Network Working Group S. Jiang
Internet Draft Huawei Technologies Co., Ltd
Intended status: Informational D. Conrad
Expires: May 29, 2012 Cloudflare, Inc.
B. Carpenter
Univ. of Auckland
November 26, 2011
Moving A6 to Historic Status
draft-jiang-a6-to-historic-00.txt
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Abstract
This document provides a summary of issues and discusses the current
usage status of A6 DNS records and moves the A6 specifications to
Historic status, providing clarity to implementers and operators.
Table of Contents
1. Introduction & Background .................................... 3
2. A6 Issues .................................................... 3
2.1. Resolution Latency ...................................... 4
2.2. Resolution failure ...................................... 4
2.3. Cross administration domains ............................ 4
2.4. Difficult Maintenance ................................... 5
2.5. Existence of Multiple RR Types for one Purpose is Harmful 5
2.6. Higher Security Risks ................................... 5
3. Status of A6 current usage ................................... 5
3.1. Reasons for Current A6 Usage ............................ 6
4. Moving A6 to Historic Status ................................. 6
4.1. Impact on Current A6 Usage .............................. 6
4.2. Transition phase for current A6 ......................... 6
5. Security Considerations ...................................... 7
6. IANA Considerations .......................................... 7
7. Acknowledgments .............................................. 7
8. References ................................................... 7
8.1. Normative References .................................... 7
8.2. Informative References .................................. 7
Author's Addresses .............................................. 8
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1. Introduction & Background
The IETF began the process of standardizing two different DNS
protocol enhancements for IPv6 addresses in DNS (Domain Name System)
records: AAAA [RFC3596] in 1995 [RFC1886] and A6 [RFC2874] in 2000.
Both protocol enhancements reached Proposed Standard status.
The existence of multiple ways of representing IPv6 address in the
DNS has led to confusion and conflicts about which of these protocol
enhancements should be implemented and/or deployed. Having more than
one choice of how IPv6 addresses are to be represented within the DNS
can be argued to have led to delays in the deployment of IPv6. In
2002, "Representing IPv6 Addresses in the DNS" [RFC3363] moved A6 to
Experimental status, with an aim to clear up any confusion in this
area. [RFC3363] and [RFC3364] compared AAAA and A6, and examined many
of the issues in the A6 standard, these issues being summarized in
this document.
However, after ten years, the Experimental status of A6 has resulted
in continued confusion and parallel deployment of both A6 and AAAA,
albeit AAAA predominates by a large degree. Even in recent IPv6
transition tests and deployments, some providers informally mentioned
A6 support as a possible future choice.
This document provides a brief summary of the issues related to the
use of A6 recprds and discusses the current usage status of A6. Given
the implications of A6 on the DNS architecture and the state of A6
deployment, this document moves the A6 specifications to Historic
status, thereby clarifying that implementers and operators should
represent IPv6 addresses in the DNS only by using AAAA records.
1.1 Standards Action Taken
This document requests the IESG to change the status of RFC 2874 from
Experimental to Historic.
2. A6 Issues
This section summarizes the known issues associated with the use of
A6 resource records, including the analyses explored in [RFC3363].
The reader is encouraged to review that document to fully understand
the issues relating to A6.
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2.1. Resolution Latency
Resolving an A6 Record chain can involve resolving a series of sub-
queries that are likely to be independent of each other. Each of
these sub-queries takes a non-negligible amount of time unless the
answer already happens to be in the resolver's cache. The worst-case
time resolving a N-link chain A6 record would be the sum of the
latency resulting from each of the N resolutions. As a result, long
A6 chains would be likely to increase user frustration due to
excessive waiting times for domain names to resolve.
In practice, it is very hard to derive a reasonable timeout handling
strategy for the reassembly of all the results from A6 sub-queries.
It is proved difficult to decide multiple timeout parameters,
including (1) the communication timeout for a single A6 fragment, (2)
the communication timeout for the IPv6 address itself (total time
needed for reassembly) and (3) the TTL timeout for A6 fragment
records.
2.2. Resolution Failure
The probability of A6 resolution failure during the process of
resolving an N-link A6 chain is sum of the probabilities of failure
of each sub-query, since each of the queries involved in resolving an
A6 chain has a non-zero probability of failure and an A6 resolution
cannot complete until all sub-queries have succeeded.
Furthermore, the failure may happen at any link among 1~N of a N-Link
A6 chain. Therefore, it would take an indeterminate time to return a
failure result.
2.3. Cross Administrative Domains
One of the primary motivations for the A6 RR was to facilitate
renumbering and multihoming, where the prefix name field in the A6 RR
points to a target that is not only outside the DNS zone containing
the A6 RR, but is administered by a different organization entirely.
While pointers out of zone are not a problem per se, experience both
with glue RRs and with PTR RRs in the IN-ADDR.ARPA tree suggests that
pointers to other organizations are often not maintained properly,
perhaps because they're less amenable to automation than pointers
within a single organization would be.
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2.4. Difficult Maintenance
In A6, changes to components of an RR are not isolated from the use
of the composite IPv6 address. Any change to a non-128-bit component
of an A6 RR may cause change to a large number of IPv6 addresses. The
dependence relationship actually makes the maintenance of addresses
much more complicated and difficult. Without understanding these
complicated relationships, any arbitrary change for a non-128-bit A6
RR component may result in undesired consequences.
Multiple correlative sub-components of A6 records may have different
TTLs, which can make cache maintenance very complicated.
2.5. Existence of Multiple RR Types for one Purpose is Harmful
If both AAAA and A6 records were widely deployed in the global DNS,
it would impose more query delays to the client resolvers. DNS
clients have insufficient knowledge to choose between AAAA and A6
queries, requiring local policy to determine which record type to
query. If local policy dictates parallel queries for both AAAA and A6
and if those queries returned different results for any reason, the
clients would have no knowledge about which address to choose.
2.6. Higher Security Risks
The dependency relationships inherent in A6 chains increase security
risks. An attacker may successfully attack a single sub-component of
an A6 record, which would then influence many query results, and
possibly every host on a large site. There is also the danger of
unintentionally or maliciously creating a resolution loop - an A6
chain may create an infinite loop because an out of zone pointer may
point back to another component farther down the A6 chain.
3. Current Usage of A6
Full support for IPv6 in the global DNS can be argued to have started
when the first IPv6 records were associated with root servers in
early 2008.
One of the major DNS server software packages, BIND9 [BIND], supports
both A6 and AAAA and is unique among the major DNS resolvers in that
certain versions of the BIND9 resolver will attempt to query for A6
records and follow A6 chains.
According to published statistics for two root DNS servers (the "K"
root server [KROOT] and the "L" root server [LROOT]), there are
between 9,000 and 14,000 DNS queries per second on the "K" root
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server and 13,000 to 19,000 queries per second on the "L" root
server. The distributions of those queries by RR type are similar:
roughly 60% A queries, 20~25% AAAA queries, and less than 1% A6
queries.
3.1. Reasons for Current A6 Usage
That there is A6 query traffic does not mean that A6 is actually in
use; it is likely the result of some recursive servers that issue
internally-generated A6 queries when looking up missing name server
addresses in addition to issuing A and AAAA queries.
BIND versions 9.0 through 9.2 could be configured to make A6 queries
and it is possible that some active name servers running those
versions have not yet been upgraded.
In the late 1990s, A6 was considered to be the future in preference
to AAAA [RFC2874]. As a result, A6 queries were tried by default in
BINDv9 versions. When it was pointed out that A6 had some fundamental
issues (discussed in [A6DISC] with the deprecation codified in RFC
3363), A6 was abandoned in favor of AAAA and BINDv9 no longer tried
A6 records by default. A6 was removed from the query order in the
BIND distribution in 2004 or 2005.
Some Linux/glibc versions may have had A6 query implementations in
gethostbyname() 8-10 years ago. These operating systems/libraries may
not have been replaced or upgraded everywhere yet.
4. Moving A6 to Historic Status
This document moves the A6 specification to Historic status. This
move provides a clear signal to implementers and/or operators that A6
should NOT be implemented or deployed.
4.1. Impact on Current A6 Usage
If A6 were in use and it were to be treated as an 'unknown record'
(RFC3597) as discussed below, it might lead to some interoperability
issues since resolvers that support A6 are required to do additional
section processing for these records on the wire. However, as there
are no known production uses of A6, this impact is considered
negligible.
4.2. Transition phase for current A6
Since there is no known A6-only client in production use, the
transition phase may not be strictly necessary. However, clients that
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attempt to resolve A6 before AAAA will suffer a performance penalty.
Therefore, we recommend:
* Removing A6 handling from all new or updated host stacks;
* Recommend removing all existing A6 records; and
* All resolver and server implementations return the same response as
for any unknown or deprecated RR type for all A6 queries. If an AAAA
record exists for the name being resolved, a suitable response would
be 'no answers/no error', i.e. the response packet has an answer
count of 0 but no error is indicated.
5. Security Considerations
Eliminating A6 records will eliminate any security exposure related
to that RR type, and should introduce no new vulnerabilities.
6. IANA Considerations
IANA is requested to change the annotation of the A6 RR type from
"Experimental" to "Obsolete" in the DNS Parameters registry.
7. Acknowledgments
The authors would like to thank Ralph Droms, Roy Arends, Edward
Lewis, Andreas Gustafsson, Mark Andrews, Jun-ichiro "itojun" Hagino
and other members of DNS WGs for valuable contributions.
8. References
8.1. Normative References
[RFC2874] M. Crawford, C. Huitema, "DNS Extensions to Support IPv6
Address Aggregation and Renumbering", RFC 2874, July 2000.
[RFC3596] S. Thomson, C. Huitema, V. Ksinant, M. Souissi, "DNS
Extensions to Support IP Version 6", RFC 3596, October
2003.
8.2. Informative References
[RFC1886] S. Thomson and C. Huitema, "DNS Extensions to Support IP
Version 6", RFC 1886, December 1995.
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[RFC3363] R. Bush, A. Durand, B. Fink, O. Gudmundsson, T. Hain,
"Representing Internet Protocol version 6 (IPv6) Addresses
in the Domain Name System (DNS)", RFC 3363, August 2002.
[RFC3364] R. Austein, "Tradeoffs in Domain Name System (DNS) Support
for Internet Protocol version 6 (IPv6)", RFC 3364, August
2002.
[A6DISC] J. Hagino, "Comparison of AAAA and A6 (do we really need
A6?)", working in progress, 2001.
[BIND] http://www.isc.org/software/bind
[KROOT] http://k.root-servers.org/
[LROOT] http://dns.icann.org/lroot/
Author's Addresses
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus
No.156 Beiqing Road
Hai-Dian District, Beijing 100095
P.R. China
Email: jiangsheng@huawei.com
David Conrad
Cloudflare, Inc.
665 3rd Street, Suite 207
San Francisco CA 94107
USA
Email: drc@cloudflare.com
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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