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Network Working Group                                          G. HustonRequest for Comments: 5158                                         APNICCategory: Informational                                       March 2008               6to4 Reverse DNS Delegation SpecificationStatus 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.Abstract   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.Huston                       Informational                      [Page 1]

RFC 5158                    6to4 Reverse DNS                  March 20081.  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]Huston                       Informational                      [Page 2]

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         operations      *  performs name lookup efficiently      *  does not compromise any DNS security functions2.  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   delegations.Huston                       Informational                      [Page 3]

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.Huston                       Informational                      [Page 4]

RFC 5158                    6to4 Reverse DNS                  March 20082.5.  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 192.0.2.1, then the local IPv6   clients will use a common IPv6 address prefix of the form 2002:   {192.0.2.1}::/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.Huston                       Informational                      [Page 5]

RFC 5158                    6to4 Reverse DNS                  March 2008   An example of a 6to4 network with addressing:                       +-------------+       IPv4 network (A)|             | IPv6 network    -------------------| 6to4router  |-------------              192.0.2.1|             |    |  |   | interface identifier                       +-------------+   1A  |   | local IPv6 address                                         2002:C000:201::1A                                             |   |                                             1B  |                                             2002:C000:201::1B                                                 |                                                 1C                                                 2002:C000:201::1C                                 Figure 24.  Delegation Administration   This specification uses a single delegation level in the   2.0.0.2.ip6.arpa 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 2.0.0.2.ipv6.arpa zone are to be made using dynamic DNS updates   [RFC2136].   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 athttps://6to4.nro.netHuston                       Informational                      [Page 6]

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   process.   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   client.   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      measures.   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-assignedHuston                       Informational                      [Page 7]

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   hold:   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.Huston                       Informational                      [Page 8]

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      client.   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   zone.   For a general threat analysis of 6to4, especially the additional risk   of address spoofing in 2002::/16, see [RFC3964].Huston                       Informational                      [Page 9]

RFC 5158                    6to4 Reverse DNS                  March 2008Section 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 2.0.0.2.ip6.arpa 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.  References8.1.  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.Huston                       Informational                     [Page 10]

RFC 5158                    6to4 Reverse DNS                  March 2008Author's Address   Geoff Huston   APNIC   EMail: gih@apnic.net   URI:http://www.apnic.netHuston                       Informational                     [Page 11]

RFC 5158                    6to4 Reverse DNS                  March 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Huston                       Informational                     [Page 12]