retrieved from a cache with a key obtained separately.
o A query for the key RRset returns all DNSKEY records in the zone.
As there is limited space in the UDP packet (even with EDNS0
support), dead key records must be periodically removed. (For the
same reason, the number of stand-by keys in the zone should be
restricted to the minimum required to support the key management
policy.)
o Zone "boot-strapping" events, where a zone is signed for the first
time, can be common in configurations where a large number of
zones are being served. Procedures should be able to cope with
the introduction of keys into the zone for the first time as well
as "steady-state", where the records are being replaced as part of
normal zone maintenance.
o To allow for an emergency re-signing of the zone as soon as
possible after a key compromise has been detected, stand-by keys
(additional keys over and above those used to sign the zone) need
to be present.
Management policy, e.g. how long a key is used for, also needs to be
considered. However, the point of key management logic is not to
ensure that a "rollover" is completed at a certain time but rather to
ensure that no changes are made to the state of keys published in the
zone until it is "safe" to do so ("safe" in this context meaning that
at no time during the rollover process does any part of the zone ever
go bogus). In other words, although key management logic enforces
policy, it may not enforce it strictly.
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1.2. Types of Keys
Although DNSSEC validation treats all keys equally, [RFC4033]
recognises the broad classification of zone-signing keys (ZSK) and
key-signing keys (KSK). A ZSK is used to authenticate information
within the zone; a KSK is used to authenticate the key set in the
zone. The main implication for this distinction concerns the
consistency of information during a rollover.
During operation, a validating resolver must use separate pieces of
information to perform an authentication. At the time of
authentication, each piece of information may be in the validating
resolver's cache or may need to be retrieved from the authoritative
server. The rollover process needs to happen in such a way that at
all times through the rollover the information is consistent. With a
ZSK, the information is the RRSIG (plus associated RRset) and the
DNSKEY. These are both obtained from the same zone. In the case of
the KSK, the information is the DNSKEY and DS RRset with the latter
being obtained from a different zone.
There are similarities in the rolling of ZSKs and KSKs: replace the
RRSIG (plus RR) by the DNSKEY and replace the DNSKEY by the DS, and
the ZSK rolling algorithms are virtually the same as the KSK
algorithms. However, there are a number of differences, and for this
reason the two types of rollovers are described separately in this
document.
1.3. Terminology
The terminology used in this document is as defined in [RFC4033] and
[RFC5011].
A large number of symbols are used to identify times, intervals, etc.
All are listed in Appendix A.
2. Rollover Methods
2.1. ZSK Rollovers
A ZSK can be rolled in one of three ways:
o Pre-Publication. Described in [RFC4641], 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
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new key, and the old signatures removed. During the re-signing
process (which may or may not be atomic depending on how the zone
is managed), 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
signatures anywhere that were created using the old key, and it
can can be removed from the DNSKEY RRset.
o Double-Signature. Also mentioned in [RFC4641], 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
(again, the signing process may not be atomic), 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.
o Double-RRSIG. Strictly speaking, the use of the term "Double-
Signature" above is a misnomer as the method is not only double
signature, it is also double key as well. A true Double-Signature
method (here called the Double-RRSIG method) involves introducing
new signatures in the zone (while still retaining the old ones)
but not the new key.
Once the signing process is complete and enough time has elapsed
to ensure that all caches that may contain an RR and associated
RRSIG to have a copy of both signatures, the ZSK is changed.
After a further interval during which the old DNSKEY RRset expires
from caches, the old signatures are removed from the zone.
Of three methods, Double-Signature is the simplest conceptually -
introduce the new key and new signatures, then approximately one TTL
later remove the old key and signatures. 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.
Pre-Publication is more complex - introduce the new key,
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approximately one TTL later sign the records, and approximately one
TTL after that remove the old key. However, the amount of DNSSEC
data is kept to a minimum which reduces the impact on performance.
The Double-RRSIG combines the increase in data volume of the Double-
Signature method with the complexity of Pre-Publication. It has few
(if any) advantages and a description is only included here for
completeness.
2.2. KSK Rollovers
In the ZSK case the issue for the validating resolver is to ensure
that it has access to the ZSK that corresponds to a particular
signature. In the KSK case this can never be a problem as the KSK is
only used for one signature (that over the DNSKEY RRset) and both the
key the signature travel together. Instead, the issue is to ensure
that the KSK is trusted.
Trust in the KSK is either due to the existence of an explicitly
configured trust anchor in the validating resolver or DS record in
the parent zone (which is itself trusted). If the former, [RFC5011]
timings will be needed to roll the keys. If the latter, the rollover
algorithm will need to involve the parent zone in the addition and
removal of DS records, so timings are not wholly under the control of
the zone manager. (The zone manager may elect to include [RFC5011]
timings in the key rolling process so as to cope with the possibility
that the key has also been explicitly configured as a trust anchor.)
It is important to note that this does not preclude the development
of key rollover logic; in accordance with the goal of the rollover
logic being able to determine when a state change is "safe", the only
effect of being dependent on the parent is that there may be a period
of waiting for the parent to respond in addition to any delay the key
rollover logic requires. Although this introduces additional delays,
even with a parent that is less than ideally responsive the only
effect will be a slowdown in the rollover state transitions. This
may cause a policy violation, but will not cause any operational
problems.
Like the ZSK case, there are three methods for rolling a KSK:
o Double-Signature: Also known as Double-DNSKEY, 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. (The name "Double-
Signature" is used because, like the ZSK method of the same name,
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the new key is introduced and immediately used for signing.)
o Double-DS: 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 a further interval during which the old
DNSKEY RRset expires from caches, the old DS record is removed.
o 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.
In essence, "Double-Signature" means that the new KSK is introduced
first and used to sign the DNSKEY RRset. The DS record is changed,
and finally the old KSK removed. With "Double-DS" it is the other
way around. Finally, Double-RRset does both updates more or less in
parallel.
The strategies have different advantages and disadvantages:
o Double-Signature minimizes the number of interactions with the
parent zone. However, for the period of the rollover the DNSKEY
RRset is signed with two KSKs, so increasing the size of the
response to a query for this data.
o Double-DS keeps the size of the DNSKEY RRset to a minimum, but
does require the additional administrative overhead of two
interactions with the parent to roll a KSK. (Although this can be
mitigated if the parent has the ability for a child zone to
schedule the withdrawal of the old key at the same time as the
introduction of the new one.)
o Finally, Double-RRset allows the rollover to be done in roughly
half the time of the other two methods; it also supports policies
that require a period of running with old and new KSKs
simultaneously. However, it does have the disadvantages of both
the other two methods - it requires two signatures during the
period of the rollover and two interactions with the parent.
2.3. Summary
The methods can be summarised as follows:
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+------------------+------------------+-----------------------------+
| ZSK Method | KSK Method | Description |
+------------------+------------------+-----------------------------+
| Pre-Publication | (not applicable) | Publish the DNSKEY before |
| | | the RRSIG. |
| Double-Signature | Double-Signature | Publish the DNSKEY and |
| | | RRSIG at same time. (For a |
| | | KSK, this happens before |
| | | the DS is published.) |
| Double-RRSIG | (not applicable) | Publish RRSIG before the |
| | | DNSKEY. |
| (not applicable) | Double-DS | Publish DS before the |
| | | DNSKEY. |
| (not applicable) | Double-RRset | Publish DNSKEY and DS in |
| | | parallel. |
+------------------+------------------+-----------------------------+
Table 1
3. Key Rollover Timelines
3.1. Key States
During the rolling process, a key moves through different states.
These states are:
Generated The key has been created, but has not yet been used for
anything.
Published The DNSKEY record - or information associated with it -
is published in the zone, but predecessors of the key (or
associated information) may be held in resolver caches.
The idea of "associated information" is used in rollover
methods where RRSIG or DS records are published first and
the DNSKEY is changed in an atomic operation. It allows
the rollover still to be thought of as moving through a
set of states. In the rest of this section, the term
"key" should be taken to mean "key or associated
information".
Ready The key has been published for long enough to guarantee
that all caches that might contain a copy of the key
RRset have a copy that includes this key.
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Active The key is in the zone and has started to be used to sign
RRsets or authenticate the DNSKEY RRset. Note that when
this state is entered, it might not be possible for every
validating resolver to use the key for validation: the
zone signing may not have finished, or the data might not
have reached the resolver because of propagation delays
and/or caching issues. If this is the case, the resolver
will have to rely on the key's predecessor instead.
Retired The key is in the zone but a successor key has become
active. As there may still be information in caches that
that require use of the key, it is being retained until
this information expires.
Dead The key is published in the zone but there is no
information anywhere that requires its presence.
Removed The key has been removed from the zone.
There is one additional state, used where [RFC5011] considerations
are in effect (see Section 3.3.4):
Revoked The key is published for a period with the "revoke" bit
set as a way of notifying validating resolvers that have
configured it as a trust anchor that it is about to be
removed from the zone.
3.2. Zone-Signing Key Timelines
3.2.1. Pre-Publication Method
The following diagram shows the time line of a particular ZSK and its
replacement by its successor using the Pre-Publication method. Time
increases along the horizontal scale from left to right and the
vertical lines indicate events in the life of the key. The events
are numbered; significant times and time intervals are marked.
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|1| |2| |3| |4| |5| |6| |7| |8| |9|
| | | | | | | | |
Key N | |<-Ipub->|<--->|<-------Lzsk----->|<-Iret->|<--->|
| | | | | | | | |
Key N+1 | | | | |<-Ipub->|<->|<---Lzsk-- - -
| | | | | | | | |
Tgen Tpub Trdy Tact TpubS Tret Tdea Trem
---- Time ---->
Figure 1: Timeline for a Pre-Publication ZSK rollover.
Event 1: key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to publication in the zone (Event 2), some implementations may find
it convenient to create a pool of keys in one operation and draw from
that pool as required. For this reason, it is shown as a separate
event. Keys that are available for use but not published are said to
be generated.
Event 2: key N's DNSKEY record is put into the zone, i.e. it is added
to the DNSKEY RRset which is then re-signed with the current key-
signing key. The time at which this occurs is the key's publication
time (Tpub), and the key is now said to be published. Note that the
key is not yet used to sign records.
Event 3: before it can be used, the key must be published for long
enough to guarantee that any resolver that has a copy of the DNSKEY
RRset from the zone in its cache will have a copy of the RRset that
includes this key: in other words, that any prior cached information
about the DNSKEY RRset has expired.
This interval is the publication interval (Ipub) and, for the second
or subsequent keys in the zone, is given by:
Ipub = Dprp + TTLkey
Here, Dprp is the propagation delay - the time taken for any change
introduced at the master to replicate to all slave servers - which
depends on the depth of the master-slave hierarchy. TTLkey is the
time-to-live (TTL) for the DNSKEY records in the zone. The sum is
therefore the time taken for existing DNSKEY records to expire from
the caches of downstream validating resolvers, regardless of the
nameserver from which they were retrieved.
In the case of the first key in the zone, Ipub is slightly different
because it is not information about a DNSKEY RRset that may be
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cached, it is information about its absence. In this case:
Ipub = Dprp + Ingc
where Ingc is the negative cache interval from the zone's SOA record,
calculated according to [RFC2308] as the minimum of the TTL of the
SOA record itself (TTLsoa), and the "minimum" field in the record's
parameters (SOAmin), i.e.
Ingc = min(TTLsoa, SOAmin)
After a delay of Ipub, the key is said to be ready and could be used
to sign records. The time at which this event occurs is the key's
ready time (Trdy), which is given by:
Trdy = Tpub + Ipub
Event 4: at some later time, the key starts being used to sign
RRsets. This point is the active time (Tact) and after this, the key
is said to be active.
Event 5: while this key is active, thought must be given to its
successor (key N+1). As with the introduction of the currently
active key into the zone, the successor key will need to be published
at least Ipub before it is used. Denoting the publication time of
the successor key by TpubS, then:
TpubS <= Tact + Lzsk - Ipub
Here, Lzsk is the length of time for which a ZSK will be used (the
ZSK lifetime). It should be noted that unlike the publication
interval, Lzsk is not determined by timing logic, but by key
management policy. Lzsk will be set by the operator according to
their assessment of the risks posed by continuing to use a key and
the risks associated with key rollover. However, operational
considerations may mean a key is active for slightly more or less
than Lzsk.
Event 6: while the key N is still active, its successor becomes
ready. From this time onwards, it could be used to sign the zone.
Event 7: at some point the decision is made to 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, hence:
Tret = Tact + Lzsk
This point in time is the retire time (Tret) of key N; after this the
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key is said to be retired. (This time is also the point at which the
successor key becomes active.)
Event 8: the retired key needs to be retained in the zone whilst any
RRSIG records created using this key are still published in the zone
or held in resolver caches. (It is possible that a resolver could
have an unexpired RRSIG record and an expired DNSKEY RRset in the
cache when it is asked to provide both to a client. In this case the
DNSKEY RRset would need to be looked up again.) This means that once
the key is no longer used to sign records, it should be retained in
the zone for at least the retire interval (Iret) given by:
Iret = Dsgn + Dprp + TTLsig
Dsgn is the delay needed to ensure that all existing RRsets have been
re-signed with the new key. Dprp is (as described above) the
propagation delay, required to guarantee that the updated zone
information has reached all slave servers, and TTLsig is the TTL of
the RRSIG records.
(It should be noted that an upper limit on the retire interval is
given by:
Iret = Lsig + Dskw
where Lsig is the lifetime of a signature (i.e. the interval between
the time the signature was created and the signature end time), and
Dskw is the clock skew - the maximum difference in time between the
server and a validating resolver. The reasoning here is that
whatever happens, a key only has to be available while any signatures
created with it are valid. Wherever a signature record is held -
published in the zone and/or held in a resolver cache - it won't be
valid for longer than Lsig after it was created. The Dskw term is
present to account for the fact that the signature end time is an
absolute time rather than interval, and systems may not agree exactly
about the time.)
The time at which all RRSIG records created with this key have
expired from resolver caches is the dead time (Tdea), given by:
Tdea = Tret + Iret
...at which point the key is said to be dead.
Event 9: at any time after the key becomes dead, it can be removed
from the zone and the DNSKEY RRset re-signed with the current key-
signing key. This time is the removal time (Trem), given by:
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Trem >= Tdea
...at which time the key is said to be removed.
3.2.2. Double-Signature Method
In the Double-Signature method, both the new key and signatures
created using it are introduced at the same time. After a period
during which this information propagates to validating resolver
caches, the old key and signature are removed. The time line for the
method is shown below:
|1| |2| |3| |4| |5| |6|
| | | | | |
Key N | |<-------Lzsk------>|<-----Iret----->| |
| | | | | |
Key N+1 | | | |<----------Lzsk------- - -
| | | | | |
Tgen Tact Tret Tdea Trem
---- Time ---->
Figure 2: Timeline for a Double-Signature ZSK rollover.
Event 1: key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to publication in the zone (Event 2), some implementations may find
it convenient to create a pool of keys in one operation and draw from
that pool as required. For this reason, it is shown as a separate
event. Keys that are available for use but not published are said to
be generated.
Event 2: key N is added to the DNSKEY RRset and is immediately used
to sign the zone; existing signatures in the zone are not removed.
This is the active time (Tact) and the key is said to be active.
Event 3: at some time later, the predecessor key (key N-1) and its
signatures can be withdrawn from the zone. (The timing of key
removal is discussed further in the description of event 5.)
Event 4: the successor key (key N+1) is introduced into the zone and
starts being used to sign RRsets. The successor is key is now active
and the current key (key N) is said to be retired. This time is the
retire time of the key (Tret); it is also the active time of the
successor key (TactS).
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Tret = Tact + Lzsk
Event 5: before key N can be withdrawn from the zone, all RRsets that
need to be signed must have been signed by the successor key (as a
result, all these RRsets are now signed twice, once by key N and once
by its successor) and the information must have reached all
validating resolvers that may have RRsets from this zone cached.
This takes Iret, the retire interval, given by the expression:
Iret = Dsgn + Dprp + max(TTLkey, TTLsig)
As before, Dsgn is the time taken to sign the zone with the new key
and Dprp is the propagation delay. The final term is the period to
wait for old key and signature data to expire from caches. After the
end of this interval, key N is said to be dead. This occurs at the
dead time (Tdea) so:
Tdea = Tret + Iret
Event 6: at some later time key N and its signatures can be removed
from the zone. This is the removal time Trem, given by:
Trem >= Tdea
3.2.3. Double-RRSIG Method
As noted above, "Double-Signature" is simultaneous rollover of both
signature and key. The time line for a pure Double-Signature ZSK
rollover (the Double-RRSIG method) - where new signatures are
introduced, the key changed, and finally the old signatures removed -
is shown below:
|1||2| |3| |4||5| |6||7| |8||9| |10| |11|
| | | | | | | | | | |
Key N | |<-Dsgn->| | |<-----------Lzsk-------->|<-Iret->| |
| |<---IpubG-->| |<-IpubK->| | | | | |
| | | | | | | | | | |
Key N+1 | | | | | | |<-IpubG->| | | |
| | | | | | | | | | |
Tgen Tpub Trdy Tact TpubS TrdyS Tret Tdea Trem
---- Time ---->
Figure 3: Timeline for a Double-Signature ZSK rollover.
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Event 1: key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to publication in the zone (Event 2), some implementations may find
it convenient to create a pool of keys in one operation and draw from
that pool as required. For this reason, it is shown as a separate
event. Keys that are available for use but not published are said to
be generated.
Event 2: key N is used to sign the zone but existing signatures are
retained. Although the new ZSK is not published in the zone at this
point, for analogy with the other ZSK rollover methods and because
this is the first time that key information is visible (albeit
indirectly through the created signatures) this time is called the
publish time (Tpub).
Event 3: after the signing interval, Dsgn, all RRsets that need to be
signed have been signed by the new key. (As a result, all these
RRsets are now signed twice, once by the existing key and once by the
(still-absent) key N.
Event 4: there is now a delay while the this information reaches all
validating resolvers that may have RRsets from the zone cached. This
interval is given by the expression:
Dprp + TTLsig
...comprising the delay for the information to propagate through the
nameserver infrastructure plus the time taken for signature
information to expire from caches.
Again in analogy with other key rollover methods, this is defined as
key N's ready time (Trdy) and the key is said to be in the ready
state. (Although the key is not in the zone, it is ready to be
used.) The interval between the publication and ready times is the
publication interval of the signature, IpubG, i.e.
Trdy = Tpub + IpubG
where
IpubG = Dsgn + Dprp + TTLsig
Event 5: at some later time the predecessor key is removed and the
key N added to the DNSKEY RRset. As all the RRs have signatures
created by the old and new keys, the records can still be
authenticated. This time is the active time (Tact) and the key is
now said to be active.
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Event 6: After IpubK - the publication interval of the key - the
newly added DNSKEY RRset has propagated through to all validating
resolvers. At this point the old signatures can be removed from the
zone. IpubK is given by:
IpubK = Dprp + TTLkey
Event 7: as before, at some later time thought must be given to
rolling the key. The first step is to publish signatures created by
the successor key (key N+1) early enough so that key N can be
replaced after it has been active for its scheduled lifetime. This
occurs at TpubS (the publication time of the successor), given by:
TpubS <= Tact + Lzsk - IpubG
Event 8: the signatures have propagated through the zone and the new
key could be added to the zone. This time is the ready time of the
successor (TrdyS).
TrdyS = TpubS + IpubG
... where IpubG is as defined above.
Event 9: at some later time key N is removed from the zone and the
successor key added. This is the retire time of the key (Tret).
Event 10: The signatures must remain in the zone for long enough that
the new DNSKEY RRset has had enough time to propagate to all caches.
Once caches contain the new DNSKEY, the old signatures are no longer
of use and can be considered to be dead. The time at which this
occurs is the dead time (Tdea), given by:
Tdea = Tret + Iret
... where Iret is the retire interval, given by:
Iret = IpubK
Event 11: At some later time the signatures can be removed from the
zone. Although the key has is not longer in the zone, this is
information associated with it and so the time can be regarded as the
key's remove time (Trem), given by:
Trem >= Tdea
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3.3. Key-Signing Key Rollover Timelines
3.3.1. Double-Signature Method
The Double-Signature method (also knows as the double-DNSKEY method)
involves introducing the new KSK to the zone and waiting until its
presence has been registered by all validating resolvers. At this
point, the DS record in the parent is changed. Once that change has
propagated to all validating resolvers, the old KSK is removed.
The timing diagram for such a rollover is:
|1| |2| |3| |4| |5| |6|
| | | | | |
Key N | |<-Ipub->|<--->|<-Dreg->|<---------Lksk--- - -
| | | | | |
Key N+1 | | | | | |
| | | | | |
Tgen Tpub Trdy Tsub Tact
---- Time ---->
(continued...)
|7| |8| |9| |10| |11| |12|
| | | | | |
Key N - - -------------Lksk------->|<-Iret->| |
| | | | | |
Key N+1 |<-Ipub->|<--->|<-Dreg->|<--------Lksk----- - -
| | | | | |
TpubS TrdyS TsubS Tret Tdea Trem
---- Time (cont) ---->
Figure 4: Timeline for a Double-Signature KSK rollover.
Event 1: key N is generated at time Tgen. As before, although there
is no reason why the key cannot be generated immediately prior to
publication, some implementations may find it convenient to create a
central pool of keys and draw from it. For this reason, it is again
shown as a separate event.
Event 2: key N is introduced into the zone; it is added to the DNSKEY
RRset, which is then signed by key N and all currently active KSKs.
(So at this point, the DNSKEY RRset is signed by both key N and its
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predecessor KSK. If other KSKs were active, it is signed by these as
well.) This is the publication time (Tpub); after this the key is
said to be published.
Event 3: before it can be used, the key must be published for long
enough to guarantee that any validating resolver that has a copy of
the DNSKEY RRset from the zone in its cache will have a copy of the
RRset that includes this key: in other words, that any prior cached
information about the DNSKEY RRset has expired.
The interval is the publication interval (Ipub) and, for the second
or subsequent KSKs in the zone, is given by:
Ipub = Dprp + TTLkey
... where Dprp is the propagation delay for the zone and TTLkey the
TTL for the DNSKEY RRset. The time at which this occurs is the key's
ready time, Trdy, given by:
Trdy = Tpub + Ipub
Event 4: at some later time, the DS RR corresponding to the new KSK
is submitted to the parent zone for publication. This time is the
submission time, Tsub.
Event 5: the DS record is published in the parent zone. As this is
the point at which all information for authentication - both DNSKEY
and DS record - is available in the two zones, it is the active time
of the key:
Tact = Tsub + Dreg
... where Dreg is the registration delay, the time taken after the DS
record has been received by the parent zone manager for it to be
placed in the zone. (Parent zones are often managed by different
entities, and this term accounts of the organisational overhead of
transferring a record.)
Event 6: at some time later, all validating resolvers that have the
DS RRset cached will have a copy that includes the new DS record.
For the second or subsequent DS records, this interval is given by
the expression:
DprpP + TTLds
... where DprpP is the propagation delay in the parent zone and TTLds
the TTL assigned to DS records in that zone.
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In the case of the first DS record for the zone in question, the
expression is slightly different because it is not information about
a DS RRset that may be cached, it is information about its absence.
In this case, the interval is:
DprpP + IngcP
where IngcP is the negative cache interval from the zone's SOA
record, calculated according to [RFC2308] as the minimum of the TTL
of the parent SOA record itself (TTLsoaP), and the "minimum" field in
the record's parameters (SOAminP), i.e.
IngcP = min(TTLsoaP, SOAminP)
Event 7: while key N is active, thought needs to be given to its
successor (key N+1). At some time before the scheduled end of the
KSK lifetime, the successor KSK is introduced into the zone and is
used to sign the DNSKEY RRset. (As before, this means that the
DNSKEY RRset is signed by both the current and successor KSK.) This
is the publication time of the successor key, TpubS.
Event 8: after an interval Ipub, the successor key becomes ready (in
that all validating resolvers that have a copy of the DNSKEY RRset
have a copy of this key). This is the successor ready time, TrdyS.
Event 9: at the successor submission time (TsubS), the DS record
corresponding to the successor key is submitted to the parent zone.
Event 10: the successor DS record is published in the parent zone and
the current DS record withdrawn. The current key is said to be
retired and the time at which this occurs is Tret, given by:
The relationships between these times are:
TpubS <= Tact + Lksk - Dreg - Ipub
Tret = Tact + Lksk
... where Lksk is the scheduled lifetime of the KSK.
Event 11: key N must remain in the zone until any validators that
have the DS RRset cached have a copy of the DS RRset containing the
new DS record. This interval is the retire interval, given by:
Iret = DprpP + TTLds
... where DprpP is the propagation delay in the parent zone and TTLds
the TTL of a DS record.
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As the key is no longer used for anything, it can also be said to be
dead, in which case:
Tdea = Tret + Iret
Event 12: at some later time, key N is removed from the zone (at the
remove time Trem); the key is now said to be removed.
Trem >= Tdea
3.3.2. Double-DS Method
The Double-DS method is the reverse of the Double-Signature method is
that it is the DS record that is pre-published (in the parent), and
not the DNSKEY.
The timeline for the key rollover is shown below:
|1| |2| |3| |4| |5| |6|
| | | | | |
Key N | |<-Dreg->|<-IpubP->|<-->|<---------Lksk------- - -
| | | | | |
Key N+1 | | | | |<---->|<--Dreg+IpubP- - -
| | | | | |
Tgen Tsub Tpub Trdy Tact TsubS
---- Time ---->
(continued...)
|7| |8| |9| |10|
| | | |
Key N - - -----Lksk---------->|<-Iret->| |
| | | |
Key N+1 - - --Dreg+IpubP->|<--->|<------Lksk------ - -
| | | |
TrdyS Tret Tdea Trem
---- Time ---->
Figure 5: Timeline for a Double-DS KSK rollover.
Event 1: key N is generated at time Tgen. As before, although there
is no reason why the key cannot be generated immediately prior to
publication, some implementations may find it convenient to create a
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central pool of keys and draw from it. For this reason, it is again
shown as a separate event.
Event 2: the DS record corresponding to key N is submitted for
publication in the parent zone. This time is the submission time
(Tsub).
Event 3: after the registration delay, Dreg, the DS record is
published in the parent zone. This is the publication time Tpub,
given by:
Tpub = Tsub + Dreg
Event 4: at some later time, any validating resolver that has copies
of the DS RRset in its cache will have a copy of the DS record for
key N. At this point, key N, if introduced into the DNSKEY RRset,
could be used to validate the zone. For this reason, this time is
known as the key's ready time, Trdy, and is given by:
Trdy = Tpub + IpubP
IpubP is the parent publication interval and is given by the
expression:
IpubP = DprpP + TTLds
... where DprpP is the propagation delay in the parent zone and TTLds
the TTL assigned to DS records in that zone.
Event 5: at some later time, the key rollover takes place. The
predecessor key is withdrawn from the DNSKEY RRset and the new key
(key N) introduced and used to sign the RRset.
As both DS records have been in the parent zone long enough to ensure
that they are in the cache of any validating resolvers that have the
DS RRset cached, the zone can be authenticated throughout the
rollover - either the resolver has a copy of the DNSKEY RRset (and
associated RRSIGs) authenticated by the predecessor key, or it has a
copy of the updated RRset authenticated with the new key.
This time is the key's active time (Tact) and at this point the key
is said to be active.
Event 6: at some point thought must be given to key replacement. The
DS record for the successor key must be submitted to the parent zone
at a time such that when the current key is withdrawn, any validating
resolver that has DS records in its cache will have data about the DS
record of the successor key. The time at which this occurs is the
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submission time of the successor, given by:
TsubS <= Tact + Lksk - IpubP - Dreg
... where Lksk is the lifetime of the KSK.
Event 7: the successor key (key N+1) enters the ready state i.e. its
DS record is now in the caches of all validating resolvers that have
the parent DS RRset cached. (This is the ready time of the
successor, TrdyS.)
Event 8: when the current key has been active for its lifetime
(Lksk), the current key is removed from the DNSKEY RRset and the
successor key added; the RRset is then signed with the successor key.
This point is the retire time of the key, Tret, given by:
Tret = Tact + Lksk
Event 9: at some later time, all copies of the old DNSKEY RRset have
expired from caches and the old DS record is no longer needed. This
is called the dead time, Tdea, and is given by:
Tdea = Tret + Iret
... where Iret is the retire interval, given by:
Iret = Dprp + TTLkey
As before, this term includes the time taken to propagate the RRset
change through the master-slave hierarchy and the time take for the
DNSKEY RRset to expire from caches.
Event 10: at some later time, the DS record is removed from the
parent zone. This is the removal time (Trem), given by:
Trem >= Tdea
3.3.3. Double-RRset Method
In the Double-RRset method, both the DS and DNSKEY records are
changed at the same time, so for a period the zone can be
authenticated with either key. The advantage of this method is its
applicability in cases where zone management policy requires overlap
of authentication keys during a roll.
The timeline for this rollover is shown below:
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|1| |2| |3| |4| |5| |6| |7|
| | | | | | |
Key N | |<-Dreg->|<-----Lksk----->|<-Iret->| |
| | | | | | |
Key N+1 | | | |<-Dreg->|<-----Lksk-- - -
| | | | | | |
Tgen Tpub Tact TpubS Tret Tdea Trem
---- Time ---->
Figure 6: Timeline for a Double-RRset KSK rollover.
Event 1: key N is created at time Tgen and thereby immediately
becomes generated. As before, although there is no reason why the
key cannot be generated immediately prior to publication, some
implementations may find it convenient to create a central pool of
keys and draw from it. For this reason, it is again shown as a
separate event.
Event 2: the key is added to and used for signing the DNSKEY RRset
and is thereby published in the zone. At the same time the
corresponding DS record is submitted to the parent zone for
publication. This time is the publish time (Tpub) and the key is now
said to be published.
Event 3: after Dreg, the registration delay, the DS record is
published in the parent zone. At this point, the zones have all the
information needed for a validating resolver to authenticate the
zone, although the information may not yet have reached all
validating resolver caches. This time is the active time (Tact) and
the key is said to be active.
Tact = Tpub + Dreg
Event 4: at some point we need to give thought to key replacement.
The successor key must be introduced into the zone (and its DS record
submitted to the parent) at a time such that it becomes active when
the current key has been active for its lifetime, Lksk. This time is
TpubS, the publication time of the successor key, and is given by:
TpubS <= Tact + Lksk - Dreg
... where Lksk is the lifetime of the KSK.
Event 5: the successor key's DS record appears in the parent zone and
the successor key becomes active. At this point, the current key
becomes retired. This occurs at Tret, given by:
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Tret = Tact + Lksk
Event 6: the current DNSKEY and DS record must be retained in the
zones until any any validating resolver that has cached the DNSKEY
and/or DS RRsets will have a copy of the data for the successor key.
At this point the current key information is dead, as any validating
resolver can perform authentication using the successor key. This is
the dead time, Tdea, given by:
Tdea = Tret + Iret
... where Iret is the retire interval. This depends on how long both
the successor DNSKEY and DS records take to propagate through the
nameserver infrastructure and thence into validator caches. These
delays are the publication intervals of the child and parent zones
respectively, so a suitable expression for Iret is:
Iret = max(IpubP, IpubC)
IpubC is the publication interval of the DNSKEY in the child zone,
IpubP that of the DS record in the parent.
The child term comprises two parts - the time taken for the
introduction of the DNSKEY record to be propagated to the downstream
secondary servers (= DprpC, the child propagation delay) and the time
taken for information about the DNSKEY RRset to expire from the
validating resolver cache, i.e.
IpubC = DprpC + TTLkey
TTLkey is the TTL for a DNSKEY record in the child zone. The parent
term is similar:
IpubP = DprpP + TTLds
DprpP the propagation delay in the parent zone and TTLds the TTL for
a DS record in the parent zone.
Event 7: at some later time, the DNSKEY record can be removed from
the child zone and a request can be made to remove the DS record from
the parent zone. This is the removal time, Trem and is given by:
Trem >= Tdea
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3.3.4. Interaction with Configured Trust Anchors
Although the preceding sections have been concerned with rolling KSKs
where the trust anchor is a DS record in the parent zone, zone
managers may want to take account of the possibility that some
validating resolvers may have configured trust anchors directly.
Rolling a configured trust anchor is dealt with in [RFC5011]. It
requires introducing the KSK to be used as the trust anchor into the
zone for a period of time before use, and retaining it (with the
"revoke" bit set) for some time after use. The Double-Signature and
Double-RRset methods can be adapted to include [RFC5011]
recommendations so that the rollover will also be signalled to
validating resolvers with configured trust anchors. (The
recommendations are not suitable for the Double-DS method.
Introducing the new key early and retaining the old key after use
effectively converts it into a form of Double-RRset.)
3.3.4.1. Addition of KSK
When the new key is introduced, the publication interval (Ipub) in
the Double-Signature method should also be subject to the condition:
Ipub >= max(30 days, TTLkey)
... where the right had side of the expression is the add hold-down
time defined in section 2.4.1 of [RFC5011].
In the Double-RRSIG method, the key should not be regarded as being
active until the add hold-down time has passed. In other words, the
following condition should be enforced:
Tact >= Tpub + max(30 days, TTLkey)
(Effectively, this means extending the lifetime of the key by an
appropriate amount.)
3.3.4.2. Removal of KSK
The timeline for the removal of the key in both methods is modified
by introducing a new state, "revoked". When the key reaches the end
of the retire period, instead of being declared "dead", it is
revoked; the "revoke" bit is set on the DNSKEY RR and is published in
(and used to sign) the DNSKEY RRset. The key is maintained in this
state for the "revoke" interval, Irev, given by:
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Irev >= 30 days
... 30 days being the [RFC5011] remove hold-down time. After this
time, the key is dead and can be removed from the zone when
convenient.
3.3.5. Introduction of First KSK
There is an additional consideration when introducing a KSK into a
zone for the first time, and that is that no validating resolver
should be in a position where it can access the trust anchor before
the KSK appears in the zone. To do so will cause the validating
resolver to declare the zone to be bogus.
This is important: in the case of a secure parent, it means ensuring
that the DS record is not published in the parent zone until there is
no possibility that a validating resolver can obtain the record yet
not be able to obtain the corresponding DNSKEY. In the case of an
insecure parent, i.e. the initial creation of a new security apex, it
is important to not configure trust anchors in validating resolvers
until the DNSKEY RRset has had sufficient time to propagate. In both
cases, this time is the trust anchor availability time (Ttaa) given
by:
Ttaa >= Tpub + IpubC
where
IpubC = DprpC + TTLkeyC
or
IpubC = DprpC + IngcC
The first expression applies if there was previously a DNSKEY RRset
in the child zone, the expression for IpubC including the TTLkeyC
term to account for the time taken for that RRset to expire from
caches. (It is possible that the zone was signed but that the trust
anchor had not been submitted to the parent.)
If the introduction of the KSK caused the appearance of the first
DNSKEY RRset in the child zone, the second expression applies in
which the TTLkeyC term is replaced by Ingc to allow for the effect of
negative caching.
As before, IngcC is the negative cache interval from the child zone's
SOA record, calculated according to [RFC2308] as the minimum of the
TTL of the SOA record itself (TTLsoaC), and the "minimum" field in
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the record's parameters (SOAminC), i.e.
IngcC = min(TTLsoaC, SOAminC)
4. Standby 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 in the
ready state to sign the zone, having at least one additional key (a
standby key) in this state at all times will minimise delay.
In the case of a ZSK, a standby key only really makes sense with the
Pre-Publication method. A permanent standby DNSKEY RR should be
included in zone or successor keys could be introduced as soon as
possible after a key becomes active. Either way results in an
additional ZSK in the DNSKEY RRset that can immediately be used to
sign the zone if the current key is compromised.
(Although in theory the mechanism could be used with both the Double-
Signature and Double-RRSIG methods, it would require Pre-Publication
of the signatures. Essentially, the standby key would be permanently
active, as it would have to be periodically used to renew signatures.
Zones would also permanently require two sets of signatures,
something that could have a performance impact in large zones.)
A standby key can also be used with the Double-Signature and
Double-DS methods of rolling a KSK. (The idea of a standby key in
the Double-RRset effectively means having two active keys.) The
Double-Signature method requires that the standby KSK be included in
the DNSKEY RRset; rolling the key then requires just the introduction
of the DS record in the parent. (Note that the DNSKEY should also be
used to sign the DNSKEY RRset. As the RRset and its signatures
travel together, merely adding the DNSKEY does not provide the
desired time saving; to be used in a rollover requires that the
DNSKEY RRset be signed with the standby key, and this introduces a
delay whilst the RRset and its signatures propagate to the caches of
validating resolvers. There is no time advantage over introducing a
new DNSKEY and signing the RRset with it at the same time.)
In the Double-DS method of rolling a KSK, it is not a standby key
that is present, it is a standby DS record in the parent zone.
Whatever algorithm is used, the standby item of data can be
introduced as a permanent standby, or be a successor introduced as
early as possible.
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5. Algorithm Considerations
The preceding sections have implicitly assumed that all keys and
signatures are created using a single algorithm. However, [RFC4035]
(section 2.4) states that "There MUST be an RRSIG for each RRset
using at least one DNSKEY of each algorithm in the zone apex DNSKEY
RRset".
Except in the case of an algorithm rollover - where the algorithms
used to create the signatures are being changed - there is no
relationship between the keys of different algorithms. This means
that they can be rolled independently of one another. In other
words, the key rollover logic described above should be run
separately for each algorithm; the union of the results is included
in the zone, which is signed using the active key for each algorithm.
6. Summary
For ZSKs, "Pre-Publication" is generally considered to be the
preferred way of rolling keys. As shown in this document, the time
taken to roll is wholly dependent on parameters under the control of
the zone manager.
In contrast, "Double-RRset" is the most efficient method for KSK
rollover due to the ability to have new DS records and DNSKEY RRsets
propagate in parallel. The time taken to roll KSKs may depend on
factors related to the parent zone if the parent is signed. For
zones that intend to comply with the recommendations of [RFC5011], in
virtually all cases the rollover time will be determined by the
RFC5011 "add hold-down" and "remove hold-down" times. It should be
emphasized that this delay is a policy choice and not a function of
timing values and that it also requires changes to the rollover
process due to the need to manage revocation of trust anchors.
Finally, the treatment of emergency key rollover is significantly
simplified by the introduction of stand-by keys as standard practice
during all types of rollovers.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
This document does not introduce any new security issues beyond those
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already discussed in [RFC4033], [RFC4034], [RFC4035] and [RFC5011].
9. Acknowledgements
The authors gratefully acknowledge help and contributions from Roy
Arends and Wouter Wijngaards.
10. Change History
o draft-morris-dnsop-dnssec-key-timing-02
* General restructuring.
* Added descriptions of more rollovers (IETF-76 meeting).
* Improved description of key states and removed diagram.
* Provided simpler description of standby keys.
* Added section concerning first key in a zone.
* Moved [RFC5011] to a separate section.
* Various nits fixed (Alfred Hones, Jeremy Reed, Scott Rose, Sion
Lloyd, Tony FinchX).
o draft-morris-dnsop-dnssec-key-timing-01
* Use latest boilerplate for IPR text.
* List different ways to roll a KSK (acknowledgements to Mark
Andrews).
* Restructure to concentrate on key timing, not management
procedures.
* Change symbol notation (Diane Davidowicz and others).
* Added key state transition diagram (Diane Davidowicz).
* Corrected spelling, formatting, grammatical and style errors
(Diane Davidowicz, Alfred Hoenes and Jinmei Tatuya).
* Added note that in the case of multiple algorithms, the
signatures and rollovers for each algorithm can be considered as
more or less independent (Alfred Hoenes).
* Take account of the fact that signing a zone is not atomic
(Chris Thompson).
* Add section contrasting pre-publication rollover with double
signature rollover (Matthijs Mekking).
* Retained distinction between first and subsequent keys in
definition of initial publication interval (Matthijs Mekking).
o draft-morris-dnsop-dnssec-key-timing-00
Initial draft.
11. References
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11.1. 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.
11.2. Informative References
[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
RFC 4641, September 2006.
Appendix A. List of Symbols
The document defines a number of symbols, all of which are listed
here. All are of the form:
All symbols used in the text are of the form:
<TYPE><id><INST>
where:
<TYPE> is an upper-case character indicating what type the symbol is.
Defined types are:
D delay: interval that is a feature of the process
I interval between two events
L lifetime: interval set by the zone manager
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SOA parameter related to SOA RR
T a point in time
TTL TTL of a record
T and I are self-explanatory. D, and L are also time periods, but
whereas I values are intervals between two events (even if the events
are defined in terms of the interval, e.g. the dead time occurs
"retire interval" after the retire time), D, and L are fixed
intervals. An "L" interval (lifetime) is chosen by the zone manager
and is a feature of policy. A "D" interval (delay) is a feature of
the process, probably outside control of the zone manager. SOA and
TTL are used just because they are common terms.
<id> is lower-case and defines what object or event the variable is
related to, e.g.
act active
ngc negative cache
pub publication
Finally, <INST> is a capital letter that distinguishes between the
same variable applying to different instances of an object and is one
of:
C child
G signature
K key
P parent
S successor
The list of variables used in the text is:
Dprp Propagation delay. The amount of time for a change made at
a master nameserver to propagate to all the slave
nameservers.
DprpC Propagation delay in the child zone.
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DprpP Propagation delay in the parent zone.
Dreg Registration delay. As a parent zone is often managed by a
different organisation to that managing the child zone, the
delays associated with passing data between zones is
captured by this term.
Dskw Clock skew. The maximum difference in time between the
signing system and the resolver.
Dsgn Signing delay. After the introduction of a new ZSK, the
amount of time taken for all the RRs in the zone to be
signed with it.
Ingc Negative cache interval.
IngcP Negative cache interval of the child zone.
IngcP Negative cache interval of the parent zone.
Ipub Publication interval. The amount of time that must elapse
after the publication of a key before it can be considered
to have entered the ready state.
IpubC Publication interval in the child zone.
IpubG Publication interval for the signature.
IpubK Publication interval for the key.
IpubP Publication interval in the parent zone.
Iret Retire interval. The amount of time that must elapse after
a key enters the retire state for any signatures created
with it to be purged from validating resolver caches.
Irev Revoke interval. The amount of time that a KSK must remain
published with the revoke bit set to satisfy [RFC5011]
considerations.
Lksk Lifetime of a key-signing key. This is the intended amount
of time for which this particular KSK is regarded as the
active KSK. Depending on when the key is rolled-over, the
actual lifetime may be longer or shorter than this.
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Lzsk Lifetime of a zone-signing key. This is the intended
amount of time for which the ZSK is used to sign the zone.
Depending on when the key is rolled-over, the actual
lifetime may be longer or shorter than this.
Lsig Lifetime of a signature: the difference in time between the
signature's expiration time and the time at which the
signature was created. (Note that this is not the
difference between the signature's expiration and inception
times: the latter is usually set a small amount of time
before the signature is created to allow for clock skew
between the signing system and the validating resolver.)
SOAmin Value of the "minimum" field from an SOA record.
SOAminC Value of the "minimum" field from an SOA record in the
child zone.
SOAminP Value of the "minimum" field from an SOA record in the
parent zone.
Tact Active time of the key; the time at which the key is
regarded as the principal key for the zone.
TactS Active time of the successor key.
Tdea Dead time of a key. Applicable only to ZSKs, this is the
time at which any record signatures held in validating
resolver caches are guaranteed to be created with the
successor key.
Tgen Generate time of a key. The time that a key is created.
Tpub Publish time of a key. The time that a key appears in a
zone for the first time.
TpubS Publish time of the successor key.
Trem Removal time of a key. The time at which a key is removed
from the zone.
Tret Retire time of a key. The time at which a successor key
starts being used to sign the zone.
Trdy Ready time of a key. The time at which it can be
guaranteed that validating resolvers that have key
information from this zone cached have a copy of this key
in their cache. (In the case of KSKs, should the
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validating resolvers also have DS information from the
parent zone cached, the cache must include information
about the DS record corresponding to the key.)
TrdyS Ready time of a successor key.
Tsub Submit time - the time at which the DS record of a KSK is
submitted to the parent.
TsubS Submit time of the successor key.
TTLds Time to live of a DS record (in the parent zone).
TTLkey Time to live of a DNSKEY record.
TTLkeyC Time to live of a DNSKEY record in the child zone.
TTLsoa Time to live of a SOA record.
TTLsoaC Time to live of a SOA record in the child zone.
TTLsoaP Time to live of a SOA record in the parent zone.
TTLsig Time to live of an RRSIG record.
Ttaa Trust anchor availability time. The time at which a trust
anchor record can be made available when a KSK is first
introduced into a zone.
Authors' Addresses
Stephen Morris
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
USA
Phone: +1 650 423 1300
Email: stephen@isc.org
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Johan Ihren
Netnod
Franzengatan 5
Stockholm, SE-112 51
Sweden
Phone: +46 8615 8573
Email: johani@autonomica.se
John Dickinson
Sinodun Internet Technologies Ltd
Stables 4 Suite 11, Howbery Park
Wallingford, Oxfordshire OX10 8BA
UK
Phone: +44 1491 818120
Email: jad@sinodun.com
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