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RFC 6781

DNSSEC Operational Practices, Version 2

Pages: 71
Informational
Errata
Obsoletes:  4641
Part 3 of 4 – Pages 39 to 54
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Top   ToC   RFC6781 - Page 39   prevText

4.3. Parent Policies

4.3.1. Initial Key Exchanges and Parental Policies Considerations

The initial key exchange is always subject to the policies set by the parent. It is specifically important in a registry-registrar- registrant model where a registry maintains the parent zone, and the registrant (the user of the child-domain name) deals with the registry through an intermediary called a registrar (see [RFC3375] for a comprehensive definition). The key material is to be passed from the DNS operator to the parent via a registrar, where both the DNS operator and registrar are selected by the registrant and might be different organizations. 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 the parent and child. That is, there is no implicit need in DNSSEC to make the authentication process stronger than it is for regular DNS. Using the DNS itself as the source for the actual DNSKEY material has the benefit that it reduces the chances of user error. A DNSKEY query tool can make use of the SEP bit [RFC4035] to select the proper key(s) 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 that the child is secure. Note: Out-of-band verification is still needed when the key material is fetched for the first time, even via DNS. The parent can never be sure whether or not the DNSKEY RRs have been spoofed. With some types of key rollovers, the DNSKEY is not pre-published, and a DNSKEY query tool is not able to retrieve the successor key. In this case, the out-of-band method is required. This also allows the child to determine the digest algorithm of the DS record.
Top   ToC   RFC6781 - Page 40

4.3.2. Storing Keys or Hashes?

When designing a registry system, one should consider whether to store the DNSKEYs and/or the corresponding DSs. 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 suggest that registry systems should be able to store DS RRs, even if they also store DNSKEYs (see also "DNSSEC Trust Anchor Configuration and Maintenance" [DNSSEC-TRUST-ANCHOR]). The storage considerations also relate to the design of the customer interface and the method by which data is transferred between the 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? When registries support the Extensible Provisioning Protocol (EPP) [RFC5910], that can be used for registrar-registry interactions, since that protocol allows the transfer of both DS and, optionally, DNSKEY RRs. There is no standardized way to move the data between the customer and the registrar. Different registrars have different mechanisms, ranging from simple web interfaces to various APIs. In some cases, the use of the DNSSEC extensions to EPP may be applicable. 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.

4.3.3. Security Lameness

Security lameness is defined as the state whereby the parent has a DS RR pointing to a nonexistent DNSKEY RR. Security lameness may occur temporarily during a Double-DS rollover scheme. However, care should be taken that not all DS RRs are pointing to a nonexistent DNSKEY RR, which will cause the child's zone to 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 the child, the parental checks are limited to only comparing the key id.
Top   ToC   RFC6781 - Page 41
   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.

4.3.4. DS Signature Validity Period

Since the DS can be replayed as long as it has a valid signature, a short signature validity period for the DS RRSIG minimizes the time that 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 the case of a configuration error in the signer. There may not be enough time to fix the problems before signatures expire (this is a generic argument; also see Section 4.4.2). Something as mundane as zone administrator unavailability during weekends shows the need for DS signature validity periods longer than two days. Just like any signature validity period, we suggest an absolute minimum for the 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 period increases the operational risk for the parent. Therefore, the parent may have a policy to use a signature validity period that is considerably longer than the child would hope for. A compromise between the policy/operational constraints of the parent and minimizing damage for the child may result in a DS signature validity period somewhere between a week and several months. In addition to the signature validity period, which sets a lower bound on the number of times the zone administrator will need to sign the zone data and an upper bound on the time that a child is vulnerable after key compromise, there is the TTL value on the DS RRs. Shortening the TTL reduces the damage of a successful replay attack. It does mean 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.
Top   ToC   RFC6781 - Page 42

4.3.5. Changing DNS Operators

The parent-child relationship is often described in terms of a registry-registrar-registrant model, where a registry maintains the parent zone and the registrant (the user of the child-domain name) deals with the registry through an intermediary called a registrar [RFC3375]. Registrants may outsource the maintenance of their DNS system, including the maintenance of DNSSEC key material, to the registrar or to another third party, referred to here as the DNS operator. For various reasons, a registrant may want to move between DNS operators. How easy this move will be depends principally on the DNS operator from which the registrant is moving (the losing operator), as the losing operator has control over the DNS zone and its keys. The following sections describe the two cases: where the losing operator cooperates with the new operator (the gaining operator), and where the two do not cooperate.
4.3.5.1. Cooperating DNS Operators
In this scenario, it is assumed that the losing operator will not pass any private key material to the gaining operator (that would constitute a trivial case) but is otherwise fully cooperative. In this environment, the change could be made with a Pre-Publish ZSK rollover, whereby the losing operator pre-publishes the ZSK of the gaining operator, combined with a Double-Signature KSK rollover where the two registrars exchange public keys and independently generate a signature over those key sets that they combine and both publish in their copy of the zone. Once that is done, they can use their own private keys to sign any of their zone content during the transfer.
Top   ToC   RFC6781 - Page 43
    ------------------------------------------------------------
    initial            |        pre-publish                    |
    ------------------------------------------------------------
    Parent:
     NS_A                            NS_A
     DS_A                            DS_A
    ------------------------------------------------------------
    Child at A:            Child at A:        Child at B:
     SOA_A0                 SOA_A1             SOA_B0
     RRSIG_Z_A(SOA)         RRSIG_Z_A(SOA)     RRSIG_Z_B(SOA)

     NS_A                   NS_A               NS_B
     RRSIG_Z_A(NS)          NS_B               RRSIG_Z_B(NS)
                            RRSIG_Z_A(NS)

     DNSKEY_Z_A             DNSKEY_Z_A         DNSKEY_Z_A
                            DNSKEY_Z_B         DNSKEY_Z_B
     DNSKEY_K_A             DNSKEY_K_A         DNSKEY_K_A
                            DNSKEY_K_B         DNSKEY_K_B
     RRSIG_K_A(DNSKEY)      RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)
                            RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)
    ------------------------------------------------------------

    ------------------------------------------------------------
          re-delegation                |   post-migration      |
    ------------------------------------------------------------
    Parent:
              NS_B                           NS_B
              DS_B                           DS_B
    ------------------------------------------------------------
    Child at A:        Child at B:           Child at B:

     SOA_A1             SOA_B0                SOA_B1
     RRSIG_Z_A(SOA)     RRSIG_Z_B(SOA)        RRSIG_Z_B(SOA)

     NS_A               NS_B                  NS_B
     NS_B               RRSIG_Z_B(NS)         RRSIG_Z_B(NS)
     RRSIG_Z_A(NS)

     DNSKEY_Z_A         DNSKEY_Z_A
     DNSKEY_Z_B         DNSKEY_Z_B            DNSKEY_Z_B
     DNSKEY_K_A         DNSKEY_K_A
     DNSKEY_K_B         DNSKEY_K_B            DNSKEY_K_B
     RRSIG_K_A(DNSKEY)  RRSIG_K_A(DNSKEY)
     RRSIG_K_B(DNSKEY)  RRSIG_K_B(DNSKEY)     RRSIG_K_B(DNSKEY)
    ------------------------------------------------------------

               Figure 10: Rollover for Cooperating Operators
Top   ToC   RFC6781 - Page 44
   In this figure, A denotes the losing operator and B the gaining
   operator.  RRSIG_Z is the RRSIG produced by a ZSK, RRSIG_K is
   produced with a KSK, and the appended A or B indicates the producers
   of the key pair.  "Child at A" is how the zone content is represented
   by the losing DNS operator, and "Child at B" is how the zone content
   is represented by the gaining DNS operator.

   The zone is initially delegated from the parent to the name servers
   of operator A.  Operator A uses his own ZSK and KSK to sign the zone.
   The cooperating operator A will pre-publish the new NS record and the
   ZSK and KSK of operator B, including the RRSIG over the DNSKEY RRset
   generated by the KSK of operator B.  Operator B needs to publish the
   same DNSKEY RRset.  When that DNSKEY RRset has populated the caches,
   the re-delegation can be made, which involves adjusting the NS and DS
   records in the parent zone to point to operator B.  And after all
   DNSSEC records related to operator A have expired from the caches,
   operator B can stop publishing the keys and signatures belonging to
   operator A, and vice versa.

   The requirement to exchange signatures has a couple of drawbacks.  It
   requires more operational overhead, because not only do the operators
   have to exchange public keys but they also have to exchange the
   signatures of the new DNSKEY RRset.  This drawback does not exist if
   the Double-Signature KSK rollover is replaced with a Double-DS KSK
   rollover.  See Figure 15 in Appendix D for the diagram.

   Thus, if the registry and registrars allow DS records to be published
   that do not point to a published DNSKEY in the child zone, the
   Double-DS KSK rollover is preferred (see Figure 5), in combination
   with the Pre-Publish ZSK rollover.  This does not require sharing the
   KSK signatures between the operators, but both operators still have
   to publish each other's ZSKs.

4.3.5.2. Non-Cooperating DNS Operators
In the non-cooperating case, matters are more complicated. The losing operator may not cooperate and leave the data in the DNS as is. In extreme cases, the losing operator may become obstructive and publish a DNSKEY RR with a high TTL and corresponding signature validity period so that registrar A's DNSKEY could end up in caches for (in theory at least) decades. The problem arises when a validator tries to validate with the losing operator's key and there is no signature material produced with the losing operator available in the delegation path after re-delegation from the losing operator to the gaining operator has taken place. One could imagine a rollover scenario where the gaining operator takes a copy of all RRSIGs created by the losing operator and
Top   ToC   RFC6781 - Page 45
   publishes those in conjunction with its own signatures, but that
   would not allow any changes in the zone content.  Since a
   re-delegation took place, the NS RRset has by definition changed, so
   such a rollover scenario will not work.  Besides, if zone transfers
   are not allowed by the losing operator and NSEC3 is deployed in the
   losing operator's zone, then the gaining operator's zone will not
   have certainty that all of the losing operator's RRSIGs have been
   copied.

   The only viable operation for the registrant is to have his zone go
   Insecure for the duration of the change.  The registry should be
   asked to remove the DS RR pointing to the losing operator's DNSKEY
   and to change the NS RRset to point to the gaining operator.  Once
   this has propagated through the DNS, the registry should be asked to
   insert the DS record pointing to the (newly signed) zone at
   operator B.

   Note that some behaviors of resolver implementations may aid in the
   process of changing DNS operators:

   o  TTL sanity checking, as described in RFC 2308 [RFC2308], will
      limit the impact of the actions of an obstructive losing operator.
      Resolvers that implement TTL sanity checking will use an upper
      limit for TTLs on RRsets in responses.

   o  If RRsets at the zone cut (are about to) expire, the resolver
      restarts its search above the zone cut.  Otherwise, the resolver
      risks continuing to use a name server that might be un-delegated
      by the parent.

   o  Limiting the time that DNSKEYs that seem to be unable to validate
      signatures are cached and/or trying to recover from cases where
      DNSKEYs do not seem to be able to validate data also reduce the
      effects of the problem of non-cooperating registrars.

   However, there is no operational methodology to work around this
   business issue, and proper contractual relationships between all
   involved parties seem to be the only solution to cope with these
   problems.  It should be noted that in many cases, the problem with
   temporary broken delegations already exists when a zone changes from
   one DNS operator to another.  Besides, it is often the case that when
   operators are changed, the services that are referenced by that zone
   also change operators, possibly involving some downtime.

   In any case, to minimize such problems, the classic configuration is
   to have relatively short TTLs on all involved Resource Records.  That
   will solve many of the problems regarding changes to a zone,
   regardless of whether DNSSEC is used.
Top   ToC   RFC6781 - Page 46

4.4. 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 that has elapsed after a slave server synchronized with a master server. The TTL value and the SOA RR minimum TTL parameter [RFC2308] are used to determine how long a forwarder should cache data (or negative responses) 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. The considerations in this section are all qualitative and focused on the operational and managerial issues. A more thorough quantitative analysis of rollover timing parameters can be found in "DNSSEC Key Timing Considerations" [DNSSEC-KEY-TIMING].

4.4.1. Time Considerations

Because of the expiration of signatures, one should consider the following: o We suggest that the Maximum Zone TTL value of your zone data be smaller than your signature validity period. If the TTL duration was similar to that of the signature validity period, then all RRsets fetched during the validity period would be cached until the signature expiration time. Section 8.1 of RFC 4033 [RFC4033] 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, the query load on authoritative servers would peak at the signature expiration time, as this is also the time at which records simultaneously expire from caches. Having a TTL that is at least a few times smaller than your signature validity period avoids query load peaks. o We suggest that the signature publication period end at least one Maximum Zone TTL duration (but preferably a minimum of a few days) before the end of the signature validity period. Re-signing a zone shortly before the end of the signature validity period may cause the simultaneous expiration of data from caches. This in turn may lead to peaks in the load on authoritative servers. To avoid this, schemes are deployed
Top   ToC   RFC6781 - Page 47
         whereby the zone is periodically visited for a re-signing
         operation, and those signatures that are within a so-called
         Refresh Period from signature expiration are recreated.  Also
         see Section 4.4.2 below.

         In the case of an operational error, you would have one Maximum
         Zone TTL duration to resolve the problem.  Re-signing a zone a
         few days before the end of the signature validity period
         ensures that the signatures will survive at least a (long)
         weekend in case of such operational havoc.  This is called the
         Refresh Period (see Section 4.4.2).

   o  We suggest that the Minimum Zone TTL be long enough to both fetch
      and verify all the RRs in the trust chain.  In workshop
      environments, it has been demonstrated [NIST-Workshop] 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: DS, DNSKEY, RRSIG, and the final
          answers, i.e., the RRset that is returned for the initial
          query.

      2.  Frequent verification causes load on recursive name servers.
          Data at delegation points, DS, DNSKEY, and RRSIG RRs benefits
          from caching.  The TTL on those should be relatively long.
          Data at the leaves in the DNS tree has less impact on
          recursive name servers.

   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 synchronization 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.
Top   ToC   RFC6781 - Page 48
         If the server serves a DNSSEC-secured zone, then it may happen
         that the signatures expire well before the SOA expiration timer
         counts down to zero.  It is not possible to completely prevent
         this by modifying the SOA parameters.

         However, the effects can be minimized where the SOA expiration
         time is equal to or shorter than the Refresh Period (see
         Section 4.4.2).

         The consequence of an authoritative server not being able to
         update a zone for an extended period of time is that signatures
         may expire.  In this case, 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 that the SOA expiration timer be approximately one
         third or a quarter of the signature validity period.  It will
         allow problems with transfers from the master server to be
         noticed before signatures time out.

         We also suggest that operators of name servers that supply
         secondary services develop systems to identify upcoming
         signature expirations in zones they slave and take appropriate
         action where such an event is detected.

         When determining the value for the expiration parameter, one
         has to take the following into account: What are the chances
         that all secondaries expire the zone?  How quickly can the
         administrators of the secondary servers be reached to load a
         valid zone?  These questions are not DNSSEC-specific but may
         influence the choice of your signature validity periods.

4.4.2. Signature Validity Periods

4.4.2.1. Maximum Value
The first consideration for choosing a maximum signature validity period is the risk of a replay attack. For low-value, long-term stable resources, the risks may be minimal, and the signature validity period may be several months. Although signature validity periods of many years are allowed, the same "operational habit" arguments as those given in Section 3.2.2 play a role: When a zone is re-signed with some regularity, then zone administrators remain conscious of the operational necessity of re-signing.
Top   ToC   RFC6781 - Page 49
4.4.2.2. Minimum Value
The minimum value of the signature validity period is set for the time by which one would like to survive operational failure in provisioning: At what time will a failure be noticed, and at what time is action expected to be taken? By answering these questions, availability of zone administrators during (long) weekends or time taken to access backup media can be taken into account. The result could easily suggest a minimum signature validity period of a few days. Note, however, that the argument above is assuming that zone data has just been signed and published when the problem occurred. In practice, it may be that a zone is signed according to a frequency set by the Re-Sign Period, whereby the signer visits the zone content and only refreshes signatures that are within a given amount of time (the Refresh Period) of expiration. The Re-Sign Period must be smaller than the Refresh Period in order for zone data to be signed in a timely fashion. If an operational problem occurs during re-signing, then the signatures in the zone to expire first are the ones that have been generated longest ago. In the worst case, these signatures are the Refresh Period minus the Re-Sign Period away from signature expiration. To make matters slightly more complicated, some signers vary the signature validity period over a small range (the jitter interval) so that not all signatures expire at the same time. In other words, the minimum signature validity period is set by first choosing the Refresh Period (usually a few days), then defining the Re-Sign Period in such a way that the Refresh Period minus the Re-Sign Period, minus the maximum jitter sets the time in which operational havoc can be resolved.
Top   ToC   RFC6781 - Page 50
   The relationship between signature times is illustrated in Figure 11.

   Inception          Signing                                 Expiration
   time               time                                    time
   |                  |                                 |     |     |
   |------------------|---------------------------------|.....|.....|
   |                  |                                 |     |     |
                                                          +/-jitter

   | Inception offset |                                       |
   |<---------------->|            Validity Period            |
   |               |<---------------------------------------->|


   Inception          Signing Reuse   Reuse   Reuse   New     Expiration
   time               time                            RRSIG   time
   |                  |       |       |       |       |       |
   |------------------|-------------------------------|-------|
   |                  |       |       |       |       |       |
                       <-----> <-----> <-----> <----->
                     Re-Sign Period

                                                |   Refresh   |
                                                |<----------->|
                                                |   Period    |

                  Figure 11: Signature Timing Parameters

   Note that in the figure the validity of the signature starts shortly
   before the signing time.  That is done to deal with validators that
   might have some clock skew.  This is called the inception offset, and
   it should be chosen so that false negatives are minimized to a
   reasonable level.

4.4.2.3. Differentiation between RRsets
It is possible to vary signature validity periods between signatures over different RRsets in the zone. In practice, this could be done when zones contain highly volatile data (which may be the case in dynamic-update environments). Note, however, that the risk of replay (e.g., by stale secondary servers) should be the leading factor in determining the signature validity period, since the TTLs on the data itself are still the primary parameter for cache expiry. In some cases, the risk of replaying existing data might be different from the risk of replaying the denial of data. In those cases, the signature validity period on NSEC or NSEC3 records may be tweaked accordingly.
Top   ToC   RFC6781 - Page 51
   When a zone contains secure delegations, then a relatively short
   signature validity period protects the child against replay attacks
   in the case where the child's key is compromised (see Section 4.3.4).
   Since there is a higher operational risk for the parent registry when
   choosing a short validity period and a higher operational risk for
   the child when choosing a long validity period, some (price)
   differentiation may occur for validity periods between individual DS
   RRs in a single zone.

   There seem to be no other arguments for differentiation in validity
   periods.

5. "Next Record" Types

One of the design tradeoffs made during the development of DNSSEC was to separate the signing and serving operations instead of performing cryptographic operations as DNS requests are being serviced. It is therefore necessary to create records that cover the very large number of nonexistent names that lie between the names that do exist. There are two mechanisms to provide authenticated proof of nonexistence of domain names in DNSSEC: a clear-text one and an obfuscated-data one. Each mechanism: o includes a list of all the RRTYPEs present, which can be used to prove the nonexistence of RRTYPEs at a certain name; o stores only the name for which the zone is authoritative (that is, glue in the zone is omitted); and o uses a specific RRTYPE to store information about the RRTYPEs present at the name: The clear-text mechanism uses NSEC, and the obfuscated-data mechanism uses NSEC3.

5.1. Differences between NSEC and NSEC3

The clear-text mechanism (NSEC) is implemented using a sorted linked list of names in the zone. The obfuscated-data mechanism (NSEC3) is similar but first hashes the names using a one-way hash function, before creating a sorted linked list of the resulting (hashed) strings. The NSEC record requires no cryptographic operations aside from the validation of its associated signature record. It is human readable and can be used in manual queries to determine correct operation. The disadvantage is that it allows for "zone walking", where one can request all the entries of a zone by following the linked list of NSEC RRs via the "Next Domain Name" field. Though all agree that DNS
Top   ToC   RFC6781 - Page 52
   data is accessible through query mechanisms, for some zone
   administrators this behavior is undesirable for policy, regulatory,
   or other reasons.

   Furthermore, NSEC requires a signature over every RR in the zone
   file, thereby ensuring that any denial of existence is
   cryptographically signed.  However, in a large zone file containing
   many delegations, very few of which are to signed zones, this may
   produce unacceptable additional overhead, especially where insecure
   delegations are subject to frequent updates (a typical example might
   be a TLD operator with few registrants using secure delegations).
   NSEC3 allows intervals between two secure delegations to "opt out",
   in which case they may contain one or more insecure delegations, thus
   reducing the size and cryptographic complexity of the zone at the
   expense of the ability to cryptographically deny the existence of
   names in a specific span.

   The NSEC3 record uses a hashing method of the requested name.  To
   increase the workload required to guess entries in the zone, the
   number of hashing iterations can be specified in the NSEC3 record.
   Additionally, a salt can be specified that also modifies the hashes.
   Note that NSEC3 does not give full protection against information
   leakage from the zone (you can still derive the size of the zone,
   which RRTYPEs are in there, etc.).

5.2. NSEC or NSEC3

The first motivation to deploy NSEC3 -- prevention of zone enumeration -- only makes sense when zone content is not highly structured or trivially guessable. Highly structured zones, such as in-addr.arpa., ip6.arpa., and e164.arpa., can be trivially enumerated using ordinary DNS properties, while for small zones that only contain records in the apex of the zone and a few common names such as "www" or "mail", guessing zone content and proving completeness is also trivial when using NSEC3. In these cases, the use of NSEC is preferred to ease the work required by signers and validating resolvers. For large zones where there is an implication of "not readily available" names, such as those where one has to sign a non-disclosure agreement before obtaining it, NSEC3 is preferred. The second reason to consider NSEC3 is "Opt-Out", which can reduce the number of NSEC3 records required. This is discussed further below (Section 5.3.4).
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5.3. NSEC3 Parameters

NSEC3 is controlled by a number of parameters, some of which can be varied: This section discusses the choice of those parameters.

5.3.1. NSEC3 Algorithm

The NSEC3 hashing algorithm is performed on the Fully Qualified Domain Name (FQDN) in its uncompressed form. This ensures that brute force work done by an attacker for one FQDN cannot be reused for another FQDN attack, as these entries are by definition unique. At the time of this writing, there is only one NSEC3 hash algorithm defined. [RFC5155] specifically states: "When specifying a new hash algorithm for use with NSEC3, a transition mechanism MUST also be defined". Therefore, this document does not consider NSEC3 hash algorithm transition.

5.3.2. NSEC3 Iterations

One of the concerns with NSEC3 is that a pre-calculated dictionary attack could be performed in order to assess whether or not certain domain names exist within a zone. Two mechanisms are introduced in the NSEC3 specification to increase the costs of such dictionary attacks: iterations and salt. The iterations parameter defines the number of additional times the hash function has been performed. A higher value results in greater resiliency against dictionary attacks, at a higher computational cost for both the server and resolver. RFC 5155 Section 10.3 [RFC5155] considers the tradeoffs between incurring cost during the signing process and imposing costs to the validating name server, while still providing a reasonable barrier against dictionary attacks. It provides useful limits of iterations for a given RSA key size. These are 150 iterations for 1024-bit keys, 500 iterations for 2048-bit keys, and 2,500 iterations for 4096-bit keys. Choosing a value of 100 iterations is deemed to be a sufficiently costly, yet not excessive, value: In the worst-case scenario, the performance of name servers would be halved, regardless of key size [NSEC3-HASH-PERF].
Top   ToC   RFC6781 - Page 54

5.3.3. NSEC3 Salt

While the NSEC3 iterations parameter increases the cost of hashing a dictionary word, the NSEC3 salt reduces the lifetime for which that calculated hash can be used. A change of the salt value by the zone administrator would cause an attacker to lose all pre-calculated work for that zone. There must be a complete NSEC3 chain using the same salt value, that matches the salt value in the NSEC3PARAM record. NSEC3 salt changes do not need special rollover procedures. Since changing the salt requires that all the NSEC3 records be regenerated and thus requires generating new RRSIGs over these NSEC3 records, it makes sense to align the change of the salt with a change of the Zone Signing Key, as that process in itself already usually requires that all RRSIGs be regenerated. If there is no critical dependency on incremental signing and the zone can be signed with little effort, there is no need for such alignment.

5.3.4. Opt-Out

The Opt-Out mechanism was introduced to allow for a gradual introduction of signed records in zones that contain mostly delegation records. The use of the Opt-Out flag changes the meaning of the NSEC3 span from authoritative denial of the existence of names within the span to proof that DNSSEC is not available for the delegations within the span. This allows for the addition or removal of the delegations covered by the span without recalculating or re-signing RRs in the NSEC3 RR chain. Opt-Out is specified to be used only over delegation points and will therefore only bring relief to zones with a large number of insecure delegations. This consideration typically holds for large TLDs and similar zones; in most other circumstances, Opt-Out should not be deployed. Further considerations can be found in Section 12.2 of RFC 5155 [RFC5155].


(page 54 continued on part 4)

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