draft-ietf-dnsop-bad-dns-res-05.txt

非常好的dns解析软件

   authoritative server.

   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 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



Larson & Barber          Expires August 14, 2006               [Page 12]

Internet-Draft     Observed DNS Resolution Misbehavior     February 2006


   zone parent/child relationships we are aware of, there is typically
   some delay involved in effecting changes.  Further, 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 SOA 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 success.  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
   "internal root" design, there could conceivably be private, non-



Larson & Barber          Expires August 14, 2006               [Page 13]

Internet-Draft     Observed DNS Resolution Misbehavior     February 2006


   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 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 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 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".

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



Larson & Barber          Expires August 14, 2006               [Page 14]

Internet-Draft     Observed DNS Resolution Misbehavior     February 2006


   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 [8],
   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 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 immediately notified 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.

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.



Larson & Barber          Expires August 14, 2006               [Page 15]

Internet-Draft     Observed DNS Resolution Misbehavior     February 2006


   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.  I.e., 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 12 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          Expires August 14, 2006               [Page 16]

Internet-Draft     Observed DNS Resolution Misbehavior     February 2006


3.  Acknowledgments

   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.