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IETF DNSOP Working Group Y. Morishita
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Expires: July 11, 2004 T. Jinmei
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Common Misbehavior against DNS Queries for IPv6 Addresses
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draft-ietf-dnsop-misbehavior-against-aaaa-00.txt
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This document is an Internet-Draft and is in full conformance with
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all provisions of Section 10 of RFC2026.
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Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF), its areas, and its working groups. Note that other
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groups may also distribute working documents as Internet-Drafts.
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Internet-Drafts are draft documents valid for a maximum of six months
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and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than as "work in progress."
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The list of current Internet-Drafts can be accessed at http://
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www.ietf.org/ietf/1id-abstracts.txt.
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The list of Internet-Draft Shadow Directories can be accessed at
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http://www.ietf.org/shadow.html.
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This Internet-Draft will expire on July 11, 2004.
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Copyright (C) The Internet Society (2004). All Rights Reserved.
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There is some known misbehavior of DNS authoritative servers when
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they are queried for AAAA resource records. Such behavior can block
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IPv4 communication which should actually be available, cause a
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significant delay in name resolution, or even make a denial of
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service attack. This memo describes details of the known cases and
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discusses the effect of the cases.
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Many DNS clients (resolvers) that support IPv6 first search for AAAA
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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
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specifications, which if not obeyed by authoritative servers can
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produce unpleasant results. In some cases, for example, a web browser
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fails to connect to a web server it could otherwise. In the following
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sections, this memo describes some typical cases of the misbehavior
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and its (bad) effects.
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Note that the misbehavior is not specific to AAAA RRs. In fact, all
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known examples also apply to the cases of queries for MX, NS, and SOA
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RRs. The authors even believe this can be generalized for all types
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of queries other than those for A RRs. In this memo, however, we
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concentrate on the case for AAAA queries, since the problem is
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particularly severe for resolvers that support IPv6, which thus
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affects many end users. Resolvers at end users normally send A and/or
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AAAA queries only, and so the problem for the other cases is
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In this memo, we assume a typical network model of name resolution
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environment using DNS. It consists of three components; stub
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resolvers, caching servers, and authoritative servers. A stub
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resolver issues a recursive query to a caching server, which then
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handles the entire name resolution procedure recursively. The caching
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server caches the result of the query as well as sends the result to
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the stub resolver. The authoritative servers respond to queries for
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names for which they have the authority, normally in a non-recursive
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Suppose that an authoritative server has an A RR but not a AAAA RR
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for a host name. Then the server should return a response to a query
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for a AAAA RR of the name with the RCODE being 0 (indicating no
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error) and with an empty answer section [1]. Such a response
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indicates that there is at least one RR of a different type than AAAA
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for the queried name, and the stub resolver can then look for A RRs.
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This way, the caching server can cache the fact that the queried name
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does not have a AAAA RR (but may have other types of RRs), and thus
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can improve the response time to further queries for a AAAA RR of the
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4. Problematic Behaviors
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There are some known cases at authoritative servers that do not
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conform to the expected behavior. This section describes those
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This type of server returns a response with the RCODE being 3
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(NXDOMAIN) to a query for a AAAA RR, indicating it does not have any
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RRs of any type for the queried name.
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With this response, the stub resolver may immediately give up and
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never fall back. Even if the resolver retries with a query for an A
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RR, the negative response for the name has been cached in the caching
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server, and the caching server will simply return the negative
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response. As a result, the stub resolver considers this as a fatal
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error in name resolution.
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There have been several known examples of this behavior, but all the
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examples that the authors know have changed their behavior as of this
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Other authoritative servers return a response with the RCODE being 4
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(NOTIMP), indicating the servers do not support the requested type of
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This case is less harmful than the previous one; if the stub resolver
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falls back to querying for an A RR, the caching server will process
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the query correctly and return an appropriate response.
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In this case, the caching server does not cache the fact that the
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queried name has no AAAA RR, resulting in redundant queries for AAAA
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RRs in the future. The behavior will waste network bandwidth and
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increase the load of the authoritative server.
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Using SERVFAIL or FORMERR would cause the same effect, though the
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authors have not seen such implementations yet.
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4.3 Return a Broken Response
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Another different type of authoritative servers returns broken
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responses to AAAA queries. A known behavior of this category is to
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return a response whose RR type is AAAA, but the length of the RDATA
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is 4 bytes. The 4-byte data looks like the IPv4 address of the
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queried host name. That is, the RR in the answer section would be
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www.bad.example. 600 IN AAAA 192.0.2.1
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which is, of course, bogus (or at least meaningless).
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A widely deployed caching server implementation transparently returns
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the broken response (as well as caches it) to the stub resolver.
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Another known server implementation parses the response by
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themselves, and sends a separate response with the RCODE being 2
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In either case, the broken response does not affect queries for an A
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RR of the same name. If the stub resolver falls back to A queries, it
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will get an appropriate response.
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The latter case, however, causes the same bad effect as that
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described in the previous section: redundant queries for AAAA RRs.
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4.4 Make Lame Delegation
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Some authoritative servers respond to AAAA queries in a way causing
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lame delegation. In this case the parent zone specifies that the
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authoritative server should have the authority of a zone, but the
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server does not return an authoritative response for AAAA queries
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within the zone (i.e., the AA bit in the response is not set). On the
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other hand, the authoritative server returns an authoritative
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response for A queries.
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When a caching server asks the server for AAAA RRs in the zone, it
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recognizes the delegation is lame, and return a response with the
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RCODE being 2 (SERVFAIL) to the stub resolver.
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Furthermore, some caching servers record the authoritative server as
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lame for the zone and will not use it for a certain period of time.
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With this type of caching server, even if the stub resolver falls
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back to querying for an A RR, the caching server will simply return a
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response with the RCODE being SERVFAIL, since all the servers are
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There is also an implementation that relaxes the behavior a little
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bit. It basically tries to avoid using the lame server, but still
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continues to try it as a last resort. With this type of caching
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server, the stub resolver will get a correct response if it falls
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back after SERVFAIL. However, this still causes redundant AAAA
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queries as explained in the previous sections.
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4.5 Ignore Queries for AAAA
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Some authoritative severs seem to ignore queries for a AAAA RR,
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causing a delay at the stub resolver to fall back to a query for an A
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RR. This behavior may even cause a fatal timeout at the resolver.
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5. Security Considerations
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The CERT/CC pointed out that the response with NXDOMAIN described in
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Section 4.1 can be used for a denial of service attack [2]. The same
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argument applies to the case of "lame delegation" described in
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Section 4.4 with a certain type of caching server.
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Erik Nordmark encouraged the authors to publish this document as an
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Internet Draft. Akira Kato and Paul Vixie reviewed a preliminary
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version of this document. Pekka Savola carefully reviewed a previous
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version and provided detailed comments.
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Informative References
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[1] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC
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[2] The CERT Coordination Center, "Incorrect NXDOMAIN responses from
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AAAA queries could cause denial-of-service conditions", March
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2003, <http://www.kb.cert.org/vuls/id/714121>.
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MORISHITA Orange Yasuhiro
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Research and Development Department, Japan Registry Service Co.,Ltd.
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Fuundo Bldg 3F, 1-2 Kanda-Ogawamachi
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Chiyoda-ku, Tokyo 101-0052
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EMail: yasuhiro@jprs.co.jp
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Corporate Research & Development Center, Toshiba Corporation
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1 Komukai Toshiba-cho, Saiwai-ku
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Kawasaki-shi, Kanagawa 212-8582
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EMail: jinmei@isl.rdc.toshiba.co.jp
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Appendix A. Live Examples
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In this appendix, we show concrete implementations and domain names
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that may cause problematic cases so that the behavior can be
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reproduced in a practical environment. The examples are for
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informational purposes only, and the authors do not intend to accuse
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any implementations or zone administrators.
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The behavior described in Section 4.2 (return NOTIMP) can be found by
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looking for a AAAA RR of www.css.vtext.com at 66.174.3.4.
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The behavior described in Section 4.3 (broken responses) can be seen
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by querying for a AAAA RR of "www.gslb.mainichi.co.jp," which is an
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alias of "www.mainichi.co.jp," at 210.173.172.2. The same behavior
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can be found with the name "vip.alt.ihp.sony.co.jp," an alias of
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"www.sony.co.jp," at 210.139.255.204.
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The behavior described in Section 4.4 (lame delegation) can be found
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by querying for a AAAA RR of "www.ual.com" at 209.87.113.4.
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The behavior described in Section 4.5 (ignore queries) can be seen by
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trying to ask for a AAAA RR of "ad.3jp.doubleclick.net," which is an
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alias of "ad.jp.doubleclick.net," at 210.153.90.9.
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Many authoritative server implementations show the expected behavior
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described in Section 3. Some DNS load balancers reportedly have a
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problematic behavior shown in Section 4, but the authors do not have
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a concrete example. The CERT/CC provides a list of implementations
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that behave as described in Section 4.1 [2].
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The BIND9 caching server implementation is an example of the latter
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cases described in Section 4.3 and Section 4.4, respectively. The
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BIND8 caching server implementation is an example of the former case
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described in Section 4.3. As for the issue shown in Section 4.4,
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BIND8 caching servers prior to 8.3.5 show the behavior described as
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the former case in this section. The versions 8.3.5 and later of
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BIND8 caching server behave like the BIND9 caching server
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implementation with this matter.
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Regarding resolver implementations, the authors are only familiar
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with the ones derived from the BIND implementation. These
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implementations always fall back regardless of the RCODE; NXDOMAIN,
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NOTIMP, or SERVFAIL. It even falls back when getting a broken
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response. However, the behavior does not help the situation in the
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NXDOMAIN case (see Section 4.1). Lame delegation (Section 4.4) also
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causes a fatal error at the resolver side if the resolver is using
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some older versions of BIND8 caching server.
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The authors hear that a stub resolver routine implemented in some web
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browsers interprets the broken response described in Section 4.3 as a
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fatal error and does not fall back to A queries. However, we have not
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confirmed this information.
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Appendix B. Change History
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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
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that we can remove them when this document is (ever) officially
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o Revised some live examples based on the recent status.
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o Noted in introduction that the misbehavior is not specific to AAAA
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and that this document still concentrates on the AAAA case.
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o Changed the section title of "delegation loop" to "lame
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delegation" in order to reflect the essential point of the issue.
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Wording on this matter was updated accordingly.
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o Updated the Acknowledgements list.
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o Changed the reference category from normative to informative (this
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is an informational document after all).
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o Changed the draft name to an IETF dnsop working group document (as
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o Applied several editorial fixes.
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