DNSext Working Group O. Sury
Internet-Draft CZ.NIC
Updates: 1995 (if approved) S. Kerr, Ed.
Intended status: Standards Track ISC
Expires: January 8, 2010 July 7, 2009
IXFR-ONLY to Prevent IXFR Fallback to AXFR
draft-kerr-ixfr-only-00
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Abstract
Presents IXFR-ONLY, a way for a DNS slave to prevent a DNS master
from falling back from IXFR to AXFR.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Master Side . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Slave Side . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability of IXFR-ONLY . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5
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1. Introduction
For large DNS zones, RFC 1995 [RFC1995] defines Incremental Zone
Transfer (IXFR), which is an efficient way to transfer changes in
zones from masters to slaves.
However, when a slave has multiple masters for a single zone, it is
possible that not all masters has the same set of serials for that
zone. In this case, if a slave attempts an IXFR from a master which
does not have the serial in use by the slave, the master will fall
back to a full zone transfer (AXFR).
For example, master NS1 may have serials 1, 2, and 3 for a zone, and
master NS2 may have serials 1 and 3. A slave that is at serial 2
which sends an IXFR request to NS2 will get a full AXFR of the zone
at serial 3. This is because NS2 does not know the zone at serial 2,
and therefore does not know what the differences are between serial 2
and serial 3.
If the slave in this example had known to send the query to NS1, then
it could have gotten an incremental transfer. But slaves can only
know what the latest version of the zone is at a master (this
information is available via an SOA query).
The IXFR-ONLY type provides a way for the slaves to ask each master
to return an error instead of sending the current version of the
zone. By using this, a slave can check each master 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].
2. Master Side
A master receiving a DNS message requesting IXFR-ONLY will reply
exactly as described in RFC 1995 [RFC1995] if it is able to produce
an IXFR for the serial requested.
If the master is is not able to reply with an IXFR it MUST NOT reply
with an AXFR. Instead, it MUST reply with RCODE CannotIXFR.
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3. Slave Side
A slave who wishes to use IXFR-ONLY will send a message to one of the
masters. The format is exactly the same as for IXFR, except the
IXFR-ONLY type code is used instead of the IXFR type code.
If the master replies with IXFR, then the slave is done.
If the master replies with an RCODE of CannotIXFR, then the slave
proceeds on to a different master. In this case the master
implements IXFR-ONLY, but does not have the serial requested.
If the master replies with any RCODE other than CannotIXFR or
NoError, then the slave proceeds on to a different master. In this
case the server does not implement IXFR-ONLY.
If the slave attempts IXFR-ONLY to each master and none reply with an
incremental transfer, then it should attempt an IXFR as described in
RFC 1995 [RFC1995] to each of the servers which replied with an RCODE
other than CannotIXFR or NoError.
The method described above allows slaves to operate normally to have
some masters who support IXFR-ONLY, and some who do not. Slaves MAY
remember which masters support IXFR-ONLY and query those masters
first. However since masters may change software or even run a mix
of software, slaves MUST attempt to query each master periodically
when they attempt to get new versions of a zone.
Implementations SHOULD allow slaves to disable IXFR-ONLY for a given
master, if this is known in advance. These masters 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 MUST allow slaves to disable IXFR-ONLY completely.
Implementations SHOULD allow slaves 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 slaves to prefer particular masters, by
shifting load to ones that support IXFR-ONLY. If this a problem,
then administrators can disable IXFR-ONLY.
If a slave has a single master for a zone, it SHOULD use IXFR rather
than IXFR-ONLY.
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5. IANA Considerations
IANA allocates the new IXFR-ONLY TYPE, 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 masters by providing a quick and efficient way to check which
versions of a zone each master supports. Zones should be secured via
TSIG [RFC2845] prevent unauthorized information exposure. However,
even administrators of master servers may not want this information
given to slaves, 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
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
CZ.NIC, z. s. p. o. Americka 23
120 00 Praha 2
CZ
Email: ondrej.sury@nic.cz
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Shane Kerr (editor)
ISC
Bennebrokestraat 17-I
1015 PE Amsterdam
NL
Phone: +31 64 6336297
Email: shane@isc.org
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