IPv6 operations J. Linkova
Internet-Draft Google
Updates: 8585 (if approved) T. Jensen
Intended status: Informational Microsoft
Expires: 27 August 2025 23 February 2025
464XLAT Customer-side Translator (CLAT): Node Recommendations
draft-ietf-v6ops-claton-03
Abstract
464XLAT [RFC6877] defines an architecture for providing IPv4
connectivity across an IPv6-only network. The solution contains two
key elements: provider-side translator (PLAT) and customer-side
translator (CLAT). This document complements [RFC6877] and updates
[RFC8585] by providing recommendations for the node developers on
enabling and disabling CLAT functions.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 27 August 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Multiple Interfaces Considerations . . . . . . . . . . . . . 4
5. Enabling CLAT . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Disabling CLAT . . . . . . . . . . . . . . . . . . . . . . . 5
7. CLAT Addresses Considerations . . . . . . . . . . . . . . . . 6
7.1. CLAT IPv4 Addresses . . . . . . . . . . . . . . . . . . . 6
7.2. CLAT IPv6 Addresses . . . . . . . . . . . . . . . . . . . 7
8. CLAT and Multiple Prefixes per Interface . . . . . . . . . . 8
8.1. Link Renumbering . . . . . . . . . . . . . . . . . . . . 10
9. MTU Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Updates to RFC8585 . . . . . . . . . . . . . . . . . . . . . 11
11. Security Considerations . . . . . . . . . . . . . . . . . . . 13
12. Privacy Considerations . . . . . . . . . . . . . . . . . . . 14
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
14.1. Normative References . . . . . . . . . . . . . . . . . . 14
14.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Enabling and Disabling CLAT: Flowchart . . . . . . . 18
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
464XLAT is widely deployed in 3GPP networks (as described in
Section 4.2 of [RFC6877]) where User Equipment (UE) devices (such as
mobile phones and CE routers) perform the CLAT function, providing a
private IPv4 address and default route for the applications and
tethered devices. Enabling 464XLAT allowed mobile operators to
transition UE devices to IPv6-only mode, where those devices are only
assigned IPv6 addresses on their WAN interfaces.
Until recently, IPv6-only hosts were rather uncommon outside of
mobile networks and datacenters. Even if the network provides PLAT
in the form of NAT64 ([RFC6146]), hosts (desktops, laptops, etc.)
still needed the network to provide IPv4 addresses, as otherwise
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IPv4-only applications fail. However, as more and more operating
systems outside of the 3GPP world support CLAT, it becomes possible
to migrate those devices to IPv6-only mode, while still providing
IPv4 as a service via 464XLAT. Networks such as public Wi-Fi,
enterprise networks, or even home networks can deploy 464XLAT as
described in Section 4.2 of [RFC6877]:
* PLAT functions are performed by NAT64 network devices.
* CLAT is performed by the UA itself.
Another 464XLAT deployment model is a Wireline one (section 4.1 of
[RFC6877]), when a CPE router is connected to an IPv6-only network
and provides CLAT functions for IPv4-enabled downstream devices.
[RFC8585] specifies 464XLAT support requirements for such devices.
For both scenarios to work, the node performing CLAT (such as a host
or a CPE router) need to enable the CLAT when connecting to an
IPv6-only network. This document complements [RFC6877] and updates
[RFC8585] by providing recommendations for the node developers on
enabling and disabling CLAT functions.
2. Requirements Language
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. Terminology
This document reuses most of Terminology section from [RFC6877].
* CLAT Node: a node (a host or a router) which performs CLAT
functions by running one or multiple CLAT instances (e.g. one CLAT
instance per interface).
* IPv6-only network: A network that does not assign IPv4 addresses
to hosts and facilitates connectivity to IPv4-only destinations
using NAT64 ([RFC6146]). In this document, the term "IPv6-only
network" specifically refers to networks that provide NAT64 as
it's required by the 464XLAT architecture ([RFC6877]).
* Native IPv4 (such as in 'native IPv4 connectivity' or 'native IPv4
default gateway'): IPv4 connectivity or default gateway provided
by the network without using any form of IPv4-as-a-service or
translation mechanisms (such as 464XLAT).
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* ULA: Unique Local Addresses, [RFC4193].
Currently the only avalable form of PLAT for 464XLAT deployments is
NAT64. Therefore this document uses those terms interchangeably.
4. Multiple Interfaces Considerations
A node may have multiple IPv6-only interfaces (for example, a mobile
phone can be connected to an IPv6-only Wi-Fi network and to an
IPv6-only mobile network). In that case the node SHOULD run an
independent, dedicated CLAT instance on each interface connected to a
network equipped with PLAT. Consequently, each CLAT instance SHOULD
install a separate default IPv4 route on each CLAT-enabled interface.
The metrics of IPv4 routes SHOULD be consistent with the metrics of
IPv6 default routes.
5. Enabling CLAT
For performance and security reasons CLAT MUST NOT be enabled if the
node has IPv4 native connectivity over the given interface.
Therefore recommendations provided in this section are only
applicable to an IPv6-only node (a node which does not have a native
IPv4 default route configured).
To enable CLAT, an IPv6-only node needs to discover the PLAT-side
translation IPv6 prefix, also known as the NAT64 prefix (see
Section 6.3 of [RFC6877]). The PREF64 Router Advertisement (RA)
option ([RFC8781]) provides that information and can be used as a
strong indication that the network supports PLAT (NAT64)
functionality. Therefore an IPv6-only node SHOULD enable CLAT as
soon as a Router Advertisement containing PREF64 option is received.
If RAs received by the node do not contain a PREF64 option, the node
MAY use other mechanisms to detect the PLAT presence and obtain the
NAT64 prefix (such as [RFC7050]). When discovering the NAT64 prefix
using the mechanism defined in [RFC7050], the node MUST follow
recommendations provided in [RFC8880]. Specifically, the node MUST
send the query to the DNS servers for the specific network interface
per Section 7.1 of [RFC8880]. In particular, queries for AAAA
resource records of "ipv4only.arpa." MUST be sent to the recursive
resolvers provided by the network, not to the resolvers configured
manually, local recursive resolvers etc.
Any delay in enabling CLAT on an IPv6-only node would be impactful
for IPv4-only applications, as such applications cannot benefit from
464XLAT until CLAT is operational. Therefore it's desirable that the
node enables CLAT as soon as network support for PLAT is detected
while IPv4 connectivity is not yet detected. The node SHOULD enable
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CLAT after discovering the NAT64 prefix, unless by that time the node
has already obtained a non-link-local IPv4 address. The node SHOULD
NOT wait for an explicit (DHCP Option 108) or an implicit (DHCP
timeouts) indication that native IPv4 connectivity is not available.
However, to mitigate attacks described in Section 7 of [RFC7050], the
node MAY delay enabling CLAT if the NAT64 prefix was discovered via
DNS ([RFC7050]) only. The delay is implementation specific. If IPv4
connectivity becomes available later, the node MUST disable CLAT
(unless explicitly configured to keep it running) as discussed in the
following section.
If the node supports multiple IPv4 continuity solutions, the node
MUST follow recommendations from Section 4 of [RFC7335] to avoid IPv4
address space conflicts.
6. Disabling CLAT
It is possible that after the CLAT instance has started, native IPv4
becomes available (e.g. an IPv4 address received via DHCP). Unless
explicitly configured otherwise, the node MUST disable CLAT
immediately upon obtaining a native IPv4 default gateway.
While disabling CLAT is impactful for all applications and traffic
flows already utilizing CLAT, it is recommended not only from a
performance perspective, but also from a security point of view.
Because IPv4-only networks are inherently IPv6-ignorant, they might
lack IPv6 layer2 security features, such as RA Guard, that would
prevent spoofed RAs. An attacker can send an RA containing the
PREF64 option, while the network doesn't provide any PLAT
functionality. If the node keeps CLAT enabled and uses it for
IPv4-only destinations, that traffic could be dropped (availability
attack) or routed through an attacker-controlled route (active attack
on insecure traffic or hoarding secure traffic for "Harvest Now,
Decrypt Later" attacks for non-PQC secure traffic, see Section 8 of
[I-D.ietf-pquip-pqc-engineers]). Rogue RAs can also disturb traffic
to destinations that support both IPv4 and IPv6 by causing IPv6
through an attacker's PLAT to be used instead of the legitimate
network owner's IPv4 path.
There are some corner cases when the administrator might prefer the
node to use CLAT even if the native IPv4 connectivity is available
(e.g. for performance reasons, if IPv4 as a Service performs better
than native IPv4). However for the reasons described above such
behaviour MUST be explicitly enabled by the administrator via a
configuration knob and MUST NOT be a default behaviour, especially
for unmanaged nodes.
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7. CLAT Addresses Considerations
7.1. CLAT IPv4 Addresses
There are two different models in 464XLAT deployments:
* A dedicated prefix model, which Section 4.1 of [RFC6877] calls
"wireline network architecture". In that case, the node
performing CLAT functions also extends the network downstream and
provides network connectivity services to other connected systems.
Those systems can be physical (e.g. various clients connected to a
CPE router), or logical (e.g. virtual systems running on a node,
while the host system acts as a router and performs CLAT). In all
those cases, systems behind the CLAT node usually use [RFC1918]
addresses. Despite the word "wireline", this architecture is
applicable for wireless or 3GPP routers: such router can provide
IPv4 connectivity to connected devices, performing CLAT functions
for traffic sent/received via the IPv6-only uplink interface.
* A single-address model, which section 4.2 of [RFC6877] calls
"wireless network architecture". In that case, the CLAT instance
provides an IPv4 address and the default route to the local node's
network stack only. It should be noted that [RFC6877] implies
that the second deployment scenario is limited to 3GPP cases,
while currently it is also deployed in other types of networks,
such as enterprise networks and Wi-Fi hotspots, where hosts (as
mobile phones, laptops and desktops) use CLAT to provide
connectivity to IPv4-only local applications. Despite the word
"wireless" in the name, this architecture is applicable for wired
networks as well (e.g. desktops or servers using wired
connections).
In the single-address model, the CLAT instance needs a single IPv4
CLAT address and a single CLAT-only IPv6 address (which is distinct
from the one or more IPv6 addresses used by the node running CLAT for
its own native IPv6 connectivity, see Section 7.2). The node
providing CLAT functions to local applications SHOULD use IPv4
addresses from the dedicated 192.0.0.0/29 range ([RFC7335]), reserved
for IPv4 continuity solutions including but not limited to 464XLAT.
If the node runs multiple CLAT instances (see Section 4), the node
SHOULD use different local IPv4 addresses for each CLAT instance.
This approach limits the number of CLAT instances per node to 8,
which seems to be more that sufficient at the time of writing. If in
the future some deployment scenarios require more that 8 CLAT
instances per node, a new larger IPv4 range will be requested from
IANA.
The node MUST NOT send packets on wire from the local CLAT address.
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The host SHOULD use 255.255.255.255 as a netmask for the CLAT
address. That allows all 8 addresses from 192.0.0.0/29 to be used,
if needed.
It should be noted that 192.0.0.0/29 is shared between multiple IPv4
continuity solutions such as 464XLAT and DS-Lite (see [RFC7335]. For
example, Section 10 of [RFC6333] reserves 192.0.0.1 for the Dual-
Stack Lite default router. However, as per Section 4 of [RFC7335],
the host MUST NOT enable two active IPv4 continuity solutions
simultaneously in a way that would cause a node to have overlapping
192.0.0.0/29 address space. Therefore, as long as the host is not
using DS-Lite, it MAY use 192.0.0.1 as a CLAT address.
7.2. CLAT IPv6 Addresses
Section 6.3 of [RFC6877] recommends that the CLAT instance acquires a
dedicated /64 for translating between IPv4 and IPv6, and only uses a
single interface IPv6 address if a dedicated prefix is not available
via DHCPv6-PD. However, deployments where each node can obtain a
dedicated /64 just for CLAT are rather uncommon, especially in
environments like enterprise networks, Wi-Fi hotspots, etc. Quite
often the CLAT instance uses a single IPv6 address as a source for
all IPv4 traffic translated by CLAT. In particular, in a single
address model (see Section 7.1) the CLAT instance only need a single
CLAT IPv6 address. Even in a dedicated prefix model, the CLAT
instance can first perform stateful NAT44 to translate all IPv4
addresses from the dedicated prefix to a single IPv4 address, and
then perform stateless CLAT.
The CLAT instance SHOULD obtain a dedicated IPv6 address used
exclusively for CLAT functions. This is required as when the node
receives a packet from a NAT64 source, the node needs to
differentiate between native IPv6 traffic and traffic which needs to
be passed to the CLAT instance. For example, an ICMPv6 Echo Reply
packet from 64:ff9b::203.0.113.1 can be a response to either an IPv6
ping to 64:ff9b::203.0.113.1, or an IPv4 ping to 203.0.113.1,
translated by CLAT. Using a dedicated IPv6 source address for CLAT
traffic allows the node to make that distinction without keeping
state and operate in the stateless mode (see Section 1.3 of
[RFC6145].
In a dedicated prefix model, the instance MAY obtain a dedicated IPv6
address for each CLAT IPv4 address.
The node MUST treat the CLAT addresses as any other address and
comply with [RFC4861] and [RFC4862]. In particular:
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* Performing Duplicate Address Detection for each dedicated CLAT
address (Section 5.4 of [RFC4862]);
* Processing received Neighbor Solicitations sent to the solicited-
node multicast address ([RFC4861]) for the node CLAT addresses.
* Sending Gratuitous Neighbor Advertisements ([RFC9131]) for the
CLAT addresses.
* Registering the CLAT addresses using DHCPv6 ([RFC9686]), if the
network supports the registration.
If the dedicated CLAT address is obtained via Stateless Address
Autoconfiguration (SLAAC, [RFC4862]), the CLAT instance SHOULD ensure
that the address is checksum-neutral for the given local IPv4 CLAT
address and the NAT64 prefix (this means that the local IPv4 address
needs to be assigned/known before the IPv6 address is configured).
Using a checksum-neutral CLAT address provides the following
benefits:
* Better performance as CLAT doesn't need to recalculate the
checksum.
* If a protocol uses the standard IP checksum, CLAT doesn't need to
recalculate the checksum. That improves the chances of the
protocol working via CLAT even if CLAT is not aware of the
protocol's semantics.
To protect user privacy and prevent user tracking through CLAT
addresses, the node SHOULD generate a different interface id for the
CLAT address when connecting to different networks, even if the NAT64
prefix and the local IPv4 CLAT address do not change. In particular,
the node SHOULD generate a random CLAT address every time the network
attachement changes to another network.
8. CLAT and Multiple Prefixes per Interface
IPv6 multihoming, particularly when multiple routers on the same link
advertise different prefixes PIOs, presents a complex and not yet
fully resolved challenge.
When routers on a given link are managed independently (e.g., by
different ISPs), the resulting set of configuration parameters
received by a host can be difficult to utilize without creating a
complex and fragile state machine. For example, if router_A
advertises a PIO with prefix_A and PREF64_A, while router_B
advertises a PIO with prefix_B and a PREF64_B, it is crucial that the
CLAT bundles the information received from each router. A CLAT
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instance must use PREF64_A and generate a CLAT address from prefix_A,
sending translated packets to router_A. Alternatively, it must use
PREF64_B, generate an address from prefix_B, and send translated
packets to router_B. Mixing configuration information from different
routers (e.g., generating a CLAT address from prefix_A but using
PREF64_B for translation) can lead to packet loss. For example, if
packets with source addresses from prefix_A are sent to router_B,
that router (or the uplink network) might drop the packets according
to BCP 38 ([RFC2827]). Similarly, if the CLAT instance uses
PREF64_A, advertised by router_A, but those packets are sent to
router_B, that router might not be configured to translate packets
for that prefix.
This document does not aim to define CLAT behavior for every possible
multi-router/multi-prefix scenario. Instead, this section provides
recommendations for common scenarios, leaving numerous corner cases
out of scope.
A node discovering multiple routers on the same interface advertising
the _same PIOs and NAT64 prefix_, SHOULD only create one CLAT
instance using one of the PIOs to form a CLAT address.
A node discovering multiple routers on the same interface signalling
the _different PIOs and NAT64 prefixes_, MAY create one CLAT instance
for each tuple of PIOs and NAT64 prefix, or only a single CLAT
instance using the NAT64 prefix discovered through the selected IPv6
default router and the address formed from a PIO advertised by that
router.
When a node creates a single CLAT instance and must choose between
multiple PIOs, the node SHOULD select a single PIO using the same
algorithm as for choosing the source address for a destination within
the selected NAT64 prefix ([RFC6724], updated by
[I-D.ietf-6man-rfc6724-update]).
Discussion: This approach, leveraging the default source address
selection algorithm (Section 5 of [RFC6724]), typically results in
the policy table (rule 6) and longest prefix match (rule 8) being
used for prefix selection. This ensures CLAT address selection
aligns with default source address selection for native IPv6 flows,
offering the following advantages:
* When using the well-known NAT64 prefix (64:ff9b::/96), non-ULA
prefixes are preferred over ULA prefixes by default. This is
beneficial as ULA source packets may not reach PLAT devices.
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* For network-specific NAT64 prefixes within the known-local ULA
range ([I-D.ietf-6man-rfc6724-update]), the ULA prefix is
preferred. This can be advantageous in home and enterprise
environments where administrators intend to perform NAT64 for
specific source prefixes only.
* For network-specific NAT64 prefixes within the operator's global
non-ULA range, the longest prefix match selects the PIO, ensuring
CLAT uses the operator's source address for traffic to the
operator's PLAT in multi-prefix environments.
* In managed environments, operators can customize CLAT behavior by
modifying the policy table if the default prefix selection is
unsuitable.
Creating a single CLAT instance significantly simplifies the CLAT
state machine. However, this approach may concentrate all traffic
from that instance onto the same first-hop router and NAT64 device in
some multihomed topologies. As traffic shifts from CLAT to native
IPv6, this drawback becomes less significant and does not justify the
added complexity of multiple instances.
8.1. Link Renumbering
As discussed above, a single CLAT instance per interface, using a
single PIO, is typically sufficient, even if the link has multiple
assigned subnets. However, PIO selection can significantly impact
user experience during link renumbering.
[RFC8978] discusses various examples of "flash renumbering," where
the IPv6 prefix assigned to the link changes without explicit host
notification. [I-D.ietf-6man-slaac-renum] and [I-D.link-6man-gulla]
discuss methods to mitigate the impact of flash renumbering. These
methods generally rely on hosts with addresses from both old and new
prefixes ceasing use of the old prefix and adopting the new prefix.
For nodes running CLAT instances, this requires disabling instances
using addresses from the old prefix and creating an instance using an
address from the new prefix.
The CLAT node SHOULD use at least the following signals to detect
link renumbering events:
* A prefix used to form the CLAT address becomes deperecated or
invalid ([RFC4862]).
* The router (or routers) advertizing the PIO used to form the CLAT
address
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- has changed it state from being reachable or probably reachable
to being unknown or suspect (i.e., it's neighbor cache entry
moved to the INCOMPLETE state or ceased to exist, see
Section 6.3.6 of [RFC4861]).
- ceased to be a router (see Section 7.3.3 of [RFC4861]).
Upon receiving a signal indicating a possible renumbering event, the
node SHOULD disable the CLAT instance(s) affected by the renumbering,
and create new instance(s). In case of implicit signals (provided by
the Neighbor Unreachability Detection, [RFC4861], rather than by a
Router Advertisement deprecating or invalidating a prefix), the node
MAY send Router Solicitations to obtain the most up-to-date network
configuration information. When sending Router Solicitations the
node MUST follow recommendations specified in Section 6.3.7 of
[RFC4861]. The node MAY react to a potential renumebring event in a
"make-before-break" manner, when old instances are still running
until all required information to enable new ones becomes available.
9. MTU Considerations
The IPv4 header is 20 bytes long (or longer if IP options are
present), while the IPv6 header is 40 bytes. This means that when
CLAT translates an IPv4 packet to IPv6, it usually adds 20 bytes to
the packet size. However, when CLAT transates a fragmented IPv4
packet, then Fragment Header needs to be added to the resulting IPv6
packet (Section 4.1 of [RFC7915]). The length of IPv6 Fragment
Extension header is 8 bytes (Section 4.5 of [RFC8200]). Therefore,
to minimize undesirable IP fragmentation ([RFC8900]), the CLAT
instance in a single-address mode SHOULD present IPv4-only
applications with an IPv4 MTU which is 28 bytes smaller than the IPv6
MTU of the interface the instance is running on.
10. Updates to RFC8585
This document makes the following changes to Section 3.2.1 of
[RFC8585]:
OLD TEXT:
===
464XLAT requirements:
===
NEW TEXT:
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===
464XLAT requirements:
464XLAT-0: The IPv6 Transition CE Router SHOULD follow
recommendations provided in draft-link-v6ops-claton.
===
OLD TEXT:
===
464XLAT-4: The IPv6 Transition CE Router MUST implement [RFC7050]
("Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis") in
order to discover the provider-side translator (PLAT) translation
IPv4 and IPv6 prefix(es)/suffix(es).
===
NEW TEXT:
===
464XLAT-4: The IPv6 Transition CE Router MUST implement [RFC8781]
("Discovering PREF64 in Router Advertisements") and SHOULD implement
[RFC7050] ("Discovery of the IPv6 Prefix Used for IPv6 Address
Synthesis") in order to discover the provider-side translator (PLAT)
translation IPv4 and IPv6 prefix(es)/suffix(es).
===
OLD TEXT:
===
464XLAT-6: If the network provides several choices for the discovery/
learning of the NAT64 prefix, the priority to use one or the other
MUST follow this order: 1) [RFC7225] and 2) [RFC7050].
The NAT64 prefix could be discovered by means of the method defined
in [RFC7050] only if the service provider uses DNS64 [RFC6147]. It
may be the case that the service provider does not use or does not
trust DNS64 [RFC6147] because the DNS configuration at the CE (or
hosts behind the CE) can be modified by the customer. In that case,
the service provider may opt to configure the NAT64 prefix by means
of the option defined in [RFC7225]. This can also be used if the
service provider uses DNS64 [RFC6147].
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===
NEW TEXT
===
464XLAT-6: If the network provides several choices for the discovery/
learning of the NAT64 prefix, the priority to use one or the other
MUST follow this order: 1)[RFC7225] 2)[RFC8781] and 3) [RFC7050].
464XLAT-7: If the IPv6 Transition CE Router performs CLAT functions
it SHOULD also include the PLAT prefix in Router Advertisements
([RFC8781]) sent via the LAN interfaces. If the IPv6 Transition CE
Router acts as a DHCP server it SHOULD enable DHCP Option 108
([RFC8925]) processing. The router SHOULD have a configuration knob
to disable DHCP Option 108 processing.
[RFC8781] allows the service provider to signal NAT64 prefix
independently from DNS64 presence. At the same time the NAT64 prefix
could be discovered by means of the method defined in [RFC7050] only
if the service provider uses DNS64 [RFC6147]. It may be the case
that the service provider does not use or does not trust DNS64
[RFC6147] because the DNS configuration at the CE (or hosts behind
the CE) can be modified by the customer. In that case, the service
provider may opt to configure the NAT64 prefix by means of the option
defined in [RFC7225]. This can also be used if the service provider
uses DNS64 [RFC6147].
===
11. Security Considerations
If a malicious actor spoofs PLAT presence signals (such as an RA with
PREF64 option) or DNS responses for DNS-based NAT64 prefix detection
([RFC7050]), traffic of IPv4-only applications using CLAT can be
affected:
* if there is no PLAT (NAT64) devices, traffic to NAT64 destinations
would be dropped.
* If the attacker intercepts traffic for the NAT64 prefix (e.g. by
providing the victim with a bogus NAT64 prefix and steering
traffic for those destinations towards themselves), the attacker
might be able to perform man-in-the-middle attacks.
Using the PREF64 RA option to detect PLAT presence and the NAT64
prefix is less prone to such attacks than DNS-based detection
([RFC7050]), as the attacker needs to be on-link and be able to
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bypass layer-2 security features such as RA Guard. Therefore
Section 5 recommends the PREF64 RA option as a preferred way to
detect PLAT presence.
In networks lacking explicit IPv6 deployment and consequently IPv6
security features, administrators may inadvertently expose link-local
IPv6 connectivity when endpoints have IPv6 enabled. This unintended
exposure can facilitate PLAT presence signal falsification, enabling
malicious actors to conduct Man-in-the-Middle (MitM) or Denial-of-
Service (DoS) attacks. This document mitigates this risk by
requiring endpoints to disable CLAT when the network provides non-
link-local IPv4 connectivity, as outlined in Section 6.
12. Privacy Considerations
If the instance utilizes the same CLAT address for an extensive
period of time or, much worse, uses the same CLAT address when
connecting to different networks, eavesdroppers and information
collectors can correlate various network activity to the same node.
To mitigate that risk and make address-based network-activity
correlation more difficult, the node SHOULD generate a different
interface id for the CLAT address when connecting to different
networks (see Section 7.2).
It should be noted that the node's CLAT IPv6 address is only used
(and visible to observers) when the traffic is carried from the CLAT
node to the PLAT device. In the vast majority of the cases it means
that address is never visible outside of the network Internet edge,
so to perform address-based network-activity correlation the observer
needs to be located in the same network as the CLAT node.
To summarize, this document does not introduce any new privacy
considerations.
13. IANA Considerations
This memo does not introduce any requests to IANA.
14. References
14.1. 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>.
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[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>.
[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>.
[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>.
[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
<https://www.rfc-editor.org/info/rfc6333>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[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>.
[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>.
[RFC7335] Byrne, C., "IPv4 Service Continuity Prefix", RFC 7335,
DOI 10.17487/RFC7335, August 2014,
<https://www.rfc-editor.org/info/rfc7335>.
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[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>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[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>.
[RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by
Hosts in a Multi-Prefix Network", RFC 8028,
DOI 10.17487/RFC8028, November 2016,
<https://www.rfc-editor.org/info/rfc8028>.
[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>.
[RFC8585] Palet Martinez, J., Liu, H. M.-H., and M. Kawashima,
"Requirements for IPv6 Customer Edge Routers to Support
IPv4-as-a-Service", RFC 8585, DOI 10.17487/RFC8585, May
2019, <https://www.rfc-editor.org/info/rfc8585>.
[RFC8925] Colitti, L., Linkova, J., Richardson, M., and T.
Mrugalski, "IPv6-Only Preferred Option for DHCPv4",
RFC 8925, DOI 10.17487/RFC8925, October 2020,
<https://www.rfc-editor.org/info/rfc8925>.
[RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981,
DOI 10.17487/RFC8981, February 2021,
<https://www.rfc-editor.org/info/rfc8981>.
[RFC9131] Linkova, J., "Gratuitous Neighbor Discovery: Creating
Neighbor Cache Entries on First-Hop Routers", RFC 9131,
DOI 10.17487/RFC9131, October 2021,
<https://www.rfc-editor.org/info/rfc9131>.
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[RFC9686] Kumari, W., Krishnan, S., Asati, R., Colitti, L., Linkova,
J., and S. Jiang, "Registering Self-Generated IPv6
Addresses Using DHCPv6", RFC 9686, DOI 10.17487/RFC9686,
December 2024, <https://www.rfc-editor.org/info/rfc9686>.
[I-D.ietf-6man-rfc6724-update]
Buraglio, N., Chown, T., and J. Duncan, "Prioritizing
known-local IPv6 ULAs through address selection policy",
Work in Progress, Internet-Draft, draft-ietf-6man-rfc6724-
update-17, 27 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-
rfc6724-update-17>.
14.2. Informative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>.
[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
<https://www.rfc-editor.org/info/rfc6145>.
[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>.
[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/info/rfc8900>.
[RFC8978] Gont, F., Žorž, J., and R. Patterson, "Reaction of IPv6
Stateless Address Autoconfiguration (SLAAC) to Flash-
Renumbering Events", RFC 8978, DOI 10.17487/RFC8978, March
2021, <https://www.rfc-editor.org/info/rfc8978>.
[I-D.ietf-pquip-pqc-engineers]
Banerjee, A., Reddy.K, T., Schoinianakis, D., Hollebeek,
T., and M. Ounsworth, "Post-Quantum Cryptography for
Engineers", Work in Progress, Internet-Draft, draft-ietf-
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pquip-pqc-engineers-09, 13 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-pquip-
pqc-engineers-09>.
[I-D.ietf-6man-slaac-renum]
Gont, F., Zorz, J., Patterson, R., and J. Linkova,
"Improving the Robustness of Stateless Address
Autoconfiguration (SLAAC) to Flash Renumbering Events",
Work in Progress, Internet-Draft, draft-ietf-6man-slaac-
renum-08, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-
slaac-renum-08>.
[I-D.link-6man-gulla]
Linkova, J., "Using Prefix-Specific Link-Local Addresses
to Improve SLAAC Robustness", Work in Progress, Internet-
Draft, draft-link-6man-gulla-00, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-link-6man-
gulla-00>.
Appendix A. Enabling and Disabling CLAT: Flowchart
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+-----------------------------------------+
| |
| |
v |
+----------------------------------+ +-----------------+------------+
| | YES | |
| Has native IPv4 default route? +------> | CLAT Disabled |
| | | |
+------------------+---------------+ +------------------------------+
| NO ^ ^
+---------+-------------+ | NO |
| | | |
+--------v--------+ +-----------v------------+ +--+----------+ |
| | | | NO | PLAT prefix | |
| Start DHCP | | Received PREF64 in RA? +------>| discovered | |
| | | | | via RFC7050?| |
+---------+-------+ +-------------------+----+ +-------+-----+ |
| ^ | | |
| | | YES | |
v | v | YES |
+---------------------+ | +---------------+ | |
| | | | | | |
| Wait for DHCP Offer |<----+------+ Enable CLAT |<---------+ |
| | | | | |
+----------+----------+ | +---------------+ |
| | |
| | |
v | YES |
+---------------------+ | |
| | | |
| +-----+ |
| Option 108 Present? | |
| | NO |
| +--------------------------------------------------+
+---------------------+
Figure 1: Enabling and Disabling CLAT Instance
Acknowledgements
Thanks to Ondrej Caletka, Stuart Cheshire, Lorenzo Colitti, Jeremy
Duncan, Jason Healy, Ed Horley, KAWASHIMA Masanobu, Ted Lemon, George
Michaelson, Jordi Palet, Dieter Siegmund, Philipp S. Tiesel, Eric
Vyncke for the discussions, the input, and all contribution.
Authors' Addresses
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Jen Linkova
Google
1 Darling Island Rd
Pyrmont NSW 2009
Australia
Email: furry13@gmail.com, furry@google.com
Tommy Jensen
Microsoft
Email: tojens@microsoft.com
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