[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03

BEHAVE Working Group                                             D. Wing
Internet-Draft                                                     Cisco
Intended status:  Standards Track                      February 14, 2011
Expires:  August 18, 2011


     IPv6-only and Dual Stack Hosts on the Same Network with DNS64
                   draft-wing-behave-dns64-config-03

Abstract

   Some networks are expected to support IPv4-only, dual-stack, and
   IPv6-only hosts at the same time.  Such networks also want to IPv6/
   IPv4 translation for the IPv6-only host so it can access servers on
   the IPv4 Internet.  On such a network, the synthesized AAAA responses
   from a DNS64 can cause traffic to be translated.  This document
   describes a solution to avoid that translation when the application
   uses DNS.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 18, 2011.

Copyright Notice

   Copyright (c) 2011 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



Wing                     Expires August 18, 2011                [Page 1]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Mechanism  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  5
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  6
   Appendix A.  Other Techniques  . . . . . . . . . . . . . . . . . .  7
     A.1.  New DHCP option for 'normal' DNS server  . . . . . . . . .  7
       A.1.1.  Host Transition  . . . . . . . . . . . . . . . . . . .  7
       A.1.2.  Advantages and Disadvantages . . . . . . . . . . . . .  7
     A.2.  Modify Host's Address Selection Rules  . . . . . . . . . .  8
       A.2.1.  Host Transition  . . . . . . . . . . . . . . . . . . .  9
       A.2.2.  Limitations and Advantages . . . . . . . . . . . . . .  9
       A.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . .  9
     A.3.  Indicating AAAA synthesis using DNS response flag  . . . . 10
     A.4.  New A64 record . . . . . . . . . . . . . . . . . . . . . . 10
     A.5.  Use DHCP to Assign Appropriate DNS Server  . . . . . . . . 10
       A.5.1.  Host Requirements  . . . . . . . . . . . . . . . . . . 11
       A.5.2.  DHCPv4 and DHCPv6 Server Requirements  . . . . . . . . 11
       A.5.3.  DHCP Server Operation  . . . . . . . . . . . . . . . . 12
       A.5.4.  Host Transition  . . . . . . . . . . . . . . . . . . . 12
       A.5.5.  Advantages and Disadvantages . . . . . . . . . . . . . 13
     A.6.  New DHCP option for DNS64 server . . . . . . . . . . . . . 14
       A.6.1.  Advantages and Disadvantages . . . . . . . . . . . . . 14
     A.7.  New DHCP option to identify dual-stack host  . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14










Wing                     Expires August 18, 2011                [Page 2]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


1.  Introduction

   In order to access IPv4 servers, an IPv6-only host needs to use an
   IPv6/IPv4 translator.  Typically, the IPv6-only host performs a DNS
   query to a DNS64 recursive resolver, which synthesizes an AAAA when
   necessary.  However, if a dual-stack host uses that same DNS64
   recursive resolver and normal address selection rules [RFC3484], the
   dual-stack host will send traffic through the IPv6/IPv4 translator
   when such traffic could have been sent using IPv4.  Thus, as an
   optimization, it is desirable that a dual-stack host avoid IPv6/IPv4
   translation.

   Note:  If the dual-stack host's IPv4 traffic is being NATted the
   difference is NAT44 versus NAT64, so the performance and saleability
   concern is nearly identical.  However, at least one application
   breaks when translated between IP address families unless special
   measures are taken [I-D.ietf-behave-ftp64].  The IETF should decide
   if it is worthwhile to avoid NAT64 for dual-stack hosts that are
   connected to a network operating a DNS64.

      Note:  Windows XP can only be configured with IPv4 DNS servers
      [XP-DNS].  This means a Windows XP host is always dual-stack and
      requires an IPv4 address in order to send its DNS queries.  While
      it is possible to work around this issue by running BIND on the
      Windows XP device itself, this is complex.  Thus, Windows XP
      should not be considered a viable operating system to join an
      IPv6-only network.


2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   "IPv4-only" means a host that has only IPv4 address(es) assigned to
   its interface(s).  "Dual-stack" means a host that has an IPv4 address
   and an IPv6 address assigned to its interface(es).  "IPv6-only" means
   a host that has only IPv6 address(es) assigned to its interface(s).


3.  Mechanism

   It has been observed that some common operating systems, when
   configured as dual-stack, will successfully use an IPv4-mapped
   address (and send an IPv4 packet).  But when configured as IPv6-only,
   they will not successfully use an IPv4-mapped address (because they
   lack an IPv4 address) [experiment].



Wing                     Expires August 18, 2011                [Page 3]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


   We take advantage of this by configuring the 'normal' DNS server
   using an IPv4-mapped IPv6 address (that is, an IPv6 address starting
   with ::ffff:/96), and configuring the DNS64 server using a normal
   IPv6 address.

   DNS servers are used in the order listed [RFC3646], so a dual-stack
   host will use the 'normal' DNS server (which is accessible over IPv4)
   and an IPv6-only host will be unable to use that 'normal' DNS server
   and will use the next server on its list.

      Note:  Non-compliant IPv6 stacks might send a packet to the IPv4-
      mapped IPv6 address (::ffff:c000:0201, using the example below).
      To deal with such non-compliant IPv6 implementations the network
      can filter (drop) traffic to that IPv6 address, which will force
      those stacks to timeout when attempting to contact the first DNS
      server and fall back to using the second DNS server.

   For example, a dual-stack host and an IPv6-only host would be
   configured with the following DNS servers, in this order, where the
   first one is the normal DNS server (192.0.2.1) and the second one is
   the DNS64 server (2001:db8:dddd::1234)

                ::ffff:192.0.2.1       # 'normal' DNS server
                2001:db8:dddd::1234    # DNS64 server

   This technique requires no change to host operating systems or host
   applications.

   When transitioning from dual-stack to IPv6-only, nothing needs to
   occur - the higher-priority DNS server (with the IPv4-mapped IPv6
   address) will become inaccessible and the DNS client will fail over
   to the next-higher priority DNS server (which is the DNS64 server).
   This does mean the host will take a few extra sections to fully
   initialize, as it will have to timeout its attempts to communicate
   with the first DNS server.

   When transitioning from IPv6-only to dual-stack, nothing
   automatically causes the host to start querying the 'normal' DNS
   server.  Thus, a host that transitions from IPv6-only to dual-stack
   will continue to query the DNS64 until the host's stack re-
   initializes.

      Operating System Note:  On Linux systems, this technique is not
      effective if the sysctl net.ipv6.bindv6only is set, as setting
      this parameter causes dual-stack systems to not send packets to
      IPv4-mapped IPv6 addresses.

   If the first DNS server is unavailable (e.g., link failure or DNS



Wing                     Expires August 18, 2011                [Page 4]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


   server failure) and the host's resolver times out, it will try the
   second DNS server, which is a DNS64 server.  This is unavoidable with
   this technique.  Thus, it is important that a robust infrastructure
   be used for the DNS servers, especially the first DNS server.


4.  Security Considerations

   TBD.


5.  Acknowledgements

   Thanks to Mohamed Boucadair, Marcelo Braun, Ralph Droms, Dave Thaler,
   Bernie Volz, and Andrew Yourtchenko for their review comments.


6.  IANA Considerations

   This document has no actions for IANA.


7.  References

7.1.  Normative References

   [I-D.ietf-behave-dns64]
              Bagnulo, M., Sullivan, A., Matthews, P., and I. Beijnum,
              "DNS64: DNS extensions for Network Address Translation
              from IPv6 Clients to IPv4 Servers",
              draft-ietf-behave-dns64-11 (work in progress),
              October 2010.

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

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,



Wing                     Expires August 18, 2011                [Page 5]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


              December 2003.

   [RFC4242]  Venaas, S., Chown, T., and B. Volz, "Information Refresh
              Time Option for Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 4242, November 2005.

   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, February 2006.

7.2.  Informative References

   [6man]     IETF, "IPv6 Maintenance Working Group", 2009,
              <http://www.ietf.org/dyn/wg/charter/6man-charter>.

   [I-D.arifumi-6man-rfc3484-revise]
              Matsumoto, A., Fujisaki, T., and R. Hiromi, "Things To Be
              Considered for RFC 3484 Revision",
              draft-arifumi-6man-rfc3484-revise-03 (work in progress),
              July 2010.

   [I-D.boucadair-behave-dns-a64]
              Boucadair, M. and E. Burgey, "A64: DNS Resource Record for
              IPv4-Embedded IPv6 Address",
              draft-boucadair-behave-dns-a64-02 (work in progress),
              September 2010.

   [I-D.ietf-behave-ftp64]
              Beijnum, I., "An FTP ALG for IPv6-to-IPv4 translation",
              draft-ietf-behave-ftp64-07 (work in progress),
              January 2011.

   [I-D.savolainen-mif-dns-server-selection]
              Savolainen, T. and J. Kato, "Improved DNS Server Selection
              for Multi-Homed Nodes",
              draft-savolainen-mif-dns-server-selection-06 (work in
              progress), January 2011.

   [I-D.wing-behave-learn-prefix]
              Wing, D., "Learning the IPv6 Prefix of a Network's IPv6/
              IPv4 Translator", draft-wing-behave-learn-prefix-04 (work
              in progress), October 2009.

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

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



Wing                     Expires August 18, 2011                [Page 6]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


              October 2010.

   [XP-DNS]   Microsoft, "Windows XP: IPv6 configuration items", 5 2005,
              <http://www.microsoft.com/resources/documentation/windows/
              xp/all/proddocs/en-us/sag_ip_v6_imp_config_items.mspx>.

   [experiment]
              Braun, "practical issues with using v4-mapped addresses
              for nat64", Jul 2008, <http://www.ietf.org/mail-archive/
              web/int-area/current/msg01476.html>.

   [mif]      IETF, "Multiple Interfaces Working Group", 2009,
              <http://www.ietf.org/dyn/wg/charter/mif-charter>.


Appendix A.  Other Techniques

      [Editor's Note:  This section will be removed in a later version
      of this document.  It is kept, in this appendix, for reference.]

   This section discusses other techniques which had been considered to
   avoid DNS64/NAT64 by dual-stack hosts.

A.1.  New DHCP option for 'normal' DNS server

   Another approach, which requires modification of dual-stack hosts
   which want to avoid the DNS64, is to introduce a new DHCP option.

   This approach feels a little backwards at first.  The idea is to
   support unmodified hosts (which might be dual-stack but might be
   IPv6-only) by placing DNS64 servers into the normal DHCPv6 option for
   DNS servers [RFC3646].  Then, place the 'normal' DNS servers into a
   *new* DHCPv6 option.

A.1.1.  Host Transition

   TBD.

A.1.2.  Advantages and Disadvantages

   Disadvantages:

   o  If dual-stack hosts want to avoid NAT64, they need to be modified
      to understand this new DHCP option.  If they aren't modified, they
      will use NAT64.






Wing                     Expires August 18, 2011                [Page 7]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


A.2.  Modify Host's Address Selection Rules

   The default address selection rules [RFC3484] prefer IPv6 over IPv4.
   This means, for a dual-stack host, that IPv6 will be preferred (if
   available) over IPv4.  If a dual-stack host is configured to use a
   DNS64 server, that DNS64 server will synthesize an AAAA response if
   there is an A record.  Thus, the dual-stack host will always use IPv6
   if a DNS lookup was involved, even if IPv4 could have been used more
   optimally.

      Note:  If both a NAT44 and NAT64 are deployed on the same network,
      roughly the same inefficiency occurs (that is, NAT state is
      created).  However, it is generally considered better to perform
      NAT44 than NAT64, because NAT64 translates between IP address
      families which can have side effects (e.g., FTP).

   To avoid this, the host's default address selection rules [RFC3484]
   can be modified so that IPv4 is preferred over the IPv6/IPv4
   translator's prefix.  At the same time, native IPv6 can still be
   preferred over IPv4.  This is accomplished by adding the network's
   IPv6/IPv4 translator's prefix as the lowest Precedence in the address
   selection rules.

   If the IPv6/IPv4 translator's prefix is the IANA-assigned well-known
   prefix (64:FF9B::/96, as assigned in [RFC6052]), this can be hard-
   coded or easily scripted into the system startup.  However, if the
   IPv6/IPv4 translator's prefix is a network-specific prefix (NSP, as
   described in [RFC6052]), the default address selection rules can be
   modified only after the host learns its currently-connected network's
   IPv6/IPv4 translator's prefix (e.g., using
   [I-D.wing-behave-learn-prefix]).

   On some operating systems, the address selection rules can be
   configured using a command line utility (e.g., Windows, FreeBSD),
   without new software in the host's IP stack.  Other operating systems
   are not as accommodating of this solution (see Appendix A.2.2).

      Note:  it may be desirable to create a standard to adjust a host's
      address selection rules based on the translator's prefix.  This is
      a topic for the IPv6 maintenance working group [6man].  This
      automatic mechanism may involve modifications to the host's IP
      stack, depending on how the IETF chooses to standardize such a
      mechanism.  FOR EXAMPLE, it may be useful to consider
      [I-D.wing-behave-learn-prefix] (which proposes using either DNS or
      DHCPv6) in conjunction with adjusting the host's address selection
      rules.





Wing                     Expires August 18, 2011                [Page 8]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


A.2.1.  Host Transition

   An IPv6-only and a dual-stack host can both be configured with the
   same address selection rules (namely, both can add the network's
   translator as the lowest Precedence).  This is because the IPv6-only
   host will never use IPv4 (because it lacks an IPv4 address) and will
   thus fall through and use the IPv6 address synthesized by the DNS64
   containing the IPv6/IPv4 translator's prefix (that is, as shown in
   the examples, the IPv6-only host will use the Precedence 3 entry in
   the default policy table).  The dual-stack host, if it receives an
   AAAA response, will prefer use IPv6; if it receives only an A
   response, it will prefer to use IPv4 (using Precedence 10 for IPv4-
   mapped addresses defined in Section 2.5.4 of [RFC3513]).

A.2.2.  Limitations and Advantages

   The following limitations are observed:

   o  OSX does not implement a [RFC3484] or [RFC3484]-like policy table.

   o  Some applications implement their own address selection rules,
      effectively ignoring the OS's address selection rules.

   The following advantages are observed:

   o  Causes IPv4 to be preferred over IPv6/IPv4 translator addresses,
      even if DNS was not used to obtain the IPv4 or IPv6 address (e.g.,
      applications which do not use DNS).

A.2.3.  Examples

   For example, if a network is using the WKP 64:FF9B::/96 [RFC6052] and
   a host is using the new default policy table from
   [I-D.arifumi-6man-rfc3484-revise] (which added Precedence 5 for
   Teredo), the host's new policy table would contain one new entry with
   Precedence 3, as shown below:

        Prefix        Precedence Label
        ::1/128               50     0     # localhost
        ::/0                  30     2     # IPv6 native
        2002::/16             20     3     # 6to4
        ::ffff:0:0/96         10     4     # IPv4-mapped
        2001::/32              5     5     # Teredo
        64:FF9B::/96           3     6     # 6/4 translator's prefix







Wing                     Expires August 18, 2011                [Page 9]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


   As another example, if a network has the prefix 2001:0DB8::/32 and
   the NAT64 is using the Network-Specific Prefix (NSP) 2001:0DB8:
   AAAA::/96, and the host is using the new default policy table from
   [I-D.arifumi-6man-rfc3484-revise] (which added Precedence 5 for
   Teredo), the host's new policy table would contain one new entry with
   Precedence 3, as shown below:

        Prefix        Precedence Label
        ::1/128               50     0     # localhost
        ::/0                  30     2     # IPv6 native
        2002::/16             20     3     # 6to4
        ::ffff:0:0/96         10     4     # IPv4-mapped
        2001::/32              5     5     # Teredo
        2001:0DB8:AAAA::/96    3     6     # 6/4 translator's prefix

A.3.  Indicating AAAA synthesis using DNS response flag

   Dacheng Zhang's idea.

A.4.  New A64 record

   [I-D.boucadair-behave-dns-a64]

A.5.  Use DHCP to Assign Appropriate DNS Server

      Note:  due to the limitations of this solution (see
      Appendix A.5.5), it may have little or no value.

   To avoid unnecessary traffic through a translator, it is desirable to
   configure IPv4-only and dual-stack hosts with a 'normal' DNS
   recursive resolver.

   However, it is necessary to configure IPv6-only hosts with a DNS64
   [I-D.ietf-behave-dns64] recursive resolver so those hosts can use an
   IPv6/IPv4 translator and access servers on the IPv4 Internet.

   It is difficult to provide different DNS servers to those types of
   hosts, because there is no existing protocol that declares a host is
   IPv4-only, dual-stack, or IPv6-only.

   This document describes how a network's DHCPv4 and DHCPv6 servers,
   combined with a client-identifiers [RFC4361] chosen by the host, can
   determine if a host is IPv4-only, dual-stack, or IPv6-only, and
   assign the correct DNS server according to that determination.

      Note:  the DHCP mechanism described in this section have some
      overlap with the Multiple Interfaces Working Group [mif] and with
      split-zone DNS [I-D.savolainen-mif-dns-server-selection].



Wing                     Expires August 18, 2011               [Page 10]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


   Both an IPv4-only host and a dual-stack host obtain an IPv4 network
   address.  Today, hosts most commonly obtain an IPv4 address using
   DHCPv4 [RFC2131].  An IPv6-only host does not obtain an IPv4 address;
   however, it may be using DHCPv6 to obtain other information (e.g.,
   NTP servers).  The following procedure takes advantage of that
   difference to determine if a host is IPv4-only, dual-stack, or IPv6-
   only.

A.5.1.  Host Requirements

   The host has the following requirements:

   1.  if the host uses IPv4, it MUST use DHCPv4 to learn its IPv4
       address and its DNS server address(es); and,

   2.  if the host uses IPv6, it MUST use DHCPv6 to learn its IPv6 DNS
       resolver, using the Information-Request message described in
       Section 18.1.5 of [RFC3315] and using [RFC3646]; and,

   3.  the host MUST use client-identifiers [RFC4361] to identify itself
       to its DHCP server(s), and MUST use the same client-identifier
       for both DHCPv4 and DHCPv6

          Note:  This last requirement is stronger than the SHOULD in
          Section 6.2 of [RFC4361]

       If the host does not support DHCP authentication, and acquires/
       releases its IPv4 address while keeping its IPv6 address, it MUST
       support the procedure described in Appendix A.5.4; and,

   4.  the host MUST support the DHCP Information Refresh Time Option
       [RFC4242].

A.5.2.  DHCPv4 and DHCPv6 Server Requirements

   The DHCPv4 and DHCPv6 servers have the following requirements:

   1.  the DHCPv4 and DHCPv6 servers MUST be able to communicate with
       each other both client-identifiers [RFC4361] and if an IPv4
       address is assigned to that client-identifier; and,

   2.  If the DHCP server and the host support DHCP authentication, the
       DHCP server MUST support the procedure described in
       Appendix A.5.4.

   3.  MUST support the DHCP Information Refresh Time Option [RFC4242].





Wing                     Expires August 18, 2011               [Page 11]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


A.5.3.  DHCP Server Operation

   If the DHCP server first receives a DHCPv4 request for a particular
   client-identifier, it responds with the 'normal' DNS resolver.  The
   DHCPv6 server remembers that RFC4361 client identity and if the
   DHCPv6 server sees a DHCPv6 request from that same client identity,
   it responds to the DHCPv6 request with a 'normal' DNS resolver.

   If the DHCP server first receives a DHCPv6 request for a particular
   client-identifier, it responds with a short information refresh time
   [RFC4242] (e.g., 30 seconds) and a DNS64 recursive resolver.

      Note-1:  This means that during the short information refresh
      time, both a dual-stack host and an IPv6-only will have their DNS
      queries processed by the DNS64 recursive resolver.  During that
      time, both the dual-stack host and the IPv6-only host will get
      connectivity to IPv4 servers, but the dual-stack host will use the
      IPv6/IPv4 translator until the information refresh time expires.

      Note-2:  for discussion:  Consider have DHCP server slightly delay
      (e.g., 100ms) responding to a DHCPv6 request.  This gives a chance
      for the DHCPv4 request to be received, thus avoiding the issue
      described in Note-1.

   After the short information refresh time, the DHCPv6 client will send
   a new request.  By that time, the DHCPv6 server will have either:

   a.  have seen a DHCPv4 request from the same RFC4361 host.  This
       indicates the host supports dual-stack.  The DHCP server should
       extend the DHCPv6 lease, and provide a 'normal' DNS server
       (instead of the DNS64 server).

   b.  have not seen a DHCPv4 request from the same RFC4361 host.  This
       indicates the host is IPv6-only.  The DHCP server should extend
       the DHCPv6 lease and continue providing the same DNS64 server.

A.5.4.  Host Transition

   During natural evolution of a network or because of debugging/
   troubleshooting, a host might transition between IPv4-only, dual-
   stack, or IPv6-only.  When the host acquires or releases its IPv4
   address it transitions to needing a different DNS server; if the host
   has an IPv4 address, it needs a 'normal' DNS server and if it does
   not have an IPv4 address it needs a DNS64 server.

   There are two transitions considered, where the host transitions:





Wing                     Expires August 18, 2011               [Page 12]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


   1.  from IPv6-only to IPv4-supporting (that is, IPv4-only or dual-
       stack),

   2.  from IPv4-supporting (that is, IPv4-only or dual-stack) to IPv6-
       only.

   When doing (1), the DHCPv4 server will provide a 'normal' DNS server
   (because the DHCPv4 server sees the same client-identifier as seen by
   the DHCPv6 server).  So case (1) is solved.

   However, when doing (2), the host is giving up its IPv4 address and
   is currently using a normal DNS server, but needs to be told to use a
   DNS64 server instead.  There are two mechanisms to provide that
   function, based on the network and host's support of DHCP
   authentication (Section 19.1.1 of [RFC3315])

   1.  with DHCP authentication:  When a certain client identifier loses
       or acquires its IPv4 address and also has an IPv6 address, the
       DHCPv6 server MUST send a DHCP RECONFIGURE message [RFC3315] to
       the host and SHOULD include the Option Request option indicating
       the DNS server information has changed.  The RECONFIGURE message
       triggers the host to send a new Information-Request message to
       the DHCPv6 server.

   2.  without DHCP authentication:  the host, when keeping its IPv6
       address and releasing its IPv4 address, MUST also issue a new
       DHCPv6 Information-Request message to the DHCPv6 server.

   In both cases, the Information-Request message causes the DHCPv6
   server to reply with a DNS64 recursive resolver, as discussed in
   Appendix A.5.2.

A.5.5.  Advantages and Disadvantages

   Advantages:

   o  Dual-stack applications, which perform DNS lookups, will
      effectively avoid NAT64 when using the 'normal' DNS server.

   Disadvantages:

   o  A network with mixed IPv4-only/dual-stack hosts and IPv6-only
      hosts needs to have a mix of DNS configurations for those hosts.
      Thus, mechanisms that advertise the same DNS servers to all hosts
      cannot be used on such networks (e.g., IPv6 router
      advertisements).





Wing                     Expires August 18, 2011               [Page 13]

Internet-Draft      IPv6-only & Dual-Stack with DNS64      February 2011


   o  If separate networks operate DHCPv4 and DHCPv6 (e.g., as with
      Dual-Stack Lite where the ISP operates DHCPv4 and the customer
      premise router operates DHCPv6), it is likely impossible for the
      DHCPv4 and DHCPv6 servers to communicate necessary information
      with each other.

   o  Windows does not support [RFC4361].

   o  OSX does not support DHCPv6.

A.6.  New DHCP option for DNS64 server

   Another approach, which requires modification of IPv6-only hosts
   which need to use the DNS64, is to introduce a new DHCP option.

   The idea is to support unmodified dual-stack hosts (which use the
   normal DNS server provided via [RFC3646]), but to modify IPv6-only
   hosts to look for the DNS64 server in a newly-defined DHCPv6 option.

A.6.1.  Advantages and Disadvantages

   Disadvantages:

   o  Requires modifying IPv6-only hosts, and without this modification
      they won't work at all with a DNS64.

A.7.  New DHCP option to identify dual-stack host

   Dacheng Zhang's idea.


Author's Address

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email:  dwing@cisco.com











Wing                     Expires August 18, 2011               [Page 14]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/