Negative Caching of DNS Queries (DNS NCACHE)
RFC 2308

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Network Working Group                                          M. Andrews
Request for Comments: 2308                                          CSIRO
Updates: 1034, 1035                                            March 1998
Category: Standards Track

              Negative Caching of DNS Queries (DNS NCACHE)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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 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.

Andrews                     Standards Track                     [Page 1]
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 [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 [RFC2065]) 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 reading this document.

Andrews                     Standards Track                     [Page 2]
RFC 2308                       DNS NCACHE                     March 1998

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 the authority section may have SOA, NXT [RFC2065] and SIG RRsets.