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4074: Common Misbehavior Against DNS Queries for IPv6 Addresses

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4033: DNS Security Introduction and Requirements
4034: Resource Records for the DNS Security Extensions
4035: Protocol Modifications for the DNS Security Extensions
4074: Common Misbehavior Against DNS Queries for IPv6 Addresses
Network Working Group Y. Morishita
Request for Comments: 4074 JPRS
Category: Informational T. Jinmei
May 2005
Common Misbehavior Against DNS Queries for IPv6 Addresses
Status 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).
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.
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RFC 4074 Common Misbehavior Against DNS Queries May 2005
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 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
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
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.
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.
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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.
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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
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.
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RFC 4074 Common Misbehavior Against DNS Queries May 2005
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
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, <>.
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
Corporate Research & Development Center, Toshiba Corporation
1 Komukai Toshiba-cho, Saiwai-ku
Kawasaki-shi, Kanagawa 212-8582
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RFC 4074 Common Misbehavior Against DNS Queries May 2005
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Copyright (C) The Internet Society (2005).
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Morishita & Jinmei Informational [Page 6]
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