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BEST CURRENT PRACTICE

Network Working Group                                          M. Larson
Request for Comments: 4697                                     P. Barber
BCP: 123                                                  VeriSign, Inc.
Category: Best Current Practice                             October 2006


                  Observed DNS Resolution Misbehavior

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo describes DNS iterative resolver behavior that results in a
   significant query volume sent to the root and top-level domain (TLD)
   name servers.  We offer implementation advice to iterative resolver
   developers to alleviate these unnecessary queries.  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.

Table of Contents

   1. Introduction ....................................................2
      1.1. A Note about Terminology in this Memo ......................3
      1.2. Key Words ..................................................3
   2. Observed Iterative Resolver Misbehavior .........................3
      2.1. Aggressive Requerying for Delegation Information ...........3
           2.1.1. Recommendation ......................................5
      2.2. Repeated Queries to Lame Servers ...........................6
           2.2.1. Recommendation ......................................6
      2.3. Inability to Follow Multiple Levels of Indirection .........7
           2.3.1. Recommendation ......................................7
      2.4. Aggressive Retransmission when Fetching Glue ...............8
           2.4.1. Recommendation ......................................9
      2.5. Aggressive Retransmission behind Firewalls .................9
           2.5.1. Recommendation .....................................10
      2.6. Misconfigured NS Records ..................................10
           2.6.1. Recommendation .....................................11




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      2.7. Name Server Records with Zero TTL .........................11
           2.7.1. Recommendation .....................................12
      2.8. Unnecessary Dynamic Update Messages .......................12
           2.8.1. Recommendation .....................................13
      2.9. Queries for Domain Names Resembling IPv4 Addresses ........13
           2.9.1. Recommendation .....................................14
      2.10. Misdirected Recursive Queries ............................14
           2.10.1. Recommendation ....................................14
      2.11. Suboptimal Name Server Selection Algorithm ...............15
           2.11.1. Recommendation ....................................15
   3. Security Considerations ........................................16
   4. Acknowledgements ...............................................16
   5. Internationalization Considerations ............................16
   6. References .....................................................16
      6.1. Normative References ......................................16
      6.2. Informative References ....................................16

1.  Introduction

   Observation of query traffic received by two root name servers and
   the thirteen com/net Top-Level Domain (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
   (i.e., the same query received simultaneously from multiple IP
   addresses).

   By analyzing requery events, we have found that the cause of the
   duplicate traffic is almost always a deficient iterative resolver,
   stub resolver, 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 indirection must be followed, and aggressive retry by stub
   resolvers 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 (Denial of Service (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.  None of the changes recommended affects the core DNS protocol
   specification; instead, this document consists of guidelines to
   implementors of iterative resolvers.





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1.1.  A Note about Terminology in This Memo

   To recast an old saying about standards, the nice thing about DNS
   terms is that there are so many of them to choose from.  Writing or
   talking about DNS can be difficult and can cause confusion resulting
   from a lack of agreed-upon terms for its various components.  Further
   complicating matters are implementations that combine multiple roles
   into one piece of software, which makes naming the result
   problematic.  An example is the entity that accepts recursive
   queries, issues iterative queries as necessary to resolve the initial
   recursive query, caches responses it receives, and which is also able
   to answer questions about certain zones authoritatively.  This entity
   is an iterative resolver combined with an authoritative name server
   and is often called a "recursive name server" or a "caching name
   server".

   This memo is concerned principally with the behavior of iterative
   resolvers, which are typically found as part of a recursive name
   server.  This memo uses the more precise term "iterative resolver",
   because the focus is usually on that component.  In instances where
   the name server role of this entity requires mentioning, this memo
   uses the term "recursive name server".  As an example of the
   difference, the name server component of a recursive name server
   receives DNS queries and the iterative resolver component sends
   queries.

   The advent of IPv6 requires mentioning AAAA records as well as A
   records when discussing glue.  To avoid continuous repetition and
   qualification, this memo uses the general term "address record" to
   encompass both A and AAAA records when a particular situation is
   relevant to both types.

1.2.  Key Words

   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].

2.  Observed Iterative Resolver 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 an iterative resolver
   that attempts to resolve a query for a domain name in such a zone and



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   discovers that none of the zone's name servers can provide an answer.
   We have observed a recursive name server implementation whose
   iterative resolver 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.  (Note
   that queries with QTYPE=NS are not required by the standard
   resolution algorithm described in Section 4.3.2 of RFC 1034 [2].
   These NS queries represent this implementation's addition to that
   algorithm.)

   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 iterative resolver implementation immediately queries a
   "com" zone name server for the "example.com" NS RRSet to verify that
   it has the proper delegation information.  This 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
   recursive 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 on the com/net name
   servers increased by 50%.

   We believe this verification query is not reasonable.  Consider the
   circumstances: when an iterative resolver 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 is reachable, a verification query to the
   parent would be 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 iterative resolver 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 RFC 2181 [3], 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 iterative resolver discovers that none of the name servers



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   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 or decommissioned AND the
   delegation information in the parent zone would have to be updated
   with the new set of name servers, all within the Time to Live (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 when at all possible, with servers
   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.

   This section should not be understood to claim that all queries to a
   zone's parent are bad.  In some cases, such queries are not only
   reasonable but required.  Consider the situation when required
   information, such as the address of a name server (i.e., the address
   record corresponding to the RDATA of an NS record), has timed out of
   an iterative resolver's cache before the corresponding NS record.  If
   the name of the name server is below the apex of the zone, then the
   name server's address record is only available as glue in the parent
   zone.  For example, consider this NS record:

     example.com.        IN   NS   ns.example.com.

   If a cache has this NS record but not the address record for
   "ns.example.com", it is unable to contact the "example.com" zone
   directly and must query the "com" zone to obtain the address record.
   Note, however, that such a query would not have QTYPE=NS according to
   the standard resolution algorithm.

2.1.1.  Recommendation

   An iterative resolver 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 [4].




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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 when a subset of a zone's name servers is
   unavailable or misconfigured.  Different failure modes have different
   expected durations.  Some symptoms indicate problems that are
   potentially transient, for example, 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 do not 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.

   It should also be noted, however, that some authoritative name server
   implementations appear to be lame only for queries of certain types
   as described in RFC 4074 [5].  In this case, it makes sense to retry
   the "lame" servers for other types of queries, particularly when all
   known authoritative name servers appear to be "lame".

2.2.1.  Recommendation

   Iterative resolvers SHOULD cache name servers that they discover are
   not authoritative for zones delegated to them (i.e., lame servers).
   If this caching is performed, 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 for a configurable time
   interval after the server is discovered to be lame.  A minimum
   interval of thirty minutes is RECOMMENDED.








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   An exception to this recommendation occurs if all name servers for a
   zone are marked lame.  In that case, the iterative resolver SHOULD
   temporarily ignore the servers' lameness status and query one or more
   servers.  This behavior is a workaround for the type-specific
   lameness issue described in the previous section.

   Implementors should take care not to make lame server avoidance logic
   overly broad: note that a name server could be lame for a parent zone
   but not a child zone, e.g., lame for "example.com" but properly
   authoritative for "sub.example.com".  Therefore, a name server should
   not be automatically considered lame for subzones.  In the case
   above, even if a name server is known to be lame for "example.com",
   it should be queried for QNAMEs at or below "sub.example.com" if an
   NS record indicates that it should be authoritative for that zone.

2.3.  Inability to Follow Multiple Levels of Indirection

   Some iterative resolver implementations are unable to follow
   sufficient levels of indirection.  For example, consider the
   following delegations:

     foo.example.        IN   NS   ns1.example.com.
     foo.example.        IN   NS   ns2.example.com.

     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.

   An iterative resolver resolving the name "www.foo.example" must
   follow two levels of indirection, first obtaining address records for
   "ns1.test.example.net" or "ns2.test.example.net" in order to obtain
   address records for "ns1.example.com" or "ns2.example.com" in order
   to query those name servers for the address records of
   "www.foo.example".  Although this situation may appear contrived, we
   have seen multiple similar occurrences and expect more as new generic
   top-level domains (gTLDs) become active.  We anticipate many zones in
   new gTLDs will use name servers in existing gTLDs, increasing the
   number of delegations using out-of-zone name servers.

2.3.1.  Recommendation

   Clearly constructing a delegation that relies on multiple levels of
   indirection is not a good administrative practice.  However, the
   practice is widespread enough to require that iterative resolvers be
   able to cope with it.  Iterative resolvers SHOULD be able to handle
   arbitrary levels of indirection resulting from out-of-zone name



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   servers.  Iterative resolvers SHOULD implement a level-of-effort
   counter to avoid loops or otherwise performing too much work in
   resolving pathological cases.

   A best practice that avoids this entire issue of indirection is to
   name one or more of a zone's name servers in the zone itself.  For
   example, if the zone is named "example.com", consider naming some of
   the name servers "ns{1,2,...}.example.com" (or similar).

2.4.  Aggressive Retransmission when Fetching Glue

   When an authoritative name server responds with a referral, it
   includes NS records in the authority section of the response.
   According to the algorithm in Section 4.3.2 of RFC 1034 [2], the name
   server should also "put whatever addresses are available into the
   additional section, using glue RRs if the addresses are not available
   from authoritative data or the cache."  Some name server
   implementations take this address inclusion a step further with a
   feature called "glue fetching".  A name server that implements glue
   fetching attempts to include address records for every NS record in
   the authority section.  If necessary, the name server issues multiple
   queries of its own to obtain any missing address records.

   Problems with glue fetching can arise in the context of
   "authoritative-only" name servers, which only serve authoritative
   data and ignore requests for recursion.  Such an entity will not
   normally generate any queries of its own.  Instead it answers non-
   recursive queries from iterative resolvers looking for information in
   zones it serves.  With glue fetching enabled, however, an
   authoritative server invokes an iterative resolver to look up an
   unknown address record to complete the additional section of a
   response.

   We have observed situations where the iterative resolver of a glue-
   fetching name server can send queries that reach other name servers,
   but is apparently prevented from receiving the responses.  For
   example, perhaps the name server is authoritative-only and therefore
   its administrators expect it to receive only queries and not
   responses.  Perhaps unaware of glue fetching and presuming that the
   name server's iterative resolver will generate no queries, its
   administrators place the name server behind a network device that
   prevents it from receiving responses.  If this is the case, all
   glue-fetching queries will go unanswered.

   We have observed name server implementations whose iterative
   resolvers retry excessively when glue-fetching queries are
   unanswered.  A single com/net name server has received hundreds of
   queries per second from a single such source.  Judging from the



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   specific queries received and based on additional analysis, we
   believe these queries result from overly aggressive glue fetching.

2.4.1.  Recommendation

   Implementers whose name servers support glue fetching SHOULD take
   care to avoid sending queries at excessive rates.  Implementations
   SHOULD support throttling logic to detect when queries are sent but
   no responses are received.

2.5.  Aggressive Retransmission behind Firewalls

   A common occurrence and one of the largest sources of repeated
   queries at the com/net and root name servers appears to result from
   resolvers behind misconfigured firewalls.  In this situation, an
   iterative resolver is apparently allowed to send queries through a
   firewall to other name servers, but not receive the responses.  The
   result is more queries than necessary because of retransmission, all
   of which are useless because the responses are never received.  Just
   as with the glue-fetching scenario described in Section 2.4, the
   queries are sometimes sent at excessive rates.  To make matters
   worse, sometimes the responses, sent in reply to legitimate queries,
   trigger an alarm on the originator's intrusion detection system.  We
   are frequently contacted by administrators responding to such alarms
   who believe our name servers are attacking their systems.

   Not only do some resolvers in this situation retransmit queries at an
   excessive rate, but they continue to do so for days or even weeks.
   This scenario could result from an organization with multiple
   recursive name servers, only a subset of whose iterative resolvers'
   traffic is improperly filtered in this manner.  Stub resolvers in the
   organization could be configured to query multiple recursive name
   servers.  Consider the case where a stub resolver queries a filtered
   recursive name server first.  The iterative resolver of this
   recursive name server sends one or more queries whose replies are
   filtered, so it cannot respond to the stub resolver, which times out.
   Then the stub resolver retransmits to a recursive name server that is
   able to provide an answer.  Since resolution ultimately succeeds the
   underlying problem might not be recognized or corrected.  A popular
   stub resolver implementation has a very aggressive retransmission
   schedule, including simultaneous queries to multiple recursive name
   servers, which could explain how such a situation could persist
   without being detected.








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2.5.1.  Recommendation

   The most obvious recommendation is that administrators SHOULD take
   care not to place iterative resolvers behind a firewall that allows
   queries, but not the resulting replies, to pass through.

   Iterative resolvers SHOULD take care to avoid sending queries at
   excessive rates.  Implementations SHOULD support throttling logic to
   detect when queries are sent but no responses are received.

2.6.  Misconfigured NS Records

   Sometimes a zone administrator forgets to add the trailing dot on the
   domain names in the RDATA of a zone's NS records.  Consider this
   fragment of the zone file for "example.com":

     $ORIGIN example.com.
     example.com.      3600   IN   NS   ns1.example.com  ; Note missing
     example.com.      3600   IN   NS   ns2.example.com  ; trailing dots

   The zone's authoritative servers will parse the NS RDATA as
   "ns1.example.com.example.com" and "ns2.example.com.example.com" and
   return NS records with this incorrect RDATA in responses, including
   typically the authority section of every response containing records
   from the "example.com" zone.

   Now consider a typical sequence of queries.  An iterative resolver
   attempting to resolve address records for "www.example.com" with no
   cached information for this zone will query a "com" authoritative
   server.  The "com" server responds with a referral to the
   "example.com" zone, consisting of NS records with valid RDATA and
   associated glue records.  (This example assumes that the
   "example.com" zone delegation information is correct in the "com"
   zone.)  The iterative resolver caches the NS RRSet from the "com"
   server and follows the referral by querying one of the "example.com"
   authoritative servers.  This server responds with the
   "www.example.com" address record in the answer section and,
   typically, the "example.com" NS records in the authority section and,
   if space in the message remains, glue address records in the
   additional section.  According to Section 5.4.1 of RFC 2181 [3], NS
   records in the authority section of an authoritative answer are more
   trustworthy than NS records from the authority section of a non-
   authoritative answer.  Thus, the "example.com" NS RRSet just received
   from the "example.com" authoritative server overrides the
   "example.com" NS RRSet received moments ago from the "com"
   authoritative server.





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   But the "example.com" zone contains the erroneous NS RRSet as shown
   in the example above.  Subsequent queries for names in "example.com"
   will cause the iterative resolver to attempt to use the incorrect NS
   records and so it will try to resolve the nonexistent names
   "ns1.example.com.example.com" and "ns2.example.com.example.com".  In
   this example, since all of the zone's name servers are named in the
   zone itself (i.e., "ns1.example.com.example.com" and
   "ns2.example.com.example.com" both end in "example.com") and all are
   bogus, the iterative resolver cannot reach any "example.com" name
   servers.  Therefore, attempts to resolve these names result in
   address record queries to the "com" authoritative servers.  Queries
   for such obviously bogus glue address records occur frequently at the
   com/net name servers.

2.6.1.  Recommendation

   An authoritative server can detect this situation.  A trailing dot
   missing from an NS record's RDATA always results by definition in a
   name server name that exists somewhere under the apex of the zone
   that the NS record appears in.  Note that further levels of
   delegation are possible, so a missing trailing dot could
   inadvertently create a name server name that actually exists in a
   subzone.

   An authoritative name server SHOULD issue a warning when one of a
   zone's NS records references a name server below the zone's apex when
   a corresponding address record does not exist in the zone AND there
   are no delegated subzones where the address record could exist.

2.7.  Name Server Records with Zero TTL

   Sometimes a popular com/net subdomain's zone is configured with a TTL
   of zero on the zone's NS records, which prohibits these records from
   being cached and will result in a higher query volume to the zone's
   authoritative servers.  The zone's administrator should understand
   the consequences of such a configuration and provision resources
   accordingly.  A zero TTL on the zone's NS RRSet, however, carries
   additional consequences beyond the zone itself: if an iterative
   resolver cannot cache a zone's NS records because of a zero TTL, it
   will be forced to query that zone's parent's name servers each time
   it resolves a name in the zone.  The com/net authoritative servers do
   see an increased query load when a popular com/net subdomain's zone
   is configured with a TTL of zero on the zone's NS records.

   A zero TTL on an RRSet expected to change frequently is extreme but
   permissible.  A zone's NS RRSet is a special case, however, because
   changes to it must be coordinated with the zone's parent.  In most
   zone parent/child relationships that we are aware of, there is



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   typically some delay involved in effecting changes.  Furthermore,
   changes to the set of a zone's authoritative name servers (and
   therefore to the zone's NS RRSet) are typically relatively rare:
   providing reliable authoritative service requires a reasonably stable
   set of servers.  Therefore, an extremely low or zero TTL on a zone's
   NS RRSet rarely makes sense, except in anticipation of an upcoming
   change.  In this case, when the zone's administrator has planned a
   change and does not want iterative resolvers throughout the Internet
   to cache the NS RRSet for a long period of time, a low TTL is
   reasonable.

2.7.1.  Recommendation

   Because of the additional load placed on a zone's parent's
   authoritative servers resulting from a zero TTL on a zone's NS RRSet,
   under such circumstances authoritative name servers SHOULD issue a
   warning when loading a zone.

2.8.  Unnecessary Dynamic Update Messages

   The UPDATE message specified in RFC 2136 [6] allows an authorized
   agent to update a zone's data on an authoritative name server using a
   DNS message sent over the network.  Consider the case of an agent
   desiring to add a particular resource record.  Because of zone cuts,
   the agent does not necessarily know the proper zone to which the
   record should be added.  The dynamic update process requires that the
   agent determine the appropriate zone so the UPDATE message can be
   sent to one of the zone's authoritative servers (typically the
   primary master as specified in the zone's Start of Authority (SOA)
   record's MNAME field).

   The appropriate zone to update is the closest enclosing zone, which
   cannot be determined only by inspecting the domain name of the record
   to be updated, since zone cuts can occur anywhere.  One way to
   determine the closest enclosing zone entails walking up the name
   space tree by sending repeated UPDATE messages until successful.  For
   example, consider an agent attempting to add an address record with
   the name "foo.bar.example.com".  The agent could first attempt to
   update the "foo.bar.example.com" zone.  If the attempt failed, the
   update could be directed to the "bar.example.com" zone, then the
   "example.com" zone, then the "com" zone, and finally the root zone.

   A popular dynamic agent follows this algorithm.  The result is many
   UPDATE messages received by the root name servers, the com/net
   authoritative servers, and presumably other TLD authoritative
   servers.  A valid question is why the algorithm proceeds to send
   updates all the way to TLD and root name servers.  This behavior is
   not entirely unreasonable: in enterprise DNS architectures with an



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   "internal root" design, there could conceivably be private, non-
   public TLD or root zones that would be the appropriate targets for a
   dynamic update.

   A significant deficiency with this algorithm is that knowledge of a
   given UPDATE message's failure is not helpful in directing future
   UPDATE messages to the appropriate servers.  A better algorithm would
   be to find the closest enclosing zone by walking up the name space
   with queries for SOA or NS rather than "probing" with UPDATE
   messages.  Once the appropriate zone is found, an UPDATE message can
   be sent.  In addition, the results of these queries can be cached to
   aid in determining the closest enclosing zones for future updates.
   Once the closest enclosing zone is determined with this method, the
   update will either succeed or fail and there is no need to send
   further updates to higher-level zones.  The important point is that
   walking up the tree with queries yields cacheable information,
   whereas walking up the tree by sending UPDATE messages does not.

2.8.1.  Recommendation

   Dynamic update agents SHOULD send SOA or NS queries to progressively
   higher-level names to find the closest enclosing zone for a given
   name to update.  Only after the appropriate zone is found should the
   client send an UPDATE message to one of the zone's authoritative
   servers.  Update clients SHOULD NOT "probe" using UPDATE messages by
   walking up the tree to progressively higher-level zones.

2.9.  Queries for Domain Names Resembling IPv4 Addresses

   The root name servers receive a significant number of A record
   queries where the QNAME looks like an IPv4 address.  The source of
   these queries is unknown.  It could be attributed to situations where
   a user believes that an application will accept either a domain name
   or an IP address in a given configuration option.  The user enters an
   IP address, but the application assumes that any input is a domain
   name and attempts to resolve it, resulting in an A record lookup.
   There could also be applications that produce such queries in a
   misguided attempt to reverse map IP addresses.

   These queries result in Name Error (RCODE=3) responses.  An iterative
   resolver can negatively cache such responses, but each response
   requires a separate cache entry; i.e., a negative cache entry for the
   domain name "192.0.2.1" does not prevent a subsequent query for the
   domain name "192.0.2.2".







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2.9.1.  Recommendation

   It would be desirable for the root name servers not to have to answer
   these queries: they unnecessarily consume CPU resources and network
   bandwidth.  A possible solution is to delegate these numeric TLDs
   from the root zone to a separate set of servers to absorb the
   traffic.  The "black hole servers" used by the AS 112 Project
   (http://www.as112.net), which are currently delegated the
   in-addr.arpa zones corresponding to RFC 1918 [7] private use address
   space, would be a possible choice to receive these delegations.  Of
   course, the proper and usual root zone change procedures would have
   to be followed to make such a change to the root zone.

2.10.  Misdirected Recursive Queries

   The root name servers receive a significant number of recursive
   queries (i.e., queries with the Recursion Desired (RD) bit set in the
   header).  Since none of the root servers offers recursion, the
   servers' response in such a situation ignores the request for
   recursion and the response probably does not contain the data the
   querier anticipated.  Some of these queries result from users
   configuring stub resolvers to query a root server.  (This situation
   is not hypothetical: we have received complaints from users when this
   configuration does not work as hoped.)  Of course, users should not
   direct stub resolvers to use name servers that do not offer
   recursion, but we are not aware of any stub resolver implementation
   that offers any feedback to the user when so configured, aside from
   simply "not working".

2.10.1.  Recommendation

   When the IP address of a name server that supposedly offers recursion
   is configured in a stub resolver using an interactive user interface,
   the resolver could send a test query to verify that the server indeed
   supports recursion (i.e., verify that the response has the RA bit set
   in the header).  The user could be notified immediately if the server
   is non-recursive.

   The stub resolver could also report an error, either through a user
   interface or in a log file, if the queried server does not support
   recursion.  Error reporting SHOULD be throttled to avoid a
   notification or log message for every response from a non-recursive
   server.








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2.11.  Suboptimal Name Server Selection Algorithm

   An entire document could be devoted to the topic of problems with
   different implementations of the recursive resolution algorithm.  The
   entire process of recursion is woefully under-specified, requiring
   each implementor to design an algorithm.  Sometimes implementors make
   poor design choices that could be avoided if a suggested algorithm
   and best practices were documented, but that is a topic for another
   document.

   Some deficiencies cause significant operational impact and are
   therefore worth mentioning here.  One of these is name server
   selection by an iterative resolver.  When an iterative resolver wants
   to contact one of a zone's authoritative name servers, how does it
   choose from the NS records listed in the zone's NS RRSet?  If the
   selection mechanism is suboptimal, queries are not spread evenly
   among a zone's authoritative servers.  The details of the selection
   mechanism are up to the implementor, but we offer some suggestions.

2.11.1.  Recommendation

   This list is not conclusive, but reflects the changes that would
   produce the most impact in terms of reducing disproportionate query
   load among a zone's authoritative servers.  That is, these changes
   would help spread the query load evenly.

   o  Do not make assumptions based on NS RRSet order: all NS RRs SHOULD
      be treated equally.  (In the case of the "com" zone, for example,
      most of the root servers return the NS record for
      "a.gtld-servers.net" first in the authority section of referrals.
      Apparently as a result, this server receives disproportionately
      more traffic than the other twelve authoritative servers for
      "com".)

   o  Use all NS records in an RRSet.  (For example, we are aware of
      implementations that hard-coded information for a subset of the
      root servers.)

   o  Maintain state and favor the best-performing of a zone's
      authoritative servers.  A good definition of performance is
      response time.  Non-responsive servers can be penalized with an
      extremely high response time.

   o  Do not lock onto the best-performing of a zone's name servers.  An
      iterative resolver SHOULD periodically check the performance of
      all of a zone's name servers to adjust its determination of the
      best-performing one.




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3.  Security Considerations

   The iterative resolver misbehavior discussed in this document exposes
   the root and TLD name servers to increased risk of both intentional
   and unintentional Denial of Service attacks.

   We believe that implementation of the recommendations offered in this
   document will reduce the amount of unnecessary traffic seen at root
   and TLD name servers, thus reducing the opportunity for an attacker
   to use such queries to his or her advantage.

4.  Acknowledgements

   The authors would like to thank the following people for their
   comments that improved this document: Andras Salamon, Dave Meyer,
   Doug Barton, Jaap Akkerhuis, Jinmei Tatuya, John Brady, Kevin Darcy,
   Olafur Gudmundsson, Pekka Savola, Peter Koch, and Rob Austein.  We
   apologize if we have omitted anyone; any oversight was unintentional.

5.  Internationalization Considerations

   There are no new internationalization considerations introduced by
   this memo.

6.  References

6.1.  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.

6.2.  Informative References

   [3]  Elz, R. and R. Bush, "Clarifications to the DNS Specification",
        RFC 2181, July 1997.

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

   [5]  Morishita, Y. and T. Jinmei, "Common Misbehavior Against DNS
        Queries for IPv6 Addresses", RFC 4074, May 2005.

   [6]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic
        Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April
        1997.



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   [7]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., and E.
        Lear, "Address Allocation for Private Internets", BCP 5, RFC
        1918, February 1996.

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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