BEHAVE WG M. Bagnulo
Internet-Draft UC3M
Intended status: Standards Track A. Sullivan
Expires: August 19, 2010 Shinkuro
P. Matthews
Alcatel-Lucent
I. van Beijnum
IMDEA Networks
February 15, 2010
DNS64: DNS extensions for Network Address Translation from IPv6 Clients
to IPv4 Servers
draft-ietf-behave-dns64-06
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 Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 19, 2010.
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Copyright Notice
Copyright (c) 2010 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
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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 BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background to DNS64-DNSSEC interaction . . . . . . . . . . . . 7
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. DNS64 Normative Specification . . . . . . . . . . . . . . . . 9
5.1. Resolving AAAA queries and the answer section . . . . . . 10
5.1.1. The answer when there is AAAA data available . . . . . 10
5.1.2. The answer when there is an error . . . . . . . . . . 10
5.1.3. Special exclusion set for AAAA records . . . . . . . . 10
5.1.4. Dealing with CNAME and DNAME . . . . . . . . . . . . . 11
5.1.5. Data for the answer when performing synthesis . . . . 11
5.1.6. Performing the synthesis . . . . . . . . . . . . . . . 12
5.1.7. Querying in parallel . . . . . . . . . . . . . . . . . 12
5.2. Generation of the IPv6 representations of IPv4
addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3. Handling other RRs and the Additional Section . . . . . . 13
5.3.1. PTR queries . . . . . . . . . . . . . . . . . . . . . 13
5.3.2. Handling the additional section . . . . . . . . . . . 14
5.3.3. Other records . . . . . . . . . . . . . . . . . . . . 15
5.4. Assembling a synthesized response to a AAAA query . . . . 15
5.5. DNSSEC processing: DNS64 in recursive server mode . . . . 16
6. Deployment notes . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. DNS resolvers and DNS64 . . . . . . . . . . . . . . . . . 17
6.2. DNSSEC validators and DNS64 . . . . . . . . . . . . . . . 17
6.3. DNS64 and multihomed and dual-stack hosts . . . . . . . . 17
6.3.1. IPv6 multihomed hosts . . . . . . . . . . . . . . . . 17
6.3.2. Accidental dual-stack DNS64 use . . . . . . . . . . . 18
6.3.3. Intentional dual-stack DNS64 use . . . . . . . . . . . 18
7. Deployment scenarios and examples . . . . . . . . . . . . . . 19
7.1. Example of An-IPv6-network-to-IPv4-Internet setup with
DNS64 in DNS server mode . . . . . . . . . . . . . . . . . 20
7.2. An example of an-IPv6-network-to-IPv4-Internet setup
with DNS64 in stub-resolver mode . . . . . . . . . . . . . 21
7.3. Example of IPv6-Internet-to-an-IPv4-network setup
DNS64 in DNS server mode . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 25
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
12.1. Normative References . . . . . . . . . . . . . . . . . . . 26
12.2. Informative References . . . . . . . . . . . . . . . . . . 26
Appendix A. Motivations and Implications of synthesizing AAAA
RR when real AAAA RR exists . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
This document specifies DNS64, a mechanism that is part of the
toolbox for IPv6-IPv4 transition and co-existence. DNS64, used
together with an IPv6/IPv4 translator such as stateful NAT64
[I-D.ietf-behave-v6v4-xlate-stateful], 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 FQDN of the server.
These mechanisms are expected to play a critical role in the IPv4-
IPv6 transition and co-existence. 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 advanced
features require performing the DNS64 function directly in the end-
hosts themselves.
2. Overview
This section provides a non-normative 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, 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, servers that
can only use IPv4, as well as cases where only IPv4 connectivity is
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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 address 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
that IPv6 packets addressed to an IPv6 destination address that
contains the Pref64::/n be delivered to an IPv6/IPv4 translator, 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
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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 [I-D.ietf-behave-address-format]. There are two types
of prefixes that can be used as Pref64::/n.
The Pref64::/n can be the Well-Known Prefix 64:FF9B::/96 reserved
by [I-D.ietf-behave-address-format] for the purpose of
representing IPv4 addresses in IPv6 address space.
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 first option is to locate the DNS64 function in authoritative
servers for a zone. In this case, the authoritative server provides
a synthetic AAAA RRs for an IPv4-only host in its zone. This is one
type of DNS64 server.
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 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 advanced 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
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mode is called "DNS64 in stub-resolver mode". This is the second
type of DNS64 resolver.
3. Background to DNS64-DNSSEC interaction
DNSSEC 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 name server can be validating or
non-validating, according to operator policy. In the cases below,
the recursive server 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 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.
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.
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],
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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 resolver is
supposed to pass on all the data it gets to the query initiator
(see section 3.2.2 of [RFC4035]). This case will be problematic
with DNS64. If the DNS64 server 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 node receives a query with
the DO bit clear and CD 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 node receives a query with
the DO bit set and CD clear. This ought to work like the
previous case, except that the resolver should also set the
"Authentic Data" (AD) bit on the response.
7. A security-aware and validating DNS64 node receives a query with
the DO bit set and CD 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.
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 question.
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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 recursor: 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.
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.
Synthetic RR: A DNS resource record (RR) that is not contained in
any zone data file, but has been synthesized from other RRs. 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], [RFC1035] and
current NAT terminology from [RFC4787]. Some parts of this document
assume familiarity with the terminology of the DNS security
extensions outlined in [RFC4035].
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.
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]. Support of
multicast address handling is out of the scope of this document. A
possible approach is specified in [I-D.venaas-behave-mcast46].
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. 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, considered in
Section 5.1.3. 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 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 than would usually
happen in DNS operation.
Any other RCODE is treated as though the RCODE were 0 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.
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.
5.1.3. 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
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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 Pref::64/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 MAY 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.
5.1.4. 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, the original CNAME or DNAME RR is
included as part of the answer, and the synthetic AAAA, if
appropriate, is included.
5.1.5. 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
authortiative 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. (Transient errors may result in retrying the query,
depending on the mode and operation of the underlying resolver; this
is just as in Section 5.1.2.) If instead the query results in one or
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more A RRs, the DNS64 synthesizes AAAA RRs based on the A RRs
according to the procedure outlined in Section 5.1.6. The DNS64
returns the synthesized AAAA records in the answer section, removing
the A records that form the basis of the synthesis.
5.1.6. 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 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.)
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 SHOULD
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.
5.1.7. 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. However, this would 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
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generation algorithms are the following:
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 a 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.
The algorithm MUST be reversible; i.e., it MUST be possible to
derive the original IPv4 address from the IPv6 representation.
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 fixed string to
be used as a suffix).
For each prefix Pref64::/n, n MUST the less than or equal to
96. If one or more Pref64::/n are configured in the DNS64
through any means in the DNS64 (such as manually configured, or
other automatic means not specified in this document), the
default algorithm MUST use these prefixes (and not use the
Well-Know Prefix). If no prefix is available, the algorithm
MUST use the Well-Known prefix 64:FF9B::/96 defined in
[I-D.ietf-behave-address-format] to represent the IPv4 unicast
address range
[[anchor9: Note in document: The value 64:FF9B::/96 is proposed as
the value for the Well-Known prefix and needs to be confirmed
whenis published as RFC.]][I-D.ietf-behave-address-format]
A DNS64 MUST support the algorithm for generating IPv6
representations of IPv4 addresses defined in Section 2 of
[I-D.ietf-behave-address-format]. Moreover, the aforementioned
algorithm MUST be the default algorithm used by the DNS64. While the
normative description of the algorithm is provided in
[I-D.ietf-behave-address-format], a sample description of the
algorithm and its application to different scenarios is provided in
Section 7 for illustration purposes.
5.3. Handling other RRs and the Additional Section
5.3.1. PTR queries
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
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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. 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.
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.
The second option is for the DNS64 nameserver to synthesize a CNAME
mapping the IP6.ARPA namespace to the corresponding IN-ADDR.ARPA
name. 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 disadvantage is 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. In addition, the generated CNAME could
correspond to an unpopulated in-addr.arpa zone, so the CNAME would
provide a reference to a non-existent record.
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 nameserver 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.
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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 nameserver that is unaware of the
NAT64 in question. The result in this case will be resolution
failure anyway, only later in the resolution operation.
5.3.3. Other records
If the DNS64 is in recursive resolver mode, then considerations
outlined in [I-D.ietf-dnsop-default-local-zones] may be relevant.
All other RRs MUST be returned unchanged.
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 AAAA 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 AAAA
query. The answer section is populated according to the rules in
Section 5.1.6. The authority and additional sections are copied from
the response to the A query that the DNS64 performed.
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5.5. DNSSEC processing: DNS64 in recursive server mode
We consider the case where a recursive server 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.
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
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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.
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 realise 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
Existing DNSSEC validators (i.e. that are 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 recursor 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
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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.
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. For more discussion of
this topic, see [I-D.savolainen-mif-dns-server-selection].
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
emphasising that this arises because there are two networks involved.
Since it is most likely that the host will attempt AAAA resolution
first, in this arrangement the host will often use the NAT64 when
native IPv4 would be preferable. 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 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
to a LAN, with an IPv6-only connection on i1 to the Internet,
connecting to the IPv4 Internet via NAT64. Traffic to the LAN may
not be routable from the global Internet, as is often the case (for
instance) with LANs using RFC1918 addresses. 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
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another, and for failures to occur because nodes accessible from one
context are not accessible from the other.
It is important for deployers of DNS64 to realise 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 walk through some sample scenarios that are
expected to be common deployment cases. It should be noted that this
is provided for illustrative purposes and this section is not
normative. The normative definition of DNS64 is provided in
Section 5 and the normative definition of the address transformation
algorithm is provided in [I-D.ietf-behave-address-format].
There are two main different setups where DNS64 is expected to be
used (other setups are possible as well, but these two are the main
ones identified at the time of this writing).
One possible setup that is expected to be common is the case of an
end site or an ISP that is providing IPv6-only connectivity or
connectivity to IPv6-only hosts that wants to allow the
communication from these IPv6-only connected hosts to the IPv4
Internet. This case is called An-IPv6-network-to-IPv4-Internet
[I-D.ietf-behave-v6v4-framework]. In this case, the IPv6/IPv4
Translator is used to connect the end site or the ISP to the IPv4
Internet and the DNS64 function is provided by the end site or the
ISP.
The other possible setup that is expected is an IPv4 site that
wants that its IPv4 servers to be reachable from the IPv6
Internet. This case is called IPv6-Internet-to-an-IPv4-network
[I-D.ietf-behave-v6v4-framework]. It should be noted that the
IPv4 addresses used in the IPv4 site can be either public or
private. In this case, the IPv6/IPv4 translator is used to
connect the IPv4 end site to the IPv6 Internet and the DNS64
function is provided by the IPv4 end site itself.
In this section we illustrate how the DNS64 behaves in the different
scenarios that are expected to be common. We consider then 3
possible scenarios, namely:
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1. An-IPv6-network-to-IPv4-Internet setup with DNS64 in DNS server
mode
2. An-IPv6-network-to-IPv4-Internet setup with DNS64 in stub-
resolver mode
3. IPv6-Internet-to-an-IPv4-network setup with DNS64 in DNS server
mode
7.1. Example of An-IPv6-network-to-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 |
| | +-------------+ | Network |
| |--| Name server |--| |
| | | with DNS64 | | +----+ |
| +----+ | +-------------+ | | H2 | |
| | H1 |---| | | +----+ |
| +----+ | +-------+ | 192.0.2.1 |
| |------| NAT64 |----| |
| | +-------+ | |
| | | | |
+---------------------+ +---------------+
The figure shows an IPv6 node H1 and an IPv4 node H2 with IPv4
address 192.0.2.1 and FQDN h2.example.com
A IPv6/IPv4 Translator connects the IPv6 network to the IPv4
Internet. This IPv6/IPv4 Translator has an IPv4 address 203.0.113.1
assigned to its IPv4 interface and it is using the WKP 64:FF9B::/96
to create IPv6 representations of IPv4 addresses, as defined in
[I-D.ietf-behave-address-format].
The other element involved is the local name server. The name server
is a dual-stack node, so that H1 can contact it via IPv6, while it
can contact IPv4-only name servers via IPv4.
The local name server is configured to represent the whole IPv4
unicast space with using the WKP 64:FF9B::/96. For the purpose of
this example, we assume it learns this through manual configuration.
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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 nameserver").
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 queries for A records for H2 and gets
back a single A records containing the IPv4 address 192.0.2.1.
The name server then synthesizes a AAAA records. 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 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. An example of an-IPv6-network-to-IPv4-Internet setup with DNS64 in
stub-resolver mode
This case is depicted in the following figure:
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+---------------------+ +---------------+
|IPv6 network | | IPv4 |
| | +--------+ | Network |
| |-----| Name |----| |
| +-----+ | | server | | +----+ |
| | H1 | | +--------+ | | H2 | |
| |with |---| | | +----+ |
| |DNS64| | +-------+ | 192.0.2.1 |
| +----+ |------| NAT64 |----| |
| | +-------+ | |
| | | | |
+---------------------+ +---------------+
The figure shows an IPv6 node H1 implementing the DNS64 function and
an IPv4 node H2 with IPv4 address 192.0.2.1 and FQDN h2.example.com
A IPv6/IPv4 Translator connects the IPv6 network to the IPv4
Internet. This IPv6/IPv4 Translator is using the WKP 64:FF9B::/96
and an IPv4 address T 203.0.113.1 assigned to its IPv4 interface.
H1 needs to know the prefix assigned to the local IPv6/IPv4
Translator (64:FF9B::/96). For the purpose of this example, we
assume it learns this through manual configuration.
Also shown is a name server. For the purpose of this example, we
assume that the name server is a dual-stack node, so that H1 can
contact it via IPv6, while it can contact IPv4-only name servers via
IPv4.
For this example, assume the typical situation where IPv6 hosts have
only stub resolvers and always query a name server that provides
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
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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 i.e. 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 IPv6-Internet-to-an-IPv4-network setup 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.
In some cases, this scenario can be addressed without using any form
of DNS64 function. This is so because in principle 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 [I-D.ietf-behave-address-format] 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
[I-D.ietf-behave-address-format]). 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 modify all 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 --
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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 and so on 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 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.
The scenario for this case is depicted in the following figure:
+-----------+ +----------------------+
| | | IPv4 site |
| IPv6 | +-------+ | +----+ |
| Internet |------| NAT64 |-----|---| H2 | |
| | +-------+ | +----+ |
| | | | 192.0.2.1 |
| | +------------+ | |
| |----| Name server|--| |
| | | with DNS64 | | |
+-----------+ +------------+ | |
| | | |
+----+ | |
| H1 | +----------------------+
+----+
The figure shows an IPv6 node H1 and an IPv4 node H2 with IPv4
address X 192.0.2.1 and FQDN h2.example.com.
A IPv6/IPv4 translator connects the IPv4 network to the IPv6
Internet. This IPv6/IPv4 translator has a NSP 2001:DB8::/96.
Also shown is the authoritative name server for the local domain with
DNS64 functionality. For the purpose of this example, we assume that
the name server is a dual-stack node, so that H1 or a recursive
resolver acting on the request of H1 can contact it via IPv6, while
it can be contacted by IPv4-only nodes to receive dynamic DNS updates
via IPv4.
The local name server needs to know the prefix assigned to the local
IPv6/IPv4 Translator (2001:DB8::/96). For the purpose of this
example, we assume it learns this through manual configuration.
The steps by which H1 establishes communication with H2 are:
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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 from. 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.
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
See the discussion on the usage of DNSSEC and DNS64 described in
section 3, section 5.5, and section 6.2. .
9. IANA Considerations
This memo makes no request of IANA.
10. Contributors
Dave Thaler
Microsoft
dthaler@windows.microsoft.com
11. Acknowledgements
This draft 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: Mark Andrews,
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Jari Arkko, Rob Austein, Timothy Baldwin, Fred Baker, Doug Barton,
Marc Blanchet, Cameron Byrne, Brian Carpenter, Hui Deng, Francis
Dupont, Patrik Faltstrom, Ed Jankiewicz, Peter Koch, Suresh Krishnan,
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, Florian
Weimer, Dan Wing, Xu Xiaohu, Xiangsong Cui.
Marcelo Bagnulo and Iljitsch van Beijnum are partly funded by
Trilogy, a research project supported by the European Commission
under its Seventh Framework Program.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[I-D.ietf-behave-address-format]
Huitema, C., Bao, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators",
draft-ietf-behave-address-format-04 (work in progress),
January 2010.
12.2. Informative References
[I-D.ietf-behave-v6v4-xlate-stateful]
Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-08 (work in
progress), January 2010.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
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[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.
[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.
[RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to
Historic Status", RFC 4966, July 2007.
[RFC5735] Cotton, M. and L. Vegoda, "iSpecial Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[I-D.ietf-behave-v6v4-framework]
Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation",
draft-ietf-behave-v6v4-framework-06 (work in progress),
February 2010.
[I-D.venaas-behave-mcast46]
Venaas, S., Asaeda, H., SUZUKI, S., and T. Fujisaki, "An
IPv4 - IPv6 multicast translator",
draft-venaas-behave-mcast46-01 (work in progress),
July 2009.
[I-D.ietf-dnsop-default-local-zones]
Andrews, M., "Locally-served DNS Zones",
draft-ietf-dnsop-default-local-zones-09 (work in
progress), November 2009.
[I-D.savolainen-mif-dns-server-selection]
Savolainen, T., "DNS Server Selection on Multi-Homed
Hosts", draft-savolainen-mif-dns-server-selection-01 (work
in progress), October 2009.
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Appendix A. Motivations and Implications of synthesizing AAAA RR when
real AAAA RR exists
The motivation for synthesizing AAAA RRs when a real AAAA RRs exist
is to support the following scenario:
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 also running on the same host. That host has fully
routable IPv4 and IPv6 addresses and hence the authoritative DNS
server has an A and a AAAA record as a result.
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.
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 RR when real AAAA RR
exist is that translated connectivity may be preferred over native
connectivity in some cases where the DNS64 is operated in DNS server
mode.
RFC3484 [RFC3484] rules use longest prefix match 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 (called an NSP in
[I-D.ietf-behave-address-format]) is used, then a synthetic AAAA RR
is likely to be preferred.
This means that without further configuration:
In "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 [I-D.ietf-behave-address-format] is used,
it will probably prefer native connectivity.
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
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is used (the Well-Known Prefix usage is not supported in this
case)
In "an IPv6 network to 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 prefix match rule will select native connectivity.
So this option introduces problems in the following cases:
An IPv6 network to the IPv4 internet with an NSP
IPv6 to IPv4 in the same network when reaching external
destinations and an NSP is used.
In any case, the problem can be solved by properly configuring the
RFC3484 [RFC3484] policy table, but this requires effort on the part
of the site operator.
Authors' Addresses
Marcelo Bagnulo
UC3M
Av. Universidad 30
Leganes, Madrid 28911
Spain
Phone: +34-91-6249500
Fax:
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
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Philip Matthews
Unaffiliated
600 March Road
Ottawa, Ontario
Canada
Phone: +1 613-592-4343 x224
Fax:
Email: philip_matthews@magma.ca
URI:
Iljitsch van Beijnum
IMDEA Networks
Av. Universidad 30
Leganes, Madrid 28911
Spain
Phone: +34-91-6246245
Email: iljitsch@muada.com
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