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Internet Engineering Task Force P. Barber
Internet-Draft J. Brady
M. Larson
VeriSign, Inc.
November 9, 2001 Expires: May 9, 2002
Observed DNS Resolution Misbehavior
<draft-larson-bad-dns-res-00.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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.
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
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This Internet-Draft describes DNS name server and stub resolver
behavior that results in a significant query volume sent to the
root and top-level domain (TLD) name servers. In some cases we
recommend minor additions to the DNS protocol specification and
corresponding changes in name server implementations to alleviate
these unnecessary queries. In one case, we have highlighted
behavior of a popular name server implementation that does not
conform to the DNS specification. The recommendations made in this
document are a direct byproduct of observation and analysis of
abnormal query traffic patterns seen at two of the thirteen root
name servers and all thirteen com/net/org TLD name servers.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC2119].
Table of contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
2. Observed name server misbehavior . . . . . . . . . . . . . 4
2.1 Aggressive requerying for delegation information . . . . 4
2.1.1 Recommendation . . . . . . . . . . . . . . . . . . . . 5
2.2 Repeated queries to lame servers . . . . . . . . . . . . 5
2.2.1 Recommendation . . . . . . . . . . . . . . . . . . . . 6
2.3 Incomplete negative caching implementation . . . . . . . 6
2.3.1 Recommendation . . . . . . . . . . . . . . . . . . . . 6
2.4 Inability to follow multiple levels of out-of-zone glue . 6
2.4.1 Recommendation . . . . . . . . . . . . . . . . . . . . 7
3. Observed client misbehavior . . . . . . . . . . . . . . . 8
4. IANA considerations . . . . . . . . . . . . . . . . . . . 9
5. Security considerations . . . . . . . . . . . . . . . . . 10
6. Internationalization considerations . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Author's addresses . . . . . . . . . . . . . . . . . . . . 13
A. Full copyright statement . . . . . . . . . . . . . . . . . 14
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1. Introduction
Observation of query traffic received by two root name servers and
the thirteen com/net/org TLD name servers has revealed that a large
proportion of the total traffic often consists of "requeries". A
requery is the same question (<qname, qtype, qclass>) asked
repeatedly at an unexpectedly high rate. We have observed
requeries from both a single IP address and multiple IP addresses.
By analyzing requery events we have found that the cause of the
duplicate traffic is almost always a deficient name server, stub
resolver and/or application implementation combined with an
operational anomaly. The implementation deficiencies we have
identified to date include well-intentioned recovery attempts gone
awry, insufficient caching of failures, early abort when multiple
levels of glue records must be followed, and aggressive retry by
stub resolvers and/or applications. Anomalies that we have seen
trigger requery events include lame delegations, unusual glue
records, and anything that makes all authoritative name servers for
a zone unreachable (DoS attacks, crashes, maintenance, routing
failures, congestion, etc.).
In the following sections, we provide a detailed explanation of the
observed behavior and recommend changes that will reduce the
requery rate. Some of the changes recommended affect the core DNS
protocol specification, described principally in [RFC1034],
[RFC1035] and [RFC2181].
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2. Observed name server misbehavior
2.1 Aggressive requerying for delegation information
There can be times when every name server in a zone's NS RRset is
unreachable (e.g., during a network outage), unavailable (e.g., the
name server process is not running on the server host) or
misconfigured (e.g., the name server is not authoritative for the
given zone, also known as "lame"). Consider a name server that
attempts to resolve a recursive query for a domain name in such a
zone and discovers that none of the zone's name servers can provide
an answer. We have observed a recursive name server implementation
that then verifies the zone's NS RRset in its cache by querying for
the zone's delegation information: it sends a query for the zone's
NS RRset to one of the parent zone's name servers.
For example, suppose that example.com has the following NS RRset:
example.com. IN NS ns1.example.com.
example.com. IN NS ns2.example.com.
Upon receipt of a query for www.example.com and assuming that
neither ns1.example.com nor ns2.example.com can provide an answer,
this recursing name server implementation immediately queries a com
zone name server for the example.com NS RRset to verify it has the
proper delegation information. This name server implementation
performs this query to a zone's parent zone for each recursive
query it receives that fails because of a completely unresponsive
set of name servers for the target zone. Consider the effect when
a popular zone experiences a catastrophic failure of all its name
servers: now every recursive query for domain names in that zone
sent to this name server implementation results in a query to the
failed zone's parent name servers. On one occasion when several
dozen popular zones became unreachable, the query load to the
com/net/org name servers increased by 50%.
We believe this verification query is not reasonable. Consider the
circumstances: When a recursing name server is resolving a query
for a domain name in a zone it has not previously searched, it uses
the list of name servers in the referral from the target zone's
parent. If on its first attempt to search the target zone, none of
the name servers in the referral are reachable, a verification
query to the parent is pointless: this query to the parent would
come so quickly on the heels of the referral that it would be
almost certain to contain the same list of name servers. The
chance of discovering any new information is slim.
The other possibility is that the recursing name server
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successfully contacts one of the target zone's name servers and
then caches the NS RRset from the authority section of a response,
the proper behavior according to section 5.4.1 of [RFC2181],
because the NS RRset from the target zone is more trustworthy than
delegation information from the parent zone. If, while processing
a subsequent recursive query, the recursing name server discovers
that none of the name servers specified in the cached NS RRset is
available or authoritative, querying the parent would be wrong. An
NS RRset from the parent zone would now be less trustworthy than
data already in the cache.
For this query of the parent zone to be useful, the target zone's
entire set of name servers would have to change AND the former set
of name servers would have to be deconfigured and/or decomissioned
AND the delegation information in the parent zone would have to be
updated with the new set of name servers, all within the TTL of the
target zone's NS RRset. We believe this scenario is uncommon:
administrative best practices dictate that changes to a zone's set
of name servers happen gradually, with servers that are removed
from the NS RRset left authoritative for the zone as long as
possible. The scenarios that we can envision that would benefit
from the parent requery behavior do not outweigh its damaging
effects.
2.1.1 Recommendation
Name servers offering recursion MUST NOT send a query for the NS
RRset of a non-responsive zone to any of the name servers for that
zone's parent zone. For the purposes of this injunction, a non-
responsive zone is defined as a zone for which every name server
listed in the zone's NS RRset:
(1) is not authoritative for the zone (i.e., lame), or,
(2) returns a server failure response (SERVFAIL), or,
(3) is dead or unreachable according to section 7.2 of [RFC2308].
2.2 Repeated queries to lame servers
Section 2.1 describes a catastrophic failure: when every name
server for a zone is unable to provide an answer for one reason or
another. A more common occurrence is a subset of a zone's name
servers being unavailable or misconfigured. Different failure
modes have different expected durations. Some symptoms indicate
problems that are potentially transient: various types of ICMP
unreachable messages because a name server process is not running
or a host or network is unreachable, or a complete lack of a
response to a query. Such responses could be the result of a host
rebooting or temporary outages; these events don't necessarily
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require any human intervention and can be reasonably expected to be
temporary.
Other symptoms clearly indicate a condition requiring human
intervention, such as lame server: if a name server is
misconfigured and not authoritative for a zone delegated to it, it
is reasonable to assume that this condition has potential to last
longer than unreachability or unresponsiveness. Consequently,
repeated queries to known lame servers are not useful. In this
case of a condition with potential to persist for a long time, a
better practice would be to maintain a list of known lame servers
and avoid querying them repeatedly in a short interval.
2.2.1 Recommendation
Name servers offering recursion SHOULD cache name servers that they
discover are not authoritative for zones delegated to them (i.e.
lame servers). Lame servers MUST be cached against the specific
query tuple <zone name, class, server IP address>. Zone name can
be derived from the owner name of the NS record that was referenced
to query the name server that was discovered to be lame.
Implementations that perform lame server caching MUST refrain from
sending queries to known lame servers based on a time interval from
when the server is discovered to be lame. A minimum interval of
thirty minutes is RECOMMENDED.
2.3 Incomplete negative caching implementation
A widely distributed name server implementation does not properly
implement negative caching as described in [RFC2308]. In
particular, this implementation does not cache NODATA responses.
Such a response indicates that the queried domain name exists but
has no records of the desired type. See section 2.2 of [RFC2308]
for information on how NODATA responses are indicated.
2.3.1 Recommendation
Vendors of any name server implementations that do not comply with
[RFC2308] are encouraged to bring their software into conformance.
2.4 Inability to follow multiple levels of out-of-zone glue
Some name server implementations are unable to follow more than one
level of out-of-zone glue. For example, consider the following
delegations:
foo.example. IN NS ns1.example.com.
foo.example. IN NS ns2.example.com.
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example.com. IN NS ns1.test.example.net.
example.com. IN NS ns2.test.example.net.
test.example.net. IN NS ns1.test.example.net.
test.example.net. IN NS ns2.test.example.net.
A name server processing a recursive query for www.foo.example must
follow two levels of indirection, first obtaining address records
for ns1.test.example.net and/or ns2.test.example.net in order to
obtain address records for ns1.example.com and/or ns2.example.com
in order to query those name servers for the address records of
www.foo.example. While this situation may appear contrived, we
have seen multiple similar occurrences and expect more as the new
generic top-level domains (gTLDs) become active. We anticipate
many zones in the new gTLDs will use name servers in other gTLDs,
increasing the amount of inter-zone glue.
2.4.1 Recommendation
Certainly constructing a delegation that relies on multiple levels
of out-of-zone glue is not a good administrative practice. This
issue could be mitigated with an operational injunction in an RFC
to refrain from construction of such delegations. In our opinion
the practice is widespread enough to merit clarifications to the
DNS protocol specification to permit it on a limited basis.
Name servers offering recursion SHOULD be able to handle at least
three levels of indirection resulting from out-of-zone glue.
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3. Observed client misbehavior
We have observed situations where a zone's name servers are
misconfigured or unavailable, resulting in a SERVFAIL response from
a recursive name server in response to queries for domain names in
that zone. In some instances, we then observe many repeated
queries (on the order of hundreds per second) to the com/net/org
name servers for domain names in the affected zones. Sometimes the
queries originate from multiple source IP addresses, while at other
times a single source address sends many repeated queries. This
behavior appears to be triggered by a SERVFAIL response (i.e., upon
investigation, the <qname, qtype, qclass> of a repeated query at
the com/net/org name servers produces a SERVFAIL response when sent
to a local recursive name server.)
We suspect that some DNS clients (i.e., stub resolvers) and/or
application programs have overzealous retransmission algorithms
that are trigged by a SERVFAIL response. Unfortunately, we have
not been able to isolate particular implementations. The authors
encourage and welcome reports of DNS clients and applications with
overzealous retransmission algorithms.
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4. IANA considerations
There are no new IANA considerations introduced by this Internet-
Draft.
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5. Security considerations
Nameserver, stub resolver and application misbehaviors identical or
similar to those observed and discussed in this document expose
root and TLD name server constellations to increased risk of both
intentional and unintentional denial of service.
We believe that implementation of the recommendations offered in
this document will reduce the requery traffic seen at root and TLD
name servers, thus reducing the opportunity for an attacker to use
such requerying to his or her advantage.
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6. Internationalization considerations
We do not believe this document introduces any new
internationalization considerations to the DNS protocol
specification.
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7. References
[RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
November 1987.
[RFC1035] - Domain Names - Implementation and Specifications, P.
Mockapetris, November 1987.
[RFC2119] - Key Words for Use in RFCs to Indicate Requirement
Levels, S. Bradner, March 1997.
[RFC2181] - Clarifications to the DNS Specification, R. Elz, R.
Bush, July 1997.
[RFC2308] - Negative Caching of DNS Queries (DNS NCACHE), M.
Andrews, March 1998.
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8. Authors' addresses
Piet Barber
VeriSign Global Registry Services
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
pbarber@verisign.com
John Brady
VeriSign Global Registry Services
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
jbrady@verisign.com
Matt Larson
VeriSign Global Registry Services
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
mlarson@verisign.com
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A. Full copyright statement
Copyright (C) The Internet Society 2001. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain
it or assist in its implementation may be prepared, copied,
published and distributed, in whole or in part, without restriction
of any kind, provided that the above copyright notice and this
paragraph are included on all such copies and derivative works.
However, this document itself may not be modified in any way, such
as by removing the copyright notice or references to the Internet
Society or other Internet organizations, except as needed for the
purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet Standards process
must be followed, or as required to translate it into languages
other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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