RFC4074 - Common Misbehavior Against DNS Queries for IPv6 Addresses

王朝other·作者佚名 2008-05-31

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Network Working Group Y. Morishita

Request for Comments: 4074 JPRS

Category: Informational T. Jinmei

Toshiba

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.

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.

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

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

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.

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.

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.

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

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

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.jp

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