Network Working Group Y. Morishita
Request for Comments: 4074 JPRS
Category: Informational T. Jinmei
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 (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
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
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 ). 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
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
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