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

非常好的dns解析软件

   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



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   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].

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
   are 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 don't 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.



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   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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