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INTERNET-DRAFT                                            Edward Lewis
draft-ietf-dnsext-axfr-clarify-06.txt                    NeuStar, Inc.
DNSEXT WG                                                 January 2008
Updates: 1034, 1035 (if approved)     Intended status: Standards Track

                     DNS Zone Transfer Protocol (AXFR)
Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2008).


The Domain Name System standard facilities for maintaining coherent
servers for a zone consist of three elements.  The Authoritative
Transfer (AXFR) is defined in RFC 1034 and RFC 1035.  The Incremental
Zone Transfer (IXFR) is defined in RFC 1995.  A mechanism for prompt
notification of zone changes (NOTIFY) is defined in RFC 1996.  The base
definition of these facilities, that of the AXFR, has proven
insufficient in detail, resulting in no implementation complying with
it. Yet today we have a satisfactory set of implementations that do
interoperate. This document is a new definition of the AXFR, new in the
sense that is it recording an accurate definition of an interoperable
AXFR mechanism.

1 Introduction

The Domain Name System standard facilities for maintaining coherent
servers for a zone consist of three elements.  The Authoritative
Transfer (AXFR) is defined in RFC 1034 [RFC1034] and RFC 1035 [RFC1035].
The Incremental Zone Transfer (IXFR) is defined in RFC 1995 [RFC1995].
A mechanism for prompt notification of zone changes (NOTIFY) is defined
in RFC 1996 [RFC1996].  The goal of these mechanisms is to enable a set
of DNS name servers to remain coherently authoritative for a given

Comments on this draft should be addresses to the editor or to

1.1 Definition of Terms

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in "Key words for use in
RFCs to Indicate Requirement Levels" [BCP14].

1.2 Scope

In the greater context, there are many ways to achieve coherency among a
set of name servers.  These mechanisms form just one, the one defined in
the RFCs cited.  For example, there are DNS implementations that
assemble answers from data riding in commercial database instances, and
rely on the database's proprietary or otherwise external-to-DNS means to
synchronize the database instances.  Some of these non-DNS solutions may
even interoperate in some fashion.  As far as it is known, AXFR, IXFR,
and NOTIFY are the only mechanisms that provide an interoperable
solution to the desire for coherency within the definition of DNS.

This document does not cover incoherent DNS situations.  There are
applications of the DNS in which servers for a zone are designed to be
incoherent.  For these configurations, a coherency mechanism as
described here would be unsuitable.

"General purpose" DNS implementation refers to DNS software developed
for wide spread use.  This includes resolvers and servers freely
accessible as libraries and standalone processes.  This also includes
proprietary implementations used only in support of DNS service

"Turnkey" DNS implementation refers to custom made, single use
implementations of DNS.  Such implementations consist of software the
use the DNS protocol message format but does not conform to entire range
of DNS functionality.

A DNS implementation is not required to support AXFR, IXFR, and NOTIFY.
A DNS implementation SHOULD have some means for maintaining name server
coherency.  A general purpose DNS implementation SHOULD include AXFR,
IXFR, and NOTIFY, but turnkey DNS implementations MAY operate without

1.3 Context

Besides describing the mechanisms themselves, there is the context in
which they operate to consider.  When AXFR, IXFR, and NOTIFY were
defined, there was little consideration given to security and privacy
issues.  Since the original definition of AXFR, new opinions have
appeared on the access to an entire zone's contents.  In this document,
the basic mechanisms will be discussed separately from the permission to
use these mechanisms.

1.4 Coverage

This document concentrates on just the definition of AXFR.  Any effort
to update the IXFR or NOTIFY mechanisms would be done in different
documents.  This is not strictly a clarification of the definition in
RFC 1034 and RFC 1035.  This document will update those sections,
invalidate at least one part of that definition.  The goal of this
document is define AXFR as it exists, or should exist, currently.

2 AXFR Messages

An AXFR message exchange (or session) consists of an AXFR Query message
and a set of AXFR Response messages.  In this document, AXFR client is
the sender of the AXFR Query and the AXFR server is the responder.  (Use
of terms such as master, slave, primary, secondary are not important to
defining the AXFR exchange.)  The reason for the imbalance in number of
messages derives from large zones whose contents cannot be fit into the
limited permissible size of a DNS message.

The upper limit on the permissible size of a DNS message is defined in
RFC 1035 [RFC1035], section 2.3.4, and supplemented in RFC 2671
[RFC2671], see section 4.5.

The basic format of an AXFR message is the DNS message as defined in RFC
1035, Section 4 ("MESSAGES") [RFC 1035], updated by the following
documents: RFC3425 [RFC3425], RFC1996 [RFC 1996], RFC2136 [RFC2136],
RFC2671 [RFC2671], RFC2845 [RFC2845], RFC2930 [RFC2930], RFC4035
[RFC4035], RFC4635 [RFC4635].  In addition, one change is credited to
IANA, the reserving of OPCODE = 3.

Field names used in this document will correspond to the names as the
appear in the IANA registry for DNS Header Flags [DNS-FLAGS].

2.1 AXFR Query

An AXFR Query is sent by a client whenever there is a reason to ask.
This may be because of zone maintenance activities or as a result of a
command line request, say for debugging.

2.1.1 Header Values

ID          See note 2.1.1.a
QR          MUST be 0 (Query)
OPCODE      MUST be 0 (Standard Query)
AA          See note 2.1.1.b
TC          See note 2.1.1.b
RD          See note 2.1.1.b
RA          See note 2.1.1.b
Z           See note 2.1.1.c
AD          See note 2.1.1.b
CD          See note 2.1.1.b
RCODE       MUST be 0 (No error)
ARCOUNT     MUST be either 0 or 1, the latter only if EDNS0 [RFC2671]
            is in use

Note 2.1.1.a Set to any value that the client desires.  There
is no specified means for selecting the value in this field.  However,
consideration can be given to making it harder for forged messages to be
accepted by referencing the work in progress "Measures for making DNS
more resilient against forged answers" [D-FORGERY].

Note 2.1.1.b The value in this field has no meaning in the
context of AXFR.  For the client, RECOMMENDED that the value be zero.
For the server, RECOMMENDED ignoring this value.

Note 2.1.1.c The Z bit is no longer registered with IANA (no document
cited for change).  RECOMMENDED client set to 0, server MUST ignore.

2.1.2 Query Section

The Query section of the AXFR query MUST conform to section 4.1.2 of RFC
1035 contain the following values:

QNAME       the name of the zone requested
QCLASS      the class of the zone requested

2.1.3 Answer Section

MUST be empty.

2.1.4 Authority Section

MUST be empty.

2.1.5 Additional Section

The client MAY include an EDNS0 section.  If the server has indicated
that it does not support EDNS0, the client MUST send this section empty
if there is a retry.

If the client is aware that the server does not support EDNS0,
RECOMMENDED that this section be sent empty.  A client MAY become aware
of a server's abilities via a configuration setting.

An implementation of a general purpose client and server is RECOMMENDED
to support EDNS0.

2.2 AXFR Response

The AXFR Response will consist of 0 or more messages.  A server MAY
elect to ignore the request altogether.  The first response MUST begin
with the SOA resource record of the zone, the last response MUST
conclude with the same SOA resource record.  Intermediate responses MUST
not contain the SOA resource record.

2.2.1 Header Values

ID          See note 2.2.1.a
QR          MUST be 1 (Response)
OPCODE      MUST be 0 (Standard Query)
AA          See note 2.2.1.b
TC          MUST be 0 (Not truncated)
RD          RECOMMENDED copy request's value, MAY be set to 0
RA          See note 2.2.1.c
Z           See note 2.2.1.d
AD          See note 2.2.1.e
CD          See note 2.2.1.e
RCODE       See note 2.2.1.f
QDCOUNT     MUST be 1 in the first message; MUST be 0 or 1 in all
ANCOUNT     See note 2.2.1.g
ARCOUNT     MUST be either 0 or 1, the latter only if EDNS0 [RFC2671]
            is in use

Note 2.2.1.a Because of old implementations, the requirement
on this section is stated in detail.  New DNS servers MUST set this
field to the value of the AXFR Query ID in each AXFR Response message
for the session.  New DNS clients MUST be able to accept sessions in
which the responses do not have the same ID field.

If a client detects or is aware that the server is new, that is, all of
the responses have the same ID value as the query, the client MAY issue
other DNS queries (of any type) to the server using the same transport.
Unless the client is sure that the server will consistently set the ID
field to the query's ID, the client is NOT RECOMMENDED to issue any
other queries until the end of the zone transfer.  A client MAY become
aware of a server's abilities via a configuration setting.

Note 2.2.1.b If the RCODE is 0 (no error), then the AA bit
MUST be 1.

For any other value of RCODE, the AA bit MUST be set according to rules
for that error code.  If in
doubt, RECOMMENDED setting to 1, RECOMMENDED ignoring the value

Note 2.2.1.c RECOMMENDED server setting value to 0,
RECOMMENDED client ignoring this value.

The server MAY set this value according to the local policy regarding
recursive service, but doing so may confuse the interpretation of the
response as AXFR MAY NOT be retrieved recursively.  A client MAY note
the server's policy regarding recursive from this value, but SHOULD NOT
conclude that the AXFR response was obtained recursively even if the RD
bit was 1 in the query.

Note 2.2.1.d The Z bit is no longer registered with IANA (no document
cited for change).  RECOMMENDED client set to 0, server MUST ignore.

Note 2.2.1.e If the implementation is implementing DNSSEC [RFC4033-5],
this value MUST be set according to the rules in RFC 4035 [RFC4035],
section 3.1.6, "The AD and CD Bits in an Authoritative Response."  If
the implementation is not implementing DNSSEC, then this value MUST be
set to 0 an MUST be ignored.

Note 2.2.1.f In the absence of an error, the server MUST set the value
of this field to NoError.  If a server is not authoritative for the
queried zone, the server SHOULD set the value to NotAuth.  (Reminder,
consult the appropriate IANA registry [DNS-VALUES].)  If a client
receives any other value in response, it MUST act according to the
error.  For example, a malformed AXFR query or the presence of an EDNS0
OPT resource record sent to an old server will garner a FormErr value.
This value is not set as part of the AXFR response processing.  The same
is true for other error-indicating values.

Note 2.2.1.g The count of answer records MUST equal the number of
resource records in the AXFR Answer Section.  When a server is aware
that a client will only accept one resource record per response message,
then the value MUST be 1.  A server MAY be made aware of a client's
limitations via configuration data.

2.2.2 Query Section

In the first response message, this section MUST be copied from the
query.  In subsequent messages this section MAY be copied from the
query, MAY be empty.  The content of this section MAY be used to
determine the context of the message, that is, the name of the zone
being transfered.

2.2.3 Answer Section

MUST be populated with the zone contents.  See later section on encoding
zone contents.

2.2.4 Authority Section

MUST be empty.

2.2.5 Additional Section

If the query included an EDNS0 OPT RR this section MAY include an OPT RR
in reply.  If the query had an empty Additional Section, this MUST be
empty.  A client MAY ignore the contents of this section.

3 Zone Contents

The objective of the AXFR session is to request and transfer the
contents of a zone.  The objective is to permit the client to
reconstruct the zone as it exists at the server for the given zone
serial number.  Over time the definition of a zone has evolved from a
static set of records to a dynamically updated set of records to a
continually regenerated set of records.

3.1 Records to Include

In the answer section of AXFR response messages the resource records
within a zone for the given serial number MUST appear.  The definition
of what belongs in a zone is described in RFC 1034, Section 4.2, "How
the database is divided into zones", and in particular, section 4.2.1.,
"Technical considerations."

The first resource record of the first AXFR response message sent by the
AXFR server MUST be the zone's SOA resource record.   The last resource
record of the final AXFR response message sent by the AXFR server MUST
be the zone's SOA resource record.  The order and grouping of all other
records in the AXFR is arbitrary, but the AXFR server SHOULD group
resource record sets together and transmit in the same AXFR message.

Unless the AXFR server knows that the AXFR client expects just one
resource record per AXFR response message, an AXFR server SHOULD
populate an AXFR response message with as many complete resource records
as will fit within the limited permissible message size.

Zones for which it is impractical to list the entire zones for a serial
number (because changes happen too quickly) are not suitable for AXFR

3.2 Delegation Records

In RFC 1034, section 4.2.1, this text appears (keep in mind that the use
of the word "should" in the quotation is exempt from the interpretation
in section 1.1) "The RRs that describe cuts ... should be exactly the
same as the corresponding RRs in the top node of the subzone." There has
been some controversy over this statement and the impact on which NS
resource records are included in a zone transfer.

The issue is that in operations there are times when the NS resource
records for a zone might be different at a cut point in the parent and
at the apex of a zone.  Sometimes this is the result of an error and
sometimes it is part of an ongoing change in name servers.  The DNS
protocol is robust enough to overcome inconsistencies up to there being
no parent indicated NS resource record referencing a server that is able
to serve the child zone.  This robustness is one quality that has fueld
the success of the DNS.  Still, the inconsistency is a error state and
steps need to be taken to make it apparent (if it is unplanned) and to
make it clear once the inconsistency has been removed.

Another issue is that the AXFR server could be authoritative for a
different set of zones than the AXFR client.  It is possible that the
AXFR server may be authoritative for both halves of an inconsistent cut
point and that the AXFR client is authoritative for just the parent of
the cut point.

The question that arises is, when facing a situation in which a cut
point's NS resource records do not match the authoritative set, whether
an AXFR server responds with the NS resource record set that is in the
zone or is at the authoritative location.

The AXFR response MUST contain the cut point NS resource record set
registered with the zone whether it agrees with the authoritative set or
not.  "Registered with" can interpreted as residing in the zone file of
the zone for the particular serial number (in zone file environments) or
as any data configured to be in the zone, statically or dynamically.

The reasons for this requirement are:

1) The AXFR server might not be able to determine that there is an
inconsistency given local data, hence requiring consistency would mean a
lot more needed work and even network retrieval of data.  An
authoritative server ought not be required to perform any queries.

2) By transferring the inconsistent NS resource records from a server
that is authoritative for both the cut point and the apex to a client
that is not authoritative for both, the error is exposed.  For example,
an authorized administrator can manually request the AXFR and inspect
the results to see the inconsistent records.  (A server authoritative
for both halves would otherwise always answer from the more
authoritative set, concealing the error.)

3) The inconsistent NS resource record set might indicate a problem in a
registration database.  The DNS shouldn't cover this over.
3.3 Glue Records

As in the previous section, RFC 1034, section 4.2.1, provides guidance
and rationale for the inclusion of glue records as part of an AXFR
transfer.  And, as also argued in the previous section of this document,
even when there is an inconsistency between the address in a glue record
and the authoritative copy of the name server's address, the glue
resource record that is registered as part of the zone for that serial
number is to be included.

This applies for glue records for any address family.

3.4 Name Compression

Compression of names in DNS messages is described in RFC 1035, section 4.1.4, "Message compression".  The issue highlighted here relates to a
comment made in RFC 1034, section 3.1, "Name space specifications and
terminology" which says "When you receive a domain name or label, you
should preserve its case."

Name compression in an AXFR message MUST preserve the case of the
original domain name.  That is, although when comparing a domain name,
"a" equals "A", when comparing for the purposes of message comparison,
"a" is not equal to "A".

Name compression of RDATA in an AXFR message MAY only be done on
resource record types which explicitly permit such compression.

4 Transport

AXFR sessions are restricted by RFC 1034, section 4.3.5's "because
accuracy is essential, TCP or some other reliable protocol must be used
for AXFR requests."  With the addition of EDNS0 and applications which
require many small zones such in web hosting and some ENUM scenarios,
AXFR sessions on UDP are now possible and desirable.  In addition, it is
conceivable to interleave requests for other data or AXFRs of other
zones during one session in TCP if the ID values are consistently

4.1 TCP

In the original definition there is an implicit assumption that a TCP
connection is used for one and only one AXFR session.  This is evidenced
in no requirement to maintain neither the query section nor the message
ID in responses and the lack of an explicit bit indicating that a zone
transfer continues in the next message.

Once an AXFR client opens a connection and sends an AXFR query, the AXFR
server MAY close the connection without a reply. Such an action is to be
interpreted as refusal to honor the request.  This option was not
originally defined but has proven to be one way to stop abusive
behaviors by clients attempting to use up the server's available
resources for TCP activity.

Accommodation for implementations assuming this can be maintained, but
newer implementations MAY choose to use the open TCP connection for
other queries and AXFR sessions of other zones.

An AXFR client MAY send a subsequent request to the AXFR server while
the AXFR server is responding to a previous query.  If this action
causes the AXFR server to stop the original AXFR, the AXFR client SHOULD
not try this again with that AXFR server.

An AXFR server MAY opt to respond to other queries while responding the
original AXFR query that opened the connection.  An AXFR server MAY
ignore or even close the connection if there are two outstanding AXFR
queries for the same zone on a connection, as this could be evidence of
an abusive AXFR client.

4.2 UDP

AXFR sessions over UDP are not included in the base specification of
DNS.  Given the definition of AXFR, probably for good reason.  But there
are applications in which AXFR over UDP just might work.  With expanded
DNS messages made possible by EDNS0, it can be possible to fit an entire
zone's contents in to one DNS message.

Reasons not to do AXFR over UDP include cases where multiple AXFR
messages are needed for a zone, there is no way to guarantee all AXFR
messages will arrive at the AXFR client and no way to detect a dropped
AXFR message.

If an AXFR server cannot place the entire contents of the requested zone
in one AXFR response message, the AXFR server MAY silently drop the
request or MAY send a response with an return code of SERVFAIL.

If an AXFR client does not receive a reply to an AXFR query over UDP or
receives a SERVFAIL response code, the client SHOULD retry the request
via TCP.

5 Authorization

A zone administrator has the option to restrict AXFR access to a zone.
This was not envisioned in the original design of the DNS but has
emerged as a requirement as the DNS has evolved.  Restrictions on AXFR
could be for various reasons including a desire to keep the bulk version
of the zone concealed or to prevent the servers from handling the load
incurred in serving AXFR.  All reasons are arguable, but the fact
remains that there is a requirement to provide mechanisms to restrict

A DNS implementation SHOULD provide means to restrict AXFR sessions to
specific clients.  By default, a DNS implementation SHOULD only allow
the designated authoritative servers to have access to the zone.

An implementation SHOULD allow access to be granted to Internet Protocol
addresses and ranges, regardless of whether a source address could be
spoofed.  Combining this with techniques such as Virtual Private
Networks (VPN) [RFC2764] or Virtual LANs has proven to be effective.

An implementation SHOULD allow access to be granted based upon "Secret
Key Transaction Authentication for DNS" [RFC2845] and/or "DNS Request
and Transaction Signatures ( SIG(0)s )" [RFC2931].

An implementation SHOULD allow access to be open to all requests.

6 Zone Integrity

Ensuring that an AXFR client does not accept a forged copy of a zone is
important to the security of a zone.  If a zone operator has the
opportunity, protection can be afforded via dedicated links, physical or
virtual via a VPN among the authoritative servers.  But there are
instances in which zone operators have no choice but to run AXFR
sessions over the global public Internet.

Besides best attempts at securing TCP sessions, DNS implementations
SHOULD provide means to make use of "Secret Key Transaction
Authentication for DNS" [RFC2845] and/or "DNS Request and Transaction
Signatures ( SIG(0)s )" [RFC2931] to allow AXFR clients to verify the
contents.  These techniques MAY also be used for authorization.

7 Backwards Compatibility

Describing backwards compatibility is difficult because of a lack of
specifics in the original definition.  In this section some hints at
building in backwards compatibility are given, mostly repeated from the
earlier sections.

Backwards compatibility is not necessary, but the greater extent of an
implementation's compatibility increases it's interoperability.  For
turnkey implementations this is not usually a concern.  For general
purpose implementations this takes on varying levels of importance
depending on the implementers desire to maintain interoperability.

It is unfortunate that needs to fall back to older behavior cannot be
discovered, hence need to be noted in a configuration file.  An
implementation SHOULD, in it's documentation, encourage operators to
periodically review AXFR clients and servers it has made notes about as
old software periodically gets updated.

7.1 Server

An AXFR server has the luxury of being able to react to an AXFR client's
abilities with the exception of knowing if the client can accept
multiple resource records per AXFR response message.  The knowledge that
a client is so restricted apparently cannot be discovered, hence it has
to set by configuration.

An implementation of an AXFR server SHOULD permit configuring on a per
AXFR client basis a need to revert to single resource record per
message.  The default SHOULD be to use multiple records per message.

7.2 Client

An AXFR client has the opportunity to try extensions when querying an
AXFR server.

The use of EDNS0 to increase the DNS message size, offer authorizing
proof, or to invoke message integrity can be tried and rejected by the
AXFR server via the methods already described as part of the EDNS0

If an AXFR client attempts to use the UDP transport, non-response from
the AXFR server or other error message can indicate not to retry that.

Attempting to issue multiple DNS queries over a TCP transport for an
AXFR session SHOULD be aborted if it interrupts the original request and
SHOULD take into consideration whether the AXFR server intends to close
the connection immediately upon completion of the original
(connection-causing) zone transfer.

8 Security Considerations

Concerns regarding authorization, traffic flooding, and message
integrity are mentioned in "Authorization" (section 5), "TCP" (section
4.2) and Zone Integrity (section 6).

9 IANA Considerations

No new registries or new registrations are included in this document.

10 Internationalization Considerations

It is assumed that supporting of international domain names has been
solved via "Internationalizing Domain Names in Applications (IDNA)"

11 Acknowledgements

Earlier editions of this document have been edited by Andreas
Gustafsson. In his latest version, this acknowledgement appeared.

"Many people have contributed input and commentary to earlier versions
of this document, including but not limited to Bob Halley, Dan
Bernstein, Eric A. Hall, Josh Littlefield, Kevin Darcy, Robert Elz,
Levon Esibov, Mark Andrews, Michael Patton, Peter Koch, Sam Trenholme,
and Brian Wellington."

12 References

12.1 Normative

[RFC1034]    "Domain names - concepts and facilities.", P.V. Mockapetris.
[RFC1035]    "Domain names - implementation and specification." P.V.
             Mockapetris. Nov-01-1987.
[RFC1995]    "Incremental Zone Transfer in DNS." M. Ohta. August 1996.
[RFC1996]    "A Mechanism for Prompt Notification of Zone Changes (DNS
             NOTIFY)." P. Vixie. August 1996.
[RFC2136]    "Dynamic Updates in the Domain Name System (DNS UPDATE)."
             P. Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound. April 1997.
[RFC2671]    "Extension Mechanisms for DNS (EDNS0)." P. Vixie.
             August 1999.
[RFC2845]    "Secret Key Transaction Authentication for DNS (TSIG)."
             P.  Vixie, O. Gudmundsson, D. Eastlake, B. Wellington. May 2000.
[RFC2930]    "Secret Key Establishment for DNS (TKEY RR)." D. Eastlake.
             September 2000.
[RFC3425]    "Obsoleting IQUERY." D. Lawrence. November 2002.
[RFC4033-5]  "DNS Security Introduction and Requirements," "Resource
             Records for the DNS Security Extensions," and "Protocol
             Modifications for the DNS Security Extensions." R. Arends,
             R. Austein, M. Larson, D.  Massey, S. Rose. March 2005.
[RFC4035]    "Protocol Modifications for the DNS Security Extensions."
             R.  Arends, R. Austein, M. Larson, D. Massey, S. Rose. March
[RFC4635]    "HMAC SHA (Hashed Message Authentication Code, Secure Hash
             Algorithm) TSIG Algorithm Identifiers." D. Eastlake 3rd.
             August 2006.
[DNS-FLAGS]  http://www.iana.org/assignments/dns-header-flags
[DNS-VALUES] http://www.iana.org/assignments/dns-parameters

12.2 Informative

[BCP14]      "Key words for use in RFCs to Indicate Requirement Levels."
             S. Bradner. March 1997.
[RFC2764]    "A Framework for IP Based Virtual Private Networks." B.
             Gleeson, A. Lin, J. Heinanen, G. Armitage, A. Malis. February
[RFC3490]    "Internationalizing Domain Names in Applications (IDNA)." P.
             Faltstrom, P. Hoffman, A. Costello. March 2003.
[D-FORGERY]  "Measures for making DNS more resilient against forged
             answers." A. Hubert, R. van Mook. Work in Progress.

13 Editor's Address

Edward Lewis
46000 Center Oak Plaza
Sterling, VA, 22033, US

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