Internet Engineering Task Force                                P. Barber
Internet-Draft                                                  J. Brady
DNS Operations                                                 M. Larson
                                                          VeriSign, Inc.
November 9, 2001
Internet-Draft                                                 P. Barber
Expires: May 9, 2002 December 22, 2003                                      VeriSign
                                                           June 23, 2003

                  Observed DNS Resolution Misbehavior

Status of this memo Memo

   This document is an Internet-Draft and is in full conformance with
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Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   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 com/net TLD
   name servers.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119]. RFC 2119 [1].

Table of contents 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
     8. Author's addresses . . . . . . . . .
         Authors' Addresses . . . . . . . . . . .  13
     A. Full copyright statement . . . . . . . . . . 12
         Intellectual Property and Copyright Statements . . . . . . .  14 13

1. Introduction

   Observation of query traffic received by two root name servers and
   the thirteen com/net/org 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 [RFC1034],
     [RFC1035] RFC 1034 [2], RFC 1035 [3]
   and [RFC2181]. RFC 2181 [4].

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 has the following NS RRset:   IN   NS   IN   NS

   Upon receipt of a query for and assuming that neither nor can provide an answer, this recursing
   recursive name server implementation immediately queries a com zone
   name server for the 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 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
   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], 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-
   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), (RCODE=2), or,

   3.  is dead or unreachable according to section 7.2 of [RFC2308]. 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
   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 [RFC2308]. 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 Section 2.2 of [RFC2308] 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

     foo.example.        IN   NS
     foo.example.        IN   NS        IN   NS        IN   NS   IN   NS   IN   NS

   A name server processing a recursive query for must
   follow two levels of indirection, first obtaining address records for and/or in order to obtain
   address records for and/or in order
   to query those name servers for the address records of 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.

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 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/org 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.

4. IANA considerations

   There are no new IANA considerations introduced by this Internet-

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.

6. Internationalization considerations

   We do not believe this document introduces any new
   internationalization considerations to the DNS protocol


Normative References

     [RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
     November 1987.

     [RFC1035] - Domain Names - Implementation and Specifications, P.
     Mockapetris, November 1987.

     [RFC2119] - Key Words

   [1]  Bradner, S., "Key words for Use use in RFCs to Indicate Requirement
     Levels, S. Bradner,
        Levels", BCP 14, RFC 2119, March 1997.


   [2]  Mockapetris, P., "Domain names - Clarifications to the DNS Specification, R. 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.

     [RFC2308] - Negative

   [5]  Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE), M.
     Andrews, NCACHE)", RFC
        2308, March 1998.


Authors' addresses

     Piet Barber
     VeriSign Global Registry Services
     21345 Ridgetop Circle
     Dulles, VA 20166-6503

     John Brady
     VeriSign Global Registry Services Addresses

   Matt Larson
   VeriSign, Inc.
   21345 Ridgetop Circle
   Dulles, VA  20166-6503

     Matt Larson
     VeriSign Global Registry Services


   Piet Barber
   VeriSign, Inc.
   21345 Ridgetop Circle
   Dulles, VA  20166-6503



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