Internet Engineering Task Force (IETF) S. Cheshire
Request for Comments: 8880 Apple Inc.
Updates: 7050 D. Schinazi
Category: Standards Track Google LLC
ISSN: 2070-1721 August 2020
Special Use Domain Name 'ipv4only.arpa'
Abstract
NAT64 (Network Address and Protocol Translation from IPv6 Clients to
IPv4 Servers) allows client devices using IPv6 to communicate with
servers that have only IPv4 connectivity.
The specification for how a client discovers its local network's
Network Address and Protocol Translation from IPv6 Clients to IPv4
Servers (NAT64)
NAT64 prefix (RFC 7050) defines the special name 'ipv4only.arpa' for
this purpose, but purpose. However, in its Domain Name Reservation Considerations
section (Section 8.1 of RFC 7050), 8.1), that specification (RFC 7050) indicates that
the name actually has no particularly special properties that would
require special handling and does not request
IANA to record the name in the "Special-Use Domain Names" registry. handling.
Consequently, despite the well-articulated special purpose of the
name, 'ipv4only.arpa' was not recorded in the "Special-Use Special-Use Domain
Names"
Names registry as a name with special properties.
This document updates RFC 7050. It describes the special treatment required,
required and formally declares the special properties of the name, name.
It also adds similar declarations for the corresponding reverse
mapping names, and updates
RFC 7050. names.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8880.
Copyright Notice
Copyright (c) 2020 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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.
Table of Contents
1. Introduction
1.1. Conventions and Terminology
2. Specialness of Reasons to Declare 'ipv4only.arpa' as Special
3. Consequences of 'ipv4only.arpa' Not Being Declared Special
3.1. Consequences for Name Resolution APIs and Libraries
3.2. Consequences for DNS64 Implementations
4. Remedies
5. Security Considerations
6. IANA Considerations
7. Domain Name Reservation Considerations
7.1. Special Use Domain Name 'ipv4only.arpa'
7.2. Names '170.0.0.192.in-addr.arpa' and
'171.0.0.192.in-addr.arpa'
7.2.1. ip6.arpa Reverse Mapping PTR Records
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Example BIND 9 Configuration
Acknowledgements
Authors' Addresses
1. Introduction
NAT64 (Network Address and Protocol Translation from IPv6 Clients to
IPv4 Servers) [RFC6146] allows client devices using IPv6 to
communicate with servers that have only IPv4 connectivity.
DNS64 (DNS Extensions for Network Address Translation from IPv6
Clients to IPv4 Servers) [RFC6147] facilitates use of NAT64 by
clients by generating synthesized IPv6 addresses for IPv4 servers
that have no IPv6 address of their own, or by communicating the local
network's NAT64 prefix to clients so that they can perform the IPv6
address synthesis themselves.
The specification for how a client discovers its local network's
NAT64 prefix [RFC7050] defines the special name 'ipv4only.arpa' for
this purpose, but in its Domain Name Reservation Considerations
section (Section 8.1 of [RFC7050]), 8.1), that specification [RFC7050] indicates that
the name actually has no particularly special properties that would
require special handling and does not request IANA to record the name
in the Special-Use Domain Names registry [SUDN].
Consequently, despite the well-articulated special purpose of the
name, 'ipv4only.arpa' was not recorded in the Special-Use Domain
Names registry [SUDN] as a name with special properties.
This omission was discussed in the document "Special-Use Domain Names
Problem Statement" [RFC8244].
As a result of this omission, in cases where software needs to give
this name special treatment in order for it to work correctly, there
was no clear mandate authorizing software authors to implement that
special treatment. Software implementers were left with the choice
between not implementing the special behavior necessary for the name
queries to work correctly or implementing the special behavior and
being accused of being noncompliant with some RFC. IETF DNS specifications.
This document describes the special treatment required, formally
declares the special properties of the name, and adds similar
declarations for the corresponding reverse mapping names.
1.1. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Specialness of Reasons to Declare 'ipv4only.arpa' as Special
The hostname 'ipv4only.arpa' is peculiar in that it was never
intended to be treated like a normal hostname.
A typical client never has any reason to look up the IPv4 address
records for 'ipv4only.arpa': no normal user is ever trying to view a
website hosted at that domain name or trying to send email to an
email address at that domain name. The name 'ipv4only.arpa' is
already known, by IETF specification [RFC7050], to have exactly two
IPv4 address records: 192.0.0.170 and 192.0.0.171. No client ever
has to look up the name in order to learn those two addresses.
In contrast, clients often look up the IPv6 AAAA address records for
'ipv4only.arpa', which is contrary to general DNS expectations, given
that it is already known, by IETF specification [RFC7050], that
'ipv4only.arpa' is an IPv4-only name, which and it has no IPv6 AAAA address
records. And yet, clients expect to receive, and do in fact receive,
positive answers for these IPv6 AAAA address records that apparently
should not exist.
This odd query behavior comes not because clients are using DNS to
learn legitimate answers from the name's legitimate authoritative
server, but because the DNS protocol has, in effect, been co-opted as
an improvised client-to-middlebox communication protocol to look for
a DNS64/NAT64 [RFC6146] [RFC6147] [RFC6146] gateway and, if one is present, to
request that it disclose the prefix it is using for IPv6 address
synthesis.
This use of specially crafted DNS queries as an improvised client-to-
middlebox communication protocol has a number of specific
consequences, outlined below, which client software needs to take
into account if the queries are to produce the desired results. This
is particularly true when used on a multihomed host or when a VPN
tunnel is in use. The name 'ipv4only.arpa' is most definitely a
special name and needs to be listed in IANA's registry along with
other DNS names that have special uses [SUDN].
3. Consequences of 'ipv4only.arpa' Not Being Declared Special
As a result of the original specification [RFC7050] not formally
declaring 'ipv4only.arpa' to have special properties, there was no
clear mandate for DNS software to treat this name specially. In
particular, this lack of mandate for special treatment is relevant
(a) to the name resolution APIs and libraries on client devices and
(b) to DNS64 [RFC6147] implementations. These two aspects are
discussed in more detail below.
3.1. Consequences for Name Resolution APIs and Libraries
A serious problem can occur with DNS64/NAT64 when a device is
configured to use a recursive resolver other than the one it learned
from the network.
A device joining a NAT64 network will learn the recursive resolver
recommended for that network, typically via "IPv6 IPv6 Router Advertisement
Options for DNS Configuration" [RFC8106] or via "DNS
Configuration Options for DHCPv6" DHCPv6 [RFC3646]. On a NAT64 network, it is
essential that the client use the DNS64 recursive resolver
recommended for that network, since only that recursive resolver can
be relied upon to know the appropriate prefix(es) to use for
synthesizing IPv6 addresses that will be acceptable to that NAT64
gateway.
However, it is becoming increasingly common for users to manually
override their default DNS configuration because they wish to use
some other public recursive resolver on the Internet, which may offer
better speed, reliability, or privacy than the local network's
default recursive resolver. At the time of writing, examples of
widely known public recursive resolver services include Cloudflare
Public DNS [DNS1], Google Public DNS [DNS8], and Quad9 [DNS9].
Another common scenario is the use of corporate or personal VPN
client software. Both for privacy reasons, reasons and also because the local
network's recursive resolver will typically be unable to provide
answers for the company's private internal host names, so VPN client
software usually overrides the local network's default configuration, configuration
to divert some or all DNS requests so that they go to the company's
own private internal recursive resolver, resolver instead of to the default
recursive resolver the client learned from its local network. (The
company's own private internal recursive resolvers typically have
addresses that are themselves reached through the VPN tunnel. tunnel, not via
the public Internet.) As with the case described above of public
recursive resolver services, the company's private internal recursive
resolver cannot be expected to be able to synthesize IPv6 addresses
correctly for use with the local network's NAT64 gateway, because the
company's private internal recursive resolver is unlikely to be aware
of the NAT64 prefix in use on the NAT64 network to which the client
device is currently attached. It is clear that a single recursive
resolver cannot meet both needs. The local network's recursive
resolver cannot be expected to give answers for some unknown
company's private internal host names, and some a company's private
internal recursive resolver cannot be expected to give correctly
synthesized IPv6 addresses suitable for the some unknown local network's
NAT64 gateway.
Note that multiple NAT64 services may be simultaneously available to
a client. For example, the local network may provide NAT64 service
(to allow an IPv6-only client device to access IPv4-only Internet
services), while at the same time, a corporate VPN may also provide
NAT64 service (to allow a client connecting via an IPv6-only VPN
tunnel to access IPv4-only corporate services). The NAT64 address
synthesis prefixes for the two NAT64 services may be different. In
this case, it is essential that the NAT64 address synthesis prefix
used on the local network be the prefix learned from the local
network's recursive resolver, and the NAT64 address synthesis prefix
used on the VPN tunnel be the prefix learned from the VPN tunnel's
recursive resolver.
The conflict difficulty here arises because DNS is being used for two
unrelated purposes. The first purpose is retrieving data from a
(nominally) global database, generally retrieving the IP address(es)
associated with a hostname. The second purpose is using the DNS
protocol as a middlebox communication protocol protocol, to interrogate the
local network infrastructure to discover the IPv6 prefix(es) in use
by the local NAT64 gateway for address synthesis.
3.2. Consequences for DNS64 Implementations
As a result of there being no mandate for special treatment, queries
for 'ipv4only.arpa' had to be handled normally, resulting in DNS64
gateways performing unnecessary queries to the authoritative 'arpa'
name servers, both unnecessary IPv6 address record queries (DNS qtype
"AAAA", always returning negative responses) and unnecessary IPv4
address record queries (DNS qtype "A", always returning the same
positive
responses) to the authoritative 'arpa' name servers. responses).
Having DNS64 gateways around the world issue these queries generated
additional load on the authoritative 'arpa' name servers, which was
redundant when the name 'ipv4only.arpa' is defined, by IETF
specification [RFC7050], to have exactly two IPv4 address records,
192.0.0.170 and 192.0.0.171, and no other IPv4 or IPv6 address
records.
Also, at times, for reasons that remain unclear, the authoritative
'arpa' name servers have been observed to be slow or unresponsive.
The failures of these 'ipv4only.arpa' queries result in unnecessary
failures of the DNS64 gateways and of the client devices that depend
on them for DNS64 [RFC6147] address synthesis.
Even when the authoritative 'arpa' name servers are operating
correctly, having to perform an unnecessary query to obtain an answer
that is already known in advance can add precious milliseconds of
delay, affecting user experience on the client devices waiting for
those synthesized replies.
4. Remedies
This document leverages operational experience to update the "Domain Domain
Name Reservation Considerations" Considerations section [RFC6761] of the earlier
prefix discovery specification [RFC7050] with one that more
accurately lists the actual special properties of the name
'ipv4only.arpa', so that software can legitimately implement the
correct behavior necessary for better performance, better
reliability, and correct operation.
These changes affect two bodies of software: (a) the name resolution
APIs and libraries on client devices, and (b) DNS64 implementations.
The new special rules specified in this document for name resolution
APIs and libraries state how they should select which recursive
resolver to query to learn the IPv6 address synthesis prefix in use
on a particular physical or virtual interface. Specifically, when
querying for the name 'ipv4only.arpa', name resolution APIs and
libraries should use the recursive resolver recommended by the
network for the interface in question rather than a recursive
resolver configured manually, a recursive resolver configured by VPN
software, or a full-service recursive resolver running on the local
host. Superficially, this might seem like a security issue, since
the user might have explicitly configured the particular DNS resolver
they wish to use, and rather than using that, the name resolution
code ignores the user's stated preference and uses untrusted input
received from the network instead. However, the 'ipv4only.arpa'
query is not really a DNS query in the usual sense; even though it
may look like a DNS query, it is actually an improvised client-to-
middlebox communication protocol in disguise. For NAT64 to work at
all, it is necessary for the interface on which NAT64 translation is
being performed to tell the host the address of the DNS64 recursive
resolver the host must use to learn the NAT64 prefix being used by
that NAT64. This is typically done via IPv6 Router Advertisement
Options for DNS Configuration [RFC8106] or via DNS Configuration
options for DHCPv6 [RFC3646].
The new special rules specified in this document for DNS64
implementations recommend that they avoid performing run-time network
queries for values that are known to be fixed by specification.
A useful property of the way NAT64 Prefix Discovery [RFC7050] was
originally specified was that it allowed for incremental deployment.
Even if existing DNS64 gateways, that were unaware of the special
'ipv4only.arpa' name, were already deployed, once IANA created the
appropriate 'ipv4only.arpa' records, clients could begin to use the
new facility immediately. Clients could send their special queries
for 'ipv4only.arpa' to an ipv4only-unaware DNS64 gateway, and and, as a
side effect of its usual query processing (after a query to IANA's servers)
servers), the DNS64 gateway would then generate the correct
synthesized response.
While this was a useful transition strategy to enable rapid adoption,
it is not the ideal end situation. For better performance, better
reliability, and lower load in IANA's servers, it is preferable for
DNS64 gateways to be aware of the special 'ipv4only.arpa' name so
that they can avoid issuing unnecessary queries. Network operators
who wish to provide reliable, high-performance service to their
customers are motivated to prefer DNS64 gateways that recognize the
special 'ipv4only.arpa' name and apply the appropriate optimizations.
5. Security Considerations
One of the known concerns with DNS64 is that it conflicts with
DNSSEC. If DNSSEC is used to assert cryptographically that a name
has no IPv6 AAAA records, then this interferes with using DNS64
address synthesis to assert tell a client that those nonexistent IPv6 AAAA
records do exist.
Section 3 of the DNS64 specification [RFC6147] discusses this:
| ... 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. This case will not work with DNS64,
| unless the validating resolver is prepared to do DNS64 itself.
The NAT64 Prefix Discovery specification [RFC7050] provides the
mechanism for the query initiator to learn the NAT64 prefix so that
it can do its own validation and DNS64 synthesis as described above.
With this mechanism, the client can (i) interrogate the local DNS64/
NAT64 gateway with (with an 'ipv4only.arpa' query query) to learn the IPv6
address synthesis prefix, (ii) query for the (signed) IPv4 address
records
itself for the desired hostname and validate the response, and then
(iii) perform its own IPv6 address synthesis locally, combining the
IPv6 address synthesis prefix learned from the local DNS64/NAT64
gateway with the validated DNSSEC-signed data learned from the global
Domain Name System.
It is conceivable that, over time, if DNSSEC adoption continues to
grow, the majority of clients could move to this validate-and-
synthesize-locally model, which reduces the DNS64 machinery to the
vestigial role of simply responding to the 'ipv4only.arpa' query to
report the local IPv6 address synthesis prefix. At the time of
publication, network operators have been observed "in the wild"
deploying NAT64 service with DNS recursive resolvers that reply to
'ipv4only.arpa' queries but otherwise perform no other NAT64 address
synthesis. In no case does the client care what answer(s) the
authoritative 'arpa' name servers might give for that query. The
'ipv4only.arpa' query is being used purely as a local client-to-middlebox client-to-
middlebox communication message.
This validate-and-synthesize-locally approach is even more attractive
if it does not create an additional dependency on the authoritative
'arpa' name servers to answer a query that is unnecessary because the
DNS64/NAT64 gateway already knows the answer before it even issues
the query. Avoiding this unnecessary query improves performance and
reliability for the client and reduces unnecessary load for the
authoritative 'arpa' name servers.
Hard coding
Hardcoding the known answers for 'ipv4only.arpa' IPv4 address record
queries (DNS qtype "A") in recursive resolvers also reduces the risk
of malicious devices intercepting those queries and returning
incorrect answers. Because the 'ipv4only.arpa' zone has to be an
insecure delegation (see below), DNSSEC cannot be used to protect
these answers from tampering by malicious devices on the path.
With respect to the question of whether 'ipv4only.arpa' should be a
secure or insecure delegation, we need to consider two paths of
information flow through the network: the
1. The path from the server authoritative for 'ipv4only.arpa' to the
DNS64 recursive resolver and
the
2. The path from the DNS64 recursive resolver to the ultimate client. client
The path from the authoritative server to the DNS64 recursive
resolver (queries for IPv4 address records) need not be protected by
DNSSEC, because the DNS64 recursive resolver already knows, by
specification, what the answers are. In principle, if this were a
secure delegation, and 'ipv4only.arpa' were a signed zone, then the
path from the authoritative server to the DNS64 recursive resolver
would still work, but DNSSEC is not necessary here. Run-time
cryptographic signatures are not needed to verify compile-time
constants. Validating the signatures could only serve to introduce
potential failures into the system for minimal benefit.
The path from the DNS64 recursive resolver to the ultimate client
(queries for IPv6 address records) *cannot* be protected by DNSSEC
because the DNS64 recursive resolver is synthesizing IPv6 address
answers and does not possess the DNSSEC secret key required to sign
those answers.
Consequently, the 'ipv4only.arpa' zone MUST be an insecure delegation
to give DNS64/NAT64 gateways the freedom to synthesize answers to
those queries at will, without the answers being rejected by DNSSEC-
capable resolvers. DNSSEC-capable resolvers that follow this
specification MUST NOT attempt to validate answers received in
response to queries for the IPv6 AAAA address records for
'ipv4only.arpa'. Note that the name 'ipv4only.arpa' has no use
outside of being used for this special DNS pseudo-query used to learn
the DNS64/NAT64 address synthesis prefix, so the lack of DNSSEC
security for that name is not a problem.
The original NAT64 Prefix Discovery specification [RFC7050] stated,
incorrectly:
| A signed "ipv4only.arpa." allows validating DNS64 servers (see
| [RFC6147] Section 3, Case 5, for example) to detect malicious AAAA
| resource records. Therefore, the zone serving the well-known name
| has to be protected with DNSSEC.
This document updates the previous specification [RFC7050] to correct
that error. The 'ipv4only.arpa' zone MUST be an insecure delegation.
6. IANA Considerations
IANA has created an insecure delegation for 'ipv4only.arpa' to allow
DNS64 recursive resolvers to create synthesized AAAA answers within
that zone.
IANA has recorded the following names in the "Special-Use Special-Use Domain
Names" Names
registry [SUDN]:
ipv4only.arpa.
170.0.0.192.in-addr.arpa.
171.0.0.192.in-addr.arpa.
IANA has recorded the following IPv4 addresses in the "IANA IANA IPv4
Special-Purpose Address Registry" Registry [SUv4]:
192.0.0.170
192.0.0.171
7. Domain Name Reservation Considerations
7.1. Special Use Domain Name 'ipv4only.arpa'
The name 'ipv4only.arpa' is defined, by IETF specification [RFC7050],
to have two IPv4 address records with rdata 192.0.0.170 and
192.0.0.171.
When queried via a DNS64 [RFC6147] recursive resolver, resolver [RFC6147], the name
'ipv4only.arpa' is also defined to have IPv6 AAAA records, with rdata
synthesized from a combination of the NAT64 IPv6 prefix(es) and the
IPv4 addresses 192.0.0.170 and 192.0.0.171. This can return more
than one pair of IPv6 addresses if there are multiple NAT64 prefixes.
The name 'ipv4only.arpa' has no other IPv4 or IPv6 address records.
There are no subdomains of 'ipv4only.arpa'. All names falling below
'ipv4only.arpa' are defined to be nonexistent (NXDOMAIN).
The name 'ipv4only.arpa' is special to
a. client software wishing to perform DNS64 address synthesis,
b. APIs responsible for retrieving the correct information, and
c. the DNS64 recursive resolver responding to such requests.
These three considerations are listed in items 2, 3, and 4 below:
1. Normal users should never have reason to encounter the
'ipv4only.arpa' domain name. If they do, they should expect
queries for 'ipv4only.arpa' to result in the answers required by
the specification [RFC7050]. Normal users have no need to know
that 'ipv4only.arpa' is special.
2. Application software may explicitly use the name 'ipv4only.arpa'
for DNS64/NAT64 address synthesis and expect to get the answers
required by the specification [RFC7050]. If application software
encounters the name 'ipv4only.arpa' in the normal course of
handling user input, the application software should resolve that
name as usual and need not treat it in any special way.
3. Name resolution APIs and libraries MUST recognize 'ipv4only.arpa'
as special and MUST give it special treatment.
Learning a network's NAT64 prefix is, by its nature, an
interface-specific operation, and the special DNS query used to
learn this interface-specific NAT64 prefix MUST be sent to the
DNS recursive resolver address(es) the client learned via the
configuration machinery for that specific client interface. The
NAT64 prefix is a per-interface property, not a per-device
property.
Regardless of any manual client DNS configuration, DNS overrides
configured by VPN client software, or any other mechanisms that
influence the choice of the client's recursive resolver
address(es) (including client devices that run their own local
recursive resolver and use the loopback address as their
configured recursive resolver address), all queries for
'ipv4only.arpa' and any subdomains of that name MUST be sent to
the recursive resolver learned from the network interface in
question via IPv6 Router Advertisement Options for DNS
Configuration [RFC8106], DNS Configuration options for DHCPv6
[RFC3646], or other configuration mechanisms. Because DNS
queries for 'ipv4only.arpa' are actually a special middlebox
communication protocol, it is essential that they go to the
correct middlebox for the interface in question, and failure to
honor this requirement would cause failure of the NAT64 Prefix
Discovery mechanism [RFC7050].
One implication of this is that, on multihomed devices (devices
that allow more than one logical or physical IP interface to be
active at the same time, e.g., cellular data and Wi-Fi, or one
physical interface plus a VPN connection), clients MUST use
interface-aware name resolution APIs. On different (logical or
physical) interfaces, different DNS64 answers may be received,
and DNS64 answers are only valid for the interface on which they
were received. On multihomed devices (including devices that
support VPN), name resolution APIs that do not include interface
parameters will not work reliably with NAT64. On single-homed
devices, interface-unaware name resolution APIs are acceptable
since when only one interface can be active at a time, there is
no need to specify an interface.
DNSSEC-capable resolvers MUST NOT attempt to validate answers
received in response to queries for the IPv6 AAAA address records
for 'ipv4only.arpa' since, by definition, any such answers are
generated by the local network's DNS64/NAT64 gateway, not the
authoritative server responsible for that name.
4. For the purposes of this section, recursive resolvers fall into
two categories. The first category is traditional recursive
resolvers, which includes *forwarding* recursive resolvers, as
commonly implemented in residential home gateways, and
*iterative* recursive resolvers, as commonly deployed by ISPs.
More information on these terms can be found in DNS Terminology
[RFC8499].
The second category is DNS64 recursive resolvers, whose purpose
is to synthesize IPv6 address records. These may be *forwarding*
DNS64 recursive resolvers or *iterative* DNS64 recursive
resolvers, and they work in partnership with a companion NAT64
gateway to communicate the appropriate NAT64 address synthesis
prefix to clients. More information on these terms can be found
in the DNS Terminology document [RFC8499].
Traditional forwarding recursive resolvers SHOULD NOT recognize
'ipv4only.arpa' as special or give that name, or subdomains of
that name, any special treatment. The rationale for this is that
a traditional forwarding recursive resolver, such as built in to
a residential home gateway, may itself be downstream of a DNS64
recursive resolver. Passing through the 'ipv4only.arpa' queries
to the upstream DNS64 recursive resolver will allow the correct
NAT64 prefix to be discovered.
Traditional iterative recursive resolvers that are not explicitly
configured to synthesize IPv6 prefixes on behalf of a companion
NAT64 gateway need not recognize 'ipv4only.arpa' as special or
take any special action.
Forwarding or iterative recursive resolvers that have been
explicitly configured to perform DNS64 address synthesis in
support of a companion NAT64 gateway (i.e., "DNS64 recursive
resolvers") MUST recognize 'ipv4only.arpa' as special. The
authoritative name servers for 'ipv4only.arpa' cannot be expected
to know the local network's NAT64 address synthesis prefix, so
consulting the authoritative name servers for IPv6 address
records for 'ipv4only.arpa' is futile. All DNS64 recursive
resolvers MUST recognize 'ipv4only.arpa' (and all of its
subdomains) as special, and they MUST NOT attempt to look up NS
records for 'ipv4only.arpa' or otherwise query authoritative name
servers in an attempt to resolve this name. Instead, DNS64
recursive resolvers MUST act as authoritative for this zone, by
generating immediate responses for all queries for
'ipv4only.arpa' (and any subdomain of 'ipv4only.arpa'), with the
one exception of queries for the DS record. Queries for the DS
record are resolved the usual way to allow a client to securely
verify that the 'ipv4only.arpa' zone has an insecure delegation.
Note that this exception is not expected to receive widespread
usage, since any client compliant with this specification already
knows that 'ipv4only.arpa' is an insecure delegation and will not
attempt DNSSEC validation for this name.
DNS64 recursive resolvers MUST generate the 192.0.0.170 and
192.0.0.171 responses for IPv4 address queries (DNS qtype "A"),
the appropriate synthesized IPv6 address record responses for
IPv6 address queries (DNS qtype "AAAA"), and a negative
("no error no answer") response for all other query types except
DS.
For all subdomains of 'ipv4only.arpa', DNS64 recursive resolvers
MUST generate immediate NXDOMAIN responses. All names falling
below 'ipv4only.arpa' are defined to be nonexistent.
An example configuration for BIND 9 showing how to achieve the
desired result is given in Appendix A.
Note that this is *not* a locally served zone in the usual sense
of that term [RFC6303] because this rule applies *only* to DNS64
recursive resolvers, not to traditional forwarding DNS or iterative
recursive resolvers.
5. Authoritative name server software need not recognize
'ipv4only.arpa' as special or handle it in any special way.
6. Generally speaking, operators of authoritative name servers need
not know anything about the name 'ipv4only.arpa', just as they do
not need to know anything about any other names they are not
responsible for. Only the administrators of the 'arpa' namespace
need to be aware of this name's purpose and how it should be
configured. In particular, 'ipv4only.arpa' MUST have the
required records, and MUST be an insecure delegation, to allow
DNS64 recursive resolvers to create synthesized AAAA answers
within that zone. Making the 'ipv4only.arpa' zone a secure
delegation would make it impossible for DNS64 recursive resolvers
to create synthesized AAAA answers that will be accepted by
DNSSEC validating clients, thereby defeating the entire purpose
of the 'ipv4only.arpa' name.
7. DNS Registries/Registrars need not know anything about the name
'ipv4only.arpa', just as they do not need to know anything about
any other name they are not responsible for.
7.2. Names '170.0.0.192.in-addr.arpa' and '171.0.0.192.in-addr.arpa'
Since the IPv4 addresses 192.0.0.170 and 192.0.0.171 are defined to
be special, and are listed in the "IANA IANA IPv4 Special-Purpose Address
Registry"
Registry [SUv4], the corresponding reverse mapping names in the
in-addr.arpa domain are similarly special.
The name '170.0.0.192.in-addr.arpa' is defined, by IETF specification
[RFC7050], to have only one DNS record, type PTR, with rdata
'ipv4only.arpa'.
The name '171.0.0.192.in-addr.arpa' is defined, by IETF specification
[RFC7050], to have only one DNS record, type PTR, with rdata
'ipv4only.arpa'.
There are no subdomains of '170.0.0.192.in-addr.arpa' or
'171.0.0.192.in-addr.arpa'. All names falling below these names are
defined to be nonexistent (NXDOMAIN).
Practically speaking, these two names are rarely used, but to the
extent that they may be, they are special only to resolver APIs and
libraries, as described in item 3 below:
1. Normal users should never have reason to encounter these two
reverse mapping names. However, if they do, queries for these
reverse mapping names should return the expected answer
'ipv4only.arpa'. Normal users have no need to know that these
reverse mapping names are special.
2. Application software SHOULD NOT recognize these two reverse
mapping names as special and SHOULD NOT treat them differently.
For example, if the user were to issue the Unix command
"host 192.0.0.170", then the "host" command should call the name
resolution API or library as usual and display the result that is
returned.
3. Name resolution APIs and libraries SHOULD recognize these two
reverse mapping names as special and generate the required
responses locally. For the names '170.0.0.192.in-addr.arpa' and
'171.0.0.192.in-addr.arpa', PTR queries yield the result
'ipv4only.arpa'; all other query types yield a negative
("no error no answer") response. For all subdomains of these two
reverse mapping domains, all queries yield an NXDOMAIN response.
All names falling below these two reverse mapping domains are
defined to be nonexistent.
This local self-contained generation of these responses is to
avoid placing unnecessary load on the authoritative
'in-addr.arpa' name servers.
4. Recursive resolvers SHOULD NOT recognize these two reverse
mapping names as special and SHOULD NOT, by default, give them
any special treatment.
5. Authoritative name server software need not recognize these two
reverse mapping names as special or handle them in any special
way.
6. Generally speaking, most operators of authoritative name servers
need not know anything about these two reverse mapping names,
just as they do not need to know anything about any other names
they are not responsible for. Only the operators of the
authoritative name servers for these two reverse mapping names
need to be aware that these names are special, and require fixed
answers specified by IETF specification.
7. DNS Registries/Registrars need not know anything about these two
reverse mapping names, just as they do not need to know anything
about any other name they are not responsible for.
7.2.1. ip6.arpa Reverse Mapping PTR Records
For all IPv6 addresses synthesized by a DNS64 recursive resolver, the
DNS64 recursive resolver is responsible for synthesizing the
appropriate 'ip6.arpa' reverse mapping PTR records too, if it chooses
to provide reverse mapping PTR records. The same applies to the
synthesized IPv6 addresses corresponding to the IPv4 addresses
192.0.0.170 and 192.0.0.171.
Generally, a DNS64 recursive resolver synthesizes appropriate
'ip6.arpa' reverse mapping PTR records by extracting the embedded
IPv4 address from the encoded IPv6 address, performing a reverse
mapping PTR query for that IPv4 address, and then synthesizing a
corresponding 'ip6.arpa' reverse mapping PTR record containing the
same rdata.
In the case of synthesized IPv6 addresses corresponding to the IPv4
addresses 192.0.0.170 and 192.0.0.171, the DNS64 recursive resolver
does not issue reverse mapping queries for those IPv4 addresses, but
instead, according to rule 3 above, immediately returns the answer
'ipv4only.arpa'.
In the case of a client that uses the 'ipv4only.arpa' query to
discover the IPv6 prefixes in use by the local NAT64 gateway, and
then proceeds to perform its own address synthesis locally (which has
benefits such as allowing DNSSEC validation), that client MUST also
synthesize 'ip6.arpa' reverse mapping PTR records for those
discovered prefix(es), according to the rules above: When a client's
name resolution APIs and libraries receive a request to look up an
'ip6.arpa' reverse mapping PTR record for an address that falls
within one of the discovered NAT64 address synthesis prefixes, the
software extracts the embedded IPv4 address and then, for IPv4
addresses 192.0.0.170 and 192.0.0.171, returns the fixed answer
'ipv4only.arpa', and for all other IPv4 addresses, performs a reverse
mapping PTR query for the IPv4 address and then synthesizes a
corresponding 'ip6.arpa' reverse mapping PTR record containing the
same rdata.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
DOI 10.17487/RFC3646, December 2003,
<https://www.rfc-editor.org/info/rfc3646>.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <https://www.rfc-editor.org/info/rfc6146>.
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
<https://www.rfc-editor.org/info/rfc7050>.
[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 8106, DOI 10.17487/RFC8106, March 2017,
<https://www.rfc-editor.org/info/rfc8106>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163,
RFC 6303, DOI 10.17487/RFC6303, July 2011,
<https://www.rfc-editor.org/info/rfc6303>.
[RFC8244] Lemon, T., Droms, R., and W. Kumari, "Special-Use Domain
Names Problem Statement", RFC 8244, DOI 10.17487/RFC8244,
October 2017, <https://www.rfc-editor.org/info/rfc8244>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[SUDN] IANA, "Special-Use Domain Names",
<https://www.iana.org/assignments/special-use-domain-
names/>.
[SUv4] IANA, "IANA IPv4 Special-Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv4-special-
registry/>.
[DNS1] Cloudflare, "1.1.1.1 - The free app that makes your
Internet safer.", <https://1.1.1.1/>.
[DNS8] Google, "Google Public DNS",
<https://developers.google.com/speed/public-dns/>.
[DNS9] Quad9, "Internet Security and Privacy In a Few Easy
Steps", <https://quad9.net/>.
Appendix A. Example BIND 9 Configuration
A BIND 9 recursive resolver can be configured to act as authoritative
for the necessary DNS64 names as described below.
In /etc/named.conf, the following line is added:
zone "ipv4only.arpa" { type master; file "ipv4only"; };
The file /var/named/ipv4only is created with the following content:
$TTL 86400 ; Default TTL 24 hours
@ IN SOA nameserver.example. admin.nameserver.example. (
2016052400 ; Serial
7200 ; Refresh ( 7200 = 2 hours)
3600 ; Retry ( 3600 = 1 hour)
15724800 ; Expire (15724800 = 6 months)
60 ; Minimum
)
@ IN NS nameserver.example.
@ IN A 192.0.0.170
@ IN A 192.0.0.171
@ IN AAAA 64:ff9b::192.0.0.170 ; If not using Well-Known Prefix
@ IN AAAA 64:ff9b::192.0.0.171 ; place chosen NAT64 prefix here
Acknowledgements
Thanks to Jouni Korhonen, Teemu Savolainen, and Dan Wing Wing, for
devising the NAT64 Prefix Discovery mechanism [RFC7050] and for their
feedback on this document.
Thanks to Geoff Huston for his feedback on this document.
Thanks to Erik Kline for pointing out that the in-addr.arpa names are
special, too.
Thanks to Mark Andrews for conclusively pointing out the reasons why
the 'ipv4only.arpa' zone must be an insecure delegation in order for
the NAT64 Prefix Discovery mechanism [RFC7050] to work and for many
other very helpful comments.
Thanks particularly to Lorenzo Colitti for an especially spirited
hallway discussion at IETF 96 in Berlin, which lead directly to
significant improvements in how this document presents the issues.
Thanks to Scott Bradner, Bernie Volz, Barry Leiba, Mirja Kuehlewind,
Suresh Krishnan, Benjamin Kaduk, Roman Danyliw, Eric Vyncke, and the
other IESG reviewers for their thoughtful feedback.
Thanks to Dave Thaler and Warren Kumari for generously helping
shepherd this document through the publication process.
Authors' Addresses
Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Phone: +1 (408) 996-1010
Email: cheshire@apple.com
David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, California 94043
United States of America
Email: dschinazi.ietf@gmail.com