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Updated by: 8996 Errata Exist
Network Working Group                                          G. Huston
Request for Comments: 5158                                         APNIC
Category: Informational                                       March 2008

               6to4 Reverse DNS Delegation Specification

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.


   This memo describes the service mechanism for entering a delegation
   of DNS servers that provide reverse lookup of 6to4 IPv6 addresses
   into the 6to4 reverse zone file.  The mechanism is based on a
   conventional DNS delegation service interface, allowing the service
   client to enter the details of a number of DNS servers for the
   delegated domain.  In the context of a 6to4 reverse delegation, the
   client is primarily authenticated by its source address used in the
   delegation request, and is authorized to use the function if its IPv6
   address prefix corresponds to an address from within the requested
   6to4 delegation address block.

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RFC 5158                    6to4 Reverse DNS                  March 2008

1.  Introduction

   6to4 [RFC3056] 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 an IPv6 network without
   necessarily obtaining IPv6 services from its adjacent upstream
   network provider.

   As noted in [6to4-dns], 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
      [DNS] 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." [6to4-dns]

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RFC 5158                    6to4 Reverse DNS                  March 2008

   The desired characteristics of a reverse lookup delegation mechanism
   are that it:

      *  is deployable with minimal overhead or tool development

      *  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 behaviors 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 [6to4-dns], 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 an end site would use 6to4
   is commonly that the upstream Internet Service Provider does not
   support an IPv6 transit service and the end site is using 6to4 to
   tunnel through to IPv6 connectivity.  A conventional end site
   environment of this form would have the end site using provider-based
   IPv4 addresses, where the end site's IPv4 address is a more specific
   address block drawn from the upstream provider's address block, and
   the end site would have an entry in the upstream provider's reverse
   DNS zone file, or it would have authoritative local name servers that
   are delegated from the upstream provider's DNS zone.  In the case of
   the 6to4 mapped IPv6 space, the upstream may not be providing any
   IPv6-based services at all, and therefore would not be expected to
   have a 6to4 reverse DNS delegation for its IPv4 address block.  The
   general observation is that 6to4 IPv6 reverse DNS delegations cannot
   necessarily always precisely match the corresponding IPv4 reverse DNS

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RFC 5158                    6to4 Reverse DNS                  March 2008

   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
   behavior 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 behaviors for this domain are
   not to be taken lightly, and the delegation chain for IPv4 will 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 delegation for IPv4 would parallel that of 6to4.

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 considered here 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.

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RFC 5158                    6to4 Reverse DNS                  March 2008

2.5.  Selecting a Reasonable Approach

   It would appear that the most reasonable approach from this set of
   potential candidates 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

                                 Figure 1

   The IPv4 address used as part of the generation of 6to4 addresses for
   the local IPv6 network is that of the external IPv4 network interface
   address (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 mode.

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RFC 5158                    6to4 Reverse DNS                  March 2008

   An example of a 6to4 network with addressing:

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

                                 Figure 2

4.  Delegation Administration

   This specification uses a single delegation level in the zone (the ip6.arpa zone is specified in [RFC3596]),
   delegating zones only at the 48th bit position.  This corresponds
   with individual delegations related to a single /32 IPv4 address, or
   the equivalent of a single 6to4 local site.

   The zone files containing the end site delegations are to be operated
   with a low TTL (configured to be a time value in the scale of hours
   rather than days or weeks), and updates for delegation requests in
   the zone are to be made using dynamic DNS updates

   The delegation system is to be self-driven by clients residing within
   6to4 networks.  The 6to4 reverse DNS delegation function is 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 client.

   This service is to operate the delegation management service using
   web-based server access using Transport Layer Security (TLS)
   [RFC4346] (accessible via a "https:" URL).  This is intended to
   ensure that the source address-driven delegation selection function
   cannot be disrupted through proxy caching of the web server's
   responses, and also to ensure that the delegation service cannot be
   readily mimicked.

   The service is to be found at https://6to4.nro.net

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RFC 5158                    6to4 Reverse DNS                  March 2008

   This service is implemented by web servers that are operated on a
   dual-stack IPv4 / IPv6 server, accessible via SSL.  The web 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

   When accessed by a 6to4 source address, the interface presented by
   the delegation service is a conventional DNS delegation interface,
   allowing the client to enter the details of a number of DNS servers
   for the corresponding reverse domain.  The targets of the DNS
   delegation are checked by the delegation manager using IPv4 and IPv6,
   according to the addresses of the targets, 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 zone at the primary
   master.  In order to avoid the server being used as a denial of
   service platform, the server should throttle the number of DNS
   delegation requests made to any single IP address, and also throttle
   the number of redelegation requests for any single 6to4 zone.

   In other cases the system provides diagnostic information to the

   The benefits of this 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, or any 6to4 site that uses
      dynamically-assigned external IPv4 interface addresses, could
      inherit nonsense reverse entries created by previous users of the
      dynamically assigned address.  In this case, the client site could
      request delegation of the reverse zone as required.  More
      generally, given the potentially for inheritance of 'stale'
      reverse DNS information in this context, in those cases where the
      issues of potential inheritance of 'stale' reverse DNS information
      is a concern, it is recommended that a 6to4 site either use a
      static IPv4 address in preference to a dynamically-assigned

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RFC 5158                    6to4 Reverse DNS                  March 2008

      address, or ensure that the reverse delegation information is
      updated using the service mechanism described here upon each
      dynamic address assignment event.

   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 an isolated
      network where other forms of a 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 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.

   There may be circumstances where an IPv4 address controller wishes to
   "block" the ability for users of these addresses to use this 6to4
   scheme.  Scenarios that would motivate this concern would include
   situations when the IPv4 provider is also offering an IPv6 service,
   and native IPv6 should be deployed instead of 6to4.  In such
   circumstances the 6to4 reverse zone operator should allow for such a
   6to4 reverse delegation blocking function upon application to the
   delegation zone operator.

   For a delegation to be undertaken the following conditions should

   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 must be online, reachable, correctly configured, and in a
      mutually consistent state with respect to the 6to4 reverse zone.
      In this context, "mutually consistent" implies the same SOA RR and
      identical NS RRSets.

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RFC 5158                    6to4 Reverse DNS                  March 2008

   o  The 6to4 reverse delegation service will only accept delegation
      requests associated with the 6to4 source address of the requesting

   The approach described here, of a fully automated system driven by
   the site administrators of the 6to4 client networks, appears to
   represent an appropriate match of the operational requirements of
   managing reverse DNS domains for 6to4 addresses.

   For maintenance of the reverse delegation zones the service maintains
   an email contact point for each active delegation, derived from the
   zone's SOA contact address (SOA RNAME), or explicitly entered in the
   delegation interface.  This contact point would be informed upon any
   subsequent change of delegation details.

   The 6to4 reverse DNS management system also undertakes 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
   is scheduled for 14 days later.  If this second check fails, the
   delegation is automatically removed, and a further notice is issued
   to the contact point.

5.  Security Considerations

   This system offers a rudimentary level of assurance in attempting to
   ensure that delegation requests from a 6to4 site can only direct the
   delegation of the corresponding 6to4 reverse DNS domain and no other.

   Address-based authentication is not a very robust method from a
   security perspective, as addresses can be readily spoofed.
   Accordingly, reverse delegation information does not provide reliable
   information in either validating a domain name or in validating an IP
   address, and no conclusions should be drawn from the presence or
   otherwise of a reverse DNS 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 delegation
   test system to make a DNS query to any nominated system.  The server
   throttles the number of requests made to any single IP address to
   mitigate the attendant risk of a high volume of bogus DNS queries
   being generated by the server.  For similar reasons, the server also
   throttles the number of redelegation requests for any single 6to4

   For a general threat analysis of 6to4, especially the additional risk
   of address spoofing in 2002::/16, see [RFC3964].

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RFC 5158                    6to4 Reverse DNS                  March 2008

   Section 4 notes that the local site administrator could take
   appropriate access control measures to prevent clients inside a 6to4
   site performing unauthorized changes to the delegation details.  This
   may be in the form of a firewall configuration, regarding control of
   access to the service from the interior of a 6to4 site, or a similar
   mechanism that enforces local access policies.

6.  IANA Considerations

   The IANA has delegated the domain according to
   delegation instructions provided by the Internet Architecture Board.

7.  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 Peter Koch, Pekka Savola,
   Jun-ichiro Itojun Hagino, and Catherine Meadows for their helpful
   review comments.

8.  References

8.1.  Normative References

   [RFC2136]   Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
               "Dynamic Updates in the Domain Name System (DNS UPDATE)",
               RFC 2136, April 1997.

   [RFC3056]   Carpenter, B. and K. Moore, "Connection of IPv6 Domains
               via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3596]   Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
               "DNS Extensions to Support IP Version 6", RFC 3596,
               October 2003.

   [RFC4346]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.1", RFC 4346, April 2006.

8.2.  Informative References

   [6to4-dns]  Moore, K., "6to4 and DNS", Work in Progress, April 2003.

   [RFC3964]   Savola, P. and C. Patel, "Security Considerations for
               6to4", RFC 3964, December 2004.

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RFC 5158                    6to4 Reverse DNS                  March 2008

Author's Address

   Geoff Huston

   EMail: gih@apnic.net
   URI:   http://www.apnic.net

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RFC 5158                    6to4 Reverse DNS                  March 2008

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