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Versions: (draft-savolainen-heuristic-nat64-discovery) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 17 RFC 7050

Behave WG                                                  T. Savolainen
Internet-Draft                                                     Nokia
Intended status: Standards Track                             J. Korhonen
Expires: May 8, 2013                              Nokia Siemens Networks
                                                                 D. Wing
                                                           Cisco Systems
                                                        November 4, 2012


      Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis
           draft-ietf-behave-nat64-discovery-heuristic-12.txt

Abstract

   This document describes a method for detecting the presence of DNS64
   and for learning the IPv6 prefix used for protocol translation on an
   access network.  The method depends on the 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 NAT64 on dual-stack 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   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 May 8, 2013.

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.  Validation of Discovered Pref64::/n  . . . . . . . . . . .  6
       3.1.1.  DNSSEC Requirements for the Network  . . . . . . . . .  6
       3.1.2.  DNSSEC Requirements for the Node . . . . . . . . . . .  7
     3.2.  Connectivity Check . . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  No Connectivity Checks Against ipv4only.arpa . . . . .  9
     3.3.  Alternative Domain Names . . . . . . . . . . . . . . . . . 10
     3.4.  Message Flow Illustration  . . . . . . . . . . . . . . . . 10
   4.  Operational Considerations for Hosting the IPv4-Only
       Well-Known Name  . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Operational Considerations for DNS64 Operator  . . . . . . . . 12
     5.1.  Mapping of IPv4 Address Ranges to IPv6 Prefixes  . . . . . 12
   6.  Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Example of DNS Record Configuration . . . . . . . . . 16
   Appendix B.  About the IPv4 Address for the Well-Known Name  . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
















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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].  DNS64
   utilizes IPv6 address synthesis to create local IPv6 addresses for
   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 addresses that would be translated
   to the 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.  A prefix, which may
   be a Network Specific Prefix (NSP) or Well-Known Prefix (WKP)
   [RFC6052], is referred to 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 a 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 used in dual-stack enabled access
   networks or if a node is multi-interfaced [RFC6418].  In such cases,
   nodes may choose to prefer IPv4 or an alternative network interface
   in order to avoid traversal through protocol translators.

   It is important to note that use of this approach will not result in
   a system that is as robust, secure, and well-behaved 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
   transition 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: a fully qualified domain name for a NAT64 protocol
   translator.

   Pref64::/n: a IPv6 prefix used for IPv6 address synthesis [RFC6146].

   Pref64::WKA: an IPv6 address consisting of Pref64::/n and WKA at any
   of the locations allowed by RFC 6052 [RFC6052].

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

   Well-Known IPv4 Address (WKA): an IPv4 address that is well-known and
   present in an A record for the well-known name.  Two well-known IPv4
   addresses are defined for Pref64::/n discovery purposes: 192.0.0.170
   and 192.0.0.171.


3.  Node Behavior

   A node requiring information about the presence (or absence) of
   NAT64, and one or more Pref64::/n used for protocol translation SHALL
   send a DNS query for AAAA resource records of the Well-Known IPv4-
   only Name (WKN) "ipv4only.arpa".  The node MAY perform the DNS query
   in both IPv6-only and dual-stack access networks.

   When sending a DNS AAAA resource record query for the WKN, a node
   MUST set the "Checking Disabled (CD)" bit to zero [RFC4035], as
   otherwise the DNS64 server will not perform IPv6 address synthesis
   (Section 3 of [RFC6147]) and hence would not reveal the Pref64::/n
   used for protocol translation.

   A DNS reply with one or more AAAA resource records indicates that the
   access network is utilizing IPv6 address synthesis.  In some
   scenarios captive portals, or NXDOMAIN and NODATA hijacking,
   performed by the access network may result in a false positive.  One
   method to detect such hijacking is to query a Fully Qualified Domain
   Name (FQDN) that is known to be invalid (and normally return an empty
   response or an error response) and see if it returns a valid resource
   record.  However, as long as the hijacked domain does not result in
   AAAA resource record responses that contain well-known IPv4 address



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   in any location defined by RFC6052, the response will not disturb the
   Pref64::/n learning procedure.

   A node MUST look through all of the received AAAA resource records to
   collect one or more Pref64::/n.  The Pref64::/n list might include
   the Well-Known Prefix 64:ff9b::/96 [RFC6052] or one or more Network-
   Specific Prefixes.  In the case of NSPs, the node SHALL determine the
   used address format by searching the received IPv6 addresses for the
   WKN's well-known IPv4 addresses.  The node SHALL assume the well-
   known IPv4 addresses might be found at the locations specified by
   [RFC6052] section 2.2.  The node MUST check on octet boundaries to
   ensure a 32-bit well-known IPv4 address value is present only once in
   an IPv6 address.  In case another instance of the value is found
   inside the IPv6 address, the node SHALL repeat the search with the
   other well-known IPv4 address.

   If 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 server on the network.

   If more than one Pref64::/n was present in the DNS response: a node
   SHOULD use all of them when determining whether other received IPv6
   addresses are synthetic.  The node MUST use all learned Pref64::/n
   when performing local IPv6 address synthesis, and use the prefixes in
   the order received from the DNS64 server.  That is, when the node is
   providing a list of locally synthesized IPv6 addresses to upper
   layers, IPv6 addresses MUST be synthesized by using all discovered
   Pref64::/n in the received order.

   If the well-known IPv4 addresses are not found within the standard
   locations, it indicates that the network is not using a standard
   address format and the Pref64::/n cannot be determined.  Developers
   can over time learn of 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
   procedure.  The additional steps are outside the scope of the present
   document.

   In case a node does not receive a positive DNS reply to the AAAA
   resource record query, the node MAY perform a DNS A resource record
   query for the well-known name.  If the node receives a positive reply
   to the DNS A resource record query it means the used recursive DNS
   server is not a DNS64 server.

   In the case of a negative response (NXDOMAIN, NODATA) or a DNS query
   timeout: a DNS64 server is not available on the access network, the
   access network filtered out the well-known query, or something went
   wrong in the DNS resolution.  All unsuccessful cases result in a node



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   being unable to perform local IPv6 address synthesis.  In the case of
   timeout, the node SHOULD retransmit the DNS query like any other DNS
   query the node makes [RFC1035].  In the case of a negative response
   (NXDOMAIN, NODATA), the node MUST obey the Time-To-Live [RFC1035] of
   the response before resending the AAAA resource record query.  The
   node MAY monitor for DNS replies with IPv6 addresses constructed from
   the WKP, in which case if any are observed the node SHOULD use the
   WKP as if it were learned during the query for the well-known name.

   To save Internet resources if possible, a node should perform
   Pref64::/n discovery only when needed (e.g., when local synthesis is
   required, a new network interface is connected to a new network, and
   so forth).  The node SHALL cache the replies it receives during the
   Pref64::/n discovery procedure and it SHOULD repeat the discovery
   process ten seconds before the Time-To-Live of the Well-Known Name's
   synthetic AAAA resource record expires.

3.1.  Validation of Discovered Pref64::/n

   If a node is using an insecure channel between itself and a DNS64
   server, or the DNS64 server is untrusted, it is possible for an
   attacker to influence the node's Pref64::/n discovery procedures.
   This may result in denial-of-service, redirection, man-in-the-middle,
   or other attacks.

   To mitigate against attacks, the node SHOULD communicate with a
   trusted DNS64 server over a secure channel, or use DNSSEC.  NAT64
   operators SHOULD provide facilities for validating discovery of
   Pref64::/n via a secure channel and/or DNSSEC protection.

   It is important to understand that DNSSEC only validates that the
   discovered Pref64::/n is the one that belongs to a domain used by
   NAT64 FQDN.  Importantly, the DNSSEC validation does not tell if the
   node is at the network where the Pref64::/n is intended to be used.
   Furthermore, DNSSEC validation cannot be utilized in the case of WKP.

3.1.1.  DNSSEC Requirements for the Network

   If the operator has chosen to support nodes performing validation of
   discovered Pref64::/n 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 a dedicated



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       NAT64 FQDN per Pref64::/n.

   2.  Each NAT64 FQDN MUST have one or more DNS AAAA resource records
       containing Pref64::WKA (Pref64::/n combined with WKA).

   3.  Each Pref64::WKA MUST have a PTR resource record that points to
       the corresponding NAT64 FQDN.

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

3.1.2.  DNSSEC Requirements for the Node

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

   1.  Heuristically find Pref64::/n candidates by making a AAAA
       resource record query for "ipv4only.arpa" by following the
       procedure in Section 3.  This will result in IPv6 addresses
       consisting of Pref64::/n combined with WKA, i.e., Pref64::WKA.
       For each Pref64::/n that the node wishes to validate, the node
       performs the following steps.

   2.  Send a DNS PTR resource record query for the IPv6 address of the
       translator (for "ip6.arpa"), using the Pref64::WKA learned in
       step 1.  CNAME and DNAME results should be followed according to
       the rules in RFC 1034 [RFC1034], RFC 1034 [RFC1035], and RFC 6672
       [RFC6672].  The ultimate response will include one or more NAT64
       FQDNs.

   3.  The node SHOULD compare the domains of learned NAT64 FQDNs to a
       list of the node's trusted domains and choose a NAT64 FQDN that
       matches.  The means for a node to learn the trusted domains is
       implementation-specific.  If the node has no list of trusted
       domains, the node MAY query the user whether the domain can be
       trusted and MAY remember the answer for future use.  If the node
       has no trust for the domain, the discovery procedure is not
       secure and the remaining steps described below MUST NOT be
       performed.

   4.  Send a DNS AAAA resource record query for the NAT64 FQDN.

   5.  Verify the DNS AAAA resource record contains Pref64::WKA
       addresses received at the step 1.  It is possible that the NAT64
       FQDN has multiple AAAA records, in which case the node MUST check
       if any of the addresses match the ones obtained in step 1.  The
       node MUST ignore other responses and not use them for local IPv6



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       address synthesis.

   6.  Perform DNSSEC validation of the DNS AAAA response.

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

3.2.  Connectivity Check

   After learning a Pref64::/n, the node SHOULD perform a 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-noticeable delay), and
   implementations should carefully weigh which approach is appropriate
   for their application and the network.

   The node SHOULD use an implementation-specific connectivity check
   server and a protocol of the implementation's choice, but if that is
   not possible, a node MAY do a PTR resource record query of the
   Pref64::WKA to get a NAT64 FQDN.  The node then does an A resource
   query of the NAT64 FQDN, which will return zero or more A resource
   records pointing to connectivity check servers used by the network
   operator.  A negative response to the PTR or A resource query means
   there are no connectivity check servers available.  A network
   operator that provides NAT64 services for a mix of nodes with and
   without implementation-specific connectivity check servers SHOULD
   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).  The Pref64::/n -based connectivity
   check approach works only with NSPs, as it is not possible to
   register A records for each different domain using WKP.  The network
   operator MUST disable ICMPv6 rate limiting for connectivity check
   messages.

   In case of multiple connectivity check servers being available for
   use, the node chooses the first one, preferring implementation-
   specific servers.

   The connectivity check protocol used with implementation-specific
   connectivity check servers is implementation-specific.




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   The connectivity check protocol used with connectivity check servers
   pointed to by the NAT64 FQDN's A resource records is ICMPv6
   [RFC4443].  The node performing a connectivity check against these
   servers SHALL send an ICMPv6 Echo Request to an IPv6 address
   synthesized by combining discovered Pref64::/n with an IPv4 address
   of the server as specified in [RFC6052].  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.  If no response is received for
   the ICMPv6 Echo Request, the node SHALL send another ICMPv6 Echo
   Request, a second later.  If still no response is received, the node
   SHALL send a third ICMPv6 Echo Request two 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 and three seconds since the last ICMPv6 Echo
   Request, no response is received, the node learns this Pref64::/n
   might not be functioning, and the node MAY choose a different
   Pref64::/n (if available), choose to alert the user, or proceed
   anyway hoping the problem is temporary or only with the connectivity
   check itself.  After all, the ICMPv6 is by design unreliable and
   failure to receive ICMPv6 responses may not indicate anything other
   than network failure to transport ICMPv6 messages through.

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

3.2.1.  No Connectivity Checks Against ipv4only.arpa

   Clients MUST NOT send a connectivity check to an address returned by
   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 "ipv4only.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.




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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
   function similar to "ipv4only.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.  For example, a company named Example might have their
   application query "ipv4only.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.

3.4.  Message Flow Illustration

   The figure below gives an example illustration of a message flow in
   the case of prefix discovery utilizing Pref64::/n validation.  The
   figure shows also a step where the procedure ends if no Pref64::/n
   validation is performed.

   In this example, three Pref64::/n are provided by the DNS64 server.
   The first Pref64::/n is using an NSP, in this example "2001:db8:
   42::/96".  The second Pref64::/n is using an NSP, in this example
   "2001:db8:43::/96".  The third Pref64::/n is using the WKP.  Hence,
   when the Pref64::/n are combined with the WKA to form Pref64::WKA,
   the synthetic IPv6 addresses returned by DNS64 server are "2001:db8:
   42::192.0.0.170", "2001:db8:43::192.0.0.170", and "64:ff9b::
   192.0.0.170".  The DNS64 server could also return synthetic addresses
   containing the IPv4 address 192.0.0.171.

   The validation is not done for the WKP, see Section 3.1.

   Node                                           DNS64 server
     |                                                |
     |  "AAAA" query for "ipv4only.arpa"              |
     |----------------------------------------------->| "A" query for
     |                                                | "ipv4only.arpa"
     |                                                |---------------->
     |                                                |
     |                                                | "A" response:
     |                                                | "192.0.0.170"
     |                                                | "192.0.0.171"
     |                                                |<----------------
     |                                +----------------------------+
     |                                | "AAAA" synthesis using     |
     |                                | three Pref64::/n.          |
     |                                +----------------------------+
     |  "AAAA" response with:                         |
     |  "2001:db8:42::192.0.0.170"                    |



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     |  "2001:db8:43::192.0.0.170"                    |
     |  "64:ff9b::192.0.0.170"                       |
     |<-----------------------------------------------|
     |                                                |
  +----------------------------------------------+    |
  | If Pref64::/n validation is not performed, a |    |
  | node can fetch prefixes from AAAA responses  |    |
  | at this point and skip the steps below.      |    |
  +----------------------------------------------+    |
     |                                                |
     |  "PTR" query #1 for "2001:db8:42::192.0.0.170  |
     |----------------------------------------------->|
     |  "PTR" query #2 for "2001:db8:43::192.0.0.170  |
     |----------------------------------------------->|
     |                                                |
     |  "PTR" response #1 "nat64_1.example.com"       |
     |<-----------------------------------------------|
     |  "PTR" response #2 "nat64_2.example.com"       |
     |<-----------------------------------------------|
     |                                                |
  +----------------------------------------------+    |
  | Compare received domains to a trusted domain |    |
  | list and if matches are found, continue.     |    |
  +----------------------------------------------+    |
     |                                                |
     |  "AAAA" query #1 for "nat64_1.example.com"     |
     |----------------------------------------------->|
     |  "AAAA" query #2 for "nat64_2.example.com"     |
     |----------------------------------------------->|
     |                                                |
     | "AAAA" resp. #1 with "2001:db8:42::192.0.0.170 |
     |<-----------------------------------------------|
     | "AAAA" resp. #2 with "2001:db8:43::192.0.0.170 |
     |<-----------------------------------------------|
     |                                                |
  +----------------------------------------------+    |
  | Validate AAAA responses and compare the IPv6 |    |
  | addresses to those previously learned.       |    |
  +----------------------------------------------+    |
     |                                                |
  +----------------------------------------------+    |
  | Fetch the Pref64::/n from the validated      |    |
  | responses and take into use.                 |    |
  +----------------------------------------------+    |
     |                                                |

                      Pref64::/n discovery procedure




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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 at least 60 minutes in order to
   improve effectiveness of DNS caching.  The exact TTL value will be
   determined and tuned based on operational experiences.

   The domain serving the well-known name MUST be signed with DNSSEC.
   See also section 7.


5.  Operational Considerations for DNS64 Operator

   A network operator of a DNS64 server can guide nodes utilizing
   heuristic discovery procedures by managing the responses a DNS64
   server provides.

   If the network operator would like nodes to utilize multiple
   Pref64::/n, the operator needs to configure DNS64 servers to respond
   with multiple synthetic AAAA records.  As per Section 3 the nodes can
   then use them all.

   There are no guarantees on which of the Pref64::/n nodes will end up
   using.  If the operator wants nodes to specifically use a certain
   Pref64::/n or periodically change the Pref64::/n they use, for
   example for load balancing reasons, the only guaranteed method is to
   make DNS64 servers return only a single synthetic AAAA resource
   record, and have the Time-To-Live of that synthetic record such that
   the node repeats the Pref64::/n discovery when required.

   Besides choosing how many Pref64::/n to respond and what Time-To-Live
   to use, DNS64 servers MUST NOT interfere with or perform other
   special procedures for the queries related to the well-known name.

5.1.  Mapping of IPv4 Address Ranges to IPv6 Prefixes

   RFC 6147 [RFC6147] allows DNS64 implementations to be able to map
   specific IPv4 address ranges to separate Pref64::/n.  That allows
   handling of special use IPv4 addresses [RFC5735].  The example setup
   where this might be used is illustrated in Figure 1.  The NAT64 "A"
   is used when accessing IPv4-only servers in the datacenter, and the
   NAT64 "B" is used for Internet access.









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                       NAT64 "A" ----- IPv4-only servers in a datacenter
                      /
   IPv6-only node----<
                      \
                       NAT64 "B" ----- IPv4 Internet

                 Figure 1: NAT64s with IPv4 Address Ranges

   The heuristic discovery method described herein does not support
   learning of the possible rules used by a DNS64 server for mapping
   specific IPv4 address ranges to separate Pref64::/n.  Therefore,
   nodes will use the same discovered Pref64::/n to synthesize IPv6
   addresses from any IPv4 address.  This can cause issues for routing
   and connectivity establishment procedures.  The operator of the NAT64
   and the DNS64 ought to take this into account in the network design.

   The network operators can help IPv6-only nodes by ensuring the nodes
   do not have to work with IPv4 address literals for which special
   mapping rules are used.  That is, the IPv4-only servers addressed
   from the special IPv4 address ranges ought to have signed AAAA
   records, which allows IPv6-only nodes to avoid local address
   synthesis.  If the IPv6-only nodes are not using DNSSEC, then it is
   enough if the network's DNS64 server returns synthetic AAAA resource
   records pointing to IPv4-only servers.  Avoiding the need for IPv6-
   only nodes to perform address synthesis for IPv4 addresses belonging
   to special ranges is the best approach to assist nodes.

   If the IPv6-only nodes have no other choice than using IPv4-address
   literals belonging to special IPv4 address ranges, and the IPv6-only
   node will perform local synthesis by using the discovered Pref64::/n,
   then the network ought to ensure with routing that the packets are
   delivered to the correct NAT64.  For example, a router in the path
   from an IPv6-only host to NAT64s can forward the IPv6 packets to the
   correct NAT64 as illustrated in Figure 2.  The routing could be based
   on the last 32-bits of the IPv6 address, but the network operator can
   also use some other IPv6 address format allowed by RFC 6052
   [RFC6052], if it simplifies routing setup.  This setup requires
   additional logic on the NAT64 providing connectivity to special IPv4
   address ranges: it needs to be able to translate packets it receives
   that are using the Pref64::/n used with Internet connections.

                       NAT64 "A" ----- IPv4-only servers in a datacenter
                      /
   IPv6-only host----router
                      \
                       NAT64 "B" ----- IPv4 Internet

                  Figure 2: NAT64s with Assisting Router



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6.  Exit Strategy

   A day will come when this tool is no longer needed.  At that point
   the best suited techniques for implementing an exit strategy will be
   documented.

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


7.  Security Considerations

   The security considerations follow closely those of RFC 6147
   [RFC6147].  The possible attacks are very similar in the case where
   an attacker controls a DNS64 server and returns tampered IPv6
   addresses to a node and in the case where an attacker causes the node
   to use tampered Pref64::/n for local address synthesis.  DNSSEC
   cannot be used to validate responses created by a DNS64 server the
   node has no trust relationship with.  Hence this document does not
   change the big picture for untrusted network scenarios.  If an
   attacker alters the Pref64::/n used by a DNS64 server or a node, the
   traffic generated by the node will be delivered to an 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 a target for attackers who
   wish to alter nodes' Pref64::/n discovery procedures.

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

   For Pref64::/n discovery validation, the access network SHOULD sign
   the NAT64 translator's fully qualified domain name.  A node SHOULD
   use the algorithm described in Section 3.1 to validate each
   discovered Pref64::/n.

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




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8.  IANA Considerations

   According to procedures described in [RFC3172] this document directs
   IANA 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 needs to map to two different global IPv4
   addresses.  The addresses are to be taken from the 192.0.0.0/24
   address block, and can be, for example 192.0.0.170 and 192.0.0.171.
   The addresses are to be documented to be of global scope, but they do
   not need to be routable within local or global scopes.


9.  Acknowledgements

   Authors would like to thank Dmitry Anipko, Cameron Byrne, Aaron Yi
   Ding Christian Huitema, Washam Fan, Peter Koch, Stephan Lagerholm,
   Zhenqiang Li, Simon Perreault, Marc Petit-Huguenin, Andrew Sullivan,
   and Dave Thaler, for significant improvement ideas and comments.


10.  References

10.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

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

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [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



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              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.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, June 2012.

10.2.  Informative References

   [RFC3172]  Huston, G., "Management Guidelines & Operational
              Requirements for the Address and Routing Parameter Area
              Domain ("arpa")", BCP 52, RFC 3172, September 2001.

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

   [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 a NAT64 operator.

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

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














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   $ORIGIN A.A.0.0.0.0.0.C\
   .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.

   $ORIGIN B.A.0.0.0.0.0.C\
   .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 example.com does not use DNSSEC, the following configuration file
   could be used.  Please note that nat64.example.com has both an AAAA
   record with the Pref64::/n and an 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:C000:00AA
                 IN AAAA  2001:db8:0:0:0:0:C000:00AB
                 IN A  192.0.2.1

   To DNSSEC sign the records, the owner of the example.com zone would
   have RRSIG records for both the AAAA and A records for
   nat64.example.com.  As a normal DNSSEC requirement, the zone and its
   parent also need to be signed.


Appendix B.  About the IPv4 Address for the Well-Known Name

   The IPv4 addresses for the well-known name cannot be non-global IPv4
   addresses as listed in the Section 3 of [RFC5735].  Otherwise DNS64
   servers might not perform AAAA record synthesis when the well-known
   prefix is used, as stated in Section 3.1 of [RFC6052].  However, the



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   addresses do not have to be routable or allocated to any real node,
   as no communications will be initiated to these IPv4 address.

   Allocation of at least two IPv4 addresses improves the heuristics in
   cases where the bit pattern of the primary IPv4 address appears more
   than once in the synthetic IPv6 address (i.e., the NSP prefix
   contains the same bit pattern as the IPv4 address).

   If no well-known IPv4 addresses would be statically allocated for
   this method, the heuristic would require sending of an additional A
   query to learn the IPv4 addresses that would be then searched from
   inside of the received IPv6 address.


Authors' Addresses

   Teemu Savolainen
   Nokia
   Hermiankatu 12 D
   FI-33720 Tampere
   Finland

   Email: teemu.savolainen@nokia.com


   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










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