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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 2308

   INTERNET-DRAFT                                      Mark Andrews (CSIRO)
   <draft-ietf-dnsind-ncache-09.txt>                          November 1997
   Updates: 1034, 1035
   
   
                 Negative Caching of DNS Queries (DNS NCACHE)
   
   
   Status of This Memo
   
           This document is an Internet-Draft.  Internet-Drafts are working
           documents of the Internet Engineering Task Force (IETF), its
           areas, and its working groups.  Note that other groups may also
           distribute working documents as Internet-Drafts.
   
           Internet-Drafts are draft documents valid for a maximum of six
           months and may be updated, replaced, or obsoleted by other
           documents at any time.  It is inappropriate to use Internet-
           Drafts as reference material or to cite them other than as "work
           in progress."
   
           To learn the current status of any Internet-Draft, please check
           the "1id-abstracts.txt" listing contained in the Internet-Drafts
           Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net
           (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
           Coast), or ftp.isi.edu (US West Coast).
   
   
   Abstract
   
           [RFC1034] provided a description of how to cache negative
           responses.  It however had a fundamental flaw in that it did not
           allow a name server to hand out those cached responses to other
           resolvers, thereby greatly reducing the effect of the caching.
           This document addresses issues raise in the light of experience
           and replaces [RFC1034 Section 4.3.4].
   
           Negative caching was an optional part of the DNS specification
           and deals with the caching of the non-existence of an RRset
           [RFC2181] or domain name.
   
           Negative caching is useful as it reduces the response time for
           negative answers.  It also reduces the number of messages that
           have to be sent between resolvers and name servers hence overall
           network traffic.  A large proportion of DNS traffic on the
   
   
   
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           Internet could be eliminated if all resolvers implemented
           negative caching.  With this in mind negative caching should no
           longer be seen as an optional part of a DNS resolver.
   
   1 - Terminology
   
   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 in [RFC2119].
   
   "Negative caching" - the storage of knowledge that something does not
   exist.  We can store the knowledge that a record has a particular value.
   We can also do the reverse, that is, to store the knowledge that a
   record does not exist.  It is the storage of knowledge that something
   does not exist, cannot or does not give an answer that we call negative
   caching.
   
   "QNAME" - the name in the query section of an answer, or where this
   resolves to a CNAME, or CNAME chain, the data field of the last CNAME.
   The last CNAME in this sense is that which contains a value which does
   not resolve to another CNAME.  Implementations should note that
   including CNAME records in responses in order, so that the first has the
   label from the query section, and then each in sequence has the label
   from the data section of the previous (where more than one CNAME is
   needed) allows the sequence to be processed in one pass, and
   considerably eases the task of the receiver.  Other relevant records
   (such as SIG RRs) can be interspersed amongst the CNAMEs.
   
   "NXDOMAIN" - an alternate expression for the "Name Error" RCODE as
   described in [RFC1035 Section 4.1.1] and the two terms are used
   interchangeably in this document.
   
   "NODATA" - a pseudo RCODE which indicates that the name is valid, for
   the given class, but are no records of the given type.  A NODATA
   response has to be inferred from the answer.
   
   "FORWARDER" - a nameserver used to resolve queries instead of directly
   using the authoritative nameserver chain.  The forwarder typically
   either has better access to the internet, or maintains a bigger cache
   which may be shared amongst many resolvers.  How a server is identified
   as a FORWARDER, or knows it is a FORWARDER is outside the scope of this
   document.  However if you are being used as a forwarder the query will
   have the recursion desired flag set.
   
   An understanding of [RFC1034], [RFC1035] and [RFC2065] is expected when
   
   
   
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   reading this document.
   
   2 - Negative Responses
   
   The most common negative responses indicate that a particular RRset does
   not exist in the DNS.  The first sections of this document deal with
   this case.  Other negative responses can indicate failures of a
   nameserver, those are dealt with in section 7 (Other Negative
   Responses).
   
   A negative response is indicated by one of the following conditions:
   
   2.1 - Name Error
   
   Name errors (NXDOMAIN) are indicated by the presence of "Name Error" in
   the RCODE field.  In this case the domain referred to by the QNAME does
   not exist.  Note: the answer section may have SIG and CNAME RRs and
   authority section may have SOA, NXT and SIG RRsets.
   
   It is possible to distinguish between a referral and a NXDOMAIN response
   by the presense of NXDOMAIN in the RCODE regardless of the presence of
   NS or SOA records in the authority section.
   
   NXDOMAIN responses can be categorised into four types by the contents of
   the authority section.  These are shown below along with a referral for
   comparison.  Fields not mentioned are not important in terms of the
   examples.
   
           NXDOMAIN RESPONSE: TYPE 1.
   
           Header:
               RDCODE=NXDOMAIN
           Query:
               AN.EXAMPLE. A
           Answer:
               AN.EXAMPLE. CNAME TRIPPLE.XX.
           Authority:
               XX. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
               XX. NS NS1.XX.
               XX. NS NS2.XX.
           Additional:
               NS1.XX. A 127.0.0.2
               NS2.XX. A 127.0.0.3
   
   
   
   
   
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           NXDOMAIN RESPONSE: TYPE 2.
   
           Header:
               RDCODE=NXDOMAIN
           Query:
               AN.EXAMPLE. A
           Answer:
               AN.EXAMPLE. CNAME TRIPPLE.XX.
           Authority:
               XX. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
           Additional:
               <empty>
   
   
           NXDOMAIN RESPONSE: TYPE 3.
   
           Header:
               RDCODE=NXDOMAIN
           Query:
               AN.EXAMPLE. A
           Answer:
               AN.EXAMPLE. CNAME TRIPPLE.XX.
           Authority:
               <empty>
           Additional:
               <empty>
   
   
           NXDOMAIN RESPONSE: TYPE 4
   
           Header:
               RDCODE=NXDOMAIN
           Query:
               AN.EXAMPLE. A
           Answer:
               AN.EXAMPLE. CNAME TRIPPLE.XX.
           Authority:
               XX. NS NS1.XX.
               XX. NS NS2.XX.
           Additional:
               NS1.XX. A 127.0.0.2
               NS2.XX. A 127.0.0.3
   
   
   
   
   
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           REFERRAL RESPONSE.
   
           Header:
               RDCODE=NOERROR
           Query:
               AN.EXAMPLE. A
           Answer:
               AN.EXAMPLE. CNAME TRIPPLE.XX.
           Authority:
               XX. NS NS1.XX.
               XX. NS NS2.XX.
           Additional:
               NS1.XX. A 127.0.0.2
               NS2.XX. A 127.0.0.3
   
   
   Note, in the four examples of NXDOMAIN responses, it is known that the
   name "AN.EXAMPLE." exists, and has as its value a CNAME record.  The
   NXDOMAIN refers to "TRIPPLE.XX", which is then known not to exist.  On
   the other hand, in the referral example, it is shown that "AN.EXAMPLE"
   exists, and has a CNAME RR as its value, but nothing is known one way or
   the other about the existence of "TRIPPLE.XX", other than that "NS1.XX"
   or "NS2.XX" can be consulted as the next step in obtaining information
   about it.
   
   Where no CNAME records appear, the NXDOMAIN response refers to the name
   in the label of the RR in the question section.
   
   2.1.1 Special Handling of Name Error
   
   This section deals with errors encountered when implementing negative
   caching of NXDOMAIN responses.
   
   There are a large number of resolvers currently in existence that fail
   to correctly detect and process all forms of NXDOMAIN response.  Some
   resolvers treat a TYPE 1 NXDOMAIN response as a referral.  To alleviate
   this problem it is recommended that servers that are authoritative for
   the NXDOMAIN response only send TYPE 2 NXDOMAIN responses, that is the
   authority section contains a SOA record and no NS records.  If a non-
   authoritative server sends a type 1 NXDOMAIN response to one of these
   old resolvers, the result will be an unnecessary query to an
   authoritative server.  This is undesirable, but not fatal except when
   the server is being used a FORWARDER.  If however the resolver is using
   the server as a FORWARDER to such a resolver it will be necessary to
   
   
   
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   disable the sending of TYPE 1 NXDOMAIN response to it, use TYPE 2
   NXDOMAIN instead.
   
   Some resolvers incorrectly continue processing if the authoritative
   answer flag is not set.  This is a problem when your nameserver is
   listed as a FORWARDER for such resolvers.  If the nameserver is used as
   a FORWARDER by such resolver, the authority flag will have to be forced
   on for NXDOMAIN responses to these resolvers.
   
   2.2 - No Data
   
   NODATA is indicated by an answer with the RCODE set to NOERROR and no
   relevant answers in the answer section.  The authority section will
   contain an SOA record, or there will be no NS records there.
   
   NODATA responses have to be algorithmically determined from the
   response's contents as there is no RCODE value to indicate NODATA.  In
   some cases to determine with certainty that NODATA is the correct
   response it can be necessary to send another query.
   
   The authority section may contain NXT and SIG RRsets in addition to NS
   and SOA records.  CNAME and SIG records may exist in the answer section.
   
   It is possible to distinguish between a NODATA and a referral response
   by the presence of a SOA record in the authority section or the absence
   of NS records in the authority section.
   
   NODATA responses can be categorised into three types by the contents of
   the authority section.  These are shows below along with a referral for
   comparison.  Fields not mentioned are not important in terms of the
   examples.
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
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           NODATA RESPONSE: TYPE 1.
   
           Header:
               RDCODE=NOERROR
           Query:
               ANOTHER.EXAMPLE. A
           Answer:
               <empty>
           Authority:
               EXAMPLE. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
               EXAMPLE. NS NS1.XX.
               EXAMPLE. NS NS2.XX.
           Additional:
               NS1.XX. A 127.0.0.2
               NS2.XX. A 127.0.0.3
   
   
           NO DATA RESPONSE: TYPE 2.
   
           Header:
               RDCODE=NOERROR
           Query:
               ANOTHER.EXAMPLE. A
           Answer:
               <empty>
           Authority:
               EXAMPLE. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
           Additional:
               <empty>
   
   
           NO DATA RESPONSE: TYPE 3.
   
           Header:
               RDCODE=NOERROR
           Query:
               ANOTHER.EXAMPLE. A
           Answer:
               <empty>
           Authority:
               <empty>
           Additional:
               <empty>
   
   
   
   
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           REFERRAL RESPONSE.
   
           Header:
               RDCODE=NOERROR
           Query:
               ANOTHER.EXAMPLE. A
           Answer:
               <empty>
           Authority:
               EXAMPLE. NS NS1.XX.
               EXAMPLE. NS NS2.XX.
           Additional:
               NS1.XX. A 127.0.0.2
               NS2.XX. A 127.0.0.3
   
   
   These examples, unlike the NXDOMAIN examples above, have no CNAME
   records, however they could, in just the same way that the NXDOMAIN
   examples did, in which case it would be the value of the last CNAME (the
   QNAME) for which NODATA would be concluded.
   
   2.2.1 - Special Handling of No Data
   
   There are a large number of resolvers currently in existence that fail
   to correctly detect and process all forms of NODATA response.  Some
   resolvers treat a TYPE 1 NODATA response as a referral.  To alleviate
   this problem it is recommended that servers that are authoritative for
   the NODATA response only send TYPE 2 NODATA responses, that is the
   authority section contains a SOA record and no NS records.  Sending a
   TYPE 1 NODATA response from a non-authoritative server to one of these
   resolvers will only result in an unnecessary query.  If a server is
   listed as a FORWARDER for another resolver it may also be necessary to
   disable the sending of TYPE 1 NODATA response for non-authoritative
   NODATA responses.
   
   Some name servers fail to set the RCODE to NXDOMAIN in the presence of
   CNAMEs in the answer section.  If a definitive NXDOMAIN / NODATA answer
   is required in this case the resolver must query again using the QNAME
   as the query label.
   
   3 - Negative Answers from Authoritative Servers
   
   Name servers authoritative for a zone MUST include the SOA record of the
   zone in the authority section of the response when reporting an NXDOMAIN
   
   
   
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   or indicating that no data of the requested type exists.  This is
   required so that the response may be cached.  The TTL of this record is
   set from the minimum of the MINIMUM field of the SOA record and the TTL
   of the SOA itself, and indicates how long a resolver may cache the
   negative answer.  The TTL SIG record associated with the SOA record
   should also be trimmed in line with the SOA's TTL.
   
   If the containing zone is signed [RFC2065] the SOA and appropriate NXT
   and SIG records MUST be added.
   
   4 - SOA Minimum Field
   
   The SOA minimum field has been overloaded in the past to have three
   different meanings, the minimum TTL value of all RRs in a zone, the
   default TTL of RRs which did not contain a TTL value and the TTL of
   negative responses.
   
   Despite being the original defined meaning, the first of these, the
   minimum TTL value of all RRs in a zone, has never in practice been used
   and is hereby deprecated.
   
   The second, the default TTL of RRs which contain no explicit TTL in the
   master zone file, is relevant only at the primary server.  After a zone
   transfer all RRs have explicit TTLs and it is impossible to determine
   whether the TTL for a record was explicitly set or derived from the
   default after a zone transfer.  Where a server does not require RRs to
   include the TTL value explicitly, it should provide a mechanism, not
   being the value of the MINIMUM field of the SOA record, from which the
   missing TTL values are obtained.  How this is done is implementation
   dependent.
   
   The Master File format [RFC 1035 Section 5] is extended to include the
   following directive:
   
           $TTL <TTL> [comment]
   
   All resource records appearing after the directive, and which do not
   explicitly include a TTL value, have their TTL set to the TTL given in
   the $TTL directive.  SIG records without a explicit TTL get their TTL
   from the "original TTL" of the SIG record [RFC 2065 Section 4.5].
   
   The remaining of the current meanings, of being the TTL to be used for
   negative responses, is the new defined meaning of the SOA minimum field.
   
   
   
   
   
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   5 - Caching Negative Answers
   
   Like normal answers negative answers have a time to live (TTL).  As
   there is no record in the answer section to which this TTL can be
   applied, the TTL must be carried by another method.  This is done by
   including the SOA record from the zone in the authority section of the
   reply.  When the authoritative server creates this record its TTL is
   taken from the minimum of the SOA.MINIMUM field and SOA's TTL.  This TTL
   decrements in a similar manner to a normal cached answer and upon
   reaching zero (0) indicates the cached negative answer MUST NOT be used
   again.
   
   A negative answer that resulted from a name error (NXDOMAIN) should be
   cached such that it can be retrieved and returned in response to another
   query for the same <QNAME, QCLASS> that resulted in the cached negative
   response.
   
   A negative answer that resulted from a no data error (NODATA) should be
   cached such that it can be retrieved and returned in response to another
   query for the same <QNAME, QTYPE, QCLASS> that resulted in the cached
   negative response.
   
   The NXT record, if it exists in the authority section of a negative
   answer received, MUST be stored such that it can be be located and
   returned with SOA record in the authority section, as should any SIG
   records in the authority section.  For NXDOMAIN answers there is no
   "necessary" obvious relationship between the NXT records and the QNAME.
   The NXT record MUST have the same owner name as the query name for
   NODATA responses.
   
   Negative responses without SOA records SHOULD NOT be cached as there is
   no way to prevent the negative responses looping forever between a pair
   of servers even with a short TTL.
   
   As with caching positive responses it is sensible for a resolver to
   limit for how long it will cache a negative response as the protocol
   supports caching for up to 68 years.  Such a limit should not be greater
   than that applied to positive answers and preferably be tunable.  Values
   of one to three hours have been found to work well and would make
   sensible a default.  Values exceeding one day have been found to be
   problematic.
   
   
   
   
   
   
   
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   6 - Negative answers from the cache
   
   When a server, in answering a query, encounters a cached negative
   response it MUST add the cached SOA record to the authority section of
   the response with the TTL decremented by the amount of time it was
   stored in the cache.  This allows the NXDOMAIN / NODATA response to time
   out correctly.
   
   If a NXT record was cached along with SOA record it MUST be added to the
   authority section.  If a SIG record was cached along with a NXT record
   it SHOULD be added to the authority section.
   
   As with all answers coming from the cache, negative answers SHOULD have
   an implicit referral built into the answer.  This enables the resolver
   to locate an authoritative source.  An implicit referral is
   characterised by NS records in the authority section referring the
   resolver towards a authoritative source.  NXDOMAIN types 1 and 4
   responses contain implicit referrals as does NODATA type 1 response.
   
   7 - Other Negative Responses
   
   Caching of other negative responses is not covered by any existing RFC.
   There is no way to indicate a desired TTL in these responses.  Care
   needs to be taken to ensure that there are not forwarding loops.
   
   7.1 Server Failure (OPTIONAL)
   
   Server failures fall into two major classes.  The first is where a
   server can determine that it has been misconfigured for a zone.  This
   may be where it has been listed as a server, but not configured to be a
   server for the zone, or where it has been configured to be a server for
   the zone, but cannot obtain the zone data for some reason.  This can
   occur either because the zone file does not exist or contains errors, or
   because another server from which the zone should have been available
   either did not respond or was unable or unwilling to supply the zone.
   
   The second class is where the server needs to obtain an answer from
   elsewhere, but is unable to do so, due to network failures, other
   servers that don't reply, or return server failure errors, or similar.
   
   In either case a resolver MAY cache a server failure response.  If it
   does so it MUST NOT cache it for longer than five (5) minutes, and it
   MUST be cached against the specific query tuple <query name, type,
   class, server IP address>.
   
   
   
   
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   7.2 Dead / Unreachable Server (OPTIONAL)
   
   Dead / Unreachable servers are servers that fail to respond in any way
   to a query or where the transport layer has provided an indication that
   the server does not exist or is unreachable.  A server may be deemed to
   be dead or unreachable if it has not responded to an outstanding query
   within 120 seconds.
   
   Examples of transport layer indications are:
   
           ICMP error messages indicating host, net or port unreachable.
           TCP resets
           IP stack error messages providing similar indications to those above.
   
   
   A server MAY cache a dead server indication.  If it does so it MUST NOT
   be deemed dead for longer than five (5) minutes.  The indication MUST be
   stored against query tuple <query name, type, class, server IP address>
   unless there was a transport layer indication that the server does not
   exist, in which case it applies to all queries to that specific IP
   address.
   
   8 - Changes from RFC 1034
   
   Negative caching in resolvers is no-longer optional, if a resolver
   caches anything it must also cache negative answers.
   
   Non-authoritative negative answers MAY be cached.
   
   The SOA record from the authority section MUST be cached.  Name error
   indications must be cached against the tuple <query name, QCLASS>.  No
   data indications must be cached against <query name, QTYPE, QCLASS>
   tuple.
   
   A cached SOA record must be added to the response.  This was explicitly
   not allowed because previously the distinction between a normal cached
   SOA record, and the SOA cached as a result of a negative response was
   not made, and simply extracting a normal cached SOA and adding that to a
   cached negative response causes problems.
   
   The $TTL TTL directive was added to the master file format.
   
   
   
   
   
   
   
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   9 History of Negative Caching
   
   The following is a potted history of negative caching in the DNS and
   forms no part of the technical specification of negative caching.
   
   It is interesting to note that the same concepts were re-invented in
   both the CHIVES and BIND servers.
   
   The history of the early CHIVES work (Section 9.1) was supplied by Rob
   Austein <sra@epilogue.com> and is reproduced here in the form in which
   he supplied it [MPA].
   
   
   Sometime around the spring of 1985, I mentioned to Paul Mockapetris that
   our experience with his JEEVES DNS resolver had pointed out the need for
   some kind of negative caching scheme.  Paul suggested that we simply
   cache authoritative errors, using the SOA MINIMUM value for the zone
   that would have contained the target RRs.  I'm pretty sure that this
   conversation took place before RFC-973 was written, but it was never
   clear to me whether this idea was something that Paul came up with on
   the spot in response to my question or something he'd already been
   planning to put into the document that became RFC-973.  In any case,
   neither of us was entirely sure that the SOA MINIMUM value was really
   the right metric to use, but it was available and was under the control
   of the administrator of the target zone, both of which seemed to us at
   the time to be important feature.
   
   Late in 1987, I released the initial beta-test version of CHIVES, the
   DNS resolver I'd written to replace Paul's JEEVES resolver.  CHIVES
   included a search path mechanism that was used pretty heavily at several
   sites (including my own), so CHIVES also included a negative caching
   mechanism based on SOA MINIMUM values.  The basic strategy was to cache
   authoritative error codes keyed by the exact query parameters (QNAME,
   QCLASS, and QTYPE), with a cache TTL equal to the SOA MINIMUM value.
   CHIVES did not attempt to track down SOA RRs if they weren't supplied in
   the authoritative response, so it never managed to completely eliminate
   the gratuitous DNS error message traffic, but it did help considerably.
   Keep in mind that this was happening at about the same time as the
   near-collapse of the ARPANET due to congestion caused by exponential
   growth and the the "old" (pre-VJ) TCP retransmission algorithm, so
   negative caching resulted in drasticly better DNS response time for our
   users, mailer daemons, etcetera.
   
   As far as I know, CHIVES was the first resolver to implement negative
   caching.  CHIVES was developed during the twilight years of TOPS-20, so
   
   
   
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   it never ran on very many machines, but the few machines that it did run
   on were the ones that were too critical to shut down quickly no matter
   how much it cost to keep them running.  So what few users we did have
   tended to drive CHIVES pretty hard.  Several interesting bits of DNS
   technology resulted from that, but the one that's relevant here is the
   MAXTTL configuration parameter.
   
   Experience with JEEVES had already shown that RRs often showed up with
   ridiculously long TTLs (99999999 was particularly popular for many
   years, due to bugs in the code and documentation of several early
   versions of BIND), and that robust software that blindly believed such
   TTLs could create so many strange failures that it was often necessary
   to reboot the resolver frequently just to clear this garbage out of the
   cache.  So CHIVES had a configuration parameter "MAXTTL", which
   specified the maximum "reasonable" TTL in a received RR.  RRs with TTLs
   greater than MAXTTL would either have their TTLs reduced to MAXTTL or
   would be discarded entirely, depending on the setting of another
   configuration parameter.
   
   When we started getting field experience with CHIVES's negative caching
   code, it became clear that the SOA MINIMUM value was often large enough
   to cause the same kinds of problems for negative caching as the huge
   TTLs in RRs had for normal caching (again, this was in part due to a bug
   in several early versions of BIND, where a secondary server would
   authoritatively deny all knowledge of its zones if it couldn't contact
   the primaries on reboot).  So we started running the negative cache TTLs
   through the MAXTTL check too, and continued to experiment.
   
   The configuration that seemed to work best on WSMR-SIMTEL20.ARMY.MIL
   (last of the major Internet TOPS-20 machines to be shut down, thus the
   last major user of CHIVES, thus the place where we had the longest
   experimental baseline) was to set MAXTTL to about three days.  Most of
   the traffic initiated by SIMTEL20 in its last years was mail-related,
   and the mail queue timeout was set to one week, so this gave a "stuck"
   message several tries at complete DNS resolution, without bogging down
   the system with a lot of useless queries.  Since (for reasons that now
   escape me) we only had the single MAXTTL parameter rather than separate
   ones for positive and negative caching, it's not clear how much effect
   this setting of MAXTTL had on the negative caching code.
   
   CHIVES also included a second, somewhat controversial mechanism which
   took the place of negative caching in some cases.  The CHIVES resolver
   daemon could be configured to load DNS master files, giving it the
   ability to act as what today would be called a "stealth secondary".
   That is, when configured in this way, the resolver had direct access to
   
   
   
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   authoritative information for heavily-used zones.  The search path
   mechanisms in CHIVES reflected this: there were actually two separate
   search paths, one of which only searched local authoritative zone data,
   and one which could generate normal iterative queries.  This cut down on
   the need for negative caching in cases where usage was predictably heavy
   (e.g., the resolver on XX.LCS.MIT.EDU always loaded the zone files for
   both LCS.MIT.EDU and AI.MIT.EDU and put both of these suffixes into the
   "local" search path, since between them the hosts in these two zones
   accounted for the bulk of the DNS traffic).  Not all sites running
   CHIVES chose to use this feature; C.CS.CMU.EDU, for example, chose to
   use the "remote" search path for everything because there were too many
   different sub-zones at CMU for zone shadowing to be practical for them,
   so they relied pretty heavily on negative caching even for local
   traffic.
   
   Overall, I still think the basic design we used for negative caching was
   pretty reasonable: the zone administrator specified how long to cache
   negative answers, and the resolver configuration chose the actual cache
   time from the range between zero and the period specified by the zone
   administrator.  There are a lot of details I'd do differently now (like
   using a new SOA field instead of overloading the MINIMUM field), but
   after more than a decade, I'd be more worried if we couldn't think of at
   least a few improvements.
   
   9.2 BIND
   
   While not the first attempt to get negative caching into BIND, in July
   1993, BIND 4.9.2 ALPHA, Anant Kumar of ISI supplied code that
   implemented, validation and negative caching (NCACHE).  This code had a
   10 minute TTL for negative caching and only cached the indication that
   there was a negative response, NXDOMAIN or NOERROR_NODATA. This is the
   origin of the NODATA pseudo response code mentioned above.
   
   Mark Andrews of CSIRO added code (RETURNSOA) that stored the SOA record
   such that it could be retrieved by a similar query.  UUnet complained
   that they were getting old answers after loading a new zone, and the
   option was turned off, BIND 4.9.3-alpha5, April 1994.  In reality this
   indicated that the named needed to purge the space the zone would
   occupy.  Functionality to do this was added in BIND 4.9.3 BETA11 patch2,
   December 1994.
   
   RETURNSOA was re-enabled by default, BIND 4.9.5-T1A, August 1996.
   
   
   
   
   
   
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   10 Example
   
   The following example is based on a signed zone that is empty apart from
   the nameservers.  We will query for WWW.XX.EXAMPLE showing initial
   response and again 10 minutes later.
   Note 1: during the intervening 10 minutes the NS records for XX.EXAMPLE
   have expired.
   Note 2: the TTL of the SIG records are not explicitly set in the zone
   file and are hence the TTL of the RRset they are the signature for.
   
           Zone File:
   
           $TTL 86400
           $ORIGIN XX.EXAMPLE.
           @       IN      SOA     NS1.XX.EXAMPLE. HOSTMATER.XX.EXAMPLE. (
                                   1997102000      ; serial
                                   1800    ; refresh (30 mins)
                                   900     ; retry (15 mins)
                                   604800  ; expire (7 days)
                                   1200 ) ; minimum (20 mins)
                   IN      SIG     SOA ...
             1200  IN      NXT     NS1.XX.EXAMPLE. A NXT SIG SOA NS KEY
                   IN      SIG     NXT ... XX.EXAMPLE. ...
              300  IN      NS      NS1.XX.EXAMPLE.
              300  IN      NS      NS2.XX.EXAMPLE.
                   IN      SIG     NS ... XX.EXAMPLE. ...
                   IN      KEY     0x4100 1 1 ...
                   IN      SIG     KEY ... XX.EXAMPLE. ...
                   IN      SIG     KEY ... EXAMPLE. ...
           NS1     IN      A       10.0.0.1
                   IN      SIG     A ... XX.EXAMPLE. ...
             1200  IN      NXT     NS2.XX.EXAMPLE. A NXT SIG
                   IN      SIG     NXT ...
           NS2     IN      A       10.0.0.2
                   IN      SIG     A ... XX.EXAMPLE. ...
             1200  IN      NXT     XX.EXAMPLE. A NXT SIG
                   IN      SIG     NXT ... XX.EXAMPLE. ...
   
   
   
   
   
   
   
   
   
   
   
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           Initial Response:
   
           Header:
               RDCODE=NXDOMAIN, AA=1, QR=1, TC=0
           Query:
               WWW.XX.EXAMPLE. IN A
           Answer:
               <empty>
           Authority:
               XX.EXAMPLE.      1200 IN SOA NS1.XX.EXAMPLE. ...
               XX.EXAMPLE.      1200 IN SIG SOA ... XX.EXAMPLE. ...
               NS2.XX.EXAMPLE.  1200 IN NXT XX.EXAMPLE. NXT A NXT SIG
               NS2.XX.EXAMPLE.  1200 IN SIG NXT ... XX.EXAMPLE. ...
               XX.EXAMPLE.     86400 IN NS  NS1.XX.EXAMPLE.
               XX.EXAMPLE.     86400 IN NS  NS2.XX.EXAMPLE.
               XX.EXAMPLE.     86400 IN SIG NS ... XX.EXAMPLE. ...
           Additional
               XX.EXAMPLE.     86400 IN KEY 0x4100 1 1 ...
               XX.EXAMPLE.     86400 IN SIG KEY ... EXAMPLE. ...
               NS1.XX.EXAMPLE. 86400 IN A   10.0.0.1
               NS1.XX.EXAMPLE. 86400 IN SIG A ... XX.EXAMPLE. ...
               NS2.XX.EXAMPLE. 86400 IN A   10.0.0.2
               NS3.XX.EXAMPLE. 86400 IN SIG A ... XX.EXAMPLE. ...
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
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           After 10 Minutes:
   
           Header:
               RDCODE=NXDOMAIN, AA=0, QR=1, TC=0
           Query:
               WWW.XX.EXAMPLE. IN A
           Answer:
               <empty>
           Authority:
               XX.EXAMPLE.       600 IN SOA NS1.XX.EXAMPLE. ...
               XX.EXAMPLE.       600 IN SIG SOA ... XX.EXAMPLE. ...
               NS2.XX.EXAMPLE.   600 IN NXT XX.EXAMPLE. NXT A NXT SIG
               NS2.XX.EXAMPLE.   600 IN SIG NXT ... XX.EXAMPLE. ...
               EXAMPLE.        65799 IN NS  NS1.YY.EXAMPLE.
               EXAMPLE.        65799 IN NS  NS2.YY.EXAMPLE.
               EXAMPLE.        65799 IN SIG NS ... XX.EXAMPLE. ...
           Additional
               XX.EXAMPLE.     65800 IN KEY 0x4100 1 1 ...
               XX.EXAMPLE.     65800 IN SIG KEY ... EXAMPLE. ...
               NS1.YY.EXAMPLE. 65799 IN A   10.100.0.1
               NS1.YY.EXAMPLE. 65799 IN SIG A ... EXAMPLE. ...
               NS2.YY.EXAMPLE. 65799 IN A   10.100.0.2
               NS3.YY.EXAMPLE. 65799 IN SIG A ... EXAMPLE. ...
               EXAMPLE.        65799 IN KEY 0x4100 1 1 ...
               EXAMPLE.        65799 IN SIG KEY ... . ...
   
   
   11 Security Considerations
   
   It is believed that this document does not introduce any significant
   additional security threats other that those that already exist when
   using data from the DNS.
   
   With negative caching it might be possible to propagate a denial of
   service attack by spreading a NXDOMAIN message with a very high TTL.
   Without negative caching that would be much harder.  A similar effect
   could be achieved previously by spreading a bad A record, so that the
   server could not be reached - which is almost the same.  It has the same
   effect as far as what the end user is able to do, but with a different
   psychological effect.  With the bad A, I feel "damn the network is
   broken again" and try again tomorrow.  With the "NXDOMAIN" I feel "Oh,
   they've turned off the server and it doesn't exist any more" and
   probably never bother trying this server again.
   
   
   
   
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   For such an attack to be successful, the NXDOMAIN indiction injected
   into a parent server (or a busy caching resolver).  One way this might
   be done by the use of a CNAME which results in the parent server
   querying an attackers server.  Resolvers that wish to prevent such
   attacks can query again the final QNAME ignoring any NS data in the
   query responses it has received for this query.
   
   Implementing TTL sanity checking will reduce the effectiveness of such
   an attack, because a successful attack would require re-injection of the
   bogus data at more frequent intervals.
   
   DNS Security [RFC2065] provides a mechanism to verify whether a negative
   response is valid or not, through the use of NXT and SIG records.  This
   document supports the use of that mechanism by promoting the
   transmission of the relevant security records even in a non security
   aware server.
   
   
   I would like to thank Rob Austein for his history of the CHIVES
   nameserver. The DNSIND working group, in particular Robert Elz for his
   valuable technical and editorial contributions to this document.
   
   References
   
   [RFC1034]
           Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES," STD
           13, RFC 1034, November 1987.
   
   
   [RFC1035]Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
           SPECIFICATION," STD 13, RFC 1035, November 1987.
   
   
   [RCF2065]
           Eastlake, D. 3rd. and Kaufman, C,. "Domain Name System Security
           Extensions," RFC 2065, January 1997
   
   
   [RFC2119]
           Bradner, S., "Key words for use in RFCs to Indicate Requirement
           Levels," BCP 14, RFC 2119, March 1997
   
   
   [RFC2181]
           Elz, R., and Bush, R., "Clarifications to the DNS
   
   
   
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           Specification," RFC 2181, July 1997.
   
   
   Author's Address
   
           Mark Andrews
              CSIRO - Mathematical and Information Sciences
              Locked Bag 17
              North Ryde NSW 2113
              AUSTRALIA
              +61 2 9325 3148
              <Mark.Andrews@cmis.csiro.au>
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
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