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IETF RFC 6303
Last modified on Thursday, July 14th, 2011
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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
 Abstract
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.
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
http://www.rfc-editor.org/info/RFC 6303.
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
Provisions Relating to IETF Documents
(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.
Andrews Best Current Practice PAGE 1 
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, [RFC 1034] and [RFC 1035])
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 [RFC 1918] and the IP6.ARPA zones for locally assigned
unique local IPv6 addresses defined in [RFC 4193].
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RFC 6303 Locally Served DNS Zones July 2011
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] [RFC 6304] [RFC 6305]. 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.
[RFC 4193] recommends that queries for D.F.IP6.ARPA be handled
locally. This document extends the recommendation to cover the
IN-ADDR.ARPA zones for [RFC 1918] 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 [RFC 1918] addresses but do not follow the last
paragraph in Section 3 of [RFC 1918].
1.1. Reserved Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
2. Effects on Sites Using RFC 1918 Addresses
For most sites using [RFC 1918] 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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RFC 6303 Locally Served DNS Zones July 2011
The most significant impact will be felt at sites that make use of
delegations for [RFC 1918] 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." [RFC 2606]. 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 [RFC 2308] 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 [RFC 2136] 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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RFC 6303 Locally Served DNS Zones July 2011
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
[RFC 1918].
+----------------------+
| 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 [RFC 5735]
and [RFC 5737] 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
1.0.0.127.IN-ADDR.ARPA 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 |
| 255.255.255.255.IN-ADDR.ARPA | IPv4 BROADCAST |
+------------------------------+-----------------------+
4.3. Local IPv6 Unicast Addresses
The reverse mappings ([RFC 3596], Section 2.5 ("IP6.ARPA Domain")) for
the IPv6 Unspecified (::) and Loopback (::1) addresses ([RFC 4291],
Sections 2.4, 2.5.2, and 2.5.3) are covered by these two zones:
+-------------------------------------------+
| Zone |
+-------------------------------------------+
| 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.\ |
| 0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA |
| 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.\ |
| 0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA |
+-------------------------------------------+
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 [RFC 4193] already required special treatment of:
+--------------+
| Zone |
+--------------+
| D.F.IP6.ARPA |
+--------------+
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RFC 6303 Locally Served DNS Zones July 2011
4.5. IPv6 Link-Local Addresses
IPv6 Link-Local Addresses as described in [RFC 4291], 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 [RFC 3849].
+--------------------------+
| Zone |
+--------------------------+
| 8.B.D.0.1.0.0.2.IP6.ARPA |
+--------------------------+
Note: 8.B.D.0.1.0.0.2.IP6.ARPA is not being used as an example here.
5. Zones That Are Out of Scope
IPv6 site-local addresses (deprecated, see [RFC 4291] Sections 2.4 and
2.5.7), and IPv6 non-locally assigned local addresses ([RFC 4193]) 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) [RFC 4193],
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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RFC 6303 Locally Served DNS Zones July 2011
IP6.INT was once used to provide reverse mapping for IPv6. IP6.INT
was deprecated in [RFC 4159] 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 [RFC 5226].
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 [RFC 1918]
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 [RFC 4035] 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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RFC 6303 Locally Served DNS Zones July 2011
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
[RFC 1034] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION", STD 13, RFC 1035, November 1987.
[RFC 1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC 2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC 2606] Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
Names", BCP 32, RFC 2606, June 1999.
[RFC 3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[RFC 4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC 4159] Huston, G., "Deprecation of "ip6.int"", BCP 109, RFC 4159,
August 2005.
[RFC 4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
Andrews Best Current Practice PAGE 9
RFC 6303 Locally Served DNS Zones July 2011
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 5226] 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/>.
[RFC 3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC 3849, July 2004.
[RFC 5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[RFC 5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737, January 2010.
[RFC 6304] Abley, J. and W. Maton, "AS112 Nameserver Operations",
RFC 6304, July 2011.
[RFC 6305] 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
US
EMail: marka@isc.org
Andrews Best Current Practice PAGE 10
RFC TOTAL SIZE: 22503 bytes
PUBLICATION DATE: Thursday, July 14th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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