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rfc:rfc6147

Internet Engineering Task Force (IETF) M. Bagnulo Request for Comments: 6147 UC3M Category: Standards Track A. Sullivan ISSN: 2070-1721 Shinkuro

                                                           P. Matthews
                                                        Alcatel-Lucent
                                                        I. van Beijnum
                                                        IMDEA Networks
                                                            April 2011
       DNS64: DNS Extensions for Network Address Translation
                 from IPv6 Clients to IPv4 Servers

Abstract

 DNS64 is a mechanism for synthesizing AAAA records from A records.
 DNS64 is used with an IPv6/IPv4 translator to enable client-server
 communication between an IPv6-only client and an IPv4-only server,
 without requiring any changes to either the IPv6 or the IPv4 node,
 for the class of applications that work through NATs.  This document
 specifies DNS64, and provides suggestions on how it should be
 deployed in conjunction with IPv6/IPv4 translators.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6147.

Bagnulo, et al. Standards Track [Page 1] RFC 6147 DNS64 April 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Bagnulo, et al. Standards Track [Page 2] RFC 6147 DNS64 April 2011

Table of Contents

 1. Introduction ....................................................4
 2. Overview ........................................................5
 3. Background to DNS64-DNSSEC Interaction ..........................7
 4. Terminology .....................................................9
 5. DNS64 Normative Specification ..................................10
    5.1. Resolving AAAA Queries and the Answer Section .............11
         5.1.1. The Answer when There is AAAA Data Available .......11
         5.1.2. The Answer when There is an Error ..................11
         5.1.3. Dealing with Timeouts ..............................12
         5.1.4. Special Exclusion Set for AAAA Records .............12
         5.1.5. Dealing with CNAME and DNAME .......................12
         5.1.6. Data for the Answer when Performing Synthesis ......13
         5.1.7. Performing the Synthesis ...........................13
         5.1.8. Querying in Parallel ...............................14
    5.2. Generation of the IPv6 Representations of IPv4 Addresses ..14
    5.3. Handling Other Resource Records and the Additional
         Section ...................................................15
         5.3.1. PTR Resource Record ................................15
         5.3.2. Handling the Additional Section ....................16
         5.3.3. Other Resource Records .............................17
    5.4. Assembling a Synthesized Response to a AAAA Query .........17
    5.5. DNSSEC Processing: DNS64 in Validating Resolver Mode ......17
 6. Deployment Notes ...............................................19
    6.1. DNS Resolvers and DNS64 ...................................19
    6.2. DNSSEC Validators and DNS64 ...............................19
    6.3. DNS64 and Multihomed and Dual-Stack Hosts .................19
         6.3.1. IPv6 Multihomed Hosts ..............................19
         6.3.2. Accidental Dual-Stack DNS64 Use ....................20
         6.3.3. Intentional Dual-Stack DNS64 Use ...................21
 7. Deployment Scenarios and Examples ..............................21
    7.1. Example of "an IPv6 Network to the IPv4 Internet"
         Setup with DNS64 in DNS Server Mode .......................22
    7.2. Example of "an IPv6 Network to the IPv4 Internet"
         Setup with DNS64 in Stub-Resolver Mode ....................23
    7.3. Example of "the IPv6 Internet to an IPv4 Network"
         Setup with DNS64 in DNS Server Mode .......................24
 8. Security Considerations ........................................27
 9. Contributors ...................................................27
 10. Acknowledgements ..............................................27
 11. References ....................................................28
    11.1. Normative References .....................................28
    11.2. Informative References ...................................28
 Appendix A.  Motivations and Implications of Synthesizing AAAA
              Resource Records when Real AAAA Resource Records
              Exist ................................................30

Bagnulo, et al. Standards Track [Page 3] RFC 6147 DNS64 April 2011

1. Introduction

 This document specifies DNS64, a mechanism that is part of the
 toolbox for IPv4-IPv6 transition and coexistence.  DNS64, used
 together with an IPv6/IPv4 translator such as stateful NAT64
 [RFC6146], allows an IPv6-only client to initiate communications by
 name to an IPv4-only server.
 DNS64 is a mechanism for synthesizing AAAA resource records (RRs)
 from A RRs.  A synthetic AAAA RR created by the DNS64 from an
 original A RR contains the same owner name of the original A RR, but
 it contains an IPv6 address instead of an IPv4 address.  The IPv6
 address is an IPv6 representation of the IPv4 address contained in
 the original A RR.  The IPv6 representation of the IPv4 address is
 algorithmically generated from the IPv4 address returned in the A RR
 and a set of parameters configured in the DNS64 (typically, an IPv6
 prefix used by IPv6 representations of IPv4 addresses and,
 optionally, other parameters).
 Together with an IPv6/IPv4 translator, these two mechanisms allow an
 IPv6-only client to initiate communications to an IPv4-only server
 using the Fully Qualified Domain Name (FQDN) of the server.
 These mechanisms are expected to play a critical role in the IPv4-
 IPv6 transition and coexistence.  Due to IPv4 address depletion, it
 is likely that in the future, many IPv6-only clients will want to
 connect to IPv4-only servers.  In the typical case, the approach only
 requires the deployment of IPv6/IPv4 translators that connect an
 IPv6-only network to an IPv4-only network, along with the deployment
 of one or more DNS64-enabled name servers.  However, some features
 require performing the DNS64 function directly in the end hosts
 themselves.
 This document is structured as follows: Section 2 provides a
 non-normative overview of the behavior of DNS64.  Section 3 provides
 a non-normative background required to understand the interaction
 between DNS64 and DNS Security Extensions (DNSSEC).  The normative
 specification of DNS64 is provided in Sections 4, 5, and 6.
 Section 4 defines the terminology, Section 5 is the actual DNS64
 specification, and Section 6 covers deployment issues.  Section 7 is
 non-normative and provides a set of examples and typical deployment
 scenarios.

Bagnulo, et al. Standards Track [Page 4] RFC 6147 DNS64 April 2011

2. Overview

 This section provides an introduction to the DNS64 mechanism.
 We assume that we have one or more IPv6/IPv4 translator boxes
 connecting an IPv4 network and an IPv6 network.  The IPv6/IPv4
 translator device provides translation services between the two
 networks, enabling communication between IPv4-only hosts and
 IPv6-only hosts.  (NOTE: By "IPv6-only hosts", we mean hosts running
 IPv6-only applications and hosts that can only use IPv6, as well as
 cases where only IPv6 connectivity is available to the client.  By
 "IPv4-only servers", we mean servers running IPv4-only applications
 and servers that can only use IPv4, as well as cases where only IPv4
 connectivity is available to the server).  Each IPv6/IPv4 translator
 used in conjunction with DNS64 must allow communications initiated
 from the IPv6-only host to the IPv4-only host.
 To allow an IPv6 initiator to do a standard AAAA RR DNS lookup to
 learn the address of the responder, DNS64 is used to synthesize a
 AAAA record from an A record containing a real IPv4 address of the
 responder, whenever the DNS64 cannot retrieve a AAAA record for the
 queried name.  The DNS64 service appears as a regular DNS server or
 resolver to the IPv6 initiator.  The DNS64 receives a AAAA DNS query
 generated by the IPv6 initiator.  It first attempts a resolution for
 the requested AAAA records.  If there are no AAAA records available
 for the target node (which is the normal case when the target node is
 an IPv4-only node), DNS64 performs a query for A records.  For each A
 record discovered, DNS64 creates a synthetic AAAA RR from the
 information retrieved in the A RR.
 The owner name of a synthetic AAAA RR is the same as that of the
 original A RR, but an IPv6 representation of the IPv4 address
 contained in the original A RR is included in the AAAA RR.  The IPv6
 representation of the IPv4 address is algorithmically generated from
 the IPv4 address and additional parameters configured in the DNS64.
 Among those parameters configured in the DNS64, there is at least one
 IPv6 prefix.  If not explicitly mentioned, all prefixes are treated
 equally, and the operations described in this document are performed
 using the prefixes available.  So as to be general, we will call any
 of these prefixes Pref64::/n, and describe the operations made with
 the generic prefix Pref64::/n.  The IPv6 addresses representing IPv4
 addresses included in the AAAA RR synthesized by the DNS64 contain
 Pref64::/n, and they also embed the original IPv4 address.
 The same algorithm and the same Pref64::/n prefix(es) must be
 configured both in the DNS64 device and the IPv6/IPv4 translator(s),
 so that both can algorithmically generate the same IPv6
 representation for a given IPv4 address.  In addition, it is required

Bagnulo, et al. Standards Track [Page 5] RFC 6147 DNS64 April 2011

 that IPv6 packets addressed to an IPv6 destination address that
 contains the Pref64::/n be delivered to an IPv6/IPv4 translator that
 has that particular Pref64::/n configured, so they can be translated
 into IPv4 packets.
 Once the DNS64 has synthesized the AAAA RRs, the synthetic AAAA RRs
 are passed back to the IPv6 initiator, which will initiate an IPv6
 communication with the IPv6 address associated with the IPv4
 receiver.  The packet will be routed to an IPv6/IPv4 translator,
 which will forward it to the IPv4 network.
 In general, the only shared state between the DNS64 and the IPv6/IPv4
 translator is the Pref64::/n and an optional set of static
 parameters.  The Pref64::/n and the set of static parameters must be
 configured to be the same on both; there is no communication between
 the DNS64 device and IPv6/IPv4 translator functions.  The mechanism
 to be used for configuring the parameters of the DNS64 is beyond the
 scope of this memo.
 The prefixes to be used as Pref64::/n and their applicability are
 discussed in [RFC6052].  There are two types of prefixes that can be
 used as Pref64::/n.
 o  The Pref64::/n can be the Well-Known Prefix 64:ff9b::/96 reserved
    by [RFC6052] for the purpose of representing IPv4 addresses in
    IPv6 address space.
 o  The Pref64::/n can be a Network-Specific Prefix (NSP).  An NSP is
    an IPv6 prefix assigned by an organization to create IPv6
    representations of IPv4 addresses.
 The main difference in the nature of the two types of prefixes is
 that the NSP is a locally assigned prefix that is under control of
 the organization that is providing the translation services, while
 the Well-Known Prefix is a prefix that has a global meaning since it
 has been assigned for the specific purpose of representing IPv4
 addresses in IPv6 address space.
 The DNS64 function can be performed in any of three places.  The
 terms below are more formally defined in Section 4.
 The first option is to locate the DNS64 function in authoritative
 servers for a zone.  In this case, the authoritative server provides
 synthetic AAAA RRs for an IPv4-only host in its zone.  This is one
 type of DNS64 server.

Bagnulo, et al. Standards Track [Page 6] RFC 6147 DNS64 April 2011

 Another option is to locate the DNS64 function in recursive name
 servers serving end hosts.  In this case, when an IPv6-only host
 queries the name server for AAAA RRs for an IPv4-only host, the name
 server can perform the synthesis of AAAA RRs and pass them back to
 the IPv6-only initiator.  The main advantage of this mode is that
 current IPv6 nodes can use this mechanism without requiring any
 modification.  This mode is called "DNS64 in DNS recursive-resolver
 mode".  This is a second type of DNS64 server, and it is also one
 type of DNS64 resolver.
 The last option is to place the DNS64 function in the end hosts,
 coupled to the local (stub) resolver.  In this case, the stub
 resolver will try to obtain (real) AAAA RRs, and in case they are not
 available, the DNS64 function will synthesize AAAA RRs for internal
 usage.  This mode is compatible with some functions like DNSSEC
 validation in the end host.  The main drawback of this mode is its
 deployability, since it requires changes in the end hosts.  This mode
 is called "DNS64 in stub-resolver mode".  This is the second type of
 DNS64 resolver.

3. Background to DNS64-DNSSEC Interaction

 DNSSEC ([RFC4033], [RFC4034], [RFC4035]) presents a special challenge
 for DNS64, because DNSSEC is designed to detect changes to DNS
 answers, and DNS64 may alter answers coming from an authoritative
 server.
 A recursive resolver can be security-aware or security-oblivious.
 Moreover, a security-aware recursive resolver can be validating or
 non-validating, according to operator policy.  In the cases below,
 the recursive resolver is also performing DNS64, and has a local
 policy to validate.  We call this general case vDNS64, but in all the
 cases below, the DNS64 functionality should be assumed to be needed.
 DNSSEC includes some signaling bits that offer some indicators of
 what the query originator understands.
 If a query arrives at a vDNS64 device with the "DNSSEC OK" (DO) bit
 set, the query originator is signaling that it understands DNSSEC.
 The DO bit does not indicate that the query originator will validate
 the response.  It only means that the query originator can understand
 responses containing DNSSEC data.  Conversely, if the DO bit is
 clear, that is evidence that the querying agent is not aware of
 DNSSEC.

Bagnulo, et al. Standards Track [Page 7] RFC 6147 DNS64 April 2011

 If a query arrives at a vDNS64 device with the "Checking Disabled"
 (CD) bit set, it is an indication that the querying agent wants all
 the validation data so it can do checking itself.  By local policy,
 vDNS64 could still validate, but it must return all data to the
 querying agent anyway.
 Here are the possible cases:
 1.  A DNS64 (DNSSEC-aware or DNSSEC-oblivious) receives a query with
     the DO bit clear.  In this case, DNSSEC is not a concern, because
     the querying agent does not understand DNSSEC responses.  The
     DNS64 can do validation of the response, if dictated by its local
     policy.
 2.  A security-oblivious DNS64 receives a query with the DO bit set,
     and the CD bit clear or set.  This is just like the case of a
     non-DNS64 case: the server doesn't support it, so the querying
     agent is out of luck.
 3.  A security-aware and non-validating DNS64 receives a query with
     the DO bit set and the CD bit clear.  Such a resolver is not
     validating responses, likely due to local policy (see [RFC4035],
     Section 4.2).  For that reason, this case amounts to the same as
     the previous case, and no validation happens.
 4.  A security-aware and non-validating DNS64 receives a query with
     the DO bit set and the CD bit set.  In this case, the DNS64 is
     supposed to pass on all the data it gets to the query initiator
     (see Section 3.2.2 of [RFC4035]).  This case will not work with
     DNS64, unless the validating resolver is prepared to do DNS64
     itself.  If the DNS64 modifies the record, the client will get
     the data back and try to validate it, and the data will be
     invalid as far as the client is concerned.
 5.  A security-aware and validating DNS64 resolver receives a query
     with the DO bit clear and the CD bit clear.  In this case, the
     resolver validates the data.  If it fails, it returns RCODE 2
     (Server failure); otherwise, it returns the answer.  This is the
     ideal case for vDNS64.  The resolver validates the data, and then
     synthesizes the new record and passes that to the client.  The
     client, which is presumably not validating (else it should have
     set DO and CD), cannot tell that DNS64 is involved.
 6.  A security-aware and validating DNS64 resolver receives a query
     with the DO bit set and the CD bit clear.  This works like the
     previous case, except that the resolver should also set the
     "Authentic Data" (AD) bit on the response.

Bagnulo, et al. Standards Track [Page 8] RFC 6147 DNS64 April 2011

 7.  A security-aware and validating DNS64 resolver receives a query
     with the DO bit set and the CD bit set.  This is effectively the
     same as the case where a security-aware and non-validating
     recursive resolver receives a similar query, and the same thing
     will happen: the downstream validator will mark the data as
     invalid if DNS64 has performed synthesis.  The node needs to do
     DNS64 itself, or else communication will fail.

4. Terminology

 This section provides definitions for the special terms used in the
 document.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].
 Authoritative server:  A DNS server that can answer authoritatively a
    given DNS request.
 DNS64:  A logical function that synthesizes DNS resource records
    (e.g., AAAA records containing IPv6 addresses) from DNS resource
    records actually contained in the DNS (e.g., A records containing
    IPv4 addresses).
 DNS64 recursive resolver:  A recursive resolver that provides the
    DNS64 functionality as part of its operation.  This is the same
    thing as "DNS64 in recursive-resolver mode".
 DNS64 resolver:  Any resolver (stub resolver or recursive resolver)
    that provides the DNS64 function.
 DNS64 server:  Any server providing the DNS64 function.  This
    includes the server portion of a recursive resolver when it is
    providing the DNS64 function.
 IPv4-only server:  Servers running IPv4-only applications and servers
    that can only use IPv4, as well as cases where only IPv4
    connectivity is available to the server.
 IPv6-only hosts:  Hosts running IPv6-only applications and hosts that
    can only use IPv6, as well as cases where only IPv6 connectivity
    is available to the client.

Bagnulo, et al. Standards Track [Page 9] RFC 6147 DNS64 April 2011

 Recursive resolver:  A DNS server that accepts requests from one
    resolver, and asks another server (of some description) for the
    answer on behalf of the first resolver.  Full discussion of DNS
    recursion is beyond the scope of this document; see [RFC1034] and
    [RFC1035] for full details.
 Synthetic RR:  A DNS resource record (RR) that is not contained in
    the authoritative servers' zone data, but which is instead
    synthesized from other RRs in the same zone.  An example is a
    synthetic AAAA record created from an A record.
 IPv6/IPv4 translator:  A device that translates IPv6 packets to IPv4
    packets and vice versa.  It is only required that the
    communication initiated from the IPv6 side be supported.
 For a detailed understanding of this document, the reader should also
 be familiar with DNS terminology from [RFC1034] and [RFC1035] and
 with current NAT terminology from [RFC4787].  Some parts of this
 document assume familiarity with the terminology of the DNS security
 extensions outlined in [RFC4035].  It is worth emphasizing that while
 DNS64 is a logical function separate from the DNS, it is nevertheless
 closely associated with that protocol.  It depends on the DNS
 protocol, and some behavior of DNS64 will interact with regular DNS
 responses.

5. DNS64 Normative Specification

 DNS64 is a logical function that synthesizes AAAA records from A
 records.  The DNS64 function may be implemented in a stub resolver,
 in a recursive resolver, or in an authoritative name server.  It
 works within those DNS functions, and appears on the network as
 though it were a "plain" DNS resolver or name server conforming to
 [RFC1034] and [RFC1035].
 The implementation SHOULD support mapping of separate IPv4 address
 ranges to separate IPv6 prefixes for AAAA record synthesis.  This
 allows handling of special use IPv4 addresses [RFC5735].
 DNS messages contain several sections.  The portion of a DNS message
 that is altered by DNS64 is the answer section, which is discussed
 below in Section 5.1.  The resulting synthetic answer is put together
 with other sections, and that creates the message that is actually
 returned as the response to the DNS query.  Assembling that response
 is covered below in Section 5.4.
 DNS64 also responds to PTR queries involving addresses containing any
 of the IPv6 prefixes it uses for synthesis of AAAA RRs.

Bagnulo, et al. Standards Track [Page 10] RFC 6147 DNS64 April 2011

5.1. Resolving AAAA Queries and the Answer Section

 When the DNS64 receives a query for RRs of type AAAA and class IN, it
 first attempts to retrieve non-synthetic RRs of this type and class,
 either by performing a query or, in the case of an authoritative
 server, by examining its own results.  The query may be answered from
 a local cache, if one is available.  DNS64 operation for classes
 other than IN is undefined, and a DNS64 MUST behave as though no
 DNS64 function is configured.

5.1.1. The Answer when There is AAAA Data Available

 If the query results in one or more AAAA records in the answer
 section, the result is returned to the requesting client as per
 normal DNS semantics, except in the case where any of the AAAA
 records match a special exclusion set of prefixes, as considered in
 Section 5.1.4.  If there is (non-excluded) AAAA data available, DNS64
 SHOULD NOT include synthetic AAAA RRs in the response (see Appendix A
 for an analysis of the motivations for and the implications of not
 complying with this recommendation).  By default, DNS64
 implementations MUST NOT synthesize AAAA RRs when real AAAA RRs
 exist.

5.1.2. The Answer when There is an Error

 If the query results in a response with an RCODE other than 0 (No
 error condition), then there are two possibilities.  A result with
 RCODE=3 (Name Error) is handled according to normal DNS operation
 (which is normally to return the error to the client).  This stage is
 still prior to any synthesis having happened, so a response to be
 returned to the client does not need any special assembly other than
 what would usually happen in DNS operation.
 Any other RCODE is treated as though the RCODE were 0 (see
 Sections 5.1.6 and 5.1.7) and the answer section were empty.  This is
 because of the large number of different responses from deployed name
 servers when they receive AAAA queries without a AAAA record being
 available (see [RFC4074]).  Note that this means, for practical
 purposes, that several different classes of error in the DNS are all
 treated as though a AAAA record is not available for that owner name.
 It is important to note that, as of this writing, some servers
 respond with RCODE=3 to a AAAA query even if there is an A record
 available for that owner name.  Those servers are in clear violation
 of the meaning of RCODE 3, and it is expected that they will decline
 in use as IPv6 deployment increases.

Bagnulo, et al. Standards Track [Page 11] RFC 6147 DNS64 April 2011

5.1.3. Dealing with Timeouts

 If the query receives no answer before the timeout (which might be
 the timeout from every authoritative server, depending on whether the
 DNS64 is in recursive-resolver mode), it is treated as RCODE=2
 (Server failure).

5.1.4. Special Exclusion Set for AAAA Records

 Some IPv6 addresses are not actually usable by IPv6-only hosts.  If
 they are returned to IPv6-only querying agents as AAAA records,
 therefore, the goal of decreasing the number of failure modes will
 not be attained.  Examples include AAAA records with addresses in the
 ::ffff:0:0/96 network, and possibly (depending on the context) AAAA
 records with the site's Pref64::/n or the Well-Known Prefix (see
 below for more about the Well-Known Prefix).  A DNS64 implementation
 SHOULD provide a mechanism to specify IPv6 prefix ranges to be
 treated as though the AAAA containing them were an empty answer.  An
 implementation SHOULD include the ::ffff/96 network in that range by
 default.  Failure to provide this facility will mean that clients
 querying the DNS64 function may not be able to communicate with hosts
 that would be reachable from a dual-stack host.
 When the DNS64 performs its initial AAAA query, if it receives an
 answer with only AAAA records containing addresses in the excluded
 range(s), then it MUST treat the answer as though it were an empty
 answer, and proceed accordingly.  If it receives an answer with at
 least one AAAA record containing an address outside any of the
 excluded range(s), then it by default SHOULD build an answer section
 for a response including only the AAAA record(s) that do not contain
 any of the addresses inside the excluded ranges.  That answer section
 is used in the assembly of a response as detailed in Section 5.4.
 Alternatively, it MAY treat the answer as though it were an empty
 answer, and proceed accordingly.  It MUST NOT return the offending
 AAAA records as part of a response.

5.1.5. Dealing with CNAME and DNAME

 If the response contains a CNAME or a DNAME, then the CNAME or DNAME
 chain is followed until the first terminating A or AAAA record is
 reached.  This may require the DNS64 to ask for an A record, in case
 the response to the original AAAA query is a CNAME or DNAME without a
 AAAA record to follow.  The resulting AAAA or A record is treated
 like any other AAAA or A case, as appropriate.
 When assembling the answer section, any chains of CNAME or DNAME RRs
 are included as part of the answer along with the synthetic AAAA (if
 appropriate).

Bagnulo, et al. Standards Track [Page 12] RFC 6147 DNS64 April 2011

5.1.6. Data for the Answer when Performing Synthesis

 If the query results in no error but an empty answer section in the
 response, the DNS64 attempts to retrieve A records for the name in
 question, either by performing another query or, in the case of an
 authoritative server, by examining its own results.  If this new A RR
 query results in an empty answer or in an error, then the empty
 result or error is used as the basis for the answer returned to the
 querying client.  If instead the query results in one or more A RRs,
 the DNS64 synthesizes AAAA RRs based on the A RRs according to the
 procedure outlined in Section 5.1.7.  The DNS64 returns the
 synthesized AAAA records in the answer section, removing the A
 records that form the basis of the synthesis.

5.1.7. Performing the Synthesis

 A synthetic AAAA record is created from an A record as follows:
 o  The NAME field is set to the NAME field from the A record.
 o  The TYPE field is set to 28 (AAAA).
 o  The CLASS field is set to the original CLASS field, 1.  Under this
    specification, DNS64 for any CLASS other than 1 is undefined.
 o  The Time to Live (TTL) field is set to the minimum of the TTL of
    the original A RR and the SOA RR for the queried domain.  (Note
    that in order to obtain the TTL of the SOA RR, the DNS64 does not
    need to perform a new query, but it can remember the TTL from the
    SOA RR in the negative response to the AAAA query.  If the SOA RR
    was not delivered with the negative response to the AAAA query,
    then the DNS64 SHOULD use the TTL of the original A RR or
    600 seconds, whichever is shorter.  It is possible instead to
    query explicitly for the SOA RR and use the result of that query,
    but this will increase query load and time to resolution for
    little additional benefit.)  This is in keeping with the approach
    used in negative caching [RFC2308].
 o  The RDLENGTH field is set to 16.
 o  The RDATA field is set to the IPv6 representation of the IPv4
    address from the RDATA field of the A record.  The DNS64 MUST
    check each A RR against configured IPv4 address ranges and select
    the corresponding IPv6 prefix to use in synthesizing the AAAA RR.
    See Section 5.2 for discussion of the algorithms to be used in
    effecting the transformation.

Bagnulo, et al. Standards Track [Page 13] RFC 6147 DNS64 April 2011

5.1.8. Querying in Parallel

 The DNS64 MAY perform the query for the AAAA RR and for the A RR in
 parallel, in order to minimize the delay.
    NOTE: Querying in parallel will result in performing unnecessary A
    RR queries in the case where no AAAA RR synthesis is required.  A
    possible trade-off would be to perform them sequentially but with
    a very short interval between them, so if we obtain a fast reply,
    we avoid doing the additional query.  (Note that this discussion
    is relevant only if the DNS64 function needs to perform external
    queries to fetch the RR.  If the needed RR information is
    available locally, as in the case of an authoritative server, the
    issue is no longer relevant.)

5.2. Generation of the IPv6 Representations of IPv4 Addresses

 DNS64 supports multiple algorithms for the generation of the IPv6
 representation of an IPv4 address.  The constraints imposed on the
 generation algorithms are the following:
 o  The same algorithm to create an IPv6 address from an IPv4 address
    MUST be used by both a DNS64 to create the IPv6 address to be
    returned in the synthetic AAAA RR from the IPv4 address contained
    in an original A RR, and by an IPv6/IPv4 translator to create the
    IPv6 address to be included in the source address field of the
    outgoing IPv6 packets from the IPv4 address included in the source
    address field of the incoming IPv4 packet.
 o  The algorithm MUST be reversible; i.e., it MUST be possible to
    derive the original IPv4 address from the IPv6 representation.
 o  The input for the algorithm MUST be limited to the IPv4 address;
    the IPv6 prefix (denoted Pref64::/n) used in the IPv6
    representations; and, optionally, a set of stable parameters that
    are configured in the DNS64 and in the NAT64 (such as a fixed
    string to be used as a suffix).
  • For each prefix Pref64::/n, n MUST be less than or equal to 96.

If one or more Pref64::/n are configured in the DNS64 through

       any means (such as manual configuration, or other automatic
       means not specified in this document), the default algorithm
       MUST use these prefixes (and not use the Well-Known Prefix).
       If no prefix is available, the algorithm MUST use the
       Well-Known Prefix 64:ff9b::/96 defined in [RFC6052] to
       represent the IPv4 unicast address range.

Bagnulo, et al. Standards Track [Page 14] RFC 6147 DNS64 April 2011

 A DNS64 MUST support the algorithm for generating IPv6
 representations of IPv4 addresses defined in Section 2 of [RFC6052].
 Moreover, the aforementioned algorithm MUST be the default algorithm
 used by the DNS64.  While the normative description of the algorithm
 is provided in [RFC6052], a sample description of the algorithm and
 its application to different scenarios are provided in Section 7 for
 illustration purposes.

5.3. Handling Other Resource Records and the Additional Section

5.3.1. PTR Resource Record

 If a DNS64 server receives a PTR query for a record in the IP6.ARPA
 domain, it MUST strip the IP6.ARPA labels from the QNAME, reverse the
 address portion of the QNAME according to the encoding scheme
 outlined in Section 2.5 of [RFC3596], and examine the resulting
 address to see whether its prefix matches any of the locally
 configured Pref64::/n or the default Well-Known Prefix.  There are
 two alternatives for a DNS64 server to respond to such PTR queries.
 A DNS64 server MUST provide one of these, and SHOULD NOT provide both
 at the same time unless different IP6.ARPA zones require answers of
 different sorts:
 1.  The first option is for the DNS64 server to respond
     authoritatively for its prefixes.  If the address prefix matches
     any Pref64::/n used in the site, either a NSP or the Well-Known
     Prefix (i.e., 64:ff9b::/96), then the DNS64 server MAY answer the
     query using locally appropriate RDATA.  The DNS64 server MAY use
     the same RDATA for all answers.  Note that the requirement is to
     match any Pref64::/n used at the site, and not merely the locally
     configured Pref64::/n.  This is because end clients could ask for
     a PTR record matching an address received through a different
     (site-provided) DNS64, and if this strategy is in effect, those
     queries should never be sent to the global DNS.  The advantage of
     this strategy is that it makes plain to the querying client that
     the prefix is one operated by the (DNS64) site, and that the
     answers the client is getting are generated by DNS64.  The
     disadvantage is that any useful reverse-tree information that
     might be in the global DNS is unavailable to the clients querying
     the DNS64.
 2.  The second option is for the DNS64 name server to synthesize a
     CNAME mapping the IP6.ARPA namespace to the corresponding
     IN-ADDR.ARPA name.  In this case, the DNS64 name server SHOULD
     ensure that there is RDATA at the PTR of the corresponding
     IN-ADDR.ARPA name, and that there is not an existing CNAME at
     that name.  This is in order to avoid synthesizing a CNAME that
     makes a CNAME chain longer or that does not actually point to

Bagnulo, et al. Standards Track [Page 15] RFC 6147 DNS64 April 2011

     anything.  The rest of the response would be the normal DNS
     processing.  The CNAME can be signed on the fly if need be.  The
     advantage of this approach is that any useful information in the
     reverse tree is available to the querying client.  The
     disadvantages are that it adds additional load to the DNS64
     (because CNAMEs have to be synthesized for each PTR query that
     matches the Pref64::/n), and that it may require signing on
     the fly.
 If the address prefix does not match any Pref64::/n, then the DNS64
 server MUST process the query as though it were any other query;
 i.e., a recursive name server MUST attempt to resolve the query as
 though it were any other (non-A/AAAA) query, and an authoritative
 server MUST respond authoritatively or with a referral, as
 appropriate.

5.3.2. Handling the Additional Section

 DNS64 synthesis MUST NOT be performed on any records in the
 additional section of synthesized answers.  The DNS64 MUST pass the
 additional section unchanged.
    NOTE: It may appear that adding synthetic records to the
    additional section is desirable, because clients sometimes use the
    data in the additional section to proceed without having to
    re-query.  There is in general no promise, however, that the
    additional section will contain all the relevant records, so any
    client that depends on the additional section being able to
    satisfy its needs (i.e., without additional queries) is
    necessarily broken.  An IPv6-only client that needs a AAAA record,
    therefore, will send a query for the necessary AAAA record if it
    is unable to find such a record in the additional section of an
    answer it is consuming.  For a correctly functioning client, the
    effect would be no different if the additional section were empty.
    The alternative of removing the A records in the additional
    section and replacing them with synthetic AAAA records may cause a
    host behind a NAT64 to query directly a name server that is
    unaware of the NAT64 in question.  The result in this case will be
    resolution failure anyway, but later in the resolution operation.
    The prohibition on synthetic data in the additional section
    reduces, but does not eliminate, the possibility of resolution
    failures due to cached DNS data from behind the DNS64.  See
    Section 6.

Bagnulo, et al. Standards Track [Page 16] RFC 6147 DNS64 April 2011

5.3.3. Other Resource Records

 If the DNS64 is in recursive-resolver mode, then considerations
 outlined in [DEFAULT-LOCAL-ZONES] may be relevant.
 All other RRs MUST be returned unchanged.  This includes responses to
 queries for A RRs.

5.4. Assembling a Synthesized Response to a AAAA Query

 A DNS64 uses different pieces of data to build the response returned
 to the querying client.
 The query that is used as the basis for synthesis results either in
 an error, an answer that can be used as a basis for synthesis, or an
 empty (authoritative) answer.  If there is an empty answer, then the
 DNS64 responds to the original querying client with the answer the
 DNS64 received to the original (initiator's) query.  Otherwise, the
 response is assembled as follows.
 The header fields are set according to the usual rules for recursive
 or authoritative servers, depending on the role that the DNS64 is
 serving.  The question section is copied from the original
 (initiator's) query.  The answer section is populated according to
 the rules in Section 5.1.7.  The authority and additional sections
 are copied from the response to the final query that the DNS64
 performed, and used as the basis for synthesis.
 The final response from the DNS64 is subject to all the standard DNS
 rules, including truncation [RFC1035] and EDNS0 handling [RFC2671].

5.5. DNSSEC Processing: DNS64 in Validating Resolver Mode

 We consider the case where a recursive resolver that is performing
 DNS64 also has a local policy to validate the answers according to
 the procedures outlined in [RFC4035], Section 5.  We call this
 general case vDNS64.
 The vDNS64 uses the presence of the DO and CD bits to make some
 decisions about what the query originator needs, and can react
 accordingly:
 1.  If CD is not set and DO is not set, vDNS64 SHOULD perform
     validation and do synthesis as needed.  See the next item for
     rules about how to do validation and synthesis.  In this case,
     however, vDNS64 MUST NOT set the AD bit in any response.

Bagnulo, et al. Standards Track [Page 17] RFC 6147 DNS64 April 2011

 2.  If CD is not set and DO is set, then vDNS64 SHOULD perform
     validation.  Whenever vDNS64 performs validation, it MUST
     validate the negative answer for AAAA queries before proceeding
     to query for A records for the same name, in order to be sure
     that there is not a legitimate AAAA record on the Internet.
     Failing to observe this step would allow an attacker to use DNS64
     as a mechanism to circumvent DNSSEC.  If the negative response
     validates, and the response to the A query validates, then the
     vDNS64 MAY perform synthesis and SHOULD set the AD bit in the
     answer to the client.  This is acceptable, because [RFC4035],
     Section 3.2.3 says that the AD bit is set by the name server side
     of a security-aware recursive name server if and only if it
     considers all the RRSets in the answer and authority sections to
     be authentic.  In this case, the name server has reason to
     believe the RRSets are all authentic, so it SHOULD set the AD
     bit.  If the data does not validate, the vDNS64 MUST respond with
     RCODE=2 (Server failure).
     A security-aware end point might take the presence of the AD bit
     as an indication that the data is valid, and may pass the DNS
     (and DNSSEC) data to an application.  If the application attempts
     to validate the synthesized data, of course, the validation will
     fail.  One could argue therefore that this approach is not
     desirable, but security-aware stub resolvers must not place any
     reliance on data received from resolvers and validated on their
     behalf without certain criteria established by [RFC4035],
     Section 4.9.3.  An application that wants to perform validation
     on its own should use the CD bit.
 3.  If the CD bit is set and DO is set, then vDNS64 MAY perform
     validation, but MUST NOT perform synthesis.  It MUST return the
     data to the query initiator, just like a regular recursive
     resolver, and depend on the client to do the validation and the
     synthesis itself.
     The disadvantage to this approach is that an end point that is
     translation-oblivious but security-aware and validating will not
     be able to use the DNS64 functionality.  In this case, the end
     point will not have the desired benefit of NAT64.  In effect,
     this strategy means that any end point that wishes to do
     validation in a NAT64 context must be upgraded to be
     translation-aware as well.

Bagnulo, et al. Standards Track [Page 18] RFC 6147 DNS64 April 2011

6. Deployment Notes

 While DNS64 is intended to be part of a strategy for aiding IPv6
 deployment in an internetworking environment with some IPv4-only and
 IPv6-only networks, it is important to realize that it is
 incompatible with some things that may be deployed in an IPv4-only or
 dual-stack context.

6.1. DNS Resolvers and DNS64

 Full-service resolvers that are unaware of the DNS64 function can be
 (mis)configured to act as mixed-mode iterative and forwarding
 resolvers.  In a native IPv4 context, this sort of configuration may
 appear to work.  It is impossible to make it work properly without it
 being aware of the DNS64 function, because it will likely at some
 point obtain IPv4-only glue records and attempt to use them for
 resolution.  The result that is returned will contain only A records,
 and without the ability to perform the DNS64 function the resolver
 will be unable to answer the necessary AAAA queries.

6.2. DNSSEC Validators and DNS64

 An existing DNSSEC validator (i.e., one that is unaware of DNS64)
 might reject all the data that comes from DNS64 as having been
 tampered with (even if it did not set CD when querying).  If it is
 necessary to have validation behind the DNS64, then the validator
 must know how to perform the DNS64 function itself.  Alternatively,
 the validating host may establish a trusted connection with a DNS64,
 and allow the DNS64 recursive resolver to do all validation on its
 behalf.

6.3. DNS64 and Multihomed and Dual-Stack Hosts

6.3.1. IPv6 Multihomed Hosts

 Synthetic AAAA records may be constructed on the basis of the network
 context in which they were constructed.  If a host sends DNS queries
 to resolvers in multiple networks, it is possible that some of them
 will receive answers from a DNS64 without all of them being connected
 via a NAT64.  For instance, suppose a system has two interfaces, i1
 and i2.  Whereas i1 is connected to the IPv4 Internet via NAT64, i2
 has native IPv6 connectivity only.  I1 might receive a AAAA answer
 from a DNS64 that is configured for a particular NAT64; the IPv6
 address contained in that AAAA answer will not connect with anything
 via i2.

Bagnulo, et al. Standards Track [Page 19] RFC 6147 DNS64 April 2011

           +---------------+                 +-------------+
           |      i1 (IPv6)+----NAT64--------+IPv4 Internet|
           |               |                 +-------------+
           | host          |
           |               |                 +-------------+
           |      i2 (IPv6)+-----------------+IPv6 Internet|
           +---------------+                 +-------------+
                   Figure 1:  IPv6 Multihomed Hosts
 This example illustrates why it is generally preferable that hosts
 treat DNS answers from one interface as local to that interface.  The
 answer received on one interface will not work on the other
 interface.  Hosts that attempt to use DNS answers globally may
 encounter surprising failures in these cases.
 Note that the issue is not that there are two interfaces, but that
 there are two networks involved.  The same results could be achieved
 with a single interface routed to two different networks.

6.3.2. Accidental Dual-Stack DNS64 Use

 Similarly, suppose that i1 has IPv6 connectivity and can connect to
 the IPv4 Internet through NAT64, but i2 has native IPv4 connectivity.
 In this case, i1 could receive an IPv6 address from a synthetic AAAA
 that would better be reached via native IPv4.  Again, it is worth
 emphasizing that this arises because there are two networks involved.
           +---------------+                 +-------------+
           |      i1 (IPv6)+----NAT64--------+IPv4 Internet|
           |               |                 +-------------+
           | host          |
           |               |                 +-------------+
           |      i2 (IPv4)+-----------------+IPv4 Internet|
           +---------------+                 +-------------+
              Figure 2:  Accidental Dual-Stack DNS64 Use
 The default configuration of dual-stack hosts is that IPv6 is
 preferred over IPv4 ([RFC3484]).  In that arrangement, the host will
 often use the NAT64 when native IPv4 would be more desirable.  For
 this reason, hosts with IPv4 connectivity to the Internet should
 avoid using DNS64.  This can be partly resolved by ISPs when
 providing DNS resolvers to clients, but that is not a guarantee that

Bagnulo, et al. Standards Track [Page 20] RFC 6147 DNS64 April 2011

 the NAT64 will never be used when a native IPv4 connection should be
 used.  There is no general-purpose mechanism to ensure that native
 IPv4 transit will always be preferred, because to a DNS64-oblivious
 host, the DNS64 looks just like an ordinary DNS server.  Operators of
 a NAT64 should expect traffic to pass through the NAT64 even when it
 is not necessary.

6.3.3. Intentional Dual-Stack DNS64 Use

 Finally, consider the case where the IPv4 connectivity on i2 is only
 with a LAN, and not with the IPv4 Internet.  The IPv4 Internet is
 only accessible using the NAT64.  In this case, it is critical that
 the DNS64 not synthesize AAAA responses for hosts in the LAN, or else
 that the DNS64 be aware of hosts in the LAN and provide context-
 sensitive answers ("split view" DNS answers) for hosts inside the
 LAN.  As with any split view DNS arrangement, operators must be
 prepared for data to leak from one context to another, and for
 failures to occur because nodes accessible from one context are not
 accessible from the other.
           +---------------+                 +-------------+
           |      i1 (IPv6)+----NAT64--------+IPv4 Internet|
           |               |                 +-------------+
           | host          |
           |               |
           |      i2 (IPv4)+---(local LAN only)
           +---------------+
              Figure 3:  Intentional Dual-Stack DNS64 Use
 It is important for deployers of DNS64 to realize that, in some
 circumstances, making the DNS64 available to a dual-stack host will
 cause the host to prefer to send packets via NAT64 instead of via
 native IPv4, with the associated loss of performance or functionality
 (or both) entailed by the NAT.  At the same time, some hosts are not
 able to learn about DNS servers provisioned on IPv6 addresses, or
 simply cannot send DNS packets over IPv6.

7. Deployment Scenarios and Examples

 In this section, we illustrate how the DNS64 behaves in different
 scenarios that are expected to be common.  In particular, we will
 consider the following scenarios defined in [RFC6144]: the "an IPv6
 network to the IPv4 Internet" scenario (both with DNS64 in DNS server
 mode and in stub-resolver mode) and the "IPv6 Internet to an IPv4
 network" setup (with DNS64 in DNS server mode only).

Bagnulo, et al. Standards Track [Page 21] RFC 6147 DNS64 April 2011

 In all the examples below, there is an IPv6/IPv4 translator
 connecting the IPv6 domain to the IPv4 one.  Also, there is a name
 server that is a dual-stack node, so it can communicate with IPv6
 hosts using IPv6 and with IPv4 nodes using IPv4.  In addition, we
 assume that in the examples, the DNS64 function learns which IPv6
 prefix it needs to use to map the IPv4 address space through manual
 configuration.

7.1. Example of "an IPv6 Network to the IPv4 Internet" Setup with DNS64

    in DNS Server Mode
 In this example, we consider an IPv6 node located in an IPv6-only
 site that initiates a communication to an IPv4 node located in the
 IPv4 Internet.
 The scenario for this case is depicted in the following figure:
           +---------------------+         +---------------+
           |IPv6 network         |         |    IPv4       |
           |           |  +-------------+  |  Internet     |
           |           |--| Name server |--|               |
           |           |  | with DNS64  |  |  +----+       |
           |  +----+   |  +-------------+  |  | H2 |       |
           |  | H1 |---|         |         |  +----+       |
           |  +----+   |   +------------+  |  192.0.2.1    |
           |           |---| IPv6/IPv4  |--|               |
           |           |   | Translator |  |               |
           |           |   +------------+  |               |
           |           |         |         |               |
           +---------------------+         +---------------+
        Figure 4:  "An IPv6 Network to the IPv4 Internet" Setup
                     with DNS64 in DNS Server Mode
 The figure shows an IPv6 node H1 and an IPv4 node H2 with the IPv4
 address 192.0.2.1 and FQDN h2.example.com.
 The IPv6/IPv4 translator has an IPv4 address 203.0.113.1 assigned
 to its IPv4 interface, and it is using the Well-Known Prefix
 64:ff9b::/96 to create IPv6 representations of IPv4 addresses.  The
 same prefix is configured in the DNS64 function in the local name
 server.
 For this example, assume the typical DNS situation where IPv6 hosts
 have only stub resolvers, and they are configured with the IP address
 of a name server that they always have to query and that performs
 recursive lookups (henceforth called "the recursive name server").

Bagnulo, et al. Standards Track [Page 22] RFC 6147 DNS64 April 2011

 The steps by which H1 establishes communication with H2 are:
 1.  H1 does a DNS lookup for h2.example.com.  H1 does this by sending
     a DNS query for a AAAA record for H2 to the recursive name
     server.  The recursive name server implements DNS64
     functionality.
 2.  The recursive name server resolves the query, and discovers that
     there are no AAAA records for H2.
 3.  The recursive name server performs an A-record query for H2 and
     gets back an RRSet containing a single A record with the IPv4
     address 192.0.2.1.  The name server then synthesizes a AAAA
     record.  The IPv6 address in the AAAA record contains the prefix
     assigned to the IPv6/IPv4 translator in the upper 96 bits and the
     received IPv4 address in the lower 32 bits; i.e., the resulting
     IPv6 address is 64:ff9b::192.0.2.1.
 4.  H1 receives the synthetic AAAA record and sends a packet towards
     H2.  The packet is sent to the destination address 64:ff9b::
     192.0.2.1.
 5.  The packet is routed to the IPv6 interface of the IPv6/IPv4
     translator, and the subsequent communication flows by means of
     the IPv6/IPv4 translator mechanisms.

7.2. Example of "an IPv6 Network to the IPv4 Internet" Setup with DNS64

    in Stub-Resolver Mode
 This case is depicted in the following figure:
           +---------------------+         +---------------+
           |IPv6 network         |         |    IPv4       |
           |           |     +--------+    |  Internet     |
           |           |-----| Name   |----|               |
           | +-----+   |     | server |    |  +----+       |
           | | H1  |   |     +--------+    |  | H2 |       |
           | |with |---|         |         |  +----+       |
           | |DNS64|   |   +------------+  |  192.0.2.1    |
           | +----+    |---| IPv6/IPv4  |--|               |
           |           |   | Translator |  |               |
           |           |   +------------+  |               |
           |           |         |         |               |
           +---------------------+         +---------------+
        Figure 5:  "An IPv6 Network to the IPv4 Internet" Setup
                     with DNS64 in Stub-Resolver Mode

Bagnulo, et al. Standards Track [Page 23] RFC 6147 DNS64 April 2011

 The figure shows an IPv6 node H1 implementing the DNS64 function and
 an IPv4 node H2 with the IPv4 address 192.0.2.1 and FQDN
 h2.example.com.
 The IPv6/IPv4 translator has an IPv4 address 203.0.113.1 assigned
 to its IPv4 interface, and it is using the Well-Known Prefix
 64:ff9b::/96 to create IPv6 representations of IPv4 addresses.  The
 same prefix is configured in the DNS64 function in H1.
 For this example, assume the typical DNS situation where IPv6 hosts
 have only stub resolvers, and they are configured with the IP address
 of a name server that they always have to query and that performs
 recursive lookups (henceforth called "the recursive name server").
 The recursive name server does not perform the DNS64 function.
 The steps by which H1 establishes communication with H2 are:
 1.  H1 does a DNS lookup for h2.example.com.  H1 does this by sending
     a DNS query for a AAAA record for H2 to the recursive name
     server.
 2.  The recursive DNS server resolves the query, and returns the
     answer to H1.  Because there are no AAAA records in the global
     DNS for H2, the answer is empty.
 3.  The stub resolver at H1 then queries for an A record for H2 and
     gets back an A record containing the IPv4 address 192.0.2.1.  The
     DNS64 function within H1 then synthesizes a AAAA record.  The
     IPv6 address in the AAAA record contains the prefix assigned to
     the IPv6/IPv4 translator in the upper 96 bits, then the received
     IPv4 address in the lower 32 bits; the resulting IPv6 address is
     64:ff9b::192.0.2.1.
 4.  H1 sends a packet towards H2.  The packet is sent to the
     destination address 64:ff9b::192.0.2.1.
 5.  The packet is routed to the IPv6 interface of the IPv6/IPv4
     translator and the subsequent communication flows using the IPv6/
     IPv4 translator mechanisms.

7.3. Example of "the IPv6 Internet to an IPv4 Network" Setup with DNS64

    in DNS Server Mode
 In this example, we consider an IPv6 node located in the IPv6
 Internet that initiates a communication to an IPv4 node located in
 the IPv4 site.

Bagnulo, et al. Standards Track [Page 24] RFC 6147 DNS64 April 2011

 In some cases, this scenario can be addressed without using any form
 of DNS64 function.  This is because it is possible to assign a fixed
 IPv6 address to each of the IPv4 nodes.  Such an IPv6 address would
 be constructed using the address transformation algorithm defined in
 [RFC6052] that takes as input the Pref64::/96 and the IPv4 address of
 the IPv4 node.  Note that the IPv4 address can be a public or a
 private address; the latter does not present any additional
 difficulty, since an NSP must be used as Pref64::/96 (in this
 scenario, the usage of the Well-Known Prefix is not supported as
 discussed in [RFC6052]).  Once these IPv6 addresses have been
 assigned to represent the IPv4 nodes in the IPv6 Internet, real AAAA
 RRs containing these addresses can be published in the DNS under the
 site's domain.  This is the recommended approach to handle this
 scenario, because it does not involve synthesizing AAAA records at
 the time of query.
 However, there are some more dynamic scenarios, where synthesizing
 AAAA RRs in this setup may be needed.  In particular, when DNS Update
 [RFC2136] is used in the IPv4 site to update the A RRs for the IPv4
 nodes, there are two options.  One option is to modify the DNS server
 that receives the dynamic DNS updates.  That would normally be the
 authoritative server for the zone.  So the authoritative zone would
 have normal AAAA RRs that are synthesized as dynamic updates occur.
 The other option is to modify all of the authoritative servers to
 generate synthetic AAAA records for a zone, possibly based on
 additional constraints, upon the receipt of a DNS query for the AAAA
 RR.  The first option -- in which the AAAA is synthesized when the
 DNS update message is received, and the data published in the
 relevant zone -- is recommended over the second option (i.e., the
 synthesis upon receipt of the AAAA DNS query).  This is because it is
 usually easier to solve problems of misconfiguration when the DNS
 responses are not being generated dynamically.  However, it may be
 the case where the primary server (that receives all the updates)
 cannot be upgraded for whatever reason, but where a secondary can be
 upgraded in order to handle the (comparatively small amount of) AAAA
 queries.  In such a case, it is possible to use the DNS64 as
 described next.  The DNS64 behavior that we describe in this section
 covers the case of synthesizing the AAAA RR when the DNS query
 arrives.

Bagnulo, et al. Standards Track [Page 25] RFC 6147 DNS64 April 2011

 The scenario for this case is depicted in the following figure:
            +-----------+          +----------------------+
            |           |          |   IPv4 site          |
            |   IPv6    |    +------------+  |   +----+   |
            | Internet  |----| IPv6/IPv4  |--|---| H2 |   |
            |           |    | Translator |  |   +----+   |
            |           |    +------------+  |            |
            |           |          |         | 192.0.2.1  |
            |           |    +------------+  |            |
            |           |----| Name server|--|            |
            |           |    | with DNS64 |  |            |
            +-----------+    +------------+  |            |
              |                    |         |            |
            +----+                 |                      |
            | H1 |                 +----------------------+
            +----+
        Figure 6:  "The IPv6 Internet to an IPv4 Network" Setup
                     with DNS64 in DNS Server Mode
 The figure shows an IPv6 node H1 and an IPv4 node H2 with the IPv4
 address 192.0.2.1 and FQDN h2.example.com.
 The IPv6/IPv4 translator is using an NSP 2001:db8::/96 to create IPv6
 representations of IPv4 addresses.  The same prefix is configured in
 the DNS64 function in the local name server.  The name server that
 implements the DNS64 function is the authoritative name server for
 the local domain.
 The steps by which H1 establishes communication with H2 are:
 1.  H1 does a DNS lookup for h2.example.com.  H1 does this by sending
     a DNS query for a AAAA record for H2.  The query is eventually
     forwarded to the server in the IPv4 site.
 2.  The local DNS server resolves the query (locally), and discovers
     that there are no AAAA records for H2.
 3.  The name server verifies that h2.example.com and its A RR are
     among those that the local policy defines as allowed to generate
     a AAAA RR.  If that is the case, the name server synthesizes a
     AAAA record from the A RR and the prefix 2001:db8::/96.  The IPv6
     address in the AAAA record is 2001:db8::192.0.2.1.
 4.  H1 receives the synthetic AAAA record and sends a packet towards
     H2.  The packet is sent to the destination address 2001:db8::
     192.0.2.1.

Bagnulo, et al. Standards Track [Page 26] RFC 6147 DNS64 April 2011

 5.  The packet is routed through the IPv6 Internet to the IPv6
     interface of the IPv6/IPv4 translator and the communication flows
     using the IPv6/IPv4 translator mechanisms.

8. Security Considerations

 DNS64 operates in combination with the DNS, and is therefore subject
 to whatever security considerations are appropriate to the DNS mode
 in which the DNS64 is operating (i.e., authoritative, recursive, or
 stub-resolver mode).
 DNS64 has the potential to interfere with the functioning of DNSSEC,
 because DNS64 modifies DNS answers, and DNSSEC is designed to detect
 such modifications and to treat modified answers as bogus.  See the
 discussion above in Sections 3, 5.5, and 6.2.
 Additionally, for the correct functioning of the translation
 services, the DNS64 and the NAT64 need to use the same Pref64.  If an
 attacker manages to change the Pref64 used by the DNS64, the traffic
 generated by the host that receives the synthetic reply will be
 delivered to the altered Pref64.  This can result in either a denial-
 of-service (DoS) attack (if the resulting IPv6 addresses are not
 assigned to any device), a flooding attack (if the resulting IPv6
 addresses are assigned to devices that do not wish to receive the
 traffic), or an eavesdropping attack (in case the Pref64 is routed
 through the attacker).

9. Contributors

 Dave Thaler
 Microsoft
 dthaler@windows.microsoft.com

10. Acknowledgements

 This document contains the result of discussions involving many
 people, including the participants of the IETF BEHAVE Working Group.
 The following IETF participants made specific contributions to parts
 of the text, and their help is gratefully acknowledged: Jaap
 Akkerhuis, Mark Andrews, Jari Arkko, Rob Austein, Timothy Baldwin,
 Fred Baker, Doug Barton, Marc Blanchet, Cameron Byrne, Brian
 Carpenter, Zhen Cao, Hui Deng, Francis Dupont, Patrik Faltstrom,
 David Harrington, Ed Jankiewicz, Peter Koch, Suresh Krishnan, Martti
 Kuparinen, Ed Lewis, Xing Li, Bill Manning, Matthijs Mekking, Hiroshi
 Miyata, Simon Perrault, Teemu Savolainen, Jyrki Soini, Dave Thaler,
 Mark Townsley, Rick van Rein, Stig Venaas, Magnus Westerlund, Jeff
 Westhead, Florian Weimer, Dan Wing, Xu Xiaohu, and Xiangsong Cui.

Bagnulo, et al. Standards Track [Page 27] RFC 6147 DNS64 April 2011

 Marcelo Bagnulo and Iljitsch van Beijnum are partly funded by
 Trilogy, a research project supported by the European Commission
 under its Seventh Framework Program.

11. References

11.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, November 1987.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
            RFC 2671, August 1999.
 [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
            (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
            RFC 4787, January 2007.
 [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
            Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
            October 2010.

11.2. Informative References

 [DEFAULT-LOCAL-ZONES]
            Andrews, M., "Locally-served DNS Zones", Work in Progress,
            March 2011.
 [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
            "Dynamic Updates in the Domain Name System (DNS UPDATE)",
            RFC 2136, April 1997.
 [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
            NCACHE)", RFC 2308, March 1998.
 [RFC3484]  Draves, R., "Default Address Selection for Internet
            Protocol version 6 (IPv6)", RFC 3484, February 2003.
 [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
            "DNS Extensions to Support IP Version 6", RFC 3596,
            October 2003.

Bagnulo, et al. Standards Track [Page 28] RFC 6147 DNS64 April 2011

 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements",
            RFC 4033, March 2005.
 [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Resource Records for the DNS Security Extensions",
            RFC 4034, March 2005.
 [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Protocol Modifications for the DNS Security
            Extensions", RFC 4035, March 2005.
 [RFC4074]  Morishita, Y. and T. Jinmei, "Common Misbehavior Against
            DNS Queries for IPv6 Addresses", RFC 4074, May 2005.
 [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
            BCP 153, RFC 5735, January 2010.
 [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
            IPv4/IPv6 Translation", RFC 6144, April 2011.
 [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
            NAT64: Network Address and Protocol Translation from IPv6
            Clients to IPv4 Servers", RFC 6146, April 2011.

Bagnulo, et al. Standards Track [Page 29] RFC 6147 DNS64 April 2011

Appendix A. Motivations and Implications of Synthesizing AAAA Resource

           Records when Real AAAA Resource Records Exist
 The motivation for synthesizing AAAA RRs when real AAAA RRs exist is
 to support the following scenario:
 o  An IPv4-only server application (e.g., web server software) is
    running on a dual-stack host.  There may also be dual-stack server
    applications running on the same host.  That host has fully
    routable IPv4 and IPv6 addresses, and hence the authoritative DNS
    server has an A record and a AAAA record.
 o  An IPv6-only client (regardless of whether the client application
    is IPv6-only, the client stack is IPv6-only, or it only has an
    IPv6 address) wants to access the above server.
 o  The client issues a DNS query to a DNS64 resolver.
 If the DNS64 only generates a synthetic AAAA if there's no real AAAA,
 then the communication will fail.  Even though there's a real AAAA,
 the only way for communication to succeed is with the translated
 address.  So, in order to support this scenario, the administrator of
 a DNS64 service may want to enable the synthesis of AAAA RRs even
 when real AAAA RRs exist.
 The implication of including synthetic AAAA RRs when real AAAA RRs
 exist is that translated connectivity may be preferred over native
 connectivity in some cases where the DNS64 is operated in DNS server
 mode.
 RFC 3484 [RFC3484] rules use "longest matching prefix" to select the
 preferred destination address to use.  So, if the DNS64 resolver
 returns both the synthetic AAAA RRs and the real AAAA RRs, then if
 the DNS64 is operated by the same domain as the initiating host, and
 a global unicast prefix (referred to as a Network-Specific Prefix
 (NSP) in [RFC6052]) is used, then a synthetic AAAA RR is likely to be
 preferred.
 This means that without further configuration:
 o  In the "an IPv6 network to the IPv4 Internet" scenario, the host
    will prefer translated connectivity if an NSP is used.  If the
    Well-Known Prefix defined in [RFC6052] is used, it will probably
    prefer native connectivity.

Bagnulo, et al. Standards Track [Page 30] RFC 6147 DNS64 April 2011

 o  In the "IPv6 Internet to an IPv4 network" scenario, it is possible
    to bias the selection towards the real AAAA RR if the DNS64
    resolver returns the real AAAA first in the DNS reply, when an NSP
    is used (the Well-Known Prefix usage is not supported in this
    case).
 o  In the "an IPv6 network to an IPv4 network" scenario, for local
    destinations (i.e., target hosts inside the local site), it is
    likely that the NSP and the destination prefix are the same, so we
    can use the order of RR in the DNS reply to bias the selection
    through native connectivity.  If the Well-Known Prefix is used,
    the "longest matching prefix" rule will select native
    connectivity.
 The problem can be solved by properly configuring the RFC 3484
 [RFC3484] policy table.

Bagnulo, et al. Standards Track [Page 31] RFC 6147 DNS64 April 2011

Authors' Addresses

 Marcelo Bagnulo
 UC3M
 Av. Universidad 30
 Leganes, Madrid  28911
 Spain
 Phone: +34-91-6249500
 EMail: marcelo@it.uc3m.es
 URI:   http://www.it.uc3m.es/marcelo
 Andrew Sullivan
 Shinkuro
 4922 Fairmont Avenue, Suite 250
 Bethesda, MD  20814
 USA
 Phone: +1 301 961 3131
 EMail: ajs@shinkuro.com
 Philip Matthews
 Unaffiliated
 600 March Road
 Ottawa, Ontario
 Canada
 Phone: +1 613-592-4343 x224
 EMail: philip_matthews@magma.ca
 Iljitsch van Beijnum
 IMDEA Networks
 Avda. del Mar Mediterraneo, 22
 Leganes, Madrid  28918
 Spain
 Phone: +34-91-6246245
 EMail: iljitsch@muada.com

Bagnulo, et al. Standards Track [Page 32]

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