INTERNET-DRAFT                                      Mark Andrews (CSIRO)
   <draft-ietf-dnsind-ncache-03.txt>                             March 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
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           When [RFC1034] was written there were no DNS servers that imple-
           mented negative caching [RFC1034 Section 4.3.4]. This document
           replaces [RFC1034 Section 4.3.4] in the light of experience.
           Negative caching was an optional part of the DNS specification
           and deals with the caching of the non-existence of a 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 servers hence overall network traffic. A
           large proportion of DNS traffic on the Internet could be elim-
           inated if all servers implemented negative caching. With this in
           mind negative caching should no longer be seen as an optional
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           part of a DNS server.
   0 - History
           [ Anyone want to say why negative caching wasn't implemented
           in the early servers or why not in BIND? If I don't get anything
           this section will go. MPA ]
   This is mainly the history of negative caching support in BIND.
   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 pesudo 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?. I reality this
   indicated that the server 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?.
   1 - Terminology
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFCXXXX].
   Negative caching is the store of knowledge that something does not
   exist.  For example we can storage 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" this 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
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   SIG records may be intermixed with CNAME records.
   "NODATA" is a pseudo RCODE which indicated that the name is valid, for
   the given class, but there were no records of the given type. A NODATA
   response has to be inferred from the answer.
   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
   these responses. There are other negative responses that indicate
   failures of servers.  They are dealt with from section ``7 - Other nega-
   tive 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 answer section may have SIG and CNAME RRs and authority
   section may have SOA, NXT and SIG RRsets.
   Some nameservers do not look at the RCODE and continue processing in the
   presence of NS records in the authority section. NS records MUST NOT be
   sent in the authority section where the RCODE is NXDOMAIN.
   Some nameservers incorrectly continue processing if the authority flag
   is not set. The authority flag SHOULD be set even in non-authoritative
   answers. It is hoped that this restriction can be relaxed at some future
   time. With this in mind all servers MUST accept and correctly process
   messages with the authority flag not set.
   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 records in the authority section.
   2.1 - 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.
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   No data is indicated by an answer with a RCODE of NOERROR, no relevant
   answers in the answer section and no NS records in the authority sec-
   tion. The authority section may contain SOA, NXT and SIG RRsets. CNAMEs
   and SIG records may exist in the answer section.
   It is possible to distiguish 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.
   [MC] Some nameservers fail to stop processing if there is a SOA record
   in the authority section along with NS record.  To prevent interopera-
   bility problems NS records SHOULD NOT be added to the authority section
   of a NODATA response.  Nameervers MUST ensure that they stop processing
   in the presence of the SOA record in the authority section so that this
   restriction can be lifted at a further date.
   Some nameservers 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 server must query again with QNAME.
   3 - Negative Answers from Authoritative Servers
   Authoritative servers SHOULD 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
   the MINIMUM field of the SOA record and is 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 [RFC1034 Section ...].
   When the TTL reaches zero (0) the record 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
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   server 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 server threshold and the received TTL.  A server threshold of one
   (1) hour is often appropriate.
   As this TTL is different to the TTL value of the SOA record itself, this
   SOA record MUST NOT be used to answer SOA queries.
   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
   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, MUST be stored such that it can be be
   located as should a SIG record. For NXDOMAIN answers there is no "neces-
   sary" 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.
   5 - Negative answers from the cache
   Whan a server, in answering a query, encounters a cached negative
   response it MUST add the cached SOA to the authority section of the
   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.
   NS records MUST NOT be added to the authority section as existing
   servers do not look for the SOA record that would indicate the differ-
   ence between a NODATA response and a referal.
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   6 - Changes from RFC 1034
   Negative caching in servers 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>
   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
   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.
   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 server 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 Server (OPTIONAL)
   Dead 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
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   exist. A server is deemed to be dead if it has not responded to an out-
   standing query within 120 seconds.
   Examples of transport layer indications are:
           ICMP error messages
           TCP resets
           Kernel 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
   8 Security
   It is believed that this document does not introduce any significant
   additional 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"
   and probably never bother trying this server again.
   For such an attack to be sucessful you need to get the NXDOMAIN indic-
   tion injected into a parent server (or a busy caching server). This can
   only be done by the use of a CNAME which results in the parent server
   quering an attackers server.  Servers 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.
   Impementing TTL sanity check will reduce the impact of such an attack
   and cause it to be come an active attack rather than a passive attack,
   i.e. it needs to be reprimed regularly.
   DNS Security [RFC2065] provides a mechanism to verify whether a negative
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   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
           P. Mockapetris, ``DOMAIN NAMES - CONCEPTS AND FACILITIES,'' RFC
           1034, ISI, November 1987.
           TION,'' RFC 1035, ISI, November 1987.
           D. Eastlake, 3rd, C. Kaufman, ``Domain Name System Security
           Extensions,'' RFC 2065, CyberCash, Iris, January 1997
           S. Bradner, ``Key words for use in RFCs to Indicate Requirement
           Levels,'' RFC XXXX, Harvard University, January 1997
   Authors' Addresses
           Mark Andrews
              CSIRO - Mathematical and Information Sciences
              Locked Bag 17
              North Ryde NSW 2113
              +61 2 9325 3148
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