draft-ietf-dnsop-bad-dns-res-04.txt

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   referenced to query the name server that was discovered to be lame.
   Implementations that perform lame server caching MUST refrain from
   sending queries to known lame servers based on a time interval from
   when the server is discovered to be lame.  A minimum interval of
   thirty minutes is RECOMMENDED.

   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 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".  While 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.





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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
   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



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   prevents it from receiving responses.  If this is the case, all glue-
   fetching queries will go answered.

   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
   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 can't 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



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   servers, which could explain how such a situation could persist
   without being detected.

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 to pass through but not the resulting replies.

   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 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"



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   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



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   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-



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   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



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