| Internet Engineering Task Force | S. Morris |
| Internet-Draft | ISC |
| Intended status: Informational | J. Ihren |
| Expires: March 21, 2015 | Netnod |
| J. Dickinson | |
| Sinodun | |
| W. Mekking | |
| NLnet Labs | |
| September 17, 2014 |
DNSSEC Key Rollover Timing Considerations
draft-ietf-dnsop-dnssec-key-timing-05.txt
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.
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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:
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.
A high-level overview of key rollover can be found in [RFC6781]. In contrast, this document focuses on the low-level timing detail of two classes of operations described there, the rollover of zone-signing keys (ZSKs), and the rollover of key-signing keys (KSKs).
Although DNSSEC validation treats all keys equally, [RFC4033] recognises the broad classification of ZSKs and KSKs. A ZSK is used to authenticate information within the zone; a KSK is used to authenticate the zone's DNSKEY RRset. 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 its cache or may need to be retrieved from an authoritative server. The rollover process needs to happen in such a way that at all times during 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.
Although there are similarities in the algorithms to roll ZSKs and KSKs, there are a number of differences. For this reason, the two types of rollovers are described separately. It is also possible to use a single key as both the ZSK and KSK. However, the rolling of this type of key is not treated in this document.
The terminology used in this document is as defined in [RFC4033] and [RFC5011].
A number of symbols are used to identify times, intervals, etc. All are listed in Appendix A.
For ZSKs, the issue for the zone operator/signer is to ensure that any caching validator has access to a particular signature that corresponds to a valid ZSK.
A ZSK can be rolled in one of three ways:
Of the three methods, Double-Signature is conceptually the simplest - introduce the new key and new signatures, then approximately one TTL later remove the old key and old signatures. It is also the fastest, but suffers from increasing the size of the zone and the size of responses.
Pre-Publication is more complex - introduce the new key, approximately one TTL later sign the records, and approximately one TTL after that remove the old key. It does however keep the zone and response sizes to a minimum.
Double-RRSIG is essentially the reverse of Pre-Publication - introduce the new signatures, approximately one TTL later change the key, and approximately one TTL after that remove the old signatures. However, it has the disadvantage of the Pre-Publication method in terms of time taken to perform the rollover, the disadvantage of the Double-Signature rollover in terms of zone and response sizes, and none of the advantages of either. For these reasons, it is unlikely to be used in any real-world situations and so will not be considered further in this document.
In the KSK case, there should not be a problem that a caching validator does not have access to a particular signature that corresponds to a valid KSK. The KSK is only used for one signature (that over the DNSKEY RRset) and both the key and 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 a signed and validated DS record in the parent zone or an explicitly configured trust anchor. If the former, 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. If the latter, [RFC5011] timings will be needed to roll the keys. (Even in the case where authentication is via a DS record, 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:
In essence, Double-KSK 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. It limits interactions with the parent to a minimum but, for the duration of the rollover, the size of the DNSKEY RRset is increased.
With Double-DS, the order of operations is the other way round: introduce the new DS, change the DNSKEY, then remove the old DS. The size of the DNSKEY RRset is kept to a minimum, but two interactions are required with the parent.
Finally, Double-RRset is the fastest way to roll the KSK, but has the drawbacks of both of the other methods: a larger DNSKEY RRset and two interactions with the parent.
A DNSSEC key contributes two pieces of information to the validation process: the DNSKEY itself and the data created from it. In the case of the validation of an RR, the data created from the DNSKEY is the RRSIG. Where there is a need to validate a chain or trust, the data created from the DNSKEY is the DS. In this section, the term "associated data" refers to the RRSIGs created from a DNSKEY when discussing a ZSK, or to the DNSKEY's corresponding DS record when referring to a KSK.
During the rolling process, keys move through different states. The defined states are:
There is one additional state, used where [RFC5011] considerations are in effect (see Section 3.3.4):
The following sections describe the rolling of a ZSK. They show the events in the lifetime of a key (referred to as "key N") and cover its replacement by its successor (key N+1).
In this method, the new key is introduced into the DNSKEY RRset. After enough time to ensure that any cached DNSKEY RRsets contain both keys, the zone is signed using the new key and the old signatures are removed. Finally, when all signatures created with the old key have expired from caches, the old key is removed.
The following diagram shows the timeline of a Pre-Publication rollover. Time increases along the horizontal scale from left to right and the vertical lines indicate events in the process. Significant times and time intervals are marked.
|1| |2| |3| |4| |5| |6| |7| |8|
| | | | | | | |
Key N |<-Ipub->|<--->|<-------Lzsk------>|<-Iret->|<--->|
| | | | | | | |
Key N+1 | | | |<-Ipub->|<-->|<---Lzsk---- - -
| | | | | | | |
Key N Tpub Trdy Tact Tret Tdea Trem
Key N+1 Tpub Trdy Tact
---- Time ---->
Figure 1: Timeline for a Pre-Publication ZSK rollover.
Event 1: 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 currently active key-signing keys. The time at which this occurs is the publication time (Tpub), and the key is now said to be published. Note that the key is not yet used to sign records.
Event 2: Before it can be used, the key must be published for long enough to guarantee that any cached version of the zone's DNSKEY RRset includes this key.
This interval is the publication interval (Ipub) and, for the second or subsequent keys in the zone, is given by:
Here, Dprp is the propagation delay - the time taken for a change introduced at the master to replicate to all name servers. TTLkey is the time-to-live (TTL) for the DNSKEY records in the zone. The sum is therefore the maximum time taken for existing DNSKEY records to expire from caches, regardless of the nameserver from which they were retrieved.
(The case of introducing the first ZSK into the zone is discussed in Section 3.3.5.)
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 key N's ready time (Trdy), which is given by:
Event 3: At some later time, the key starts being used to sign RRsets. This point is the activation time (Tact) and after this, key N is said to be active.
Event 4: At some point 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 activated. The publication time of key N+1 depends on the activation time of key N:
Here, Lzsk is the length of time for which a ZSK will be used (the ZSK lifetime). It should be noted that in the diagrams the actual key lifetime is represented; this may differ slightly from the intended lifetime set by key management policy.
Event 5: While key N is still active, its successor becomes ready. From this time onwards, key N+1 could be used to sign the zone.
Event 6: When key N has been in use for an interval equal to the ZSK lifetime, it is retired (i.e. it will never again be used to generate new signatures) and key N+1 activated and used to sign the zone. This is the retire time of key N (Tret), and is given by:
It is also the activation time of the successor key N+1. Note that operational considerations may cause key N to remain in use for a longer (or shorter) time than the lifetime set by the key management policy.
Event 7: 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 caches. (It is possible that a validating resolver could have an old RRSIG record in the cache, but the old DNSKEY RRset has expired 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:
Dsgn is the delay needed to ensure that all existing RRsets have been re-signed with the new key. Dprp is the propagation delay, required to guarantee that the updated zone information has reached all slave servers, and TTLsig is the maximum TTL of all the RRSIG records in the zone created with the retiring key.
The time at which all RRSIG records created with this key have expired from resolver caches is the dead time (Tdea), given by:
... at which point the key is said to be dead.
Event 8: At any time after the key becomes dead, it can be removed from the zone's DNSKEY RRset, which must then be re-signed with the current key-signing key. This time is the removal time (Trem), given by:
... at which time the key is said to be removed.
In this rollover, a new key is introduced and used to sign the zone; the old key and signatures are retained. Once all cached DNSKEY and/or RRSIG information contains copies of the new DNSKEY and RRSIGs created with it, the old DNSKEY and RRSIGs can be removed from the zone.
The timeline for a double-signature rollover is shown below. The diagram follows the convention described in Section 3.2.1
|1| |2| |3| |4|
| | | |
Key N |<-------Lzsk----------->|<--->|
| | | |
| |<--Iret-->| |
| | | |
Key N+1 | |<----Lzsk------- - -
| | | |
Key N Tact Tdea Trem
Key N+1 Tact
---- Time ---->
Figure 2: Timeline for a Double-Signature ZSK rollover.
Event 1: Key N is added to the DNSKEY RRset and is then used to sign the zone; existing signatures in the zone are not removed. The key is published and active: this is key N's activation time (Tact), after which the key is said to be active.
Event 2: As the current key (key N) approaches the end of its actual lifetime (Lzsk), the successor key (key N+1) is introduced into the zone and starts being used to sign RRsets: neither the current key nor the signatures created with it are removed. The successor key is now also active.
Event 3: Before key N can be withdrawn from the zone, all RRsets that need to be signed must have been signed by the successor key (key N+1) and any old RRsets that do not include the new key or new RRSIGs must have expired from caches. Note that the signatures are not replaced - each RRset is signed by both the old and new key.
This takes Iret, the retire interval, given by the expression:
As before, Dsgn is the delay needed to ensure that all existing RRsets have been signed with the new key and Dprp is the propagation delay, required to guarantee that the updated zone information has reached all slave servers. The final term (the maximum of TTLkey and TTLsig) is the period to wait for key and signature data associated with key N to expire from caches. (TTLkey is the TTL of the DNSKEY RRset and TTLsig is the maximum TTL of all the RRSIG records in the zone created with the ZSK. The two may be different: although the TTL of an RRSIG is equal to the TTL of the RRs in the associated RRset [RFC4034], the DNSKEY RRset only needs to be signed with the KSK.)
At the end of this interval, key N is said to be dead. This occurs at the dead time (Tdea) so:
Event 4: At some later time key N and the signatures generated with it can be removed from the zone. This is the removal time (Trem), given by:
The following sections describe the rolling of a KSK. They show the events in the lifetime of a key (referred to as "key N") and cover it replacement by its successor (key N+1). (The case of introducing the first KSK into the zone is discussed in Section 3.3.5.)
In this rollover, The new DNSKEY is added to the zone. After an interval long enough to guarantee that any cached DNSKEY RRsets contain the new DNSKEY, the DS record in the parent zone is changed. After a further interval to allow the old DS record to expire from caches, the old DNSKEY is removed from the zone.
The timeline for a Double-KSK rollover is shown below. The diagram follows the convention described in Section 3.2.1.
|1| |2| |3| |4|
| | | |
Key N |<-IpubC->|<--->|<-Dreg->|<-----Lksk--- - -
| | | |
Key N+1 | | | |
| | | |
Key N Tpub Trdy Tsbm Tact
Key N+1
---- Time ---->
(continued ...)
|5| |6| |7| |8| |9| |10|
| | | | | |
Key N - - --------------Lksk------->|<-Iret->|<----->|
| | | | | |
Key N+1 |<-IpubC->|<--->|<-Dreg->|<--------Lksk----- - -
| | | | | |
Key N Tret Tdea Trem
Key N+1 Tpub Trdy Tsbm Tact
---- Time (cont) ---->
Figure 3: Timeline for a Double-KSK rollover.
Event 1: Key N is introduced into the zone; it is added to the DNSKEY RRset, which is then signed by all currently active KSKs. (So at this point, the DNSKEY RRset is signed by both key N and its predecessor KSK. If other KSKs were active, it is signed by these as well.) This is the publication time of key N (Tpub); after this the key is said to be published.
Event 2: 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 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 in the child zone (IpubC) and is given by:
... where DprpC is the propagation delay for the child zone (the zone containing the KSK being rolled) and TTLkey the TTL for the DNSKEY RRset. The time at which this occurs is the key N's ready time, Trdy, given by:
Event 3: At some later time, the DS record corresponding to the new KSK is submitted to the parent zone for publication. This time is the submission time, Tsbm:
Event 4: 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, in analogy with other rollover methods, this is called the activation time of key N (Tact):
... where Dreg is the registration delay, the time taken after the DS record has been submitted to the parent zone manager for it to be placed in the zone. (Parent zones are often managed by different entities, and this term accounts for the organisational overhead of transferring a record. In practice, Dreg will not be a fixed time: instead, the end of Dreg will be signalled by the appearance of the DS record in the parent zone.)
Event 5: 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 published in the zone. (As before, this means that the DNSKEY RRset is signed by all KSKs.) This time is the publication time of the successor key N+1, given by:
... where Lksk is the actual lifetime of the KSK, and Dreg the registration delay.
Event 6: After an interval IpubC, key N+1 becomes ready (in that all caches that have a copy of the DNSKEY RRset have a copy of this key). This time is the ready time of the successor key N+1 (Trdy).
Event 7: At the submission time of the successor key N+1, Tsbm(N+1), the DS record corresponding to key N+1 is submitted to the parent zone.
Event 8: The successor DS record is published in the parent zone and the current DS record withdrawn. Key N is said to be retired and the time at which this occurs is Tret(N), given by:
Event 9: Key N must remain in the zone until any caches that contain a copy of the DS RRset have a copy containing the new DS record. This interval is the retire interval, given by:
... where DprpP is the propagation delay in the parent zone and TTLds the TTL of a DS record in the parent zone.
As the key is no longer used for anything, it is said to be dead. This point is the dead time (Tdea), given by:
Event 10: At some later time, key N is removed from the zone's DNSKEY RRset (at the remove time Trem); the key is now said to be removed.
In this rollover, the new DS record is published in the parent zone. When any caches that contain the DS RRset contain a copy of the new record, the KSK in the zone is changed. After a further interval for the old DNSKEY RRset to expire from caches, the old DS record is removed from the parent.
The timeline for a Double-DS rollover is shown below. The diagram follows the convention described in Section 3.2.1
|1| |2| |3| |4| |5|
| | | | |
Key N |<-Dreg->|<-IpubP->|<-->|<-------Lksk----- - -
| | | | |
Key N+1 | | | | |<--Dreg-- - -
| | | | |
Key N Tsbm Tpub Trdy Tact
Key N+1 Tsbm
---- Time ---->
(continued ...)
|6| |7| |8| |9| |10|
| | | | |
Key N - - -----Lksk--------->|<-Iret->|<---->|
| | | | |
Key N+1 - - --Dreg-->|<-IpubP->|<------>|<------Lksk------ - -
| | | | |
Key N Tret Tdea Trem
Key N+1 Tpub Trdy Tact
---- Time ---->
Figure 4: Timeline for a Double-DS KSK rollover.
Event 1: The DS RR is submitted to the parent zone for publication. This time is the submission time, Tsbm.
Event 2: After the registration delay, Dreg, the DS record is published in the parent zone. This is the publication time (Tpub) of key N, given by:
As before, in practice Dreg will not be a fixed time. Instead, the end of Dreg will be signalled by the appearance of the DS record in the parent zone.
Event 3: At some later time, any cache that has a copy of the DS RRset 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 ready time, Trdy, and is given by:
IpubP is the publication interval of the DS record (in the parent zone) and is given by the expression:
... where DprpP is the propagation delay for the parent zone and TTLds the TTL assigned to DS records in that zone.
Event 4: At some later time, the key rollover takes place and the new key (key N) is introduced into the DNSKEY RRset and used to sign it. This time is key N's activation time (Tact) and at this point key N is said to be active:
Event 5: 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 cache that contains the zone's DS records has data about the DS record of the successor key. The time at which this occurs is the submission time of the successor key N+1, given by:
... where Lksk is the actual lifetime of key N (which may differ slightly from the lifetime set in the key management policy) and Dreg is the registration delay.
Event 6. After an interval Dreg, the successor DS record is published in the zone.
Event 7: The successor key (key N+1) enters the ready state, i.e. its DS record is now in caches that contain the parent DS RRset.
Event 8: When key N has been active for its lifetime (Lksk), it is replaced in the DNSKEY RRset by key N+1; the RRset is then signed with the new key. At this point, as both the old and new DS records have been in the parent zone long enough to ensure that they are in caches that contain the DS RRset, the zone can be authenticated throughout the rollover. A validating resolver can authenticate either the old or new KSK.
This time is the retire time (Tret) of key N, given by:
This is also the activation time of the successor key N+1.
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. In analogy with other rollover methods, this is called the dead time, Tdea, and is given by:
... where Iret is the retire interval of the key, given by:
As before, this term includes DprpC, the time taken to propagate the RRset change through the master-slave hierarchy of the child zone and TTLkey, the time taken for the DNSKEY RRset to expire from caches.
Event 10: At some later time, the DS record is removed from the parent zone. In analogy with other rollover methods, this is the removal time (Trem), given by:
In the Double-RRset rollover, the new DNSKEY and DS records are published simultaneously in the appropriate zones. Once enough time has elapsed for the old DNSKEY and DS RRsets to expire from caches, the old DNSKEY and DS records are removed from their respective zones.
The timeline for this rollover is shown below. The diagram follows the convention described in Section 3.2.1
|1| |2| |3| |4| |5|
| | | | |
Key N |<-Ipub->|<-----Lksk----->|<------>|
| | | | |
Key N+1 | | |<-Ipub->|<------Lksk--- - -
| | | | |
Key N Tpub Tact Tret Trem
Key N+1 Tpub Tact
---- Time ---->
Figure 5: Timeline for a Double-RRset KSK rollover.
Event 1: 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 for key N (Tpub) and the key is said to be published.
Event 2: At some later time, the DS record is published in the parent zone and at some time after that, the updated information has reached all caches: any cache that holds a DNSKEY RRset from the child zone will have a copy that includes the new KSK, and any cache that has a copy of the parent DS RRset will have a copy that includes the new DS record.
The time at which this occurs is called the activation time of key N (Tact), given by:
... where Ipub is the composite publication interval for the DNSKEY and DS records, given by:
IpubP being the publication interval of the DS record in the parent zone and IpubC the publication interval of the DNSKEY in the child zone. The parent zone's publication interval is given by:
where Dreg is the registration delay, the time taken for the DS record to be published in the parent zone. DprpP is the parent zone's propagation delay and TTLds is the TTL of the DS record in that zone.
The child zone's publication interval is given by a similar equation:
... where DprpC is the propagation delay in the child zone and TTLkey the TTL of a DNSKEY record.
Event 3: At some point we need to give thought to key replacement. The successor key (key N+1) 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 actual lifetime, Lksk. This is the publication time (Tpub) of the successor key, and is given by:
... where Lksk is the actual lifetime of the KSK and Ipub is as defined above.
Event 4: Key N+1's DNSKEY and DS records are now in caches that contain the child zone DNSKEY and/or the parent zone DS RR, and so the zone can be validated with the new key. This is the activation time (Tact) of the successor key N+1 and by analogy with other rollover methods, it is also the dead time of key N:
Event 5: At some later time, the key N's DS and DNSKEY records are removed from their respective zones. In analogy with other rollover methods, this is the removal time (Trem), given by:
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.
When the new key is introduced, the expression for the publication interval of the DNSKEY(IpubC) in the Double-KSK and Double-RRset methods is modified to:
... where the right hand side of the expression now includes the "trust point" interval. This term is the interval required to guarantee that a resolver configured for the automatic update of keys from a particular trust point will see at least two validated DNSKEY RRsets containing the new key (a requirement from [RFC5011], section 2.4.1). It is defined by the expression:
... where queryInterval and retryTime are as defined in section 2.3 of [RFC5011]. "n" is the total number of retries needed by the resolver during the two attempts to get the DNSKEY RRset.
The first term of the expression (2 * queryInterval) represents the time to obtain two validated DNSKEY RRsets. The second term (n * retryTime) is a safety margin, with the value of "n" reflecting the degree of confidence in the communication between a resolver and the trust point.
In the Double-DS method, instead of swapping the KSK RRs in a single step, there must now be a period of overlap. In other words, the new KSK must be introduced into the zone at least:
... before the switch is made.
The timeline for the removal of the key in all methods is modified by introducing a new state, "revoked". When the key reaches its dead time, instead of being declared "dead", it is revoked; the "revoke" bit is set in the published DNSKEY RR, and the DNSKEY RRset re-signed with the current and revoked keys. The key is maintained in this state for the "revoke" interval, Irev, given by:
... 30 days being the [RFC5011] remove hold-down time. After this time, the key is dead and can be removed from the zone.
There are no timing considerations associated with the introduction of the first keys into a zone other that they must be introduced and the zone validly signed before a chain of trust to the zone is created.
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 is not 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 not possible to guarantee this. It is up to the operator of the validating resolver to wait for the new KSK to appear at all servers for the zone before configuring the trust anchor.
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 the zone or successor keys could be introduced as soon as possible after a key becomes active. Either way results in one or more additional ZSKs 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.)
It is also possible to have a standby KSK. The Double-KSK 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 standby KSK should also be used to sign the DNSKEY RRset. As the RRset and its signatures travel together, merely adding the KSK without using it to sign the DNSKEY RRset does not provide the desired time saving: for a KSK to be used in a rollover the DNSKEY RRset must be signed with it, and this would introduce a delay while the old RRset (not signed with the new key) expires from caches.
The idea of a standby KSK in the Double-RRset rollover method effectively means having two active keys (as the standby KSK and associated DS record would both be published at the same time in their respective zones).
Finally, 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 included in the zone on a permanent basis, or be a successor introduced as early as possible.
The preceding sections have implicitly assumed that all keys and signatures are created using a single algorithm. However, [RFC4035] (section 2.4) requires that there 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.
This document represents current thinking at the time of publication. However, the subject matter is evolving and it is more than likely that this document will need to be revised in the future.
Some of the techniques and ideas that DNSSEC operators are considering differ from this those described in this document. Of particular interest are alternatives to the strict split into KSK and ZSK key roles and the consequences for rollover logic from partial signing (i.e. when the new key initially only signs a fraction of the zone while leaving other signatures generated by the old key in place).
Furthermore, as noted in section 5, this document covers only rolling keys of the same algorithm: it does not cover transitions between algorithms. The timing issues associated with algorithm rollovers will require a separate document.
The reader is therefore reminded that DNSSEC is, as of date of publication, in the early stages of deployment, and best practices may further develop over time.
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 standby keys as standard practice during all types of rollovers.
This memo includes no request to IANA.
This document does not introduce any new security issues beyond those already discussed in [RFC4033], [RFC4034], [RFC4035] and [RFC5011].
The authors gratefully acknowledge help and contributions from Roy Arends and Wouter Wijngaards.
| [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", STD 74, RFC 5011, September 2007. |
| [RFC6781] | Kolkman, O., Mekking, W. and R. Gieben, "DNSSEC Operational Practices, Version 2", RFC 6781, December 2012. |
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:
where:
<TYPE> is an upper-case character indicating what type the symbol is. Defined types are:
I, T and TTL are self-explanatory. Like I, both D and L are 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: a "D" interval (delay) is a feature of the process, probably outside control of the zone manager, whereas an "L" interval (lifetime) is chosen by the zone manager and is a feature of policy.
<id> is lower-case and defines what object or event the variable is related to, e.g.,
<ZONE> is an optional capital letter that distinguishes between the same variable applied to different zones and is one of:
Within the rollover descriptions, times may be suffixed by a number in brackets indicating the instance of the key to which they apply, e.g. Tact(N) is the activation time of key N, Tpub(N+1) the publication time of key N+1 etc.
The list of variables used in the text given below.