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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 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
	      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 (effective only if glibc
		     was built with debug support; see resolver(3)).

	      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.

	      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
		     request.

       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.

       Lines that contain a semicolon (;) or hash character (#) in  the	 first
       column are treated as comments.

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 3.54 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	  2013-07-31			RESOLV.CONF(5)
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