Commit 2ed540c8 authored by Mark Andrews's avatar Mark Andrews

new draft

parent 99252499
INTERNET-DRAFT Andreas Gustafsson
draft-ietf-dnsext-axfr-clarify-05.txt Nominum Inc.
November 2002
DNS Zone Transfer Protocol Clarifications
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
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In the Domain Name System, zone data is replicated among
authoritative DNS servers by means 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 in
1. Introduction
The original definition of the DNS zone transfer protocol consists of
a single paragraph in [RFC1034] section 4.3.5 and some additional
notes in [RFC1035] section 6.3. It is not sufficiently detailed to
serve as the sole basis for constructing interoperable
implementations. This document is an attempt to provide a more
complete definition of the protocol. Where the text in RFC1034
conflicts with existing practice, the existing practice has been
codified in the interest of interoperability.
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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 to the master server over a reliable transport such as TCP.
The form of this request is specified in sufficient detail in RFC1034
and needs no further clarification.
Implementers are advised that one server implementation 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 the master server is unable or unwilling to provide a zone
transfer, it MUST respond with a single DNS message containing an
appropriate RCODE other than NOERROR. If the master is not
authoritative for the requested zone, the RCODE SHOULD be 9
Slave servers should note that some master server implementations
will simply close the connection when denying the slave access to the
zone. Therefore, slaves MAY interpret an immediate graceful close of
the TCP connection as equivalent to a "Refused" response (RCODE 5).
If a zone transfer can be provided, the master server sends one or
more DNS messages containing the zone data as described below.
3.1. Multiple answers per message
The zone data in a zone transfer response is a sequence of answer
RRs. These RRs are transmitted in the answer section(s) of one or
more DNS response messages.
The AXFR protocol definition in RFC1034 does not make a clear
distinction between response messages and answer RRs. Historically,
DNS servers always transmitted a single answer RR per message. This
encoding is wasteful due to the overhead of repeatedly sending DNS
message headers and the loss of domain name compression
opportunities. To improve efficiency, some newer servers support a
mode where multiple RRs are transmitted in a single DNS response
A master MAY transmit multiple answer RRs per response message up to
the largest number that will fit within the 65535 byte limit on TCP
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DNS message size. In the case of a small zone, this can cause the
entire transfer to be transmitted in a single response message.
Slaves MUST accept messages containing any number of answer RRs. For
compatibility with old slaves, masters that support sending multiple
answers per message SHOULD be configurable to revert to the
historical mode of one answer per message, and the configuration
SHOULD be settable on a per-slave basis.
3.2. DNS message header contents
RFC1034 does not specify the contents of the DNS message header of
the zone transfer response messages. The header of each message MUST
be as follows:
ID Copy from request
QR 1
AA 1, but MAY be 0 when RCODE is not NOERROR
TC 0
RD Copy from request, or 0
RA Set according to availability of recursion, or 0
Z 0
AD 0
CD 0
RCODE NOERROR on success, error code otherwise
The slave MUST check the RCODE in each message and abort the transfer
if it is not NOERROR. It SHOULD check the ID of the first message
received and abort the transfer if it does not match the ID of the
request. The ID SHOULD be ignored in subsequent messages, and fields
other than RCODE and ID SHOULD be ignored in all messages, to ensure
interoperability 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 to any kind of additional
section processing or automatic inclusion of SIG records. SIG RRs in
the zone data are treated exactly the same as any other RR type.
3.4. The question section
RFC1034 does not specify whether zone transfer response messages have
a question section or not. The initial message of a zone transfer
response SHOULD have a question section identical to that in the
request. Subsequent messages SHOULD NOT have a question section,
though the final message MAY. The receiving slave server MUST accept
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any combination of messages with and without a question section.
3.5. The authority section
The master server MUST transmit messages with an empty authority
section. Slaves 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 MUST ignore any unexpected RRs in the
additional section. It MUST NOT treat additional section RRs as zone
4. Zone data
The purpose of the zone transfer mechanism is to exactly replicate at
each slave the set of RRs associated with a particular zone at its
primary master. An RR is associated with a zone by being loaded from
the master file 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
or do not serve, and regardless of what data may be cached in
resolvers associated with the intermediate masters/slaves.
Therefore, in a zone transfer the master MUST send exactly those
records that are associated with the zone, whether or not their owner
names would be considered to be "in" the zone for purposes of
resolution, and whether or not they would be eligible for use as glue
in responses. The transfer MUST NOT include any RRs that are not
associated with the zone, such as RRs associated with zones other
than the one being transferred or present in the cache of the local
resolver, even if their owner names are in the zone being transferred
or are pointed to by NS records in the zone being transferred.
The slave MUST associate the RRs received in a zone transfer with the
specific zone being transferred, and maintain that association for
purposes of acting as a master in outgoing transfers.
5. Transmission order
RFC1034 states that "The first and last messages must contain the
data for the top authoritative node of the zone". This is not
consistent with existing practice. All known master implementations
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send, and slave implementations expect to receive, the zone's SOA RR
as the first and last record of the transfer.
Therefore, the quoted sentence is hereby superseded by the sentence
"The first and last RR transmitted must be the SOA record of the
The initial and final SOA record MUST be identical, with the possible
exception of case and compression. In particular, they MUST have the
same serial number. The slave MUST consider the transfer to be
complete when, and only when, it has received the message containing
the second SOA record.
The transmission order of all other RRs in the zone is undefined.
Each of them SHOULD be transmitted only once, and slaves MUST ignore
any duplicate RRs received.
6. Security Considerations
The zone transfer protocol as defined in [RFC1034] and clarified by
this memo does not have any built-in mechanisms for the slave to
securely verify the identity of the master server and the integrity
of the transferred zone data. The use of a cryptographic mechanism
for ensuring authenticity and integrity, such as TSIG [RFC2845],
The zone transfer protocol allows read-only public access to the
complete zone data. Since data in the DNS is public by definition,
this is generally acceptable. Sites 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.
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.
[RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
November 1987.
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[RFC1035] - Domain Names - Implementation and Specifications, P.
Mockapetris, November 1987.
[RFC2119] - Key words for use in RFCs to Indicate Requirement Levels,
S. Bradner, BCP 14, March 1997.
[RFC2845] - Secret Key Transaction Authentication for DNS (TSIG). P.
Vixie, O. Gudmundsson, D. Eastlake, B. Wellington, May 2000.
Author's Address
Andreas Gustafsson
Nominum Inc.
2385 Bay Rd
Redwood City, CA 94063
Phone: +1 650 381 6004
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