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Versions: 00 01 02 03 04 05 06 07 rfc5158                               
Individual Submission                                          G. Huston
Internet-Draft                                                     APNIC
Expires: April 5, 2005                                   October 5, 2004

                      6to4 Reverse DNS Delegation

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
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   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on April 5, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).


   This memo describes a potential mechanism for entering a description
   of DNS servers which provide "reverse lookup" of 6to4 addresses into
   the 6to4 reverse zone file.  The proposed mechanism is a conventional
   DNS delegation interface, allowing the client to enter the details of
   a number of DNS servers for the delegated domain.  The client is
   authenticated by its source address and is authorised to use the
   function if its IPv6 /48 address prefix corresponds to the requested
   delegation point.

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1.  Introduction

   6to4 [1] defines a mechanism for allowing isolated IPv6 sites to
   communicate using IPv6 over the public IPv4 Internet.  This is
   achieved through the use of a dedicated IPv6 global unicast address
   prefix.  A 6to4 'router' can use its IPv4 address value in
   conjunction with this global prefix to create a local IPv6 site
   prefix.  Local IPv6 hosts use this site prefix to form their local
   IPv6 address.

   This address structure allows any site that is connected to the IPv4
   Internet the ability to use IPv6 via automatically created IPv6 over
   IPv4 tunnels.  The advantage of this approach is that it allows the
   piecemeal deployment of IPv6 using tunnels to traverse IPv4 network
   segments.  A local site can connect to a IPv6 network without
   necessarily obtaining IPv6 services from its adjacent upstream
   network provider.

   As noted in [3], the advantage of this approach is that: "it
   decouples deployment of IPv6 by the core of the network (e.g.
   Internet Service Providers or ISPs) from deployment of IPv6 at the
   edges (e.g.  customer sites), allowing each site or ISP to deploy
   IPv6 support in its own time frame according to its own priorities.
   With 6to4, the edges may communicate with one another using IPv6 even
   if one or more of their ISPs do not yet provide native IPv6 service."

   The particular question here is the task of setting up a set of
   delegations that allows "reverse lookups" for this address space.

      "[This] requires that there be a delegation path for the IP
      address being queried, from the DNS root to the servers for the
      DNA zone which provides the PTR records for that IP address.  For
      ordinary IPv6 addresses, the necessary DNS servers and records for
      IPv6 reverse lookups would be maintained by the each organization
      to which an address block is delegated; the delegation path of DNS
      records reflects the delegation of address blocks themselves.
      However, for IPv6 addresses beginning with the 6to4 address
      prefix, the DNS records would need to reflect IPv4 address
      delegation.  Since the entire motivation of 6to4 is to decouple
      site deployment of IPv6 from infrastructure deployment of IPv6,
      such records cannot be expected to be present for a site using
      6to4 - especially for a site whose ISP did not yet support IPv6 in
      any form." [3]

   The desired characteristics of a reverse lookup delegation mechanism
   are that it:
      *  is deployable with minimal overhead or tool development

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      *  has no impact on existing DNS software and existing DNS
      *  performs name lookup efficiently
      *  does not compromise any DNS security functions

2.  Potential Approaches

   There are a number of approaches to this problem, ranging from a
   conventional explicit delegation structure to various forms of
   modified server behaviours that attempt to guess the location of non-
   delegated servers for fragments of this address space.  These
   approaches have been explored in some detail in terms of their
   advantages and drawbacks in [3], so only a summary of these
   approaches will be provided here.

2.1  Conventional Address Delegation

   The problem with this form of delegation is the anticipated piecemeal
   deployment of 6to4 sites.  The reason why a site would use 6to4 is
   commonly that the upstream provider does not support a IPv6 transit
   service and the end site is using 6to4 to tunnel through to IPv6
   connectivity.  A conventional environment would have the 6to4 site
   using provider-based IPv4 addresses.  In the IPv4 "in-addr.arpa"
   domain the local site would have an entry in the upstream's reverse
   DNS zone file, or would have authoritative local name servers that
   are delegated from the upstream's DNS zone.  In the case of the
   mapped IPv6 space the upstream is not using IPv6 and therefore would
   not be expected to have a 6to4 delegation for its IPv4 address block.

   Sub-delegations of IPv4 provider address space are not consistently
   recorded, and any 6to4 reverse zone operator would be required to
   undertake reverse zone delegations in the absence of reliable current
   address assignment information, undertaking a "hop over" of the
   upstream provider's address block.  Similarly, a delegated entity may
   need to support the same "hop over" when undertaking further
   delegations in their reverse zone.

2.2  Guessing a Non-Delegated 6to4 Reverse Server

   One way to avoid such unreliable delegations is to alter server
   behaviour for reverse servers in this zone.  Where no explicit
   delegation information exists in the zone file the server could look
   up the in-addr.arpa domain for the servers for the equivalent IPv4
   address root used in the 6to4 address.  These servers could then be
   queried for the IPv6 PTR query.

   The issues with fielding altered server behaviours for this domain
   are not to be taken lightly, and the delegation chain for IPv4 will

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   not be the same for 6to4 in any case.  An isolated 6to4 site uses a
   single IPv4 /32 address, and it is improbable that a single address
   would have explicit in-addr.arpa delegation.  In other words it is
   not likely that the server delegation for IPv4 would parallel that of

2.3  Locating Local Servers at Reserved Addresses

   Another approach uses an altered server to resolve non-delegated 6to4
   reverse queries.  The 6to4 query is decoded to recover the original
   6to4 IP address.  The site-specific part of the address is rewritten
   to a constant value, and this value is used as the target of a lookup
   query.  This requires that a 6to4 site should reserve local
   addresses, and configure reverse servers on these addresses.  Again
   this is a weak approach in that getting the DNS to query
   non-delegated addresses is a case of generation of spurious traffic.

2.4  Synthesized Responses

   The final approach is to synthesize an answer when no explicit
   delegation exists.  This approach would construct a pseudo host name
   using the IPv6 query address as the seed.  Given that the host name
   has no valid forward DNS mapping, then this becomes a case of
   transforming one invalid DNS object into another.

2.5  Selecting a Reasonable Approach

   It would appear that the most reasonable approach is to support a
   model of conventional standard delegation.  The consequent task is to
   reduce the administrative overheads in managing the zone, supporting
   delegation of reverse zone files on a basis of providing a delegation
   capability directly to each 6to4 site.

3.  6to4 Networks Address Use

   A 6to4 client network is an isolated IPv6 network composed as a set
   of IPv6 hosts and a dual stack (IPv4 and IPv6) local router connected
   to the local IPv6 network and the external IPv4 network.

   An example of a 6to4 network is as follows:

   IPv6-in-IPv4 packets (A)|             |     IPv6 packets
   ------------------------| 6to4router  |--------------------------
                           |             |    |  |   |     |   |
                           +-------------+   local IPv6 clients

      IPv4 network                              IPv6 network

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                                Figure 1

   The IPv4 address used as part of the generation of 6to4 addresses for
   the local IPv6 network is the external IPv4 network (labelled '(A)'
   in the above diagram).  For example, if the interface (A) has the
   IPv4 address, then the local IPv6 clients will use a common
   IPv6 address prefix of the form 2002:{}::/48 (or
   (2002:C000:201::/48 in hex notation).  All the local IPv6 clients
   share this common /48 address prefix, irrespective of any local IPv4
   address that such host may use if they are operating in a dual stack

   An example of a 6to4 network with addressing:

         IPv4 network  |             | IPv6 network
    -------------------| 6to4router  |-------------
    |             |    |  |   | interface identifier
                       +-------------+   1A  |   | local IPv6 address
                                             |   |
                                             1B  |

                                Figure 2

4.  Delegation Administration

   This document proposes to use a a single delegation level in the zone, delegating zones only at the 48th bit
   position.  The corresponds with individual delegations corresponding
   to a /32 IPv4 address, or the equivalent of a single 6to4 local site.

   The zone files containing the end site delegations are proposed to be
   operated with a TTL (configured to be a time value in the scale of
   hours rather than days or weeks), and updates from delegation
   requests are to be made using incremental DNS updates [2].

   The delegation system is proposed to be self-driven by clients
   residing within 6to4 networks.  The server's delegation function is
   proposed to be accessible only by clients using 6to4 IPv6 source
   addresses, and the only delegation that can be managed is that
   corresponding to the /48 prefix of the IPv6 source address of the

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   It is proposed to operate the delegation management service using
   secure web-based servers.  This will ensure that the source address-
   driven delegation selection function cannot be disrupted through
   proxy caching of the server's responses.

   The URL of this service is https://6to4.nro.net

   It is proposed that the secure web servers be operated on a
   dual-stack IPv4 / IPv6 server.  The service is to be available on a)
   an IPv4 address (instructions only), b) a native IPv6 address
   (instructions plus delegation service) and c) a 6to4 address
   (instructions plus delegation service).

   The server's actions will be determined by the source address of the
   client.  If the client uses a 6to4 source address the server will
   present a delegation interface for the corresponding 6to4 reverse
   zone.  Otherwise the server will provide a description of the
   delegation process.

   When accessed by a 6to4 source address, the interface presented by
   the delegation server is a standard DNS delegation interface,
   allowing the client to enter the details of a number of DNS servers
   for the corresponding reverse domain.  The delegation servers are
   checked by the delegation manager to ensure that they are responding,
   that they are configured consistently and are authoritative for the
   delegated domain.  If these conditions are met the delegation details
   are entered into the primary zone.  In order to avoid the server
   being used as a denial of service platform the server should throttle
   the number of DNS requests made to any single IP address, and also
   throttle the number of redelegation requests for any single 6to4

   In other cases the system provides diagnostic information to the

   The benefits of this proposed structure include a fully automated
   mode of operation.  The service delivery is on demand and the system
   only permits self-operation of the delegation function.

   The potential issues with this structure include:
   o  Clients inside a 6to4 site could alter the delegation details
      without the knowledge of the site administrator.  It is noted that
      this is intended for small-scale sites.  Where there are potential
      issues of unauthorized access to this delegation function the
      local site administrator could take appropriate access control
   o  IPv4 DHCP-based 6to4 sites could inherit nonsense reverse entries
      created by previous users of the DHCP address.  In this case the

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      client site could request delegation of the reverse zone as
   o  The approach does not scale efficiently, as there is the potential
      that the flat zone file may grow considerably.  However it is
      noted that 6to4 is intended to be a transition mechanism useful
      for a limited period of time in a limited context of isolated
      network where other forms of tunnelled connection is not feasible.
      It is envisaged that at some point the density of IPv6 adoption in
      stub network would provide adequate drivers for widespread
      adoption of native IPv6 services, obviating the need for continued
      scaling of 6to4 support services.  An estimate of the upper bound
      of the size of the 6to4 reverse delegation zone would be of the
      order of millions of entries.  It is also noted that the value of
      a reverse delegation is a questionable proposition and many
      deployment environments have no form of reverse delegation.
   o  It is also conceivable that an enterprise network could decide to
      use 6to4 internally in some form of private context, with the
      hosts only visible in internal DNS servers.  In this proposed
      mechanism the reverse delegation, if desired, would need to be
      implemented in an internal private (non-delegated) corresponding
      zone of the 6to4 reverse domain space.

   It is envisaged that there may be circumstances with an IPv4 address
   controller wishes to "block" the ability for "children" to use this
   6to4 scheme.  It is envisaged that scenarios that would motivate this
   concern would include when the IPv4 provider is also offering an IPv6
   service, and native IPv6 should be deployed instead of 6to4.  In such
   circumstances the 2002 zone operator should allow for such a
   delegation blocking function upon application to the delegation zone

   For a delegation to be undertaken the following must hold:
   o  The 6to4 site must have connectivity to the global IPv6 network
   o  The 6to4 site must have configured a minimum of one primary and
      one secondary server for the 6to4 IPv6 reverse address zone.
   o  At the time of the delegation request, the primary and secondary
      servers should be online, reachable, correctly configured,  and in
      a mutually consistent state with respect to the 6to4 reverse zone.
   o  The delegation server will only accept delegation requests
      associated with the 6to4 source address of the requesting client.

   The approach suggested here, of a fully automated system driven by
   the site administrators of the 6to4 client networks, appears to
   represent an appropriate match the requirements of reverse DNS

   For maintenance of the reverse delegation zones it is proposed to
   maintain an email contact point for each active delegation, derived

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   from the zone's SOA contact address, or explicitly entered in the
   delegation interface.  This contact point would be informed upon any
   subsequent change of delegation details.

   The management system will also undertake a periodic sweep of all
   active delegations, so that each delegation is checked every 30 days.
   If the delegation fails this integrity check the email contact point
   is informed of the problem, and a further check scheduled in a
   further 14 days.  If this second check fails, the delegation is
   automatically removed, and a further notice is issued to the contact

5.  Security Considerations

   The system proposed here offers a moderate level of assurance in
   attempting to ensure that a 6to4 site can only direct the delegation
   of the corresponding reverse domain and no other.

   Address-based authentication is not useful in a security sense.
   Accordingly, reverse delegation information does not provide useful
   information in either validating a domain name or in validating an IP
   address, and that no conclusions should be drawn from the presence or
   otherwise of a reverse mapping for any IP address.

   The service management interface allows a 6to4 client to insert any
   server name as a DNS server, potentially directing the server to make
   a DNS query to any nominated system.  The server should throttle the
   number of requests made to any single IP address to mitigate this
   risk of a high volume of bogus DNS queries being generated by the
   server.  For similar reasons, the server should also throttle the
   number of redelegation requests for any single 6to4 zone.

6.  Acknowledgements

   The author acknowledges the prior work of Keith Moore in preparing a
   document that enumerated a number of possible approaches to undertake
   the delegation and discovery of reverse zones.  The author
   acknowledges the assistance of George Michaelson and Andrei
   Robachevsky in preparing this document, and Pekka Savola and
   Jun-ichiro itojun Hagino for their review comments.

7  References

   [1]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4
        Clouds", RFC 3056, February 2001.

   [2]  Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
        Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April

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   [3]  Moore, K., "Work in progress: 6to4 and DNS", April 2003.

Author's Address

   Geoff Huston

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