Skip to main content

Common Misbehavior Against DNS Queries for IPv6 Addresses

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 4074.
Authors Yasuhiro Orange Morishita , Tatsuya Jinmei
Last updated 2013-03-02 (Latest revision 2004-10-25)
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state (None)
Document shepherd (None)
IESG IESG state RFC 4074 (Informational)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD David Kessens
IESG note
Send notices to,
IETF DNSOP Working Group                                    Y. Morishita
Internet-Draft                                                      JPRS
Expires: April 23, 2005                                        T. Jinmei
                                                        October 23, 2004

       Common Misbehavior against DNS Queries for IPv6 Addresses

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on April 23, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).


   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.

Morishita & Jinmei       Expires April 23, 2005                 [Page 1]
Internet-Draft    Common Misbehavior against AAAA Queries   October 2004

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 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 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
   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

Morishita & Jinmei       Expires April 23, 2005                 [Page 2]
Internet-Draft    Common Misbehavior against AAAA Queries   October 2004

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 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 or
   at the application which 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 the RCODE being 3 ("Name
   Error") 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 fixed their behavior as of this

4.3  Return Other Erroneous Codes

   Other authoritative servers return a response with other erroneous
   response codes than RCODE 3 ("Name Error").  One well-known such
   RCODE is 4 ("Not Implemented"), indicating 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

Morishita & Jinmei       Expires April 23, 2005                 [Page 3]
Internet-Draft    Common Misbehavior against AAAA Queries   October 2004

   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 failure of message delivery.

   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 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

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

   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
   ("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 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.

Morishita & Jinmei       Expires April 23, 2005                 [Page 4]
Internet-Draft    Common Misbehavior against AAAA Queries   October 2004

   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 ("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 the RCODE being 2, 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 Sever failure.  However, this still causes redundant AAAA
   queries as explained in the previous sections.

5.  Security Considerations

   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.

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.  Bill Fenner, Scott
   Hollenbeck, Thomas Narten, and Alex Zinin reviewed and helped improve
   the document at the last stage for publication.

7  Informative References

        1034, November 1987.

   [2]  The CERT Coordination Center, "Incorrect NXDOMAIN responses from
        AAAA queries could cause denial-of-service conditions", March
        2003, <>.

Morishita & Jinmei       Expires April 23, 2005                 [Page 5]
Internet-Draft    Common Misbehavior against AAAA Queries   October 2004

Authors' Addresses

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


   JINMEI Tatuya
   Corporate Research & Development Center, Toshiba Corporation
   1 Komukai Toshiba-cho, Saiwai-ku
   Kawasaki-shi, Kanagawa  212-8582


Appendix A.  Change History


   Changes since draft-morishita-dnsop-misbehavior-against-aaaa-00 are:

   o  Made a separate appendix and moved live examples to appendix so
      that we can remove them when this document is (ever) officially
   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
   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

Morishita & Jinmei       Expires April 23, 2005                 [Page 6]
Internet-Draft    Common Misbehavior against AAAA Queries   October 2004

   Changes since draft-ietf-dnsop-misbehavior-against-aaaa-01 are:

   o  Used the standard keywords for describing RCODEs.
   o  Provided more specific references for RFC1034.
   o  Described an additional known issue regarding RCODE 2 ("Server
      failure").  Also changed the section title accordingly.
   o  Moved the "Ignore Queries" section to the first of Section 4,
      since it looks the most widely seen misbehavior.

Morishita & Jinmei       Expires April 23, 2005                 [Page 7]
Internet-Draft    Common Misbehavior against AAAA Queries   October 2004

Intellectual Property Statement

   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 in BCP 78 and BCP 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 at

   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

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Morishita & Jinmei       Expires April 23, 2005                 [Page 8]