Discovering PREF64 in Router Advertisements
draft-ietf-6man-ra-pref64-09

Abstract

This document specifies a Neighbor Discovery option to be used in Router Advertisements to communicate NAT64 prefixes to hosts.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on June 21, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 1.2. Terminology
• 2. Use cases for communicating the NAT64 prefix to hosts
• 3. Why include the NAT64 prefix in Router Advertisements
• 4. Option format
• 4.1. Scaled Lifetime Processing
• 5. Usage Guidelines
• 5.1. Handling Multiple NAT64 Prefixes
• 5.2. PREF64 Consistency
• 6. IANA Considerations
• 7. Security Considerations
• 8. Acknowledgements
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

NAT64 [RFC6146] with DNS64 [RFC6147] is a widely-deployed mechanism to provide IPv4 access on IPv6-only networks. In various scenarios, the host must be aware of the NAT64 prefix in use by the network. This document specifies a Neighbor Discovery [RFC4861] option to be used in Router Advertisements to communicate NAT64 prefixes to hosts.

1.1. 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.

1.2. Terminology

PREF64 (or NAT64 prefix): an IPv6 prefix used for IPv6 address synthesis [RFC6146];

NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers [RFC6146];

RA: Router Advertisement, a message used by IPv6 routers to advertise their presence together with various link and Internet parameters [RFC4861];

DNS64: a mechanism for synthesizing AAAA records from A records [RFC6147];

2. Use cases for communicating the NAT64 prefix to hosts

On networks employing NAT64, it is useful for hosts to know the NAT64 prefix for several reasons, including the following:

• Enabling DNS64 functions on end hosts. In particular:
  • Local DNSSEC validation (DNS64 in stub-resolver mode). As discussed in [RFC6147] section 2, the stub resolver in the host "will try to obtain (real) AAAA RRs, and in case they are not available, the DNS64 function will synthesize AAAA RRs for internal usage." Therefore to perform the DNS64 function the stub resolver needs to know the NAT64 prefix. This is required in order to use DNSSEC on a NAT64 network.
  • Trusted DNS server. AAAA synthesis is required for the host to be able to use a DNS server not provided by the network (e.g., a DNS-over-TLS [RFC7858] or DNS-over-HTTPS [RFC8484] server with which the host has an existing trust relationship).
  • Networks with no DNS64 server. Hosts that support AAAA synthesis and that are aware of the NAT64 prefix in use do not need the network to perform the DNS64 function at all.
• Enabling NAT64 address translation functions on end hosts. For example:
  • IPv4 address literals on an IPv6-only host. As described in [RFC8305] section 7.1, IPv6-only hosts connecting to IPv4 address literals can translate the IPv4 literal to an IPv6 literal.
  • 464XLAT [RFC6877]. 464XLAT requires the host be aware of the NAT64 prefix.

3. Why include the NAT64 prefix in Router Advertisements

Fate sharing: NAT64 requires routing to be configured. IPv6 routing configuration requires receiving an IPv6 Router Advertisement [RFC4861]. Therefore using Router Advertisements to provide hosts with NAT64 prefix ensures that NAT64 reachability information shares fate with the rest of network configuration on the host.

Atomic configuration: including the NAT64 prefix in the Router Advertisement minimizes the number of packets required to configure a host. Only one packet (a Router Advertisement) is required to complete the network configuration. This speeds up the process of connecting to a network that supports NAT64/DNS64, and simplifies host implementation by removing the possibility that the host can have an incomplete layer 3 configuration (e.g., IPv6 addresses and prefixes, but no NAT64 prefix).

Updatability: it is possible to change the NAT64 prefix at any time, because when it changes, it is possible to notify hosts by sending a new Router Advertisement.

Deployability: all IPv6 hosts and networks are required to support Neighbor Discovery [RFC4861] so just a minor extension to the existing implementation is required. Other options such as [RFC7225] require implementing other protocols (e.g. PCP [RFC7225]) which could be considered an obstacle for deployment.

4. Option format

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |     Scaled Lifetime     | PLC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|              Highest 96 bits of the Prefix                    |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

				      

Figure 1: NAT64 Prefix Option Format

Fields:

4.1. Scaled Lifetime Processing

It would be highly undesirable for the NAT64 prefix to have a lifetime shorter than the Router Lifetime, which is defined in the Section 4.2 of [RFC4861] as 16-bit unsigned integer. If the NAT64 prefix lifetime is not at least equal to the default router lifetime it might lead to scenarios when the NAT64 prefix lifetime expires before the arrival of the next unsolicited RA. Therefore the Scaled Lifetime encodes the NAT64 prefix lifetime in units of 8 seconds. The receiver MUST multiply the Scaled Lifetime value by 8 (for example, by logical left shift) to calculate the maximum time in seconds the prefix MAY be used. The maximum lifetime of the NAT64 prefix is thus 65528 seconds. To ensure that the NAT64 prefix does not expire before the default router, when using this option it is NOT RECOMMENDED to configure default router lifetimes greater than 65528 seconds. Lifetime of 0 indicates that the prefix SHOULD NOT be used anymore.

The value of the Scaled Lifetime field SHOULD by default be set to the lesser of 3 x MaxRtrAdvInterval ([RFC4861]) divided by 8, or 8191.

Router vendors SHOULD allow administrators to specify non-zero lifetime values which are not divisible by 8. In such cases the router SHOULD round the provided value up to the nearest integer that is divisible by 8 and smaller than 65536, then divide the result by 8 (or perform a logical right-shift by 3), and set the Scaled Lifetime field to the resulting value. If such a non-zero lifetime value to be divided by 8 (to be subjected to a logical right-shift by 3) is less than 8 then the Scaled Lifetime field SHOULD be set to 1. This last step ensures that lifetimes under 8 seconds are encoded as a non-zero Scaled Lifetime.

5. Usage Guidelines

This option specifies exactly one NAT64 prefix for all IPv4 destinations. If the network operator desires to route different parts of the IPv4 address space to different NAT64 devices, this can be accomplished by routing more specific sub-prefixes of the NAT64 prefix to those devices. For example, suppose an operator is using the [RFC1918] address space 10.0.0.0/8 internally. That operator might want to route 10.0.0.0/8 through NAT64 device A, and the rest of the IPv4 space through NAT64 device B. If the operator's NAT64 prefix is 2001:db8:a:b::/96, then the operator can route 2001:db8:a:b::a00:0/104 to NAT64 A and 2001:db8:a:b::/96 to NAT64 B.

This option may appear more than once in a Router Advertisement (e.g. in case of graceful renumbering the network from one NAT64 prefix to another). Host behaviour with regards to synthesizing IPv6 addresses from IPv4 addresses SHOULD follow the recommendations given in Section 3 of [RFC7050], limited to the NAT64 prefixes that have non-zero lifetime.

In a network (or a provisioning domain) that provides both IPv4 and NAT64, it may be desirable for certain IPv4 addresses not to be translated. An example might be private address ranges that are local to the network/provisioning domain and should not be reached through the NAT64. This type of configuration cannot be conveyed to hosts using this option, or through other NAT64 prefix provisioning mechanisms such as [RFC7050] or [RFC7225]. This problem does not apply in IPv6-only networks, because in such networks, the host does not have an IPv4 address and cannot reach any IPv4 destinations without the NAT64.

5.1. Handling Multiple NAT64 Prefixes

In some cases a host may receive multiple NAT64 prefixes from different sources. Possible scenarios include (but are not limited to):

• the host is using multiple mechanisms to discover PREF64 prefixes (e.g. by using PCP [RFC7225]) and/or by resolving IPv4-only fully qualified domain name [RFC7050] in addition to receiving the PREF64 RA option);
• the PREF64 option presents in a single RA more than once;
• the host receives multiple RAs with different PREF64 prefixes on a given interface.

When multiple PREF64 were discovered via RA PREF64 Option (the Option presents more than once in a single RA or multiple RAs were received), host behaviour with regards to synthesizing IPv6 addresses from IPv4 addresses SHOULD follow the recommendations given in Section 3 of [RFC7050], limited to the NAT64 prefixes that have non-zero lifetime.

When different PREF64 are discovered by using multiple mechanisms, hosts SHOULD select one source of information only. The RECOMMENDED order is:

• PCP-discovered prefixes [RFC7225], if supported;
• PREF64 discovered via RA Option;
• PREF64 resolving IPv4-only fully qualified domain name [RFC7050]

Note that if the network provides PREF64 both via this RA option and [RFC7225], hosts that receive the PREF64 via RA option may choose to use it immediately before waiting for PCP to complete, and therefore some traffic may not reflect any more detailed configuration provided by PCP.

The host SHOULD treat the PREF64 as being specific to the network interface it was received on. Provisioning Domain (PvD, [RFC7556]) aware hosts MUST treat the PREF64 as being scoped to the implicit or explicit PvD.

5.2. PREF64 Consistency

Section 6.2.7 of [RFC4861] recommends that routers inspect RAs sent by other routers to ensure that all routers onlink advertise consistent information. Routers SHOULD inspect valid PREF64 options received on a given link and verify the consistency. Detected inconsistencies indicate that one or more routers might be misconfigured. Routers SHOULD log such cases to system or network management. Routers SHOULD check and compare the following information: [RFC7556]) aware routers MUST only compare information scoped to the same implicit or explicit PvD.

• set of PREF64 with non-zero lifetime;
• set of PREF64 with zero lifetime.

Provisioning Domain (PvD,

6. IANA Considerations

The IANA is requested to assign a new IPv6 Neighbor Discovery Option type for the PREF64 option defined in this document.

The IANA registry for these options is:

• https://www.iana.org/assignments/icmpv6-parameters

7. Security Considerations

Because Router Advertisements are required in all IPv6 configuration scenarios, on IPv6-only networks, Router Advertisements must already be secured, e.g., by deploying RA guard [RFC6105]. Providing all configuration in Router Advertisements reduces the attack surface to be targeted by malicious attackers to provide hosts with invalid configuration as compared to distributing the configuration through multiple different mechanisms that need to be secured independently.

If a host is provided with an incorrect NAT64 prefix the IPv6-only host might not be able to communicate with IPv4-only destinations. Connectivity to destinations reachable over IPv6 would not be impacted just by providing a host with an incorrect prefix (however if attackers are capable of sending rogue RAs they can perform denial-of-service or man-in-the-middle attacks, as described in [RFC6104]).

The security measures that must already be in place to ensure that Router Advertisements are only received from legitimate sources eliminate the problem of NAT64 prefix validation described in section 3.1 of [RFC7050].

8. Acknowledgements

Thanks to the following people (in alphabetical order) for their review and feedback: Mikael Abrahamsson, Mark Andrews, Brian E Carpenter, David Farmer, Nick Heatley, Robert Hinden, Martin Hunek, Tatuya Jinmei, Benjamin Kaduk, Erik Kline, Suresh Krishnan, Warren Kumari, David Lamparter, Barry Leiba, Jordi Palet Martinez, Tommy Pauly, Alexandre Petrescu, Michael Richardson, David Schinazi, Ole Troan, Eric Vynke, Bernie Volz.

9. References

9.1. Normative References

9.2. Informative References

Authors' Addresses