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.
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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.
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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
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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
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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.
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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.
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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.
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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.
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+---------------+ +-------------+
| 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
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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).
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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").
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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]
RFC, FYI, BCP