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

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 characteristics.

Status of This Memo

This memo documents an Internet Best Current Practice.



Copyright Notice




Table of Contents

   2. Effects on Sites Using RFC 1918 Addresses .......................3
   3. Changes to Iterative Resolver Behaviour .........................4
      4.2. RFC 5735 and RFC 5737 Zones ................................5
      4.4. IPv6 Locally Assigned Local Addresses ......................6
   5. Zones That Are Out of Scope .....................................7

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].

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 Internet.

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


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.

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.

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 [RFC1918].

                         | 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.

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 |

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. |


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.

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 re-configured.

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.

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.
              BCP 5, RFC 1918, February 1996.
              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.
              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.
              Addresses", RFC 4193, October 2005.
              Architecture", RFC 4291, February 2006.
              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