Commit 33ea08b5 authored by Mark Andrews's avatar Mark Andrews
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4074: Common Misbehavior Against DNS Queries for IPv6 Addresses

parent 81bdad57
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
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which he or she become aware will be disclosed, in accordance with
RFC 3668.
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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.
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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
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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
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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.
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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, <>.
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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
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
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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.
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