DNSOP O. Kolkman
Internet-Draft RIPE NCC
Expires: August 30, 2004 R. Gieben
NLnet Labs
March 2004
DNSSEC Operational Practices
draft-ietf-dnsop-dnssec-operational-practices-01.txt
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This Internet-Draft will expire on August 30, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes a set of practices for operating a DNSSEC
aware environment. The target audience is zone administrators
deploying DNSSEC that need a guide to help them chose appropriate
values for DNSSEC parameters. It also discusses operational matters
such as key rollovers, KSK and ZSK considerations and related
matters.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 The Use of the Term 'key' . . . . . . . . . . . . . . . . 3
1.2 Keeping the Chain of Trust Intact . . . . . . . . . . . . 3
2. Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Time Definitions . . . . . . . . . . . . . . . . . . . . . 4
2.2 Time Considerations . . . . . . . . . . . . . . . . . . . 5
3. Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Motivations for the KSK and ZSK Functions . . . . . . . . 7
3.2 Key Security Considerations . . . . . . . . . . . . . . . 8
3.2.1 Key Validity Period . . . . . . . . . . . . . . . . . 8
3.2.2 Key Algorithm . . . . . . . . . . . . . . . . . . . . 8
3.2.3 Key Sizes . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Key Rollovers . . . . . . . . . . . . . . . . . . . . . . 9
3.3.1 Zone-signing Key Rollovers . . . . . . . . . . . . . . 10
3.3.2 Key-signing Key Rollovers . . . . . . . . . . . . . . 13
4. Planning for Emergency Key Rollover . . . . . . . . . . . . . 14
4.1 KSK Compromise . . . . . . . . . . . . . . . . . . . . . . 15
4.2 ZSK Compromise . . . . . . . . . . . . . . . . . . . . . . 15
4.3 Compromises of Keys Anchored in Resolvers . . . . . . . . 16
5. Parental Policies . . . . . . . . . . . . . . . . . . . . . . 16
5.1 Initial Key Exchanges and Parental Policies
Considerations . . . . . . . . . . . . . . . . . . . . . . 16
5.2 Storing Keys So Hashes Can Be Regenerated . . . . . . . . 16
5.3 Security Lameness Checks . . . . . . . . . . . . . . . . . 17
5.4 DS Signature Validity Period . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 18
8.2 Informative References . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19
A. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 19
B. Zone-signing Key Rollover Howto . . . . . . . . . . . . . . . 20
C. Typographic Conventions . . . . . . . . . . . . . . . . . . . 20
D. Document Details and Changes . . . . . . . . . . . . . . . . . 22
D.1 draft-ietf-dnsop-dnssec-operational-practices-00 . . . . . 22
D.2 draft-ietf-dnsop-dnssec-operational-practices-01 . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . 23
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1. Introduction
During workshops and early operational deployment tests, operators
and system administrators gained experience about operating DNSSEC
aware DNS services. This document translates these experiences into
a set of practices for zone administrators. At the time of writing,
there exists very little experience with DNSSEC in production
environments, this document should therefore explicitly not be seen
as represented 'Best Current Practices'.
The procedures herein are focused on the maintenance of signed zones
(i.e. signing and publishing zones on authoritative servers). It is
intended that maintenance of zones such as resigning or key rollovers
be transparent to any verifying clients on the Internet.
The structure of this document is as follows: It begins with
discussing some of the considerations with respect to timing
parameters of DNS in relation to DNSSEC (Section 2). Aspects of key
management such as key rollover schemes are described in Section 3.
Emergency rollover considerations are addressed in Section 4. The
typographic conventions used in this document are explained in
Appendix C.
Since this is a document with operational suggestions and there are
no protocol specifications, the RFC2119 [5] language does not apply.
1.1 The Use of the Term 'key'
It is assumed that the reader is familiar with the concept of
asymmetric keys on which DNSSEC is based (Public Key Cryptography
[Ref to Schneider?]). Therefore, this document will use the term
'key' rather loosely. Where it is written that 'a key is used to sign
data' it is assumed that the reader understands that it is the
private part of the key-pair that is used for signing. It is also
assumed that the reader understands that the public part of the
key-pair is published in the DNSKEY resource record and that it is
used in key-exchanges.
1.2 Keeping the Chain of Trust Intact
Maintaining a valid chain of trust is important because broken chains
of trust will result in data being marked as bogus, which may cause
entire (sub)domains to become invisible to verifying clients. The
administrators of secured zones have to realise that their zone is,
to their clients, part of a chain of trust.
As mentioned in the introduction, the procedures herein are intended
to ensure maintenance of zones, such as resigning or key rollovers,
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be transparent to the verifying clients on the Internet.
Administrators of secured zones will have to keep in mind that data
published on an authoritative primary server will not be immediately
seen by verifying clients; it may take some time for the data to be
transfered to other secondary authoritative nameservers, during which
period clients may be fetching data from caching non-authoritative
servers. For the verifying clients it is important that data from
secured zones can be used to build chains of trust regardless of
whether the data came directly from an authoritative server, a
caching nameserver or some middle box. Only by carefully using the
available timing parameters can a zone administrator assure that the
data necessary for verification can be obtained.
The responsibility for maintaining the chain of trust is shared by
administrators of secured zones in the chain of trust. This is most
obvious in the case of a 'key compromise' when a trade off between
maintaining a valid chain of trust and the fact that the key has been
stolen, must be made.
The zone administrator will have to make a tradeoff between keeping
the chain of trust intact -thereby allowing for attacks with the
compromised key- or to deliberately break the chain of trust thereby
making secured subdomains invisible to security aware resolvers. Also
see Section 4.
2. Time in DNSSEC
Without DNSSEC all times in DNS are relative. The SOA's refresh,
retry and expiration timers are counters that are used to determine
the time elapsed after a slave server syncronised (or tried to
syncronise) with a master server. The Time to Live (TTL) value and
the SOA minimum TTL parameter [6] are used to determine how long a
forwarder should cache data after it has been fetched from an
authoritative server. DNSSEC introduces the notion of an absolute
time in the DNS. Signatures in DNSSEC have an expiration date after
which the signature is marked as invalid and the signed data is to be
considered bogus.
2.1 Time Definitions
In this document we will be using a number of time related terms.
Within the context of this document the following definitions apply:
o "Signature validity period"
The period that a signature is valid. It starts at the time
specified in the signature inception field of the RRSIG RR and
ends at the time specified in the expiration field of the RRSIG
RR.
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o "Signature publication period"
Time after which a signature (made with a specific key) is
replaced with a new signature (made with the same key). This
replacement takes place by publishing the relevant RRSIG in the
master zone file. If a signature is published at time T0 and a
new signature is published at time T1, the signature
publication period is T1 - T0.
If all signatures are refreshed at zone (re)signing then the
signature publication period is equal signature validity
period.
o "Maximum/Minimum Zone TTL"
The maximum or minimum value of all the TTLs in a zone.
2.2 Time Considerations
Because of the expiration of signatures, one should consider the
following.
o The Maximum Zone TTL of your zone data should be a fraction of
your signature validity period.
If the TTL would be of similar order as the signature validity
period, then all RRsets fetched during the validity period
would be cached until the signature expiration time. As a
result query load on authoritative servers would peak at
signature expiration time.
To avoid query load peaks we suggest the TTL on all the RRs in
your zone to be at least a few times smaller than your
signature validity period.
o The signature publication period should be at least one maximum
TTL smaller than the signature validity period.
Resigning a zone shortly before the end of the signature
validity period may cause simultaneous expiration of data from
caches. This in turn may lead to peaks in the load on
authoritative servers.
o The Minimum zone TTL should be long enough to both fetch and
verify all the RRs in the authentication chain.
1. During validation, some data may expire before the
validation is complete. The validator should be able to keep
all data, until is completed. This applies to all RRs needed
to complete the chain of trust: DSs, DNSKEYs, RRSIGs, and
the final answers i.e. the RR that is returned for the
initial query.
2. Frequent verification causes load on recursive
nameservers. Data at delegation points, DSs, DNSKEYs and
RRSIGs benefit from caching. The TTL on those should be
relatively long.
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We have seen events where data needed for verification of an
authentication chain had expired from caches.
We suggest the TTL on DNSKEY and DSs to be between ten minutes
and one hour. We recommend zone administrators to chose TTLs
longer than half a minute.
[Editor's Note: this observation could be implementation
specific. We are not sure if we should leave this item]
o Slave servers will need to be able to fetch newly signed zones
well before the data expires from your zone.
'Better no answers than bad answers.'
If a properly implemented slave server is not able to contact a
master server for an extended period the data will at some
point expire and the slave server will not hand out any data.
If the server serves a DNSSEC zone than it may well happen that
the signatures expire well before the SOA expiration timer
counts down to zero. It is not possible to completely prevent
this from happening by tweaking the SOA parameters. However,
the effects can be minimized where the SOA expiration time is
equal or smaller than the signature validity period.
The consequence of an authoritative server not being able to
update a zone, whilst that zone includes expired signaturs, is
that non-secure resolvers will continue to be able to resolve
data served by the particular slave servers. Security aware
resolvers will experience problems.
We suggest the SOA expiration timer being approximately one
third or one fourth of the signature validity period. It will
allow problems with transfers from the master server to be
noticed before the actual signature time out.
We suggest that operators of nameservers with slave zones
develop 'watch dogs' to spot upcoming signature expirations in
slave zones, and take appropriate action.
When determining the value for the expiration parameter one has
to take the following into account: What are the chances that
all my secondary zones expire; How quickly can I reach an
administrator and load a valid zone? All these arguments are
not DNSSEC specific.
3. Keys
In the DNSSEC protocol there is only one type of key, the zone key.
With this key, the data in a zone is signed.
To make zone re-signing and key rollovers procedures easier to
implement, it is possible to use one or more keys as Key Signing Keys
(KSK) these keys will only sign the apex DNSKEY RRs in a zone. Other
keys can be used to sign all the RRsets in a zone and are referred to
as Zone Signing Keys (ZSK). In this document we assume that KSKs are
the subset of keys that are used for key exchanges with the parents
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and potentially for configuration as trusted anchors - the so called
Secure Entry Point keys (SEP). In this document we assume a
one-to-one mapping between KSK and SEP keys and we assume the SEP
flag [4] to be set on KSKs.
3.1 Motivations for the KSK and ZSK Functions
Differentiating between the KSK to ZSK functions has several
advantages:
o Making the KSK stronger (i.e. using more bits in the key material)
has little operational impact since it is only used to sign a
small fraction of the zone data.
o As the KSK is only used to sign a keyset, which is most probably
updated less frequently than other data in the zone, it can be
stored separately from (and thus in a safer location than) the
ZSK.
o A KSK can be used for longer periods.
o No parent/child interaction is required when ZSKs are updated.
The KSK is used less than ZSK, once a keyset is signed with the KSK
all the keys in the keyset can be used as ZSK. If a ZSK is
compromised, it can be simply dropped from the keyset. The new keyset
is then resigned with the KSK.
Given the assumption that for KSKs the SEP flag is set, the KSK can
be distinguished from a ZSK by examining the flag field in the DNSKEY
RR. If the flag field is an odd number it is a KSK if it is an even
number it is a ZSK e.g. a value of 256 and a key signing key has 257.
The zone-signing key can be used to sign all the data in a zone on a
regular basis. When a zone-signing key is to be rolled, no
interaction with the parent is needed. This allows for relatively
short "Signature Validity Periods". That is, Signature Validity
Periods of the order of days.
The key-signing key is only to be used to sign the Key RR set from
the zone apex. If a key-signing key is to be rolled over, there will
be interactions with parties other than the zone administrator such
as the registry of the parent zone or administrators of verifying
resolvers that have the particular key configured as trusted entry
points. Hence, the "Key Usage Time" of these keys can and should be
made much longer. Although, given a long enough key, the "Key Usage
Time" can be on the order of years we suggest to plan for a "Key
Usage Time" of the order of a few months so that a key rollover
remains an operational routine.
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3.2 Key Security Considerations
Keys in DNSSEC have a number of parameters which should all be chosen
with care, the most important once are: size, algorithm and the key
validity period (its lifetime).
3.2.1 Key Validity Period
RFC2541 [2] describes a number of considerations with respect to the
security of keys. The document deals with the generation, lifetime,
size and storage of private keys.
In Section 3 of RFC2541 [2] there are some suggestions for a key
validity period: 13 months for long-lived keys and 36 days for
transaction keys but suggestions for key sizes are not made.
If we say long-lived keys are key-signing keys and transactions keys
are zone-signing keys, these recommendations will lead to rollovers
occurring frequently enough to become part of 'operational habits';
the procedure does not have to be reinvented every time a key is
replaced.
3.2.2 Key Algorithm
We recommend you choose RSA/SHA-1 as the preferred algorithm for the
key. RSA has been developed in an open and transparent manner. As the
patent on RSA expired in 2001, its use is now also free. The current
known attacks on RSA can be defeated by making your key longer. As
the MD5 hashing algorithm is showing (theoretical) cracks, we
recommend the usage of SHA1.
3.2.3 Key Sizes
When choosing key sizes, zone administrators will need to take into
account how long a key will be used and how much data will be signed
during the key publication period. It is hard to give precise
recommendations but Lenstra and Verheul [9] supplied the following
table with lower bound estimates for cryptographic key sizes. Their
recommendations are based on a set of explicitly formulated parameter
settings, combined with existing data points about cryptosystems. For
details we refer to the original paper.
[Editor's Note: DSA???]
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Year RSA Key Sizes Elliptic Curve Key Size
2000 952 132
2001 990 135
2002 1028 139
2003 1068 140
2004 1108 143
2005 1149 147
2006 1191 148
2007 1235 152
2008 1279 155
2009 1323 157
2010 1369 160
2011 1416 163
2012 1464 165
2013 1513 168
2014 1562 172
2015 1613 173
2016 1664 177
2017 1717 180
2018 1771 181
2019 1825 185
2020 1881 188
2021 1937 190
2022 1995 193
2023 2054 197
2024 2113 198
2025 2174 202
2026 2236 205
2027 2299 207
2028 2362 210
2029 2427 213
For example, should you wish your key to last three years from 2003,
check the RSA keysize values for 2006 in this table. In this case
1191.
3.3 Key Rollovers
Key rollovers are a fact of life when using DNSSEC. A DNSSEC key
cannot be used forever (see RFC2541 [2] and Section 3.2 ). Zone
administrators who are in the process of rolling their keys have to
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take into account that data published in previous versions of their
zone still lives in caches. When deploying DNSSEC, this becomes an
important consideration; ignoring data that may be in caches may lead
to loss of service for clients.
The most pressing example of this is when zone material signed with
an old key is being validated by a resolver which does not have the
old zone key cached. If the old key is no longer present in the
current zone, this validation fails, marking the data bogus.
Alternatively, an attempt could be made to validate data which is
signed with a new key against an old key that lives in a local cache,
also resulting in data being marked bogus.
To appreciate the situation one could think of a number of
authoritative servers that may not be instantaneously running the
same version of a zone and a security aware non-recursive resolver
that sits behind security aware caching forwarders.
Note that KSK rollovers and ZSK rollovers are different. A zone-key
rollover can be handled in two different ways: pre-publish (Section
Section 3.3.1.1) and double signature (Section Section 3.3.1.2). The
pre-publish technique works because the key-signing key stays the
same during this ZSK rollover. With this KSK a cache is able to
validate the new keyset of a zone. With a KSK rollover a cache can
not validate the new keyset, because it does not trust the new KSK.
[Editors note: This needs more verbose explanation, nobody will
appreciate the situation just yet. Help with text and examples is
appreciated]
3.3.1 Zone-signing Key Rollovers
For zone-signing key rollovers there are two ways to make sure that
during the rollover data still cached can be verified with the new
keysets or newly generated signatures can be verified with the keys
still in caches. One schema uses double signatures, it is described
in Section 3.3.1.2, the other uses key pre-publication (Section
3.3.1.1). The pros, cons and recommendations are described in Section
3.3.1.3.
3.3.1.1 Pre-publish Keyset Rollover
This section shows how to perform a ZSK rollover without the need to
sign all the data in a zone twice - the so called "prepublish
rollover". We recommend this method because it has advantages in the
case of key compromise. If the old key is compromised, the new key
has already been distributed in the DNS. The zone administrator is
then able to quickly switch to the new key and remove the compromised
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key from the zone. Another major advantage is that the zone size does
not double, as is the case with the double signature ZSK rollover. A
small "HOWTO" for this kind of rollover can be found in Appendix B.
normal pre-roll roll after
SOA0 SOA1 SOA2 SOA3
RRSIG10(SOA0) RRSIG10(SOA1) RRSIG11(SOA2) RRSIG11(SOA3)
DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY1
DNSKEY10 DNSKEY10 DNSKEY10 DNSKEY11
DNSKEY11 DNSKEY11
RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) RRSIG1(DNSKEY) RRSIG1 (DNSKEY)
RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG11(DNSKEY) RRSIG11(DNSKEY)
normal: Version 0 of the zone: DNSKEY 1 is the key-signing key.
DNSKEY 10 is used to sign all the data of the zone, the
zone-signing key.
pre-roll: DNSKEY 11 is introduced into the keyset. Note that no
signatures are generated with this key yet, but this does not
secure against brute force attacks on the public key. The minimum
duration of this pre-roll phase is the time it takes for the data
to propagate to the authoritative servers plus TTL value of the
keyset. This equates to two times the Maximum Zone TTL.
roll: At the rollover stage (SOA serial 1) DNSKEY 11 is used to sign
the data in the zone exclusively (i.e. all the signatures from
DNSKEY 10 are removed from the zone). DNSKEY 10 remains published
in the keyset. This way data that was loaded into caches from
version 1 of the zone can still be verified with key sets fetched
from version 2 of the zone.
The minimum time that the keyset including DNSKEY 10 is to be
published is the time that it takes for zone data from the
previous version of the zone to expire from old caches i.e. the
time it takes for this zone to propagate to all authoritative
servers plus the Maximum Zone TTL value of any of the data in the
previous version of the zone.
after: DNSKEY 10 is removed from the zone. The keyset, now only
containing DNSKEY 11 is resigned with the DNSKEY 1.
The above scheme can be simplified by always publishing the "future"
key immediately after the rollover. The scheme would look as follows
(we show two rollovers); the future key is introduced in "after" as
DNSKEY 12 and again a newer one, numbered 13, in "2nd after":
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normal roll after 2nd roll 2nd after
SOA0 SOA2 SOA3 SOA4 SOA5
RRSIG10(SOA0) RRSIG11(SOA2) RRSIG11(SOA3) RRSIG12(SOA4) RRSIG12(SOA5)
DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY1
DNSKEY10 DNSKEY10 DNSKEY11 DNSKEY11 DNSKEY12
DNSKEY11 DNSKEY11 DNSKEY12 DNSKEY12 DNSKEY13
RRSIG1(DNSKEY) RRSIG1 (DNSKEY) RRSIG1(DNSKEY) RRSIG1(DNSKEY) RRSIG1(DNSKEY)
RRSIG10(DNSKEY) RRSIG11(DNSKEY) RRSIG11(DNSKEY) RRSIG12(DNSKEY) RRSIG12(DNSKEY)
Note that the key introduced after the rollover is not used for
production yet; the private key can thus be stored in a physically
secure manner and does not need to be 'fetched' every time a zone
needs to be signed.
This scheme has the benefit that the key that is intended for future
use: immediately during an emergency rollover assuming that the
private key was stored in a physically secure manner.
3.3.1.2 Double Signature Zone-signing Key Rollover
This section shows how to perform a ZSK key rollover using the double
zone data signature scheme, aptly named "double sig rollover".
During the rollover stage the new version of the zone file will need
to propagate to all authoritative servers and the data that exists in
(distant) caches will need to expire, this will take at least the
maximum Zone TTL .
normal roll after
SOA0 SOA1 SOA2
RRSIG10(SOA0) RRSIG10(SOA1) RRSIG11(SOA2)
RRSIG11(SOA1)
DNSKEY1 DNSKEY1 DNSKEY1
DNSKEY10 DNSKEY10 DNSKEY11
DNSKEY11
RRSIG1(DNSKEY) RRSIG1(DNSKEY) RRSIG1(DNSKEY)
RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG11(DNSKEY)
RRSIG11(DNSKEY)
normal: Version 0 of the zone: DNSKEY 1 is the key-signing key.
DNSKEY 10 is used to sign all the data of the zone, the
zone-signing key.
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roll: At the rollover stage (SOA serial 1) DNSKEY 11 is introduced
into the keyset and all the data in the zone is signed with DNSKEY
10 and DNSKEY 11. The rollover period will need to exist until all
data from version 0 of the zone has expired from remote caches.
This will take at least the maximum Zone TTL of version 0 of the
zone.
after: DNSKEY 10 is removed from the zone. All the signatures from
DNSKEY 10 are removed from the zone. The keyset, now only
containing DNSKEY 11, is resigned with DNSKEY 1.
At every instance the data from the previous version of the zone can
be verified with the key from the current version and vice verse. The
data from the current version can be verified with the data from the
previous version of the zone. The duration of the rollover phase and
the period between rollovers should be at least the "Maximum Zone
TTL".
Making sure that the rollover phase lasts until the signature
expiration time of the data in version 0 of the zone is recommended.
However, this date could be considerably longer than the Maximum Zone
TTL, making the rollover a lengthy procedure.
Note that in this example we assumed that the zone was not modified
during the rollover. New data can be introduced in the zone as long
as it is signed with both keys.
3.3.1.3 Pros and Cons of the Schemes
Prepublish-keyset rollover: This rollover does not involve signing
the zone data twice. Instead, just before the actual rollover, the
new key is published in the keyset and thus available for
cryptanalysis attacks. A small disavantage is that this process
requires four steps. Also the prepublish scheme will not work for
KSKs as explained in Section 3.3.
Double signature rollover: The drawback of this signing scheme is
that during the rollover the number of signatures in your zone
doubles, this may be prohibitive if you have very big zones. An
advantage is that it only requires three steps.
3.3.2 Key-signing Key Rollovers
For the rollover of a key-signing key the same considerations as for
the rollover of a zone-signing key apply. However we can use a double
signature scheme to guarantee that old data (only the apex keyset) in
caches can be verified with a new keyset and vice versa.
Since only the keyset is signed with a KSK, zone size considerations
do not apply.
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normal roll after
SOA0 SOA1 SOA2
RRSIG10(SOA0) RRSIG10(SOA1) RRSIG10(SOA2)
DNSKEY1 DNSKEY1 DNSKEY2
DNSKEY2
DNSKEY10 DNSKEY10 DNSKEY10
RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) RRSIG2(DNSKEY)
RRSIG2 (DNSKEY)
RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY)
normal: Version 0 of the zone. The parental DS points to DNSKEY1.
Before the rollover starts the child will have to verify what the
TTL is of the DS RR that points to DNSKEY1 - it is needed during
the rollover and we refer to the value as TTL_DS.
roll: During the rollover phase the zone administrator generates a
second KSK, DNSKEY2. The key is provided to the parent and the
child will have to wait until a new DS RR has been generated that
points to DNSKEY2. After that DS RR has been published on _all_
servers authoritative for the parents zone, the zone administrator
has to wait at least TTL_DS to make sure that the old DS RR has
expired from distant caches.
after: DNSKEY1 has been removed.
The scenario above puts the responsibility for maintaining a valid
chain of trust with the child. It also is based on the premises that
the parent only has one DS RR (per algorithm) per zone. St John [The
draft has expired] proposed a mechanism where using an established
trust relation, the interaction can be performed in-band. In this
mechanism there are periods where there are two DS RRs at the parent.
[Editors note: We probably need to mention more]
4. Planning for Emergency Key Rollover
This section deals with preparation for a possible key compromise.
Our advice is to have a documented procedure ready for when a key
compromise is suspected or confirmed.
[Editors note: We are much in favor of a rollover tactic that keeps
the authentication chain intact as long as possible. This means that
one has to take all the regular rollover properties into account.]
When the private material of one of your keys is compromised it can
be used for as long as a valid authentication chain exists. An
authentication chain remains intact for:
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o as long as a signature over the compromised key in the
authentication chain is valid,
o as long as a parental DS RR (and signature) points to the
compromised key,
o as long as the key is anchored in a resolver and is used as a
starting point for validation. (This is the hardest to update.)
While an authentication chain to your compromised key exists, your
name-space is vulnerable to abuse by the malicious key holder (i.e.
the owner of the compromised key). Zone operators have to make a
trade off if the abuse of the compromised key is worse than having
data in caches that cannot be validated. If the zone operator chooses
to break the authentication chain to the compromised key, data in
caches signed with this key cannot be validated. However, if the zone
administrator chooses to take the path of a regular roll-over, the
malicious key holder can spoof data so that it appears to be valid,
note that this kind of attack will usually be localised in the
Internet topology.
4.1 KSK Compromise
When the KSK has been compromised the parent must be notified as soon
as possible using secure means. The keyset of the zone should be
resigned as soon as possible. Care must be taken to not break the
authentication chain. The local zone can only be resigned with the
new KSK after the parent's zone has been updated with the new KSK.
Before this update takes place it would be best to drop the security
status of a zone all together: the parent removes the DS of the child
at the next zone update. After that the child can be made secure
again.
An additional danger of a key compromise is that the compromised key
can be used to facilitate a legitimate DNSKEY/DS and/or nameserver
rollover at the parent. When that happens the domain can be in
dispute. An out of band and secure notify mechanism to contact a
parent is needed in this case.
4.2 ZSK Compromise
Primarily because there is no parental interaction required when a
ZSK is compromised, the situation is less severe than with with a KSK
compromise. The zone must still be resigned with a new ZSK as soon
as possible. As this is a local operation and requires no
communication between the parent and child this can be achieved
fairly quickly. However, one has to take into account that just as
with a normal rollover the immediate disappearance from the old
compromised key may lead to verification problems. The
pre-publication scheme as discussed above minimises such problems.
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4.3 Compromises of Keys Anchored in Resolvers
A key can also be pre-configured in resolvers. If DNSSEC is rolled
out as planned the root key should be pre-configured in every secure
aware resolver on the planet. [Editors Note: add more about
authentication of a newly received resolver key]
If trust-anchor keys are compromised, the resolvers using these keys
should be notified of this fact. Zone administrators may consider
setting up a mailing list to communicate the fact that a SEP key is
about to be rolled over. This communication will of course need to be
authenticated e.g. by using digital signatures.
5. Parental Policies
5.1 Initial Key Exchanges and Parental Policies Considerations
The initial key exchange is always subject to the policies set by the
parent (or its registry). When designing a key exchange policy one
should take into account that the authentication and authorisation
mechanisms used during a key exchange should be as strong as the
authentication and authorisation mechanisms used for the exchange of
delegation information between parent and child.
Using the DNS itself as the source for the actual DNSKEY material,
with an off-band check on the validity of the DNSKEY, has the benefit
that it reduces the chances of user error. A parental DNSKEY download
tool can make use of the SEP bit [4] to select the proper key from a
DNSSEC keyset; thereby reducing the chance that the wrong DNSKEY is
sent. It can validate the self-signature over a key; thereby
verifying the ownership of the private key material. Fetching the
DNSKEY from the DNS ensures that the child will not become bogus once
the parent publishes the DS RR indicating the child is secure.
Note: the off-band verification is still needed when the key-material
is fetched by a tool. The parent can not be sure whether the DNSKEY
RRs have been spoofed.
5.2 Storing Keys So Hashes Can Be Regenerated
When designing a registry system one should consider if the DNSKEYs
and/or the corresponding DSs are stored. Storing DNSKEYs will help
during troubleshooting while the overhead of calculating DS records
from them is minimal.
Having an out-of-band mechanism, such as a Whois database, to find
out which keys are used to generate DS Resource Records for specific
owners may also help with troubleshooting.
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5.3 Security Lameness Checks
Security Lameness is defined as what happens when a parent has a DS
Resource Record pointing to a non-existing DNSKEY RR. During key
exchange a parent should make sure that the child's key is actually
configured in the DNS before publishing a DS RR in its zone. Failure
to do so would render the child's zone being marked as bogus.
Child zones should be very careful removing DNSKEY material,
specifically SEP keys, for which a DS RR exists.
Once a zone is "security lame" a fix (e.g. by removing a DS RR) will
take time to propagate through the DNS.
5.4 DS Signature Validity Period
Since the DS can be replayed as long as it has a valid signature a
short signature validity period over the DS minimises the time a
child is vulnerable in the case of a compromise of the child's
KSK(s). A signature validity period that is too short introduces the
possibility that a zone is marked bogus in case of a configuration
error in the signer; there may not be enough time to fix the problems
before signatures expire. Something as mundane as operator
unavailability during weekends shows the need for DS signature
lifetimes longer than 2 days. We recommend the minimum for a DS
signature validity period to be a few days.
The maximum signature lifetime of the DS record depends on how long
child zones are willing to be vulnerable after a key compromise. We
consider a signature validity period of around one week to be a good
compromise between the operational constraints of the parent and
minimising damage for the child.
6. Security Considerations
DNSSEC adds data integrity to the DNS. This document tries to assess
considerations to operate a stable and secure DNSSEC service. Not
taking into account the 'data propagation' properties in the DNS will
cause validation failures and may make secured zones unavailable to
security aware resolvers.
7. Acknowledgments
We, the folk mentioned as authors, only acted as editors. Most of the
ideas in this draft were the result of collective efforts during
workshops, discussions and try outs.
At the risk of forgetting individuals who where the original
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contributors of the ideas we would like to acknowledge people who
where actively involved in the compilation of this document. In
random order: Olafur Gudmundsson, Wesley Griffin, Michael Richardson,
Scott Rose, Rick van Rein, Tim McGinnis, Gilles Guette and Olivier
Courtay, Sam Weiler.
Emma Bretherick and Adrian Bedford corrected many of the spelling and
style issues.
Kolkman and Gieben take the blame for introducing all miscakes(SIC).
8. References
8.1 Normative References
[1] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[2] Eastlake, D., "DNS Security Operational Considerations", RFC
2541, March 1999.
[3] Lewis, E., "DNS Security Extension Clarification on Zone
Status", RFC 3090, March 2001.
[4] Lewis, E., Kolkman, O. and J. Schlyter, "KEY RR Key-Signing Key
(KSK) Flag", draft-ietf-dnsext-keyrr-key-signing-flag-06 (work
in progress), February 2003.
8.2 Informative References
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC
2308, March 1998.
[7] Gudmundsson, O., "Delegation Signer Resource Record",
draft-ietf-dnsext-delegation-signer-13 (work in progress), March
2003.
[8] Arends, R., "Protocol Modifications for the DNS Security
Extensions", draft-ietf-dnsext-dnssec-protocol-01 (work in
progress), March 2003.
[9] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key Sizes",
The Journal of Cryptology 14 (255-293), 2001.
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Authors' Addresses
Olaf M. Kolkman
RIPE NCC
Singel 256
Amsterdam 1016 AB
The Netherlands
Phone: +31 20 535 4444
EMail: olaf@ripe.net
URI: http://www.ripe.net/
Miek Gieben
NLnet Labs
Kruislaan 419
Amsterdam 1098 VA
The Netherlands
EMail: miek@nlnetlabs.nl
URI: http://www.nlnetlabs.nl
Appendix A. Terminology
In this document there is some jargon used that is defined in other
documents. In most cases we have not copied the text from the
documents defining the terms but given a more elaborate explanation
of the meaning. Note that these explanations should not be seen as
authoritative.
Private and Public Keys: DNSSEC secures the DNS through the use of
public key cryptography. Public key cryptography is based on the
existence of two keys, a public key and a private key. The public
keys are published in the DNS by use of the DNSKEY Resource Record
(DNSKEY RR). Private keys should remain private i.e. should not be
exposed to parties not-authorised to do the actual signing.
Signer: The system that has access to the private key material and
signs the Resource Record sets in a zone. A signer may be
configured to sign only parts of the zone e.g. only those RRsets
for which existing signatures are about to expire.
KSK: A Key-Signing Key (KSK) is a key that is used exclusively for
signing the apex keyset. The fact that a key is a KSK is only
relevant to the signing tool.
ZSK: A Zone Signing Key (ZSK) is a key that is used for signing all
data in a zone. The fact that a key is a ZSK is only relevant to
the signing tool.
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SEP Key: A KSK that has a parental DS record pointing to it. Note:
this is not enforced in the protocol. A SEP Key with no parental
DS is security lame.
Anchored Key: A DNSKEY configured in resolvers around the globe. This
Key is hard to update, hence the term anchored.
Bogus: [Editors Note: a reference here] An RRset in DNSSEC is marked
"Bogus" when a signature of a RRset does not validate against the
DNSKEY. Even if the key itself was not marked Bogus. A cache may
choose to cache Bogus data for various reasons.
Singing the Zone File: The term used for the event where an
administrator joyfully signs its zone file while producing melodic
sound patterns.
Zone Administrator: The 'role' that is responsible for signing a zone
and publishing it on the primary authoritative server.
Appendix B. Zone-signing Key Rollover Howto
Using the pre-published signature scheme and the most conservative
method to assure oneself that data does not live in distant caches
here follows the "HOWTO". [WES: has some comments about this]
Key notation:
Step 0: The preparation: Create two keys and publish both in your
keyset. Mark one of the keys as "active" and the other as
"published". Use the "active" key for signing your zone data.
Store the private part of the "published" key, preferably
off-line.
Step 1: Determine expiration: At the beginning of the rollover make a
note of the highest expiration time of signatures in your zone
file created with the current key marked as "active".
Wait until the expiration time marked in Step 1 has passed
Step 2: Then start using the key that was marked as "published" to
sign your data i.e. mark it as "active". Stop using the key that
was marked as "active", mark it as "rolled".
Step 3: It is safe to engage in a new rollover (Step 1) after at
least one "signature validity period".
Appendix C. Typographic Conventions
The following typographic conventions are used in this document:
Key notation: A key is denoted by KEYx, where x is a number, x could
be thought of as the key id.
RRset notations: RRs are only denoted by the type. All other
information - owner, class, rdata and TTL - is left out. Thus:
example.com 3600 IN A 192.168.1.1 is reduced to: A. RRsets are a
list of RRs. A example of this would be: A1,A2, specifying the
RRset containing two A records. This could again be abbreviated to
just: A.
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Signature notation: Signatures are denoted as RRSIGx(RRset), which
means that RRset is signed with DNSKEYx.
Zone representation: Using the above notation we have simplified the
representation of a signed zone by leaving out all unnecessary
details such as the names and by representing all data by "SOAx"
SOA representation: SOA's are represented as SOAx, where x is the
serial number.
Using this notation the following zone :
example.net. 600 IN SOA ns.example.net. ernie.example.net. (
10 ; serial
450 ; refresh (7 minutes 30 seconds)
600 ; retry (10 minutes)
345600 ; expire (4 days)
300 ; minimum (5 minutes)
)
600 RRSIG SOA 5 2 600 20130522213204 (
20130422213204 14 example.net.
cmL62SI6iAX46xGNQAdQ... )
600 NS a.iana-servers.net.
600 NS b.iana-servers.net.
600 RRSIG NS 5 2 600 20130507213204 (
20130407213204 14 example.net.
SO5epiJei19AjXoUpFnQ ... )
3600 DNSKEY 256 3 5 (
EtRB9MP5/AvOuVO0I8XDxy0...
) ; key id = 14
3600 DNSKEY 256 3 5 (
gsPW/Yy19GzYIY+Gnr8HABU...
) ; key id = 15
3600 RRSIG DNSKEY 5 2 3600 20130522213204 (
20130422213204 14 example.net.
J4zCe8QX4tXVGjV4e1r9... )
3600 RRSIG DNSKEY 5 2 3600 20130522213204 (
20130422213204 15 example.net.
keVDCOpsSeDReyV6O... )
600 NSEC a.example.net. NS SOA TXT RRSIG DNSKEY NSEC
600 RRSIG NSEC 5 2 600 20130507213204 (
20130407213204 14 example.net.
obj3HEp1GjnmhRjX... )
a.example.net. 600 IN TXT "A label"
600 RRSIG TXT 5 3 600 20130507213204 (
20130407213204 14 example.net.
IkDMlRdYLmXH7QJnuF3v... )
600 NSEC b.example.com. TXT RRSIG NSEC
600 RRSIG NSEC 5 3 600 20130507213204 (
20130407213204 14 example.net.
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bZMjoZ3bHjnEz0nIsPMM... )
...
is reduced to the following represenation:
SOA10
RRSIG14(SOA10)
DNSKEY14
DNSKEY15
RRSIG14(KEY)
RRSIG15(KEY)
The rest of the zone data has the same signature as the SOA record,
i.e a RRSIG created with DNSKEY 14.
Appendix D. Document Details and Changes
This section is to be removed by the RFC editor if and when the
document is published.
$Header: /var/cvs/dnssec-key/
draft-ietf-dnsop-dnssec-operational-practices.xml,v 1.22 2004/05/12
08:29:11 dnssec Exp $
D.1 draft-ietf-dnsop-dnssec-operational-practices-00
Submission as working group document. This document is a modified and
updated version of draft-kolkman-dnssec-operational-practices-00.
D.2 draft-ietf-dnsop-dnssec-operational-practices-01
changed the definition of "Bogus" to reflect the one in the protocol
draft.
Bad to Bogus
Style and spelling corrections
KSK - SEP mapping made explicit.
Updates from Sam Weiler added
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Kolkman & Gieben Expires August 30, 2004 [Page 24]
