Learning the IPv6 Prefix of a NAT64
draft-wing-behave-learn-prefix-01

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Abstract

In some IPv6/IPv4 translation scenarios it is necessary for an IPv6 host to know the IPv6 prefix used by its NAT64. In some of the IPv6/IPv4 translation proposals, the prefix is not fixed; that is, the prefix is chosen by the network operator. This specification provides two methods for a host learn its NAT64's IPv6 prefix and length.

Table of Contents

1.  Terminology
2.  Introduction
3.  Learning IPv6 Prefix and Length
    3.1.  Using DNS to Learn IPv6 Prefix and Length
    3.2.  Using DHCP to Learn IPv6 Prefix and Length
4.  Authenticating the Learned Prefix
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgements
8.  References
    8.1.  Normative References
    8.2.  Informative References
Appendix A.  For future study
    A.1.  multi-homed hosts
Appendix B.  Changes
    B.1.  Changes from -00 to -01
§  Author's Address

1.  Terminology

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

AFT: Address Family Translator. A device that translates between IP address families.

DNS64: The function of synthesizing an AAAA response from an A record.

2.  Introduction

Several variations of Address Family Translators (AFT) have been proposed for IPv6/IPv6 coexistence. For IPv6 hosts to access IPv4 hosts, a DNS function exists which synthesizes DNS AAAA records -- this function generally called "DNS64" (also "DNS rewriting") [I‑D.bagnulo‑behave‑dns64] (Bagnulo, M., Sullivan, A., Matthews, P., Beijnum, I., and M. Endo, “DNS64: DNS extensions for Network Address Translation from IPv6 Clients to IPv4 Servers,” March 2009.). The DNS64 function, when used in conjunction with an NAT64, allows a IPv6-only host to access IPv4-only hosts. This access, for the most part, is transparent to the IPv6 host -- to much the same degree that today's widely-deployed NATs are transparent to IPv4 hosts. But, like with today's NATs, there are applications which do not work with NAT64 or do not work with DNS64, and require IPv6 hosts to implement additional functionality.

So far, two applications have been identified which can benefit from knowing the IPv6 prefix of the host's NAT64:

• host-based DNSSEC validation (as described in Section 4.3 of [I‑D.bagnulo‑behave‑dns64] (Bagnulo, M., Sullivan, A., Matthews, P., Beijnum, I., and M. Endo, “DNS64: DNS extensions for Network Address Translation from IPv6 Clients to IPv4 Servers,” March 2009.))
• BitTorrent (Section 2.2 of (Wing, D., “Referrals Across an IPv6/IPv4 Translator,” October 2009.) [I‑D.wing‑behave‑nat64‑referrals])

3.  Learning IPv6 Prefix and Length

Both the IPv6 prefix and the prefix length need to be learned This can be done using DNS or DHCP, as described in the following sections.

3.1.  Using DNS to Learn IPv6 Prefix and Length

This specification defines a new U-NAPTR (Daigle, L., “Domain-Based Application Service Location Using URIs and the Dynamic Delegation Discovery Service (DDDS),” April 2007.) [RFC4848] application to discover the NAT64's IPv6 prefix and length. The input domain name is the exact same as would be used for a reverse DNS lookup, derived from the host's IPv6 in the ".ip6.arpa." tree and follows the construction rules in Section 2.5 of (Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, “DNS Extensions to Support IP Version 6,” October 2003.) [RFC3596]. This is shortened to 20 labels (representing a /64 network prefix) and, if DNS returns an error is shortened to 16 labels (representing a /48 network prefix).

If a NAT64 is present on the network, the successful result of one of those queries will produce a NAPTR record with the desired service tag "NAT64:" which contains the IPv6 prefix and prefix length o the NAT64, separated by a "/" (the same syntax as specified in Section 2.3 of (Hinden, R. and S. Deering, “IP Version 6 Addressing Architecture,” February 2006.) [RFC4291]).

For example, a host with the IP address 2001:db8:1:2:3:4:567:89ab would first send an NAPTR query for 3.0.0.0.2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.IP6.ARPA (20 elements, representing a /64 network prefix). If that fails (returns NXDOMAIN), it would send an NAPTR query for 2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.IP6.ARPA (16 elements, representing a /48 network prefix).

Note: Both /64 and /48 prefix lengths are shown in this version of the document for illustrative purposes. The number of elements of this query will depend on the prefix length(s) defined by the BEHAVE working group for a NAT64. If the BEHAVE working group decides that all NAT64's will have a certain prefix length, then only one DNS query is sent.

If the host needs to authenticate the prefix it just learned (e.g., because the host is running a DNSSEC validator) the host performs the additional authentication steps described in Section 4 (Authenticating the Learned Prefix).

3.2.  Using DHCP to Learn IPv6 Prefix and Length

A new DHCP option, OPTION_AFT_PREFIX, is defined. It contains the IPv6 prefix and its length.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       OPTION_AFT_PREFIX       |         option-length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prefix-length |                                               |
+-+-+-+-+-+-+-+-+          IPv6 prefix                          |
|                        (up to 16 octets)                      |
|                                                               |
|                                                               |
|                                                               |
|               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               |
+-+-+-+-+-+-+-+-+

    option-code:      OPTION_AFT_PREFIX (TBD)

    option-length:    17

    prefix-length:    Length for this prefix in bits

    IPv6-prefix:      An IPv6 prefix

In order to conserve space, it is RECOMMENDED that only the significant bits of the IPv6 prefix be sent in the DHCP option.

If the host needs to authenticate the prefix it just learned (e.g., because the host is running a DNSSEC validator) the host performs the additional authentication steps described in Section 4 (Authenticating the Learned Prefix).

4.  Authenticating the Learned Prefix

In some cases (e.g., a host performing DNSSEC validation), the host needs to authenticate the NAT64's IPv6 prefix learned via one of the mechanisms described earlier. To allow such authentication the operator of the NAT64 first creates a PTR record for the NAT64 (with 0's for the elements after the NAT64's IPv6 prefix) which points to a hostname. The hostname has a signed AAAA record for the same 0-padded IPv6 address returned by the PTR query. Once those configuration steps are done, a host can validate the NAT64 IPv6 prefix by performing the following steps:

a.

The host sends a DNS PTR query for the IPv6 address of the NAT64 (for "ipv6.arpa"), using 0 for the elements after the prefix length. This will return the fully-qualified hostname of that NAT64 device.

b.

Verify the full-qualified hostname is on the host's configured list of authorized translators (e.g., seattle.nat64.example.net).

c.

Send a DNS AAAA query for that hostname.

d.

Verify the AAAA response matches the IPv6 address obtained in step 1.

e.

Perform DNSSEC validation of the AAAA response.

For example, if the NAT64's IPv6 prefix length is /48, the host would send a PTR query for 2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.IP6.ARPA which would return a hostname, seattle.nat64.example.net. The host verifies that seattle.nat64.example.net is on its configured list of authorized NAT64 hosts, as maintained in a text file. The host sends an AAAA query for seattle.nat64.example.net and verifies the AAAA response contains the same IPv6 address. The host then validates the DNSSEC signature for seattle.nat64.example.net.

5.  Security Considerations

After learning the IPv6 prefix of its translator by following the procedures in this specification, the IPv6 host will utilize this information for subsequent actions (e.g., sending a packet to it, or using that information to synthesize DNS records or to perform DNSSEC validation). If an attacker provides a fraudulent IPv6 to the IPv6 host, the attacker can become on-path for traffic to/from that IPv6 host and preform passive or active eavesdropping or traffic analysis. To protect against this attack, it is RECOMMENDED that IPv6 hosts be configured with the names of authorized translators and RECOMMENDED that IPv6 hosts uses DNSSEC to validate that name matches the IPv6 prefix learned via DNS or DHCPv6, as described in Section 4 (Authenticating the Learned Prefix).

6.  IANA Considerations

A new DHCPv6 option, OPTION_AFT_PREFIX, needs to be assigned by IANA.

The new NAPTR Application Service tag "NAT64" is registered with IANA.

7.  Acknowledgements

This draft was fostered by discussion on the 46translation mailing list and at the v4v6 Interim in Montreal. Special thanks to Iljitsch van Beijnum, Andrew Sullivan, Marcelo Bagnulo Braun, Fred Baker, and Xing Li for their comments and dialog.

The mechanism to perform a shortened NAPTR query was described first by Martin Thomson [I‑D.thomson‑geopriv‑res‑gw‑lis‑discovery] (Thomson, M. and R. Bellis, “Location Information Server (LIS) Discovery using IP address and Reverse DNS,” January 2010.).

Thanks to Ralph Droms for his help with DHCPv6. Thanks to John Schnizlein for improving the DNS learning algorithm.

8.  References

8.1. Normative References

8.2. Informative References

Appendix A.  For future study

A.1.  multi-homed hosts

A multi-homed host may have different NAT64 devices available on each of its networks, and can learn those via DNS or via DHCP.

When using DNS to learn the NAT64 prefix (Section 3.1 (Using DNS to Learn IPv6 Prefix and Length)) or using DNS to authenticate the NAT64 prefix (Section 4 (Authenticating the Learned Prefix), it is possible a split horizon DNS exists. Such a split DNS requires the host to query the DNS server associated with that network prefix as described in [I‑D.savolainen‑mif‑dns‑server‑selection] (Savolainen, T., “DNS Server Selection on Multi-Homed Hosts,” February 2010.).

Appendix B.  Changes

B.1.  Changes from -00 to -01

• made clearer this is for NAT64 prefix (changed title and some text).
• changed from querying for "_aft_prefix" TXT record to querying ipv6.arpa NAPTR record.
• BitTorrent is another application that benefits from knowing the NAT64 prefix; previously only DNSSEC was listed.
• changed to standards track.

Author's Address