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Network Working Group                                          M. LarsonRequest for Comments: 4697                                     P. BarberBCP: 123                                                  VeriSign, Inc.Category: Best Current Practice                             October 2006Observed DNS Resolution MisbehaviorStatus 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 Contents1. Introduction ....................................................21.1. A Note about Terminology in this Memo ......................31.2. Key Words ..................................................32. Observed Iterative Resolver Misbehavior .........................32.1. Aggressive Requerying for Delegation Information ...........32.1.1. Recommendation ......................................52.2. Repeated Queries to Lame Servers ...........................62.2.1. Recommendation ......................................62.3. Inability to Follow Multiple Levels of Indirection .........72.3.1. Recommendation ......................................72.4. Aggressive Retransmission when Fetching Glue ...............82.4.1. Recommendation ......................................92.5. Aggressive Retransmission behind Firewalls .................92.5.1. Recommendation .....................................102.6. Misconfigured NS Records ..................................102.6.1. Recommendation .....................................11Larson & Barber          Best Current Practice                  [Page 1]

RFC 4697          Observed DNS Resolution Misbehavior       October 20062.7. Name Server Records with Zero TTL .........................112.7.1. Recommendation .....................................122.8. Unnecessary Dynamic Update Messages .......................122.8.1. Recommendation .....................................132.9. Queries for Domain Names Resembling IPv4 Addresses ........132.9.1. Recommendation .....................................142.10. Misdirected Recursive Queries ............................142.10.1. Recommendation ....................................142.11. Suboptimal Name Server Selection Algorithm ...............152.11.1. Recommendation ....................................153. Security Considerations ........................................164. Acknowledgements ...............................................165. Internationalization Considerations ............................166. References .....................................................166.1. Normative References ......................................166.2. Informative References ....................................161.  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 20061.1.  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 inRFC 2119 [1].2.  Observed Iterative Resolver Misbehavior2.1.  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 andLarson & 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 inSection 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 toSection 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 serversLarson & 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 toSection 7.2 of RFC 2308 [4].Larson & Barber          Best Current Practice                  [Page 5]

RFC 4697          Observed DNS Resolution Misbehavior       October 20062.2.  Repeated Queries to Lame ServersSection 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 inRFC 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 nameLarson & 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 inSection 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 theLarson & 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 inSection 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 20062.5.1.  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 toSection 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 isLarson & 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 inRFC 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 anLarson & Barber          Best Current Practice                 [Page 12]

RFC 4697          Observed DNS Resolution Misbehavior       October 2006   "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 20062.9.1.  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 toRFC 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 20062.11.  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 20063.  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.  References6.1.  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)",RFC2308, 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,RFC1918, 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.comLarson & Barber          Best Current Practice                 [Page 17]

RFC 4697          Observed DNS Resolution Misbehavior       October 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Larson & Barber          Best Current Practice                 [Page 18]