Commit 7b18de74 authored by Mark Andrews's avatar Mark Andrews

new draft

parent 13f4bdc9
INTERNET-DRAFT Edward Lewis
draft-ietf-dnsext-axfr-clarify-09.txt NeuStar, Inc.
DNSEXT WG July 2008
Updates: 1034, 1035 (if approved) Intended status: Standards Track
DNS Extensions Working Group Edward Lewis
INTERNET-DRAFT NeuStar, Inc.
Expires: July 1, 2009 January 2009
Updates: 1034, 1035 (if approved)
Intended status: Standards Track
DNS Zone Transfer Protocol (AXFR)
Status of this Memo
DNS Zone Transfer Protocol (AXFR)
draft-ietf-dnsext-axfr-clarify-10.txt
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Status of this Memo
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Copyright (C) The IETF Trust (2008).
Copyright (c) 2008 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Abstract
......@@ -139,7 +146,7 @@ documents.
The original "specification" of the AXFR sub-protocol is scattered
through RFC 1034 and RFC 1035. Section 2.2 of RFC 1035 on page 5
depicts the scenario for which AXFR has been designed. Section 4.3.5
of RFC 1034 describes the zone synchronisation strategies in general
of RFC 1034 describes the zone synchronization strategies in general
and rules for the invocation of a full zone transfer via AXFR; the
fifth paragraph of that section contains a very short sketch of the
AXFR protocol. Section 3.2.3 of RFC 1035 has assigned the code point
......@@ -162,13 +169,12 @@ define AXFR as it exists, or is supposed to exist, currently.
2 AXFR Messages
An AXFR message exchange (or session) consists of an AXFR query message
and a set of AXFR response messages. In this document, AXFR client is
An AXFR session consists of an exchange of a AXFR query message and a
set of AXFR response messages. In this document, the AXFR client is
the sender of the AXFR query and the AXFR server is the responder.
(Use of terms such as master, slave, primary, secondary are not
important to defining the AXFR exchange.) The reason for the imbalance
in number of messages derives from large zones whose contents cannot be
fit into the limited permissible size of a DNS message.
important to defining AXFR.) The use of the word "session" without
qualification refers to an AXFR session.
An important aspect to keep in mind is that the definition of AXFR is
restricted to TCP [RFC0793]. The design of the AXFR process has
......@@ -177,7 +183,7 @@ certain inherent features that are not easily ported to UDP [RFC0768].
The basic format of an AXFR message is the DNS message as defined in
RFC 1035, Section 4 ("MESSAGES") [RFC1035], updated by the following:
- "A Mechanism for Prompt Notification of Zone Changes (...)" [RFC1996]
- "Domain Name System (DNS) IANA Considerations" [RFC2929]
- "Domain Name System (DNS) IANA Considerations" [RFC5395]
- "Dynamic Updates in the Domain Name System (DNS UPDATE)" [RFC2136]
- "Extension Mechanisms for DNS (EDNS0)" [RFC2671]
- "Secret Key Transaction Authentication for DNS (TSIG)" [RFC2845]
......@@ -188,8 +194,11 @@ RFC 1035, Section 4 ("MESSAGES") [RFC1035], updated by the following:
- "HMAC SHA TSIG Algorithm Identifiers" [RFC4635]
The upper limit on the permissible size of a DNS message over TCP is
defined in RFC 1035, section 4.2.2. Unlike DNS messages over UDP,
this limit is not changed by EDNS0.
only restricted by the TCP framing defined in RFC 1035, section 4.2.2
which specifies a two-octet message length field, understood to be
unsigned, and thus causing a limit of 65535 octets. Unlike DNS
messages over UDP, this limit is not changed by EDNS0.
Field names used in this document will correspond to the names as they
appear in the IANA registry for DNS Header Flags [DNSFLGS].
......@@ -294,7 +303,7 @@ additional section might change over time. If either a change to an
existing resource record (like the OPT RR for EDNS0) is made or
a new additional section record is created, the new definitions ought
to include a discussion on the impact upon AXFR. Although this is not
predictale, future additional section residing records may have an
predictable, future additional section residing records may have an
effect that is orthogonal to AXFR, so can ride through the session as
opaque data. In this case, a "wise" implementation ought to be able
to pass these records through without disruption.
......@@ -323,7 +332,8 @@ necessary.
An AXFR client might receive a number of AXFR response messages
free of an error condition before the message indicating an error
is received. But once an error is reported, the AXFR client can
assume that the error reporting message is the last.
assume that the error reporting message is the last message sent by
the AXFR server in the current AXFR session.
2.2.1 "0 Message" Response
......@@ -333,33 +343,16 @@ is unhealthy for there to be 0 responses in a protocol that is designed
around a query - response paradigm over an unreliable transport. The
lack of a response could be a sign of underlying network problems and
cause the protocol state machine to react accordingly. However, AXFR
uses TCP and not UDP, eliminating undetected network errors.
uses TCP and not UDP, eliminating undetectable network errors.
A "0 message response" is reserved for situations in which the server
has a reason to suspect that the query is sent for the purpose of
abuse. Due to the use of this being so controversial, a "0 message
response" is not being defined as a legitimate part of the protocol
but the use of it is being acknowledge as a warning to AXFR client
but the use of it is being acknowledged as a warning to AXFR client
implementations. Any earnest query has the expectation of some
response but may not get one.
If an AXFR client sends a query on a TCP connection and the connection
is closed at any point, the AXFR client MUST consider the session
terminated. The message ID MAY be used again on a new connection,
even if the question and AXFR server are the same. Facing a dropped
connection a client SHOULD try to make some determination whether the
connection closure was the result of network activity or a decision
by the AXFR server. This determination is not an exact science. It
is up to the AXFR client implementor to react, but the reaction
SHOULD NOT be an endless cycle of retires nor an increasing (in
frequency) retry rate.
An AXFR server implementor SHOULD take into consideration what this
dilemma described above when a connection is closed with an outstanding
query in the pipeline. For this reason, a server ought to reserve
this course of action for situations in which it believes beyond a
doubt that the AXFR client is attemping abusive behavior.
2.2.2 Header Values
ID See note 2.2.2.a
......@@ -446,11 +439,14 @@ resource records appearing in the additional section. See Section
2.2.3 Query Section
In the first response message, this section MUST be copied from the
query. In subsequent messages this section MAY be copied from the
query, MAY be empty. The content of this section MAY be used to
query. In subsequent messages, this section MAY be copied from the
query or it MAY be empty. The content of this section MAY be used to
determine the context of the message, that is, the name of the zone
being transferred.
>| [...]. In subsequent messages, this section MAY be copied from the
>| query, or it MAY be empty. [...]
2.2.4 Answer Section
MUST be populated with the zone contents. See later section on
......@@ -460,7 +456,7 @@ encoding zone contents.
MUST be empty.
2.2.5 Additional Section
2.2.6 Additional Section
The contents of this section MUST follow the guidelines for EDNS0,
TSIG, SIG(0), or what ever other future record is possible here. The
......@@ -469,10 +465,29 @@ contents of section 2.1.5 apply here as well.
Note that TSIG and SIG(0), if in use, will treat each individual
AXFR response message within a session as a unit of data. That is,
each message will have a TSIG or SIG(0) (if in use) and the
crytpographic check will cover just that message. The same rule
cryptographic check will cover just that message. The same rule
will apply to future alternatives and documents covering them ought
to consider the impact on AXFR response messages.
2.3 TCP Connection Aborts
If an AXFR client sends a query on a TCP connection and the connection
is closed at any point, the AXFR client MUST consider the AXFR session
terminated. The message ID MAY be used again on a new connection,
even if the question and AXFR server are the same. Facing a dropped
connection a client SHOULD try to make some determination whether the
connection closure was the result of network activity or a decision
by the AXFR server. This determination is not an exact science. It
is up to the AXFR client implementor to react, but the reaction
SHOULD NOT be an endless cycle of retries nor an increasing (in
frequency) retry rate.
An AXFR server implementor SHOULD take into consideration the dilemma
described above when a connection is closed with an outstanding query
in the pipeline. For this reason, a server ought to reserve this
course of action for situations in which it believes beyond a doubt
that the AXFR client is attempting abusive behavior.
3 Zone Contents
The objective of the AXFR session is to request and transfer the
......@@ -497,7 +512,11 @@ records as will fit within a DNS message.
Zones for which it is impractical to list the entire zones for a serial
number (because changes happen too quickly) are not suitable for AXFR
retrieval.
retrieval. A typical (but not limiting) description of such a zone
is a zone consisting of responses generated via other database lookups
and/or computed based upon ever changing data. In essence, if the
zone changes (on average) more frequently than and AXFR session can be
finished, the zone is not a good candidate for AXFR.
3.2 Delegation Records
......@@ -511,11 +530,12 @@ included in a zone transfer.
The phrase "that describe cuts" is a reference to the NS set and
applicable glue records. It does not mean that the cut points and the
apex resource records are identical. For example, the SOA resource
record is only found at the apex, as well as a slew of DNSSEC resource
records. There are also some DNSSEC resource record sets that are
explicitly different between the cut point and the apex. The
discussion here is restricted to just the NS resource record set and
glue as these "describe cuts."
record is only found at the apex, as well as DNSSEC resource records.
The is even a DNSSEC resource record found only at the zone cut and not
at the corresponding apex. There are also some DNSSEC resource record
sets that are explicitly different between the cut point and the apex.
The discussion here is restricted to just the NS resource record set
and glue as these "describe cuts."
The issue is that in operations there are times when the NS resource
records for a zone might be different at a cut point in the parent and
......@@ -525,7 +545,7 @@ protocol is robust enough to overcome inconsistencies up to (but not
including) there being no parent indicated NS resource record
referencing a server that is able to serve the child zone. This
robustness is one quality that has fueled the success of the DNS.
Still, the inconsistency is a error state and steps need to be taken
Still, the inconsistency is an error state and steps need to be taken
to make it apparent (if it is unplanned) and to make it clear once
the inconsistency has been removed.
......@@ -538,7 +558,7 @@ the cut point.
The question that arises is, when facing a situation in which a cut
point's NS resource records do not match the authoritative set, whether
an AXFR server responds with the NS resource record set that is in the
zone or is at the authoritative location.
zone being transferred or is at the authoritative location.
The AXFR response MUST contain the cut point NS resource record set
registered with the zone whether it agrees with the authoritative set
......@@ -567,7 +587,7 @@ in a registration database.
4) Beginning with an error state of two servers for a zone having
inconsistent zone contents for a given zone serial number, if a client
requests and recieves an IXFR transfer from one server followed by
requests and receives an IXFR transfer from one server followed by
another IXFR transfer from the other server, the client can encounter
an IXFR protocol error state where an attempt is made to incrementally
add a record that already exists or to delete a record that does not
......@@ -578,7 +598,7 @@ it relates. A nudge for updated text on this.)
3.3 Glue Records
As quoted in the previous section, RFC 1034, section 4.2.1, provides
As quoted in the previous section, section 4.2.1 of RFC 1034 provides
guidance and rationale for the inclusion of glue records as part of
an AXFR transfer. And, as also argued in the previous section of this
document, even when there is an inconsistency between the address in a
......@@ -586,7 +606,7 @@ glue record and the authoritative copy of the name server's address,
the glue resource record that is registered as part of the zone for
that serial number is to be included.
This applies for glue records for any address family.
This applies to glue records for any address family [RFC1700].
The AXFR response MUST contain the appropriate glue records as
registered with the zone. The interpretation of "registered with"
......@@ -615,12 +635,12 @@ Compression."
3.5 Occluded Names
Dynamic Update [RFC2136] (and including DNAME [2672]) operations can
Dynamic Update [RFC2136] (and including DNAME [RFC2672]) operations can
have a side effect of occluding names in a zone. The addition of a
delegation point via dynamic update will render all subordinate domain
names to be in a limbo, still part of the zone but not available for to
the look up process. The addition of a DNAME resource record set has
the same impact. The subordinate names are said to be "occluded."
names to be in a limbo, still part of the zone but not available
to the lookup process. The addition of a DNAME resource record has the
same impact. The subordinate names are said to be "occluded."
Occluded names MUST be included in AXFR responses. An AXFR client MUST
be able to identify and handle occluded names. The rationale for this
......@@ -631,20 +651,20 @@ was in error and is to be undone.
AXFR sessions are currently restricted to TCP by section 4.3.5 of RFC
1034 that states: "Because accuracy is essential, TCP or some other
reliable protocol must be used for AXFR requests." The most common
scenario is for an AXFR client to open a TCP connection to the AXFR
server, send an AXFR query, receive the AXFR response, and then
close the connection. There are variations on this, such as a query
for the zone's SOA resource record first, and so on.
Two issues have emerged since the original specification of AXFR.
One is that lack of specificity has yielded some implementations
that assume the TCP connection is dedicated to the single AXFR
session, which has led to implementation choices that prevent either
multiple concurrent zone transfers or the use of the open connection
for other queries. The other issue is the prospect of using UDP as a
transport has come to look promising because of trends in the past
two decades.
reliable protocol must be used for AXFR requests." The restriction to
TCP is also mentioned in section 6.1.3.2. of "Requirements for Internet
Hosts - Application and Support" [RFC1123].
The most common scenario is for an AXFR client to open a TCP connection
to the AXFR server, send an AXFR query, receive the AXFR response, and
then close the connection. There are variations on this, such as a
query for the zone's SOA resource record first, and so on. Note that
this is documented as a most common scenario.
The assumption that a TCP connection is dedicated to the single AXFR
session is incorrect, this as has led to implementation choices that
prevent either multiple concurrent zone transfers or the use of the
open connection for other queries.
Being able to have multiple concurrent zone transfers is considered
desirable by operators who have sets of name servers that are
......@@ -690,10 +710,11 @@ reason. An AXFR client SHOULD close the connection when there is
no apparent need to use the connection for some time period. The
AXFR server ought not have to maintain idle connections, the burden
of connection closure ought to be on the client. Apparent need for
the connection is a judgement for the AXFR client and the DNS
client. If the connection is used for multiple sessions, or it is
known sessions will be coming or is there is other query/response
traffic on the open connection, that is "apparent need."
the connection is a judgment for the AXFR client and the DNS
client. If the connection is used for multiple sessions, or if it is
known sessions will be coming or if there is other query/response
traffic anticipated or currently on the open connection, then there
is "apparent need."
An AXFR client MAY cancel delivery of a zone only by closing the
connection. However, this action will also cancel all other outstanding
......@@ -719,7 +740,7 @@ opening a new connection. (Non-AXFR session traffic can also use an
open connection.) If in doing so the AXFR client realizes that
the responses cannot be properly differentiated (lack of matching
query IDs for example) or the connection is terminated for a remote
reason, then the AXFR client SHOULD not attempt to reuse an open
reason, then the AXFR client SHOULD NOT attempt to reuse an open
connection with the specific AXFR server until the AXFR server is
updated (which is of course, not an event captured in the DNS
protocol).
......@@ -758,8 +779,7 @@ serving AXFR. All reasons are arguable, but the fact remains that
there is a requirement to provide mechanisms to restrict AXFR.
A DNS implementation SHOULD provide means to restrict AXFR sessions to
specific clients. By default, a DNS implementation SHOULD only allow
the designated authoritative servers to have access to the zone.
specific clients.
An implementation SHOULD allow access to be granted to Internet
Protocol addresses and ranges, regardless of whether a source address
......@@ -777,10 +797,32 @@ all AXFR requests. I.e., an operator ought to be able to allow any
AXFR query to be granted.
A general purpose implementation SHOULD NOT have a default policy
for AXFR requests to be "open to all."
for AXFR requests to be "open to all." For example, a default could
be to restrict transfers to loopback address(es) and such.
6 Zone Integrity
An AXFR client MUST ensure that only a successfully transferred
copy of the zone data can be used to serve this zone. Previous
description and implementation practice have introduced a two-stage
model of the whole zone synchronization procedure: Upon a trigger
event (e.g., polling of SOA resource record detects change in the
SOA serial number, or via DNS NOTIFY [RFC1996]), the AXFR session
is initiated, whereby the zone data are saved in a zone file or
data base (this latter step is necessary anyway to ensure proper
restart of the server); upon successful completion of the AXFR
operation and some sanity checks, this data set is 'loaded' and
made available for serving the zone in an atomic operation, and
flagged 'valid' for use during the next restart of the DNS server;
if any error is detected, this data set MUST be deleted, and the
AXFR client MUST continue to serve the previous version of the zone,
if it did before. The externally visible behavior of an AXFR client
implementation MUST be equivalent to that of this two-stage model.
If a server rejects data contained in an AXFR session, the server
SHOULD remember the serial number and not attempt to retrieve the
same zone version again.
Ensuring that an AXFR client does not accept a forged copy of a zone is
important to the security of a zone. If a zone operator has the
opportunity, protection can be afforded via dedicated links, physical
......@@ -794,7 +836,34 @@ Authentication for DNS" [RFC2845] and/or "DNS Request and Transaction
Signatures ( SIG(0)s )" [RFC2931] to allow AXFR clients to verify the
contents. These techniques MAY also be used for authorization.
7 Backwards Compatibility
7 Zone Expiry Timer
Section 4.3.5 of RFC 1034 contains the following paragraph:
"The periodic polling of the secondary servers is controlled by
parameters in the SOA RR for the zone, which set the minimum acceptable
polling intervals. The parameters are called REFRESH, RETRY, and
EXPIRE. Whenever a new zone is loaded in a secondary, the secondary
waits REFRESH seconds before checking with the primary for a new serial.
If this check cannot be completed, new checks are started every RETRY
seconds. The check is a simple query to the primary for the SOA RR of
the zone. If the serial field in the secondary's zone copy is equal to
the serial returned by the primary, then no changes have occurred, and
the REFRESH interval wait is restarted. If the secondary finds it
impossible to perform a serial check for the EXPIRE interval, it must
assume that its copy of the zone is obsolete an discard it."
Perhaps what is not clear in the paragraph regarding the EXPIRE
interval timer is that it is only reset to the EXPIRE parameter when
a new zone is loaded. A new zone means a zone with a higher serial
number than the most recently loaded zone. The EXPIRE interval timer
is not reset automatically as a result of a zone transfer as a zone
could be (mistakenly) transferred with the same or lower serial number.
I.e., successively transferring a zone from server to server does not
permit the zone to avoid expiration.
8 Backwards Compatibility
Describing backwards compatibility is difficult because of the lack of
specifics in the original definition. In this section some hints at
......@@ -813,7 +882,7 @@ implementation SHOULD, in it's documentation, encourage operators to
periodically review AXFR clients and servers it has made notes about as
old software periodically gets updated.
7.1 Server
8.1 Server
An AXFR server has the luxury of being able to react to an AXFR
client's abilities with the exception of knowing if the client can
......@@ -825,7 +894,7 @@ An implementation of an AXFR server SHOULD permit configuring, on a per
AXFR client basis, a need to revert to single resource record per
message. The default SHOULD be to use multiple records per message.
7.2 Client
8.2 Client
An AXFR client has the opportunity to try extensions when querying
an AXFR server.
......@@ -836,23 +905,23 @@ and SHOULD take into consideration whether the AXFR server intends to
close the connection immediately upon completion of the original
(connection-causing) zone transfer.
8 Security Considerations
9 Security Considerations
Concerns regarding authorization, traffic flooding, and message
integrity are mentioned in "Authorization" (section 5), "TCP" (section
4.2) and Zone Integrity (section 6).
4.2) and "Zone Integrity" (section 6).
9 IANA Considerations
10 IANA Considerations
No new registries or new registrations are included in this document.
10 Internationalization Considerations
11 Internationalization Considerations
It is assumed that supporting of international domain names has been
solved via "Internationalizing Domain Names in Applications (IDNA)"
[RFC3490].
11 Acknowledgements
12 Acknowledgements
Earlier editions of this document have been edited by Andreas
Gustafsson. In his latest version, this acknowledgement appeared.
......@@ -868,15 +937,15 @@ Alfred Hoenes, Mark Andrews, Paul Vixie, Wouter Wijngaards, Iain
Calder, Tony Finch, Ian Jackson, Andreas Gustafsson, Brian Wellington,
...
12 References
13 References
All references prefixed by "RFC" can be obtained from the RFC Editor,
All references prefixed by "RFC" can be obtained from the RFC Editor
web site at the URLs: http://rfc-editor.org/rfc.html
or http://rfc-editor.org/rfcsearch.html ;
information regarding this organization can be found at the following
URL:
http://rfc-editor.org/
Additionally, these documents can be obtained via the IETF web site.
URL: http://rfc-editor.org/
12.1 Normative
13.1 Normative
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
......@@ -886,6 +955,8 @@ Additionally, these documents can be obtained via the IETF web site.
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
August 1996.
[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
......@@ -900,9 +971,8 @@ Additionally, these documents can be obtained via the IETF web site.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
[RFC2929] Eastlake 3rd, D., Brunner-Williams, E., and B. Manning,
"Domain Name System (DNS) IANA Considerations", BCP 42, RFC
2929, September 2000.
[RFC5395] Eastlake 3rd, "Domain Name System (DNS) IANA Considerations",
BCP 42, RFC 5395, November 2008.
[RFC2930] Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000.
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
......@@ -925,10 +995,12 @@ Additionally, these documents can be obtained via the IETF web site.
[DNSFLGS] http://www.iana.org/assignments/dns-header-flags
[DNSVALS] http://www.iana.org/assignments/dns-parameters
12.2 Informative
13.2 Informative
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1700] J. Reynolds and J. Postel, "Assigned Numbers", RFC 1700,
October 1994.
[RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private Networks",
RFC 2764, February 2000.
......@@ -936,7 +1008,7 @@ Additionally, these documents can be obtained via the IETF web site.
"Internationalizing Domain Names in Applications (IDNA)", RFC
3490, March 2003.
13 Editor's Address
14 Editor's Address
Edward Lewis
46000 Center Oak Plaza
......@@ -944,49 +1016,7 @@ Sterling, VA, 22033, US
+1-571-434-5468
ed.lewis@neustar.biz
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