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Versions: 00 01 02 03 04 05 06 07 08 RFC 4641
DNSOP O. Kolkman
Internet-Draft RIPE NCC
Expires: March 1, 2004 R. Gieben
NLnet Labs
September 2003
DNSSEC Operational Practices
draft-ietf-dnsop-dnssec-operational-practices-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on March 1, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document intends to describe a set of practices for operating a
DNSSEC aware enviroment. Its target audience is zone administrators
who are deploying DNSSEC and need a guide to help them chose sensible
values for DNSSEC parameters. Is also discusses operational matters
like key rollovers, KSK and ZSK considerations and more.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 The use of the term 'key' . . . . . . . . . . . . . . . . . 3
2. Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Time definitions . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Time considerations . . . . . . . . . . . . . . . . . . . . 4
3. Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Motivations for the KSK and ZSK functions . . . . . . . . . 6
3.2 Key security considerations . . . . . . . . . . . . . . . . 7
3.3 Key rollovers . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.1 Zone-signing key rollovers . . . . . . . . . . . . . . . . . 9
3.3.2 Key-signing key rollovers . . . . . . . . . . . . . . . . . 12
4. Planning for emergency key rollover. . . . . . . . . . . . . 13
4.1 KSK compromise . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 ZSK compromise . . . . . . . . . . . . . . . . . . . . . . . 14
4.3 Compromises of keys anchored in resolvers . . . . . . . . . 14
5. Parental policies. . . . . . . . . . . . . . . . . . . . . . 14
5.1 Initial key exchanges and parental policies
considerations. . . . . . . . . . . . . . . . . . . . . . . 14
5.2 Storing keys so hashes can be regenerated . . . . . . . . . 15
5.3 Security lameness checks. . . . . . . . . . . . . . . . . . 15
5.4 SIG DS validity period. . . . . . . . . . . . . . . . . . . 15
6. Security considerations . . . . . . . . . . . . . . . . . . 16
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 16
Normative References . . . . . . . . . . . . . . . . . . . . 16
Informative References . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17
A. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 17
B. Zone-signing key rollover howto . . . . . . . . . . . . . . 18
C. Typographic conventions . . . . . . . . . . . . . . . . . . 19
D. Document Details and Changes . . . . . . . . . . . . . . . . 20
D.1 draft-ietf-dnsop-dnssec-operational-practices-00 . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . 22
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1. Introduction
During workshops and early operational deployment tests, operators
and system administrators gained knowledge about operating DNSSEC
aware DNS services. This document describes these practices.
The structure of the document is as follows. It starts 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 is 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. Therefore this document
will use the term key rather loosely. Wherever we write that 'a key
is used to sign data' it is assumed that the reader knows that it is
the private part of the key-pair that is used for signing. It is also
assumed that the reader will know that the public part of the
key-pair is published in the DNSKEY resource record and that it is
the public part of a key-pair that is used in key-exchanges.
2. Time in DNSSEC
Without DNSSEC all times in DNS are relative. The SOA's refresh,
retry and expiration timers are counters that are being used to
determine the time elapsed after a slave server synced (or tried to
sync) with a master server. The TTL value and the SOA minimum TTL
parameter [6] are used to 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 invalid and the signed data is to be considered BAD.
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"
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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.
o "Signature publication period"
Time after which a signature made with a 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 on time T0 and a new signature is
published on 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 to the period between
two consecutive zone signing operations.
o "Key publication period"
The period for which the public part of the key is published in
the DNS. The public part of the key can be published in the DNS
while it has not yet been used to sign data. As soon as a
public key is published a brute force attack can be attempted
to recover the private key. Publishing the public key in
advance (and not signing any data with it) does not guard
against this attack.
[Editor's Note: We don't use this term in the doc yet, is it
needed elsewhere and handy to define here? No:1 Yes:0]
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 behavior might become bursty.
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We suggest the TTL on all the RRs in your zone to be at least
an order of magnitude smaller than your signature validity
period.
o The signature publication period should at least be one maximum
TTL smaller than the signature validity period.
If a zone is resigned shortly before the end of the signature
validity period this may cause simultaneous expiration of data
from caches which leads to bursty query behavior and increase
the load on authoritative servers.
o The Minimum zone TTL should be long enough to fetch and verify all
the RRs in the authentication chain.
1. During validation, some data may expire before validation
is complete. The validator should be able to keep all the
data, until validation is complete. This applies to all data
in 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.
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 at least of the
order 10 minutes to an hour and all the other RRs in your zone
to be at least 30 seconds. These are absolute minimum, we
recommend zone administrators to chose longer ones.
[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.
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
counted down to zero. It is not possible to fully prevent this
from happening by tweaking the SOA parameters. But the effects
can be minimized if the SOA expiration time is of the same of
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order of magnitude as or smaller than the signature validity
period.
When a zone cannot be updated while signatures in that zone
have expired non-secure resolvers will continue to be able to
resolve the data served by the particular slave servers. Only
security aware resolvers that receive data with expired
signatures will experience problems.
We suggest the SOA expiration timer being approximately one
third or one fourth of the signature validity period.
We also suggest that operators of nameservers with slave zones
develop watchdogs to be able to spot these upcoming signature
expirations in slave zones, so that appropriate action can be
taken.
o [Editor's Note: Need examples here]
3. Keys
3.1 Motivations for the KSK and ZSK functions
Delegation Signer [7] introduced the concept of key-signing and
zone-signing keys.The Key-signing-flag [4] introduced the concept of
a key with the Secure Entry Point flag set; a key that is the first
key from the zone when following an authentication chain. When using
a key-signing key with the SEP flag set (the parent has a DS RR
pointing to that DNSKEY) and when using zone-signing keys without the
SEP flag set (a practice which we recommend ) one can use the
following operational procedures.
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 over no
interactions with the parent is needed. This allows for relatively
short "Signature Validity Periods" (order of days).
The key-signing key (with the SEP flag set) 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 maintainer 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
In RFC2541 [2] a number of considerations with respect to the
security of keys are described. That document deals with the
generation, lifetime, size and storage of private keys.
In Section 3 of RFC2541 [2], Eastlake does have some suggestions: 13
months for long-lived keys and 36 days for transaction keys but
suggestions for key sizes are not made.
If we read the long-lived key being a key that is used as key-signing
key and transaction keys being zone signing keys, then these
recommendations are good starting points for an operational
procedure. These recommendations will lead to rollovers occurring
frequently enough so that they can become part of 'operational
habits' and the procedure does not have to be reinvented every time a
key is replaced.
When choosing a 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.
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
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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
Suppose you want your key to last 3 years and the current year is
2003. Add 3 to 2003 equals 2006 and read of the sizes: 1191 for
asymmetric keys and 148 bits for elliptic curve keys.
Note that adding only a "handful of bits" to the key size will
increase the key's resistance against brute force attacks.
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
maintainers who are in the process of rolling their keys have to take
into account that data they have 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 which is
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 BAD.
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 BAD.
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.
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Note that KSK rollovers and ZSK rollovers are different. A zone-key
rollover can be handled in two different way: pre-publish and
[Editors note: ref please] double-sig. 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 the data still in caches can be verified with the
new keysets or the newly generated signatures can be verified with
the keys still in caches. One schema uses double signatures, it is
described in Section 3.3.1.1, the other uses key pre-publication
(Section 3.3.1.2). The pros, cons and recommendations are described
in Section 3.3.1.3.
3.3.1.1 A double signature zone-signing key rollover
This section shows how to perform a ZSK key rollover using the double
zone data signature scheme.
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)
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normal: Version 0 of the zone: DNSKEY 1 is a key-signing key. DNSKEY
10 is used to sign all the data of the zone, it is the
zone-signing key.
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 the 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 the 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".
To be on the safe side one could make sure that the rollover phase
lasts until the signature expiration time of the data in version 0 of
the zone. But this date could be considerable longer than the Maximum
Zone TTL, making the rollover a lengthly procedure.
Note that in this example we assumed that the zone did not get
modified during the rollover. New data can be introduced in the zone
as long as it is signed with both keys.
3.3.1.2 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. We recommend this method because
it has advantages in the case of key compromises. If the old key gets
compromised the new key is already distributed in the DNS. The zone
administrator is then able to quickly switch to the new key and
remove the compromised 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)
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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 a key-signing key. DNSKEY
10 is used to sign all the data of the zone, its the zone-signing
key.
pre-roll: DNSKEY 11 is introduced in the keyset. Note that no
signatures are generated with this key yet, but this will not
prevent 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 on the
keyset. This would boil down 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 that includes 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 a bit by always publishing the
"future" key immediately after the rollover. The scheme would look
like this (we show 2 rollovers); the future key is introduced in
"after" as DNSKEY 12 and again a newer one, numbered 13, in "2nd
after":
normal roll after 2nd roll 2nd after
SOA0 SOA2 SOA3 SOA4 SOA5
RRSIG10(SOA0) RRSIG11(SOA2) RRSIG11(SOA3) RRSIG12(SOA4) RRSIG12(SOA5)
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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, can immediately be used during an emergency rollover under the
assumption that it was stored in a physically secure manner.
3.3.1.3 Pros and cons of the schemes
A double signature rollover: The drawback of this signing scheme is
that during the rollover the number of signatures in your zone
doubles, which may be prohibitive if you have very big zones. An
advantage is that it only requires three steps.
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 is useless for
KSKs as explained in Section 3.3.
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, size considerations do not apply.
normal roll after
SOA0 SOA1 SOA2
RRSIG10(SOA0) RRSIG10(SOA1) RRSIG10(SOA2)
DNSKEY1 DNSKEY1 DNSKEY2
DNSKEY2
DNSKEY10 DNSKEY10 DNSKEY10
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RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) RRSIG2(DNSKEY)
RRSIG2 (DNSKEY)
RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY)
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 has as a
result 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 by 'blackhats' for as long as a valid authentication chain
exists. A authentication chain remains intact for:
as long as a signature over the compromised key in the
authentication chain is valid,
as long as a parental DS RR (and signature) points to the
compromised key,
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 "blackhat". 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 can not be validated. On the
other hand if the zone administrator chooses to take the path of a
regular roll-over the "blackhat" can spoof data so that it appears to
be valid, note that this kind of attack will usually be localized in
the Internet topology.
4.1 KSK compromise
When the KSK has been compromised the parent must be notified as soon
as possible and through 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.
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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 legitemate 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
Mainly 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. One has to take into account though 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 minimizes that problem.
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 that key is compromised all the resolvers 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 authorization
mechanisms used during a key exchange should be as strong as the
authentication and authorization 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 changes of operator error. A parental DNSKEY
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download tool can make use of the SEP bit [4] to select the proper
key from a DNSSEC keyset; thereby reducing the change that the wrong
DNSKEY is sent. It can validate the self-signature over a key;
thereby verifying the ownership of the private key material. Besides,
by fetching the DNSKEY from the DNS one can be sure that the child
will not become invisible once the parent indicates the child is
secure by publishing the DS RR.
Note: the off-band verification is still needed when the keymaterial
is fetched by a tool. The parent can not be sure if the DNSKEY RRs
where not spoofed.
5.2 Storing keys so hashes can be regenerated
When designing a registry system one should consider if the DNSKEYs
or the corresponding DSs are stored. Storing DNSKEYs will help during
troubleshooting while the overhead of calculating DS records from
them is minimal.
Having a 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.
5.3 Security lameness checks.
Security lameness is defined as the event that a parent has a DS
Resource Record that points to a non-existing DNSKEY RR. At key
exchange a parent should make sure that the childs 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 marked "BAD".
Child zones should be very careful removing DNSKEY material,
specifically SEP keys, for which a DS RR exist.
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 SIG DS validity period.
Since the DS can be replayed as long as it has a valid signature a
short signature validity period over the DS minimizes the time a
child is vulnerable in the case of a compromise of the child's KSK.
A signature validity period that is too short introduces the
possibility that a zone is marked BAD 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 weekends show the
need for a DS signature lifetimes longer than 2 days. We recommend
the minimum for a DS signature validity period to be about a few
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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 the order of one week a good
compromise between the operational constraints of the parent and
minimizing 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.
7. Acknowledgments
We, the folk mentioned as authors, only acted as editors. Most of the
ideas in this draft where the result of collective efforts during
workshops and discussions and try outs.
At the risk of forgetting individuals who where the original
contributors of the ideas we like to acknowledge people who where
actively involved in the compilation of this document. In
alphabetical order: Olafur Gudmundsson, Wesley Griffin, Michael
Richardson, Scott Rose, Rick van Rein, Tim McGinnis.
Kolkman and Gieben take the blame for all mistakes.
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.
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
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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.
Authors' Addresses
Olaf M. Kolkman
RIPE NCC
Singel 256
Amsterdam 1016 AB
NL
Phone: +31 20 535 4444
EMail: olaf@ripe.net
URI: http://www.ripe.net/
Miek Gieben
NLnet Labs
Kruislaan 419
Amsterdam 1098 VA
NL
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 give 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 2 keys, a public key and a private key. The public
keys are published in the DNS by use of the DNSKEY Resource Record
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(DNSKEY RR). Private keys are supposed to remain private i.e.
should not be exposed to parties not-authorized 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 for exclusively
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.
BAD: [Editors Note: a reference here] A RRset in DNSSEC is marked
"bad" when a signature of a RRset does not validate against the
DNSKEY. Even is the key itself was not marked BAD. BAD data is not
cached.
Singing the Zone File: The term used for the event where an
administrator joyfully signs its zone file while producing melodic
sound patterns.
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]
STEP 0, the preparation: Create two keys and publish them 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
zonefile created with the current key currently marked as
"active".
Wait until the expiration time marked in STEP 1
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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 abreviated to
just: A.
Signature notation: Signatures are denoted as SIGx(RRset), which
means that RRset is signed with KEYx.
Zone representation: Using the above notation we have simplify the
representation of a signed zone by leaving out all unneeded
details such as the names and by just representing all data by
"SOAx"
SOA representation: Soa's are represented as SOA x, 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.
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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.
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
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document is published.
$Header: /var/cvs/dnssec-key/
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09:49:07 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.
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