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RESOLV.CONF(5)		   Linux Programmer's Manual		RESOLV.CONF(5)

NAME
       resolv.conf - resolver configuration file

SYNOPSIS
       /etc/resolv.conf

DESCRIPTION
       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 doesn't 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
	      <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 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
	      made.)

       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
	      This  option 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.  Here is an
	      example:

		  sortlist 130.155.160.0/255.255.240.0 130.155.0.0

       options
	      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.

	      ndots:n
		     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.

	      timeout:n
		     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.

	      attempts:n
		     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 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.

	      no-check-names
		     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
		     on.

	      ip6-bytestring (since glibc 2.3.4)
		     sets   RES_USE_BSTRING   in  _res.options.	  This	causes
		     reverse IPv6 lookups to be made using the bit-label  for‐
		     mat  described  in	 RFC 2673;  if this option is not set,
		     then nibble format is used.

	      ip6-dotint/no-ip6-dotint (since glibc 2.3.4)
		     Clear/set RES_NOIP6DOTINT	in  _res.options.   When  this
		     option  is	 clear	(ip6-dotint), reverse IPv6 lookups are
		     made in the (deprecated) ip6.int zone; when  this	option
		     is	 set (no-ip6-dotint), reverse IPv6 lookups are made in
		     the ip6.arpa zone by default.   This  option  is  set  by
		     default.

	      edns0 (since glibc 2.6)
		     sets RES_USE_EDNSO in _res.options.  This enables support
		     for the DNS extensions described in RFC 2671.

       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
       options.

       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.

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

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

COLOPHON
       This page is part of release 3.27 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	  2009-03-01			RESOLV.CONF(5)
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