IPv6アドレスのDNS問合せに対する一般的な不適切動作

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

   Copyright (C) The Internet Society (2005).

   There is some known misbehavior of DNS authoritative servers when
   they are queried for AAAA resource records.  Such behavior can block
   IPv4 communication that should actually be available, cause a
   significant delay in name resolution, or even make a denial of
   service attack.  This memo describes details of known cases and
   discusses their effects.

   Many existing DNS clients (resolvers) that support IPv6 first search
   for AAAA Resource Records (RRs) of a target host name, and then for A
   RRs of 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 reach.
   In the following sections, this memo describes some typical cases of
   such 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 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, so the problem for the other cases is
   relatively minor.

   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 and 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.

   Suppose that an authoritative server has an A RR but has no AAAA RR
   for a host name.  Then, the server should return a response to a
   query for an AAAA RR of the name with the response code (RCODE) being
   0 (indicating no error) and with an empty answer section (see
   Sections 4.3.2 and 6.2.4 of [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
   has no AAAA RR (but may have other types of RRs), and thus improve
   the response time to further queries for an AAAA RR of the name.

   There are some known cases at authoritative servers that do not
   conform to the expected behavior.  This section describes those
   problematic cases.

   Some authoritative servers seem to ignore queries for an AAAA RR,
   causing a delay at the stub resolver to fall back to a query for an A
   RR.  This behavior may cause a fatal timeout at the resolver or at
   the application that calls the resolver.  Even if the resolver
   eventually falls back, the result can be an unacceptable delay for
   the application user, especially with interactive applications like
   web browsing.

   This type of server returns a response with RCODE 3 ("Name Error") to
   a query for an AAAA RR, indicating that 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 to be a
   fatal error in name resolution.

   Several examples of this behavior are known to the authors.  As of
   this writing, all have been fixed.

   Other authoritative servers return a response with erroneous response
   codes other than RCODE 3 ("Name Error").  One such RCODE is 4 ("Not
   Implemented"), indicating that the servers do not support the
   requested type of query.

   These cases are 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.

   However, these can still cause a serious effect.  There was an
   authoritative server implementation that returned RCODE 2 ("Server
   failure") to queries for AAAA RRs.  One widely deployed mail server
   implementation with a certain type of resolver library interpreted
   this result as an indication of retry and did not fall back to
   queries for A RRs, causing message delivery failure.

   If the caching server receives a response with these response codes,
   it 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 RCODE 1 ("Format error") would cause a similar effect, though
   the authors have not seen such implementations yet.

   Another type of authoritative servers returns broken responses to
   AAAA queries.  Returning a response whose RR type is AAAA with the
   length of the RDATA being 4 bytes is a known behavior of this
   category.  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 as follows:

     www.bad.example. 600 IN AAAA 192.0.2.1

   which is, of course, bogus (or at least meaningless).

   A widely deployed caching server implementation transparently returns
   the broken response (and caches it) to the stub resolver.  Another
   known server implementation parses the response by itself, and sends
   a separate response with RCODE 2 ("Server failure").

   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.

   Some authoritative servers respond to AAAA queries in a way that
   causes lame delegation.  In this case, the parent zone specifies that
   the authoritative server should have the authority of a zone, but the
   server should 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 RCODE
   2 ("Server failure") 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 RCODE 2, since all the servers are known to be "lame."

   There is also an implementation that relaxes the behavior a little
   bit.  It tries to avoid using the lame server, but 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 Server
   failure.  However, this still causes redundant AAAA queries, as
   explained in the previous sections.

   The CERT/CC pointed out that the response with RCODE 3 ("Name
   Error"), described in Section 4.2, can be used for a denial of
   service attack [2].  The same argument applies to the case of "lame
   delegation", described in Section 4.5, with a certain type of caching
   server.

   Erik Nordmark encouraged the authors to publish this document as an
   RFC.  Akira Kato and Paul Vixie reviewed a preliminary version of
   this document.  Pekka Savola carefully reviewed a previous version
   and provided detailed comments.  Bill Fenner, Scott Hollenbeck,
   Thomas Narten, and Alex Zinin reviewed and helped improve the
   document at the last stage for publication.

   [1]  Mockapetris, P., "Domain names - concepts and facilities", STD
        13, 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>.

   MORISHITA Orange Yasuhiro
   Research and Development Department, Japan Registry Services Co.,Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065
   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

   Copyright (C) The Internet Society (2005).

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