IETF DNSOP Working Group Y. Morishita
Internet-Draft JPRS
Expires: October 8, 2004 T. Jinmei
Toshiba
April 9, 2004
Common Misbehavior against DNS Queries for IPv6 Addresses
draft-ietf-dnsop-misbehavior-against-aaaa-01.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
There is some known misbehavior of DNS authoritative servers when
they are queried for AAAA resource records. Such behavior can block
IPv4 communication which should actually be available, cause a
significant delay in name resolution, or even make a denial of
service attack. This memo describes details of the known cases and
discusses the effect of the cases.
1. Introduction
Many DNS clients (resolvers) that support IPv6 first search for AAAA
Resource Records (RRs) of a target host name, and then for A RRs of
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the same name. This fallback mechanism is based on the DNS
specifications, which if not obeyed by authoritative servers can
produce unpleasant results. In some cases, for example, a web browser
fails to connect to a web server it could otherwise. In the following
sections, this memo describes some typical cases of the misbehavior
and its (bad) effects.
Note that the misbehavior is not specific to AAAA RRs. In fact, all
known examples also apply to the cases of queries for MX, NS, and SOA
RRs. The authors even believe this can be generalized for all types
of queries other than those for A RRs. In this memo, however, we
concentrate on the case for AAAA queries, since the problem is
particularly severe for resolvers that support IPv6, which thus
affects many end users. Resolvers at end users normally send A and/or
AAAA queries only, and so the problem for the other cases is
relatively minor.
2. Network Model
In this memo, we assume a typical network model of name resolution
environment using DNS. It consists of three components; stub
resolvers, caching servers, and authoritative servers. A stub
resolver issues a recursive query to a caching server, which then
handles the entire name resolution procedure recursively. The caching
server caches the result of the query as well as sends the result to
the stub resolver. The authoritative servers respond to queries for
names for which they have the authority, normally in a non-recursive
manner.
3. Expected Behavior
Suppose that an authoritative server has an A RR but not a AAAA RR
for a host name. Then the server should return a response to a query
for a AAAA RR of the name with the RCODE being 0 (indicating no
error) and with an empty answer section [1]. Such a response
indicates that there is at least one RR of a different type than AAAA
for the queried name, and the stub resolver can then look for A RRs.
This way, the caching server can cache the fact that the queried name
does not have a AAAA RR (but may have other types of RRs), and thus
can improve the response time to further queries for a AAAA RR of the
name.
4. Problematic Behaviors
There are some known cases at authoritative servers that do not
conform to the expected behavior. This section describes those
problematic cases.
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4.1 Return NXDOMAIN
This type of server returns a response with the RCODE being 3
(NXDOMAIN) to a query for a AAAA RR, indicating it does not have any
RRs of any type for the queried name.
With this response, the stub resolver may immediately give up and
never fall back. Even if the resolver retries with a query for an A
RR, the negative response for the name has been cached in the caching
server, and the caching server will simply return the negative
response. As a result, the stub resolver considers this as a fatal
error in name resolution.
There have been several known examples of this behavior, but all the
examples that the authors know have changed their behavior as of this
writing.
4.2 Return NOTIMP
Other authoritative servers return a response with the RCODE being 4
(NOTIMP), indicating the servers do not support the requested type of
query.
This case is less harmful than the previous one; if the stub resolver
falls back to querying for an A RR, the caching server will process
the query correctly and return an appropriate response.
In this case, the caching server does not cache the fact that the
queried name has no AAAA RR, resulting in redundant queries for AAAA
RRs in the future. The behavior will waste network bandwidth and
increase the load of the authoritative server.
Using SERVFAIL or FORMERR would cause the same effect, though the
authors have not seen such implementations yet.
4.3 Return a Broken Response
Another different type of authoritative servers returns broken
responses to AAAA queries. A known behavior of this category is to
return a response whose RR type is AAAA, but the length of the RDATA
is 4 bytes. The 4-byte data looks like the IPv4 address of the
queried host name. That is, the RR in the answer section would be
described like this:
www.bad.example. 600 IN AAAA 192.0.2.1
which is, of course, bogus (or at least meaningless).
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A widely deployed caching server implementation transparently returns
the broken response (as well as caches it) to the stub resolver.
Another known server implementation parses the response by
themselves, and sends a separate response with the RCODE being 2
(SERVFAIL).
In either case, the broken response does not affect queries for an A
RR of the same name. If the stub resolver falls back to A queries, it
will get an appropriate response.
The latter case, however, causes the same bad effect as that
described in the previous section: redundant queries for AAAA RRs.
4.4 Make Lame Delegation
Some authoritative servers respond to AAAA queries in a way causing
lame delegation. In this case the parent zone specifies that the
authoritative server should have the authority of a zone, but the
server does not return an authoritative response for AAAA queries
within the zone (i.e., the AA bit in the response is not set). On the
other hand, the authoritative server returns an authoritative
response for A queries.
When a caching server asks the server for AAAA RRs in the zone, it
recognizes the delegation is lame, and returns a response with the
RCODE being 2 (SERVFAIL) to the stub resolver.
Furthermore, some caching servers record the authoritative server as
lame for the zone and will not use it for a certain period of time.
With this type of caching server, even if the stub resolver falls
back to querying for an A RR, the caching server will simply return a
response with the RCODE being SERVFAIL, since all the servers are
known to be "lame."
There is also an implementation that relaxes the behavior a little
bit. It basically tries to avoid using the lame server, but still
continues to try it as a last resort. With this type of caching
server, the stub resolver will get a correct response if it falls
back after SERVFAIL. However, this still causes redundant AAAA
queries as explained in the previous sections.
4.5 Ignore Queries for AAAA
Some authoritative severs seem to ignore queries for a AAAA RR,
causing a delay at the stub resolver to fall back to a query for an A
RR. This behavior may even cause a fatal timeout at the resolver.
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5. Security Considerations
The CERT/CC pointed out that the response with NXDOMAIN described in
Section 4.1 can be used for a denial of service attack [2]. The same
argument applies to the case of "lame delegation" described in
Section 4.4 with a certain type of caching server.
6. Acknowledgements
Erik Nordmark encouraged the authors to publish this document as an
Internet Draft. Akira Kato and Paul Vixie reviewed a preliminary
version of this document. Pekka Savola carefully reviewed a previous
version and provided detailed comments.
Informative References
[1] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC
1034, November 1987.
[2] The CERT Coordination Center, "Incorrect NXDOMAIN responses from
AAAA queries could cause denial-of-service conditions", March
2003, <http://www.kb.cert.org/vuls/id/714121>.
Authors' Addresses
MORISHITA Orange Yasuhiro
Research and Development Department, Japan Registry Service Co.,Ltd.
Fuundo Bldg 3F, 1-2 Kanda-Ogawamachi
Chiyoda-ku, Tokyo 101-0052
Japan
EMail: yasuhiro@jprs.co.jp
JINMEI Tatuya
Corporate Research & Development Center, Toshiba Corporation
1 Komukai Toshiba-cho, Saiwai-ku
Kawasaki-shi, Kanagawa 212-8582
Japan
EMail: jinmei@isl.rdc.toshiba.co.jp
Appendix A. Change History
[NOTE TO RFC EDITOR: PLEASE REMOVE THIS SECTION UPON PUBLICATION.]
Changes since draft-morishita-dnsop-misbehavior-against-aaaa-00 are:
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o Made a separate appendix and moved live examples to appendix so
that we can remove them when this document is (ever) officially
published.
o Revised some live examples based on the recent status.
o Noted in introduction that the misbehavior is not specific to AAAA
and that this document still concentrates on the AAAA case.
o Changed the section title of "delegation loop" to "lame
delegation" in order to reflect the essential point of the issue.
Wording on this matter was updated accordingly.
o Updated the Acknowledgements list.
o Changed the reference category from normative to informative (this
is an informational document after all).
o Changed the draft name to an IETF dnsop working group document (as
agreed).
o Applied several editorial fixes.
Changes since draft-ietf-dnsop-misbehavior-against-aaaa-00 are:
o Removed the appendix talking about live examples since these were
not appropriate for official publication.
o Added a note to rfc editor asking to remove this section upon
publication.
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