INTERNET-DRAFT                                      Mark Andrews (CSIRO)
   <draft-ietf-dnsind-ncache-04.txt>                              July 1997
   
   Updates: RFC 1034
   
   
                 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
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           Internet-Drafts are draft documents valid for a maximum of six
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           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 nullifying 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 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
   
   
   
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           network traffic.  A large proportion of DNS traffic on the
           Internet could be eliminated if all resolvers implemented nega-
           tive 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 is the storage of knowledge that something does not
   exist.  For example we can store the knowledge that a record has a par-
   ticular 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" refers to 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. CNAMEs are expected to appear in order so that a single pass of
   the message will expose the chain before any requested records. Note:
   SIG records may be intermixed with CNAME records.
   
   "NODATA" is 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.
   
   "NXDOMAIN" is as alternate expression for the "Name Error" RCODE as
   described in [RFC1035 Section 4.1.1] and can be interchanged for each
   other in this document.
   
   "FORWARDER" is a nameserver that is used by other nameserver to resolve
   queries that it cannot answer in preference to those that would be
   queried as the result of looking at the records in the DNS. FORWARDERS
   are often used to reduce the number of queries sent across wide area
   networks and / or provide DNS services through firewalls.
   
   An understanding of [RFC1034], [RFC1035] and [RFC2065] is expected when
   reading this document.
   
   2 - Negative Responses
   
   Negative responses usually refer to the lack of existence of a particu-
   lar RRset in the DNS. The first sections of this document deal with
   
   
   
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   these responses. There are other negative responses that indicate
   failures of name servers.  They are dealt with from section ``7 - Other
   negative responses'' onwards.
   
   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 in 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 RESPONSE: TYPE 1.
   
           Header:
           RDCODE=NXDOMAIN
           Query:
           FOO.EXAMPLE. A
           Answer:
           FOO.EXAMPLE. CNAME BAR.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:
           FOO.EXAMPLE. A
           Answer:
           FOO.EXAMPLE. CNAME BAR.XX.
           Authority:
           XX. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
           Additional:
           <empty>
   
   
           NXDOMAIN RESPONSE: TYPE 3.
   
           Header:
           RDCODE=NXDOMAIN
           Query:
           FOO.EXAMPLE. A
           Answer:
           FOO.EXAMPLE. CNAME BAR.XX.
           Authority:
           <empty>
           Additional:
           <empty>
   
   
           REFERRAL RESPONSE.
   
           Header:
           RDCODE=NOERROR
           Query:
           FOO.EXAMPLE. A
           Answer:
           FOO.EXAMPLE. CNAME BAR.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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   The key differences in the NXDOMAIN types above is in the contents of
   the authority section. While the examples above have CNAMES in the
   answer section this is for example purposes only and need not be there.
   
   2.1.1 Special Handling Name Error
   
   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. Sending a
   TYPE 1 NXDOMAIN 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 be necessary to dis-
   able sending TYPE 1 NXDOMAIN response for non-authoritative NXDOMAIN
   responses.
   
   Some resolvers incorrectly continue processing if the authority flag is
   not set. This is a problem when your nameserver is listed as a FORWARDER
   for these resolvers. It is sometimes necessary to force the authority
   flag on for NXDOMAIN responses when you have such a resolver.
   
   2.2 - No Data
   
   NODATA responses have to be algorithmically determined from the
   responses contents as there is no RCODE field to indicate NODATA. In
   some cases it is necessary to query again to get a definitive answer.
   
   No data is indicated by an answer with a RCODE of NOERROR and no
   relevant answers in the answer section and either SOA record in the
   authority section or no NS records in the authority section. The author-
   ity section may contain NXT and SIG RRsets in addition to NS and SOA
   records. CNAMEs and SIG records may exist in the answer section.
   
   It is possible to distinguish between a referral and a NODATA response
   by the presence of a SOA record in the authority section or the absence
   of NS records in the authority section.
   
   
   
   
   
   
   
   
   
   
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           NO DATA RESPONSE: TYPE 1.
   
           Header:
           RDCODE=NOERROR
           Query:
           FOO.EXAMPLE. A
           Answer:
           FOO.EXAMPLE. CNAME BAR.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
   
   
           NO DATA RESPONSE: TYPE 2.
   
           Header:
           RDCODE=NOERROR
           Query:
           FOO.EXAMPLE. A
           Answer:
           FOO.EXAMPLE. CNAME BAR.XX.
           Authority:
           XX. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
           Additional:
           <empty>
   
   
           NO DATA RESPONSE: TYPE 3.
   
           Header:
           RDCODE=NOERROR
           Query:
           FOO.EXAMPLE. A
           Answer:
           FOO.EXAMPLE. CNAME BAR.XX.
           Authority:
           <empty>
           Additional:
           <empty>
   
   
   
   
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           REFERRAL RESPONSE.
   
           Header:
           RDCODE=NOERROR
           Query:
           FOO.EXAMPLE. A
           Answer:
           FOO.EXAMPLE. CNAME BAR.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
   
   
   The key differences in the NODATA types and REFERRAL RESPONSE above is
   in the contents of the authority section. While the examples above have
   CNAMES in the answer section this is for example purposes only and need
   not be there.
   
   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 the resolver must query again with QNAME.
   
   3 - Negative Answers from Authoritative Servers
   
   Authoritative name servers MUST add the SOA record of the containing
   zone to the authority section of answers containing negative responses
   to enable the response to be cached.  The TTL of this record is set from
   
   
   
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   the MINIMUM field of the SOA record, not the TTL of the SOA itself, and
   indicates how long a server may cache this negative answer.
   
   If the containing zone is signed [RFC2065] the SOA and appropriate NXT
   and SIG records MUST be added.
   
   4 - 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 using the SOA
   record from the containing zone and putting it in the authority section
   with an initial TTL set from the SOA minimum field. This TTL decrements
   in a similar manner to a normal cached answer and upon reaching zero (0)
   indicates the negative answer MUST be discarded.
   
   Often SOA minimums are set with no regard to the TTL of negative answers
   so that negative responses have TTL measured in days. Early advertise-
   ment of a service before all the secondaries have a copy of the relevant
   zone can lead to prolonged denials of service. With this in mind a
   resolver SHOULD set an upper bound on the TTL of the negative answer it
   is willing to cache.  If it does this the TTL MUST be set to the minimum
   of the resolver threshold and the received TTL.  A resolver threshold of
   one (1) hour is often appropriate.
   
   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, 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 query name. 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.
   
   
   
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   5 - 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.
   
   6 - Changes from RFC 1034
   
   Negative caching in resolvers is no-longer optional.
   
   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.
   
   7 - Other Negative Responses
   
   Caching of other negative responses is not covered by any existing RFC.
   There is no way to indicated a desired TTL of these responses. Care
   needs to be taken to ensure that there are not forwarding loops.  [ Do
   we need to have a hold down period where we cannot cache these, tran-
   sport layer indications aside? MPA ]
   
   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, 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.
   
   
   
   
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   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.
   
   A resolver MAY cache a server failure response. If it does so it MUST
   NOT cache it for longer that five (5) minutes, and it MUST be cached
   against the specific query tuple <query name, type, class, server IP
   address>.
   
   7.2 Dead / Unreachable Server (OPTIONAL)
   
   Dead / Unreachable servers are servers that fails to respond in any way
   to a query or the transport layer has provided an indication that the
   server does not exist or is unreachable. A server is 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
           TCP resets
           IP stack error messages indicating host or net unreachable.
   
   
   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 is stored against the specific IP address
   involved.
   
   8 Security
   
   We believed that this document does not introduce any significant addi-
   tional security threats.
   
   With negative caching it might be possible to propagate a denial of ser-
   vice 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 but not quite.
   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"
   
   
   
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   and probably never bother trying this server again.
   
   For such an attack to be successful you would need to get the NXDOMAIN
   indiction injected into a parent server (or a busy caching resolver).
   This can only be done by the use of a CNAME which results in the parent
   server querying an attackers server.  Resolvers that are 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 transmis-
   sion of the relevant security records even in a non security aware
   server.
   
   8 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 8.1) was supplied by Rob
   Austein <sra@epilogue.com> and is essentially untouch from what he sent
   me [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 plan-
   ning 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.
   
   
   
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   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 nega-
   tive 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
   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 ver-
   sions 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 speci-
   fied 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 confi-
   guration 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.
   
   
   
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   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 abil-
   ity to act as what today would be called a "stealth secondary".  That
   is, when configured in this way, the resolver had direct access to
   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.
   
   8.2 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
   
   
   
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   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 below.
   
   NCACHE made default XXXX. xxxx 199?.
   
   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, XXXX 199?. 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,
   XXXX 199?.
   
   RETURNSOA was re-enabled by default, BIND 4.9.5-T1A, XXXX 199?.
   
   References
   
   [RFC1034]
           P. Mockapetris, ``DOMAIN NAMES - CONCEPTS AND FACILITIES,'' RFC
           1034, ISI, November 1987.
   
   
   [RFC1035]P. Mockapetris, ``DOMAIN NAMES - IMPLEMENTATION AND SPECIFICA-
           TION,'' RFC 1035, ISI, November 1987.
   
   
   [RCF2065]
           D. Eastlake, 3rd, C. Kaufman, ``Domain Name System Security
           Extensions,'' RFC 2065, CyberCash, Iris, January 1997
   
   
   [RFC2119]
           S. Bradner, ``Key words for use in RFCs to Indicate Requirement
           Levels,'' RFC 2119, Harvard University, March 1997
   
   
   Authors' Addresses
   
   
   
   
   
   
   
   
   
   Expires January 1998                                          [Page 14]


   INTERNET-DRAFT                 DNS NCACHE                      July 1997
   
   
   
           Mark Andrews
              CSIRO - Mathematical and Information Sciences
              Locked Bag 17
              North Ryde NSW 2113
              AUSTRALIA
              +61 2 9325 3148
              <Mark.Andrews@cmis.csiro.au>
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   Expires January 1998                                          [Page 15]