draft-ietf-dnsext-axfr-clarify-00.txt   draft-ietf-dnsext-axfr-clarify-01.txt 
INTERNET-DRAFT Andreas Gustafsson INTERNET-DRAFT Andreas Gustafsson
draft-ietf-dnsext-axfr-clarify-00.txt Nominum Inc. draft-ietf-dnsext-axfr-clarify-01.txt Nominum Inc.
March 2000 November 2000
DNS Zone Transfer Protocol Clarifications DNS Zone Transfer Protocol Clarifications
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119]. document are to be interpreted as described in [RFC 2119].
2. The zone transfer request 2. The zone transfer request
To initiate a zone transfer, the slave server sends a zone transfer To initiate a zone transfer, the slave server sends a zone transfer
request to the master server over a reliable transport such as TCP. request to the master server over a reliable transport such as TCP.
The form of this request is specified in sufficient detail in RFC1034 The form of this request is specified in sufficient detail in RFC1034
and needs no further clarification. 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 3. The zone transfer response
If the master server is unable or unwilling to provide a zone If the master server is unable or unwilling to provide a zone
transfer, it MUST respond with a single DNS message containing an transfer, it MUST respond with a single DNS message containing an
appropriate RCODE other than NOERROR. appropriate RCODE other than NOERROR.
If a zone transfer can be provided, the master server sends one or If a zone transfer can be provided, the master server sends one or
more DNS messages containing the zone data as described below. more DNS messages containing the zone data as described below.
3.1. Multiple answers per message 3.1. Multiple answers per message
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RFC1034 does not specify the contents of the DNS message header of RFC1034 does not specify the contents of the DNS message header of
the zone transfer response messages. The header of each message MUST the zone transfer response messages. The header of each message MUST
be as follows: be as follows:
ID Copy from request ID Copy from request
QR 1 QR 1
OPCODE QUERY OPCODE QUERY
AA 1 (but MAY be 0 when RCODE is nonzero) AA 1 (but MAY be 0 when RCODE is nonzero)
TC 0 TC 0
RD Copy from request RD Copy from request
RA Set according to availability of recursion RA Set according to availability of recursion S Z 0
Z 000 AD 0
CD 0
RCODE 0 or error code RCODE 0 or error code
The slave MUST check the RCODE and abort the transfer if it is 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 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 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 ID SHOULD be ignored in subsequent messages, and fields other than
RCODE and ID SHOULD be ignored in all messages, to ensure RCODE and ID SHOULD be ignored in all messages, to ensure
interoperability with certain older implementations which transmit interoperability with certain older implementations which transmit
incorrect or arbitrary values in these fields. incorrect or arbitrary values in these fields.
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receive from masters that do not comply with this requirement. receive from masters that do not comply with this requirement.
3.6. The additional section 3.6. The additional section
The additional section MAY contain additional RRs such as transaction The additional section MAY contain additional RRs such as transaction
signatures. The slave MUST ignore any unexpected RRs in the signatures. The slave MUST ignore any unexpected RRs in the
additional section. additional section.
4. Glue 4. Glue
Zone transfers MAY contain glue RRs pertaining to NS records in the A master transmitting a zone transfer MUST include the full set of
zone. An RR is considered a glue RR when it is not within the zone zone data it loaded from the zone's master file, from an incoming
being transferred. A slave MUST recognize glue RRs as such; it MUST zone transfer, or other similar means of configuring zone data. This
NOT treat them as authoritative data. includes any nonauthoritative data ("glue") associated with the zone
by being present in the zone's master file or the incoming transfer
along with the authoritative data. This glue data includes any
configured zone data obscured by zone cuts or otherwise outside the
zone in case; it is not limited to RRs pointed to by NS records.
Note that classifying an RR as glue or non-glue may not be possible The glue RRs are transmitted in the answer section along with the
until the entire zone has been received so that the zone cuts defined authoritative data. This is unlike ordinary DNS responses where glue
by the zone's NS records can be determined. is transmitted in the authority or additional section.
Zone transfers MUST NOT contain RRs from the authoritative data of
zones other than the one being transferred or from the cache, even
when such RRs are pointed to by NS records in the zone being
transferred.
A slave receiving a zone transfer MUST accept glue data and recognize
it as such; glue MUST NOT be treated as authoritative data nor
entered into the cache. 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. Glue data that is not below the zone origin ("cross-zone
glue") MAY be discarded by the slave.
5. Transmission order 5. Transmission order
RFC1034 states that "The first and last messages must contain the RFC1034 states that "The first and last messages must contain the
data for the top authoritative node of the zone". This is not data for the top authoritative node of the zone". This is not
consistent with existing practice. All known master implementations consistent with existing practice. All known master implementations
send, and slave implementations expect to receive, the zone's SOA RR 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 as the first and last record of the transfer. Any other RRs at the
zone's apex are transmitted only once. zone's apex are transmitted only once.
Therefore, the quoted sentence is hereby changed to read "The first Therefore, the quoted sentence is hereby changed to read "The first
and last RR transmitted must be the SOA record of the zone". and last RR transmitted must be the SOA record of the zone".
The initial and final SOA record MUST be identical, with the possible The initial and final SOA record MUST be identical, with the possible
exception of case and compression. In particular, they MUST have the exception of case and compression. In particular, they MUST have the
same serial number. same serial number.
The transmission order of all other RRs in the zone, including glue The transmission order of all other RRs in the zone, including glue
records, is undefined. records, is undefined.
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 TSIG [RFC2845] for this
purpose is RECOMMENDED.
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
slaves.
These clarifications are not believed to themselves introduce any new
security problems, nor to solve any existing ones.
References References
[RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris, [RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
November 1987. November 1987.
[RFC1035] - Domain Names - Implementation and Specifications, P. [RFC1035] - Domain Names - Implementation and Specifications, P.
Mockapetris, November 1987. Mockapetris, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] - Key words for use in RFCs to Indicate Requirement Levels,
Requirement Levels", BCP 14, RFC 2119, March 1997. 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 Author's Address
Andreas Gustafsson Andreas Gustafsson
Nominum Inc. Nominum Inc.
950 Charter Street 950 Charter Street
Redwood City, CA 94063 Redwood City, CA 94063
USA USA
Phone: +1 650 779 6004 Phone: +1 650 779 6004
Email: gson@nominum.com Email: gson@nominum.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved. Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
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