RFC 6303

Internet Engineering Task Force (IETF)                        M. Andrews
Request for Comments: 6303                                           ISC
BCP: 163                                                       July 2011
Category: Best Current Practice
ISSN: 2070-1721

                        Locally Served DNS Zones


   Experience with the Domain Name System (DNS) has shown that there are
   a number of DNS zones that all iterative resolvers and recursive
   nameservers should automatically serve, unless configured otherwise.
   RFC 4193 specifies that this should occur for D.F.IP6.ARPA.  This
   document extends the practice to cover the IN-ADDR.ARPA zones for RFC
   1918 address space and other well-known zones with similar

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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
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   (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.

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RFC 6303                Locally Served DNS Zones               July 2011

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................2
      1.1. Reserved Words .............................................3
   2. Effects on Sites Using RFC 1918 Addresses .......................3
   3. Changes to Iterative Resolver Behaviour .........................4
   4. Lists Of Zones Covered ..........................................5
      4.1. RFC 1918 Zones .............................................5
      4.2. RFC 5735 and RFC 5737 Zones ................................5
      4.3. Local IPv6 Unicast Addresses ...............................6
      4.4. IPv6 Locally Assigned Local Addresses ......................6
      4.5. IPv6 Link-Local Addresses ..................................7
      4.6. IPv6 Example Prefix ........................................7
   5. Zones That Are Out of Scope .....................................7
   6. IANA Considerations .............................................8
   7. Security Considerations .........................................8
   8. Acknowledgements ................................................9
   9. References ......................................................9
      9.1. Normative References .......................................9
      9.2. Informative References ....................................10

1.  Introduction

   Experience with the Domain Name System (DNS, [RFC1034] and [RFC1035])
   has shown that there are a number of DNS zones that all iterative
   resolvers and recursive nameservers SHOULD automatically serve,
   unless intentionally configured otherwise.  These zones include, but
   are not limited to, the IN-ADDR.ARPA zones for the address space
   allocated by [RFC1918] and the IP6.ARPA zones for locally assigned
   unique local IPv6 addresses defined in [RFC4193].

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   This recommendation is made because data has shown that significant
   leakage of queries for these namespaces is occurring, despite
   instructions to restrict them, and because it has therefore become
   necessary to deploy sacrificial nameservers to protect the immediate
   parent nameservers for these zones from excessive, unintentional
   query load [AS112] [RFC6304] [RFC6305].  There is every expectation
   that the query load will continue to increase unless steps are taken
   as outlined here.

   Additionally, queries from clients behind badly configured firewalls
   that allow outgoing queries for these namespaces, but drop the
   responses, put a significant load on the root servers (forward zones
   but not reverse zones are configured).  They also cause operational
   load for the root server operators, as they have to reply to
   enquiries about why the root servers are "attacking" these clients.
   Changing the default configuration will address all these issues for
   the zones listed in Section 4.

   [RFC4193] recommends that queries for D.F.IP6.ARPA be handled
   locally.  This document extends the recommendation to cover the
   IN-ADDR.ARPA zones for [RFC1918] and other well-known IN-ADDR.ARPA
   and IP6.ARPA zones for which queries should not appear on the public

   It is hoped that by doing this the number of sacrificial servers
   [AS112] will not have to be increased, and may in time be reduced.

   This recommendation should also help DNS responsiveness for sites
   that are using [RFC1918] addresses but do not follow the last
   paragraph in Section 3 of [RFC1918].

1.1.  Reserved Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Effects on Sites Using RFC 1918 Addresses

   For most sites using [RFC1918] addresses, the changes here will have
   little or no detrimental effect.  If the site does not already have
   the reverse tree populated, the only effect will be that the name
   error responses will be generated locally rather than remotely.

   For sites that do have the reverse tree populated, most will either
   have a local copy of the zones or will be forwarding the queries to
   servers that have local copies of the zone.  Therefore, this
   recommendation will not be relevant.

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   The most significant impact will be felt at sites that make use of
   delegations for [RFC1918] addresses and have populated these zones.
   These sites will need to override the default configuration expressed
   in this document to allow resolution to continue.  Typically, such
   sites will be fully disconnected from the Internet and have their own
   root servers for their own non-Internet DNS tree.

3.  Changes to Iterative Resolver Behaviour

   Unless configured otherwise, an iterative resolver will now return
   authoritatively (AA=1) name errors (RCODE=3) for queries within the
   zones in Section 4, with the obvious exception of queries for the
   zone name itself where SOA, NS, and "no data" responses will be
   returned as appropriate to the query type.  One common way to do this
   all at once is to serve empty (SOA and NS only) zones.

   An implementation of this recommendation MUST provide a mechanism to
   disable this new behaviour, and SHOULD allow this decision on a zone-
   by-zone basis.

   If using empty zones one SHOULD NOT use the same NS and SOA records
   as used on the public Internet servers, as that will make it harder
   to detect the origin of the responses and thus any leakage to the
   public Internet servers.  It is RECOMMENDED that the NS record
   defaults to the name of the zone and the SOA MNAME defaults to the
   name of the only NS RR's (Resource Record's) target.  The SOA RNAME
   SHOULD default to "nobody.invalid."  [RFC2606].  Implementations
   SHOULD provide a mechanism to set these values.  No address records
   need to be provided for the nameserver.

   Below is an example of a generic empty zone in master file format.
   It will produce a negative cache Time to Live (TTL) of 3 hours.

   @ 10800 IN SOA @ nobody.invalid. 1 3600 1200 604800 10800
   @ 10800 IN NS @

   The SOA RR is needed to support negative caching [RFC2308] of name
   error responses and to point clients to the primary master for DNS
   dynamic updates.

   SOA values of particular importance are the MNAME, the SOA RR's TTL,
   and the negTTL value.  Both TTL values SHOULD match.  The rest of the
   SOA timer values MAY be chosen arbitrarily since they are not
   intended to control any zone transfer activity.

   The NS RR is needed as some UPDATE [RFC2136] clients use NS queries
   to discover the zone to be updated.  Having no address records for
   the nameserver is expected to abort UPDATE processing in the client.

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4.  Lists Of Zones Covered

   The following subsections are the initial contents of the IANA
   registry as described in the IANA Considerations section.  Following
   the caveat in that section, the list contains only reverse zones
   corresponding to permanently assigned address space.  The zone name
   is the entity to be registered.

4.1.  RFC 1918 Zones

   The following zones correspond to the IPv4 address space reserved in

                         | Zone                 |
                         | 10.IN-ADDR.ARPA      |
                         | 16.172.IN-ADDR.ARPA  |
                         | 17.172.IN-ADDR.ARPA  |
                         | 18.172.IN-ADDR.ARPA  |
                         | 19.172.IN-ADDR.ARPA  |
                         | 20.172.IN-ADDR.ARPA  |
                         | 21.172.IN-ADDR.ARPA  |
                         | 22.172.IN-ADDR.ARPA  |
                         | 23.172.IN-ADDR.ARPA  |
                         | 24.172.IN-ADDR.ARPA  |
                         | 25.172.IN-ADDR.ARPA  |
                         | 26.172.IN-ADDR.ARPA  |
                         | 27.172.IN-ADDR.ARPA  |
                         | 28.172.IN-ADDR.ARPA  |
                         | 29.172.IN-ADDR.ARPA  |
                         | 30.172.IN-ADDR.ARPA  |
                         | 31.172.IN-ADDR.ARPA  |
                         | 168.192.IN-ADDR.ARPA |

4.2.  RFC 5735 and RFC 5737 Zones

   The following zones correspond to those address ranges from [RFC5735]
   and [RFC5737] that are not expected to appear as source or
   destination addresses on the public Internet; as such, there are no
   globally unique names associated with the addresses in these ranges.

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RFC 6303                Locally Served DNS Zones               July 2011

   The recommendation to serve an empty zone 127.IN-ADDR.ARPA is not an
   attempt to discourage any practice to provide a PTR RR for locally.  In fact, a meaningful reverse
   mapping should exist, but the exact setup is out of the scope of this
   document.  Similar logic applies to the reverse mapping for ::1
   (Section 4.3).  The recommendations made here simply assume that no
   other coverage for these domains exists.

         | Zone                         | Description           |
         | 0.IN-ADDR.ARPA               | IPv4 "THIS" NETWORK   |
         | 127.IN-ADDR.ARPA             | IPv4 Loopback NETWORK |
         | 254.169.IN-ADDR.ARPA         | IPv4 LINK LOCAL       |
         | 2.0.192.IN-ADDR.ARPA         | IPv4 TEST-NET-1       |
         | 100.51.198.IN-ADDR.ARPA      | IPv4 TEST-NET-2       |
         | 113.0.203.IN-ADDR.ARPA       | IPv4 TEST-NET-3       |
         | | IPv4 BROADCAST        |

4.3.  Local IPv6 Unicast Addresses

   The reverse mappings ([RFC3596], Section 2.5 ("IP6.ARPA Domain")) for
   the IPv6 Unspecified (::) and Loopback (::1) addresses ([RFC4291],
   Sections 2.4, 2.5.2, and 2.5.3) are covered by these two zones:

               | Zone                                      |
               |\ |
               |      |
               |\ |
               |      |

   Note: Line breaks and escapes ('\') have been inserted above for
   readability and to adhere to line width constraints.  They are not
   parts of the zone names.

4.4.  IPv6 Locally Assigned Local Addresses

   Section 4.4 of [RFC4193] already required special treatment of:

                             | Zone         |
                             | D.F.IP6.ARPA |

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4.5.  IPv6 Link-Local Addresses

   IPv6 Link-Local Addresses as described in [RFC4291], Section 2.5.6
   are covered by four distinct reverse DNS zones:

                            | Zone           |
                            | 8.E.F.IP6.ARPA |
                            | 9.E.F.IP6.ARPA |
                            | A.E.F.IP6.ARPA |
                            | B.E.F.IP6.ARPA |

4.6.  IPv6 Example Prefix

   IPv6 example prefix [RFC3849].

                       | Zone                     |
                       | 8.B.D. |

   Note: 8.B.D. is not being used as an example here.

5.  Zones That Are Out of Scope

   IPv6 site-local addresses (deprecated, see [RFC4291] Sections 2.4 and
   2.5.7), and IPv6 non-locally assigned local addresses ([RFC4193]) are
   not covered here.

   It is expected that IPv6 site-local addresses will be self correcting
   as IPv6 implementations remove support for site-local addresses.
   However, sacrificial servers for the zones C.E.F.IP6.ARPA through
   F.E.F.IP6.ARPA may still need to be deployed in the short term if the
   traffic becomes excessive.

   For IPv6 non-locally assigned local addresses (L = 0) [RFC4193],
   there has been no decision made about whether the Regional Internet
   Registries (RIRs) will provide delegations in this space or not.  If
   they don't, then C.F.IP6.ARPA will need to be added to the list in
   Section 4.4.  If they do, then registries will need to take steps to
   ensure that nameservers are provided for these addresses.

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   IP6.INT was once used to provide reverse mapping for IPv6.  IP6.INT
   was deprecated in [RFC4159] and the delegation removed from the INT
   zone in June 2006.  While it is possible that legacy software
   continues to send queries for names under the IP6.INT domain, this
   document does not specify that IP6.INT be considered a local zone.

   This document has also deliberately ignored names immediately under
   the root domain.  While there is a subset of queries to the root
   nameservers that could be addressed using the techniques described
   here (e.g., .local, .workgroup, and IPv4 addresses), there is also a
   vast amount of traffic that requires a different strategy (e.g.,
   lookups for unqualified hostnames, IPv6 addresses).

6.  IANA Considerations

   IANA has established a registry of zones that require this default
   behaviour.  The initial contents of this registry are defined in
   Section 4.  Implementors are encouraged to periodically check this
   registry and adjust their implementations to reflect changes therein.

   This registry can be amended through "IETF Review" as per [RFC5226].
   As part of this review process, it should be noted that once a zone
   is added it is effectively added permanently; once an address range
   starts being configured as a local zone in systems on the Internet,
   it will be impossible to reverse those changes.

   IANA should coordinate with the RIRs to ensure that, as DNS Security
   (DNSSEC) is deployed in the reverse tree, delegations for these zones
   are made in the manner described in Section 7.

7.  Security Considerations

   During the initial deployment phase, particularly where [RFC1918]
   addresses are in use, there may be some clients that unexpectedly
   receive a name error rather than a PTR record.  This may cause some
   service disruption until their recursive nameserver(s) have been

   As DNSSEC is deployed within the IN-ADDR.ARPA and IP6.ARPA
   namespaces, the zones listed above will need to be delegated as
   insecure delegations, or be within insecure zones.  This will allow
   DNSSEC validation to succeed for queries in these spaces despite not
   being answered from the delegated servers.

   It is recommended that sites actively using these namespaces secure
   them using DNSSEC [RFC4035] by publishing and using DNSSEC trust
   anchors.  This will protect the clients from accidental import of
   unsigned responses from the Internet.

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8.  Acknowledgements

   This work was supported by the US National Science Foundation
   (research grant SCI-0427144) and DNS-OARC.

9.  References

9.1.  Normative References

              STD 13, RFC 1034, November 1987.

              SPECIFICATION", STD 13, RFC 1035, November 1987.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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

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

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, March 1998.

   [RFC2606]  Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, June 1999.

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

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4159]  Huston, G., "Deprecation of "ip6.int"", BCP 109, RFC 4159,
              August 2005.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

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RFC 6303                Locally Served DNS Zones               July 2011

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

9.2.  Informative References

   [AS112]    "AS112 Project", <http://www.as112.net/>.

   [RFC3849]  Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
              Reserved for Documentation", RFC 3849, July 2004.

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              BCP 153, RFC 5735, January 2010.

   [RFC5737]  Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
              Reserved for Documentation", RFC 5737, January 2010.

   [RFC6304]  Abley, J. and W. Maton, "AS112 Nameserver Operations",
              RFC 6304, July 2011.

   [RFC6305]  Abley, J. and W. Maton, "I'm Being Attacked by
              PRISONER.IANA.ORG!", RFC 6305, July 2011.

Author's Address

   Mark P. Andrews
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA  94063

   EMail: marka@isc.org

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