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Versions: (RFC 4641) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 6781

DNSOP                                                         O. Kolkman
Internet-Draft                                                NLnet Labs
Obsoletes: 2541 (if approved)                                  R. Gieben
Intended status: BCP
Expires: September 3, 2009                                 March 2, 2009


                      DNSSEC Operational Practices
                     draft-ietf-dnsop-rfc4641bis-00

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
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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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  The Use of the Term 'key'  . . . . . . . . . . . . . . . .  5
     1.2.  Time Definitions . . . . . . . . . . . . . . . . . . . . .  5
   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 . . . . . .  7
       3.1.2.  KSKs for High-Level Zones  . . . . . . . . . . . . . .  8
     3.2.  Key Generation . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Key Effectivity Period . . . . . . . . . . . . . . . . . .  9
     3.4.  Key Algorithm  . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Key Sizes  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.6.  Private Key Storage  . . . . . . . . . . . . . . . . . . . 12
   4.  Signature Generation, Key Rollover, and Related Policies . . . 12
     4.1.  Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . 12
       4.1.1.  Time Considerations  . . . . . . . . . . . . . . . . . 13
     4.2.  Key Rollovers  . . . . . . . . . . . . . . . . . . . . . . 15
       4.2.1.  Zone Signing Key Rollovers . . . . . . . . . . . . . . 15
         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 . . . . . . . . . . . 19
       4.2.2.  Key Signing Key Rollovers  . . . . . . . . . . . . . . 19
       4.2.3.  Difference Between ZSK and KSK Rollovers . . . . . . . 20
       4.2.4.  Automated Key Rollovers  . . . . . . . . . . . . . . . 21
     4.3.  Planning for Emergency Key Rollover  . . . . . . . . . . . 22
       4.3.1.  KSK Compromise . . . . . . . . . . . . . . . . . . . . 22



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         4.3.1.1.  Keeping the Chain of Trust Intact  . . . . . . . . 23
         4.3.1.2.  Breaking the Chain of Trust  . . . . . . . . . . . 24
       4.3.2.  ZSK Compromise . . . . . . . . . . . . . . . . . . . . 24
       4.3.3.  Compromises of Keys Anchored in Resolvers  . . . . . . 24
     4.4.  Parental Policies  . . . . . . . . . . . . . . . . . . . . 25
       4.4.1.  Initial Key Exchanges and Parental Policies
               Considerations . . . . . . . . . . . . . . . . . . . . 25
       4.4.2.  Storing Keys or Hashes?  . . . . . . . . . . . . . . . 25
       4.4.3.  Security Lameness  . . . . . . . . . . . . . . . . . . 26
       4.4.4.  DS Signature Validity Period . . . . . . . . . . . . . 26
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   6.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 27
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 27
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 28
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Terminology . . . . . . . . . . . . . . . . . . . . . 29
   Appendix B.  Zone Signing Key Rollover How-To  . . . . . . . . . . 30
   Appendix C.  Typographic Conventions . . . . . . . . . . . . . . . 31
   Appendix D.  Document Editing History  . . . . . . . . . . . . . . 33
     D.1.  draft-ietf-dnsop-rfc4641-00  . . . . . . . . . . . . . . . 33






























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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 [3] for an introduction to DNSSEC, RFC 4034 [4] for the
   newly introduced Resource Records (RRs), and RFC 4035 [5] 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 [6] language does not apply.

   This document obsoletes RFC 2541 [11] 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
   [16]).  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 [10] 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 [3]
   Section 5), which may cause entire (sub)domains to become invisible
   to verifying clients.  The administrators of secured zones have to



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   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 [8] and incremental transfer (IXFR) [7].
   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



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   so-called Secure Entry Point (SEP) [5] 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



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   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 [13].  One should carefully assess if the random number
   generator used during key generation adheres to these suggestions.




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   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



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   Technology (NIST).  The creation of signatures takes roughly the same
   time as with RSA, but is 10 to 40 times as slow for verification
   [16].

   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 [18] and
   [19]) 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 [16]), 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
   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
   [12] 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.

   [12] 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):





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      +-------------+-----------+--------------+
      | 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
   [15] and Section 7.5 of [16].  It should be noted though that [15] 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.





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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 [9], 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 RRSet,
   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 [9], 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 [10]
   are used to determine how long a forwarder should cache data after it



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   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 [3] 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 [17] 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



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         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.



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         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.




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   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

   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"



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   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.






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   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

   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.
   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



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   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)
   --------------------------------------------------------------------



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   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.




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   --------------------------------------------------------------------
     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.





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   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



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   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.



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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) [20].





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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 [5] 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



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   allow DNSKEYs?  In the registry-registrar model, one can use the
   DNSSEC extensions to the Extensible Provisioning Protocol (EPP) [14],
   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



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   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.  IANA considerations

   There are no IANA considerations with respect to this document

7.  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 [11].

   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.

8.  References



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8.1.  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]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "DNS Security Introduction and Requirements", RFC 4033,
         March 2005.

   [4]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Resource Records for the DNS Security Extensions", RFC 4034,
         March 2005.

   [5]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Protocol Modifications for the DNS Security Extensions",
         RFC 4035, March 2005.

8.2.  Informative References

   [6]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [7]   Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
         August 1996.

   [8]   Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
         (DNS NOTIFY)", RFC 1996, August 1996.

   [9]   Wellington, B., "Secure Domain Name System (DNS) Dynamic
         Update", RFC 3007, November 2000.

   [10]  Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)",
         RFC 2308, March 1998.

   [11]  Eastlake, D., "DNS Security Operational Considerations",
         RFC 2541, March 1999.

   [12]  Orman, H. and P. Hoffman, "Determining Strengths For Public
         Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
         April 2004.

   [13]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
         Requirements for Security", BCP 106, RFC 4086, June 2005.

   [14]  Hollenbeck, S., "Domain Name System (DNS) Security Extensions



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         Mapping for the Extensible Provisioning Protocol (EPP)",
         RFC 4310, December 2005.

   [15]  Lenstra, A. and E. Verheul, "Selecting Cryptographic Key
         Sizes", The Journal of Cryptology 14 (255-293), 2001.

   [16]  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.

   [17]  Rose, S., "NIST DNSSEC workshop notes",  , June 2001.

   [18]  Jansen, J., "Use of SHA-2 algorithms with RSA in DNSKEY and
         RRSIG Resource Records for  DNSSEC",
         draft-ietf-dnsext-dnssec-rsasha256-05 (work in progress),
         July 2008.

   [19]  Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS)
         Resource Records (RRs)", RFC 4509, May 2006.

   [20]  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 [3].  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.



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   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 [5] 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



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      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:










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   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,



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   i.e., an RRSIG created with DNSKEY 14.

Appendix D.  Document Editing History

   [To be removed prior to publication as an RFC]

D.1.  draft-ietf-dnsop-rfc4641-00

   Version 0 was differs from RFC4641 in the following ways.

   o  Status of this memo appropriate for I-D

   o  TOC formatting differs.

   o  Whitespaces, linebreaks, and pagebreaks may be slightly different
      because of xml2rfc generation.

   o  References slightly reordered.

   o  Applied the errata from
      http://www.rfc-editor.org/errata_search.php?rfc=4641

   o  Inserted trivial "IANA considertations" section.

   In other words it should not contain substantive changes in content
   as intended by the workinggroup for the original RFC4641.

   $Id: draft-ietf-dnsop-rfc4641bis.xml 23 2009-03-02 11:02:11Z olaf $

Authors' Addresses

   Olaf M. Kolkman
   NLnet Labs
   Kruislaan 419
   Amsterdam  1098 VA
   The Netherlands

   EMail: olaf@nlnetlabs.nl
   URI:   http://www.nlnetlabs.nl


   Miek Gieben


   EMail: miek@miek.nl






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