Domain Name System Operations W. Wijngaards
Internet-Draft O. Kolkman
Intended status: Standards Track NLnet Labs
Expires: December 31, 2010 June 29, 2010
DNSSEC Trust Anchor History Service
draft-ietf-dnsop-dnssec-trust-history-02
Abstract
When DNS validators have trusted keys, but have been offline for a
longer period, key rollover will fail and they are stuck with stale
trust anchors. History service allows validators to query for older
DNSKEY RRsets and pick up the rollover trail where they left off.
Requirements Language
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 RFC 2119 [RFC2119].
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 31, 2010.
Copyright Notice
Copyright (c) 2010 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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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
1. Introduction
This memo defines a trust history service for DNS validators -- the
component in a resolver that performs DNSSEC [RFC4034] validation,
validator for short.
A validator that has been offline or missed an (emergency) rollover
can use this service to reconfigure themselves with the current
trust-anchor. Using a newly defined resource record (RR) that links
old DNSKEYS together, the TALINK RR, a validator fetches old DNSKEY
RRsets and checks they form a chain to the latest key (see
Section 3). The lists of old DNSKEYS, linked with the TALINK RRs, do
not necessarily need to be published in the zone for which the DNSKEY
history is being maintained but can be published in any DNS domain.
We will call the entity that offers the trust history the History
Provider. The choice of the History Provider is made by the
maintainer of the validator, possibly based on a hint provided, using
the TALINK, by the zone owner.
Section 2 provides background on the mechanism and usage. It looks
at the viewpoints of publishers and consumers of trust anchors, the
use of keys with revocation flags, and SEP flags.
The algorithm that the validator uses to construct a history and
reconfigure a new key is detailed in Section 4, it uses the TALINK RR
type defined in Section 3. The algorithms for how providers of trust
history can fetch the DNSKEY data as published by the zone they track
and publish that are explained in Section 5.
2. Motivation and Description
Validators provide a service in DNSSEC that can be seen from two
ways. Seen from the publisher's point of view, they provide
assurance that the data as received is as it was when it left the
publisher's hands. In this way of looking at things, validators
provide a publication integrity service. The publisher can be
confident that nobody can alter the published data (if it is
validated), because any alteration will be detected. So it protects
a publisher from being seen to send someone to the wrong place.
From the consumer's point of view, validators provide a reason to
trust the data from the network. In this view, the validator is
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making a claim about whether the data ought to be accepted or not.
This is subtly different from the publisher's point of view, because
the question for the consumer is not whether the data is safe while
the consumer is not looking, but whether the data is safe for the
consumer at the moment of consumption. Validation protects a
consumer from going to the wrong place.
These two slightly different ways of looking at the situation result
in slightly different operational goals. Whereas publishers want to
make assertions about their data, by controlling the roll over of
keys, consumers want to get the best assurance that they can get that
the data they are consuming is correct.
If a validator has been offline during a key rollover event for one
of its trust anchors, then the validator will be unable to validate
answers that need that trust anchor. For the publisher, this state
of affairs is acceptable: the publisher is confident that no
validator ever consumes the wrong data. For the consumer, however,
this state of affairs represents an outage.
Since publishers of trust anchors already use a chained series of
keys to perform rollovers under some circumstances (see [RFC5011]),
it is possible to use the history of that chain to allow a validator
to resume service for the consumer without needing to use an out-of-
band mechanism to obtain a new trust anchor. This improves the
experience for consumers of validated data, and increases the chances
that DNSSEC is useful for consumers of DNS data.
The mechanism to do this is a double-linked list that recounts a
portion of the history of DNSKEY Resource Records. The list is used
by a validator to catch up with the changes that the validator
somehow missed. This approach may be thought of as replaying the
[RFC5011] rollover history, only at a later time.
2.1. Considerations for Using a Revoked Key
The keys that the publisher rolled are marked REVOKED by the RFC5011
protocol. At this point the publisher considers the keys revoked,
but the validators have not yet seen this or marked the keys as
revoked. In the RFC5011 protocol, the validators probe regularly and
can then see if keys are revoked. If unable to probe, they will be
unable to see if keys are revoked. Hence when using a history to
recount rollovers, the consumer's validator has also missed a number
of revocations. The goal is to pick up the right keys and also the
new revocations along the way.
Although the keys have been marked by the publisher as REVOKED a long
time ago, for the consumer these REVOKED keys are new information.
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Their storage in the history list makes it possible for consumers to
pick up key revocations if they missed the revocation announcement
because they could not probe.
This is the allowed usage of REVOKED keys. The publisher is
announcing their presence. And the validators mark them as REVOKED
after verification. The initial part of this verification is the
reverse walk through the history list, which is to avoid exposing
which key is trusted. This means that older signatures with keys
that have in the meantime been revoked are used to construct and
verify the history list by the validator.
A consequence is that once a publisher marks keys as REVOKED, there
will still be consumers who are using such keys, because they have
not seen the revocation. From the publishers point of view they are
revoked and the revocation is filed in the historical key list. From
the consumers point of view, it has not seen a revocation yet, and a
historical key list lookup algorithm is a state change where a new
trusted key is obtained while the old key is observed to be revoked.
2.2. Motivation for Requiring the SEP Bit
The SEP bit is used to differentiate Key Signing Keys from other
keys. It is defined in [RFC3757], it is used to designate trust
anchors in [RFC5011]. The protocol herein specified requires that
DNSKEYs that are subject to use for the trust history service have
the SEP bit set. The reason for this is to keep the set of keys that
need to be stored in history small.
3. The TALINK Resource Record
The DNS Resource Record type TALINK (decimal 58) ties the elements of
a linked list of DNSKEY RRs together.
The rdata consists of two domain names. The first name is the start,
or previous name, and the other name the end or next name in the
list. The empty label '.' is used at the endpoints of the list.
The presentation format is the two domain names. The wire encoding
is the two domain names, with no compression so the type can be
treated according to [RFC3597]. The list is a double linked list,
because this empowers low memory hosts to perform consistency checks.
The TALINK used at the zone apex holds the endpoints of the list.
The TALINKs that form the lists hold previous and next entries.
These TALINKs are distinguished by their usage (entrypoint or list
connection). The double linked list is not circular, because lookups
must stop when they reach the oldest entry.
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4. Trust History Lookup
This is the algorithm that a validator uses to detect and resolve the
situation in which a trust-anchor is out of sync with the DNSKEYs
published by a zone owner. The algorithm uses the TALINK RR type
which is used to link various old DNSKEYs as published by the History
Provider, to arrive from the outdated configured Trust Anchor to one
that matches the current DNSKEY. The TALINK RR type is defined in
Section 3.
When the algorithm below results in failure the trust history cannot
be built and a new trust anchor will need to be re-configured using
another mechanism.
Step 1: The validator performs a DNSKEY lookup to the target zone,
which looks like any other initial DNSKEY lookup that the
validator needs to match a trust anchor to the currently used
DNSKEY RR set. If the keyset verifies with the trust anchor
currently held, the trust-anchor is not out of sync. Otherwise,
store the DNSKEY RR set as result. The algorithm will
successfully build a linked list to this DNSKEY RR, or delete the
trust point, or fail.
All nameservers (the ones authoritative for the zone or the
upstream resolver caches when the validator is not full resolver)
SHOULD be checked to make sure the DNSKEY RR sets are the same.
The results can differ if a key-rollover is in progress and not
all nameservers are in sync yet. This case can be detected by
checking that the older keyset signs the newer and take the newer
as result keyset. If both of the keysets sign each other, the
result keyset has the newest rrsig that validates it using the
other keyset. Use the the average over the middle of the
inception and expiration dates of the signatures that are
validated (and for serial arithmetic assume all dates on these
signatures lie within 2^(SERIAL_BITS-1) distance). If the keysets
do not sign each other then this is not a secure change in the
keyset and the history lookup fails.
Step 2: Fetch the trust history list end points. Perform a query of
QTYPE TALINK and QNAME the domain name that is configured to be
the History Provider for the particular domain you are trying to
update the trust-anchor for.
Step 3: Go backwards through the trust history list as provided by
the TALINK linked list. Verify that the keyset validly signs the
next keyset. This is [RFC4034] validation, but the RRSIG
expiration date is ignored. Replace the owner domain name with
the target zone name for verification. One of the keys that signs
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the next keyset MUST have the SEP bit set. The middle of
inception and expiration date from the valid signature MUST be
older than that of the signature that validates the next keys in
the list. Take the average if multiple signatures validate (and
for serial arithmetic assume all dates on these signatures lie
within 2^(SERIAL_BITS-1) distance). Query type TALINK to get
previous and next locations.
If all SEP keys have the REVOKE flag set at this step, and the
keyset is signed by all SEP keys, then continue but store that the
end result is that the trust point is deleted, see Section 5
[RFC5011].
If all SEP keys are of an unknown algorithm at this step, continue
and at the next step, when you verify if the keyset signs validly:
if false, continue with result a failure, if true, continue with
the end result that the trust point is deleted. Thus, the keysets
with unknown algorithms are stepped over with an end result of
failure because the validator cannot determine if unknown
algorithm signatures are valid, until the oldest keyset with
unknown algorithms is signed by a known algorithm and the result
is set to deletion and step 3 continues to a known key.
Step 4: When the trust anchor currently held by the validator
verifies the keyset, the algorithm is done. The validator SHOULD
store the result on stable storage. Use the new trust anchor for
validation (if using [RFC5011], put it in state VALID).
5. Trust History Tracker
External trackers can poll the target zone DNSKEY RRset regularly.
Ignore date changes in the RRSIG. Ignore changes to keys with no SEP
flag. Copy the newly polled DNSKEY RRset and RRSIGs, change the
owner name to a new name at the history location. Publish the new
RRset and TALINK records to make it the last element in the list.
Update the TALINK that advertises the first and last name.
Integrated into the rollover, the keys are stored in the history and
the TALINK is updated when a new key is used in the rollover process.
This gives the TALINK and new historical key time to propagate.
The signer can support trust history. Trust history key sets need
only contain SEP keys. To use older signers, move historical RRSIGs
to another file. Sign the zone, including the TALINK and DNSKEY
records. Append the historical RRSIGs to the result. Signing the
zone like this obviates the need for changes to signer and server
software.
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6. Example
In this example the trust history for the 'example.net' zone is
published in the 'example.com' namespace. The DNSKEY rdata and RRSIG
rdata is omitted for brevity, it is a copy and paste of the data from
example.net.
$ORIGIN example.com.
example.com. TALINK h0.example.com. h2.example.com.
h0 TALINK . h1.example.com.
h0 DNSKEY ...
h0 RRSIG ...
h1 TALINK h0.example.com. h2.example.com.
h1 DNSKEY ...
h1 RRSIG ...
h2 TALINK h1.example.com. .
h2 DNSKEY ...
h2 RRSIG ...
The example.net zone can advertise the example.com History Provider
by providing the TALINK shown here at example.com at the apex of the
example.net zone. The TALINK at example.com is then not needed.
7. Deployment
The trust history is advertised with TALINK RRs at the zone apex.
These represent alternative history sources, that can be searched in
turn. The TALINK at the zone apex contains the first and last name
of the list of historical keys.
The historical list of keys grows perpetually. Since most validators
have recent keys, their processing time remains similar as the list
grows. If validators no longer have trust in the keys then they need
no longer be published. The oldest key entries can be omitted from
the list to shorten it.
The validator decides how long it trusts a key. A recommendation
from the zone owner can be configured for keys of that zone, or
recommendations per algorithm and key size can be used (e.g. see
[NIST800-57]). If a key is older than that, trust history lookup
fails with it and the trust point can be considered deleted. This
assumes the validator has decided on a security policy and also can
take actions when the update of the trust anchor fails. Without such
policy, or if the alternative is no DNSSEC, the approach below can be
used.
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In general, the decision can be that any key - no matter how old or
how small - is better than no security. The validator then never
considers a key too old, and the lookup algorithm becomes an
unsecured update mechanism at the time where the key can be trivially
broken. The history provider SHOULD provide these broken keys to
facilitate clients performing the unsecured update. If a key can not
be trivially broken then it provides a non-trivial amount of security
that the history lookup algorithm uses to get the current keys.
Conceivably after the update the result is stored on stable storage
and the client is thereafter safe - it performs a leap of faith. The
validator operator can opt for this set up after considering the
trade-off between loss of DNSSEC, loss of connectivity, and the
argument that perceived security is worse than no security.
The history lookup can be used on its own. Then, the trust history
is used whenever the key rolls over and no polling is performed. The
results of trust history lookup SHOULD be stored on stable storage,
so that the trust history lookup does not need to be performed if the
last results are okay and for use as trusted anchor in the next
history lookup.
If a validator is also using [RFC5011] for the target zone, then the
trust history algorithm SHOULD only be invoked if the [RFC5011]
algorithm failed due to the inability to perform probes. This is the
case when the last [RFC5011] successful probe was more than 30 days
ago. If a new key has been announced, invoke the history if no 2
probes succeeded during the add hold-down time and there was no
successful probe after the add hold-down time passed. Therefore the
time of the last successful probe MUST be stored on stable storage.
For testing the potentially very infrequently used lookup, the
following SHOULD be implemented. For the test the lookup is
triggered manually by allowing the system to be given a particular
keyset with a last successful lookup date in the past and a test
History Provider. The test History Provider provides access to a
generated back-dated test history.
8. Security Considerations
The History Provider only provides copies of old data. If that
historic data is altered or withheld the lookup algorithm fails
because of validation errors in Step 3 of the algorithm. If the
History provider or a Man in the Middle Adversary (MIMA) has access
to the original private keys (through theft, cryptanalisis, or
otherwise), history can be altered without failure of the algorithm.
Below we only consider MIMAs and assume the History Provider is a
trusted party.
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Spoofing by a MIMA of data looked up in step 2 or 3, i.e. spoofing of
TALINK and DNSKEY data, can present some alternate history. However
the DNSKEY RR set trusted that the history should arrive at is
already fixed by step 1. If an attempt is made to subvert the
algorithm at step 2 or 3, then the result keyset can not be replaced
by another keyset unnoticed.
To change the keyset trusted as the outcome, the step 1 data has to
be spoofed and the key held by the validator (or a newer historic
key) has to be compromised. Unless such spoof is targeted to a
specific victim, a spoof of the step 1 result has a high visibility.
Since most of the validators that receive the spoof have an up-to-
date trust anchor most validators that would receive this spoof
return validation failure for data from the zone that contains the
DNSKEYs. An adversary will therefore have to target the attack to
validators that are in the process of an update. Since validators do
not announce that they use trust history lookup until step 2
adversaries will not be able to select the validators.
A spoof of data in steps 2 and 3, together with a compromised (old)
key, can result in a downgrade. At steps 2 and 3 a faked trust point
deletion or algorithm rollover can be inserted in a fake history.
This avoids the high visibility of spoofing the current key (see
previous paragraph) and downgrades to insecure.
Finally there is the case that one of the keys published by the
History Providers has been compromised. Since someone spoofing at
step 1 of the lookup algorithm and presenting some fake history to a
compromised key, of course does not include key revocations and does
extend the history to contain the compromised key, it therefore is
not really useful for a History Provider to remove the key from the
published history. That only makes lookups fail for those validators
who are not under attack. Useful action could be to update
validators using some other means.
Rollover with [RFC5011] revokes keys after use. If a History
Provider is used, then such revoked keys SHOULD be used to perform
history tracking and history lookup. The trust anchor keys that the
validator has in its own storage and final current keys that it
stores MUST NOT be trusted if they are revoked.
If the validator operator chooses to operate trust history without
also using [RFC5011] the trust anchor does not get hold-down timer
protection. This has associated risks, in that the immediate
rollover without timeout that it provides could be abused (if private
keys are compromised). Such abuse could result in the stored lookup
results to become compromised. The key changes can be logged, to
inform operators and keep an audit trail.
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The SEP bit is checked to make sure that control over the KSK is
necessary to change the keyset for the target zone.
The algorithm can be used to get the inception and expiration times
of signatures on the current keyset, a clock. A MIMA can attempt to
shorten history and put back that clock, but the algorithm attempts
to make this difficult to target and highly visible to others.
If the clock of the validator can be influenced, then setting it
forward is unlikely to give advantage, but setting it backward
enables a replay attack of old DNSSEC data and signatures. This
vulnerability exists also in plain DNSSEC.
9. IANA Considerations
Resource record type TALINK has been defined using RFC5395 expert
review, it has RR type number 58 (decimal).
10. Acknowledgments
Thanks to the people who provided review and suggestions, Peter Koch,
Andrew Sullivan, Joe Abley, George Barwood, Edward Lewis, Michael
StJohns, Bert Hubert, Mark Andrews, Ted Lemon, Steve Crocker, Bill
Manning, Eric Osterweil, Wolfgang Nagele, Alfred Hoenes, Olafur
Gudmundsson, Roy Arends and Matthijs Mekking.
11. References
11.1. Informative References
[NIST800-57] Barker, E., Barker, W., Burr, W., Polk, W., and M.
Smid, "Recommendations for Key Management", NIST
SP 800-57, March 2007.
[RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name
System KEY (DNSKEY) Resource Record (RR) Secure Entry
Point (SEP) Flag", RFC 3757, April 2004.
[RFC5011] StJohns, M., "Automated Updates of DNS Security
(DNSSEC) Trust Anchors", RFC 5011, September 2007.
11.2. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource
Record (RR) Types", RFC 3597, September 2003.
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[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security
Extensions", RFC 4034, March 2005.
Authors' Addresses
Wouter Wijngaards
NLnet Labs
Science Park 140
Amsterdam 1098 XG
The Netherlands
EMail: wouter@nlnetlabs.nl
Olaf Kolkman
NLnet Labs
Science Park 140
Amsterdam 1098 XG
The Netherlands
EMail: olaf@nlnetlabs.nl
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