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rfc:rfc4697

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

Larson & Barber Best Current Practice [Page 1] RFC 4697 Observed DNS Resolution Misbehavior October 2006

    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.

Larson & Barber Best Current Practice [Page 2] RFC 4697 Observed DNS Resolution Misbehavior October 2006

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

Larson & Barber Best Current Practice [Page 3] RFC 4697 Observed DNS Resolution Misbehavior October 2006

 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

Larson & Barber Best Current Practice [Page 4] RFC 4697 Observed DNS Resolution Misbehavior October 2006

 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].

Larson & Barber Best Current Practice [Page 5] RFC 4697 Observed DNS Resolution Misbehavior October 2006

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.

Larson & Barber Best Current Practice [Page 6] RFC 4697 Observed DNS Resolution Misbehavior October 2006

 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

Larson & Barber Best Current Practice [Page 7] RFC 4697 Observed DNS Resolution Misbehavior October 2006

 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

Larson & Barber Best Current Practice [Page 8] RFC 4697 Observed DNS Resolution Misbehavior October 2006

 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.

Larson & Barber Best Current Practice [Page 9] RFC 4697 Observed DNS Resolution Misbehavior October 2006

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.

Larson & Barber Best Current Practice [Page 10] RFC 4697 Observed DNS Resolution Misbehavior October 2006

 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

Larson & Barber Best Current Practice [Page 11] RFC 4697 Observed DNS Resolution Misbehavior October 2006

 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".

Larson & Barber Best Current Practice [Page 13] RFC 4697 Observed DNS Resolution Misbehavior October 2006

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.

Larson & Barber Best Current Practice [Page 14] RFC 4697 Observed DNS Resolution Misbehavior October 2006

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.

Larson & Barber Best Current Practice [Page 15] RFC 4697 Observed DNS Resolution Misbehavior October 2006

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.

Larson & Barber Best Current Practice [Page 16] RFC 4697 Observed DNS Resolution Misbehavior October 2006

 [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

Larson & Barber Best Current Practice [Page 17] RFC 4697 Observed DNS Resolution Misbehavior October 2006

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Larson & Barber Best Current Practice [Page 18]

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