INTERNET-DRAFT Andreas Gustafsson
draft-ietf-dnsext-axfr-clarify-00.txt Nominum Inc.
March 2000
DNS Zone Transfer Protocol Clarifications
Status of this Memo
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Abstract
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
RFC1034.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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.
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 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
message.
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
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
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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
OPCODE QUERY
AA 1 (but MAY be 0 when RCODE is nonzero)
TC 0
RD Copy from request
RA Set according to availability of recursion
Z 000
RCODE 0 or error code
The slave MUST check the RCODE and abort the transfer if it is
nonzero. 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
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.
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4. Glue
Zone transfers MAY contain glue RRs pertaining to NS records in the
zone. An RR is considered a glue RR when it is not within the zone
being transferred. A slave MUST recognize glue RRs as such; it MUST
NOT treat them as authoritative data.
Note that classifying an RR as glue or non-glue may not be possible
until the entire zone has been received so that the zone cuts defined
by the zone's NS records can be determined.
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
send, and slave implementations expect to receive, the zone's SOA RR
as the first and last record of the transfer. Any other RRs at the
zone's apex are transmitted only once.
Therefore, the quoted sentence is hereby changed to read "The first
and last RR transmitted must be the SOA record of the zone".
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 transmission order of all other RRs in the zone, including glue
records, is undefined.
References
[RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
November 1987.
[RFC1035] - Domain Names - Implementation and Specifications, P.
Mockapetris, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Andreas Gustafsson
Nominum Inc.
950 Charter Street
Redwood City, CA 94063
USA
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Phone: +1 650 779 6004
Email: gson@nominum.com
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