Discovery of IPv6 Prefix Used for IPv6 Address Synthesis
draft-ietf-behave-nat64-discovery-heuristic-06

Behave WG                                                  T. Savolainen
Internet-Draft                                                     Nokia
Intended status: Standards Track                             J. Korhonen
Expires: September 13, 2012                       Nokia Siemens Networks
                                                                 D. Wing
                                                           Cisco Systems
                                                          March 12, 2012

        Discovery of IPv6 Prefix Used for IPv6 Address Synthesis
           draft-ietf-behave-nat64-discovery-heuristic-06.txt

Abstract

   This document describes a method for detecting presence of DNS64 and
   for learning IPv6 prefix used for protocol translation on an access
   network.  The method depends on existence of a well-known IPv4-only
   domain name "ipv4only.arpa".  The information learned enables nodes
   to perform local IPv6 address synthesis and to potentially avoid
   traversal through NAT64 on dual-stack accesses and multi-interface
   deployments.

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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   This Internet-Draft will expire on September 13, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements and Terminology . . . . . . . . . . . . . . . . .  3
     2.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Node Behavior  . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Secure Learning of Pref64::/n  . . . . . . . . . . . . . .  6
       3.1.1.  DNSSEC Requirements for the Network  . . . . . . . . .  6
       3.1.2.  Node Behavior  . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Connectivity Check . . . . . . . . . . . . . . . . . . . .  7
       3.2.1.  No Connectivity Checks Against ipv4only.arpa . . . . .  8
     3.3.  Alternative Domain Names . . . . . . . . . . . . . . . . .  8
   4.  Operational Considerations for Hosting the IPv4-Only
       Well-Known Name  . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  DNS(64) Entity Considerations  . . . . . . . . . . . . . . . .  9
   6.  Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  About the IPv4 Address for the Well-Known Name . . . . . . 10
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     10.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Example of DNS Record Configuration . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   As part of the transition to IPv6, NAT64 [RFC6146] and DNS64
   [RFC6147] technologies will be utilized by some access networks to
   provide IPv4 connectivity for IPv6-only nodes [RFC6144].  The DNS64
   utilizes IPv6 address synthesis to create local IPv6 presentations of
   peers having only IPv4 addresses, hence allowing DNS-using IPv6-only
   nodes to communicate with IPv4-only peers.

   However, DNS64 cannot serve applications not using DNS, such as those
   receiving IPv4 address literals as referrals.  Such applications
   could nevertheless be able to work through NAT64, provided they are
   able to create locally valid IPv6 presentations of peers' IPv4
   addresses.

   Additionally, DNS64 is not able to do IPv6 address synthesis for
   nodes running validating DNSSEC enabled DNS resolvers, but instead
   the synthesis must be done by the nodes themselves.  In order to
   perform IPv6 synthesis nodes have to learn the IPv6 prefix(es) used
   on the access network for protocol translation.  The prefixes, which
   may be Network Specific Prefixes (NSP) or Well-Known Prefixes (WKP)
   [RFC6052], are referred in this document as Pref64::/n [RFC6146].

   This document describes a best effort method for applications and
   nodes to learn the information required to perform local IPv6 address
   synthesis.  The IPv6 address synthesis procedure itself is out-of-
   scope of this document.  An example application is a browser
   encountering IPv4 address literals in an IPv6-only access network.
   Another example is a node running validating security aware DNS
   resolver in an IPv6-only access network.

   The knowledge of IPv6 address synthesis taking place may also be
   useful if DNS64 and NAT64 are present in dual-stack enabled access
   networks or if a node is multi-interfaced [RFC6418].  In such cases
   nodes may choose to prefer IPv4 or alternative network interface in
   order to avoid traversal through protocol translators.

   It is important to notice that use of this approach will not result
   in as robust, secure, and good behaving system as an all-IPv6 system
   would be.  Hence it is highly recommended to upgrade nodes'
   destinations to IPv6 and utilize the described method only as a
   short-term solution.

2.  Requirements and Terminology

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2.1.  Requirements

   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].

2.2.  Terminology

   NAT64 FQDN: One or more fully qualified domain names for NAT64
   protocol translator entity.

   Pref64::/n: The IPv6 prefix used on IPv6 address synthesis [RFC6146].

   Well-Known IPv4-only Name (WKN): a fully qualified domain name,
   "ipv4only.arpa", well-known to have only A record.

   Well-Known IPv4 Address: an IPv4 address that is well-known and
   mapped to the well-known name.

3.  Node Behavior

   A node requiring information about presence of NAT64 and the
   Pref64::/n used for protocol translation SHALL send a DNS query for
   AAAA records of a well-known IPv4-only fully qualified domain name:
   "ipv4only.arpa".  The node MAY also need to perform DNS query for the
   A record of the well-known name in order to learn what is the IPv4
   address of the well-known name.  The node may perform this check in
   both IPv6-only and dual-stack access networks.

   When sending AAAA query for the well-known name a node MUST set
   "Checking Disabled (CD)" bit to zero, as otherwise the DNS64 will not
   perform IPv6 address synthesis hence does not reveal the Pref64::/n
   used for protocol translation.

   A DNS reply with one or more non-empty AAAA records indicates that
   the access network is utilizing IPv6 address synthesis.  A node MUST
   look through all of the received AAAA records to collect one or more
   Pref64::/n.  Pref64::/n list may include Well-Known Prefix 64:
   ff9b::/96 [RFC6052] or one or more Network-Specific Prefixes.  In the
   case of NSPs the node SHALL search for the IPv4 address of the well-
   known name inside of the received IPv6 addresses to determine the
   used address format.

   An IPv4 address of the well-known name should be found inside
   synthetic IPv6 address at some of the locations described in
   [RFC6052].  If the searched IPv4 address is not found on any of the
   standard locations the network must be using different formatting.

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   Developers may over time learn on IPv6 translated address formats
   that are extensions or alternatives to the standard formats.
   Developers MAY at that point add additional steps to the described
   discovery procedures.  The additional steps are outside the scope of
   the present document.

   The node should ensure a 32-bit IPv4 address value is present only
   once in an IPv6 address.  In case another instance of the value is
   found inside the IPv6, the node shall repeat the search with another
   IPv4 address, if possible.

   In the case only one Pref64::/n was present in the DNS response: a
   node shall use that Pref64::/n for both local synthesis and for
   detecting synthesis done by the DNS64 entity on the network.

   In the case of more than one Pref64::/n were present in the DNS
   response: a node SHOULD use all of them when determining whether
   other received IPv6 addresses are synthetic.  However, for selecting
   Pref64::/n for the local IPv6 address synthesis node MUST use the
   following prioritization order, of which purpose is to avoid use of
   Pref64::/n containing suffixes reserved for the future [RFC6052]:

   1.  Use NSP having /96 prefix

   2.  Use WKP prefix

   3.  Use longest available NSP prefix

   In the case of NXDOMAIN response or an empty AAAA reply: the DNS64 is
   not available on the access network, network filtered the well-known
   query on purpose, or something went wrong in the DNS resolution.  All
   unsuccessful cases result in unavailability of a node to perform
   local IPv6 address synthesis.  The node MAY periodically resend AAAA
   query to check if DNS64 has become available or possibly temporary
   problem cleared.  The node MAY perform A query for the well-known
   name to learn whether the Pref64::/n discovery framework is available
   at all (see section 6 about Exit Strategy).  The node MAY also
   continue monitoring DNS replies with IPv6 addresses constructed from
   WKP, in which case the node MAY use the WKP as if it were learned
   during the query for well-known name.

   To save Internet's resources, if possible, a node should perform
   Pref64::/n discovery only when needed (e.g. when local synthesis is
   required, cached reply timeouts, new network interface is started,
   and so forth).  The node SHALL cache the replies it receives during
   Pref64::/n discovery procedure and it SHALL repeat the discovery
   process when one third of the Time-To-Live of the Well-Known Name's
   synthetic AAAA DNS response is remaining.

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3.1.  Secure Learning of Pref64::/n

   If a node is using insecure channel between itself and DNS64, or
   DNS64 entity itself is untrusted, it is possible for an attacker to
   influence node's Pref64::/n detection procedures.  This may result in
   denial-of-service, redirection, man-in-the-middle, or other attacks.
   To protect against these attacks the node SHOULD communicate with
   trusted DNS64 over secure channel or use DNSSEC.  NAT64 operators
   SHOULD provide facilities for secure learning of Pref64::/n: a secure
   channel and/or DNSSEC protection.

3.1.1.  DNSSEC Requirements for the Network

   If the operator has chosen to support nodes performing Pref64::/n
   learning securely with DNSSEC, the operator of the NAT64 device MUST
   perform the following configurations.

   1.  Have one or more fully qualified domain names for the NAT64
       translator entities (later referred as NAT64 FQDN).  In the case
       of more than one Pref64::/n being used in a network, e.g. for
       load-balancing purposes, it is for network administrators to
       decide whether a single NAT64's fully qualified domain name maps
       to more than one Pref64::/n, or whether there will be dedicated
       NAT64 FQDN per Pref64::/n.

   2.  Each NAT64 FQDN MUST have one or more DNS AAAA resource records
       with each IPv6 address consisting of Pref64::/n and 0's for the
       elements after the actual prefix.

   3.  Each Pref64::/n MUST HAVE PTR record that points to corresponding
       NAT64 FQDN.

   4.  Sign the NAT64 FQDNs' AAAA and A records with DNSSEC.

3.1.2.  Node Behavior

   A node SHOULD prefer secure channel to talk to DNS64, whenever
   possible.  In addition, a node that implements DNSSEC validating
   resolver MAY use the following procedure to secure discovery of the
   Pref64::/n.

   1.  Heuristically find out a Pref64::/n candidate by making AAAA
       query for the "ipv4only.arpa" by following the procedure in
       Section 3.  This will return one or more AAAA resource records.
       For each of those AAAA resource records node wishes to use
       securely, the node performs the following steps.

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   2.  Send DNS PTR query for the IPv6 address of the translator (for
       "ipv6.arpa"), using the Pref64::/n from the step 1 and zeroes for
       the elements after the actual prefix length.  This will return
       the NAT64 FQDN.

   3.  Send DNS AAAA query for the NAT64 FQDN.

   4.  Verify the DNS AAAA response matches the address obtained in step
       1.  It is possible that the NAT64 FQDN maps to multiple AAAA
       records, in which case the node has to check if any of the
       responses matches to the address obtained in step 1.  The node
       must ignore other responses and not to use those for local IPv6
       address synthesis.

   5.  Perform DNSSEC validation of the DNS AAAA response.

   After the node has successfully performed the above five steps, the
   node can consider Pref64::/n securely learned.

3.2.  Connectivity Check

   After learning Pref64::/n, the node SHOULD perform connectivity check
   to ensure the learned Pref64::/n is functional.  It could be non-
   functional for a variety of reasons -- the discovery failed to work
   as expected, the IPv6 path to the NAT64 is down, the NAT64 is down,
   or the IPv4 path beyond the NAT64 is down.

   There are two main approaches to determine if the learned Pref64::/n
   is functional.  The first approach is to perform a dedicated
   connectivity check.  The second approach is to simply attempt to use
   the learned Pref64::/n.  Each approach has some trade-offs (e.g.,
   additional network traffic or possible user-noticable delay), and
   implementations should carefully weight which approach is appropriate
   for their application and the network.

   The node SHOULD use implementation specific connectivity check
   server, but if that is not possible a node MAY do a PTR query of the
   Pref64::/n that may return a hostname.  The node then does an A query
   of that hostname, which may return zero, one or more A resource
   records pointing to connectivity check servers used by the network
   operator.  Negative response to PTR or A query means there are no
   connectivity check servers available.  The operator of a NAT64 entity
   MAY assist nodes in their connectivity checks by mapping each NAT64
   FQDN to one or more DNS A resource records with IPv4 address(es)
   pointing to connectivity check server(s).

   In case of one or more connectivity check servers being available for
   use, the node chooses the first one preferring vendor specific

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   servers, if multiple are available.  The node MAY perform separate
   connectivity check by sending an ICMPv6 Echo Request to IPv6 address
   synthesized by combining discovered Pref64::/n with an IPv4 address
   of the server used for the connectivity check.  This will test the
   IPv6 path to the NAT64, the NAT64's operation, and the IPv4 path all
   the way to the connectivity check server.  Alternatively the node MAY
   utilize implementation specific connectivity check protocol.  If no
   response is received for the ICMPv6 Echo Request, the node sends
   another ICMPv6 Echo Request, a second later.  If still no response is
   received, it sends a third ICMPv6 Echo Request 3 seconds later.  If
   an ICMPv6 Echo Response is received, the node knows the IPv6 path to
   the connectivity check server is functioning normally.  If, after the
   three transmissions of the ICMPv6 Echo Request, no response is
   received, the node learns this Pref64::/n may not be functioning, and
   the node MAY choose a different NAT64 (if a different NAT64 is
   available), choose to alert the user, or proceed anyway hoping the
   problem is temporal or only with the connectivity check itself.

   If no separate connectivity check is performed before local IPv6
   address synthesis, a node may monitor success of connection attempts
   performed with locally synthetized IPv6 addresses.  Based on success
   of these connections, and based on possible ICMPv6 error messages
   received (such as Destination Unreachable Message), the mode MAY
   cease to perform local address synthesis and MAY restart Pref64::/n
   discovery procedures.

3.2.1.  No Connectivity Checks Against ipv4only.arpa

   Clients MUST NOT send a connectivity check to the address returned in
   the ipv4only.arpa query.  This is because, by design, no server will
   be operated on the Internet at that address as such.  Similarly,
   network operators MUST NOT operate a server on that address.  The
   reason this address isn't used for connectivity checks is that
   operators who neglect to operate a connectivity check server will
   allow that traffic towards the Internet where it will be dropped and
   cause a false negative connectivity check with the client (that is,
   the NAT64 is working fine, but the connectivity check fails because a
   server is not operating at ipv4-only.arpa on the Internet and a
   server is not operated by the NAT64 operator).  Instead, for the
   connectivity check, an additional DNS resource record is looked up
   and used for the connectivity check.  This ensures that packets don't
   unnecessarily leak to the Internet and reduces the chance of a false
   negative connectivity check.

3.3.  Alternative Domain Names

   Some applications, operating systems, devices, or networks may find
   it advantageous to operate their own DNS infrastructure to perform a

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   function similar to ipv4-only.arpa, but using a different resource
   record.  The primary advantage is to ensure availability of the DNS
   infrastructure and ensure the proper configuration of the DNS record
   itself.  DNS For example, a company named Example might have their
   application query ipv4-only.example.com.  Other than the different
   DNS resource record being queried, the rest of the operations are
   anticipated to be identical to the steps described in this document.

4.  Operational Considerations for Hosting the IPv4-Only Well-Known Name

   The authoritative name server for the well-known name shall have DNS
   record Time-To-Live (TTL) set to a long value in order to improve
   effectiveness of DNS caching.  The exact TTL value depends on
   availability time for the used public IPv4 address.

   The domain serving the well-known name must be signed with DNSSEC.
   See also Security Considerations section.

   It is expected that volumes for well-known name related queries are
   roughly SOMETHING, TBD.  The infrastructure required to serve well-
   known name is SOMETHING, TBD.

5.  DNS(64) Entity Considerations

   DNS(64) servers MUST NOT interfere or perform special procedures for
   the queries related to the well-known name until the time has arrived
   for the exit strategy to be defined and deployed.

6.  Exit Strategy

   A day will come when this tool is no longer needed.  At that point
   best suited techniques for implementing exit strategy will be
   documented.  In the global scope the exit strategy may include
   sending NXDOMAIN replies by the authoritative name server of the
   well-known name with a very long TTL.

   A node implementation receiving NXDOMAIN response for the A query of
   the well-known name means SHOULD consider this tool as temporarily
   disabled.

   A node SHOULD implement configuration option for disabling the
   Pref64::/n discovery feature.

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7.  Security Considerations

   The security considerations follow closely those of RFC6147
   [RFC6147].  If an attacker manages to change the Pref64::/n node
   discovers, the traffic generated by the node will be delivered to
   altered destination.  This can result in either a denial-of-service
   (DoS) attack (if the resulting IPv6 addresses are not assigned to any
   device), a flooding attack (if the resulting IPv6 addresses are
   assigned to devices that do not wish to receive the traffic), or an
   eavesdropping attack (in case the altered NSP is routed through the
   attacker).

   The zone serving the well-known name has to be protected with DNSSEC,
   as otherwise it will be too attractive target for attackers who wish
   to alter nodes' Pref64::/n discovery procedures.

   A node SHOULD implement validating DNSSEC resolver for validating the
   A response of the well-known name query.  A node without validating
   DNSSEC resolver SHOULD request validation to be performed by the used
   recursive DNS server and use secure channel when communicating with
   the DNS64.

   For the secure Pref64::/n discovery the access network SHOULD sign
   the NAT64 translator's fully qualified domain name.  A node SHOULD
   use the algorithm described in Section 3.1 in order to securely learn
   the Pref64::/n.

   Lastly, best mitigation action against Pref64::/n discovery attacks
   is to add IPv6 support for nodes' destinations and hence reduce need
   to perform local IPv6 address synthesis.

8.  IANA Considerations

   According to procedures described in RFC3172 this document requests
   IANA and IAB to reserve a second level domain from the .ARPA zone for
   the well-known domain name.  The well-known domain name could be, for
   example, "ipv4only.arpa".

   The well-known name also needs to map to one but preferably to two
   different public IPv4 addresses.

8.1.  About the IPv4 Address for the Well-Known Name

   The IPv4 address for the well-known name, if possible, should be
   chosen so that it is unlikely to appear more than once within an IPv6
   address and also as easy as possible to find from within the
   synthetic IPv6 address.  An address not listed in the Section 3 of

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   [RFC5735] is required as otherwise DNS64 entity may not perform AAAA
   record synthesis.  The address does not have to be routable or
   allocated to any node, as no communications are initiated to the IPv4
   address.

   Allocating two IPv4 addresses would improve the heuristics in cases
   where the primary IPv4 address' bit pattern appears more than once in
   the synthetic IPv6 address (NSP prefix contains the same bit pattern
   as the IPv4 address).

   If no well-known IPv4 address is statically allocated for this
   method, the heuristic requires sending additional A query to learn
   the IPv4 address that is sought inside the received IPv6 address.
   Without knowing IPv4 address it is impossible to determine address
   format used by DNS64.

9.  Acknowledgements

   Authors would like to thank Andrew Sullivan, Washam Fan, Cameron
   Byrne, Zhenqiang Li, Dave Thaler, Peter Koch, and Christian Huitema
   for significant improvement ideas and comments.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              BCP 153, RFC 5735, January 2010.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [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,
              April 2011.

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10.2.  Informative References

   [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation", RFC 6144, April 2011.

   [RFC6418]  Blanchet, M. and P. Seite, "Multiple Interfaces and
              Provisioning Domains Problem Statement", RFC 6418,
              November 2011.

Appendix A.  Example of DNS Record Configuration

   The following BIND-style examples illustrate how A and AAAA records
   could be configured by NAT64 operator.

   The examples use Pref64::/n of 2001:db8::/96 and example.com domain.

   The PTR record for reverse queries (Section 3.1.1 bullet 3):

   $ORIGIN 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.IP6.ARPA.
   @         IN      SOA   ns1.example.com. hostmaster.example.com. (
                           2003080800 12h 15m 3w 2h)
             IN      NS    ns.example.com.

             IN      PTR   nat64.example.com.

   If the example.com does not use DNSSEC, the following configuration
   file could be used.  Please note the nat64.example.com has both AAAA
   record with the Pref64::/n and A record for the connectivity check
   (Section 3.1.1 bullet 2).

   example.com.  IN SOA  ns.example.com. hostmaster.example.com. (
                         2002050501 ; serial
                         100        ; refresh (1 minute 40 seconds)
                         200        ; retry (3 minutes 20 seconds)
                         604800     ; expire (1 week)
                         100        ; minimum (1 minute 40 seconds)
                         )

   example.com.  IN NS  ns.example.com.

   nat64.example.com.
                 IN AAAA  2001:db8:0:0:0:0:0 nat64.example.com.
                 IN A  192.0.2.1

   If the example.com does use DNSSEC, the following configuration file
   could be used for A and AAAA records:

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   example.com.  IN SOA  ns.example.com. hostmaster.example.com. (
                         2002050501 ; serial
                         100        ; refresh (1 minute 40 seconds)
                         200        ; retry (3 minutes 20 seconds)
                         604800     ; expire (1 week)
                         100        ; minimum (1 minute 40 seconds)
                         )

   example.com.  IN RRSIG SOA  5 2 100 20090803071330 (
                         20090704071330 17000 example.com.
                         TVgWsNQvsFmeNHAeccGi7+UI7KwcE9TXPuSvmV9yyJwo
                         4FvHkxVC1H+98EtrmbR4c/XcdUzdfgn+q+lBqNsnbAit
                         xFERwPxzxbX0+yeCdHbBjHe7OuOc2Gc+CH6SbT2lKwVi
                         iEx3ySqqNoVScoUyhRdnPV2A1LV0yd9GtG9mI4w= )

   example.com.  IN NS  ns.example.com.
   example.com.  IN RRSIG NS  5 2 100 20090803071330 (
                         20090704071330 17000 example.com.
                         Xuw7saDDi6+5Z7SmtC7FC2npPOiE8F9qMR87eA0egG0I
                         B+xFx7pIogoVIDpOd1h3jqYivhblpCoDSBQb2oMbVy3B
                         SX5cF0r7Iu/xKP8XrV4DjNiugpa+NnhEIaRqG5uoPFbX
                         4cYT51yNq70I5mJvvajJu7UjmdHl26ZlnK33xps= )

   nat64.example.com.
                 IN AAAA  2001:db8:0:0:0:0:0 nat64.example.com.
                 IN RRSIG SOA  5 2 100 20090803071330 (
                         20090704071330 17000 example.net.
                         TVgWsNQvsFmeNHAeccGi7+UI7KwcE9TXPuSvmV9yyJwo
                         4FvHkxVC1H+98EtrmbR4c/XcdUzdfgn+q+lBqNsnbAit
                         xFERwPxzxbX0+yeCdHbBjHe7OuOc2Gc+CH6SbT2lKwVi
                         iEx3ySqqNoVScoUyhRdnPV2A1LV0yd9GtG9mI4w= )

   nat64.example.com.
                 IN A  192.0.2.1

Authors' Addresses

   Teemu Savolainen
   Nokia
   Hermiankatu 12 D
   FI-33720 Tampere
   Finland

   Email: teemu.savolainen@nokia.com

Savolainen, et al.     Expires September 13, 2012              [Page 13]
Internet-Draft            Pref64::/n Discovery                March 2012

   Jouni Korhonen
   Nokia Siemens Networks
   Linnoitustie 6
   FI-02600 Espoo
   Finland

   Email: jouni.nospam@gmail.com

   Dan Wing
   Cisco Systems
   170 West Tasman Drive
   San Jose, California  95134
   USA

   Email: dwing@cisco.com

Savolainen, et al.     Expires September 13, 2012              [Page 14]