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Versions: 00 01 02 03 04 05 06 RFC 4697

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-ietf-dnsop-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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