INTERNET-DRAFT                                      Andreas Gustafsson
draft-ietf-dnsext-axfr-clarify-05.txt                     Nominum                                            Edward Lewis
draft-ietf-dnsext-axfr-clarify-06.txt                    NeuStar, Inc.
                                                         November 2002
DNSEXT WG                                                 January 2008
Updates: 1034, 1035 (if approved)     Intended status: Standards Track

                     DNS Zone Transfer Protocol Clarifications (AXFR)
Status of this Memo

   This document

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   In the

The Domain Name System, System standard facilities for maintaining coherent
servers for a zone data consist of three elements.  The Authoritative
Transfer (AXFR) is replicated among
   authoritative DNS servers by means defined in RFC 1034 and RFC 1035.  The Incremental
Zone Transfer (IXFR) is defined in RFC 1995.  A mechanism for prompt
notification of the "zone transfer" protocol,
   also known as the "AXFR" protocol.  This memo clarifies, updates, and
   adds missing detail to the original AXFR protocol specification zone changes (NOTIFY) is defined in

1. Introduction RFC 1996.  The original base
definition of the DNS zone transfer protocol consists these facilities, that of
   a single paragraph the AXFR, has proven
insufficient in [RFC1034] section 4.3.5 and some additional
   notes detail, resulting in [RFC1035] section 6.3.  It is not sufficiently detailed to
   serve as the sole basis for constructing interoperable
   implementations. no implementation complying with
it. Yet today we have a satisfactory set of implementations that do
interoperate. This document is an attempt to provide a more
   complete new definition of the protocol.  Where the text AXFR, new in RFC1034
   conflicts with existing practice, the existing practice has been
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 interest editor or to

1.1 Definition of interoperability. Terms

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in [RFC 2119].

2. The zone transfer request

   To initiate a zone transfer, the slave server sends a zone transfer
   request "Key words for use in
RFCs to Indicate Requirement Levels" [BCP14].

1.2 Scope

In the master server over greater context, there are many ways to achieve coherency among a reliable transport such as TCP.
   The form
set of this request is specified in sufficient detail name servers.  These mechanisms form just one, the one defined in RFC1034
   and needs no further clarification.

the RFCs cited.  For example, there are advised DNS implementations that one server implementation
assemble answers from data riding in widespread
   use sends AXFR requests where the TCP message envelope size exceeds
   the DNS request message size by two octets.

3. The zone transfer response

   If commercial database instances, and
rely on the master server is unable database's proprietary or unwilling 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
   transfer, it MUST respond with 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 containing an
   appropriate RCODE other than NOERROR.  If the master format but does not conform to entire range
of DNS functionality.

A DNS implementation is not
   authoritative for the requested zone, the RCODE required to support AXFR, IXFR, and NOTIFY.
A DNS implementation SHOULD be 9

   Slave servers should note that have some master means for maintaining name server
coherency.  A general purpose DNS implementation SHOULD include AXFR,
IXFR, and NOTIFY, but turnkey DNS implementations
   will simply close MAY operate without

1.3 Context

Besides describing the connection when denying mechanisms themselves, there is the slave access 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
   zone.  Therefore, slaves MAY interpret an immediate graceful close original definition of AXFR, new opinions have
appeared on the TCP connection as equivalent access to a "Refused" response (RCODE 5).

   If a zone transfer can an entire zone's contents.  In this document,
the basic mechanisms will be provided, discussed separately from the master server sends one or
   more DNS messages containing permission to
use these mechanisms.

1.4 Coverage

This document concentrates on just the zone data as described below.

3.1. Multiple answers per message

   The zone data definition of AXFR.  Any effort
to update the IXFR or NOTIFY mechanisms would be done in a zone transfer response different
documents.  This is not strictly a sequence clarification of answer
   RRs.  These RRs are transmitted in the answer section(s) of definition in
RFC 1034 and RFC 1035.  This document will update those sections,
invalidate at least one or
   more DNS response messages. part of that definition.  The goal of this
document is define AXFR protocol definition in RFC1034 does not make a clear
   distinction between response messages as it exists, or should exist, currently.

2 AXFR Messages

An AXFR message exchange (or session) consists of an AXFR Query message
and answer RRs.  Historically,
   DNS servers always transmitted a single answer RR per message.  This
   encoding set of AXFR Response messages.  In this document, AXFR client is wasteful due to
the overhead sender of repeatedly sending DNS
   message headers the AXFR Query and the loss AXFR server is the responder.  (Use
of domain name compression
   opportunities.  To improve efficiency, some newer servers support a
   mode where multiple RRs terms such as master, slave, primary, secondary are transmitted in a single DNS response

   A master MAY transmit multiple answer RRs per response message up not important to
defining the AXFR exchange.)  The reason for the largest imbalance in number that will of
messages derives from large zones whose contents cannot be fit within into the 65535 byte
limited permissible size of a DNS message.

The upper limit on TCP
   DNS message size.  In the case permissible size of a small zone, this can cause the
   entire transfer to be transmitted DNS message is defined in a single response message.

   Slaves MUST accept messages containing any number

[RFC2671], see section 4.5.

The basic format of answer RRs.  For
   compatibility with old slaves, masters that support sending multiple
   answers per an AXFR message SHOULD be configurable to revert to the
   historical mode of one answer per message, and is the configuration
   SHOULD be settable on a per-slave basis.

3.2. DNS message header contents

   RFC1034 does not specify as defined in RFC
1035, Section 4 ("MESSAGES") [RFC 1035], updated by the contents following
documents: RFC3425 [RFC3425], RFC1996 [RFC 1996], RFC2136 [RFC2136],

[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 message header 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
   the zone transfer response messages.  The header of each message MUST
   be maintenance activities or as follows: a result of a
command line request, say for debugging.

2.1.1 Header Values

ID      Copy from request          See note 2.1.1.a
QR      1          MUST be 0 (Query)
       AA      1, but MAY      MUST be 0 when RCODE is not NOERROR (Standard Query)
AA          See note 2.1.1.b
TC      0          See note 2.1.1.b
RD      Copy from request, or 0          See note 2.1.1.b
RA      Set according to availability of recursion, or 0          See note 2.1.1.b
Z       0           See note 2.1.1.c
AD      0          See note 2.1.1.b
CD      0          See note 2.1.1.b
RCODE   NOERROR on success, error code otherwise

   The slave       MUST check the RCODE in each message and abort be 0 (No error)
ARCOUNT     MUST be either 0 or 1, the transfer latter only if it EDNS0 [RFC2671]
            is not NOERROR.  It SHOULD check the ID of in use

Note 2.1.1.a Set to any value that the first message
   received and abort client desires.  There
is no specified means for selecting the transfer if value in this field.  However,
consideration can be given to making it does not match the ID of harder for forged messages to be
accepted by referencing the
   request. work in progress "Measures for making DNS
more resilient against forged answers" [D-FORGERY].

Note 2.1.1.b The ID SHOULD be ignored value in subsequent messages, and fields
   other than RCODE and ID SHOULD be ignored this field has no meaning in all messages, to ensure
   interoperability 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 certain older implementations which transmit
   incorrect or arbitrary values in these fields.

3.3. Additional section and SIG processing

   Zone transfer responses are not subject IANA (no document
cited for change).  RECOMMENDED client set to any kind 0, server MUST ignore.

2.1.2 Query Section

The Query section of additional the AXFR query MUST conform to section processing or automatic inclusion 4.1.2 of RFC
1035 contain the following values:

QNAME       the name of SIG records.  SIG RRs in the zone data are treated exactly requested
QCLASS      the same as any other RR type.

3.4. 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 question section

   RFC1034 client MAY include an EDNS0 section.  If the server has indicated
that it does not specify whether zone transfer response messages have support EDNS0, the client MUST send this section empty
if there is a question retry.

If the client is aware that the server does not support EDNS0,
RECOMMENDED that this section or not.  The initial message be sent empty.  A client MAY become aware
of a zone transfer
   response SHOULD have server's abilities via a question section identical to that in the
   request.  Subsequent messages SHOULD NOT have configuration setting.

An implementation of a question section,
   though the final message MAY. general purpose client and server is RECOMMENDED
to support EDNS0.

2.2 AXFR Response

The receiving slave AXFR Response will consist of 0 or more messages.  A server MAY
elect to ignore the request altogether.  The first response MUST accept
   any combination of messages begin
with and without a question section.

3.5. The authority section

   The master server the SOA resource record of the zone, the last response MUST transmit messages
conclude with an empty authority
   section.  Slaves the same SOA resource record.  Intermediate responses MUST ignore any authority section contents they may
   receive from masters that do
not comply with this requirement.

3.6. The additional section

   The additional section MAY contain additional RRs such as transaction
   signatures.  The slave the SOA resource record.

2.2.1 Header Values

ID          See note 2.2.1.a
QR          MUST ignore any unexpected RRs 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
   additional section.  It first message; MUST NOT treat additional 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 RRs 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

4. Zone data

   The purpose 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 transfer mechanism is administrator has the option to exactly replicate at
   each slave restrict AXFR access to a zone.
This was not envisioned in the set original design of RRs associated with the DNS but has
emerged as a particular zone at its
   primary master.  An RR is associated with requirement as the DNS has evolved.  Restrictions on AXFR
could be for various reasons including a zone by being loaded from desire to keep the master file bulk version
of that zone at the primary master server, or by some
   other, equivalent method for configuring zone data.

   This replication shall be complete and unaltered, regardless of how
   many and which intermediate masters/slaves are involved, and
   regardless of what other zones those intermediate masters/slaves do concealed or do not serve, and regardless of what data may be cached in
   resolvers associated with to prevent the servers from handling the intermediate masters/slaves.

   Therefore, load
incurred in a zone transfer serving AXFR.  All reasons are arguable, but the master MUST send exactly those
   records fact
remains that are associated with 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 zone, 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 not their owner
   names would Virtual LANs has proven to be considered effective.

An implementation SHOULD allow access to be "in" the zone granted based upon "Secret
Key Transaction Authentication for purposes of
   resolution, DNS" [RFC2845] and/or "DNS Request
and whether or not they would Transaction Signatures ( SIG(0)s )" [RFC2931].

An implementation SHOULD allow access to be eligible for use as glue
   in responses.  The transfer MUST NOT include any RRs open to all requests.

6 Zone Integrity

Ensuring that are an AXFR client does not
   associated with the zone, such as RRs associated with zones other
   than the one being transferred or present in the cache accept a forged copy of the local
   resolver, even if their owner names are in the zone being transferred
   or are pointed to by NS records in the a zone being transferred.

   The slave MUST associate is
important to the RRs received in security of a zone.  If a zone transfer with 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 being transferred, 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 maintain that association Transaction
Signatures ( SIG(0)s )" [RFC2931] to allow AXFR clients to verify the
contents.  These techniques MAY also be used for
   purposes authorization.

7 Backwards Compatibility

Describing backwards compatibility is difficult because of acting as a master lack of
specifics in outgoing transfers.

5. Transmission order

   RFC1034 states that "The first and last messages must contain the
   data for the top authoritative node of original definition.  In this section some hints at
building in backwards compatibility are given, mostly repeated from the zone".  This
earlier sections.

Backwards compatibility is not
   consistent with existing practice.  All known master necessary, but the greater extent of an
implementation's compatibility increases it's interoperability.  For
turnkey implementations
   send, and slave this is not usually a concern.  For general
purpose implementations expect to receive, this takes on varying levels of importance
depending on the zone's SOA RR 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 first and last record luxury of being able to react to an AXFR client's
abilities with the transfer.

   Therefore, exception of knowing if the quoted sentence client can accept
multiple resource records per AXFR response message.  The knowledge that
a client is hereby superseded by the sentence
   "The first and last RR transmitted must so restricted apparently cannot be the SOA record discovered, hence it has
to set by configuration.

An implementation of the

   The initial and final SOA an AXFR server SHOULD permit configuring on a per
AXFR client basis a need to revert to single resource record MUST per
message.  The default SHOULD be identical, with the possible
   exception of case and compression.  In particular, they MUST have to use multiple records per message.

7.2 Client

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

The slave MUST consider use of EDNS0 to increase the transfer DNS message size, offer authorizing
proof, or to invoke message integrity can be
   complete when, tried and only when, it has received rejected by the message containing
AXFR server via the second SOA record.

   The transmission order methods already described as part of all other RRs in the zone is undefined.
   Each of them 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 transmitted only once, and slaves MUST ignore
   any duplicate RRs received.

6. Security Considerations

   The zone transfer protocol as defined in [RFC1034] aborted if it interrupts the original request and clarified by
   this memo does not have any built-in mechanisms for
SHOULD take into consideration whether the slave AXFR server intends to
   securely verify close
the identity connection immediately upon completion of the master server original
(connection-causing) zone transfer.

8 Security Considerations

Concerns regarding authorization, traffic flooding, and the message
   of the transferred zone data.  The use of a cryptographic mechanism
   for ensuring authenticity are mentioned in "Authorization" (section 5), "TCP" (section
4.2) and integrity, such as TSIG [RFC2845],
   IPSEC, Zone Integrity (section 6).

9 IANA Considerations

No new registries or TLS, is RECOMMENDED.

   The zone transfer protocol allows read-only public access to the
   complete zone data.  Since data new registrations are included in the DNS is public by definition, this document.

10 Internationalization Considerations

It is generally acceptable.  Sites assumed that wish to avoid disclosing
   their full zone data MAY restrict zone transfer access to authorized

   These clarifications are not believed to themselves introduce any new
   security problems, nor to solve any existing ones. 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. Wellington."

12 References

12.1 Normative

[RFC1034]    "Domain names - Domain Names - Concepts concepts and Facilities, P. Mockapetris,
   November 1987. facilities.", P.V. Mockapetris.
[RFC1035]    "Domain names - Domain Names - Implementation implementation and Specifications, 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.
   Mockapetris, November 1987.

   [RFC2119] - 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.

12.2 Informative

[BCP14]      "Key words for use in RFCs to Indicate Requirement Levels, Levels."
             S. Bradner, BCP 14, Bradner. March 1997.

   [RFC2845] - Secret Key Transaction Authentication
[RFC2764]    "A Framework for DNS (TSIG).  P.
   Vixie, O. Gudmundsson, D. Eastlake, IP Based Virtual Private Networks." B. Wellington, May
             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

   Andreas Gustafsson
   Nominum Inc.
   2385 Bay Rd
   Redwood City, CA 94063

   Phone: +1 650 381 6004


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

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