draft-ietf-dnsop-bad-dns-res-02.txt

bind 9.3结合mysql数据库

DNS Operations                                                 M. Larson
Internet-Draft                                                 P. Barber
Expires: August 16, 2004                                        VeriSign
                                                       February 16, 2004


                  Observed DNS Resolution Misbehavior
                    draft-ietf-dnsop-bad-dns-res-02

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 16, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   This Internet-Draft describes DNS name server and resolver behavior
   that results in a significant query volume sent to the root and
   top-level domain (TLD) name servers.  In some cases we recommend
   minor additions to the DNS protocol specification and corresponding
   changes in name server implementations 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.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   document are to be interpreted as described in RFC 2119 [1].

Table of Contents

   1.     Introduction . . . . . . . . . . . . . . . . . . . . . . .   3
   2.     Observed name server misbehavior . . . . . . . . . . . . .   4
   2.1    Aggressive requerying for delegation information . . . . .   4
   2.1.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .   5
   2.2    Repeated queries to lame servers . . . . . . . . . . . . .   5
   2.2.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .   6
   2.3    Inability to follow multiple levels of out-of-zone glue  .   6
   2.3.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .   7
   2.4    Aggressive retransmission when fetching glue . . . . . . .   7
   2.4.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .   8
   2.5    Aggressive retransmission behind firewalls . . . . . . . .   8
   2.5.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .   8
   2.6    Misconfigured NS records . . . . . . . . . . . . . . . . .   9
   2.6.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .  10
   2.7    Name server records with zero TTL  . . . . . . . . . . . .  10
   2.7.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .  11
   2.8    Unnecessary dynamic update messages  . . . . . . . . . . .  11
   2.8.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .  11
   2.9    Queries for domain names resembling IP addresses . . . . .  12
   2.9.1  Recommendation . . . . . . . . . . . . . . . . . . . . . .  12
   2.10   Misdirected recursive queries  . . . . . . . . . . . . . .  12
   2.10.1 Recommendation . . . . . . . . . . . . . . . . . . . . . .  13
   2.11   Suboptimal name server selection algorithm . . . . . . . .  13
   2.11.1 Recommendation . . . . . . . . . . . . . . . . . . . . . .  13
   3.     IANA considerations  . . . . . . . . . . . . . . . . . . .  15
   4.     Security considerations  . . . . . . . . . . . . . . . . .  16
   5.     Internationalization considerations  . . . . . . . . . . .  17
          Normative References . . . . . . . . . . . . . . . . . . .  18
          Authors' Addresses . . . . . . . . . . . . . . . . . . . .  18
          Intellectual Property and Copyright Statements . . . . . .  19

















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1. Introduction

   Observation of query traffic received by two root name servers and
   the thirteen com/net 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.

   By analyzing requery events we have found that the cause of the
   duplicate traffic is almost always a deficient name server, stub
   resolver and/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 glue records must be followed, and aggressive retry by stub
   resolvers and/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 (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.  Some of the changes recommended affect the core DNS protocol
   specification, described principally in RFC 1034 [2], RFC 1035 [3]
   and RFC 2181 [4].
























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2. Observed name server 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 a recursive name server
   that attempts to resolve a query for a domain name in such a zone and
   discovers that none of the zone's name servers can provide an answer.
   We have observed a recursive name server implementation that 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.

   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 recursive name server implementation immediately queries
   a "com" zone name server for the "example.com" NS RRset to verify it
   has the proper delegation information.  This name server
   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 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 a recursive name server 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 is 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 recursive name server successfully



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   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 [4], 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 recursing name server discovers that none of the name servers
   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 and/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 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, with servers that are 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.

2.1.1 Recommendation

   Name servers offering recursion 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 [5].


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 a subset of a zone's name servers being
   unavailable or misconfigured.  Different failure modes have different
   expected durations.  Some symptoms indicate problems that are
   potentially transient: 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



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   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.

2.2.1 Recommendation

   Recursive name servers SHOULD cache name servers that they discover
   are not authoritative for zones delegated to them (i.e. lame
   servers). 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 based on a time interval from
   when the server is discovered to be lame.  A minimum interval of
   thirty minutes is RECOMMENDED.

2.3 Inability to follow multiple levels of out-of-zone glue

   Some recursive name server implementations are unable to follow more
   than one level of out-of-zone glue.  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.

   A name server processing a recursive query for "www.foo.example" must
   follow two levels of indirection, first obtaining address records for
   "ns1.test.example.net" and/or "ns2.test.example.net" in order to
   obtain address records for "ns1.example.com" and/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



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   top-level domains (gTLDs) become active.  We anticipate many zones in
   the new gTLDs will use name servers in other gTLDs, increasing the
   amount of inter-zone glue.

2.3.1 Recommendation

   Clearly constructing a delegation that relies on multiple levels of
   out-of-zone glue is not a good administrative practice.  This issue
   could be mitigated with an operational injunction in an RFC to
   refrain from construction of such delegations.  In our opinion the
   practice is widespread enough to merit clarifications to the DNS
   protocol specification to permit it on a limited basis.

   Name servers offering recursion SHOULD be able to handle at least
   three levels of indirection resulting from out-of-zone glue.

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 A records for every NS record in the
   authority section.  If necessary, the name server issues multiple
   queries of its own to obtain any missing A 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 a server will not
   generate any queries of its own.  Instead it answers non-recursive
   queries from resolvers looking for information in zones it serves.