DNS Operations M. Larson
Internet-Draft P. Barber
Expires: December 22, 2003 VeriSign
June 23, 2003
Observed DNS Resolution Misbehavior
draft-ietf-dnsop-bad-dns-res-01
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
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 TLD
name servers.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
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
Normative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . 13
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1. Introduction
Observation of query traffic received by two root name servers and
the thirteen com/net 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 RFC 1034 [2], RFC 1035 [3]
and RFC 2181 [4].
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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
recursive 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 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 successfully
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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 RFC 2181 [4], 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 (RCODE=2), or,
3. is dead or unreachable according to section 7.2 of RFC 2308 [5].
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
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outages; these events don't necessarily 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 RFC 2308 [5]. 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 RFC 2308 [5] for
information on how NODATA responses are indicated.
2.3.1 Recommendation
Vendors of any name server implementations that do not comply with
RFC 2308 [5] 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 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 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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Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[3] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[4] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
[5] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC
2308, March 1998.
Authors' Addresses
Matt Larson
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
EMail: mlarson@verisign.com
Piet Barber
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
EMail: pbarber@verisign.com
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