dnsop W. Hardaker
Internet-Draft USC/ISI
Intended status: Standards Track W. Kumari
Expires: December 29, 2017 Google
June 27, 2017
Security Considerations for RFC5011 Publishers
draft-ietf-dnsop-rfc5011-security-considerations-02
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 with
only recently added DNSKEYs. 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 December 29, 2017.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Hardaker & Kumari Expires December 29, 2017 [Page 1]
Internet-Draft RFC5011 Security Considerations June 2017
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Document History and Motivation . . . . . . . . . . . . . 3
1.2. Safely Rolling the Root Zone's KSK in 2017/2018 . . . . . 3
1.3. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Timing Associated with RFC5011 Processing . . . . . . . . . . 4
4.1. Timing Associated with Publication . . . . . . . . . . . 4
4.2. Timing Associated with Revocation . . . . . . . . . . . . 5
5. Denial of Service Attack Considerations . . . . . . . . . . . 5
5.1. Enumerated Attack Example . . . . . . . . . . . . . . . . 5
5.1.1. Attack Timing Breakdown . . . . . . . . . . . . . . . 6
6. Minimum RFC5011 Timing Requirements . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Operational Considerations . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. Normative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Real World Example: The 2017 Root KSK Key Roll . . . 10
Appendix B. Changes / Author Notes. . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
[RFC5011] defines a mechanism by which DNSSEC validators can extend
their list of trust anchors when they've seen a new key published in
a zone. However, RFC5011 [intentionally] provides no guidance to the
publishers of DNSKEYs about how long they must wait before switching
to using only recently published keys for signing records, or how
long they must wait before removing a revoked key from a zone.
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 compliment the guidance offered in RFC5011 (which
is written to provide timing guidance solely to a Validating
Resolver's point of view).
Hardaker & Kumari Expires December 29, 2017 [Page 2]
Internet-Draft RFC5011 Security Considerations June 2017
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 using a new KSK [RFC4033] that
was being rolled according to the 5011 process. All 5 experts
answered with an insecure value, and we determined that this lack of
operational guidance is causing security concerns today 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 upcoming] 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
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 Validating
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, as required
from the DNSKEY publication and revocation's points of view. Note
that it 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
using only recently added keys or removing a revoked keys too soon.
Failure of a DNSKEY publisher to follow the minimum recommendations
associated with this draft will 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 Validating Resolvers leaving a key in
their trust anchor storage beyond their expected lifetime.
Hardaker & Kumari Expires December 29, 2017 [Page 3]
Internet-Draft RFC5011 Security Considerations June 2017
3. Terminology
Trust Anchor Publisher The entity responsible for publishing a
DNSKEY 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 will become a trust anchor by validators
following the RFC5011 process.
RFC5011 Validating Resolver A DNSSEC Validating Resolver that is
using the RFC5011 processes to track and update trust anchors.
Sometimes referred to as a "RFC5011 Resolver"
Attacker An attacker intent on foiling the RFC5011 Validator's
ability to successfully adopt the Zone Signer's new DNSKEY as a
new trust anchor or to prevent the RFC5011 Validator from removing
an old DNSKEY from its list of trust anchors.
Also see Section 2 of [RFC4033] and [RFC7719] for additional
terminology.
4. Timing Associated with RFC5011 Processing
4.1. Timing Associated with Publication
RFC5011's process of safely publishing a new key and then making use
of that key falls into a number of high-level steps to be performed
by the Trust Anchor Publisher:
1. Publish a new DNSKEY in the zone, but continue to sign the zone
with the old one.
2. Wait a period of time.
3. Begin using only recently published DNSKEYs to sign the
appropriate resource records.
This document discusses 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 results in a denial of service attack against the zone
if that value is used.
Hardaker & Kumari Expires December 29, 2017 [Page 4]
Internet-Draft RFC5011 Security Considerations June 2017
4.2. Timing Associated with Revocation
RFC5011's process of advertising that an old key is to be revoked
from RFC5011 validating 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 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 staying in a
RFC5011 validating resolver long past its expected usage.
5. Denial of Service Attack Considerations
If an attacker is able to provide a RFC5011 Validating Resolver with
past responses, such as when it is in-path or able to otherwise
perform any number of cache poisoning attacks, the attacker may be
able to leave compliant RFC5011-Validating Resolvers without an
appropriate DNSKEY trust anchor. This scenario will remain until an
administrator manually fixes the situation.
The following timeline 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
DNSKEY RRSIG Signature Validity 10 days
Zone resigned every 1 day
Given these settings, the sequence of events in Section 5.1.1 depicts
how a Trust Anchor 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
Hardaker & Kumari Expires December 29, 2017 [Page 5]
Internet-Draft RFC5011 Security Considerations June 2017
Trust Anchor Publisher publishing a new Key Signing Key (KSK), with
the intent that it will later become 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 being published:
K_old An older KSK and Trust Anchor being replaced.
K_new A new KSK being transitioned into active use and becoming a
Trust Anchor via the RFC5011 process.
5.1.1. Attack Timing Breakdown
The following series of steps depicts the timeline in which an attack
occurs that foils the adoption of a new DNSKEY by a Trust Anchor
Publisher that starts signing with the new DNSKEY too quickly.
T-1 The last RRSIGs are published by the Zone Signer that signs only
K_old key using the K_old key itself. [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 talking about RRSIGs that
cover the DNSKEYs.] 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 Validator (later to be under
attack) retrieves this new key set and corresponding RRSIGs and
notices the publication of K_new. The RFC5011 Validator starts
the (30-day) hold-down timer for K_new.
T+5 The RFC5011 Validator 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
Attacker successfully replays the keyset and associated signatures
that they 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) and the RFC5011
Validator cancels the hold-down timer for K_new, per the RFC5011
algorithm.
T+10 The RFC5011 Validator queries for the zone's keyset and
discovers the new kset which includes K_new (again), signed by
K_old. Note: the attacker is unable to replay the records cached
Hardaker & Kumari Expires December 29, 2017 [Page 6]
Internet-Draft RFC5011 Security Considerations June 2017
at T-1, because they have now expired. The RFC5011 Validator
starts (anew) the hold-timer for K_new.
T+15,T+20, and T+25 The RFC5011 Validator continues checking the
zone's key set at the prescribed regular intervals. The RFC5011
Validator's hold-down timer keep running without being reset
assuming all of the validations succeed (again: 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 Validator 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 Validator 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 of 30 days + 1/2 the signature validity period
would have passed). However, the hold-down timer of our attacked
RFC5011 Validator is only at 25 days; The replay attack at T+5
means its new hold-time timer actually started at T+10, and thus
at this time it's real hold-down timer is at T+35 - T+10 = 25
days, which is less than the RFC5011 required 30 days and the
RFC5011 won't consider it a valid trust anchor addition yet.
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
K_new, and 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 Validator has a hold-down timer for K_new at only
26 days, it will fail to accept K_new as a trust anchor. Since
K_old is no longer being used, all the DNSKEY records from the
zone signed by K_new will be treated as invalid. Subsequently,
all keys in the key set are now unusable, invalidating all of the
records in the zone of any type and name.
6. Minimum RFC5011 Timing Requirements
Given the attack description in Section 5, the correct minimum length
of time required for the Zone Signer to wait before using K_new is:
Hardaker & Kumari Expires December 29, 2017 [Page 7]
Internet-Draft RFC5011 Security Considerations June 2017
waitTime = addHoldDownTime
+ (DNSKEY RRSIG Signature Validity)
+ MAX(MIN((DNSKEY RRSIG Signature Validity) / 2,
MAX(original TTL of K_old DNSKEY RRSet) / 2,
15 days),
1 hour)
+ 2 * MAX(TTL of all records)
The RFC5011 "Active Refresh" requirements state that:
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.
The important timing constraint introduced by this memo relates to
the last point at which a validating resolver may have received a
replayed the original DNSKEY set (K_old) without the new key. It's
the next query of the RFC5011 validator that the assured K_new will
be seen without a potential replay afterward. Thus, the latest time
a RFC5011 validator may begin their hold down timer is an "Active
Refresh" period after the last point that an attacker can replay the
K_old DNSKEY set.
The "Active Refresh" interval used by a RFC5011 validator is
determined by the larger of (DNSKEY RRSIG Signature Validity) and
(original TTL for the DNSKEY RRSet). The Following text assumes that
(DNSKEY RRSIG Signature Validity) is larger of the two, which is
operationally more common today.
Thus, the worst case scenario of this attack is when the attacker can
replay K_old just before (DNSKEY RRSIG Signature Validity). If a
RFC5011 validator picks up K_old at this this point, it will not have
a hold down timer started as it will have been reset by previous
replays. It's not until the next "Active Refresh" time that they'll
pick up K_new with assurance, and thus start their (final) hold down
timer. Thus, this is not at (DNSKEY RRSIG Signature Validity) time
past publication but may be significantly longer based on the zone's
DNSSEC parameters.
The extra 2 * MAX(TTL of all records) is the standard added safety
margin when dealing with DNSSEC due to caching that can take place.
Because the 5011 steps require direct validation using the signature
Hardaker & Kumari Expires December 29, 2017 [Page 8]
Internet-Draft RFC5011 Security Considerations June 2017
validity, the authors aren't yet convinced it is needed in this
particular case, but it is prudent to include it for added assurance.
For the parameters listed in Section 5.1, our example:
waitTime = 30
+ 10
+ 10 / 2
+ 2 * (1) (days)
waitTime = 47 (days)
This hold-down time of 47 days is 12 days longer than the (frequently
perceived) 35 days in the example at T+35 above.
It is important to note that this value affects not just the
publication of new DNSKEYs intended to be used as trust anchors, but
also the length of time required to publish a DNSKEY with the revoke
bit set. Both of these publication timing requirements are affected
by the attacks described in this document.
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 Trust Anchor 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 we do not suggest "good
operational practice" that might be associated with a BCP on the
subject. This document is intended only to fill a single operational
void that results in security ramifications (specifically a denial of
service attack against an RFC5011 Validator). This document does not
attempt to document any other missing operational guidance for zone
publishers.
9. Security Considerations
This document, is solely about the security considerations with
respect to the Trust Anchor Publisher of RFC5011 trust anchors /
keys. Thus the entire document is a discussion of Security
Considerations when rolling DNSKEYs using the RFC5011 process.
Hardaker & Kumari Expires December 29, 2017 [Page 9]
Internet-Draft RFC5011 Security Considerations June 2017
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, 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>.
[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>.
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 RRSIG Signature Validity: 21 days
Old DNSKEY TTL: 2 days
Thus, sticking this information into the equation in
Section Section 6 yields (in days):
Hardaker & Kumari Expires December 29, 2017 [Page 10]
Internet-Draft RFC5011 Security Considerations June 2017
waitTime = addHoldDownTime
+ (DNSKEY RRSIG Signature Validity)
+ MAX(MIN((DNSKEY RRSIG Signature Validity) / 2,
MAX(original TTL of K_old DNSKEY RRSet) / 2,
15 days),
1 hour)
+ 2 * MAX(TTL of all records)
waitTime = 30
+ (21)
+ MAX(MIN((21) / 2,
MAX(2 / 2,
15 days)),
1 hour)
+ 2 * MAX(2)
waitTime = 30 + 21 + MAX(MIN(11.5, MAX( 1, 15)), 1 hour) + 4
waitTime = 30 + 21 + 11.5 + 4
waitTime = 66.5 days
Thus, ICANN should wait 66.5 days before switching to the newly
published KSK and 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.
Appendix B. Changes / Author Notes.
From Individual-00 to DNSOP-00:
o Filename change.
From -00 to -01:
o Added Revocation processing (including "Timing Associated with
Revocation")
o Added real world example.
o Fixed some typoes and missing references.
From Ind-00 to -02:
Additional background and clarifications in abstract.
Hardaker & Kumari Expires December 29, 2017 [Page 11]
Internet-Draft RFC5011 Security Considerations June 2017
Better separation in attack description between attacked and non-
attacked resolvers.
Some language cleanup.
Clarified that this is maths ( and math is hard, let's go
shopping!)
Changed to " <?rfc include='reference....'?> " style references.
From -02 to -03:
Minor changes from Bob Harold
Clarified why 3/2 signature validity is needed
Changed min wait time math to include TTL value as well
From -03 to -04:
Fixed the waitTime equation to handle the difference between the
usage of the expiration time and the Active Refresh time.
More clarification text and text changes proposed by Petr Spacek
From -04 to -05:
Clarifications about signing using only new keys, vs old ones too
Pre-DNSOP document: From hardaker-04 to ietf-00:
Just rebranding.
From ietf-00 to ietf-01:
Added discussion surrounding revocation everywhere
Fixed the text about the formula
Another complete re-read for word-smithing
Authors' Addresses
Hardaker & Kumari Expires December 29, 2017 [Page 12]
Internet-Draft RFC5011 Security Considerations June 2017
Wes Hardaker
USC/ISI
P.O. Box 382
Davis, CA 95617
US
Email: ietf@hardakers.net
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
Hardaker & Kumari Expires December 29, 2017 [Page 13]