Internet Engineering Task Force (IETF)
Request for Comments: 7050
Category: Standards Track
ISSN: 2070-1721
T. Savolainen
J. Korhonen
D. Wing
Cisco Systems
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 "". The information learned enables nodes to perform local IPv6 address synthesis and to potentially avoid NAT64 on dual-stack and multi- interface deployments.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2013 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 ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Requirements Notation and Terminology ...........................4
      2.1. Requirements Notation ......................................4
      2.2. Terminology ................................................4
   3. Node Behavior ...................................................4
      3.1. Validation of Discovered Pref64::/n ........................6
           3.1.1. DNSSEC Requirements for the Network .................7
           3.1.2. DNSSEC Requirements for the Node ....................7
      3.2. Connectivity Check .........................................8
           3.2.1. No Connectivity Checks against "" .....9
      3.3. Alternative Fully Qualified Domain Names ..................10
      3.4. Message Flow Illustration .................................10
   4. Operational Considerations for Hosting the IPv4-Only
      Well-Known Name ................................................12
   5. Operational Considerations for DNS64 Operator ..................12
      5.1. Mapping of IPv4 Address Ranges to IPv6 Prefixes ...........13
   6. Exit Strategy ..................................................14
   7. Security Considerations ........................................14
   8. IANA Considerations ............................................15
      8.1. Domain Name Reservation Considerations ....................15
      8.2. IPv4 Address Allocation Considerations ....................16
      8.3. IAB Statement Regarding This .arpa Request ................17
   9. Acknowledgements ...............................................18
   10. References ....................................................18
      10.1. Normative References .....................................18
      10.2. Informative References ...................................19
   Appendix A.  Example of DNS Record Configuration ..................20
   Appendix B.  About the IPv4 Address for the Well-Known Name .......21

1. Introduction

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 transition solution.

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].

2.2. Terminology

NAT64 FQDN: a fully qualified domain name (FQDN) for a NAT64 protocol translator.


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, "", 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: and

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) "". 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 document.

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 well-known name.

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 WKP.

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.
  1. Each NAT64 FQDN MUST have one or more DNS AAAA resource records containing Pref64::WKA (Pref64::/n combined with WKA).
  1. Each Pref64::WKA MUST have a PTR resource record that points to the corresponding NAT64 FQDN.
  1. 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 "" 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.
  1. Send a DNS PTR resource record query for the IPv6 address of the translator (for "" 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 NAT64 FQDNs.
  1. 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.
  1. Send a DNS AAAA resource record query for the NAT64 FQDN.
  1. 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 address synthesis.
  1. 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 check messages.

If multiple connectivity check servers are available for use, the node chooses the first one, preferring implementation-specific servers.

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 ""

Clients MUST NOT send a connectivity check to an address returned by the "" 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 "" 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 "" 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 "". 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::", "2001:db8:43::", and "64:ff9b::". The DNS64 server could also return synthetic addresses containing the IPv4 address

The validation is not done for the WKP; see Section 3.1.

    Node                                           DNS64 server
      |                                                |
      |  "AAAA" query for ""             |
      |----------------------------------------------->|"A" query for
      |                                                |""
      |                                                |--------------->
      |                                                |
      |                                                | "A" response:
      |                                                | ""
      |                                                | ""
      |                                                |<---------------
      |                                +----------------------------+
      |                                | "AAAA" synthesis using     |
      |                                | three Pref64::/n.          |
      |                                +----------------------------+
      |  "AAAA" response with:                         |
      |  "2001:db8:42::"                    |
      |  "2001:db8:43::"                    |
      |  "64:ff9b::"                        |
      |                                                |
   +----------------------------------------------+    |
   | If Pref64::/n validation is not performed, a |    |
   | node can fetch prefixes from AAAA responses  |    |
   | at this point and skip the steps below.      |    |
   +----------------------------------------------+    |
      |                                                |
      |  "PTR" query #1 for "2001:db8:42::  |
      |  "PTR" query #2 for "2001:db8:43::  |
      |                                                |
      |  "PTR" response #1 ""       |
      |  "PTR" response #2 ""       |
      |                                                |
   +----------------------------------------------+    |
   | Compare received domains to a trusted domain |    |
   | list and if matches are found, continue.     |    |
   +----------------------------------------------+    |
      |                                                |
      |  "AAAA" query #1 for ""     |
      |  "AAAA" query #2 for ""     |
      |                                                |
      | "AAAA" resp. #1 with "2001:db8:42:: |
      | "AAAA" resp. #2 with "2001:db8:43:: |
      |                                                |
   +----------------------------------------------+    |
   | 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 Procedure

4. 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 server provides.

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
   IPv6-only node---<
                      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
   IPv6-only host---router
                       NAT64 "B" ----- IPv4 Internet

Figure 3: NAT64s with Assisting Router

6. 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 feature.

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 attacker).

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 "" 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 discovered Pref64::/n.

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 "". The intention is that there will not be any further delegation of names below the "" 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.

  1. 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.

  1. 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.

  1. Are developers of caching domain name servers expected to make their implementations recognize these names as special and treat them differently? If so, how?


  1. Are developers of authoritative domain name servers expected to make their implementations recognize these names as special and treat them differently? If so, how?


  1. 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.

  1. 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- designated entity?

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 and have been assigned to this document with the parameters shown below:

          | Attribute            | Value                         |
          | Address Block        |                |
          |                      |                |
          | 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

Address Registry

The zone "" is delegated from the ARPA zone to appropriate name servers chosen by the IANA. An apex A RRSet has been inserted in the "" zone as follows:



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 "" as written in this specification. However, the IAB does not take any architectural or technical position about this specification.

9. Acknowledgements

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 comments.

Jouni Korhonen would like to acknowledge his previous employer, Nokia Siemens Networks, where the majority of his work on this document was carried out.

10. References

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,
              October 2010.
   [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,
              April 2011.
   [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,
              November 2011.
   [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 domain.

The PTR record for reverse queries (Section 3.1.1, Bullet 3):

   $ORIGIN A.A.\
   @         IN      SOA (
                           2003080800 12h 15m 3w 2h)
             IN      NS
             IN      PTR
   $ORIGIN B.A.\
   @         IN      SOA (
                           2003080800 12h 15m 3w 2h)
             IN      NS
             IN      PTR

If does not use DNSSEC, the following configuration file could be used. Please note that has both a AAAA record with the Pref64::/n and an A record for the connectivity check (Section 3.1.1, Bullet 2).  IN SOA (
                         2002050501 ; serial
                         100        ; refresh (1 minute 40 seconds)
                         200        ; retry (3 minutes 20 seconds)
                         604800     ; expire (1 week)
                         100        ; minimum (1 minute 40 seconds)
                         )  IN NS

IN AAAA 2001:db8:0:0:0:0:C000:00AA

IN AAAA 2001:db8:0:0:0:0:C000:00AB


For DNSSEC to sign the records, the owner of the zone would have RRSIG records for both the AAAA and A records for 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.

Authors' Addresses

Teemu Savolainen
Hermiankatu 12 D
FI-33720 Tampere


Jouni Korhonen
Linnoitustie 6
FI-02600 Espoo

   Dan Wing
   Cisco Systems
   170 West Tasman Drive
   San Jose, California  95134