wdiff rfc9872v1.txt rfc9872.txt

Internet Engineering Task Force (IETF) N. Buraglio
Request for Comments: 9872 Energy Sciences Network
Category: Informational T. Jensen
ISSN: 2070-1721
J. Linkova
Google
September 2025

Recommendations for Discovering IPv6 Prefix Used for IPv6 Address
Synthesis

Abstract

On networks providing IPv4-IPv6 translation (RFC 7915), hosts and
other endpoints need to know the IPv6 prefix(es) used for translation
(the NAT64 prefix (RFC 6052)). This document provides guidelines for
NAT64 prefix discovery, specifically recommending obtaining the NAT64
prefix from the Router Advertisement option (RFC 8781) when
available.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

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). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see 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/rfc9872.

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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Terminology
3. Recommendations for PREF64 Discovery
3.1. Deployment Recommendations for Endpoints
3.2. Deployment Recommendations for Operators
3.2.1. Mobile Network Considerations
3.2.2. Migration Considerations
4. Existing Issues with RFC 7050
4.1. Dependency on Network-Provided Recursive Resolvers
4.2. Network Stack Initialization Delay
4.3. Latency in Updates Propagation
4.4. Multihoming Implications
4.5. Security Implications
4.5.1. Definition of Secure Channel
4.5.2. Secure Channel Example of IPsec
4.5.3. Secure Channel Example of Link Layer Encryption
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

Devices translating between IPv4 and IPv6 packet headers [RFC7915]
use a NAT64 prefix to map IPv4 addresses into the IPv6 address space,
and vice versa. When a network provides NAT64, it is advantageous
for endpoints to acquire the network's NAT64 prefixes (PREF64).
Discovering the PREF64 enables endpoints to:

* Implement the customer-side translator (CLAT) function of the
464XLAT architecture [RFC6877].

* Translate IPv4 literals to IPv6 literals (Section 7.1 of
[RFC8305]).

* Perform local DNS64 [RFC6147] functions.

* Support applications relying on IPv4 address referral
(Section 3.2.2 of [RFC7225]).

Dynamic PREF64 discovery is useful to keep the NAT64 prefix
configuration up-to-date, particularly for unmanaged endpoints or
endpoints that move between networks. [RFC7050] introduces the first
DNS64-based mechanism for PREF64 discovery based on [RFC7051]
analysis. per the analysis described
in [RFC7051]. However, subsequent methods have been developed to
address the [RFC7050] limitations. limitations of the mechanism described in [RFC7050].

For instance, [RFC8781] defines a Neighbor Discovery [RFC4861] option
for Router Advertisements (RAs) to convey PREF64 information to
hosts. This approach offers several advantages (Section 3 of
[RFC8781]), including fate sharing with other host network
configuration parameters.

Due to fundamental shortcomings of the [RFC7050] mechanism
(Section 4), defined in [RFC7050]
(see Section 4 for more details), [RFC8781] is describes the preferred
solution for new deployments. Implementations should strive for
consistent PREF64 acquisition methods. The DNS64-based mechanism of
[RFC7050] should be employed only when RA-based PREF64 delivery is
unavailable or as a fallback for legacy systems incapable of
processing the PREF64 RA Option.

2. Terminology

DNS64: A mechanism for synthesizing AAAA records from A records,
defined in [RFC6147].

NAT64: A mechanism for translating IPv6 packets to IPv4 packets, and
vice versa. The translation is done by translating the packet
headers according to the IP/ICMP Translation Algorithm defined in
[RFC7915]. NAT64 translators can operate in stateful mode
[RFC6144] or stateless mode [RFC6877] (e.g., customer-side
translator (CLAT)). This document uses "NAT64" as a generalized
term for a translator, which uses the stateless IP/ICMP
Translation Algorithm defined in [RFC7915] and operates within a
framework for IPv4/IPv6 translation described in [RFC6144].

PREF64 (Pref64::/n

PREF64: Pref64::/n or NAT64 prefix): prefix. An IPv6 prefix used for IPv6
address synthesis and for translating network addresses and
protocols from IPv6 clients to IPv4 servers using the algorithm
defined in [RFC6052].

RA: Router Advertisement (RA): Advertisement. A packet used by Neighbor Discovery
[RFC4861] and SLAAC to advertise the presence of the routers,
together with other IPv6 configuration information.

SLAAC: Stateless Address Autoconfiguration [RFC4862].

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.

3. Recommendations for PREF64 Discovery

3.1. Deployment Recommendations for Endpoints

Endpoints SHOULD attempt to obtain PREF64 information from RAs per
[RFC8781], instead of using the [RFC7050] method. method described in [RFC7050]. In
the absence of the PREF64 information in RAs, an endpoint MAY choose
to fall back to the mechanism defined in [RFC7050]. This
recommendation to prefer the [RFC8781] mechanism defined in [RFC8781] over the
one defined in [RFC7050] is consistent with Section 5.1 of [RFC8781].

3.2. Deployment Recommendations for Operators

Network operators deploying NAT64 SHOULD provide PREF64 information
in Router Advertisements per [RFC8781].

3.2.1. Mobile Network Considerations

While [RFC8781] support for the option specified in [RFC8781] is widely
integrated into modern operating systems on mobile endpoints,
equipment deployed in mobile network environments often lacks
abilities to include the PREF64 Option into RAs. Therefore, the
immediate deployment and enablement of PREF64 by mobile operators may
not currently be feasible and the recommendations outlined in this
document are not presently applicable to mobile network operators.
These environments are encouraged to incorporate the option specified
in [RFC8781] when made practical by infrastructure upgrades or
software stack feature additions.

3.2.2. Migration Considerations

Transitioning from the heurisitic procedure in [RFC7050] heuristic to using the [RFC8781]
approach in [RFC8781] might require a period of time where both
mechanisms coexist. How long this may take depends on the endpoint
footprint, particularly the presence and number of endpoints running
outdated operating systems that do not support the option in
[RFC8781]. Operators are advised to take those factors into account
prior to removing support for the [RFC7050] heuristic, heurisitc procedure in [RFC7050],
noting that it is still safe to add support for the [RFC8781] approach in
[RFC8781] since endpoints that support it will always prefer it over
[RFC7050] if they follow RFC requirements.

Migrating away from DNS64-based discovery also reduces dependency on
DNS64 in general, thereby eliminating DNSSEC and DNS64
incompatibility concerns (Section 6.2 of [RFC6147]).

4. Existing Issues with RFC 7050

DNS-based discovery of the NAT64 prefix introduces some challenges,
which make this approach less preferable than the latest developed
alternatives (such as the PREF64 RA Option [RFC8781]). This section
outlines the key issues associated with [RFC7050] with a focus on
those not discussed in [RFC7050] or in the analysis of solutions for
hosts to discover the NAT64 prefix [RFC7051].

Signalling PREF64 in the RA option Option addresses all issues outlined in
this section (see Section 3 of [RFC8781] for details).

4.1. Dependency on Network-Provided Recursive Resolvers

Fundamentally, the presence of the NAT64 function and the exact value of the prefix
used for the translation are network-specific attributes. Therefore,
[RFC7050] requires the endpoint discovering the prefix to use the
DNS64 resolvers provided by the network. If the device or an
application is configured to use other recursive resolvers or runs a
local recursive resolver, the corresponding name resolution APIs and
libraries are required to recognize 'ipv4only.arpa.' as a special
name and give it special treatment. This issue and remediation
approach are discussed in [RFC8880]. However, it has been observed
that very few [RFC7050] implementations of the method described in [RFC7050]
support the [RFC8880] requirements specified in [RFC8880] for special treatment
of 'ipv4only.arpa.'. As a result, configuring such systems and
applications to use resolvers other than the one provided by the
network breaks the PREF64 discovery, leading to degraded user
experience.

VPN applications may override the endpoint's DNS configuration, for
example, by configuring enterprise DNS servers as the node's
recursive resolvers and forcing all name resolution through the VPN.
These enterprise DNS servers typically lack DNS64 functionality and
therefore cannot provide information about the PREF64 used within the
local network. If the VPN is configured in so-called "split
tunneling" mode (when only a subset of network traffic is routed into
the VPN tunnel), endpoints may not discover the necessary PREF64,
which negatively impacts their connectivity on IPv6-only networks.

If both the network-provided DNS64 and the endpoint's resolver happen
to utilize the Well-Known Prefix (64:ff9b::/96) [RFC6052], the
endpoint would end up using a PREF64 that's valid for the current
network. However, if the endpoint changes its network attachment, it
can't detect if the new network lacks NAT64 entirely or uses a
network-specific prefix (NSP) [RFC6144] for NAT64.

Signalling PREF64 in an RA option Option decouples the PREF64 discovery from
the host's DNS resolver configuration.

4.2. Network Stack Initialization Delay

When using SLAAC, an IPv6 host typically requires a single RA to
acquire its network configuration. For IPv6-only endpoints, timely
PREF64 discovery is critical, particularly for those performing local
DNS64 or NAT64 functions, such as CLAT [RFC6877]. Until a PREF64 is
obtained, the endpoint's IPv4-only applications and communication to
IPv4-only destinations are impaired. The mechanism defined in
[RFC7050] does not bundle PREF64 information with other network
configuration parameters and requires at least one round-trip time
(to send a DNS request and receive a response) after the network
stack configuration is completed.

On the other hand, advertising PREF64 in an RA eliminates the period
when the host obtains IPv6 addresses and default routers but no
PREF64.

4.3. Latency in Updates Propagation

Section 3 of [RFC7050] states:

| 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.

As a result, once a PREF64 is discovered, it will be used until the
TTL expires or until the node disconnects from the network. There is
no mechanism for an operator to force the PREF64 rediscovery on the
node without disconnecting the node from the network. If the
operator needs to change the PREF64 value used in the network, they
need to proactively reduce the TTL value returned by the DNS64
server. This method has two significant drawbacks:

* Many networks utilize external DNS64 servers and therefore have no
control over the TTL value if the PREF64 needs to be changed or
withdrawn.

* The PREF64 changes need to be planned and executed at least TTL
seconds in advance. If the operator needs to notify nodes that a
particular prefix must not be used (e.g., during a network outage
or if the nodes learned a rogue PREF64 as a result of an attack),
it might not be possible without interrupting the network
connectivity for the affected nodes.

The mechanism defined in [RFC8781] allows notifying hosts about
PREF64 changes immediately by sending an RA with updated information.

4.4. Multihoming Implications

Section 3 of [RFC7050] requires a node to examine all received AAAA
resource records to discover one or more PREF64s and to utilize all
learned prefixes. However, this approach presents challenges in some
multihomed topologies where different DNS64 servers belonging to
different ISPs might return different PREF64s. In such cases, it is
crucial that traffic destined for synthesized addresses is sent to
the correct NAT64 and the source address selected for those flows
belongs to the prefix from that ISP's address space. In other words,
the node needs to associate each discovered PREF64 with upstream
information, including the IPv6 prefix and default gateway.
Currently, there is no reliable way for a node to map a DNS64
response (and the prefix learned from it) to a specific upstream in a
multihoming scenario. Consequently, the node might inadvertently
select an incorrect source address for a given PREF64 and/or send
traffic to the incorrect uplink.

Advertising PREF64 in RAs allows hosts to track which PREF64 was
advertised by which router and use that information to select the
correct next hop. Section 8 of [CLAT] discusses this scenario in
more details.

4.5. Security Implications

As discussed in Section 7 of [RFC7050], the DNS-based PREF64
discovery is prone to DNS spoofing attacks. In addition to creating
a wider attack surface for IPv6 deployments, [RFC7050] has other
security challenges, which are discussed below.

4.5.1. Definition of Secure Channel

[RFC7050] requires a node's communication channel with a DNS64 server
to be a "secure channel", which it defines to mean "a communication
channel a node has between itself and a DNS64 server protecting DNS
protocol-related messages from interception and tampering". This
need is redundant when another communication mechanism of
IPv6-related configuration, specifically RAs, can already be defended
against tampering, for example, by enabling RA-Guard [RFC6105].
Requiring nodes to implement two defense mechanisms when only one is
necessary when When
the mechanism defined in [RFC8781] is used in place of [RFC7050] the one
defined in [RFC7050], nodes only need to implement one defense
mechanism; requiring nodes to implement two defense mechanisms
creates an unnecessary risk.

4.5.2. Secure Channel Example of IPsec

One of the two examples that [RFC7050] defines to qualify a
communication channel with a DNS64 server is the use of an "IPsec-
based virtual private network (VPN) tunnel". As of the time of this
writing, this is not supported as a practice by any common operating
system DNS client. While they could, there have also since been
multiple mechanisms defined for performing DNS-specific encryption,
such as those defined in [RFC9499], that would be more appropriately
scoped to the applicable DNS traffic. These are also compatible with
encrypted DNS advertisement by the network using Discovery of
Network-designated Resolvers [RFC9463], which would ensure the
clients know in advance that the DNS64 server supported the
encryption mechanism.

4.5.3. Secure Channel Example of Link Layer Encryption

The other example given by [RFC7050] that would allow a communication
channel with a DNS64 server to qualify as a "secure channel" is the
use of a "link layer utilizing data encryption technologies". As of
the time of this writing, most common link layer implementations use
data encryption already with no extra effort needed on the part of
network nodes. While this appears to be a trivial way to satisfy
this requirement, it also renders the requirement meaningless since
any node along the path can still read the higher-layer DNS traffic
containing the translation prefix. This seems to be at odds with the
definition of "secure channel", as explained in Section 2.2 of
[RFC7050].

5. Security Considerations

Obtaining PREF64 information using RAs improves the overall security
of an IPv6-only endpoint as it mitigates all attack vectors related
to a spoofed or rogue DNS response, as discussed in Section 7 of
[RFC7050]. Security considerations related to obtaining PREF64
information from RAs are discussed in Section 7 of [RFC8781].

6. IANA Considerations

This document has no IANA actions.

7. References

7.1. 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>.

[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>.

[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>.

[RFC8781] Colitti, L. and J. Linkova, "Discovering PREF64 in Router
Advertisements", RFC 8781, DOI 10.17487/RFC8781, April
2020, <https://www.rfc-editor.org/info/rfc8781>.

7.2. Informative References

[CLAT] Colitti, L., Linkova, J., and T. Jensen, "464XLAT
Customer-side Translator (CLAT): Node Recommendations",
Work in Progress, Internet-Draft, draft-ietf-v6ops-claton-
08, 17 September 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
claton-08>.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.

[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.

[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<https://www.rfc-editor.org/info/rfc6052>.

[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
DOI 10.17487/RFC6105, February 2011,
<https://www.rfc-editor.org/info/rfc6105>.

[RFC6144] Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation", RFC 6144, DOI 10.17487/RFC6144,
April 2011, <https://www.rfc-editor.org/info/rfc6144>.

[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>.

[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC6877, April 2013,
<https://www.rfc-editor.org/info/rfc6877>.

[RFC7051] Korhonen, J., Ed. and T. Savolainen, Ed., "Analysis of
Solution Proposals for Hosts to Learn NAT64 Prefix",
RFC 7051, DOI 10.17487/RFC7051, November 2013,
<https://www.rfc-editor.org/info/rfc7051>.

[RFC7225] Boucadair, M., "Discovering NAT64 IPv6 Prefixes Using the
Port Control Protocol (PCP)", RFC 7225,
DOI 10.17487/RFC7225, May 2014,
<https://www.rfc-editor.org/info/rfc7225>.

[RFC7915] Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
"IP/ICMP Translation Algorithm", RFC 7915,
DOI 10.17487/RFC7915, June 2016,
<https://www.rfc-editor.org/info/rfc7915>.

[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.

[RFC8880] Cheshire, S. and D. Schinazi, "Special Use Domain Name
'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August
2020, <https://www.rfc-editor.org/info/rfc8880>.

[RFC9463] Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
and T. Jensen, "DHCP and Router Advertisement Options for
the Discovery of Network-designated Resolvers (DNR)",
RFC 9463, DOI 10.17487/RFC9463, November 2023,
<https://www.rfc-editor.org/info/rfc9463>.

[RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<https://www.rfc-editor.org/info/rfc9499>.

Acknowledgments

The authors would like to thank the following people for their
valuable contributions: Mike Bishop, Mohamed Boucadair, Lorenzo
Colitti, Tom Costello, Charles Eckel, Susan Hares, Nick Heatley, Ted
Lemon, Gábor Lencse, David Lou, Peter Schmitt, Éric Vyncke, and
Chongfeng Xie.

Authors' Addresses

Nick Buraglio
Energy Sciences Network
Email: buraglio@forwardingplane.net

Tommy Jensen
Email: tojens.ietf@gmail.com

Jen Linkova
Google
Email: furry13@gmail.com