Internet Engineering Task Force (IETF) M. Andrews
Request for Comments: 6303 ISC
BCP: 163 July 2011
Category: Best Current Practice
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.
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Table of Contents
1. Introduction ....................................................21.1. Reserved Words .............................................32. Effects on Sites Using RFC 1918 Addresses .......................33. Changes to Iterative Resolver Behaviour .........................44. Lists Of Zones Covered ..........................................54.1. RFC 1918 Zones .............................................54.2. RFC 5735 and RFC 5737 Zones ................................54.3. Local IPv6 Unicast Addresses ...............................64.4. IPv6 Locally Assigned Local Addresses ......................64.5. IPv6 Link-Local Addresses ..................................74.6. IPv6 Example Prefix ........................................75. Zones That Are Out of Scope .....................................76. IANA Considerations .............................................87. Security Considerations .........................................88. Acknowledgements ................................................99. References ......................................................99.1. Normative References .......................................99.2. Informative References ....................................101. 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
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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.
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-
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
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
| 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
184.108.40.206.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 ([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 |
| 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 |
| 220.127.116.11.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.\ |
| 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 [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.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 [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
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.
This work was supported by the US National Science Foundation
(research grant SCI-0427144) and DNS-OARC.
9.1. Normative References
[RFC1034] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
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,
[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,
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[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,
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.
Mark P. Andrews
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063