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Network Working Group                                        S. Cheshire
Internet-Draft                                               D. Schinazi
Updates: 7050 (if approved)                                   Apple Inc.
Intended status: Standards Track                           July 18, 2016
Expires: January 19, 2017


                Special Use Domain Name 'ipv4only.arpa'
                draft-cheshire-sudn-ipv4only-dot-arpa-03

Abstract

   The specification for how a client discovers its network's NAT64
   prefix [RFC7050] defines the special name 'ipv4only.arpa' for this
   purpose, but treats it as a non special name in the document's Domain
   Name Reservation Considerations section.

   Consequently, despite the well articulated special purpose of the
   name, as of July 2016 'ipv4only.arpa' still does not appear as one of
   the names with special properties that are recorded in the Special-
   Use Domain Names registry.

   This document formally declares the actual special properties of the
   name, and adds similar declarations for the corresponding reverse
   mapping names.

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 January 19, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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
   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.


1.  Introduction

   The specification for how a client discovers its network's NAT64
   prefix [RFC7050] defines the special name 'ipv4only.arpa' for this
   purpose, but treats it as a non special name in the document's Domain
   Name Reservation Considerations section.

   Consequently, despite the well articulated special purpose of the
   name, as of July 2016 'ipv4only.arpa' still does not appear as one of
   the names with special properties that are recorded in the Special-
   Use Domain Names registry [SUDN].

   This document formally declares the actual special properties of the
   name.  This document also adds similar declarations for the
   corresponding reverse mapping names.


2.  Specialness of 'ipv4only.arpa'

   The hostname 'ipv4only.arpa' is peculiar in that it was never
   intended to be treated like a normal hostname.

   A typical client never looks up the IPv4 address records for
   'ipv4only.arpa', because it is already known, by specification
   [RFC7050], to have exactly two IPv4 address records, 192.0.0.170 and
   192.0.0.171.  No client ever has to look the name in order to learn
   those two addresses.

   In contrast, clients often look up the IPv6 AAAA address records for
   'ipv4only.arpa', which is contrary to general DNS expectations, given
   that it is already known, by specification [RFC7050], that no such
   IPv6 AAAA address records exist.  And yet, clients expect to receive,
   and do in fact receive, positive answers for these IPv6 AAAA address
   records that are known to not exist.

   This is clearly not a typical DNS name.  Clients never normally query
   for the two records that do in fact exist, instead querying for



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   records that are known to not exist, and getting positive answers to
   those abnormal queries.  Clients are using DNS to perform queries for
   this name, but they are certainly not using DNS to learn legitimate
   answers from the name's legitimate authoritative server.  Instead,
   clients are using these pseudo-DNS queries as an impromptu middlebox
   communication protocol, to communicate with the NAT64/DNS64
   [RFC6146][RFC6147] gateway, if present, and request that it disclose
   the prefix it is using for IPv6 address synthesis.

   It is this use of specially-crafted DNS queries for 'ipv4only.arpa'
   as an impromptu middlebox communication protocol that makes the name
   'ipv4only.arpa' noteworthy, and legitimately qualifying to be
   described as a 'special' name.


3.  Consequences of 'ipv4only.arpa' previously being unspecial

   As a result of the original specification [RFC7050] not formally
   declaring 'ipv4only.arpa' to have special properties, there was no
   mandate for any server software to treat this name specially.
   Consequently, queries for this name had to be handled normally,
   resulting in unnecessary queries to the authoritative 'arpa' name
   servers.

   Having millions of devices around the world issue these queries
   generated pointless additional load on the authoritative 'arpa' name
   servers, which was completely unnecessary when the name
   'ipv4only.arpa' is defined, by Internet Standard, to have exactly two
   IPv4 address records, 192.0.0.170 and 192.0.0.171, and no other
   records of any type.

   Also, at times, for reasons that are as yet unclear, the
   authoritative 'arpa' name servers have been observed to be slow or
   unresponsive.  The failures of these 'ipv4only.arpa' queries result
   in unnecessary failures of software that depends on them for DNS64
   [RFC6147] address synthesis.

   Even when the authoritative 'arpa' name servers are operating
   correctly, having to perform an unnecessary query to obtain an answer
   that is already known in advance can add precious milliseconds of
   delay for no reason.

   This document leverages this operational experience to update the
   Domain Name Reservation Considerations section [RFC6761] of the
   earlier specification [RFC7050] with one that accurately lists the
   actual special properties of the name 'ipv4only.arpa' so that
   software can legitimately make appropriate performance and
   reliability optimizations.



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4.  Security Considerations

   Hard-coding the known answers for 'ipv4only.arpa' queries in
   recursive/caching DNS servers reduces the risk of malicious devices
   intercepting those queries and returning incorrect answers,
   particularly in the case of recursive/caching DNS servers that do not
   perform DNSSEC validation.

   One of the known concerns with DNS64 [RFC6147] is that it interferes
   with DNSSEC.  DNSSEC may cryptographically assert that a name has no
   IPv6 AAAA records, while at the same time DNS64 address synthesis is
   contradicting this and claiming that IPv6 AAAA records do exist.

   Section 3 of the DNS64 specification [RFC6147] discusses this:

      ... DNS64 receives a query with the DO bit set and
      the CD bit set. In this case, the DNS64 is supposed
      to pass on all the data it gets to the query initiator.
      This case will not work with DNS64, unless the
      validating resolver is prepared to do DNS64 itself.

   The NAT64 Prefix Discovery specification [RFC7050] provides the
   mechanism for the query initiator to learn the NAT64 prefix so that
   it can do its own validation and DNS64 synthesis as described above.
   With this mechanism the client can (i) interrogate the local NAT64/
   DNS64 gateway with an 'ipv4only.arpa' query to learn the IPv6 address
   synthesis prefix, (ii) query for the (signed) IPv4 address records
   itself, and then (iii) perform its own IPv6 address synthesis
   locally, combining the IPv6 address synthesis prefix learned from the
   local NAT64/DNS64 gateway with the secure DNSSEC-signed data learned
   from the global Domain Name System.

   It is conceivable that over time, if DNSSEC is successful, the
   majority of clients could move to this validate-and-synthesize-
   locally model, which reduces the DNS64 machinery to the vestigial
   role of simply responding to the 'ipv4only.arpa' query to report the
   local IPv6 address synthesis prefix.  In no case does the client care
   what answer(s) the authoritative 'arpa' name servers might give for
   that query.  The 'ipv4only.arpa' query is being used purely as a
   local client-to-middlebox communication message.

   This approach is even more attractive if it doesn't create an
   additional dependency on the authoritative 'arpa' name servers to
   answer a query that is unnecessary because the NAT64/DNS64 gateway
   already knows the answer before it even issues the query.  Avoiding
   this unnecessary query improves performance and reliability for the
   client, and reduces unnecessary load for the authoritative 'arpa'
   name servers.



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5.  IANA Considerations

   [Once published, this should say]

   IANA has recorded the following names in the
   Special-Use Domain Names registry [SUDN]:
      ipv4only.arpa.
      170.0.0.192.in-addr.arpa.
      171.0.0.192.in-addr.arpa.

   IANA has recorded the following IPv4 addresses in the
   IPv4 Special-Purpose Address Registry [SUv4]:
      192.0.0.170
      192.0.0.171


6.  Domain Name Reservation Considerations

6.1.  Conventions and Terminology Used in this Section

   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
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

6.2.  ipv4only.arpa

   The name 'ipv4only.arpa' is defined, by Internet Standard, to have
   two IPv4 address records with rdata 192.0.0.170 and 192.0.0.171.

   When queried via a DNS64 [RFC6147] recursive/caching server, the name
   'ipv4only.arpa' is also defined to have IPv6 AAAA records, with rdata
   synthesized from a combination of the NAT64 IPv6 prefix(es), and the
   IPv4 addresses 192.0.0.170 and 192.0.0.171.  This can return more
   than one pair of IPv6 addresses if there are multiple NAT64 prefixes.

   The name 'ipv4only.arpa' has no other DNS records of any type.

   The name 'ipv4only.arpa' is special only to
   (a) client software wishing to perform DNS64 address synthesis, and
   (b) the DNS64 recursive/caching server responding to such requests.
   These two considerations are listed in items 2 and 4 below:

   1.  Normal users should never have reason to encounter the
       'ipv4only.arpa' domain name.  If they do, they should expect
       queries for 'ipv4only.arpa' to result in the answers required by
       the specification [RFC7050].  Normal users have no need to know
       that 'ipv4only.arpa' is special.



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   2.  Application software may explicitly use the name 'ipv4only.arpa'
       for NAT64/DNS64 address synthesis, and expect to get the answers
       required by the specification [RFC7050].  If application software
       encounters the name 'ipv4only.arpa' in the normal course of
       handling user input, the application software should resolve that
       name as usual and need not treat it in any special way.

   3.  Name resolution APIs and libraries SHOULD NOT recognize
       'ipv4only.arpa' as special and SHOULD NOT treat it differently.
       Name resolution APIs SHOULD send queries for this name to their
       configured recursive/caching DNS server(s).

   4.  Recursive/caching DNS servers SHOULD recognize 'ipv4only.arpa' as
       special and SHOULD NOT, by default, attempt to look up NS records
       for it, or otherwise query authoritative DNS servers in an
       attempt to resolve this name.  Instead, recursive/caching DNS
       servers SHOULD, by default, act as authoritative and generate
       immediate responses for all such queries.

       Traditional recursive/caching DNS servers that act as
       authoritative for this name MUST generate only the 192.0.0.170
       and 192.0.0.171 responses for IPv4 address queries (DNS qtype
       "A"), and a "no error no answer" response for all other query
       types.

       All DNS64 recursive/caching DNS servers MUST generate the
       192.0.0.170 and 192.0.0.171 responses for IPv4 address queries
       (DNS qtype "A"), the appropriate synthesized IPv6 address record
       responses for IPv6 address queries (DNS qtype "AAAA"), and a
       "no error no answer" response for all other query types.
       This local self-contained generation of these responses is to
       avoid placing unnecessary load on the authoritative 'arpa' name
       servers.

       Example configurations for BIND 9 showing how to achieve these
       results are given in Appendix A.

   5.  Traditional authoritative DNS server software need not recognize
       'ipv4only.arpa' as special or handle it in any special way.
       Recursive/caching DNS servers SHOULD routinely act as
       authoritative for this name and return the results described
       above.  Only the administrators of the 'arpa' namespace need to
       explicitly configure their actual authoritative name servers to
       be authoritative for this name and to generate the appropriate
       answers; all other authoritative name servers will not be
       configured to know anything about this name and will reject
       queries for it as they would reject queries for any other name
       about which they have no information.



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   6.  Generally speaking, operators of authoritative DNS servers need
       not know anything about the name 'ipv4only.arpa', just as they
       don't need to know anything about any other names they are not
       responsible for.  Operators of authoritative DNS servers who are
       configuring their name servers to be authoritative for this name
       MUST understand that 'ipv4only.arpa' is a special name, with
       records rigidly specified by Internet Standard (generally this
       applies only to the administrators of the 'arpa' namespace).

   7.  DNS Registries/Registrars need not know anything about the name
       'ipv4only.arpa', just as they don't need to know anything about
       any other name they are not responsible for.  Only the
       administrators of the 'arpa' namespace need to be aware of this
       name's purpose and how it should be configured.

6.3.  170.0.0.192.in-addr.arpa and 171.0.0.192.in-addr.arpa

   Since the IPv4 addresses 192.0.0.170 and 192.0.0.171 are defined to
   be special, and are listed in the IPv4 Special-Purpose Address
   Registry [SUv4], the corresponding reverse mapping names in the
   in-addr.arpa domain are similarly special.

   The name '170.0.0.192.in-addr.arpa' is defined, by Internet Standard,
   to have only a single DNS record, type PTR, with rdata
   'ipv4only.arpa'.

   The name '171.0.0.192.in-addr.arpa' is defined, by Internet Standard,
   to have only a single DNS record, type PTR, with rdata
   'ipv4only.arpa'.

   Practically speaking these two names are rarely used, but to the
   extent that they may be, they are special only to recursive/caching
   DNS servers as described in item 4 below:

   1.  Normal users should never have reason to encounter these two
       reverse mapping names.  However, if they do, queries for these
       reverse mapping names should return the expected answer
       'ipv4only.arpa'.  Normal users have no need to know that these
       reverse mapping names are special.

   2.  Application software SHOULD NOT recognize these two reverse
       mapping names as special, and SHOULD NOT treat them differently.
       For example, if the user were to issue the Unix command
       "host 192.0.0.170" then the "host" command should issue the query
       as usual and display the result that is returned.

   3.  Name resolution APIs and libraries SHOULD NOT recognize these two
       reverse mapping names as special and SHOULD NOT treat them



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       differently.  Name resolution APIs SHOULD send queries for these
       names to their configured recursive/caching DNS server(s).

   4.  Recursive/caching DNS servers SHOULD recognize these two reverse
       mapping names as special and SHOULD NOT, by default, attempt to
       look up NS records for them, or otherwise query authoritative DNS
       servers in an attempt to resolve them.  Instead, recursive/
       caching DNS servers SHOULD, by default, act as authoritative and
       generate immediate responses for all such queries.

       Recursive/caching DNS servers that act as authoritative for these
       names MUST generate only the 'ipv4only.arpa' response for PTR
       queries, and a "no error no answer" response for all other query
       types.  This local self-contained generation of these responses
       is to avoid placing unnecessary load on the authoritative
       'in-addr.arpa' name servers.

   5.  Traditional authoritative DNS server software need not recognize
       these two reverse mapping names as special or handle them in any
       special way.
       As a practical matter, only the administrators of the
       'in-addr.arpa' namespace will configure their name servers to be
       authoritative for these names and to generate the appropriate
       answers; all other authoritative name servers will not be
       configured to know anything about these names and will reject
       queries for them as they would reject queries for any other name
       about which they have no information.

   6.  Generally speaking, operators of authoritative DNS servers need
       not know anything about these two reverse mapping names, just as
       they don't need to know anything about any other names they are
       not responsible for.  Operators of authoritative DNS servers who
       are configuring their name servers to be authoritative for this
       name MUST understand that these two reverse mapping names are
       special, with answers specified by Internet Standard (generally
       this applies only to the administrators of the 'in-addr.arpa'
       namespace).

   7.  DNS Registries/Registrars need not know anything about these two
       reverse mapping names, just as they don't need to know anything
       about any other name they are not responsible for.  Only the
       administrators of the 'in-addr.arpa' namespace need to be aware
       of the purpose of these two names.








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6.4.  ip6.arpa Reverse Mapping PTR Records

   For all IPv6 addresses synthesized by the NAT64 gateway, the DNS64
   recursive/caching server is responsible for synthesizing the
   appropriate ip6.arpa reverse mapping PTR records, if it chooses to do
   so.  The same applies to the synthesized IPv6 addresses corresponding
   to the IPv4 addresses 192.0.0.170 and 192.0.0.171.

   Generally a DNS64 recursive/caching server synthesizes appropriate
   ip6.arpa reverse mapping PTR records by extracting the embedded IPv4
   address from the encoded IPv6 address, performing a reverse mapping
   query for that IPv4 address, and then synthesizing a corresponding
   ip6.arpa reverse mapping PTR record containing the same rdata.

   In the case of synthesized IPv6 addresses corresponding to the IPv4
   addresses 192.0.0.170 and 192.0.0.171, the DNS64 recursive/caching
   server does not issue mapping queries for those IPv4 addresses, but
   instead, according to rule 3 above, immediately returns the answer
   'ipv4only.arpa'.
































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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <http://www.rfc-editor.org/info/rfc6146>.

   [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,
              DOI 10.17487/RFC6147, April 2011,
              <http://www.rfc-editor.org/info/rfc6147>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <http://www.rfc-editor.org/info/rfc6761>.

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,
              <http://www.rfc-editor.org/info/rfc7050>.

7.2.  Informative References

   [SUDN]     "Special-Use Domain Names Registry", <https://
              www.iana.org/assignments/special-use-domain-names/>.

   [SUv4]     "IANA IPv4 Special-Purpose Address Registry", <https://
              www.iana.org/assignments/iana-ipv4-special-registry/>.















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Appendix A.  Example BIND 9 Configuration

   A BIND 9 recursive/caching DNS server could be configured to act as
   authoritative for the appropriate names as follows.


   In /etc/named.conf the following lines are added:

    zone "ipv4only.arpa"            { type master; file "ipv4only"; };
    zone "170.0.0.192.in-addr.arpa" { type master; file "ipv4only-r"; };
    zone "171.0.0.192.in-addr.arpa" { type master; file "ipv4only-r"; };


   The file /var/named/ipv4only is created with the following content.
   (The lines marked "Only for DNS64 server" are omitted on a standard
   recursive/caching DNS server.)

    $TTL 86400               ; Default TTL 24 hours
    @ IN SOA nameserver.example. admin.nameserver.example. (
             2016052400      ; Serial
             7200            ; Refresh ( 7200 = 2 hours)
             3600            ; Retry   ( 3600 = 1 hour)
             15724800        ; Expire  (15724800 = 6 months)
             60              ; Minimum
             )
    @ IN NS  nameserver.example.

    @ IN A     192.0.0.170
    @ IN A     192.0.0.171
    @ IN AAAA  64:ff9b::192.0.0.170 ; Only for DNS64 server
    @ IN AAAA  64:ff9b::192.0.0.171 ; Only for DNS64 server


   The file /var/named/ipv4only-r is created with the following content:

    $TTL 86400               ; Default TTL 24 hours
    @ IN SOA nameserver.example. admin.nameserver.example. (
             2016052400      ; Serial
             7200            ; Refresh ( 7200 = 2 hours)
             3600            ; Retry   ( 3600 = 1 hour)
             15724800        ; Expire  (15724800 = 6 months)
             60              ; Minimum
             )
    @ IN NS  nameserver.example.

    @ IN PTR ipv4only.arpa.





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Authors' Addresses

   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   USA

   Phone: +1 408 974 3207
   Email: cheshire@apple.com


   David Schinazi
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   USA

   Phone: +1 669 227 9921
   Email: dschinazi@apple.com































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