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Network Working Group                                       Y. MorishitaRequest for Comments: 4074                                          JPRSCategory: Informational                                        T. Jinmei                                                                 Toshiba                                                                May 2005Common Misbehavior Against DNS Queries for IPv6 AddressesStatus of This Memo   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 Notice   Copyright (C) The Internet Society (2005).Abstract   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.1.  Introduction   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.Morishita & Jinmei           Informational                      [Page 1]

RFC 4074         Common Misbehavior Against DNS Queries         May 20052.  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 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.3.  Expected Behavior   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   Sections4.3.2 and6.2.4 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.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.4.1.  Ignore Queries for AAAA   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.4.2.  Return "Name Error"   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.Morishita & Jinmei           Informational                      [Page 2]

RFC 4074         Common Misbehavior Against DNS Queries         May 2005   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.4.3.  Return Other Erroneous Codes   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.4.4.  Return a Broken Response   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.Morishita & Jinmei           Informational                      [Page 3]

RFC 4074         Common Misbehavior Against DNS Queries         May 2005   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.4.5.  Make Lame Delegation   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.Morishita & Jinmei           Informational                      [Page 4]

RFC 4074         Common Misbehavior Against DNS Queries         May 20055.  Security Considerations   The CERT/CC pointed out that the response with RCODE 3 ("Name   Error"), described inSection 4.2, can be used for a denial of   service attack [2].  The same argument applies to the case of "lame   delegation", described inSection 4.5, with a certain type of caching   server.6.  Acknowledgements   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.7.  Informative References   [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>.Authors' Addresses   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.jpMorishita & Jinmei           Informational                      [Page 5]

RFC 4074         Common Misbehavior Against DNS Queries         May 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Morishita & Jinmei           Informational                      [Page 6]