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DNSext Working Group O. Sury
Internet-Draft CZ.NIC
Updates: 1995 (if approved) S. Kerr, Ed.
Intended status: Standards Track ISC
Expires: August 30, 2010 February 26, 2010
IXFR-ONLY to Prevent IXFR Fallback to AXFR
draft-kerr-ixfr-only-01
Abstract
This documents proposes a new QTYPE (Query pseudo RRtype) for the
Domain Name System (DNS). IXFR-ONLY is a variant of IXFR (RFC 1995)
that allows an authoritative server to incrementally update zone
content from another (primary) server without falling back from IXFR
to AXFR. This way, alternate peers can be contacted more quickly and
convergence of zone content may be achieved much faster in important,
resilient operational scenarios.
Status of this Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. IXFR Server Side . . . . . . . . . . . . . . . . . . . . . . . 4
3. IXFR Client Side . . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability of IXFR-ONLY . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
For large DNS zones, RFC 1995 [RFC1995] defines Incremental Zone
Transfer (IXFR), which allows only to transfer the changed portion(s)
of a zone.
In the document, an IXFR client and an IXFR server is defined as in
RFC 1995 [RFC1995], a secondary name server which requests IXFR is
called an IXFR client and a primary or secondary name server which
responds to the request is called an IXFR server.
IXFR is an efficient way to transfer changes in zones from IXFR
servers to IXFR clients. However, when an IXFR client has multiple
IXFR servers for a single zone, it is possible that not all IXFR
servers have the zone with same serial number for that zone. In this
case, if an IXFR client attempts an IXFR from an IXFR server which
does not have zone with the serial number used by the IXFR client,
the IXFR server will fall back to a full zone transfer (AXFR) when it
has a version of the zone with serial number greater than the serial
requested by the IXFR client.
For example, IXFR server NS1 may have serial numbers 1, 2, and 3 for
a zone, and IXFR server NS2 may have serial numbers 1 and 3 for the
same zone. An IXFR client that has the zone with serial number 2
which sends an IXFR request to IXFR server NS2 will get a full zone
transfer (AXFR) of the zone at serial number 3. This is because NS2
does not know the zone with serial number 2, and therefore does not
know what the differences are between zone with serial number 2 and
3.
If the IXFR client in this example had known to send the query to
IXFR server NS1, then it could have gotten an incremental transfer
(IXFR). But IXFR clients can only know what the latest version of
the zone is at a IXFR server (this information is available via an
SOA query).
The IXFR-ONLY query type provides a way for the IXFR client to ask
each IXFR server to return an error instead of sending the current
version of the zone via full zone transfer (AXFR). By using this, a
IXFR client can check each IXFR server until it finds one able to
provide IXFR.
1.1. Requirements Language
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 [RFC2119].
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2. IXFR Server Side
A IXFR server receiving a DNS message requesting IXFR-ONLY will reply
as described in RFC 1995 [RFC1995] if it is able to produce an IXFR
for the serial number requested.
If the IXFR server is is not able to reply with an IXFR it MUST NOT
reply with an AXFR unless AXFR result is smaller than IXFR result.
Instead, it MUST reply with RCODE CannotIXFR. (!FIXME)
If the IXFR result is larger than an AXFR, then an IXFR server MAY
reply with an AXFR result instead. This is an optimization, and IXFR
servers MAY only reply with AXFR if they are certain that the reply
using AXFR is smaller than an equivalent IXFR reply.
3. IXFR Client Side
An IXFR client who wishes to use IXFR-ONLY will send a message to one
of the IXFR servers. The format is exactly the same as for IXFR,
except the IXFR-ONLY QTYPE code is used instead of the IXFR QTYPE
code.
If the IXFR server replies with IXFR, then the IXFR client is done.
If the IXFR server replies with an RCODE of CannotIXFR, then the IXFR
client proceeds on to a different IXFR server. In this case the IXFR
server implements IXFR-ONLY, but does not have information about zone
with the serial number requested.
If the IXFR server replies with any RCODE other than CannotIXFR or
NoError, then the IXFR client proceeds on to a different IXFR server.
In this case the IXFR server does not implement IXFR-ONLY.
If the IXFR client attempts IXFR-ONLY to each IXFR server and none of
them reply with an incremental transfer (IXFR), then it should
attempt an IXFR as described in RFC 1995 [RFC1995] to each of the
IXFR servers which replied with an RCODE other than CannotIXFR or
NoError.
The method described above allows IXFR clients to operate normally in
situatians where some of the IXFR servers do support IXFR-ONLY, and
some who do not. IXFR clients MAY remember which IXFR servers
support IXFR-ONLY and query those IXFR servers first. However since
IXFR servers may change software or even run a mix of software, IXFR
clients MUST attempt to query each IXFR server periodically when they
attempt to get new versions of a zone.
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Implementations MAY allow IXFR clients to disable IXFR-ONLY for a
given IXFR server, if this is known in advance. These IXFR servers
are treated as if they replied with an RCODE other than CannotIXFR or
NoError, although no query with IXFR-ONLY is actually sent.
4. Applicability of IXFR-ONLY
Implementations SHOULD allow IXFR clients to disable IXFR-ONLY
completely.
Implementations MAY allow IXFR clients to disable IXFR-ONLY for a
specific zone. This may be useful for small zones, where fallback to
AXFR is cheap, or in other cases where IXFR-ONLY is causing problems.
Usage of IXFR-ONLY may cause IXFR clients to prefer particular IXFR
servers, by shifting load to ones that support IXFR-ONLY. If this a
problem, then administrators can disable IXFR-ONLY in implementations
that allow it.
If a IXFR client has a single IXFR server for a zone, it SHOULD use
IXFR rather than IXFR-ONLY.
5. IANA Considerations
IANA allocates the new IXFR-ONLY QTYPE, which means "incremental
transfer only". IANA allocates the CannotIXFR RCODE, which means
"Server cannot provide IXFR for zone".
6. Security Considerations
IXFR-ONLY may be used by someone to get information about the state
of IXFR servers by providing a quick and efficient way to check which
versions of a zone each IXFR server supports. Zones should be
secured via TSIG [RFC2845] to prevent unauthorized information
exposure. However, even administrators of IXFR servers may not want
this information given to IXFR clients, in which case they will need
to disable IXFR-ONLY.
7. Normative References
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
Authors' Addresses
Ondrej Sury
CZ.NIC
Americka 23
120 00 Praha 2
CZ
Phone: +420 222 745 110
Email: ondrej.sury@nic.cz
Shane Kerr (editor)
ISC
Bennebrokestraat 17-I
1015 PE Amsterdam
NL
Phone: +31 64 6336297
Email: shane@isc.org
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