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           The  resolver is a set of routines in the C library that provide access
           to the Internet Domain Name System (DNS).  The  resolver  configuration
           file  contains  information  that  is read by the resolver routines the
           first time they are invoked by a process.  The file is designed  to  be
           human readable and contains a list of keywords with values that provide
           various types of resolver information.  The configuration file is  con-
           sidered a trusted source of DNS information (e.g., DNSSEC AD-bit infor-
           mation will be returned unmodified from this source).
           If this file does not exist, only the name server on the local  machine
           will  be  queried;  the domain name is determined from the hostname and
           the domain search path is constructed from the domain name.
           The different configuration options are:
           nameserver Name server IP address
                  Internet address of a  name  server  that  the  resolver  should
                  query,  either  an  IPv4  address  (in dot notation), or an IPv6
                  address in colon (and possibly dot) notation as  per  RFC  2373.
                  Up  to  MAXNS  (currently 3, see <resolv.h>) name servers may be
                  listed, one per keyword.  If there  are  multiple  servers,  the
                  resolver  library queries them in the order listed.  If no name-
                  server entries are present, the  default  is  to  use  the  name
                  server  on  the  local machine.  (The algorithm used is to try a
                  name server, and if the query times out, try the next, until out
                  of name servers, then repeat trying all the name servers until a
                  maximum number of retries are made.)
           domain Local domain name.
                  Most queries for names within this domain can  use  short  names
                  relative to the local domain.  If set to '.', the root domain is
                  considered.  If no domain entry is present, the domain is deter-
                  mined  from  the  local hostname returned by gethostname(2); the
                  domain part is taken to  be  everything  after  the  first  '.'.
                  Finally,  if  the  hostname  does not contain a domain part, the
                  root domain is assumed.
           search Search list for host-name lookup.
                  The search list is normally determined  from  the  local  domain
                  name;  by default, it contains only the local domain name.  This
                  may be changed by listing the desired domain search path follow-
                  ing the search keyword with spaces or tabs separating the names.
                  Resolver queries having fewer than ndots dots (default is 1)  in
                  them  will  be attempted using each component of the search path
                  in turn until a match is found.  For environments with  multiple
                  subdomains  please  read  options ndots:n below to avoid man-in-
                  the-middle attacks and unnecessary  traffic  for  the  root-dns-
                  servers.  Note that this process may be slow and will generate a
                  Options allows certain internal resolver variables to  be  modi-
                  fied.  The syntax is
                         options option ...
                  where option is one of the following:
                  debug  sets  RES_DEBUG  in _res.options (effective only if glibc
                         was built with debug support; see resolver(3)).
                         sets a threshold for the number of dots which must appear
                         in  a name given to res_query(3) (see resolver(3)) before
                         an initial absolute query will be made.  The default  for
                         n is 1, meaning that if there are any dots in a name, the
                         name will be tried first as an absolute name  before  any
                         search  list  elements are appended to it.  The value for
                         this option is silently capped to 15.
                         sets the amount of time the  resolver  will  wait  for  a
                         response  from  a  remote name server before retrying the
                         query via a different name server.  Measured in  seconds,
                         the default is RES_TIMEOUT (currently 5, see <resolv.h>).
                         The value for this option is silently capped to 30.
                         sets the number of times the resolver will send  a  query
                         to  its  name  servers  before giving up and returning an
                         error  to  the  calling  application.   The  default   is
                         RES_DFLRETRY  (currently  2,  see <resolv.h>).  The value
                         for this option is silently capped to 5.
                  rotate sets RES_ROTATE in _res.options, which causes round-robin
                         selection  of name servers from among those listed.  This
                         has the effect of spreading  the  query  load  among  all
                         listed  servers,  rather  than having all clients try the
                         first listed server first every time.
                         sets RES_NOCHECKNAME in _res.options, which disables  the
                         modern BIND checking of incoming hostnames and mail names
                         for invalid characters such as underscore (_), non-ASCII,
                         or control characters.
                  inet6  sets  RES_USE_INET6 in _res.options.  This has the effect
                         of trying a AAAA query before an A query inside the geth-
                         ostbyname(3)  function,  and of mapping IPv4 responses in
                         the zone by default.   This  option  is  set  by
                  edns0 (since glibc 2.6)
                         sets RES_USE_EDNSO in _res.options.  This enables support
                         for the DNS extensions described in RFC 2671.
                  single-request (since glibc 2.10)
                         sets RES_SNGLKUP in _res.options.  By default, glibc per-
                         forms  IPv4  and  IPv6  lookups in parallel since version
                         2.9.  Some appliance  DNS  servers  cannot  handle  these
                         queries  properly  and  make the requests time out.  This
                         option disables the behavior and makes glibc perform  the
                         IPv6  and IPv4 requests sequentially (at the cost of some
                         slowdown of the resolving process).
                  single-request-reopen (since glibc 2.9)
                         The resolver uses the same socket  for  the  A  and  AAAA
                         requests.   Some  hardware mistakenly sends back only one
                         reply.  When that happens the client system will sit  and
                         wait  for  the  second  reply.   Turning  this  option on
                         changes this behavior so that if two  requests  from  the
                         same  port  are  not  handled correctly it will close the
                         socket and open a  new  one  before  sending  the  second
           The  domain  and  search keywords are mutually exclusive.  If more than
           one instance of these keywords is present, the last instance wins.
           The search keyword of a system's resolv.conf file can be overridden  on
           a  per-process basis by setting the environment variable LOCALDOMAIN to
           a space-separated list of search domains.
           The options keyword of a system's resolv.conf file can be amended on  a
           per-process  basis by setting the environment variable RES_OPTIONS to a
           space-separated list of  resolver  options  as  explained  above  under
           The  keyword  and  value  must appear on a single line, and the keyword
           (e.g., nameserver) must start the line.  The value follows the keyword,
           separated by white space.
           Lines  that  contain a semicolon (;) or hash character (#) in the first
           column are treated as comments.


           /etc/resolv.conf, <resolv.h>


           gethostbyname(3), resolver(3), hostname(7), named(8)
           Name Server Operations Guide for BIND

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