BEHAVE Working Group D. Wing
Internet-Draft Cisco
Intended status: Informational October 23, 2008
Expires: April 26, 2009
Learning the Address Family Translator's IPv6 Prefix
draft-wing-behave-learn-prefix-00
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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 address family translator.
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 several methods to learn the prefix and its
length.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Learning IPv6 Prefix and Length . . . . . . . . . . . . . . . . 3
3.1. Using DNS to Learn IPv6 Prefix and Length . . . . . . . . . 3
3.2. Using DHCP to Learn IPv6 Prefix and Length . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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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].
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. All of these operate by
synthesizing DNS AAAA responses for hosts that do not have AAAA
records -- this is generally called "DNS64" (also "DNS rewriting" or
"DNS-ALG"). The DNS64 function, when used in conjunction with an
AFT, 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 AFT or do not work with DNS64, and require IPv6 hosts
to implement additional functionality.
So far, one application has been identified which requires an IPv6
host to know the IPv6 prefix used by its address family translator
(AFT): A DNSSEC security-aware stub resolver needs to securly obtain
the IPv6 prefix. It uses this prefix to generate its own synthesized
AAAA response from the un-translated A record [Sullivan-email], or to
validate a synthesized AAAA response by removing the IPv6 prefix
[Wing-email].
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
In order for an IPv6 host to determine if a NAT64 is present on its
network, it sends a DNS query. Because a host doesn't always know
its network's default domain name, the procedure described below
provides a way for the host to learn it in order to authorize that
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network's address family translator:
1. Send a DNS AAAA query for "_aft_prefix", without a domain name.
If this does not return an IPv6 address it means a address family
translator is not present and processing MUST stop.
2. If validation of the returned IPv6 prefix is necessary, then:
a. Send a DNS PTR query for that IPv6 address. This returns a
fully-qualified hostname.
b. Verify the full-qualified hostname is on the host's
configured list of authorized translators.
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.
f. Send a DNS TXT query for the fully-qualfied name to learn the
number of bits of the prefix (e.g., a 48 bit prefix would
return the string "48").
g. Perform DNSSEC validation of the TXT response.
3. If validation of this information is not necessary, then:
a. Send a DNS TXT query for "_aft_prefix", without the domain
name, to learn the number of bits of the prefix.
Note: The DNS server being queried is the DNS64. The DNS64
performs the AAAA synthesis function, which means the DNS64 is
already aware of the prefix of the address family translator and
the prefix length. Thus, is it already capable of returning
information about the network's AFT.
Discussion: without a domain name, it is unavoidable that root
nameservers will see this query. Need to think about ways to
reduce the effect of those queries (e.g., make them authoritative
and return all 0's which will get cached).
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.
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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
Figure 1: OPTION_AFT_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 implements a security-aware DNSSEC validator, it must
validate and authorize that translator. To do this, it issues a
ipv6.arpa PTR query on the IPv6 address learned via DHCPv6. This
returns a name. The host then issues a AAAA query using that name,
and DNSSEC validates the response. If the query validates, and the
AAAA response contains an IPv6 address that matches the AFT-PREFIX
learned from DHCPv6, and the host's administrator has authorized use
of that AFT translator name, DNSSEC can use that prefix. Details of
DNSSEC operation in conjunction with AFT are in [draft to be written
providing DNSSEC details].
4. 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
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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.
5. IANA Considerations
A new DHCPv6 option, OPTION_AFT_PREFIX, needs to be assigned by IANA.
The name "_aft_translator" should be reserved by IANA for this
purpose.
6. 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.
Thanks to Ralph Droms for his help with DHCPv6. Thanks to John
Schnizlein for improving the DNS learning algorithm.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[Sullivan-email]
Sullivan, A., "DNSSEC Current Thinking", October 2008, <ht
tps://www.employees.org/mailman/private/46translation/
2008-October/000028.html>.
[Wing-email]
Wing, D., "DNSSEC, approach 2", October 2008, <https://
www.employees.org/mailman/private/46translation/
2008-October/000031.html>.
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Author's Address
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
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This document was produced using xml2rfc v1.33 (of
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