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rfc:rfc4472

Network Working Group A. Durand Request for Comments: 4472 Comcast Category: Informational J. Ihren

                                                            Autonomica
                                                             P. Savola
                                                             CSC/FUNET
                                                            April 2006
        Operational Considerations and Issues with IPv6 DNS

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 memo presents operational considerations and issues with IPv6
 Domain Name System (DNS), including a summary of special IPv6
 addresses, documentation of known DNS implementation misbehavior,
 recommendations and considerations on how to perform DNS naming for
 service provisioning and for DNS resolver IPv6 support,
 considerations for DNS updates for both the forward and reverse
 trees, and miscellaneous issues.  This memo is aimed to include a
 summary of information about IPv6 DNS considerations for those who
 have experience with IPv4 DNS.

Table of Contents

 1. Introduction ....................................................3
    1.1. Representing IPv6 Addresses in DNS Records .................3
    1.2. Independence of DNS Transport and DNS Records ..............4
    1.3. Avoiding IPv4/IPv6 Name Space Fragmentation ................4
    1.4. Query Type '*' and A/AAAA Records ..........................4
 2. DNS Considerations about Special IPv6 Addresses .................5
    2.1. Limited-Scope Addresses ....................................5
    2.2. Temporary Addresses ........................................5
    2.3. 6to4 Addresses .............................................5
    2.4. Other Transition Mechanisms ................................5
 3. Observed DNS Implementation Misbehavior .........................6
    3.1. Misbehavior of DNS Servers and Load-balancers ..............6
    3.2. Misbehavior of DNS Resolvers ...............................6

Durand, et al. Informational [Page 1] RFC 4472 Considerations with IPv6 DNS April 2006

 4. Recommendations for Service Provisioning Using DNS ..............7
    4.1. Use of Service Names instead of Node Names .................7
    4.2. Separate vs. the Same Service Names for IPv4 and IPv6 ......8
    4.3. Adding the Records Only When Fully IPv6-enabled ............8
    4.4. The Use of TTL for IPv4 and IPv6 RRs .......................9
         4.4.1. TTL with Courtesy Additional Data ...................9
         4.4.2. TTL with Critical Additional Data ..................10
    4.5. IPv6 Transport Guidelines for DNS Servers .................10
 5. Recommendations for DNS Resolver IPv6 Support ..................10
    5.1. DNS Lookups May Query IPv6 Records Prematurely ............10
    5.2. Obtaining a List of DNS Recursive Resolvers ...............12
    5.3. IPv6 Transport Guidelines for Resolvers ...................12
 6. Considerations about Forward DNS Updating ......................13
    6.1. Manual or Custom DNS Updates ..............................13
    6.2. Dynamic DNS ...............................................13
 7. Considerations about Reverse DNS Updating ......................14
    7.1. Applicability of Reverse DNS ..............................14
    7.2. Manual or Custom DNS Updates ..............................15
    7.3. DDNS with Stateless Address Autoconfiguration .............16
    7.4. DDNS with DHCP ............................................17
    7.5. DDNS with Dynamic Prefix Delegation .......................17
 8. Miscellaneous DNS Considerations ...............................18
    8.1. NAT-PT with DNS-ALG .......................................18
    8.2. Renumbering Procedures and Applications' Use of DNS .......18
 9. Acknowledgements ...............................................19
 10. Security Considerations .......................................19
 11. References ....................................................20
    11.1. Normative References .....................................20
    11.2. Informative References ...................................22
 Appendix A. Unique Local Addressing Considerations for DNS ........24
 Appendix B. Behavior of Additional Data in IPv4/IPv6
             Environments ..........................................24
    B.1. Description of Additional Data Scenarios ..................24
    B.2. Which Additional Data to Keep, If Any? ....................26
    B.3. Discussion of the Potential Problems ......................27

Durand, et al. Informational [Page 2] RFC 4472 Considerations with IPv6 DNS April 2006

1. Introduction

 This memo presents operational considerations and issues with IPv6
 DNS; it is meant to be an extensive summary and a list of pointers
 for more information about IPv6 DNS considerations for those with
 experience with IPv4 DNS.
 The purpose of this document is to give information about various
 issues and considerations related to DNS operations with IPv6; it is
 not meant to be a normative specification or standard for IPv6 DNS.
 The first section gives a brief overview of how IPv6 addresses and
 names are represented in the DNS, how transport protocols and
 resource records (don't) relate, and what IPv4/IPv6 name space
 fragmentation means and how to avoid it; all of these are described
 at more length in other documents.
 The second section summarizes the special IPv6 address types and how
 they relate to DNS.  The third section describes observed DNS
 implementation misbehaviors that have a varying effect on the use of
 IPv6 records with DNS.  The fourth section lists recommendations and
 considerations for provisioning services with DNS.  The fifth section
 in turn looks at recommendations and considerations about providing
 IPv6 support in the resolvers.  The sixth and seventh sections
 describe considerations with forward and reverse DNS updates,
 respectively.  The eighth section introduces several miscellaneous
 IPv6 issues relating to DNS for which no better place has been found
 in this memo.  Appendix A looks briefly at the requirements for
 unique local addressing.  Appendix B discusses additional data.

1.1. Representing IPv6 Addresses in DNS Records

 In the forward zones, IPv6 addresses are represented using AAAA
 records.  In the reverse zones, IPv6 address are represented using
 PTR records in the nibble format under the ip6.arpa. tree.  See
 [RFC3596] for more about IPv6 DNS usage, and [RFC3363] or [RFC3152]
 for background information.
 In particular, one should note that the use of A6 records in the
 forward tree or Bitlabels in the reverse tree is not recommended
 [RFC3363].  Using DNAME records is not recommended in the reverse
 tree in conjunction with A6 records; the document did not mean to
 take a stance on any other use of DNAME records [RFC3364].

Durand, et al. Informational [Page 3] RFC 4472 Considerations with IPv6 DNS April 2006

1.2. Independence of DNS Transport and DNS Records

 DNS has been designed to present a single, globally unique name space
 [RFC2826].  This property should be maintained, as described here and
 in Section 1.3.
 The IP version used to transport the DNS queries and responses is
 independent of the records being queried: AAAA records can be queried
 over IPv4, and A records over IPv6.  The DNS servers must not make
 any assumptions about what data to return for Answer and Authority
 sections based on the underlying transport used in a query.
 However, there is some debate whether the addresses in Additional
 section could be selected or filtered using hints obtained from which
 transport was being used; this has some obvious problems because in
 many cases the transport protocol does not correlate with the
 requests, and because a "bad" answer is in a way worse than no answer
 at all (consider the case where the client is led to believe that a
 name received in the additional record does not have any AAAA records
 at all).
 As stated in [RFC3596]:
    The IP protocol version used for querying resource records is
    independent of the protocol version of the resource records; e.g.,
    IPv4 transport can be used to query IPv6 records and vice versa.

1.3. Avoiding IPv4/IPv6 Name Space Fragmentation

 To avoid the DNS name space from fragmenting into parts where some
 parts of DNS are only visible using IPv4 (or IPv6) transport, the
 recommendation is to always keep at least one authoritative server
 IPv4-enabled, and to ensure that recursive DNS servers support IPv4.
 See DNS IPv6 transport guidelines [RFC3901] for more information.

1.4. Query Type '*' and A/AAAA Records

 QTYPE=* is typically only used for debugging or management purposes;
 it is worth keeping in mind that QTYPE=* ("ANY" queries) only return
 any available RRsets, not *all* the RRsets, because the caches do not
 necessarily have all the RRsets and have no way of guaranteeing that
 they have all the RRsets.  Therefore, to get both A and AAAA records
 reliably, two separate queries must be made.

Durand, et al. Informational [Page 4] RFC 4472 Considerations with IPv6 DNS April 2006

2. DNS Considerations about Special IPv6 Addresses

 There are a couple of IPv6 address types that are somewhat special;
 these are considered here.

2.1. Limited-Scope Addresses

 The IPv6 addressing architecture [RFC4291] includes two kinds of
 local-use addresses: link-local (fe80::/10) and site-local
 (fec0::/10).  The site-local addresses have been deprecated [RFC3879]
 but are discussed with unique local addresses in Appendix A.
 Link-local addresses should never be published in DNS (whether in
 forward or reverse tree), because they have only local (to the
 connected link) significance [WIP-DC2005].

2.2. Temporary Addresses

 Temporary addresses defined in RFC 3041 [RFC3041] (sometimes called
 "privacy addresses") use a random number as the interface identifier.
 Having DNS AAAA records that are updated to always contain the
 current value of a node's temporary address would defeat the purpose
 of the mechanism and is not recommended.  However, it would still be
 possible to return a non-identifiable name (e.g., the IPv6 address in
 hexadecimal format), as described in [RFC3041].

2.3. 6to4 Addresses

 6to4 [RFC3056] specifies an automatic tunneling mechanism that maps a
 public IPv4 address V4ADDR to an IPv6 prefix 2002:V4ADDR::/48.
 If the reverse DNS population would be desirable (see Section 7.1 for
 applicability), there are a number of possible ways to do so.
 [WIP-H2005] aims to design an autonomous reverse-delegation system
 that anyone being capable of communicating using a specific 6to4
 address would be able to set up a reverse delegation to the
 corresponding 6to4 prefix.  This could be deployed by, e.g., Regional
 Internet Registries (RIRs).  This is a practical solution, but may
 have some scalability concerns.

2.4. Other Transition Mechanisms

 6to4 is mentioned as a case of an IPv6 transition mechanism requiring
 special considerations.  In general, mechanisms that include a
 special prefix may need a custom solution; otherwise, for example,
 when IPv4 address is embedded as the suffix or not embedded at all,
 special solutions are likely not needed.

Durand, et al. Informational [Page 5] RFC 4472 Considerations with IPv6 DNS April 2006

 Note that it does not seem feasible to provide reverse DNS with
 another automatic tunneling mechanism, Teredo [RFC4380]; this is
 because the IPv6 address is based on the IPv4 address and UDP port of
 the current Network Address Translation (NAT) mapping, which is
 likely to be relatively short-lived.

3. Observed DNS Implementation Misbehavior

 Several classes of misbehavior in DNS servers, load-balancers, and
 resolvers have been observed.  Most of these are rather generic, not
 only applicable to IPv6 -- but in some cases, the consequences of
 this misbehavior are extremely severe in IPv6 environments and
 deserve to be mentioned.

3.1. Misbehavior of DNS Servers and Load-balancers

 There are several classes of misbehavior in certain DNS servers and
 load-balancers that have been noticed and documented [RFC4074]: some
 implementations silently drop queries for unimplemented DNS records
 types, or provide wrong answers to such queries (instead of a proper
 negative reply).  While typically these issues are not limited to
 AAAA records, the problems are aggravated by the fact that AAAA
 records are being queried instead of (mainly) A records.
 The problems are serious because when looking up a DNS name, typical
 getaddrinfo() implementations, with AF_UNSPEC hint given, first try
 to query the AAAA records of the name, and after receiving a
 response, query the A records.  This is done in a serial fashion --
 if the first query is never responded to (instead of properly
 returning a negative answer), significant time-outs will occur.
 In consequence, this is an enormous problem for IPv6 deployments, and
 in some cases, IPv6 support in the software has even been disabled
 due to these problems.
 The solution is to fix or retire those misbehaving implementations,
 but that is likely not going to be effective.  There are some
 possible ways to mitigate the problem, e.g., by performing the
 lookups somewhat in parallel and reducing the time-out as long as at
 least one answer has been received, but such methods remain to be
 investigated; slightly more on this is included in Section 5.

3.2. Misbehavior of DNS Resolvers

 Several classes of misbehavior have also been noticed in DNS
 resolvers [WIP-LB2005].  However, these do not seem to directly
 impair IPv6 use, and are only referred to for completeness.

Durand, et al. Informational [Page 6] RFC 4472 Considerations with IPv6 DNS April 2006

4. Recommendations for Service Provisioning Using DNS

 When names are added in the DNS to facilitate a service, there are
 several general guidelines to consider to be able to do it as
 smoothly as possible.

4.1. Use of Service Names instead of Node Names

 It makes sense to keep information about separate services logically
 separate in the DNS by using a different DNS hostname for each
 service.  There are several reasons for doing this, for example:
 o  It allows more flexibility and ease for migration of (only a part
    of) services from one node to another,
 o  It allows configuring different properties (e.g., Time to Live
    (TTL)) for each service, and
 o  It allows deciding separately for each service whether or not to
    publish the IPv6 addresses (in cases where some services are more
    IPv6-ready than others).
 Using SRV records [RFC2782] would avoid these problems.
 Unfortunately, those are not sufficiently widely used to be
 applicable in most cases.  Hence an operation technique is to use
 service names instead of node names (or "hostnames").  This
 operational technique is not specific to IPv6, but required to
 understand the considerations described in Section 4.2 and
 Section 4.3.
 For example, assume a node named "pobox.example.com" provides both
 SMTP and IMAP service.  Instead of configuring the MX records to
 point at "pobox.example.com", and configuring the mail clients to
 look up the mail via IMAP from "pobox.example.com", one could use,
 e.g., "smtp.example.com" for SMTP (for both message submission and
 mail relaying between SMTP servers) and "imap.example.com" for IMAP.
 Note that in the specific case of SMTP relaying, the server itself
 must typically also be configured to know all its names to ensure
 that loops do not occur.  DNS can provide a layer of indirection
 between service names and where the service actually is, and using
 which addresses.  (Obviously, when wanting to reach a specific node,
 one should use the hostname rather than a service name.)

Durand, et al. Informational [Page 7] RFC 4472 Considerations with IPv6 DNS April 2006

4.2. Separate vs. the Same Service Names for IPv4 and IPv6

 The service naming can be achieved in basically two ways: when a
 service is named "service.example.com" for IPv4, the IPv6-enabled
 service could either be added to "service.example.com" or added
 separately under a different name, e.g., in a sub-domain like
 "service.ipv6.example.com".
 These two methods have different characteristics.  Using a different
 name allows for easier service piloting, minimizing the disturbance
 to the "regular" users of IPv4 service; however, the service would
 not be used transparently, without the user/application explicitly
 finding it and asking for it -- which would be a disadvantage in most
 cases.  When the different name is under a sub-domain, if the
 services are deployed within a restricted network (e.g., inside an
 enterprise), it's possible to prefer them transparently, at least to
 a degree, by modifying the DNS search path; however, this is a
 suboptimal solution.  Using the same service name is the "long-term"
 solution, but may degrade performance for those clients whose IPv6
 performance is lower than IPv4, or does not work as well (see
 Section 4.3 for more).
 In most cases, it makes sense to pilot or test a service using
 separate service names, and move to the use of the same name when
 confident enough that the service level will not degrade for the
 users unaware of IPv6.

4.3. Adding the Records Only When Fully IPv6-enabled

 The recommendation is that AAAA records for a service should not be
 added to the DNS until all of following are true:
 1.  The address is assigned to the interface on the node.
 2.  The address is configured on the interface.
 3.  The interface is on a link that is connected to the IPv6
     infrastructure.
 In addition, if the AAAA record is added for the node, instead of
 service as recommended, all the services of the node should be IPv6-
 enabled prior to adding the resource record.
 For example, if an IPv6 node is isolated from an IPv6 perspective
 (e.g., it is not connected to IPv6 Internet) constraint #3 would mean
 that it should not have an address in the DNS.

Durand, et al. Informational [Page 8] RFC 4472 Considerations with IPv6 DNS April 2006

 Consider the case of two dual-stack nodes, which both are IPv6-
 enabled, but the server does not have (global) IPv6 connectivity.  As
 the client looks up the server's name, only A records are returned
 (if the recommendations above are followed), and no IPv6
 communication, which would have been unsuccessful, is even attempted.
 The issues are not always so black-and-white.  Usually, it's
 important that the service offered using both protocols is of roughly
 equal quality, using the appropriate metrics for the service (e.g.,
 latency, throughput, low packet loss, general reliability, etc.).
 This is typically very important especially for interactive or real-
 time services.  In many cases, the quality of IPv6 connectivity may
 not yet be equal to that of IPv4, at least globally; this has to be
 taken into consideration when enabling services.

4.4. The Use of TTL for IPv4 and IPv6 RRs

 The behavior of DNS caching when different TTL values are used for
 different RRsets of the same name calls for explicit discussion.  For
 example, let's consider two unrelated zone fragments:
    example.com.        300    IN    MX     foo.example.com.
    foo.example.com.    300    IN    A      192.0.2.1
    foo.example.com.    100    IN    AAAA   2001:db8::1
 ...
    child.example.com.    300  IN    NS     ns.child.example.com.
    ns.child.example.com. 300  IN    A      192.0.2.1
    ns.child.example.com. 100  IN    AAAA   2001:db8::1
 In the former case, we have "courtesy" additional data; in the
 latter, we have "critical" additional data.  See more extensive
 background discussion of additional data handling in Appendix B.

4.4.1. TTL with Courtesy Additional Data

 When a caching resolver asks for the MX record of example.com, it
 gets back "foo.example.com".  It may also get back either one or both
 of the A and AAAA records in the additional section.  The resolver
 must explicitly query for both A and AAAA records [RFC2821].
 After 100 seconds, the AAAA record is removed from the cache(s)
 because its TTL expired.  It could be argued to be useful for the
 caching resolvers to discard the A record when the shorter TTL (in
 this case, for the AAAA record) expires; this would avoid the
 situation where there would be a window of 200 seconds when
 incomplete information is returned from the cache.  Further argument

Durand, et al. Informational [Page 9] RFC 4472 Considerations with IPv6 DNS April 2006

 for discarding is that in the normal operation, the TTL values are so
 high that very likely the incurred additional queries would not be
 noticeable, compared to the obtained performance optimization.  The
 behavior in this scenario is unspecified.

4.4.2. TTL with Critical Additional Data

 The difference to courtesy additional data is that the A/AAAA records
 served by the parent zone cannot be queried explicitly.  Therefore,
 after 100 seconds the AAAA record is removed from the cache(s), but
 the A record remains.  Queries for the remaining 200 seconds
 (provided that there are no further queries from the parent that
 could refresh the caches) only return the A record, leading to a
 potential operational situation with unreachable servers.
 Similar cache flushing strategies apply in this scenario; the
 behavior is likewise unspecified.

4.5. IPv6 Transport Guidelines for DNS Servers

 As described in Section 1.3 and [RFC3901], there should continue to
 be at least one authoritative IPv4 DNS server for every zone, even if
 the zone has only IPv6 records.  (Note that obviously, having more
 servers with robust connectivity would be preferable, but this is the
 minimum recommendation; also see [RFC2182].)

5. Recommendations for DNS Resolver IPv6 Support

 When IPv6 is enabled on a node, there are several things to consider
 to ensure that the process is as smooth as possible.

5.1. DNS Lookups May Query IPv6 Records Prematurely

 The system library that implements the getaddrinfo() function for
 looking up names is a critical piece when considering the robustness
 of enabling IPv6; it may come in basically three flavors:
 1.  The system library does not know whether IPv6 has been enabled in
     the kernel of the operating system: it may start looking up AAAA
     records with getaddrinfo() and AF_UNSPEC hint when the system is
     upgraded to a system library version that supports IPv6.
 2.  The system library might start to perform IPv6 queries with
     getaddrinfo() only when IPv6 has been enabled in the kernel.
     However, this does not guarantee that there exists any useful
     IPv6 connectivity (e.g., the node could be isolated from the
     other IPv6 networks, only having link-local addresses).

Durand, et al. Informational [Page 10] RFC 4472 Considerations with IPv6 DNS April 2006

 3.  The system library might implement a toggle that would apply some
     heuristics to the "IPv6-readiness" of the node before starting to
     perform queries; for example, it could check whether only link-
     local IPv6 address(es) exists, or if at least one global IPv6
     address exists.
 First, let us consider generic implications of unnecessary queries
 for AAAA records: when looking up all the records in the DNS, AAAA
 records are typically tried first, and then A records.  These are
 done in serial, and the A query is not performed until a response is
 received to the AAAA query.  Considering the misbehavior of DNS
 servers and load-balancers, as described in Section 3.1, the lookup
 delay for AAAA may incur additional unnecessary latency, and
 introduce a component of unreliability.
 One option here could be to do the queries partially in parallel; for
 example, if the final response to the AAAA query is not received in
 0.5 seconds, start performing the A query while waiting for the
 result.  (Immediate parallelism might not be optimal, at least
 without information-sharing between the lookup threads, as that would
 probably lead to duplicate non-cached delegation chain lookups.)
 An additional concern is the address selection, which may, in some
 circumstances, prefer AAAA records over A records even when the node
 does not have any IPv6 connectivity [WIP-RDP2004].  In some cases,
 the implementation may attempt to connect or send a datagram on a
 physical link [WIP-R2006], incurring very long protocol time-outs,
 instead of quickly falling back to IPv4.
 Now, we can consider the issues specific to each of the three
 possibilities:
 In the first case, the node performs a number of completely useless
 DNS lookups as it will not be able to use the returned AAAA records
 anyway.  (The only exception is where the application desires to know
 what's in the DNS, but not use the result for communication.)  One
 should be able to disable these unnecessary queries, for both latency
 and reliability reasons.  However, as IPv6 has not been enabled, the
 connections to IPv6 addresses fail immediately, and if the
 application is programmed properly, the application can fall
 gracefully back to IPv4 [RFC4038].
 The second case is similar to the first, except it happens to a
 smaller set of nodes when IPv6 has been enabled but connectivity has
 not been provided yet.  Similar considerations apply, with the
 exception that IPv6 records, when returned, will be actually tried
 first, which may typically lead to long time-outs.

Durand, et al. Informational [Page 11] RFC 4472 Considerations with IPv6 DNS April 2006

 The third case is a bit more complex: optimizing away the DNS lookups
 with only link-locals is probably safe (but may be desirable with
 different lookup services that getaddrinfo() may support), as the
 link-locals are typically automatically generated when IPv6 is
 enabled, and do not indicate any form of IPv6 connectivity.  That is,
 performing DNS lookups only when a non-link-local address has been
 configured on any interface could be beneficial -- this would be an
 indication that the address has been configured either from a router
 advertisement, Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
 [RFC3315], or manually.  Each would indicate at least some form of
 IPv6 connectivity, even though there would not be guarantees of it.
 These issues should be analyzed at more depth, and the fixes found
 consensus on, perhaps in a separate document.

5.2. Obtaining a List of DNS Recursive Resolvers

 In scenarios where DHCPv6 is available, a host can discover a list of
 DNS recursive resolvers through the DHCPv6 "DNS Recursive Name
 Server" option [RFC3646].  This option can be passed to a host
 through a subset of DHCPv6 [RFC3736].
 The IETF is considering the development of alternative mechanisms for
 obtaining the list of DNS recursive name servers when DHCPv6 is
 unavailable or inappropriate.  No decision about taking on this
 development work has been reached as of this writing [RFC4339].
 In scenarios where DHCPv6 is unavailable or inappropriate, mechanisms
 under consideration for development include the use of [WIP-O2004]
 and the use of Router Advertisements to convey the information
 [WIP-J2006].
 Note that even though IPv6 DNS resolver discovery is a recommended
 procedure, it is not required for dual-stack nodes in dual-stack
 networks as IPv6 DNS records can be queried over IPv4 as well as
 IPv6.  Obviously, nodes that are meant to function without manual
 configuration in IPv6-only networks must implement the DNS resolver
 discovery function.

5.3. IPv6 Transport Guidelines for Resolvers

 As described in Section 1.3 and [RFC3901], the recursive resolvers
 should be IPv4-only or dual-stack to be able to reach any IPv4-only
 DNS server.  Note that this requirement is also fulfilled by an IPv6-
 only stub resolver pointing to a dual-stack recursive DNS resolver.

Durand, et al. Informational [Page 12] RFC 4472 Considerations with IPv6 DNS April 2006

6. Considerations about Forward DNS Updating

 While the topic of how to enable updating the forward DNS, i.e., the
 mapping from names to the correct new addresses, is not specific to
 IPv6, it should be considered especially due to the advent of
 Stateless Address Autoconfiguration [RFC2462].
 Typically, forward DNS updates are more manageable than doing them in
 the reverse DNS, because the updater can often be assumed to "own" a
 certain DNS name -- and we can create a form of security relationship
 with the DNS name and the node that is allowed to update it to point
 to a new address.
 A more complex form of DNS updates -- adding a whole new name into a
 DNS zone, instead of updating an existing name -- is considered out
 of scope for this memo as it could require zone-wide authentication.
 Adding a new name in the forward zone is a problem that is still
 being explored with IPv4, and IPv6 does not seem to add much new in
 that area.

6.1. Manual or Custom DNS Updates

 The DNS mappings can also be maintained by hand, in a semi-automatic
 fashion or by running non-standardized protocols.  These are not
 considered at more length in this memo.

6.2. Dynamic DNS

 Dynamic DNS updates (DDNS) [RFC2136] [RFC3007] is a standardized
 mechanism for dynamically updating the DNS.  It works equally well
 with Stateless Address Autoconfiguration (SLAAC), DHCPv6, or manual
 address configuration.  It is important to consider how each of these
 behave if IP address-based authentication, instead of stronger
 mechanisms [RFC3007], was used in the updates.
 1.  Manual addresses are static and can be configured.
 2.  DHCPv6 addresses could be reasonably static or dynamic, depending
     on the deployment, and could or could not be configured on the
     DNS server for the long term.
 3.  SLAAC addresses are typically stable for a long time, but could
     require work to be configured and maintained.
 As relying on IP addresses for Dynamic DNS is rather insecure at
 best, stronger authentication should always be used; however, this
 requires that the authorization keying will be explicitly configured
 using unspecified operational methods.

Durand, et al. Informational [Page 13] RFC 4472 Considerations with IPv6 DNS April 2006

 Note that with DHCP it is also possible that the DHCP server updates
 the DNS, not the host.  The host might only indicate in the DHCP
 exchange which hostname it would prefer, and the DHCP server would
 make the appropriate updates.  Nonetheless, while this makes setting
 up a secure channel between the updater and the DNS server easier, it
 does not help much with "content" security, i.e., whether the
 hostname was acceptable -- if the DNS server does not include
 policies, they must be included in the DHCP server (e.g., a regular
 host should not be able to state that its name is "www.example.com").
 DHCP-initiated DDNS updates have been extensively described in
 [WIP-SV2005], [WIP-S2005a], and [WIP-S2005b].
 The nodes must somehow be configured with the information about the
 servers where they will attempt to update their addresses, sufficient
 security material for authenticating themselves to the server, and
 the hostname they will be updating.  Unless otherwise configured, the
 first could be obtained by looking up the authoritative name servers
 for the hostname; the second must be configured explicitly unless one
 chooses to trust the IP address-based authentication (not a good
 idea); and lastly, the nodename is typically pre-configured somehow
 on the node, e.g., at install time.
 Care should be observed when updating the addresses not to use longer
 TTLs for addresses than are preferred lifetimes for the addresses, so
 that if the node is renumbered in a managed fashion, the amount of
 stale DNS information is kept to the minimum.  That is, if the
 preferred lifetime of an address expires, the TTL of the record needs
 to be modified unless it was already done before the expiration.  For
 better flexibility, the DNS TTL should be much shorter (e.g., a half
 or a third) than the lifetime of an address; that way, the node can
 start lowering the DNS TTL if it seems like the address has not been
 renewed/refreshed in a while.  Some discussion on how an
 administrator could manage the DNS TTL is included in [RFC4192]; this
 could be applied to (smart) hosts as well.

7. Considerations about Reverse DNS Updating

 Updating the reverse DNS zone may be difficult because of the split
 authority over an address.  However, first we have to consider the
 applicability of reverse DNS in the first place.

7.1. Applicability of Reverse DNS

 Today, some applications use reverse DNS either to look up some hints
 about the topological information associated with an address (e.g.,
 resolving web server access logs) or (as a weak form of a security
 check) to get a feel whether the user's network administrator has

Durand, et al. Informational [Page 14] RFC 4472 Considerations with IPv6 DNS April 2006

 "authorized" the use of the address (on the premise that adding a
 reverse record for an address would signal some form of
 authorization).
 One additional, maybe slightly more useful usage is ensuring that the
 reverse and forward DNS contents match (by looking up the pointer to
 the name by the IP address from the reverse tree, and ensuring that a
 record under the name in the forward tree points to the IP address)
 and correspond to a configured name or domain.  As a security check,
 it is typically accompanied by other mechanisms, such as a user/
 password login; the main purpose of the reverse+forward DNS check is
 to weed out the majority of unauthorized users, and if someone
 managed to bypass the checks, he would still need to authenticate
 "properly".
 It may also be desirable to store IPsec keying material corresponding
 to an IP address in the reverse DNS, as justified and described in
 [RFC4025].
 It is not clear whether it makes sense to require or recommend that
 reverse DNS records be updated.  In many cases, it would just make
 more sense to use proper mechanisms for security (or topological
 information lookup) in the first place.  At minimum, the applications
 that use it as a generic authorization (in the sense that a record
 exists at all) should be modified as soon as possible to avoid such
 lookups completely.
 The applicability is discussed at more length in [WIP-S2005c].

7.2. Manual or Custom DNS Updates

 Reverse DNS can of course be updated using manual or custom methods.
 These are not further described here, except for one special case.
 One way to deploy reverse DNS would be to use wildcard records, for
 example, by configuring one name for a subnet (/64) or a site (/48).
 As a concrete example, a site (or the site's ISP) could configure the
 reverses of the prefix 2001:db8:f00::/48 to point to one name using a
 wildcard record like "*.0.0.f.0.8.b.d.0.1.0.0.2.ip6.arpa. IN PTR
 site.example.com.".  Naturally, such a name could not be verified
 from the forward DNS, but would at least provide some form of
 "topological information" or "weak authorization" if that is really
 considered to be useful.  Note that this is not actually updating the
 DNS as such, as the whole point is to avoid DNS updates completely by
 manually configuring a generic name.

Durand, et al. Informational [Page 15] RFC 4472 Considerations with IPv6 DNS April 2006

7.3. DDNS with Stateless Address Autoconfiguration

 Dynamic reverse DNS with SLAAC is simpler than forward DNS updates in
 some regard, while being more difficult in another, as described
 below.
 The address space administrator decides whether or not the hosts are
 trusted to update their reverse DNS records.  If they are trusted and
 deployed at the same site (e.g., not across the Internet), a simple
 address-based authorization is typically sufficient (i.e., check that
 the DNS update is done from the same IP address as the record being
 updated); stronger security can also be used [RFC3007].  If they
 aren't allowed to update the reverses, no update can occur.  However,
 such address-based update authorization operationally requires that
 ingress filtering [RFC3704] has been set up at the border of the site
 where the updates occur, and as close to the updater as possible.
 Address-based authorization is simpler with reverse DNS (as there is
 a connection between the record and the address) than with forward
 DNS.  However, when a stronger form of security is used, forward DNS
 updates are simpler to manage because the host can be assumed to have
 an association with the domain.  Note that the user may roam to
 different networks and does not necessarily have any association with
 the owner of that address space.  So, assuming a stronger form of
 authorization for reverse DNS updates than an address association is
 generally infeasible.
 Moreover, the reverse zones must be cleaned up by an unspecified
 janitorial process: the node does not typically know a priori that it
 will be disconnected, and it cannot send a DNS update using the
 correct source address to remove a record.
 A problem with defining the clean-up process is that it is difficult
 to ensure that a specific IP address and the corresponding record are
 no longer being used.  Considering the huge address space, and the
 unlikelihood of collision within 64 bits of the interface
 identifiers, a process that would remove the record after no traffic
 has been seen from a node in a long period of time (e.g., a month or
 year) might be one possible approach.
 To insert or update the record, the node must discover the DNS server
 to send the update to somehow, similar to as discussed in
 Section 6.2.  One way to automate this is looking up the DNS server
 authoritative (e.g., through SOA record) for the IP address being
 updated, but the security material (unless the IP address-based
 authorization is trusted) must also be established by some other
 means.

Durand, et al. Informational [Page 16] RFC 4472 Considerations with IPv6 DNS April 2006

 One should note that Cryptographically Generated Addresses (CGAs)
 [RFC3972] may require a slightly different kind of treatment.  CGAs
 are addresses where the interface identifier is calculated from a
 public key, a modifier (used as a nonce), the subnet prefix, and
 other data.  Depending on the usage profile, CGAs might or might not
 be changed periodically due to, e.g., privacy reasons.  As the CGA
 address is not predictable, a reverse record can only reasonably be
 inserted in the DNS by the node that generates the address.

7.4. DDNS with DHCP

 With DHCPv4, the reverse DNS name is typically already inserted to
 the DNS that reflects the name (e.g., "dhcp-67.example.com").  One
 can assume similar practice may become commonplace with DHCPv6 as
 well; all such mappings would be pre-configured and would require no
 updating.
 If a more explicit control is required, similar considerations as
 with SLAAC apply, except for the fact that typically one must update
 a reverse DNS record instead of inserting one (if an address
 assignment policy that reassigns disused addresses is adopted) and
 updating a record seems like a slightly more difficult thing to
 secure.  However, it is yet uncertain how DHCPv6 is going to be used
 for address assignment.
 Note that when using DHCP, either the host or the DHCP server could
 perform the DNS updates; see the implications in Section 6.2.
 If disused addresses were to be reassigned, host-based DDNS reverse
 updates would need policy considerations for DNS record modification,
 as noted above.  On the other hand, if disused address were not to be
 assigned, host-based DNS reverse updates would have similar
 considerations as SLAAC in Section 7.3.  Server-based updates have
 similar properties except that the janitorial process could be
 integrated with DHCP address assignment.

7.5. DDNS with Dynamic Prefix Delegation

 In cases where a prefix, instead of an address, is being used and
 updated, one should consider what is the location of the server where
 DDNS updates are made.  That is, where the DNS server is located:
 1.  At the same organization as the prefix delegator.
 2.  At the site where the prefixes are delegated to.  In this case,
     the authority of the DNS reverse zone corresponding to the
     delegated prefix is also delegated to the site.

Durand, et al. Informational [Page 17] RFC 4472 Considerations with IPv6 DNS April 2006

 3.  Elsewhere; this implies a relationship between the site and where
     the DNS server is located, and such a relationship should be
     rather straightforward to secure as well.  Like in the previous
     case, the authority of the DNS reverse zone is also delegated.
 In the first case, managing the reverse DNS (delegation) is simpler
 as the DNS server and the prefix delegator are in the same
 administrative domain (as there is no need to delegate anything at
 all); alternatively, the prefix delegator might forgo DDNS reverse
 capability altogether, and use, e.g., wildcard records (as described
 in Section 7.2).  In the other cases, it can be slightly more
 difficult, particularly as the site will have to configure the DNS
 server to be authoritative for the delegated reverse zone, implying
 automatic configuration of the DNS server -- as the prefix may be
 dynamic.
 Managing the DDNS reverse updates is typically simple in the second
 case, as the updated server is located at the local site, and
 arguably IP address-based authentication could be sufficient (or if
 not, setting up security relationships would be simpler).  As there
 is an explicit (security) relationship between the parties in the
 third case, setting up the security relationships to allow reverse
 DDNS updates should be rather straightforward as well (but IP
 address-based authentication might not be acceptable).  In the first
 case, however, setting up and managing such relationships might be a
 lot more difficult.

8. Miscellaneous DNS Considerations

 This section describes miscellaneous considerations about DNS that
 seem related to IPv6, for which no better place has been found in
 this document.

8.1. NAT-PT with DNS-ALG

 The DNS-ALG component of NAT-PT [RFC2766] mangles A records to look
 like AAAA records to the IPv6-only nodes.  Numerous problems have
 been identified with [WIP-AD2005].  This is a strong reason not to
 use NAT-PT in the first place.

8.2. Renumbering Procedures and Applications' Use of DNS

 One of the most difficult problems of systematic IP address
 renumbering procedures [RFC4192] is that an application that looks up
 a DNS name disregards information such as TTL, and uses the result
 obtained from DNS as long as it happens to be stored in the memory of
 the application.  For applications that run for a long time, this

Durand, et al. Informational [Page 18] RFC 4472 Considerations with IPv6 DNS April 2006

 could be days, weeks, or even months.  Some applications may be
 clever enough to organize the data structures and functions in such a
 manner that lookups get refreshed now and then.
 While the issue appears to have a clear solution, "fix the
 applications", practically, this is not reasonable immediate advice.
 The TTL information is not typically available in the APIs and
 libraries (so, the advice becomes "fix the applications, APIs, and
 libraries"), and a lot more analysis is needed on how to practically
 go about to achieve the ultimate goal of avoiding using the names
 longer than expected.

9. Acknowledgements

 Some recommendations (Section 4.3, Section 5.1) about IPv6 service
 provisioning were moved here from [RFC4213] by Erik Nordmark and Bob
 Gilligan.  Havard Eidnes and Michael Patton provided useful feedback
 and improvements.  Scott Rose, Rob Austein, Masataka Ohta, and Mark
 Andrews helped in clarifying the issues regarding additional data and
 the use of TTL.  Jefsey Morfin, Ralph Droms, Peter Koch, Jinmei
 Tatuya, Iljitsch van Beijnum, Edward Lewis, and Rob Austein provided
 useful feedback during the WG last call.  Thomas Narten provided
 extensive feedback during the IESG evaluation.

10. Security Considerations

 This document reviews the operational procedures for IPv6 DNS
 operations and does not have security considerations in itself.
 However, it is worth noting that in particular with Dynamic DNS
 updates, security models based on the source address validation are
 very weak and cannot be recommended -- they could only be considered
 in the environments where ingress filtering [RFC3704] has been
 deployed.  On the other hand, it should be noted that setting up an
 authorization mechanism (e.g., a shared secret, or public-private
 keys) between a node and the DNS server has to be done manually, and
 may require quite a bit of time and expertise.
 To re-emphasize what was already stated, the reverse+forward DNS
 check provides very weak security at best, and the only
 (questionable) security-related use for them may be in conjunction
 with other mechanisms when authenticating a user.

Durand, et al. Informational [Page 19] RFC 4472 Considerations with IPv6 DNS April 2006

11. References

11.1. Normative References

 [RFC1034]     Mockapetris, P., "Domain names - concepts and
               facilities", STD 13, RFC 1034, November 1987.
 [RFC2136]     Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
               "Dynamic Updates in the Domain Name System (DNS
               UPDATE)", RFC 2136, April 1997.
 [RFC2181]     Elz, R. and R. Bush, "Clarifications to the DNS
               Specification", RFC 2181, July 1997.
 [RFC2182]     Elz, R., Bush, R., Bradner, S., and M. Patton,
               "Selection and Operation of Secondary DNS Servers",
               BCP 16, RFC 2182, July 1997.
 [RFC2462]     Thomson, S. and T. Narten, "IPv6 Stateless Address
               Autoconfiguration", RFC 2462, December 1998.
 [RFC2671]     Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
               RFC 2671, August 1999.
 [RFC2821]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
               April 2001.
 [RFC3007]     Wellington, B., "Secure Domain Name System (DNS)
               Dynamic Update", RFC 3007, November 2000.
 [RFC3041]     Narten, T. and R. Draves, "Privacy Extensions for
               Stateless Address Autoconfiguration in IPv6", RFC 3041,
               January 2001.
 [RFC3056]     Carpenter, B. and K. Moore, "Connection of IPv6 Domains
               via IPv4 Clouds", RFC 3056, February 2001.
 [RFC3152]     Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152,
               August 2001.
 [RFC3315]     Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
               and M. Carney, "Dynamic Host Configuration Protocol for
               IPv6 (DHCPv6)", RFC 3315, July 2003.
 [RFC3363]     Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
               Hain, "Representing Internet Protocol version 6 (IPv6)
               Addresses in the Domain Name System (DNS)", RFC 3363,
               August 2002.

Durand, et al. Informational [Page 20] RFC 4472 Considerations with IPv6 DNS April 2006

 [RFC3364]     Austein, R., "Tradeoffs in Domain Name System (DNS)
               Support for Internet Protocol version 6 (IPv6)",
               RFC 3364, August 2002.
 [RFC3596]     Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
               "DNS Extensions to Support IP Version 6", RFC 3596,
               October 2003.
 [RFC3646]     Droms, R., "DNS Configuration options for Dynamic Host
               Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
               December 2003.
 [RFC3736]     Droms, R., "Stateless Dynamic Host Configuration
               Protocol (DHCP) Service for IPv6", RFC 3736,
               April 2004.
 [RFC3879]     Huitema, C. and B. Carpenter, "Deprecating Site Local
               Addresses", RFC 3879, September 2004.
 [RFC3901]     Durand, A. and J. Ihren, "DNS IPv6 Transport
               Operational Guidelines", BCP 91, RFC 3901,
               September 2004.
 [RFC4038]     Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E.
               Castro, "Application Aspects of IPv6 Transition",
               RFC 4038, March 2005.
 [RFC4074]     Morishita, Y. and T. Jinmei, "Common Misbehavior
               Against DNS Queries for IPv6 Addresses", RFC 4074,
               May 2005.
 [RFC4192]     Baker, F., Lear, E., and R. Droms, "Procedures for
               Renumbering an IPv6 Network without a Flag Day",
               RFC 4192, September 2005.
 [RFC4193]     Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
               Addresses", RFC 4193, October 2005.
 [RFC4291]     Hinden, R. and S. Deering, "IP Version 6 Addressing
               Architecture", RFC 4291, February 2006.
 [RFC4339]     Jeong, J., Ed., "IPv6 Host Configuration of DNS Server
               Information Approaches", RFC 4339, February 2006.

Durand, et al. Informational [Page 21] RFC 4472 Considerations with IPv6 DNS April 2006

11.2. Informative References

 [RFC2766]     Tsirtsis, G. and P. Srisuresh, "Network Address
               Translation - Protocol Translation (NAT-PT)", RFC 2766,
               February 2000.
 [RFC2782]     Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR
               for specifying the location of services (DNS SRV)",
               RFC 2782, February 2000.
 [RFC2826]     Internet Architecture Board, "IAB Technical Comment on
               the Unique DNS Root", RFC 2826, May 2000.
 [RFC3704]     Baker, F. and P. Savola, "Ingress Filtering for
               Multihomed Networks", BCP 84, RFC 3704, March 2004.
 [RFC3972]     Aura, T., "Cryptographically Generated Addresses
               (CGA)", RFC 3972, March 2005.
 [RFC4025]     Richardson, M., "A Method for Storing IPsec Keying
               Material in DNS", RFC 4025, March 2005.
 [RFC4213]     Nordmark, E. and R. Gilligan, "Basic Transition
               Mechanisms for IPv6 Hosts and Routers", RFC 4213,
               October 2005.
 [RFC4215]     Wiljakka, J., "Analysis on IPv6 Transition in Third
               Generation Partnership Project (3GPP) Networks",
               RFC 4215, October 2005.
 [RFC4380]     Huitema, C., "Teredo: Tunneling IPv6 over UDP through
               Network Address Translations (NATs)", RFC 4380,
               February 2006.
 [TC-TEST]     Jinmei, T., "Thread "RFC2181 section 9.1: TC bit
               handling and additional data" on DNSEXT mailing list,
               Message-
               Id:y7vek9j9hyo.wl%jinmei@isl.rdc.toshiba.co.jp", August
               1, 2005, <http://ops.ietf.org/lists/namedroppers/
               namedroppers.2005/msg01102.html>.
 [WIP-AD2005]  Aoun, C. and E. Davies, "Reasons to Move NAT-PT to
               Experimental", Work in Progress, October 2005.
 [WIP-DC2005]  Durand, A. and T. Chown, "To publish, or not to
               publish, that is the question", Work in Progress,
               October 2005.

Durand, et al. Informational [Page 22] RFC 4472 Considerations with IPv6 DNS April 2006

 [WIP-H2005]   Huston, G., "6to4 Reverse DNS Delegation
               Specification", Work in Progress, November 2005.
 [WIP-J2006]   Jeong, J., "IPv6 Router Advertisement Option for DNS
               Configuration", Work in Progress, January 2006.
 [WIP-LB2005]  Larson, M. and P. Barber, "Observed DNS Resolution
               Misbehavior", Work in Progress, February 2006.
 [WIP-O2004]   Ohta, M., "Preconfigured DNS Server Addresses", Work in
               Progress, February 2004.
 [WIP-R2006]   Roy, S., "IPv6 Neighbor Discovery On-Link Assumption
               Considered Harmful", Work in Progress, January 2006.
 [WIP-RDP2004] Roy, S., Durand, A., and J. Paugh, "Issues with Dual
               Stack IPv6 on by Default", Work in Progress, July 2004.
 [WIP-S2005a]  Stapp, M., "The DHCP Client FQDN Option", Work in
               Progress, March 2006.
 [WIP-S2005b]  Stapp, M., "A DNS RR for Encoding DHCP Information
               (DHCID RR)", Work in Progress, March 2006.
 [WIP-S2005c]  Senie, D., "Encouraging the use of DNS IN-ADDR
               Mapping", Work in Progress, August 2005.
 [WIP-SV2005]  Stapp, M. and B. Volz, "Resolution of FQDN Conflicts
               among DHCP Clients", Work in Progress, March 2006.

Durand, et al. Informational [Page 23] RFC 4472 Considerations with IPv6 DNS April 2006

Appendix A. Unique Local Addressing Considerations for DNS

 Unique local addresses [RFC4193] have replaced the now-deprecated
 site-local addresses [RFC3879].  From the perspective of the DNS, the
 locally generated unique local addresses (LUL) and site-local
 addresses have similar properties.
 The interactions with DNS come in two flavors: forward and reverse
 DNS.
 To actually use local addresses within a site, this implies the
 deployment of a "split-faced" or a fragmented DNS name space, for the
 zones internal to the site, and the outsiders' view to it.  The
 procedures to achieve this are not elaborated here.  The implication
 is that local addresses must not be published in the public DNS.
 To facilitate reverse DNS (if desired) with local addresses, the stub
 resolvers must look for DNS information from the local DNS servers,
 not, e.g., starting from the root servers, so that the local
 information may be provided locally.  Note that the experience of
 private addresses in IPv4 has shown that the root servers get loaded
 for requests for private address lookups in any case.  This
 requirement is discussed in [RFC4193].

Appendix B. Behavior of Additional Data in IPv4/IPv6 Environments

 DNS responses do not always fit in a single UDP packet.  We'll
 examine the cases that happen when this is due to too much data in
 the Additional section.

B.1. Description of Additional Data Scenarios

 There are two kinds of additional data:
 1.  "critical" additional data; this must be included in all
     scenarios, with all the RRsets, and
 2.  "courtesy" additional data; this could be sent in full, with only
     a few RRsets, or with no RRsets, and can be fetched separately as
     well, but at the cost of additional queries.
 The responding server can algorithmically determine which type the
 additional data is by checking whether it's at or below a zone cut.
 Only those additional data records (even if sometimes carelessly
 termed "glue") are considered "critical" or real "glue" if and only
 if they meet the above-mentioned condition, as specified in Section
 4.2.1 of [RFC1034].

Durand, et al. Informational [Page 24] RFC 4472 Considerations with IPv6 DNS April 2006

 Remember that resource record sets (RRsets) are never "broken up", so
 if a name has 4 A records and 5 AAAA records, you can either return
 all 9, all 4 A records, all 5 AAAA records, or nothing.  In
 particular, notice that for the "critical" additional data getting
 all the RRsets can be critical.
 In particular, [RFC2181] specifies (in Section 9) that:
 a.  if all the "critical" RRsets do not fit, the sender should set
     the TC bit, and the recipient should discard the whole response
     and retry using mechanism allowing larger responses such as TCP.
 b.  "courtesy" additional data should not cause the setting of the TC
     bit, but instead all the non-fitting additional data RRsets
     should be removed.
 An example of the "courtesy" additional data is A/AAAA records in
 conjunction with MX records as shown in Section 4.4; an example of
 the "critical" additional data is shown below (where getting both the
 A and AAAA RRsets is critical with respect to the NS RR):
    child.example.com.    IN   NS ns.child.example.com.
    ns.child.example.com. IN    A 192.0.2.1
    ns.child.example.com. IN AAAA 2001:db8::1
 When there is too much "courtesy" additional data, at least the non-
 fitting RRsets should be removed [RFC2181]; however, as the
 additional data is not critical, even all of it could be safely
 removed.
 When there is too much "critical" additional data, TC bit will have
 to be set, and the recipient should ignore the response and retry
 using TCP; if some data were to be left in the UDP response, the
 issue is which data could be retained.
 However, the practice may differ from the specification.  Testing and
 code analysis of three recent implementations [TC-TEST] confirm this.
 None of the tested implementations have a strict separation of
 critical and courtesy additional data, while some forms of additional
 data may be treated preferably.  All the implementations remove some
 (critical or courtesy) additional data RRsets without setting the TC
 bit if the response would not otherwise fit.
 Failing to discard the response with the TC bit or omitting critical
 information but not setting the TC bit lead to an unrecoverable
 problem.  Omitting only some of the RRsets if all would not fit (but
 not setting the TC bit) leads to a performance problem.  These are
 discussed in the next two subsections.

Durand, et al. Informational [Page 25] RFC 4472 Considerations with IPv6 DNS April 2006

B.2. Which Additional Data to Keep, If Any?

 NOTE: omitting some critical additional data instead of setting the
 TC bit violates a 'should' in Section 9 of RFC2181.  However, as many
 implementations still do that [TC-TEST], operators need to understand
 its implications, and we describe that behavior as well.
 If the implementation decides to keep as much data (whether
 "critical" or "courtesy") as possible in the UDP responses, it might
 be tempting to use the transport of the DNS query as a hint in either
 of these cases: return the AAAA records if the query was done over
 IPv6, or return the A records if the query was done over IPv4.
 However, this breaks the model of independence of DNS transport and
 resource records, as noted in Section 1.2.
 With courtesy additional data, as long as enough RRsets will be
 removed so that TC will not be set, it is allowed to send as many
 complete RRsets as the implementations prefers.  However, the
 implementations are also free to omit all such RRsets, even if
 complete.  Omitting all the RRsets (when removing only some would
 suffice) may create a performance penalty, whereby the client may
 need to issue one or more additional queries to obtain necessary
 and/or consistent information.
 With critical additional data, the alternatives are either returning
 nothing (and absolutely requiring a retry with TCP) or returning
 something (working also in the case if the recipient does not discard
 the response and retry using TCP) in addition to setting the TC bit.
 If the process for selecting "something" from the critical data would
 otherwise be practically "flipping the coin" between A and AAAA
 records, it could be argued that if one looked at the transport of
 the query, it would have a larger possibility of being right than
 just 50/50.  In other words, if the returned critical additional data
 would have to be selected somehow, using something more sophisticated
 than a random process would seem justifiable.
 That is, leaving in some intelligently selected critical additional
 data is a trade-off between creating an optimization for those
 resolvers that ignore the "should discard" recommendation and causing
 a protocol problem by propagating inconsistent information about
 "critical" records in the caches.
 Similarly, leaving in the complete courtesy additional data RRsets
 instead of removing all the RRsets is a performance trade-off as
 described in the next section.

Durand, et al. Informational [Page 26] RFC 4472 Considerations with IPv6 DNS April 2006

B.3. Discussion of the Potential Problems

 As noted above, the temptation for omitting only some of the
 additional data could be problematic.  This is discussed more below.
 For courtesy additional data, this causes a potential performance
 problem as this requires that the clients issue re-queries for the
 potentially omitted RRsets.  For critical additional data, this
 causes a potential unrecoverable problem if the response is not
 discarded and the query not re-tried with TCP, as the nameservers
 might be reachable only through the omitted RRsets.
 If an implementation would look at the transport used for the query,
 it is worth remembering that often the host using the records is
 different from the node requesting them from the authoritative DNS
 server (or even a caching resolver).  So, whichever version the
 requestor (e.g., a recursive server in the middle) uses makes no
 difference to the ultimate user of the records, whose transport
 capabilities might differ from those of the requestor.  This might
 result in, e.g., inappropriately returning A records to an IPv6-only
 node, going through a translation, or opening up another IP-level
 session (e.g., a Packet Data Protocol (PDP) context [RFC4215]).
 Therefore, at least in many scenarios, it would be very useful if the
 information returned would be consistent and complete -- or if that
 is not feasible, leave it to the client to query again.
 The problem of too much additional data seems to be an operational
 one: the zone administrator entering too many records that will be
 returned truncated (or missing some RRsets, depending on
 implementations) to the users.  A protocol fix for this is using
 Extension Mechanisms for DNS (EDNS0) [RFC2671] to signal the capacity
 for larger UDP packet sizes, pushing up the relevant threshold.
 Further, DNS server implementations should omit courtesy additional
 data completely rather than including only some RRsets [RFC2181].  An
 operational fix for this is having the DNS server implementations
 return a warning when the administrators create zones that would
 result in too much additional data being returned.  Further, DNS
 server implementations should warn of or disallow such zone
 configurations that are recursive or otherwise difficult to manage by
 the protocol.

Durand, et al. Informational [Page 27] RFC 4472 Considerations with IPv6 DNS April 2006

Authors' Addresses

 Alain Durand
 Comcast
 1500 Market St.
 Philadelphia, PA  19102
 USA
 EMail: Alain_Durand@cable.comcast.com
 Johan Ihren
 Autonomica
 Bellmansgatan 30
 SE-118 47 Stockholm
 Sweden
 EMail: johani@autonomica.se
 Pekka Savola
 CSC/FUNET
 Espoo
 Finland
 EMail: psavola@funet.fi

Durand, et al. Informational [Page 28] RFC 4472 Considerations with IPv6 DNS April 2006

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
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Durand, et al. Informational [Page 29]

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