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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).
+
+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.
+
+
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+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
+ 4.2.1.1. Pre-Publish Key Rollover ..................15
+ 4.2.1.2. Double Signature Zone Signing Key
+ Rollover ..................................17
+ 4.2.1.3. Pros and Cons of the Schemes ..............18
+ 4.2.2. Key Signing Key Rollovers ..........................18
+ 4.2.3. Difference Between ZSK and KSK Rollovers ...........20
+ 4.2.4. Automated Key Rollovers ............................21
+ 4.3. Planning for Emergency Key Rollover .......................21
+ 4.3.1. KSK Compromise .....................................22
+ 4.3.1.1. Keeping the Chain of Trust Intact .........22
+ 4.3.1.2. Breaking the Chain of Trust ...............23
+ 4.3.2. ZSK Compromise .....................................23
+ 4.3.3. Compromises of Keys Anchored in Resolvers ..........24
+ 4.4. Parental Policies .........................................24
+ 4.4.1. Initial Key Exchanges and Parental Policies
+ Considerations .....................................24
+ 4.4.2. Storing Keys or Hashes? ............................25
+ 4.4.3. Security Lameness ..................................25
+ 4.4.4. DS Signature Validity Period .......................26
+ 5. Security Considerations ........................................26
+ 6. Acknowledgments ................................................26
+ 7. References .....................................................27
+ 7.1. Normative References ......................................27
+ 7.2. Informative References ....................................28
+ Appendix A. Terminology ...........................................30
+ Appendix B. Zone Signing Key Rollover How-To ......................31
+ Appendix C. Typographic Conventions ...............................32
+
+
+
+
+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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+RFC 4641 DNSSEC Operational Practices September 2006
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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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+RFC 4641 DNSSEC Operational Practices September 2006
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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
+ trust.
+
+ As mentioned in the introduction, the procedures herein are intended
+ to ensure that maintenance of zones, such as re-signing or key
+ rollovers, will be transparent to the verifying clients on the
+ Internet.
+
+ Administrators of secured zones will have to keep in mind that data
+ published on an authoritative primary server will not be immediately
+ seen by verifying clients; it may take some time for the data to be
+ transferred to other secondary authoritative nameservers and clients
+ may be fetching data from caching non-authoritative servers. In this
+ light, note that the time for a zone transfer from master to slave is
+ negligible when using NOTIFY [9] and incremental transfer (IXFR) [8].
+ It increases when full zone transfers (AXFR) are used in combination
+ with NOTIFY. It increases even more if you rely on full zone
+ transfers based on only the SOA timing parameters for refresh.
+
+ For the verifying clients, it is important that data from secured
+ zones can be used to build chains of trust regardless of whether the
+ data came directly from an authoritative server, a caching
+ nameserver, or some middle box. Only by carefully using the
+ available timing parameters can a zone administrator ensure that the
+ data necessary for verification can be obtained.
+
+ The responsibility for maintaining the chain of trust is shared by
+ administrators of secured zones in the chain of trust. This is most
+ obvious in the case of a 'key compromise' when a trade-off between
+ maintaining a valid chain of trust and replacing the compromised keys
+ as soon as possible must be made. Then zone administrators will have
+ to make a trade-off, between keeping the chain of trust intact --
+ thereby allowing for attacks with the compromised key -- or
+ deliberately breaking the chain of trust and making secured
+ subdomains invisible to security-aware resolvers. Also see Section
+ 4.3.
+
+
+
+
+
+
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+RFC 4641 DNSSEC Operational Practices September 2006
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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
+ 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
+
+
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+RFC 4641 DNSSEC Operational Practices September 2006
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+ compromised, it can be simply dropped from the key set. The new key
+ set is then re-signed with the KSK.
+
+ Given the assumption that for KSKs the SEP flag is set, the KSK can
+ be distinguished from a ZSK by examining the flag field in the DNSKEY
+ RR. If the flag field is an odd number it is a KSK. If it is an
+ even number it is a ZSK.
+
+ The Zone Signing Key can be used to sign all the data in a zone on a
+ regular basis. When a Zone Signing Key is to be rolled, no
+ interaction with the parent is needed. This allows for signature
+ validity periods on the order of days.
+
+ The Key Signing Key is only to be used to sign the DNSKEY RRs in a
+ zone. If a Key Signing Key is to be rolled over, there will be
+ interactions with parties other than the zone administrator. These
+ can include the registry of the parent zone or administrators of
+ verifying resolvers that have the particular key configured as secure
+ entry points. Hence, the key effectivity period of these keys can
+ and should be made much longer. Although, given a long enough key,
+ the key effectivity period can be on the order of years, we suggest
+ planning for a key effectivity on the order of a few months so that a
+ key rollover remains an operational routine.
+
+3.1.2. KSKs for High-Level Zones
+
+ Higher-level zones are generally more sensitive than lower-level
+ zones. Anyone controlling or breaking the security of a zone thereby
+ obtains authority over all of its subdomains (except in the case of
+ resolvers that have locally configured the public key of a subdomain,
+ in which case this, and only this, subdomain wouldn't be affected by
+ the compromise of the parent zone). Therefore, extra care should be
+ taken with high-level zones, and strong keys should be used.
+
+ The root zone is the most critical of all zones. Someone controlling
+ or compromising the security of the root zone would control the
+ entire DNS namespace of all resolvers using that root zone (except in
+ the case of resolvers that have locally configured the public key of
+ a subdomain). Therefore, the utmost care must be taken in the
+ securing of the root zone. The strongest and most carefully handled
+ keys should be used. The root zone private key should always be kept
+ off-line.
+
+ Many resolvers will start at a root server for their access to and
+ authentication of DNS data. Securely updating the trust anchors in
+ an enormous population of resolvers around the world will be
+ extremely difficult.
+
+
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+3.2. Key Generation
+
+ Careful generation of all keys is a sometimes overlooked but
+ absolutely essential element in any cryptographically secure system.
+ The strongest algorithms used with the longest keys are still of no
+ use if an adversary can guess enough to lower the size of the likely
+ key space so that it can be exhaustively searched. Technical
+ suggestions for the generation of random keys will be found in RFC
+ 4086 [14]. One should carefully assess if the random number
+ generator used during key generation adheres to these suggestions.
+
+ Keys with a long effectivity period are particularly sensitive as
+ they will represent a more valuable target and be subject to attack
+ for a longer time than short-period keys. It is strongly recommended
+ that long-term key generation occur off-line in a manner isolated
+ from the network via an air gap or, at a minimum, high-level secure
+ hardware.
+
+3.3. Key Effectivity Period
+
+ For various reasons, keys in DNSSEC need to be changed once in a
+ while. The longer a key is in use, the greater the probability that
+ it will have been compromised through carelessness, accident,
+ espionage, or cryptanalysis. Furthermore, when key rollovers are too
+ rare an event, they will not become part of the operational habit and
+ there is risk that nobody on-site will remember the procedure for
+ rollover when the need is there.
+
+ From a purely operational perspective, a reasonable key effectivity
+ period for Key Signing Keys is 13 months, with the intent to replace
+ them after 12 months. An intended key effectivity period of a month
+ is reasonable for Zone Signing Keys.
+
+ For key sizes that match these effectivity periods, see Section 3.5.
+
+ As argued in Section 3.1.2, securely updating trust anchors will be
+ extremely difficult. On the other hand, the "operational habit"
+ argument does also apply to trust anchor reconfiguration. If a short
+ key effectivity period is used and the trust anchor configuration has
+ to be revisited on a regular basis, the odds that the configuration
+ tends to be forgotten is smaller. The trade-off is against a system
+ that is so dynamic that administrators of the validating clients will
+ not be able to follow the modifications.
+
+ Key effectivity periods can be made very short, as in a few minutes.
+ But when replacing keys one has to take the considerations from
+ Section 4.1 and Section 4.2 into account.
+
+
+
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+RFC 4641 DNSSEC Operational Practices September 2006
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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
+ DNSSEC.
+
+ RSA has been developed in an open and transparent manner. As the
+ patent on RSA expired in 2000, its use is now also free.
+
+ DSA has been developed by the National Institute of Standards and
+ Technology (NIST). The creation of signatures takes roughly the same
+ time as with RSA, but is 10 to 40 times as slow for verification
+ [17].
+
+ We suggest the use of RSA/SHA-1 as the preferred algorithm for the
+ key. The current known attacks on RSA can be defeated by making your
+ key longer. As the MD5 hashing algorithm is showing cracks, we
+ recommend the usage of SHA-1.
+
+ At the time of publication, it is known that the SHA-1 hash has
+ cryptanalysis issues. There is work in progress on addressing these
+ issues. We recommend the use of public key algorithms based on
+ hashes stronger than SHA-1 (e.g., SHA-256), as soon as these
+ algorithms are available in protocol specifications (see [19] and
+ [20]) and implementations.
+
+3.5. Key Sizes
+
+ When choosing key sizes, zone administrators will need to take into
+ account how long a key will be used, how much data will be signed
+ during the key publication period (see Section 8.10 of [17]), and,
+ optionally, how large the key size of the parent is. As the chain of
+ trust really is "a chain", there is not much sense in making one of
+ the keys in the chain several times larger 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
+
+
+
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+ 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
+ transferred.
+
+ When relying on dynamic update to manage a signed zone [10], be aware
+ that at least one private key of the zone will have to reside on the
+ master server. This key is only as secure as the amount of exposure
+ the server receives to unknown clients and the security of the host.
+ Although not mandatory, one could administer the DNS in the following
+ way. The master that processes the dynamic updates is unavailable
+ from generic hosts on the Internet, it is not listed in the NS RR
+ set, although its name appears in the SOA RRs MNAME field. The
+ nameservers in the NS RRSet are able to receive zone updates through
+ NOTIFY, IXFR, AXFR, or an out-of-band distribution mechanism. This
+ approach is known as the "hidden master" setup.
+
+ The ideal situation is to have a one-way information flow to the
+ network to avoid the possibility of tampering from the network.
+ Keeping the zone master file on-line on the network and simply
+ cycling it through an off-line signer does not do this. The on-line
+ version could still be tampered with if the host it resides on is
+ compromised. For maximum security, the master copy of the zone file
+ should be off-net and should not be updated based on an unsecured
+ network mediated communication.
+
+ In general, keeping a zone file off-line will not be practical and
+ the machines on which zone files are maintained will be connected to
+ a network. Operators are advised to take security measures to shield
+ unauthorized access to the master copy.
+
+
+
+
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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
+ following:
+
+ o We suggest the Maximum Zone TTL of your zone data to be a fraction
+ of your signature validity period.
+
+ If the TTL would be of similar order as the signature validity
+ period, then all RRSets fetched during the validity period
+ would be cached until the signature expiration time. Section
+ 7.1 of [4] suggests that "the resolver may use the time
+ remaining before expiration of the signature validity period of
+ a signed RRSet as an upper bound for the TTL". As a result,
+ query load on authoritative servers would peak at signature
+ expiration time, as this is also the time at which records
+ simultaneously expire from caches.
+
+ To avoid query load peaks, we suggest the TTL on all the RRs in
+ your zone to be at least a few times smaller than your
+ signature validity period.
+
+ o We suggest the signature publication period to end at least one
+ Maximum Zone TTL duration before the end of the signature validity
+ period.
+
+
+
+
+
+
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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
+ problems:
+
+ 1. During validation, some data may expire before the
+ validation is complete. The validator should be able to
+ keep all data until it is completed. This applies to all
+ RRs needed to complete the chain of trust: DSes, DNSKEYs,
+ RRSIGs, and the final answers, i.e., the RRSet that is
+ returned for the initial query.
+
+ 2. Frequent verification causes load on recursive nameservers.
+ Data at delegation points, DSes, DNSKEYs, and RRSIGs
+ benefit from caching. The TTL on those should be
+ relatively long.
+
+ o Slave servers will need to be able to fetch newly signed zones
+ well before the RRSIGs in the zone served by the slave server pass
+ their signature expiration time.
+
+ When a slave server is out of sync with its master and data in
+ a zone is signed by expired signatures, it may be better for
+ the slave server not to give out any answer.
+
+ Normally, a slave server that is not able to contact a master
+ server for an extended period will expire a zone. When that
+ happens, the server will respond differently to queries for
+ that zone. Some servers issue SERVFAIL, whereas others turn
+ off the 'AA' bit in the answers. The time of expiration is set
+ in the SOA record and is relative to the last successful
+ refresh between the master and the slave servers. There exists
+ no coupling between the signature expiration of RRSIGs in the
+ zone and the expire parameter in the SOA.
+
+ If the server serves a DNSSEC zone, then it may well happen
+ that the signatures expire well before the SOA expiration timer
+ counts down to zero. It is not possible to completely prevent
+ this from happening by tweaking the SOA parameters. However,
+ the effects can be minimized where the SOA expiration time is
+ equal to or shorter than the signature validity period. The
+ consequence of an authoritative server not being able to update
+
+
+
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+
+RFC 4641 DNSSEC Operational Practices September 2006
+
+
+ a zone, whilst that zone includes expired signatures, is that
+ non-secure resolvers will continue to be able to resolve data
+ served by the particular slave servers while security-aware
+ resolvers will experience problems because of answers being
+ marked as Bogus.
+
+ We suggest the SOA expiration timer being approximately one
+ third or one fourth of the signature validity period. It will
+ allow problems with transfers from the master server to be
+ noticed before the actual signature times out. We also suggest
+ that operators of nameservers that supply secondary services
+ develop 'watch dogs' to spot upcoming signature expirations in
+ zones they slave, and take appropriate action.
+
+ When determining the value for the expiration parameter one has
+ to take the following into account: What are the chances that
+ all my secondaries expire the zone? How quickly can I reach an
+ administrator of secondary servers to load a valid zone? These
+ questions are not DNSSEC specific but may influence the choice
+ of your signature validity intervals.
+
+4.2. Key Rollovers
+
+ A DNSSEC key cannot be used forever (see Section 3.3). So key
+ rollovers -- or supercessions, as they are sometimes called -- are a
+ fact of life when using DNSSEC. Zone administrators who are in the
+ process of rolling their keys have to take into account that data
+ published in previous versions of their zone still lives in caches.
+ When deploying DNSSEC, this becomes an important consideration;
+ ignoring data that may be in caches may lead to loss of service for
+ clients.
+
+ The most pressing example of this occurs when zone material signed
+ with an old key is being validated by a resolver that does not have
+ the old zone key cached. If the old key is no longer present in the
+ current zone, this validation fails, marking the data "Bogus".
+ Alternatively, an attempt could be made to validate data that is
+ signed with a new key against an old key that lives in a local cache,
+ also resulting in data being marked "Bogus".
+
+4.2.1. Zone Signing Key Rollovers
+
+ For "Zone Signing Key rollovers", there are two ways to make sure
+ that during the rollover data still cached can be verified with the
+ new key sets or newly generated signatures can be verified with the
+ keys still in caches. One schema, described in Section 4.2.1.2, uses
+
+
+
+
+
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+
+RFC 4641 DNSSEC Operational Practices September 2006
+
+
+ double signatures; the other uses key pre-publication (Section
+ 4.2.1.1). The pros, cons, and recommendations are described in
+ Section 4.2.1.3.
+
+4.2.1.1. Pre-Publish Key Rollover
+
+ This section shows how to perform a ZSK rollover without the need to
+ sign all the data in a zone twice -- the "pre-publish key rollover".
+ This method has advantages in the case of a key compromise. If the
+ old key is compromised, the new key has already been distributed in
+ the DNS. The zone administrator is then able to quickly switch to
+ the new key and remove the compromised key from the zone. Another
+ major advantage is that the zone size does not double, as is the case
+ with the double signature ZSK rollover. A small "how-to" for this
+ kind of rollover can be found in Appendix B.
+
+ Pre-publish key rollover involves four stages as follows:
+
+ ----------------------------------------------------------------
+ initial new DNSKEY new RRSIGs DNSKEY removal
+ ----------------------------------------------------------------
+ SOA0 SOA1 SOA2 SOA3
+ RRSIG10(SOA0) RRSIG10(SOA1) RRSIG11(SOA2) RRSIG11(SOA3)
+
+ DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY1
+ DNSKEY10 DNSKEY10 DNSKEY10 DNSKEY11
+ DNSKEY11 DNSKEY11
+ RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) RRSIG1(DNSKEY) RRSIG1 (DNSKEY)
+ RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG11(DNSKEY) RRSIG11(DNSKEY)
+ ----------------------------------------------------------------
+
+ Pre-Publish Key Rollover
+
+ 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
+ zone.
+
+ DNSKEY removal: DNSKEY 10 is removed from the zone. The key set, now
+ only containing DNSKEY 1 and DNSKEY 11, is re-signed with the
+ DNSKEY 1.
+
+ The above scheme can be simplified by always publishing the "future"
+ key immediately after the rollover. The scheme would look as follows
+ (we show two rollovers); the future key is introduced in "new DNSKEY"
+ as DNSKEY 12 and again a newer one, numbered 13, in "new DNSKEY
+ (II)":
+
+ ----------------------------------------------------------------
+ initial new RRSIGs new DNSKEY
+ ----------------------------------------------------------------
+ SOA0 SOA1 SOA2
+ RRSIG10(SOA0) RRSIG11(SOA1) RRSIG11(SOA2)
+
+ DNSKEY1 DNSKEY1 DNSKEY1
+ DNSKEY10 DNSKEY10 DNSKEY11
+ DNSKEY11 DNSKEY11 DNSKEY12
+ RRSIG1(DNSKEY) RRSIG1 (DNSKEY) RRSIG1(DNSKEY)
+ RRSIG10(DNSKEY) RRSIG11(DNSKEY) RRSIG11(DNSKEY)
+ ----------------------------------------------------------------
+
+ ----------------------------------------------------------------
+ new RRSIGs (II) new DNSKEY (II)
+ ----------------------------------------------------------------
+ SOA3 SOA4
+ RRSIG12(SOA3) RRSIG12(SOA4)
+
+ DNSKEY1 DNSKEY1
+ DNSKEY11 DNSKEY12
+ DNSKEY12 DNSKEY13
+ RRSIG1(DNSKEY) RRSIG1(DNSKEY)
+ RRSIG12(DNSKEY) RRSIG12(DNSKEY)
+ ----------------------------------------------------------------
+
+ Pre-Publish Key Rollover, Showing Two Rollovers
+
+
+
+
+
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+
+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.
+
+4.2.1.2. Double Signature Zone Signing Key Rollover
+
+ This section shows how to perform a ZSK key rollover using the double
+ zone data signature scheme, aptly named "double signature rollover".
+
+ During the "new DNSKEY" stage the new version of the zone file will
+ need to propagate to all authoritative servers and the data that
+ exists in (distant) caches will need to expire, requiring at least
+ the Maximum Zone TTL.
+
+ Double signature ZSK rollover involves three stages as follows:
+
+ ----------------------------------------------------------------
+ initial new DNSKEY DNSKEY removal
+ ----------------------------------------------------------------
+ SOA0 SOA1 SOA2
+ RRSIG10(SOA0) RRSIG10(SOA1) RRSIG11(SOA2)
+ RRSIG11(SOA1)
+
+ DNSKEY1 DNSKEY1 DNSKEY1
+ DNSKEY10 DNSKEY10 DNSKEY11
+ DNSKEY11
+ RRSIG1(DNSKEY) RRSIG1(DNSKEY) RRSIG1(DNSKEY)
+ RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG11(DNSKEY)
+ RRSIG11(DNSKEY)
+ ----------------------------------------------------------------
+
+ Double Signature Zone Signing Key Rollover
+
+ 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.
+
+
+
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+
+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.
+
+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.
+
+
+
+
+
+
+
+
+
+
+
+Kolkman & Gieben Informational [Page 18]
+
+RFC 4641 DNSSEC Operational Practices September 2006
+
+
+ --------------------------------------------------------------------
+ initial new DNSKEY DS change DNSKEY removal
+ --------------------------------------------------------------------
+ Parent:
+ SOA0 --------> SOA1 -------->
+ RRSIGpar(SOA0) --------> RRSIGpar(SOA1) -------->
+ DS1 --------> DS2 -------->
+ RRSIGpar(DS) --------> RRSIGpar(DS) -------->
+
+
+ Child:
+ SOA0 SOA1 --------> SOA2
+ RRSIG10(SOA0) RRSIG10(SOA1) --------> RRSIG10(SOA2)
+ -------->
+ DNSKEY1 DNSKEY1 --------> DNSKEY2
+ DNSKEY2 -------->
+ DNSKEY10 DNSKEY10 --------> DNSKEY10
+ RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) --------> RRSIG2 (DNSKEY)
+ RRSIG2 (DNSKEY) -------->
+ RRSIG10(DNSKEY) RRSIG10(DNSKEY) --------> RRSIG10(DNSKEY)
+ --------------------------------------------------------------------
+
+ Stages of Deployment for a Double Signature Key Signing Key Rollover
+
+ initial: Initial version of the zone. The parental DS points to
+ DNSKEY1. Before the rollover starts, the child will have to
+ verify what the TTL is of the DS RR that points to DNSKEY1 -- it
+ is needed during the rollover and we refer to the value as TTL_DS.
+
+ new DNSKEY: During the "new DNSKEY" phase, the zone administrator
+ generates a second KSK, DNSKEY2. The key is provided to the
+ parent, and the child will have to wait until a new DS RR has been
+ generated that points to DNSKEY2. After that DS RR has been
+ published on all servers authoritative for the parent's zone, the
+ zone administrator has to wait at least TTL_DS to make sure that
+ the old DS RR has expired from caches.
+
+ DS change: The parent replaces DS1 with DS2.
+
+ DNSKEY removal: DNSKEY1 has been removed.
+
+ The scenario above puts the responsibility for maintaining a valid
+ chain of trust with the child. It also is based on the premise that
+ the parent only has one DS RR (per algorithm) per zone. An
+ alternative mechanism has been considered. Using an established
+ trust relation, the interaction can be performed in-band, and the
+ removal of the keys by the child can possibly be signaled by the
+ parent. In this mechanism, there are periods where there are two DS
+
+
+
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+
+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 4.2.1.1) and double signature (Section 4.2.1.2).
+
+ As the KSK is used to validate the key set and because the KSK is not
+ changed during a ZSK rollover, a cache is able to validate the new
+ key set of the zone. The pre-publish method would also work for a
+ KSK rollover. The records that are to be pre-published are the
+ parental DS RRs. The pre-publish method has some drawbacks for KSKs.
+ We first describe the rollover scheme and then indicate these
+ drawbacks.
+
+ --------------------------------------------------------------------
+ initial new DS new DNSKEY DS/DNSKEY removal
+ --------------------------------------------------------------------
+ Parent:
+ SOA0 SOA1 --------> SOA2
+ RRSIGpar(SOA0) RRSIGpar(SOA1) --------> RRSIGpar(SOA2)
+ DS1 DS1 --------> DS2
+ DS2 -------->
+ RRSIGpar(DS) RRSIGpar(DS) --------> RRSIGpar(DS)
+
+
+ Child:
+ SOA0 --------> SOA1 SOA1
+ RRSIG10(SOA0) --------> RRSIG10(SOA1) RRSIG10(SOA1)
+ -------->
+ DNSKEY1 --------> DNSKEY2 DNSKEY2
+ -------->
+ DNSKEY10 --------> DNSKEY10 DNSKEY10
+ RRSIG1 (DNSKEY) --------> RRSIG2(DNSKEY) RRSIG2 (DNSKEY)
+ RRSIG10(DNSKEY) --------> RRSIG10(DNSKEY) RRSIG10(DNSKEY)
+ --------------------------------------------------------------------
+
+ Stages of Deployment for a Pre-Publish Key Signing Key Rollover
+
+
+
+
+
+
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+
+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
+ involved.
+
+ o A KSK rollover needs interaction between parent and child. Data
+ exchange is needed to provide the new keys to the parent;
+ consequently, this data must be authenticated and integrity must
+ be guaranteed in order to avoid attacks on the rollover.
+
+4.3. Planning for Emergency Key Rollover
+
+ This section deals with preparation for a possible key compromise.
+ Our advice is to have a documented procedure ready for when a key
+ compromise is suspected or confirmed.
+
+ When the private material of one of your keys is compromised it can
+ be used for as long as a valid trust chain exists. A trust chain
+ remains intact for
+
+ o as long as a signature over the compromised key in the trust chain
+ is valid,
+
+ o as long as a parental DS RR (and signature) points to the
+ compromised key,
+
+ o as long as the key is anchored in a resolver and is used as a
+ starting point for validation (this is generally the hardest to
+ update).
+
+
+
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+
+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.
+
+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.
+
+
+
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+
+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.
+
+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
+
+
+
+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.
+
+
+
+
+
+
+
+
+
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+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
+ 2005.
+
+ [5] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
+ "Resource Records for the DNS Security Extensions", RFC 4034,
+ March 2005.
+
+ [6] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
+ "Protocol Modifications for the DNS Security Extensions", RFC
+ 4035, March 2005.
+
+
+
+
+
+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
+ 1996.
+
+ [9] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
+ (DNS NOTIFY)", RFC 1996, August 1996.
+
+ [10] Wellington, B., "Secure Domain Name System (DNS) Dynamic
+ Update", RFC 3007, November 2000.
+
+ [11] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)",
+ RFC 2308, March 1998.
+
+ [12] Eastlake, D., "DNS Security Operational Considerations", RFC
+ 2541, March 1999.
+
+ [13] Orman, H. and P. Hoffman, "Determining Strengths For Public
+ Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
+ April 2004.
+
+ [14] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
+ Requirements for Security", BCP 106, RFC 4086, June 2005.
+
+ [15] Hollenbeck, S., "Domain Name System (DNS) Security Extensions
+ Mapping for the Extensible Provisioning Protocol (EPP)", RFC
+ 4310, December 2005.
+
+ [16] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key
+ Sizes", The Journal of Cryptology 14 (255-293), 2001.
+
+ [17] Schneier, B., "Applied Cryptography: Protocols, Algorithms, and
+ Source Code in C", ISBN (hardcover) 0-471-12845-7, ISBN
+ (paperback) 0-471-59756-2, Published by John Wiley & Sons Inc.,
+ 1996.
+
+ [18] Rose, S., "NIST DNSSEC workshop notes", June 2001.
+
+ [19] Jansen, J., "Use of RSA/SHA-256 DNSKEY and RRSIG Resource
+ Records in DNSSEC", Work in Progress, January 2006.
+
+ [20] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS)
+ Resource Records (RRs)", RFC 4509, May 2006.
+
+
+
+
+
+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.
+
+
+
+
+
+
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+
+
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+Kolkman & Gieben Informational [Page 29]
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+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
+ DNSKEY.
+
+ Key Signing Key or KSK: A Key Signing Key (KSK) is a key that is used
+ exclusively for signing the apex key set. The fact that a key is
+ a KSK is only relevant to the signing tool.
+
+ Key size: The term 'key size' can be substituted by 'modulus size'
+ throughout the document. It is mathematically more correct to use
+ modulus size, but as this is a document directed at operators we
+ feel more at ease with the term key size.
+
+ Private and public keys: DNSSEC secures the DNS through the use of
+ public key cryptography. Public key cryptography is based on the
+ existence of two (mathematically related) keys, a public key and a
+ private key. The public keys are published in the DNS by use of
+ the DNSKEY Resource Record (DNSKEY RR). Private keys should
+ remain private.
+
+ Key rollover: A key rollover (also called key supercession in some
+ environments) is the act of replacing one key pair with another at
+ the end of a key effectivity period.
+
+ Secure Entry Point (SEP) key: A KSK that has a parental DS record
+ pointing to it or is configured as a trust anchor. Although not
+ required by the protocol, we recommend that the SEP flag [3] is
+ set on these keys.
+
+ Self-signature: This only applies to signatures over DNSKEYs; a
+ signature made with DNSKEY x, over DNSKEY x is called a self-
+ signature. Note: without further information, self-signatures
+ convey no trust. They are useful to check the authenticity of the
+ DNSKEY, i.e., they can be used as a hash.
+
+
+
+
+
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+
+ 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.
+
+
+
+
+
+
+
+
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+
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+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:
+ "example.com 3600 IN A 192.0.2.1" is reduced to "A". RRSets are a
+ list of RRs. A example of this would be "A1, A2", specifying the
+ RRSet containing two "A" records. This could again be abbreviated to
+ just "A".
+
+ Signature notation: Signatures are denoted as RRSIGx(RRSet), which
+ means that RRSet is signed with DNSKEYx.
+
+ Zone representation: Using the above notation we have simplified the
+ representation of a signed zone by leaving out all unnecessary
+ details such as the names and by representing all data by "SOAx"
+
+ SOA representation: SOAs are represented as SOAx, where x is the
+ serial number.
+
+ Using this notation the following signed zone:
+
+ example.net. 86400 IN SOA ns.example.net. bert.example.net. (
+ 2006022100 ; serial
+ 86400 ; refresh ( 24 hours)
+ 7200 ; retry ( 2 hours)
+ 3600000 ; expire (1000 hours)
+ 28800 ) ; minimum ( 8 hours)
+ 86400 RRSIG SOA 5 2 86400 20130522213204 (
+ 20130422213204 14 example.net.
+ cmL62SI6iAX46xGNQAdQ... )
+ 86400 NS a.iana-servers.net.
+ 86400 NS b.iana-servers.net.
+ 86400 RRSIG NS 5 2 86400 20130507213204 (
+ 20130407213204 14 example.net.
+ SO5epiJei19AjXoUpFnQ ... )
+ 86400 DNSKEY 256 3 5 (
+ EtRB9MP5/AvOuVO0I8XDxy0... ) ; id = 14
+ 86400 DNSKEY 257 3 5 (
+ gsPW/Yy19GzYIY+Gnr8HABU... ) ; id = 15
+ 86400 RRSIG DNSKEY 5 2 86400 20130522213204 (
+ 20130422213204 14 example.net.
+ J4zCe8QX4tXVGjV4e1r9... )
+
+
+
+
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+
+
+ 86400 RRSIG DNSKEY 5 2 86400 20130522213204 (
+ 20130422213204 15 example.net.
+ keVDCOpsSeDReyV6O... )
+ 86400 RRSIG NSEC 5 2 86400 20130507213204 (
+ 20130407213204 14 example.net.
+ obj3HEp1GjnmhRjX... )
+ a.example.net. 86400 IN TXT "A label"
+ 86400 RRSIG TXT 5 3 86400 20130507213204 (
+ 20130407213204 14 example.net.
+ IkDMlRdYLmXH7QJnuF3v... )
+ 86400 NSEC b.example.com. TXT RRSIG NSEC
+ 86400 RRSIG NSEC 5 3 86400 20130507213204 (
+ 20130407213204 14 example.net.
+ bZMjoZ3bHjnEz0nIsPMM... )
+ ...
+
+ is reduced to the following representation:
+
+ SOA2006022100
+ RRSIG14(SOA2006022100)
+ DNSKEY14
+ DNSKEY15
+
+ RRSIG14(KEY)
+ RRSIG15(KEY)
+
+ The rest of the zone data has the same signature as the SOA record,
+ i.e., an RRSIG created with DNSKEY 14.
+
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+
+Authors' Addresses
+
+ Olaf M. Kolkman
+ NLnet Labs
+ Kruislaan 419
+ Amsterdam 1098 VA
+ The Netherlands
+
+ EMail: olaf@nlnetlabs.nl
+ URI: http://www.nlnetlabs.nl
+
+
+ R. (Miek) Gieben
+
+ EMail: miek@miek.nl
+
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+
+
+Full Copyright Statement
+
+ Copyright (C) The Internet Society (2006).
+
+ This document is subject to the rights, licenses and restrictions
+ contained in BCP 78, and except as set forth therein, the authors
+ retain all their rights.
+
+ This document and the information contained herein are provided on an
+ "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+ OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
+ ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
+ INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
+ INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+ WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Intellectual Property
+
+ The IETF takes no position regarding the validity or scope of any
+ Intellectual Property Rights or other rights that might be claimed to
+ pertain to the implementation or use of the technology described in
+ this document or the extent to which any license under such rights
+ might or might not be available; nor does it represent that it has
+ made any independent effort to identify any such rights. Information
+ on the procedures with respect to rights in RFC documents can be
+ found in BCP 78 and BCP 79.
+
+ Copies of IPR disclosures made to the IETF Secretariat and any
+ assurances of licenses to be made available, or the result of an
+ attempt made to obtain a general license or permission for the use of
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+ specification can be obtained from the IETF on-line IPR repository at
+ http://www.ietf.org/ipr.
+
+ The IETF invites any interested party to bring to its attention any
+ copyrights, patents or patent applications, or other proprietary
+ rights that may cover technology that may be required to implement
+ this standard. Please address the information to the IETF at
+ ietf-ipr@ietf.org.
+
+Acknowledgement
+
+ Funding for the RFC Editor function is provided by the IETF
+ Administrative Support Activity (IASA).
+
+
+
+
+
+
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