Internet Engineering Task Force (IETF) T. Savolainen
Request for Comments: 7050 Nokia
Category: Standards Track J. Korhonen
ISSN: 2070-1721 Broadcom
November 2013 Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis
This document describes a method for detecting the presence of DNS64
and for learning the IPv6 prefix used for protocol translation on an
access network. The method depends on the existence of a well-known
IPv4-only fully qualified domain name "ipv4only.arpa.". The
information learned enables nodes to perform local IPv6 address
synthesis and to potentially avoid NAT64 on dual-stack and multi-
Status of This Memo
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This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................32. Requirements Notation and Terminology ...........................42.1. Requirements Notation ......................................42.2. Terminology ................................................43. Node Behavior ...................................................43.1. Validation of Discovered Pref64::/n ........................63.1.1. DNSSEC Requirements for the Network .................73.1.2. DNSSEC Requirements for the Node ....................73.2. Connectivity Check .........................................83.2.1. No Connectivity Checks against "ipv4only.arpa." .....93.3. Alternative Fully Qualified Domain Names ..................103.4. Message Flow Illustration .................................104. Operational Considerations for Hosting the IPv4-Only
Well-Known Name ................................................125. Operational Considerations for DNS64 Operator ..................125.1. Mapping of IPv4 Address Ranges to IPv6 Prefixes ...........136. Exit Strategy ..................................................147. Security Considerations ........................................148. IANA Considerations ............................................158.1. Domain Name Reservation Considerations ....................158.2. IPv4 Address Allocation Considerations ....................168.3. IAB Statement Regarding This .arpa Request ................179. Acknowledgements ...............................................1810. References ....................................................1810.1. Normative References .....................................1810.2. Informative References ...................................19Appendix A. Example of DNS Record Configuration ..................20Appendix B. About the IPv4 Address for the Well-Known Name .......21
As part of the transition to IPv6, NAT64 [RFC6146] and DNS64
[RFC6147] technologies will be utilized by some access networks to
provide IPv4 connectivity for IPv6-only nodes [RFC6144]. DNS64
utilizes IPv6 address synthesis to create local IPv6 addresses for
peers having only IPv4 addresses, hence allowing DNS-using IPv6-only
nodes to communicate with IPv4-only peers.
However, DNS64 cannot serve applications not using DNS, such as those
receiving IPv4 address literals as referrals. Such applications
could nevertheless be able to work through NAT64, provided they are
able to create locally valid IPv6 addresses that would be translated
to the peers' IPv4 addresses.
Additionally, DNS64 is not able to do IPv6 address synthesis for
nodes running validating DNS resolvers enabled by DNS Security
(DNSSEC), but instead, the synthesis must be done by the nodes
themselves. In order to perform IPv6 synthesis, nodes have to learn
the IPv6 prefix(es) used on the access network for protocol
translation. A prefix, which may be a Network-Specific Prefix (NSP)
or a Well-Known Prefix (WKP) [RFC6052], is referred to in this
document as Pref64::/n [RFC6146].
This document describes a best-effort method for applications and
nodes to learn the information required to perform local IPv6 address
synthesis. The IPv6 address synthesis procedure itself is out of the
scope of this document. An example application is a browser
encountering IPv4 address literals in an IPv6-only access network.
Another example is a node running a validating security-aware DNS
resolver in an IPv6-only access network.
The knowledge of IPv6 address synthesis taking place may also be
useful if DNS64 and NAT64 are used in dual-stack-enabled access
networks or if a node is multi-interfaced [RFC6418]. In such cases,
nodes may choose to prefer IPv4 or an alternative network interface
in order to avoid traversal through protocol translators.
It is important to note that use of this approach will not result in
a system that is as robust, secure, and well-behaved as an all-IPv6
system would be. Hence, it is highly recommended to upgrade nodes'
destinations to IPv6 and utilize the described method only as a
2. Requirements Notation and Terminology
2.1. Requirements Notation
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 [RFC2119].
NAT64 FQDN: a fully qualified domain name (FQDN) for a NAT64 protocol
Pref64::/n: an IPv6 prefix used for IPv6 address synthesis [RFC6146].
Pref64::WKA: an IPv6 address consisting of Pref64::/n and WKA at any
of the locations allowed by RFC 6052 [RFC6052].
Secure Channel: a communication channel a node has between itself and
a DNS64 server protecting DNS protocol-related messages from
interception and tampering. The channel can be, for example, an
IPsec-based virtual private network (VPN) tunnel or a link layer
utilizing data encryption technologies.
Well-Known IPv4-only Name (WKN): the fully qualified domain name,
"ipv4only.arpa.", well-known to have only A record(s).
Well-Known IPv4 Address (WKA): an IPv4 address that is well-known and
present in an A record for the well-known name. Two well-known IPv4
addresses are defined for Pref64::/n discovery purposes: 192.0.0.170
3. Node Behavior
A node requiring information about the presence (or absence) of
NAT64, and one or more Pref64::/n used for protocol translation SHALL
send a DNS query for AAAA resource records of the Well-Known
IPv4-only Name (WKN) "ipv4only.arpa.". The node MAY perform the DNS
query in both IPv6-only and dual-stack access networks.
When sending a DNS AAAA resource record query for the WKN, a node
MUST set the "Checking Disabled (CD)" bit to zero [RFC4035], as
otherwise the DNS64 server will not perform IPv6 address synthesis
(Section 3 of [RFC6147]) and hence would not reveal the Pref64::/n
used for protocol translation.
A DNS reply with one or more AAAA resource records indicates that the
access network is utilizing IPv6 address synthesis. In some
scenarios, captive portals, or NXDOMAIN and NODATA hijacking,
performed by the access network may result in a false positive. One
method to detect such hijacking is to query a fully qualified domain
name that is known to be invalid (and normally returns an empty
response or an error response) and see if it returns a valid resource
record. However, as long as the hijacked domain does not result in
AAAA resource record responses that contain a well-known IPv4 address
in any location defined by [RFC6052], the response will not disturb
the Pref64::/n learning procedure.
A node MUST look through all of the received AAAA resource records to
collect one or more Pref64::/n. The Pref64::/n list might include
the Well-Known Prefix 64:ff9b::/96 [RFC6052] or one or more Network-
Specific Prefixes. In the case of NSPs, the node SHALL determine the
used address format by searching the received IPv6 addresses for the
WKN's well-known IPv4 addresses. The node SHALL assume the well-
known IPv4 addresses might be found at the locations specified by
[RFC6052], Section 2.2. The node MUST check on octet boundaries to
ensure a 32-bit well-known IPv4 address value is present only once in
an IPv6 address. In case another instance of the value is found
inside the IPv6 address, the node SHALL repeat the search with the
other well-known IPv4 address.
If only one Pref64::/n was present in the DNS response, a node SHALL
use that Pref64::/n for both local synthesis and for detecting
synthesis done by the DNS64 server on the network.
If more than one Pref64::/n was present in the DNS response, a node
SHOULD use all of them when determining whether other received IPv6
addresses are synthetic. The node MUST use all learned Pref64::/n
when performing local IPv6 address synthesis and use the prefixes in
the order received from the DNS64 server. That is, when the node is
providing a list of locally synthesized IPv6 addresses to upper
layers, IPv6 addresses MUST be synthesized by using all discovered
Pref64::/n prefixes in the received order.
If the well-known IPv4 addresses are not found within the standard
locations, the DNS response indicates that the network is not using a
standard address format or unexpected IPv4 addresses were used in the
AAAA resource record synthesis. In either case, the Pref64::/n
cannot be determined and the heuristic procedure has failed.
Developers can, over time, learn of IPv6-translated address formats
that are extensions or alternatives to the standard formats. At that
point, developers MAY add additional steps to the described discovery
procedure. The additional steps are outside the scope of the present
In case a node does not receive a positive DNS reply to the AAAA
resource record query, the node MAY perform a DNS A resource record
query for the well-known name. Receiving a positive reply to the DNS
A resource record query indicates that the recursive DNS server that
is used is not a DNS64 server.
In the case of a negative response (NXDOMAIN, NODATA) or a DNS query
timeout, a DNS64 server is not available on the access network, the
access network filtered out the well-known query, or something went
wrong in the DNS resolution. All unsuccessful cases result in a node
being unable to perform local IPv6 address synthesis. In the case of
timeout, the node SHOULD retransmit the DNS query like any other DNS
query the node makes [RFC1035]. In the case of a negative response
(NXDOMAIN, NODATA), the node MUST obey the Time to Live (TTL)
[RFC1035] of the response before resending the AAAA resource record
query. The node MAY monitor for DNS replies with IPv6 addresses
constructed from the WKP, in which case if any are observed, the node
SHOULD use the WKP as if it were learned during the query for the
To save Internet resources if possible, a node should perform
Pref64::/n discovery only when needed (e.g., when local synthesis is
required, when a new network interface is connected to a new network,
and so forth). The node SHALL cache the replies it receives during
the Pref64::/n discovery procedure, and it SHOULD repeat the
discovery process ten seconds before the TTL of the Well-Known Name's
synthetic AAAA resource record expires.
3.1. Validation of Discovered Pref64::/n
If a node is using an insecure channel between itself and a DNS64
server or the DNS64 server is untrusted, it is possible for an
attacker to influence the node's Pref64::/n discovery procedures.
This may result in denial-of-service, redirection, man-in-the-middle,
or other attacks.
To mitigate against attacks, the node SHOULD communicate with a
trusted DNS64 server over a secure channel or use DNSSEC. NAT64
operators SHOULD provide facilities for validating discovery of
Pref64::/n via a secure channel and/or DNSSEC protection.
It is important to understand that DNSSEC only validates that the
discovered Pref64::/n is the one that belongs to a domain used by
NAT64 FQDN. Importantly, the DNSSEC validation does not tell if the
node is at the network where the Pref64::/n is intended to be used.
Furthermore, DNSSEC validation cannot be utilized in the case of a
3.1.1. DNSSEC Requirements for the Network
If the operator has chosen to support nodes performing validation of
discovered Pref64::/n with DNSSEC, the operator of the NAT64 device
MUST perform the following configurations.
1. Have one or more fully qualified domain names for the NAT64
translator entities (later referred to as NAT64 FQDN). In the
case of more than one Pref64::/n being used in a network, e.g.,
for load-balancing purposes, it is for network administrators to
decide whether a single NAT64's fully qualified domain name maps
to more than one Pref64::/n, or whether there will be a dedicated
NAT64 FQDN per Pref64::/n.
2. Each NAT64 FQDN MUST have one or more DNS AAAA resource records
containing Pref64::WKA (Pref64::/n combined with WKA).
3. Each Pref64::WKA MUST have a PTR resource record that points to
the corresponding NAT64 FQDN.
4. Sign the NAT64 FQDNs' AAAA and A resource records with DNSSEC.
3.1.2. DNSSEC Requirements for the Node
A node SHOULD prefer a secure channel to talk to a DNS64 server
whenever possible. In addition, a node that implements a DNSSEC
validating resolver MAY use the following procedure to validate
discovery of the Pref64::/n.
1. Heuristically find Pref64::/n candidates by making a AAAA
resource record query for "ipv4only.arpa." by following the
procedure in Section 3. This will result in IPv6 addresses
consisting of Pref64::/n combined with WKA, i.e., Pref64::WKA.
For each Pref64::/n that the node wishes to validate, the node
performs the following steps.
2. Send a DNS PTR resource record query for the IPv6 address of the
translator (for ".ip6.arpa." tree), using the Pref64::WKA learned
in step 1. CNAME and DNAME results should be followed according
to the rules in RFC 1034 [RFC1034], RFC 1035 [RFC1035], and RFC
6672 [RFC6672]. The ultimate response will include one or more
3. The node SHOULD compare the domains of learned NAT64 FQDNs to a
list of the node's trusted domains and choose a NAT64 FQDN that
matches. The means for a node to learn the trusted domains is
implementation specific. If the node has no trust for the
domain, the discovery procedure is not secure, and the remaining
steps described below MUST NOT be performed.
4. Send a DNS AAAA resource record query for the NAT64 FQDN.
5. Verify the DNS AAAA resource record contains Pref64::WKA
addresses received at step 1. It is possible that the NAT64 FQDN
has multiple AAAA records, in which case the node MUST check if
any of the addresses match the ones obtained in step 1. The node
MUST ignore other responses and not use them for local IPv6
6. Perform DNSSEC validation of the DNS AAAA response.
After the node has successfully performed the above five steps, the
node can consider Pref64::/n validated.
3.2. Connectivity Check
After learning a Pref64::/n, the node SHOULD perform a connectivity
check to ensure the learned Pref64::/n is functional. It could be
non-functional for a variety of reasons -- the discovery failed to
work as expected, the IPv6 path to the NAT64 is down, the NAT64 is
down, or the IPv4 path beyond the NAT64 is down.
There are two main approaches to determine if the learned Pref64::/n
is functional. The first approach is to perform a dedicated
connectivity check. The second approach is to simply attempt to use
the learned Pref64::/n. Each approach has some trade-offs (e.g.,
additional network traffic or possible user-noticeable delay), and
implementations should carefully weigh which approach is appropriate
for their application and the network.
The node SHOULD use an implementation-specific connectivity check
server and a protocol of the implementation's choice, but if that is
not possible, a node MAY do a PTR resource record query of the
Pref64::WKA to get a NAT64 FQDN. The node then does an A resource
query of the NAT64 FQDN, which will return zero or more A resource
records pointing to connectivity check servers used by the network
operator. A negative response to the PTR or A resource query means
there are no connectivity check servers available. A network
operator that provides NAT64 services for a mix of nodes with and
without implementation-specific connectivity check servers SHOULD
assist nodes in their connectivity checks by mapping each NAT64 FQDN
to one or more DNS A resource records with IPv4 address(es) pointing
to connectivity check server(s). The connectivity check approach
based on Pref64::/n works only with NSPs, as it is not possible to
register A records for each different domain using a WKP. The
network operator MUST disable ICMPv6 rate limiting for connectivity
If multiple connectivity check servers are available for use, the
node chooses the first one, preferring implementation-specific
The connectivity check protocol used with implementation-specific
connectivity check servers is implementation specific.
The connectivity check protocol used with connectivity check servers
pointed to by the NAT64 FQDN's A resource records is ICMPv6
[RFC4443]. The node performing a connectivity check against these
servers SHALL send an ICMPv6 Echo Request to an IPv6 address
synthesized by combining discovered Pref64::/n with an IPv4 address
of the server as specified in [RFC6052]. This will test the IPv6
path to the NAT64, the NAT64's operation, and the IPv4 path all the
way to the connectivity check server. If no response is received for
the ICMPv6 Echo Request, the node SHALL send another ICMPv6 Echo
Request a second later. If still no response is received, the node
SHALL send a third ICMPv6 Echo Request two seconds later. If an
ICMPv6 Echo Response is received, the node knows the IPv6 path to the
connectivity check server is functioning normally. If no response is
received after three transmissions and after three seconds have
elapsed since the last ICMPv6 Echo Request, the node learns this
Pref64::/n might not be functioning, and the node MAY choose a
different Pref64::/n (if available), choose to alert the user, or
proceed anyway assuming the failure is temporary or is caused by the
connectivity check itself. After all, ICMPv6 is unreliable by
design, and failure to receive ICMPv6 responses may not indicate
anything other than network failure to transport ICMPv6 messages.
If no separate connectivity check is performed before local IPv6
address synthesis, a node MAY monitor success of connection attempts
performed with locally synthesized IPv6 addresses. Based on success
of these connections, and based on possible ICMPv6 error messages
received (such as Destination Unreachable messages), the node MAY
cease to perform local address synthesis and MAY restart the
Pref64::/n discovery procedures.
3.2.1. No Connectivity Checks against "ipv4only.arpa."
Clients MUST NOT send a connectivity check to an address returned by
the "ipv4only.arpa." query. This is because, by design, no server
will be operated on the Internet at that address as such. Similarly,
network operators MUST NOT operate a server on that address. The
reason this address isn't used for connectivity checks is that
operators who neglect to operate a connectivity check server will
allow that traffic towards the Internet where it will be dropped and
cause a false negative connectivity check with the client (that is,
the NAT64 is working fine, but the connectivity check fails because a
server is not operating at "ipv4only.arpa." on the Internet and a
server is not operated by the NAT64 operator). Instead, for the
connectivity check, an additional DNS resource record is looked up
and used for the connectivity check. This ensures that packets don't
unnecessarily leak to the Internet and reduces the chance of a false
negative connectivity check.
3.3. Alternative Fully Qualified Domain Names
Some applications, operating systems, devices, or networks may find
it advantageous to operate their own DNS infrastructure to perform a
function similar to "ipv4only.arpa." but use a different resource
record. The primary advantage is to ensure availability of the DNS
infrastructure and ensure the proper configuration of the DNS record
itself. For example, a company named Example might have their
application query "ipv4only.example.com". Other than the different
DNS resource record being queried, the rest of the operations are
anticipated to be identical to the steps described in this document.
3.4. Message Flow Illustration
The figure below gives an example illustration of a message flow in
the case of prefix discovery utilizing Pref64::/n validation. The
figure also shows a step where the procedure ends if no Pref64::/n
validation is performed.
In this example, three Pref64::/n prefixes are provided by the DNS64
server. The first Pref64::/n is using an NSP, in this example,
"2001:db8:42::/96". The second Pref64::/n is using an NSP, in this
example, "2001:db8:43::/96". The third Pref64::/n is using the WKP.
Hence, when the Pref64::/n prefixes are combined with the WKA to form
Pref64::WKA, the synthetic IPv6 addresses returned by the DNS64
server are "2001:db8:42::192.0.0.170", "2001:db8:43::192.0.0.170",
and "64:ff9b::192.0.0.170". The DNS64 server could also return
synthetic addresses containing the IPv4 address 192.0.0.171.
The validation is not done for the WKP; see Section 3.1.
| "AAAA" resp. #1 with "2001:db8:42::192.0.0.170 |
| "AAAA" resp. #2 with "2001:db8:43::192.0.0.170 |
| Validate AAAA responses and compare the IPv6 | |
| addresses to those previously learned. | |
| Fetch the Pref64::/n from the validated | |
| responses and take into use. | |
Figure 1: Pref64::/n Discovery Procedure4. Operational Considerations for Hosting the IPv4-Only Well-Known Name
The authoritative name server for the well-known name SHALL have DNS
record TTL set to at least 60 minutes in order to improve
effectiveness of DNS caching. The exact TTL value will be determined
and tuned based on operational experiences.
The domain serving the well-known name MUST be signed with DNSSEC.
See also Section 7.
5. Operational Considerations for DNS64 Operator
A network operator of a DNS64 server can guide nodes utilizing
heuristic discovery procedures by managing the responses a DNS64
If the network operator would like nodes to utilize multiple
Pref64::/n prefixes, the operator needs to configure DNS64 servers to
respond with multiple synthetic AAAA records. As per Section 3, the
nodes can then use them all.
There are no guarantees on which of the Pref64::/n prefixes nodes
will end up using. If the operator wants nodes to specifically use a
certain Pref64::/n or periodically change the Pref64::/n they use,
for example, for load balancing reasons, the only guaranteed method
is to make DNS64 servers return only a single synthetic AAAA resource
record and have the TTL of that synthetic record such that the node
repeats the Pref64::/n discovery when required.
Besides choosing how many Pref64::/n prefixes to respond and what TTL
to use, DNS64 servers MUST NOT interfere with or perform other
special procedures for the queries related to the well-known name.
5.1. Mapping of IPv4 Address Ranges to IPv6 Prefixes
RFC 6147 [RFC6147] allows DNS64 implementations to be able to map
specific IPv4 address ranges to separate Pref64::/n prefixes. That
allows handling of special use IPv4 addresses [RFC6890]. The example
setup where this might be used is illustrated in Figure 2. The NAT64
"A" is used when accessing IPv4-only servers in the data center, and
the NAT64 "B" is used for Internet access.
NAT64 "A" ----- IPv4-only servers in a data center
NAT64 "B" ----- IPv4 Internet
Figure 2: NAT64s with IPv4 Address Ranges
The heuristic discovery method described herein does not support
learning of the possible rules used by a DNS64 server for mapping
specific IPv4 address ranges to separate Pref64::/n prefixes.
Therefore, nodes will use the same discovered Pref64::/n to
synthesize IPv6 addresses from any IPv4 address. This can cause
issues for routing and connectivity establishment procedures. The
operator of the NAT64 and the DNS64 ought to take this into account
in the network design.
The network operators can help IPv6-only nodes by ensuring the nodes
do not have to work with IPv4 address literals for which special
mapping rules are used. That is, the IPv4-only servers addressed
from the special IPv4 address ranges ought to have signed AAAA
records, which allows IPv6-only nodes to avoid local address
synthesis. If the IPv6-only nodes are not using DNSSEC, then it is
enough if the network's DNS64 server returns synthetic AAAA resource
records pointing to IPv4-only servers. Avoiding the need for
IPv6-only nodes to perform address synthesis for IPv4 addresses
belonging to special ranges is the best approach to assist nodes.
If the IPv6-only nodes have no choice other than using IPv4-address
literals belonging to special IPv4 address ranges and the IPv6-only
node will perform local synthesis by using the discovered Pref64::/n,
then the network ought to ensure with routing that the packets are
delivered to the correct NAT64. For example, a router in the path
from an IPv6-only host to NAT64s can forward the IPv6 packets to the
correct NAT64 as illustrated in Figure 3. The routing could be based
on the last 32 bits of the IPv6 address, but the network operator can
also use some other IPv6 address format allowed by RFC 6052 [RFC6052]
if it simplifies routing setup. This setup requires additional logic
on the NAT64 providing connectivity to special IPv4 address ranges:
it needs to be able to translate packets it receives that are using
the Pref64::/n used with Internet connections.
NAT64 "A" ----- IPv4-only servers in a data center
NAT64 "B" ----- IPv4 Internet
Figure 3: NAT64s with Assisting Router6. Exit Strategy
A day will come when this tool is no longer needed. A node SHOULD
implement a configuration knob for disabling the Pref64::/n discovery
7. Security Considerations
The security considerations follow closely those of RFC 6147
[RFC6147]. The possible attacks are very similar in the case where
an attacker controls a DNS64 server and returns tampered IPv6
addresses to a node and in the case where an attacker causes the node
to use tampered Pref64::/n for local address synthesis. DNSSEC
cannot be used to validate responses created by a DNS64 server with
which the node has no trust relationship. Hence, this document does
not change the big picture for untrusted network scenarios. If an
attacker alters the Pref64::/n used by a DNS64 server or a node, the
traffic generated by the node will be delivered to an altered
destination. This can result in either a denial-of-service (DoS)
attack (if the resulting IPv6 addresses are not assigned to any
device), a flooding attack (if the resulting IPv6 addresses are
assigned to devices that do not wish to receive the traffic), or an
eavesdropping attack (in case the altered NSP is routed through the
Even though a well-known name's DNS A resource record would not
necessarily need to be protected with DNSSEC as both the name and
IPv4 addresses well-known, DNSSEC protection is required for DNS AAAA
resource record queries. Without DNSSEC, fake positive AAAA
responses could cause hosts to erroneously detect Pref64::/n, thus
allowing an attacker to inject malicious Pref64::/n for hosts'
synthesis procedures. 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.
For Pref64::/n discovery validation, the access network SHOULD sign
the NAT64 translator's fully qualified domain name. A node SHOULD
use the algorithm described in Section 3.1 to validate each
The procedure described in Section 3.1.2 requires a node using DNSSEC
to validate discovery of Pref64::/n to have a list of trusted
domains. If a matching domain is not found at Step 3 in
Section 3.1.2, an implementation might be tempted to ask a user to
temporarily or permanently add a received domain as trusted. History
has shown that average users are unable to properly handle such
queries and tend to answer positively without thinking in an attempt
to move forward quickly. Therefore, unless the DNSSEC-using
implementation has a way to dynamically and reliably add trusted
domains, it is better to fail the Pref64::/n discovery procedure.
Lastly, the best mitigation action against Pref64::/n discovery
attacks is to add IPv6 support for nodes' destinations and hence
reduce the need to perform local IPv6 address synthesis.
8. IANA Considerations
8.1. Domain Name Reservation Considerations
According to procedures described in [RFC3172] and [RFC6761], IANA
has delegated a new second-level domain in the .ARPA zone for the
well-known domain name "ipv4only.arpa.". The intention is that there
will not be any further delegation of names below the
"ipv4only.arpa." domain. The administrative and operational
management of this zone is performed by IANA. The answers to the
seven questions listed in [RFC6761] are as follows:
1. Are human users expected to recognize these names as special and
use them differently? In what way?
No, although this is a domain delegated under the .arpa
infrastructural identifier top level domain.
2. Are writers of application software expected to make their
software recognize these names as special and treat them
differently? In what way?
Yes. Any application attempting to perform NAT64 discovery will
query the name.
3. Are writers of name resolution APIs and libraries expected to
make their software recognize these names as special and treat
them differently? If so, how?
Yes, to the extent the API or library is affected by NAT64.
4. Are developers of caching domain name servers expected to make
their implementations recognize these names as special and treat
them differently? If so, how?
5. Are developers of authoritative domain name servers expected to
make their implementations recognize these names as special and
treat them differently? If so, how?
6. Does this reserved Special-Use Domain Name have any potential
impact on DNS server operators? If they try to configure their
authoritative DNS server as authoritative for this reserved name,
will compliant name server software reject it as invalid? Do DNS
server operators need to know about that and understand why?
Even if the name server software doesn't prevent them from using
this reserved name, are there other ways that it may not work as
expected, of which the DNS server operator should be aware?
This name has effects for operators of NAT64/DNS64, but otherwise
is just another delegated .arpa domain.
7. How should DNS Registries/Registrars treat requests to register
this reserved domain name? Should such requests be denied?
Should such requests be allowed, but only to a specially-
The registry for .arpa is held at IANA, and only IANA needs to
take action here.
8.2. IPv4 Address Allocation Considerations
The well-known name needs to map to two different global IPv4
addresses, which have been allocated as described in [RFC6890]. The
addresses have been taken from the IANA IPv4 Special Purpose Address
Registry [RFC6890], and 192.0.0.170 and 192.0.0.171 have been
assigned to this document with the parameters shown below:
| Attribute | Value |
| Address Block | 192.0.0.170/32 |
| | 192.0.0.171/32 |
| Name | NAT64/DNS64 Discovery |
| RFC | RFC 7050, Section 2.2 |
| Allocation Date | February 2013 |
| Termination Date | N/A |
| Source | False |
| Destination | False |
| Forwardable | False |
| Global | False |
| Reserved-by-protocol | True |
The Record for IPv4 Address Allocation for IPv4 Special Purpose
The zone "ipv4only.arpa." is delegated from the ARPA zone to
appropriate name servers chosen by the IANA. An apex A RRSet has
been inserted in the "ipv4only.arpa." zone as follows:
IPV4ONLY.ARPA. IN A 192.0.0.170
IPV4ONLY.ARPA. IN A 192.0.0.171
8.3. IAB Statement Regarding This .arpa Request
With the publication of this document, the IAB approves of the
delegation of "ipv4only" in the .arpa domain. Under [RFC3172], the
IAB has requested that IANA delegate and provision "ipv4only.arpa."
as written in this specification. However, the IAB does not take any
architectural or technical position about this specification.
The authors would like to thank Dmitry Anipko, Cameron Byrne, Aaron
Yi Ding, Christian Huitema, Washam Fan, Peter Koch, Stephan
Lagerholm, Zhenqiang Li, Simon Perreault, Marc Petit-Huguenin, Andrew
Sullivan, and Dave Thaler for significant improvement ideas and
Jouni Korhonen would like to acknowledge his previous employer, Nokia
Siemens Networks, where the majority of his work on this document was
10.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[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,
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, June 2012.
10.2. Informative References
[RFC3172] Huston, G., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, September 2001.
[RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
RFC 5735, January 2010.
[RFC6144] Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation", RFC 6144, April 2011.
[RFC6418] Blanchet, M. and P. Seite, "Multiple Interfaces and
Provisioning Domains Problem Statement", RFC 6418,
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, February 2013.
[RFC6890] Cotton, M., Vegoda, L., Bonica, R., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153, RFC
6890, April 2013.
Appendix A. Example of DNS Record Configuration
The following BIND-style examples illustrate how A and AAAA records
could be configured by a NAT64 operator.
The examples use Pref64::/n of 2001:db8::/96, both WKAs, and the
The PTR record for reverse queries (Section 3.1.1, Bullet 3):
@ IN SOA ns1.example.com. hostmaster.example.com. (
2003080800 12h 15m 3w 2h)
IN NS ns.example.com.
IN PTR nat64.example.com.
@ IN SOA ns1.example.com. hostmaster.example.com. (
2003080800 12h 15m 3w 2h)
IN NS ns.example.com.
IN PTR nat64.example.com.
If example.com does not use DNSSEC, the following configuration file
could be used. Please note that nat64.example.com has both a AAAA
record with the Pref64::/n and an A record for the connectivity check
(Section 3.1.1, Bullet 2).
example.com. IN SOA ns.example.com. hostmaster.example.com. (
2002050501 ; serial
100 ; refresh (1 minute 40 seconds)
200 ; retry (3 minutes 20 seconds)
604800 ; expire (1 week)
100 ; minimum (1 minute 40 seconds)
example.com. IN NS ns.example.com.
IN AAAA 2001:db8:0:0:0:0:C000:00AA
IN AAAA 2001:db8:0:0:0:0:C000:00AB
IN A 192.0.2.1
For DNSSEC to sign the records, the owner of the example.com zone
would have RRSIG records for both the AAAA and A records for
nat64.example.com. As a normal DNSSEC requirement, the zone and its
parent also need to be signed.
Appendix B. About the IPv4 Address for the Well-Known Name
The IPv4 addresses for the well-known name cannot be non-global IPv4
addresses as listed in the Section 3 of [RFC5735]. Otherwise, DNS64
servers might not perform AAAA record synthesis when the well-known
prefix is used, as stated in Section 3.1 of [RFC6052]. However, the
addresses do not have to be routable or allocated to any real node as
no communications will be initiated to these IPv4 address.
Allocation of at least two IPv4 addresses improves the heuristics in
cases where the bit pattern of the primary IPv4 address appears more
than once in the synthetic IPv6 address (i.e., the NSP prefix
contains the same bit pattern as the IPv4 address).
If no well-known IPv4 addresses would be statically allocated for
this method, the heuristic would require sending of an additional A
query to learn the IPv4 addresses that would be then searched from
inside of the received IPv6 address.
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