dnsop W. Hardaker
Internet-Draft USC/ISI
Updates: 7583 (if approved) W. Kumari
Intended status: Standards Track Google
Expires: June 2, 2018 November 29, 2017
Security Considerations for RFC5011 Publishers
draft-ietf-dnsop-rfc5011-security-considerations-08
Abstract
This document extends the RFC5011 rollover strategy with timing
advice that must be followed in order to maintain security.
Specifically, this document describes the math behind the minimum
time-length that a DNS zone publisher must wait before signing
exclusively with recently added DNSKEYs. It contains much math and
complicated equations, but the summary is that the key rollover /
revocation time is much longer than intuition would suggest. If you
are not both publishing a DNSSEC DNSKEY, and using RFC5011 to
advertise this DNSKEY as a new Secure Entry Point key for use as a
trust anchor, you probably don't need to read this document.
This document also describes the minimum time-length that a DNS zone
publisher must wait after publishing a revoked DNSKEY before assuming
that all active RFC5011 resolvers should have seen the revocation-
marked key and removed it from their list of trust anchors.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 2, 2018.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Document History and Motivation . . . . . . . . . . . . . 3
1.2. Safely Rolling the Root Zone's KSK in 2017/2018 . . . . . 3
1.3. Requirements notation . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Timing Associated with RFC5011 Processing . . . . . . . . . . 5
4.1. Timing Associated with Publication . . . . . . . . . . . 5
4.2. Timing Associated with Revocation . . . . . . . . . . . . 6
5. Denial of Service Attack Walkthrough . . . . . . . . . . . . 6
5.1. Enumerated Attack Example . . . . . . . . . . . . . . . . 6
5.1.1. Attack Timing Breakdown . . . . . . . . . . . . . . . 7
6. Minimum RFC5011 Timing Requirements . . . . . . . . . . . . . 9
6.1. Equation Components . . . . . . . . . . . . . . . . . . . 9
6.1.1. addHoldDownTime . . . . . . . . . . . . . . . . . . . 9
6.1.2. sigExpirationTimeRemaining . . . . . . . . . . . . . 9
6.1.3. activeRefresh . . . . . . . . . . . . . . . . . . . . 9
6.1.4. activeRefreshOffset . . . . . . . . . . . . . . . . . 9
6.1.5. safetyMargin . . . . . . . . . . . . . . . . . . . . 10
6.2. Timing Requirements For Adding a New KSK . . . . . . . . 11
6.2.1. Wait Timer Based Calculation . . . . . . . . . . . . 11
6.2.2. Wall-Clock Based Calculation . . . . . . . . . . . . 12
6.2.3. Timing Constraint Summary . . . . . . . . . . . . . . 12
6.2.4. Additional Considerations for RFC7583 . . . . . . . . 13
6.2.5. Example Scenario Calculations . . . . . . . . . . . . 13
6.3. Timing Requirements For Revoking an Old KSK . . . . . . . 13
6.3.1. Wait Timer Based Calculation . . . . . . . . . . . . 14
6.3.2. Wall-Clock Based Calculation . . . . . . . . . . . . 14
6.3.3. Additional Considerations for RFC7583 . . . . . . . . 15
6.3.4. Example Scenario Calculations . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
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8. Operational Considerations . . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
11. Normative References . . . . . . . . . . . . . . . . . . . . 16
Appendix A. Real World Example: The 2017 Root KSK Key Roll . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
[RFC5011] defines a mechanism by which DNSSEC validators can update
their list of trust anchors when they've seen a new key published in
a zone or revoke a properly marked key from a trust anchor list.
However, RFC5011 [intentionally] provides no guidance to the
publishers of DNSKEYs about how long they must wait before switching
to exclusively using recently published keys for signing records, or
how long they must wait before ceasing publication of a revoked key.
Because of this lack of guidance, zone publishers may derive
incorrect assumptions about safe usage of the RFC5011 DNSKEY
advertising, rolling and revocation process. This document describes
the minimum security requirements from a publisher's point of view
and is intended to complement the guidance offered in RFC5011 (which
is written to provide timing guidance solely to a Validating
Resolver's point of view).
1.1. Document History and Motivation
To verify this lack of understanding is wide-spread, the authors
reached out to 5 DNSSEC experts to ask them how long they thought
they must wait before signing a zone exclusively with a new KSK
[RFC4033] that was being introduced according to the 5011 process.
All 5 experts answered with an insecure value, and we determined that
this lack of operational guidance might cause security concerns in
deployment and wrote this companion document to RFC5011. We hope
that this document will rectify this understanding and provide better
guidance to zone publishers that wish to make use of the RFC5011
rollover process.
1.2. Safely Rolling the Root Zone's KSK in 2017/2018
One important note about ICANN's (currently in process) 2017/2018 KSK
rollover plan for the root zone: the timing values chosen for rolling
the KSK in the root zone appear completely safe, and are not affected
by the timing concerns introduced by this draft
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1.3. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Background
The RFC5011 process describes a process by which a RFC5011 Resolver
may accept a newly published KSK as a trust anchor for validating
future DNSSEC signed records. It also describes the process for
publicly revoking a published KSK. This document augments that
information with additional constraints, from the DNSKEY publication
and revocation's points of view. Note that this document does not
define any other operational guidance or recommendations about the
RFC5011 process and restricts itself to solely the security and
operational ramifications of switching to exclusively using recently
added keys or removing a revoked keys too soon.
Failure of a DNSKEY publisher to follow the minimum recommendations
associated with this draft can result in potential denial-of-service
attack opportunities against validating resolvers. Failure of a
DNSKEY publisher to publish a revoked key for a long enough period of
time may result in RFC5011 Resolvers leaving that key in their trust
anchor storage beyond the key's expected lifetime.
3. Terminology
SEP Publisher The entity responsible for publishing a DNSKEY (with
the Secure Entry Point (SEP) bit set) that can be used as a trust
anchor.
Zone Signer The owner of a zone intending to publish a new Key-
Signing-Key (KSK) that may become a trust anchor for validators
following the RFC5011 process.
RFC5011 Resolver A DNSSEC Resolver that is using the RFC5011
processes to track and update trust anchors.
Attacker An entity intent on foiling the RFC5011 Resolver's ability
to successfully adopt the Zone Signer's new DNSKEY as a new trust
anchor or to prevent the RFC5011 Resolver from removing an old
DNSKEY from its list of trust anchors.
lastSigExpirationTime The latest value of any RRSIG Signature
Expiration field (which is a date and time) that has signed the
previous DNSKEY RRset before a new DNSKEY is introduced to a
publish DNSKEY RRset, or the DNSKEY RRset of a DNSKEY that is to
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be revoked. Note that for organizations pre-creating signatures
this time may be fairly far in the future unless they can be
significantly assured that none of their pre-generated signatures
can be replayed at a later date.
sigExpirationTime The amount of time between the DNSKEY RRSIG's
Signature Inception field and the Signature Expiration field.
sigExpirationTimeRemaining The amount of time remaining before
latestSigExpirationTime is reached.
Also see Section 2 of [RFC4033] and [RFC7719] for additional
terminology.
4. Timing Associated with RFC5011 Processing
These sections define a high-level overview of [RFC5011] processing.
These steps are not sufficient for proper RFC5011 implementation, but
provide enough background for the reader to follow the discussion in
this document. Readers need to fully understand [RFC5011] as well to
fully comprehend the content and importance of this document.
4.1. Timing Associated with Publication
RFC5011's process of safely publishing a new DNSKEY and then assuming
RFC5011 Resolvers have adopted it for trust falls into a number of
high-level steps to be performed by the SEP Publisher. This document
discusses the following scenario, which the principle way RFC5011 is
currently being used (even though Section 6 of RFC5011 suggests
having a stand-by key available):
1. Publish a new DNSKEY in a zone, but continue to sign the zone
with the old one.
2. Wait a period of time.
3. Begin to exclusively use recently published DNSKEYs to sign the
appropriate resource records.
This document discusses the time required to wait during step 2 of
the above process. Some interpretations of RFC5011 have erroneously
determined that the wait time is equal to RFC5011's "hold down time".
Section 5 describes an attack based on this (common) erroneous
belief, which can result in a denial of service attack against the
zone.
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4.2. Timing Associated with Revocation
RFC5011's process of advertising that an old key is to be revoked
from RFC5011 Resolvers falls into a number of high-level steps:
1. Set the revoke bit on the DNSKEY to be revoked.
2. Sign the revoked DNSKEY with itself.
3. Wait a period of time.
4. Remove the revoked key from the zone.
This document discusses the time required to wait in step 3 of the
above process. Some interpretations of RFC5011 have erroneously
determined that the wait time is equal to RFC5011's "hold down time".
This document describes an attack based on this (common) erroneous
belief, which results in a revoked DNSKEY potentially remaining as a
trust anchor in a RFC5011 Resolver long past its expected usage.
5. Denial of Service Attack Walkthrough
This section serves as an illustrative example of the problem being
discussed in this document. Note that in order to keep the example
simple enough to understand, some simplifications were made (such as
by not creating a set of pre-signed RRSIGs and by not using values
that result in the addHoldDownTime not being evenly divisible by the
activeRefresh value); the mathematical formulas in Section 6,
however, are complete.
If an attacker is able to provide a RFC5011 Resolver with past
responses, such as when it is in-path or able to perform any number
of cache poisoning attacks, the attacker may be able to leave
compliant RFC5011 Resolvers without an appropriate DNSKEY trust
anchor. This scenario will remain until an administrator manually
fixes the situation.
The time-line below illustrates this situation.
5.1. Enumerated Attack Example
The following example settings are used in the example scenario
within this section:
TTL (all records) 1 day
sigExpirationTime 10 days
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Zone resigned every 1 day
Given these settings, the sequence of events in Section 5.1.1 depicts
how a SEP Publisher that waits for only the RFC5011 hold time timer
length of 30 days subjects its users to a potential Denial of Service
attack. The timing schedule listed below is based on a SEP Publisher
publishing a new Key Signing Key (KSK), with the intent that it will
later be used as a trust anchor. We label this publication time as
"T+0". All numbers in this sequence refer to days before and after
this initial publication event. Thus, T-1 is the day before the
introduction of the new key, and T+15 is the 15th day after the key
was introduced into the fictitious zone being discussed.
In this dialog, we consider two keys within the example zone:
K_old: An older KSK and Trust Anchor being replaced.
K_new: A new KSK being transitioned into active use and expected to
become a Trust Anchor via the RFC5011 automated trust anchor
update process.
5.1.1. Attack Timing Breakdown
The steps shows an attack that foils the adoption of a new DNSKEY by
a 5011 Resolver when the SEP Publisher that starts signing and
publishing with the new DNSKEY too quickly.
T-1 The K_old based RRSIGs are being published by the Zone Signer.
[It may also be signing ZSKs as well, but they are not relevant to
this event so we will not talk further about them; we are only
considering the RRSIGs that cover the DNSKEYs in this document.]
The Attacker queries for, retrieves and caches this DNSKEY set and
corresponding RRSIG signatures.
T+0 The Zone Signer adds K_new to their zone and signs the zone's
key set with K_old. The RFC5011 Resolver (later to be under
attack) retrieves this new key set and corresponding RRSIGs and
notices the publication of K_new. The RFC5011 Resolver starts the
(30-day) hold-down timer for K_new. [Note that in a more real-
world scenario there will likely be a further delay between the
point where the Zone Signer publishes a new RRSIG and the RFC5011
Resolver notices its publication; though not shown in this
example, this delay is accounted for in the equation in Section 6
below]
T+5 The RFC5011 Resolver queries for the zone's keyset per the
RFC5011 Active Refresh schedule, discussed in Section 2.3 of
RFC5011. Instead of receiving the intended published keyset, the
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Attacker successfully replays the keyset and associated signatures
recorded at T-1. Because the signature lifetime is 10 days (in
this example), the replayed signature and keyset is accepted as
valid (being only 6 days old, which is less than
sigExpirationTime) and the RFC5011 Resolver cancels the (30-day)
hold-down timer for K_new, per the RFC5011 algorithm.
T+10 The RFC5011 Resolver queries for the zone's keyset and
discovers a signed keyset that includes K_new (again), and is
signed by K_old. Note: the attacker is unable to replay the
records cached at T-1, because they have now expired. Thus at
T+10, the RFC5011 Resolver starts (anew) the hold-timer for K_new.
T+11 through T+29 The RFC5011 Resolver continues checking the zone's
key set at the prescribed regular intervals. During this period,
the attacker can no longer replay traffic to their benefit.
T+30 The Zone Signer knows that this is the first time at which some
validators might accept K_new as a new trust anchor, since the
hold-down timer of a RFC5011 Resolver not under attack that had
queried and retrieved K_new at T+0 would now have reached 30 days.
However, the hold-down timer of our attacked RFC5011 Resolver is
only at 20 days.
T+35 The Zone Signer (mistakenly) believes that all validators
following the Active Refresh schedule (Section 2.3 of RFC5011)
should have accepted K_new as a the new trust anchor (since the
hold down time (30 days) + the query interval [which is just 1/2
the signature validity period in this example] would have passed).
However, the hold-down timer of our attacked RFC5011 Resolver is
only at 25 days (T+35 minus T+10); thus the RFC5011 Resolver won't
consider it a valid trust anchor addition yet, as the required 30
days have not yet elapsed.
T+36 The Zone Signer, believing K_new is safe to use, switches their
active signing KSK to K_new and publishes a new RRSIG, signed with
(only) K_new, covering the DNSKEY set. Non-attacked RFC5011
validators, with a hold-down timer of at least 30 days, would have
accepted K_new into their set of trusted keys. But, because our
attacked RFC5011 Resolver now has a hold-down timer for K_new of
only 26 days, it failed to ever accept K_new as a trust anchor.
Since K_old is no longer being used to sign the zone's DNSKEYs,
all the DNSKEY records from the zone will be treated as invalid.
Subsequently, all of the records in the DNS tree below the zone's
apex will be deemed invalid by DNSSEC.
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6. Minimum RFC5011 Timing Requirements
This section defines the minimum timing requirements for making
exclusive use of newly added DNSKEYs and timing requirements for
ceasing the publication of DNSKEYs to be revoked. First, we define
the term components used in both equations in Section 6.1.
6.1. Equation Components
6.1.1. addHoldDownTime
The addHoldDownTime is defined in Section 2.4.1 of [RFC5011] as:
The add hold-down time is 30 days or the expiration time of the
original TTL of the first trust point DNSKEY RRSet that contained
the new key, whichever is greater. This ensures that at least
two validated DNSKEY RRSets that contain the new key MUST be seen
by the resolver prior to the key's acceptance.
6.1.2. sigExpirationTimeRemaining
sigExpirationTimeRemaining is defined in Section 3.
6.1.3. activeRefresh
activeRefresh time is defined by RFC5011 by
A resolver that has been configured for an automatic update
of keys from a particular trust point MUST query that trust
point (e.g., do a lookup for the DNSKEY RRSet and related
RRSIG records) no less often than the lesser of 15 days, half
the original TTL for the DNSKEY RRSet, or half the RRSIG
expiration interval and no more often than once per hour.
This translates to:
activeRefresh = MAX(1 hour,
MIN(sigExpirationTime / 2,
MAX(TTL of K_old DNSKEY RRSet) / 2,
15 days)
)
6.1.4. activeRefreshOffset
The activeRefreshOffset term must be added for situations where the
activeRefresh value is not a factor of the addHoldDownTime.
Specifically, activeRefreshOffset will be "addHoldDownTime %
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activeRefresh", where % is the mathematical mod operator (calculating
the remainder in a division problem). This will frequently be zero,
but could be nearly as large as activeRefresh itself. For
simplicity, setting the activeRefreshOffset to the activeRefresh
value itself is always safe.
6.1.5. safetyMargin
The safetyMargin is an extra period of time to account for caching,
network delays, dropped packets, and other operational concerns
otherwise beyond the scope of this document. The value operators
should chose is highly dependent on the deployment siptuation
associated with their zone. Note that no value of a safetyMargin can
protect against resolvers that are "down". None the less, we do
offer the following as one method considering reasonable values to
select from.
The following list of variables need to be considered when selecting
an appropriate safetyMargin value:
successRate: A likely success rate for client queries and retries
numResolvers: The number of client RFC5011 Resolvers
Note that RFC5011 defines retryTime as:
If the query fails, the resolver MUST repeat the query until
satisfied no more often than once an hour and no less often
than the lesser of 1 day, 10% of the original TTL, or 10% of
the original expiration interval. That is,
retryTime = MAX (1 hour, MIN (1 day, .1 * origTTL,
.1 * expireInterval)).
With these values selected and the definition of retryTime from
RFC5011, one method for determining how many retryTime intervals to
wait in order to reduce the set of uncompleted servers to 0 assuming
normal probability is thus:
x = (1/(1 - successRate))
retryCountWait = Log_base_x(numResolvers)
To reduce the need for readers to pull out a scientific calculator,
we offer the following lookup table based on successRate and
numResolvers:
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retryCountWait lookup table
---------------------------
Number of client RFC5011 Resolvers (numResolvers)
10,000 100,000 1,000,000 10,000,000 100,000,000
0.01 917 1146 1375 1604 1833
Probability 0.05 180 225 270 315 360
of Success 0.10 88 110 132 153 175
Per Retry 0.15 57 71 86 100 114
Interval 0.25 33 41 49 57 65
(successRate) 0.50 14 17 20 24 27
0.90 4 5 6 7 8
0.95 4 4 5 6 7
0.99 2 3 3 4 4
0.999 2 2 2 3 3
Finally, a suggested value of safetyMargin can then be this
retryCountWait number multiplied by the retryTime from RFC5011:
safetyMargin = retryCountWait * retryTime
6.2. Timing Requirements For Adding a New KSK
This section defines a method for calculating the amount of time to
wait until it is safe to start signing exclusively with a new key
Section 6.2.1 (especially useful for writing code involving sleep
based timers), and an a method for calculating a wall-clock value
after which it is safe to start signing exclusively with a new key
Section 6.2.2 (especially useful for writing code based on clock-
based event triggers).
6.2.1. Wait Timer Based Calculation
Given the attack description in Section 5, the correct minimum length
of time required for the Zone Signer to wait after publishing K_new
but before exclusively using it and newer keys is:
addWaitTime = addHoldDownTime
+ sigExpirationTimeRemaining
+ activeRefresh
+ activeRefreshOffset
+ safetyMargin
6.2.1.1. Fully expanded equation
The full expanded equation is:
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addWaitTime = addHoldDownTime
+ sigExpirationTimeRemaining
+ 2 * MAX(1 hour,
MIN(sigExpirationTime / 2,
MAX(TTL of K_old DNSKEY RRSet) / 2,
15 days)
)
+ (addHoldDownTime % activeRefresh)
+ MAX(1.5 hours, 2 * MAX(TTL of all records))
+ safetyMargin
6.2.2. Wall-Clock Based Calculation
The above equations are defined based upon how long to wait from a
particular moment in time. An alternative, but equivalent, method is
to calculate the date and time before which it is unsafe to use a key
for signing. This calculation thus becomes:
addWallClockTime = lastSigExpirationTime
+ addHoldDownTime
+ activeRefresh
+ activeRefreshOffset
+ safetyMargin
where lastSigExpirationTime is the latest value of any
sigExpirationTime for which RRSIGs were created that could
potentially be replayed. Fully expanded, this becomes:
addWallClockTime = lastSigExpirationTime
+ addHoldDownTime
+ 2 * MAX(1 hour,
MIN(sigExpirationTime / 2,
MAX(TTL of K_old DNSKEY RRSet) / 2,
15 days)
)
+ (addHoldDownTime % activeRefresh)
+ MAX(1.5 hours, 2 * MAX(TTL of all records))
+ safetyMargin
6.2.3. Timing Constraint Summary
The important timing constraint introduced by this memo relates to
the last point at which a RFC5011 Resolver may have received a
replayed original DNSKEY set, containing K_old and not K_new. The
next query of the RFC5011 validator at which K_new will be seen
without the potential for a replay attack will occur after the
publication time plus sigExpirationTime. Thus, the latest time that
a RFC5011 Validator may begin their hold down timer is an "Active
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Refresh" period after the last point that an attacker can replay the
K_old DNSKEY set. The worst case scenario of this attack is if the
attacker can replay K_old just seconds before the (DNSKEY RRSIG
Signature Validity) field of the last K_old only RRSIG.
6.2.4. Additional Considerations for RFC7583
Note: our notion of addWaitTime is called "Itrp" in Section 3.3.4.1
of [RFC7583]. The equation for Itrp in RFC7583 is insecure as it
does not include the sigExpirationTime listed above. The Itrp
equation in RFC7583 also does not include the 2*TTL safety margin,
though that is an operational consideration and not necessarily as
critical.
6.2.5. Example Scenario Calculations
For the parameters listed in Section 5.1, the activeRefreshOffset is
0, since 30 days is evenly divisible by activeRefresh (1/2 day), and
our resulting addWaitTime is:
addWaitTime = 30
+ 10
+ 1 / 2
+ 2 * (1) (days)
addWaitTime = 42.5 (days)
This addWaitTime of 42.5 days is 12.5 days longer than just the hold
down timer.
6.3. Timing Requirements For Revoking an Old KSK
This issue affects not just the publication of new DNSKEYs intended
to be used as trust anchors, but also the length of time required to
continuously publish a DNSKEY with the revoke bit set.
This section defines a method for calculating the amount of time
operators need to wait until it is safe to cease publishing a DNSKEY
Section 6.2.1 (especially useful for writing code involving sleep
based timers), and an a method for calculating a minimal wall-clock
value after which it is safe to cease publishing a DNSKEY
Section 6.2.2 (especially useful for writing code based on clock-
based event triggers).
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6.3.1. Wait Timer Based Calculation
Both of these publication timing requirements are affected by the
attacks described in this document, but with revocation the key is
revoked immediately and the addHoldDown timer does not apply. Thus
the minimum amount of time that a SEP Publisher must wait before
removing a revoked key from publication is:
remWaitTime = sigExpirationTimeRemaining
+ MAX(1 hour,
MIN((sigExpirationTime) / 2,
MAX(TTL of K_old DNSKEY RRSet) / 2,
15 days),
1 hour)
+ 2 * MAX(TTL of all records)
Note that the activeRefreshOffset time does not apply to this
equation.
Note also that adding retryTime intervals to the remWaitTime may be
wise, just as it was for addWaitTime in Section 6.
6.3.2. Wall-Clock Based Calculation
Like before, the above equations are defined based upon how long to
wait from a particular moment in time. An alternative, but
equivalent, method is to calculate the date and time before which it
is unsafe to cease publishing a revoked key. This calculation thus
becomes:
remWallClockTime = lastSigExpirationTime
+ activeRefresh
+ activeRefreshOffset
+ safetyMargin
where lastSigExpirationTime is the latest value of any
sigExpirationTime for which RRSIGs were created that could
potentially be replayed. Fully expanded, this becomes:
remWallClockTime = lastSigExpirationTime
+ 2 * MAX(1 hour,
MIN(sigExpirationTime / 2,
MAX(TTL of K_old DNSKEY RRSet) / 2,
15 days)
)
+ (addHoldDownTime % activeRefresh)
+ MAX(1.5 hours, 2 * MAX(TTL of all records))
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6.3.3. Additional Considerations for RFC7583
Note that our notion of remWaitTime is called "Irev" in
Section 3.3.4.2 of [RFC7583]. The equation for Irev in RFC7583 is
insecure as it does not include the sigExpirationTime listed above.
The Irev equation in RFC7583 also does not include the 2*TTL safety
margin, though that is an operational consideration and not
necessarily as critical.
6.3.4. Example Scenario Calculations
For the parameters listed in Section 5.1, our example:
remwaitTime = 10
+ 1 / 2
+ 2 * (1) (days)
remwaitTime = 12.5 (days)
Note that for the values in this example produce a length shorter
than the recommended 30 days in RFC5011's section 6.6, step 3. Other
values of sigExpirationTime and the original TTL of the K_old DNSKEY
RRSet, however, can produce values longer than 30 days.
Note that because revocation happens immediately, an attacker has a
much harder job tricking a RFC5011 Resolver into leaving a trust
anchor in place, as the attacker must successfully replay the old
data for every query a RFC5011 Resolver sends, not just one.
7. IANA Considerations
This document contains no IANA considerations.
8. Operational Considerations
A companion document to RFC5011 was expected to be published that
describes the best operational practice considerations from the
perspective of a zone publisher and PEP Publisher. However, this
companion document has yet to be published. The authors of this
document hope that it will at some point in the future, as RFC5011
timing can be tricky as we have shown, and a BCP is clearly
warranted. This document is intended only to fill a single
operational void which, when left misunderstood, can result in
serious security ramifications. This document does not attempt to
document any other missing operational guidance for zone publishers.
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9. Security Considerations
This document, is solely about the security considerations with
respect to the SEP Publisher's ability to advertise new DNSKEYs via
the RFC5011 automated trust anchor update process. Thus the entire
document is a discussion of Security Considerations when adding or
removing DNSKEYs from trust anchor storage using the RFC5011 process.
For simplicity, this document assumes that the SEP Publisher will use
a consistent RRSIG validity period. SEP Publishers that vary the
length of RRSIG validity periods will need to adjust the
sigExpirationTime value accordingly so that the equations in
Section 6 and Section 6.3 use a value that coincides with the last
time a replay of older RRSIGs will no longer succeed.
10. Acknowledgements
The authors would like to especially thank to Michael StJohns for his
help and advice and the care and thought he put into RFC5011 itself.
We would also like to thank Bob Harold, Shane Kerr, Matthijs Mekking,
Duane Wessels, Petr Petr Spacek, Ed Lewis, and the dnsop working
group who have assisted with this document.
11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
September 2007, <http://www.rfc-editor.org/info/rfc5011>.
[RFC7583] Morris, S., Ihren, J., Dickinson, J., and W. Mekking,
"DNSSEC Key Rollover Timing Considerations", RFC 7583,
DOI 10.17487/RFC7583, October 2015, <https://www.rfc-
editor.org/info/rfc7583>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <http://www.rfc-editor.org/info/rfc7719>.
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Appendix A. Real World Example: The 2017 Root KSK Key Roll
In 2017, ICANN expects to (or has, depending on when you're reading
this) roll the key signing key (KSK) for the root zone. The relevant
parameters associated with the root zone at the time of this writing
is as follows:
addHoldDownTime: 30 days
Old DNSKEY sigExpirationTime: 21 days
Old DNSKEY TTL: 2 days
Thus, sticking this information into the equation in
Section Section 6 yields (in days):
addWaitTime = 30
+ (21)
+ MAX(MIN((21) / 2,
MAX(2 / 2,
15 days)),
1 hour)
+ 2 * MAX(2)
addWaitTime = 30 + 21 + MAX(MIN(11.5, 1, 15)), 1 hour) + 4
addWaitTime = 30 + 21 + 1 + 4
addWaitTime = 56 days
Note that we use a activeRefreshOffset of 0, since 30 days is evenly
divisible by activeRefresh (1 day).
Thus, ICANN should wait a minimum of 56 days before switching to the
newly published KSK (and 26 days before removing the old revoked key
once it is published as revoked). ICANN's current plans are to wait
70 days before using the new KEY and 69 days before removing the old,
revoked key. Thus, their current rollover plans are sufficiently
secure from the attack discussed in this memo.
Authors' Addresses
Wes Hardaker
USC/ISI
P.O. Box 382
Davis, CA 95617
US
Email: ietf@hardakers.net
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Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
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