Network Working Group O. Kolkman
Request for Comments: 4641 R. Gieben
Obsoletes: 2541 NLnet Labs
Category: Informational September 2006
DNSSEC Operational Practices
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a set of practices for operating the DNS with
security extensions (DNSSEC). The target audience is zone
administrators deploying DNSSEC.
The document discusses operational aspects of using keys and
signatures in the DNS. It discusses issues of key generation, key
storage, signature generation, key rollover, and related policies.
This document obsoletes RFC 2541, as it covers more operational
ground and gives more up-to-date requirements with respect to key
sizes and the new DNSSEC specification.
Table of Contents
1. Introduction ....................................................3
1.1. The Use of the Term 'key' ..................................4
1.2. Time Definitions ...........................................4
2. Keeping the Chain of Trust Intact ...............................5
3. Keys Generation and Storage .....................................6
3.1. Zone and Key Signing Keys ..................................6
3.1.1. Motivations for the KSK and ZSK Separation ..........6
3.1.2. KSKs for High-Level Zones ...........................7
3.2. Key Generation .............................................8
3.3. Key Effectivity Period .....................................8
3.4. Key Algorithm ..............................................9
3.5. Key Sizes ..................................................9
3.6. Private Key Storage .......................................11
4. Signature Generation, Key Rollover, and Related Policies .......12
4.1. Time in DNSSEC ............................................12
4.1.1. Time Considerations ................................12
4.2. Key Rollovers .............................................14
4.2.1. Zone Signing Key Rollovers .........................14
4.2.1.1. Pre-Publish Key Rollover ..................15
4.2.1.2. Double Signature Zone Signing Key
Rollover ..................................17
4.2.1.3. Pros and Cons of the Schemes ..............18
4.2.2. Key Signing Key Rollovers ..........................18
4.2.3. Difference Between ZSK and KSK Rollovers ...........20
4.2.4. Automated Key Rollovers ............................21
4.3. Planning for Emergency Key Rollover .......................21
4.3.1. KSK Compromise .....................................22
4.3.1.1. Keeping the Chain of Trust Intact .........22
4.3.1.2. Breaking the Chain of Trust ...............23
4.3.2. ZSK Compromise .....................................23
4.3.3. Compromises of Keys Anchored in Resolvers ..........24
4.4. Parental Policies .........................................24
4.4.1. Initial Key Exchanges and Parental Policies
Considerations .....................................24
4.4.2. Storing Keys or Hashes? ............................25
4.4.3. Security Lameness ..................................25
4.4.4. DS Signature Validity Period .......................26
5. Security Considerations ........................................26
6. Acknowledgments ................................................26
7. References .....................................................27
7.1. Normative References ......................................27
7.2. Informative References ....................................28
Appendix A. Terminology ...........................................30
Appendix B. Zone Signing Key Rollover How-To ......................31
Appendix C. Typographic Conventions ...............................32
1. Introduction
This document describes how to run a DNS Security (DNSSEC)-enabled
environment. It is intended for operators who have knowledge of the
DNS (see RFC 1034 [1] and RFC 1035 [2]) and want to deploy DNSSEC.
See RFC 4033 [4] for an introduction to DNSSEC, RFC 4034 [5] for the
newly introduced Resource Records (RRs), and RFC 4035 [6] for the
protocol changes.
During workshops and early operational deployment tests, operators
and system administrators have gained experience about operating the
DNS with security extensions (DNSSEC). 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 representing '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 re-signing or key
rollovers be transparent to any verifying clients on the Internet.
The structure of this document is as follows. In Section 2, we
discuss the importance of keeping the "chain of trust" intact.
Aspects of key generation and storage of private keys are discussed
in Section 3; the focus in this section is mainly on the private part
of the key(s). Section 4 describes considerations concerning the
public part of the keys. Since these public keys appear in the DNS
one has to take into account all kinds of timing issues, which are
discussed in Section 4.1. Section 4.2 and Section 4.3 deal with the
rollover, or supercession, of keys. Finally, Section 4.4 discusses
considerations on how parents deal with their children's public keys
in order to maintain chains of trust.
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 RFC 2119 [7] language does not apply.
This document obsoletes RFC 2541 [12] to reflect the evolution of the
underlying DNSSEC protocol since then. Changes in the choice of
cryptographic algorithms, DNS record types and type names, and the
parent-child key and signature exchange demanded a major rewrite and
additional information and explanation.
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
[17]). 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 the public part that is
used in key exchanges.
1.2. Time Definitions
In this document, we will be using a number of time-related terms.
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.
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. After one stops
publishing an RRSIG in a zone, it may take a while before the
RRSIG has expired from caches and has actually been removed from
the DNS.
o "Key effectivity period" The period during which a key pair is
expected to be effective. This period is defined as the time
between the first inception time stamp and the last expiration
date of any signature made with this key, regardless of any
discontinuity in the use of the key. The key effectivity period
can span multiple signature validity periods.
o "Maximum/Minimum Zone Time to Live (TTL)" The maximum or minimum
value of the TTLs from the complete set of RRs in a zone. Note
that the minimum TTL is not the same as the MINIMUM field in the
SOA RR. See [11] for more information.
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 (as defined in [4]
Section 5), which may cause entire (sub)domains to become invisible
to verifying clients. The administrators of secured zones have to
realize that their zone is, to verifying clients, part of a chain of
trust.
As mentioned in the introduction, the procedures herein are intended
to ensure that maintenance of zones, such as re-signing or key
rollovers, will 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
transferred to other secondary authoritative nameservers and clients
may be fetching data from caching non-authoritative servers. In this
light, note that the time for a zone transfer from master to slave is
negligible when using NOTIFY [9] and incremental transfer (IXFR) [8].
It increases when full zone transfers (AXFR) are used in combination
with NOTIFY. It increases even more if you rely on full zone
transfers based on only the SOA timing parameters for refresh.
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 ensure 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 replacing the compromised keys
as soon as possible must be made. Then zone administrators will have
to make a trade-off, between keeping the chain of trust intact --
thereby allowing for attacks with the compromised key -- or
deliberately breaking the chain of trust and making secured
subdomains invisible to security-aware resolvers. Also see Section
4.3.
3. Keys Generation and Storage
This section describes a number of considerations with respect to the
security of keys. It deals with the generation, effectivity period,
size, and storage of private keys.
3.1. Zone and Key Signing Keys
The DNSSEC validation protocol does not distinguish between different
types of DNSKEYs. All DNSKEYs can be used during the validation. In
practice, operators use Key Signing and Zone Signing Keys and use the
so-called Secure Entry Point (SEP) [3] flag to distinguish between
them during operations. The dynamics and considerations are
discussed below.
To make zone re-signing and key rollover procedures easier to
implement, it is possible to use one or more keys as Key Signing Keys
(KSKs). These keys will only sign the apex DNSKEY RRSet in a zone.
Other keys can be used to sign all the RRSets in a zone and are
referred to as Zone Signing Keys (ZSKs). In this document, we assume
that KSKs are the subset of keys that are used for key exchanges with
the parent and potentially for configuration as trusted anchors --
the SEP keys. In this document, we assume a one-to-one mapping
between KSK and SEP keys and we assume the SEP flag to be set on all
KSKs.
3.1.1. Motivations for the KSK and ZSK Separation
Differentiating between the KSK and ZSK functions has several
advantages:
o No parent/child interaction is required when ZSKs are updated.
o The KSK can be made stronger (i.e., using more bits in the key
material). This has little operational impact since it is only
used to sign a small fraction of the zone data. Also, the KSK is
only used to verify the zone's key set, not for other RRSets in
the zone.
o As the KSK is only used to sign a key set, which is most probably
updated less frequently than other data in the zone, it can be
stored separately from and in a safer location than the ZSK.
o A KSK can have a longer key effectivity period.
For almost any method of key management and zone signing, the KSK is
used less frequently than the ZSK. Once a key set is signed with the
KSK, all the keys in the key set can be used as ZSKs. If a ZSK is
compromised, it can be simply dropped from the key set. The new key
set is then re-signed 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.
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 signature
validity periods on the order of days.
The Key Signing Key is only to be used to sign the DNSKEY RRs in a
zone. If a Key Signing Key is to be rolled over, there will be
interactions with parties other than the zone administrator. These
can include the registry of the parent zone or administrators of
verifying resolvers that have the particular key configured as secure
entry points. Hence, the key effectivity period of these keys can
and should be made much longer. Although, given a long enough key,
the key effectivity period can be on the order of years, we suggest
planning for a key effectivity on the order of a few months so that a
key rollover remains an operational routine.
3.1.2. KSKs for High-Level Zones
Higher-level zones are generally more sensitive than lower-level
zones. Anyone controlling or breaking the security of a zone thereby
obtains authority over all of its subdomains (except in the case of
resolvers that have locally configured the public key of a subdomain,
in which case this, and only this, subdomain wouldn't be affected by
the compromise of the parent zone). Therefore, extra care should be
taken with high-level zones, and strong keys should be used.
The root zone is the most critical of all zones. Someone controlling
or compromising the security of the root zone would control the
entire DNS namespace of all resolvers using that root zone (except in
the case of resolvers that have locally configured the public key of
a subdomain). Therefore, the utmost care must be taken in the
securing of the root zone. The strongest and most carefully handled
keys should be used. The root zone private key should always be kept
off-line.
Many resolvers will start at a root server for their access to and
authentication of DNS data. Securely updating the trust anchors in
an enormous population of resolvers around the world will be
extremely difficult.
3.2. Key Generation
Careful generation of all keys is a sometimes overlooked but
absolutely essential element in any cryptographically secure system.
The strongest algorithms used with the longest keys are still of no
use if an adversary can guess enough to lower the size of the likely
key space so that it can be exhaustively searched. Technical
suggestions for the generation of random keys will be found in RFC
4086 [14]. One should carefully assess if the random number
generator used during key generation adheres to these suggestions.
Keys with a long effectivity period are particularly sensitive as
they will represent a more valuable target and be subject to attack
for a longer time than short-period keys. It is strongly recommended
that long-term key generation occur off-line in a manner isolated
from the network via an air gap or, at a minimum, high-level secure
hardware.
3.3. Key Effectivity Period
For various reasons, keys in DNSSEC need to be changed once in a
while. The longer a key is in use, the greater the probability that
it will have been compromised through carelessness, accident,
espionage, or cryptanalysis. Furthermore, when key rollovers are too
rare an event, they will not become part of the operational habit and
there is risk that nobody on-site will remember the procedure for
rollover when the need is there.
From a purely operational perspective, a reasonable key effectivity
period for Key Signing Keys is 13 months, with the intent to replace
them after 12 months. An intended key effectivity period of a month
is reasonable for Zone Signing Keys.
For key sizes that match these effectivity periods, see Section 3.5.
As argued in Section 3.1.2, securely updating trust anchors will be
extremely difficult. On the other hand, the "operational habit"
argument does also apply to trust anchor reconfiguration. If a short
key effectivity period is used and the trust anchor configuration has
to be revisited on a regular basis, the odds that the configuration
tends to be forgotten is smaller. The trade-off is against a system
that is so dynamic that administrators of the validating clients will
not be able to follow the modifications.
Key effectivity periods can be made very short, as in a few minutes.
But when replacing keys one has to take the considerations from
Section 4.1 and Section 4.2 into account.
3.4. Key Algorithm
There are currently three different types of algorithms that can be
used in DNSSEC: RSA, DSA, and elliptic curve cryptography. The
latter is fairly new and has yet to be standardized for usage in
DNSSEC.
RSA has been developed in an open and transparent manner. As the
patent on RSA expired in 2000, its use is now also free.
DSA has been developed by the National Institute of Standards and
Technology (NIST). The creation of signatures takes roughly the same
time as with RSA, but is 10 to 40 times as slow for verification
[17].
We suggest the use of RSA/SHA-1 as the preferred algorithm for the
key. The current known attacks on RSA can be defeated by making your
key longer. As the MD5 hashing algorithm is showing cracks, we
recommend the usage of SHA-1.
At the time of publication, it is known that the SHA-1 hash has
cryptanalysis issues. There is work in progress on addressing these
issues. We recommend the use of public key algorithms based on
hashes stronger than SHA-1 (e.g., SHA-256), as soon as these
algorithms are available in protocol specifications (see [19] and
[20]) and implementations.
3.5. Key Sizes
When choosing key sizes, zone administrators will need to take into
account how long a key will be used, how much data will be signed
during the key publication period (see Section 8.10 of [17]), and,
optionally, how large the key size of the parent is. As the chain of
trust really is "a chain", there is not much sense in making one of
the keys in the chain several times larger than the others. As
EID 791 (Verified) is as follows:Section: 3.5
Original Text:
As the chain of
trust really is "a chain", there is not much sense in making one of
the keys in the chain several times larger then the others.
Corrected Text:
As the chain of
trust really is "a chain", there is not much sense in making one of
the keys in the chain several times larger than the others.
Notes:
then -> than
from pending
always, it's the weakest link that defines the strength of the entire
chain. Also see Section 3.1.1 for a discussion of how keys serving
different roles (ZSK vs. KSK) may need different key sizes.
Generating a key of the correct size is a difficult problem; RFC 3766
[13] tries to deal with that problem. The first part of the
selection procedure in Section 1 of the RFC states:
1. Determine the attack resistance necessary to satisfy the
security requirements of the application. Do this by
estimating the minimum number of computer operations that the
attacker will be forced to do in order to compromise the
security of the system and then take the logarithm base two of
that number. Call that logarithm value "n".
A 1996 report recommended 90 bits as a good all-around choice
for system security. The 90 bit number should be increased by
about 2/3 bit/year, or about 96 bits in 2005.
[13] goes on to explain how this number "n" can be used to calculate
the key sizes in public key cryptography. This culminated in the
table given below (slightly modified for our purpose):
+-------------+-----------+--------------+
| System | | |
| requirement | Symmetric | RSA or DSA |
| for attack | key size | modulus size |
| resistance | (bits) | (bits) |
| (bits) | | |
+-------------+-----------+--------------+
| 70 | 70 | 947 |
| 80 | 80 | 1228 |
| 90 | 90 | 1553 |
| 100 | 100 | 1926 |
| 150 | 150 | 4575 |
| 200 | 200 | 8719 |
| 250 | 250 | 14596 |
+-------------+-----------+--------------+
The key sizes given are rather large. This is because these keys are
resilient against a trillionaire attacker. Assuming this rich
attacker will not attack your key and that the key is rolled over
once a year, we come to the following recommendations about KSK
sizes: 1024 bits for low-value domains, 1300 bits for medium-value
domains, and 2048 bits for high-value domains.
Whether a domain is of low, medium, or high value depends solely on
the views of the zone owner. One could, for instance, view leaf
nodes in the DNS as of low value, and top-level domains (TLDs) or the
root zone of high value. The suggested key sizes should be safe for
the next 5 years.
As ZSKs can be rolled over more easily (and thus more often), the key
sizes can be made smaller. But as said in the introduction of this
paragraph, making the ZSKs' key sizes too small (in relation to the
KSKs' sizes) doesn't make much sense. Try to limit the difference in
size to about 100 bits.
Note that nobody can see into the future and that these key sizes are
only provided here as a guide. Further information can be found in
[16] and Section 7.5 of [17]. It should be noted though that [16] is
already considered overly optimistic about what key sizes are
considered safe.
One final note concerning key sizes. Larger keys will increase the
sizes of the RRSIG and DNSKEY records and will therefore increase the
chance of DNS UDP packet overflow. Also, the time it takes to
validate and create RRSIGs increases with larger keys, so don't
needlessly double your key sizes.
3.6. Private Key Storage
It is recommended that, where possible, zone private keys and the
zone file master copy that is to be signed be kept and used in off-
line, non-network-connected, physically secure machines only.
Periodically, an application can be run to add authentication to a
zone by adding RRSIG and NSEC RRs. Then the augmented file can be
transferred.
When relying on dynamic update to manage a signed zone [10], be aware
that at least one private key of the zone will have to reside on the
master server. This key is only as secure as the amount of exposure
the server receives to unknown clients and the security of the host.
Although not mandatory, one could administer the DNS in the following
way. The master that processes the dynamic updates is unavailable
from generic hosts on the Internet, it is not listed in the NS RR
set, although its name appears in the SOA RRs MNAME field. The
nameservers in the NS RRSet are able to receive zone updates through
NOTIFY, IXFR, AXFR, or an out-of-band distribution mechanism. This
approach is known as the "hidden master" setup.
The ideal situation is to have a one-way information flow to the
network to avoid the possibility of tampering from the network.
Keeping the zone master file on-line on the network and simply
cycling it through an off-line signer does not do this. The on-line
version could still be tampered with if the host it resides on is
compromised. For maximum security, the master copy of the zone file
should be off-net and should not be updated based on an unsecured
network mediated communication.
In general, keeping a zone file off-line will not be practical and
the machines on which zone files are maintained will be connected to
a network. Operators are advised to take security measures to shield
unauthorized access to the master copy.
For dynamically updated secured zones [10], both the master copy and
the private key that is used to update signatures on updated RRs will
need to be on-line.
4. Signature Generation, Key Rollover, and Related Policies
4.1. Time in DNSSEC
Without DNSSEC, all times in the DNS are relative. The SOA fields
REFRESH, RETRY, and EXPIRATION are timers used to determine the time
elapsed after a slave server synchronized with a master server. The
Time to Live (TTL) value and the SOA RR minimum TTL parameter [11]
are used to determine how long a forwarder should cache data after it
has been fetched from an authoritative server. By using a signature
validity period, 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.
4.1.1. Time Considerations
Because of the expiration of signatures, one should consider the
following:
o We suggest the Maximum Zone TTL of your zone data to 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. Section
7.1 of [4] suggests that "the resolver may use the time
remaining before expiration of the signature validity period of
a signed RRSet as an upper bound for the TTL". As a result,
query load on authoritative servers would peak at signature
expiration time, as this is also the time at which records
simultaneously expire from caches.
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 We suggest the signature publication period to end at least one
Maximum Zone TTL duration before the end of the signature validity
period.
Re-signing 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 We suggest the Minimum Zone TTL to be long enough to both fetch
and verify all the RRs in the trust chain. In workshop
environments, it has been demonstrated [18] that a low TTL (under
5 to 10 minutes) caused disruptions because of the following two
problems:
1. During validation, some data may expire before the
validation is complete. The validator should be able to
keep all data until it is completed. This applies to all
RRs needed to complete the chain of trust: DSes, DNSKEYs,
RRSIGs, and the final answers, i.e., the RRSet that is
returned for the initial query.
2. Frequent verification causes load on recursive nameservers.
Data at delegation points, DSes, DNSKEYs, and RRSIGs
benefit from caching. The TTL on those should be
relatively long.
o Slave servers will need to be able to fetch newly signed zones
well before the RRSIGs in the zone served by the slave server pass
their signature expiration time.
When a slave server is out of sync with its master and data in
a zone is signed by expired signatures, it may be better for
the slave server not to give out any answer.
Normally, a slave server that is not able to contact a master
server for an extended period will expire a zone. When that
happens, the server will respond differently to queries for
that zone. Some servers issue SERVFAIL, whereas others turn
off the 'AA' bit in the answers. The time of expiration is set
in the SOA record and is relative to the last successful
refresh between the master and the slave servers. There exists
no coupling between the signature expiration of RRSIGs in the
zone and the expire parameter in the SOA.
If the server serves a DNSSEC zone, then 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 to or shorter than the signature validity period. The
consequence of an authoritative server not being able to update
a zone, whilst that zone includes expired signatures, is that
non-secure resolvers will continue to be able to resolve data
served by the particular slave servers while security-aware
resolvers will experience problems because of answers being
marked as Bogus.
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 times out. We also suggest
that operators of nameservers that supply secondary services
develop 'watch dogs' to spot upcoming signature expirations in
zones they slave, 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 secondaries expire the zone? How quickly can I reach an
administrator of secondary servers to load a valid zone? These
questions are not DNSSEC specific but may influence the choice
of your signature validity intervals.
4.2. Key Rollovers
A DNSSEC key cannot be used forever (see Section 3.3). So key
rollovers -- or supercessions, as they are sometimes called -- are a
fact of life when using DNSSEC. Zone administrators who are in the
process of rolling their keys have to 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 occurs when zone material signed
with an old key is being validated by a resolver that 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 that is
signed with a new key against an old key that lives in a local cache,
also resulting in data being marked "Bogus".
4.2.1. 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 key sets or newly generated signatures can be verified with the
keys still in caches. One schema, described in Section 4.2.1.2, uses
double signatures; the other uses key pre-publication (Section
4.2.1.1). The pros, cons, and recommendations are described in
Section 4.2.1.3.
4.2.1.1. Pre-Publish Key Rollover
This section shows how to perform a ZSK rollover without the need to
sign all the data in a zone twice -- the "pre-publish key rollover".
This method has advantages in the case of a 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 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 "how-to" for this
kind of rollover can be found in Appendix B.
Pre-publish key rollover involves four stages as follows:
----------------------------------------------------------------
initial new DNSKEY new RRSIGs DNSKEY removal
----------------------------------------------------------------
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)
----------------------------------------------------------------
Pre-Publish Key Rollover
EID 35 (Verified) is as follows:Section: 4.2.1.1
Original Text:
Pre-publish key rollover involves four stages as follows:
----------------------------------------------------------------
initial new DNSKEY new RRSIGs DNSKEY removal
----------------------------------------------------------------
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)
----------------------------------------------------------------
Corrected Text:
Pre-publish key rollover involves four stages as follows:
----------------------------------------------------------------
initial new DNSKEY new RRSIGs DNSKEY removal
----------------------------------------------------------------
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)
----------------------------------------------------------------
Pre-Publish Key Rollover
Notes:
The mis-alignment of the indicated line breaks the intended presentation of the procedure; cf. subsequent RFC text.
from pending
Pre-Publish Key Rollover
initial: Initial version 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.
new DNSKEY: DNSKEY 11 is introduced into the key set. 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
key set.
new RRSIGs: At the "new RRSIGs" stage (SOA serial 2), 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 key set. 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 key set 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.
DNSKEY removal: DNSKEY 10 is removed from the zone. The key set, now
only containing DNSKEY 1 and DNSKEY 11, is re-signed 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 "new DNSKEY"
as DNSKEY 12 and again a newer one, numbered 13, in "new DNSKEY
(II)":
----------------------------------------------------------------
initial new RRSIGs new DNSKEY
----------------------------------------------------------------
SOA0 SOA1 SOA2
RRSIG10(SOA0) RRSIG11(SOA1) RRSIG11(SOA2)
DNSKEY1 DNSKEY1 DNSKEY1
DNSKEY10 DNSKEY10 DNSKEY11
DNSKEY11 DNSKEY11 DNSKEY12
RRSIG1(DNSKEY) RRSIG1 (DNSKEY) RRSIG1(DNSKEY)
RRSIG10(DNSKEY) RRSIG11(DNSKEY) RRSIG11(DNSKEY)
----------------------------------------------------------------
----------------------------------------------------------------
new RRSIGs (II) new DNSKEY (II)
----------------------------------------------------------------
SOA3 SOA4
RRSIG12(SOA3) RRSIG12(SOA4)
DNSKEY1 DNSKEY1
DNSKEY11 DNSKEY12
DNSKEY12 DNSKEY13
RRSIG1(DNSKEY) RRSIG1(DNSKEY)
RRSIG12(DNSKEY) RRSIG12(DNSKEY)
----------------------------------------------------------------
Pre-Publish Key Rollover, Showing Two Rollovers
Note that the key introduced in the "new DNSKEY" phase 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.
4.2.1.2. 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 signature rollover".
During the "new DNSKEY" 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, requiring at least
the Maximum Zone TTL.
Double signature ZSK rollover involves three stages as follows:
----------------------------------------------------------------
initial new DNSKEY DNSKEY removal
----------------------------------------------------------------
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)
----------------------------------------------------------------
Double Signature Zone Signing Key Rollover
EID 790 (Verified) is as follows:Section: 4.2.1.2
Original Text:
Double signature ZSK rollover involves three stages as follows:
----------------------------------------------------------------
initial new DNSKEY DNSKEY removal
----------------------------------------------------------------
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)
----------------------------------------------------------------
Double Signature Zone Signing Key Rollover
Corrected Text:
Double signature ZSK rollover involves three stages as follows:
----------------------------------------------------------------
initial new DNSKEY DNSKEY removal
----------------------------------------------------------------
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)
----------------------------------------------------------------
Double Signature Zone Signing Key Rollover
Notes:
The mis-alignment of the indicated 3 lines breaks the intended presentation of the procedure; cf. subsequent RFC text.
The initial report was corrected by Yue Luo 2007-11-16 so that "RRSIG11" in the last row is in "New DNSKEY" stage instead of "initial" stage.
initial: Initial Version 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.
new DNSKEY: At the "New DNSKEY" stage (SOA serial 1) DNSKEY 11 is
introduced into the key set and all the data in the zone is signed
with DNSKEY 10 and DNSKEY 11. The rollover period will need to
continue 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.
DNSKEY removal: DNSKEY 10 is removed from the zone. All the
signatures from DNSKEY 10 are removed from the zone. The key set,
now only containing DNSKEY 11, is re-signed with DNSKEY 1.
At every instance, RRSIGs from the previous version of the zone can
be verified with the DNSKEY RRSet from the current version and the
other way around. The data from the current version can be verified
with the data from the previous version of the zone. The duration of
the "new DNSKEY" phase and the period between rollovers should be at
least the Maximum Zone TTL.
Making sure that the "new DNSKEY" phase lasts until the signature
| expiration time of the data in the initial version of the zone is
recommended. This way all caches are cleared of the old signatures.
EID 792 (Verified) is as follows:Section: 4.2.1.2
Original Text:
Making sure that the "new DNSKEY" phase lasts until the signature
expiration time of the data in initial version of the zone is
recommended.
Corrected Text:
Making sure that the "new DNSKEY" phase lasts until the signature
| expiration time of the data in the initial version of the zone is
recommended.
Notes:
missing article
from pending
However, this duration 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.
4.2.1.3. Pros and Cons of the Schemes
Pre-publish key rollover: This rollover does not involve signing the
zone data twice. Instead, before the actual rollover, the new key
is published in the key set and thus is available for
cryptanalysis attacks. A small disadvantage is that this process
requires four steps. Also the pre-publish scheme involves more
parental work when used for KSK rollovers as explained in Section
4.2.3.
Double signature ZSK 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.
4.2.2. 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 key
set) in caches can be verified with a new key set and vice versa.
Since only the key set is signed with a KSK, zone size considerations
do not apply.
--------------------------------------------------------------------
initial new DNSKEY DS change DNSKEY removal
--------------------------------------------------------------------
Parent:
SOA0 --------> SOA1 -------->
RRSIGpar(SOA0) --------> RRSIGpar(SOA1) -------->
DS1 --------> DS2 -------->
RRSIGpar(DS) --------> RRSIGpar(DS) -------->
Child:
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)
--------------------------------------------------------------------
Stages of Deployment for a Double Signature Key Signing Key Rollover
initial: Initial version 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.
new DNSKEY: During the "new DNSKEY" 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 parent's zone, the
zone administrator has to wait at least TTL_DS to make sure that
the old DS RR has expired from caches.
DS change: The parent replaces DS1 with DS2.
DNSKEY removal: 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 premise that
the parent only has one DS RR (per algorithm) per zone. An
alternative mechanism has been considered. Using an established
trust relation, the interaction can be performed in-band, and the
removal of the keys by the child can possibly be signaled by the
parent. In this mechanism, there are periods where there are two DS
RRs at the parent. Since at the moment of writing the protocol for
this interaction has not been developed, further discussion is out of
scope for this document.
4.2.3. Difference Between ZSK and KSK Rollovers
Note that KSK rollovers and ZSK rollovers are different in the sense
that a KSK rollover requires interaction with the parent (and
possibly replacing of trust anchors) and the ensuing delay while
waiting for it.
A zone key rollover can be handled in two different ways: pre-publish
(Section 4.2.1.1) and double signature (Section 4.2.1.2).
As the KSK is used to validate the key set and because the KSK is not
changed during a ZSK rollover, a cache is able to validate the new
key set of the zone. The pre-publish method would also work for a
KSK rollover. The records that are to be pre-published are the
parental DS RRs. The pre-publish method has some drawbacks for KSKs.
We first describe the rollover scheme and then indicate these
drawbacks.
--------------------------------------------------------------------
initial new DS new DNSKEY DS/DNSKEY removal
--------------------------------------------------------------------
Parent:
SOA0 SOA1 --------> SOA2
RRSIGpar(SOA0) RRSIGpar(SOA1) --------> RRSIGpar(SOA2)
DS1 DS1 --------> DS2
DS2 -------->
RRSIGpar(DS) RRSIGpar(DS) --------> RRSIGpar(DS)
Child:
SOA0 --------> SOA1 SOA1
RRSIG10(SOA0) --------> RRSIG10(SOA1) RRSIG10(SOA1)
-------->
DNSKEY1 --------> DNSKEY2 DNSKEY2
-------->
DNSKEY10 --------> DNSKEY10 DNSKEY10
RRSIG1 (DNSKEY) --------> RRSIG2(DNSKEY) RRSIG2 (DNSKEY)
RRSIG10(DNSKEY) --------> RRSIG10(DNSKEY) RRSIG10(DNSKEY)
--------------------------------------------------------------------
Stages of Deployment for a Pre-Publish Key Signing Key Rollover
When the child zone wants to roll, it notifies the parent during the
"new DS" phase and submits the new key (or the corresponding DS) to
the parent. The parent publishes DS1 and DS2, pointing to DNSKEY1
and DNSKEY2, respectively. During the rollover ("new DNSKEY" phase),
which can take place as soon as the new DS set propagated through the
DNS, the child replaces DNSKEY1 with DNSKEY2. Immediately after that
("DS/DNSKEY removal" phase), it can notify the parent that the old DS
record can be deleted.
The drawbacks of this scheme are that during the "new DS" phase the
parent cannot verify the match between the DS2 RR and DNSKEY2 using
the DNS -- as DNSKEY2 is not yet published. Besides, we introduce a
"security lame" key (see Section 4.4.3). Finally, the child-parent
interaction consists of two steps. The "double signature" method
only needs one interaction.
4.2.4. Automated Key Rollovers
As keys must be renewed periodically, there is some motivation to
automate the rollover process. Consider the following:
o ZSK rollovers are easy to automate as only the child zone is
involved.
o A KSK rollover needs interaction between parent and child. Data
exchange is needed to provide the new keys to the parent;
consequently, this data must be authenticated and integrity must
be guaranteed in order to avoid attacks on the rollover.
4.3. 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.
When the private material of one of your keys is compromised it can
be used for as long as a valid trust chain exists. A trust chain
remains intact for
o as long as a signature over the compromised key in the trust 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 generally the hardest to
update).
While a trust chain to your compromised key exists, your namespace is
vulnerable to abuse by anyone who has obtained illegitimate
possession of the 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
trust 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 rollover, the malicious key holder can
spoof data so that it appears to be valid.
4.3.1. KSK Compromise
A zone containing a DNSKEY RRSet with a compromised KSK is vulnerable
as long as the compromised KSK is configured as trust anchor or a
parental DS points to it.
A compromised KSK can be used to sign the key set of an attacker's
zone. That zone could be used to poison the DNS.
Therefore, when the KSK has been compromised, the trust anchor or the
parental DS should be replaced as soon as possible. It is local
policy whether to break the trust chain during the emergency
rollover. The trust chain would be broken when the compromised KSK
is removed from the child's zone while the parent still has a DS
pointing to the compromised KSK (the assumption is that there is only
one DS at the parent. If there are multiple DSes this does not apply
-- however the chain of trust of this particular key is broken).
Note that an attacker's zone still uses the compromised KSK and the
presence of a parental DS would cause the data in this zone to appear
as valid. Removing the compromised key would cause the attacker's
zone to appear as valid and the child's zone as Bogus. Therefore, we
advise not to remove the KSK before the parent has a DS to a new KSK
in place.
4.3.1.1. Keeping the Chain of Trust Intact
If we follow this advice, the timing of the replacement of the KSK is
somewhat critical. The goal is to remove the compromised KSK as soon
as the new DS RR is available at the parent. And also make sure that
the signature made with a new KSK over the key set with the
compromised KSK in it expires just after the new DS appears at the
parent, thus removing the old cruft in one swoop.
The procedure is as follows:
1. Introduce a new KSK into the key set, keep the compromised KSK in
the key set.
2. Sign the key set, with a short validity period. The validity
period should expire shortly after the DS is expected to appear
in the parent and the old DSes have expired from caches.
3. Upload the DS for this new key to the parent.
4. Follow the procedure of the regular KSK rollover: Wait for the DS
to appear in the authoritative servers and then wait as long as
the TTL of the old DS RRs. If necessary re-sign the DNSKEY RRSet
and modify/extend the expiration time.
5. Remove the compromised DNSKEY RR from the zone and re-sign the
key set using your "normal" validity interval.
An additional danger of a key compromise is that the compromised key
could be used to facilitate a legitimate DNSKEY/DS rollover and/or
nameserver changes at the parent. When that happens, the domain may
be in dispute. An authenticated out-of-band and secure notify
mechanism to contact a parent is needed in this case.
Note that this is only a problem when the DNSKEY and or DS records
are used for authentication at the parent.
4.3.1.2. Breaking the Chain of Trust
There are two methods to break the chain of trust. The first method
causes the child zone to appear 'Bogus' to validating resolvers. The
other causes the child zone to appear 'insecure'. These are
described below.
In the method that causes the child zone to appear 'Bogus' to
validating resolvers, the child zone replaces the current KSK with a
new one and re-signs the key set. Next it sends the DS of the new
key to the parent. Only after the parent has placed the new DS in
the zone is the child's chain of trust repaired.
An alternative method of breaking the chain of trust is by removing
the DS RRs from the parent zone altogether. As a result, the child
zone would become insecure.
4.3.2. ZSK Compromise
Primarily because there is no parental interaction required when a
ZSK is compromised, the situation is less severe than with a KSK
compromise. The zone must still be re-signed 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 of the old
compromised key may lead to verification problems. Also note that as
long as the RRSIG over the compromised ZSK is not expired the zone
may be still at risk.
4.3.3. Compromises of Keys Anchored in Resolvers
A key can also be pre-configured in resolvers. For instance, if
DNSSEC is successfully deployed the root key may be pre-configured in
most security aware resolvers.
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.
End-users faced with the task of updating an anchored key should
always validate the new key. New keys should be authenticated out-
of-band, for example, through the use of an announcement website that
is secured using secure sockets (TLS) [21].
4.4. Parental Policies
4.4.1. Initial Key Exchanges and Parental Policies Considerations
The initial key exchange is always subject to the policies set by the
parent. 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. That is, there is no implicit
need in DNSSEC to make the authentication process stronger than it
was in DNS.
Using the DNS itself as the source for the actual DNSKEY material,
with an out-of-band check on the validity of the DNSKEY, has the
benefit that it reduces the chances of user error. A DNSKEY query
tool can make use of the SEP bit [3] to select the proper key from a
DNSSEC key set, 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 chain of trust remains intact
once the parent publishes the DS RR indicating the child is secure.
Note: the out-of-band verification is still needed when the key
material is fetched via the DNS. The parent can never be sure
whether or not the DNSKEY RRs have been spoofed.
4.4.2. Storing Keys or Hashes?
When designing a registry system one should consider which of the
DNSKEYs and/or the corresponding DSes to store. Since a child zone
might wish to have a DS published using a message digest algorithm
not yet understood by the registry, the registry can't count on being
able to generate the DS record from a raw DNSKEY. Thus, we recommend
that registry systems at least support storing DS records.
It may also be useful to store DNSKEYs, since having them may help
during troubleshooting and, as long as the child's chosen message
digest is supported, the overhead of generating DS records from them
is minimal. Having an out-of-band mechanism, such as a registry
directory (e.g., Whois), to find out which keys are used to generate
DS Resource Records for specific owners and/or zones may also help
with troubleshooting.
The storage considerations also relate to the design of the customer
interface and the method by which data is transferred between
registrant and registry; Will the child zone administrator be able to
upload DS RRs with unknown hash algorithms or does the interface only
allow DNSKEYs? In the registry-registrar model, one can use the
DNSSEC extensions to the Extensible Provisioning Protocol (EPP) [15],
which allows transfer of DS RRs and optionally DNSKEY RRs.
4.4.3. Security Lameness
Security lameness is defined as what happens when a parent has a DS
RR pointing to a non-existing DNSKEY RR. When this happens, the
child's zone may be marked "Bogus" by verifying DNS clients.
As part of a comprehensive delegation check, the parent could, at key
exchange time, verify that the child's key is actually configured in
the DNS. However, if a parent does not understand the hashing
algorithm used by child, the parental checks are limited to only
comparing the key id.
Child zones should be very careful in removing DNSKEY material,
specifically SEP keys, for which a DS RR exists.
Once a zone is "security lame", a fix (e.g., removing a DS RR) will
take time to propagate through the DNS.
4.4.4. 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 minimizes 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
validity periods longer than 2 days. We recommend an absolute
minimum for a DS signature validity period of a few days.
The maximum signature validity period of the DS record depends on how
long child zones are willing to be vulnerable after a key compromise.
On the other hand, shortening the DS signature validity interval
increases the operational risk for the parent. Therefore, the parent
may have policy to use a signature validity interval that is
considerably longer than the child would hope for.
A compromise between the operational constraints of the parent and
minimizing damage for the child may result in a DS signature validity
period somewhere between a week and months.
In addition to the signature validity period, which sets a lower
bound on the number of times the zone owner will need to sign the
zone data and which sets an upper bound to the time a child is
vulnerable after key compromise, there is the TTL value on the DS
RRs. Shortening the TTL means that the authoritative servers will
see more queries. But on the other hand, a short TTL lowers the
persistence of DS RRSets in caches thereby increasing the speed with
which updated DS RRSets propagate through the DNS.
5. Security Considerations
DNSSEC adds data integrity to the DNS. This document tries to assess
the operational considerations to maintain 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.
6. Acknowledgments
Most of the ideas in this document were the result of collective
efforts during workshops, discussions, and tryouts.
At the risk of forgetting individuals who were the original
contributors of the ideas, we would like to acknowledge people who
were actively involved in the compilation of this document. In
random order: Rip Loomis, Olafur Gudmundsson, Wesley Griffin, Michael
Richardson, Scott Rose, Rick van Rein, Tim McGinnis, Gilles Guette
Olivier Courtay, Sam Weiler, Jelte Jansen, Niall O'Reilly, Holger
Zuleger, Ed Lewis, Hilarie Orman, Marcos Sanz, and Peter Koch.
Some material in this document has been copied from RFC 2541 [12].
Mike StJohns designed the key exchange between parent and child
mentioned in the last paragraph of Section 4.2.2
Section 4.2.4 was supplied by G. Guette and O. Courtay.
Emma Bretherick, Adrian Bedford, and Lindy Foster corrected many of
the spelling and style issues.
Kolkman and Gieben take the blame for introducing all miscakes (sic).
While working on this document, Kolkman was employed by the RIPE NCC
and Gieben was employed by NLnet Labs.
7. References
7.1. Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name System
KEY (DNSKEY) Resource Record (RR) Secure Entry Point (SEP)
Flag", RFC 3757, May 2004.
[4] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033, March
2005.
[5] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
[6] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions", RFC
4035, March 2005.
7.2. Informative References
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[8] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, August
1996.
[9] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
(DNS NOTIFY)", RFC 1996, August 1996.
[10] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[11] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)",
RFC 2308, March 1998.
[12] Eastlake, D., "DNS Security Operational Considerations", RFC
2541, March 1999.
[13] Orman, H. and P. Hoffman, "Determining Strengths For Public
Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
April 2004.
[14] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[15] Hollenbeck, S., "Domain Name System (DNS) Security Extensions
Mapping for the Extensible Provisioning Protocol (EPP)", RFC
4310, December 2005.
[16] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key
Sizes", The Journal of Cryptology 14 (255-293), 2001.
[17] Schneier, B., "Applied Cryptography: Protocols, Algorithms, and
Source Code in C", ISBN (hardcover) 0-471-12845-7, ISBN
(paperback) 0-471-59756-2, Published by John Wiley & Sons Inc.,
1996.
[18] Rose, S., "NIST DNSSEC workshop notes", June 2001.
[19] Jansen, J., "Use of RSA/SHA-256 DNSKEY and RRSIG Resource
Records in DNSSEC", Work in Progress, January 2006.
[20] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS)
Resource Records (RRs)", RFC 4509, May 2006.
[21] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
T. Wright, "Transport Layer Security (TLS) Extensions", RFC
4366, April 2006.
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 have given a more elaborate
explanation of the meaning. Note that these explanations should not
be seen as authoritative.
Anchored key: A DNSKEY configured in resolvers around the globe.
This key is hard to update, hence the term anchored.
Bogus: Also see Section 5 of [4]. An RRSet in DNSSEC is marked
"Bogus" when a signature of an RRSet does not validate against a
DNSKEY.
Key Signing Key or KSK: A Key Signing Key (KSK) is a key that is used
exclusively for signing the apex key set. The fact that a key is
a KSK is only relevant to the signing tool.
Key size: The term 'key size' can be substituted by 'modulus size'
throughout the document. It is mathematically more correct to use
modulus size, but as this is a document directed at operators we
feel more at ease with the term key size.
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 (mathematically related) 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.
Key rollover: A key rollover (also called key supercession in some
environments) is the act of replacing one key pair with another at
the end of a key effectivity period.
Secure Entry Point (SEP) key: A KSK that has a parental DS record
pointing to it or is configured as a trust anchor. Although not
required by the protocol, we recommend that the SEP flag [3] is
set on these keys.
Self-signature: This only applies to signatures over DNSKEYs; a
signature made with DNSKEY x, over DNSKEY x is called a self-
signature. Note: without further information, self-signatures
convey no trust. They are useful to check the authenticity of the
DNSKEY, i.e., they can be used as a hash.
Singing the zone file: The term used for the event where an
administrator joyfully signs its zone file while producing melodic
sound patterns.
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.
Zone Signing Key (ZSK): 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.
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 How-To
Using the pre-published signature scheme and the most conservative
method to assure oneself that data does not live in caches, here
follows the "how-to".
Step 0: The preparation: Create two keys and publish both in your key
set. Mark one of the keys "active" and the other "published".
Use the "active" key for signing your zone data. Store the
private part of the "published" key, preferably off-line. The
protocol does not provide for attributes to mark a key as active
or published. This is something you have to do on your own,
through the use of a notebook or key management tool.
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 "published" to sign
your data (i.e., mark it "active"). Stop using the key that was
marked "active"; mark it "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 DNSKEYx, where x is a number or an
identifier, 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 192.0.2.1" 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".
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: SOAs are represented as SOAx, where x is the
serial number.
Using this notation the following signed zone:
example.net. 86400 IN SOA ns.example.net. bert.example.net. (
2006022100 ; serial
86400 ; refresh ( 24 hours)
7200 ; retry ( 2 hours)
3600000 ; expire (1000 hours)
28800 ) ; minimum ( 8 hours)
86400 RRSIG SOA 5 2 86400 20130522213204 (
20130422213204 14 example.net.
cmL62SI6iAX46xGNQAdQ... )
86400 NS a.iana-servers.net.
86400 NS b.iana-servers.net.
86400 RRSIG NS 5 2 86400 20130507213204 (
20130407213204 14 example.net.
SO5epiJei19AjXoUpFnQ ... )
86400 DNSKEY 256 3 5 (
EtRB9MP5/AvOuVO0I8XDxy0... ) ; id = 14
86400 DNSKEY 257 3 5 (
gsPW/Yy19GzYIY+Gnr8HABU... ) ; id = 15
86400 RRSIG DNSKEY 5 2 86400 20130522213204 (
20130422213204 14 example.net.
J4zCe8QX4tXVGjV4e1r9... )
86400 RRSIG DNSKEY 5 2 86400 20130522213204 (
20130422213204 15 example.net.
keVDCOpsSeDReyV6O... )
86400 RRSIG NSEC 5 2 86400 20130507213204 (
20130407213204 14 example.net.
obj3HEp1GjnmhRjX... )
a.example.net. 86400 IN TXT "A label"
86400 RRSIG TXT 5 3 86400 20130507213204 (
20130407213204 14 example.net.
IkDMlRdYLmXH7QJnuF3v... )
86400 NSEC b.example.com. TXT RRSIG NSEC
86400 RRSIG NSEC 5 3 86400 20130507213204 (
20130407213204 14 example.net.
bZMjoZ3bHjnEz0nIsPMM... )
...
is reduced to the following representation:
SOA2006022100
RRSIG14(SOA2006022100)
DNSKEY14
DNSKEY15
RRSIG14(KEY)
RRSIG15(KEY)
The rest of the zone data has the same signature as the SOA record,
i.e., an RRSIG created with DNSKEY 14.
Authors' Addresses
Olaf M. Kolkman
NLnet Labs
Kruislaan 419
Amsterdam 1098 VA
The Netherlands
EMail: olaf@nlnetlabs.nl
URI: http://www.nlnetlabs.nl
R. (Miek) Gieben
EMail: miek@miek.nl
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