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RESOLV.CONF(5)		   Linux Programmers Manual	       RESOLV.CONF(5)

       resolv.conf - resolver configuration file


       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.

       If this file doesnt exist the only name server to be queried  will  be
       on  the	local machine; 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 (in dot notation) of a  name  server  that  the
	      resolver	 should   query.    Up	to  MAXNS  (currently  3,  see
	      ) name servers may be listed,  one  per  keyword.   If
	      there are multiple servers, the resolver library queries them in
	      the order listed.  If no nameserver  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

       domain Local domain name.
	      Most  queries  for  names within this domain can use short names
	      relative to the local domain.  If no domain  entry  is  present,
	      the  domain  is  determined  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
	      lot of network traffic if the servers for the listed domains are
	      not local, and that queries will time out if no server is avail
	      able for one of the domains.

	      The search list is currently limited to six domains with a total
	      of 256 characters.

	      Sortlist	allows	addresses  returned  by gethostbyname(3) to be
	      sorted.  A sortlist is specified by IP  address  netmask	pairs.
	      The  netmask  is optional and defaults to the natural netmask of
	      the net.	The IP address and optional network  pairs  are  sepa
	      rated by slashes.  Up to 10 pairs may be specified.  E.g.,

	      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.

		     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.

		     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 ).

		     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 ).

	      rotate sets RES_ROTATE in _res.options, which causes round robin
		     selection	of  nameservers 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
		     IPv6 "tunneled form" if no AAAA records are found but  an
		     A record set exists.

		     Some programs behave strangely when this option is turned

       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 systems 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 systems 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.


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

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

4th Berkeley Distribution	  2004-10-31			RESOLV.CONF(5)

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