RFC 2308 (rfc2308) - Page 2 of 19

Negative Caching of DNS Queries (DNS NCACHE)

RFC 2308                       DNS NCACHE                     March 1998


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 [RFC 2119].

   "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 [RFC 2065]) can be interspersed amongst the CNAMEs.

   "NXDOMAIN" - an alternate expression for the "Name Error" RCODE as
   described in [RFC 1035 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 [RFC 1034], [RFC 1035] and [RFC 2065] is expected
   when reading this document.









Andrews                     Standards Track