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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).


 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.

Kolkman & Gieben Informational [Page 1] RFC 4641 DNSSEC Operational Practices September 2006

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
       Pre-Publish Key Rollover ..................15
       Double Signature Zone Signing Key
                         Rollover ..................................17
       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
       Keeping the Chain of Trust Intact .........22
       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

Kolkman & Gieben Informational [Page 2] RFC 4641 DNSSEC Operational Practices September 2006

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.

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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.

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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
 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
 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

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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

3.1.1. Motivations for the KSK and ZSK Separation

 Differentiating between the KSK and ZSK functions has several
 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

Kolkman & Gieben Informational [Page 6] RFC 4641 DNSSEC Operational Practices September 2006

 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
 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.

Kolkman & Gieben Informational [Page 7] RFC 4641 DNSSEC Operational Practices September 2006

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

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.

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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
 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
 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 then the others.  As
 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

Kolkman & Gieben Informational [Page 9] RFC 4641 DNSSEC Operational Practices September 2006

       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.

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 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
 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.

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 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
 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

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       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
       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

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       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
 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, uses

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 double signatures; the other uses key pre-publication (Section  The pros, cons, and recommendations are described in
 Section 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:
  1. —————————————————————

initial new DNSKEY new RRSIGs DNSKEY removal

  1. —————————————————————


    RRSIG10(SOA0)   RRSIG10(SOA1)    RRSIG11(SOA2)   RRSIG11(SOA3)
    DNSKEY1         DNSKEY1          DNSKEY1         DNSKEY1
    DNSKEY10        DNSKEY10         DNSKEY10        DNSKEY11
    DNSKEY11         DNSKEY11
                       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

Kolkman & Gieben Informational [Page 15] RFC 4641 DNSSEC Operational Practices September 2006

    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
 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
  1. —————————————————————

initial new RRSIGs new DNSKEY

  1. —————————————————————


    RRSIG10(SOA0)       RRSIG11(SOA1)       RRSIG11(SOA2)
    DNSKEY1             DNSKEY1             DNSKEY1
    DNSKEY10            DNSKEY10            DNSKEY11
    DNSKEY11            DNSKEY11            DNSKEY12
  1. —————————————————————

new RRSIGs (II) new DNSKEY (II)

  1. —————————————————————


    RRSIG12(SOA3)       RRSIG12(SOA4)
    DNSKEY1             DNSKEY1
    DNSKEY11            DNSKEY12
    DNSKEY12            DNSKEY13
            Pre-Publish Key Rollover, Showing Two Rollovers

Kolkman & Gieben Informational [Page 16] RFC 4641 DNSSEC Operational Practices September 2006

 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. 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:
  1. —————————————————————

initial new DNSKEY DNSKEY removal

  1. —————————————————————


    RRSIG10(SOA0)       RRSIG10(SOA1)      RRSIG11(SOA2)
    DNSKEY1             DNSKEY1            DNSKEY1
    DNSKEY10            DNSKEY10           DNSKEY11
              Double Signature Zone Signing 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: 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.

Kolkman & Gieben Informational [Page 17] RFC 4641 DNSSEC Operational Practices September 2006

 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 initial version of the zone is
 recommended.  This way all caches are cleared of the old signatures.
 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. 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
 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.

Kolkman & Gieben Informational [Page 18] RFC 4641 DNSSEC Operational Practices September 2006

  1. ——————————————————————-

initial new DNSKEY DS change DNSKEY removal

  1. ——————————————————————-


     SOA0           -------->         SOA1            -------->
     RRSIGpar(SOA0) -------->         RRSIGpar(SOA1)  -------->
     DS1            -------->         DS2             -------->
     RRSIGpar(DS)   -------->         RRSIGpar(DS)    -------->
     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

Kolkman & Gieben Informational [Page 19] RFC 4641 DNSSEC Operational Practices September 2006

 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 and double signature (Section
 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
  1. ——————————————————————-

initial new DS new DNSKEY DS/DNSKEY removal

  1. ——————————————————————-


   SOA0            SOA1             -------->       SOA2
   RRSIGpar(SOA0)  RRSIGpar(SOA1)   -------->       RRSIGpar(SOA2)
   DS1             DS1              -------->       DS2
                   DS2              -------->
   RRSIGpar(DS)    RRSIGpar(DS)     -------->       RRSIGpar(DS)
   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

Kolkman & Gieben Informational [Page 20] RFC 4641 DNSSEC Operational Practices September 2006

 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
 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

Kolkman & Gieben Informational [Page 21] RFC 4641 DNSSEC Operational Practices September 2006

 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. 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.

Kolkman & Gieben Informational [Page 22] RFC 4641 DNSSEC Operational Practices September 2006

 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. 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

Kolkman & Gieben Informational [Page 23] RFC 4641 DNSSEC Operational Practices September 2006

 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.

Kolkman & Gieben Informational [Page 24] RFC 4641 DNSSEC Operational Practices September 2006

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.

Kolkman & Gieben Informational [Page 25] RFC 4641 DNSSEC Operational Practices September 2006

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

Kolkman & Gieben Informational [Page 26] RFC 4641 DNSSEC Operational Practices September 2006

 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
 [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.

Kolkman & Gieben Informational [Page 27] RFC 4641 DNSSEC Operational Practices September 2006

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
 [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.,
 [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.

Kolkman & Gieben Informational [Page 28] RFC 4641 DNSSEC Operational Practices September 2006

 [21]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
       T. Wright, "Transport Layer Security (TLS) Extensions", RFC
       4366, April 2006.

Kolkman & Gieben Informational [Page 29] RFC 4641 DNSSEC Operational Practices September 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
 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.

Kolkman & Gieben Informational [Page 30] RFC 4641 DNSSEC Operational Practices September 2006

 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.

Kolkman & Gieben Informational [Page 31] RFC 4641 DNSSEC Operational Practices September 2006

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:
 " 3600 IN A" 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:      86400  IN SOA (
                          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
                                cmL62SI6iAX46xGNQAdQ... )
                   86400  NS
                   86400  NS
                   86400  RRSIG   NS 5 2 86400 20130507213204 (
                                20130407213204 14
                                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
                                J4zCe8QX4tXVGjV4e1r9... )

Kolkman & Gieben Informational [Page 32] RFC 4641 DNSSEC Operational Practices September 2006

                   86400  RRSIG   DNSKEY 5 2 86400 20130522213204 (
                                20130422213204 15
                                keVDCOpsSeDReyV6O... )
                   86400  RRSIG   NSEC 5 2 86400 20130507213204 (
                                20130407213204 14
                                obj3HEp1GjnmhRjX... )    86400  IN TXT  "A label"
                   86400  RRSIG   TXT 5 3 86400 20130507213204 (
                                20130407213204 14
                                IkDMlRdYLmXH7QJnuF3v... )
                   86400  NSEC TXT RRSIG NSEC
                   86400  RRSIG   NSEC 5 3 86400 20130507213204 (
                                20130407213204 14
                                bZMjoZ3bHjnEz0nIsPMM... )
 is reduced to the following representation:
 The rest of the zone data has the same signature as the SOA record,
 i.e., an RRSIG created with DNSKEY 14.

Kolkman & Gieben Informational [Page 33] RFC 4641 DNSSEC Operational Practices September 2006

Authors' Addresses

 Olaf M. Kolkman
 NLnet Labs
 Kruislaan 419
 Amsterdam  1098 VA
 The Netherlands
 R. (Miek) Gieben

Kolkman & Gieben Informational [Page 34] RFC 4641 DNSSEC Operational Practices September 2006

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Kolkman & Gieben Informational [Page 35]

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