1. Introduction

   As specified in RFC 1035std13 (section 6.4), the IQUERY operation for DNS
   queries is used to look up the name(s) which are associated with the
   given value.  The value being sought is provided in the query's
   answer section and the response fills in the question section with
   one or more 3-tuples of type, name and class.

   As noted in [RFC1035], section 6.4.3, inverse query processing can
   put quite an arduous burden on a server.  A server would need to
   perform either an exhaustive search of its database or maintain a
   separate database that is keyed by the values of the primary
   database.  Both of these approaches could strain system resource use,
   particularly for servers that are authoritative for millions of
   names.

   Response packets from these megaservers could be exceptionally large,
   and easily run into megabyte sizes.  For example, using IQUERY to
   find every domain that is delegated to one of the nameservers of a
   large ISP could return tens of thousands of 3-tuples in the question
   section.  This could easily be used to launch denial of service
   attacks.

   Operators of servers that do support IQUERY in some form (such as
   very old BIND 4 servers) generally opt to disable it.  This is
   largely due to bugs in insufficiently-exercised code, or concerns
   about exposure of large blocks of names in their zones by probes such
   as inverse MX queries.

   IQUERY is also somewhat inherently crippled by being unable to tell a
   requester where it needs to go to get the information that was
   requested.  The answer is very specific to the single server that was
   queried.  This is sometimes a handy diagnostic tool, but apparently
   not enough so that server operators like to enable it, or request
   implementation where it is lacking.

   No known clients use IQUERY to provide any meaningful service.  The
   only common reverse mapping support on the Internet, mapping address
   records to names, is provided through the use of pointer (PTR)
   records in the in-addr.arpa tree and has served the community well
   for many years.

   Based on all of these factors, this document recommends that the
   IQUERY operation for DNS servers be officially obsoleted.