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

	      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


	      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 an AAAA query before  an	 A  query  inside  the
		     gethostbyname(3)  function, and of mapping IPv4 responses
		     in IPv6 "tunneled form" if no AAAA records are found  but
		     an A record set exists.

	      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

	      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)
		     Sets RES_SNGLKUPREOP in _res.options.  The resolver  uses
		     the  same socket for the A and AAAA requests.  Some hard-
		     ware mistakenly sends back only  one  reply.   When  that
		     happens  the client system will sit and wait for the sec-
		     ond reply.	 Turning this option on changes this  behavior
		     so	 that  if two requests from the same port are not han-
		     dled correctly it will close the socket and  open	a  new
		     one before sending the second request.

	      no-tld-query (since glibc 2.14)
		     Sets  RES_NOTLDQUERY in _res.options.  This option causes
		     res_nsearch() to not attempt to  resolve  an  unqualified
		     name as if it were a top level domain (TLD).  This option
		     can cause problems if the site has ``localhost'' as a TLD
		     rather  than  having localhost on one or more elements of
		     the search list.  This option has no  effect  if  neither
		     RES_DEFNAMES or RES_DNSRCH is set.

	      use-vc (since glibc 2.14)
		     Sets  RES_USEVC  in _res.options.	This option forces the
		     use of TCP for DNS resolutions.

       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),    host.conf(5),    hosts(5),	  resolver(3),	  nss-
       witch.conf(5), hostname(7), named(8)
       Name Server Operations Guide for BIND

       This page is part of release 4.04 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at

4th Berkeley Distribution	  2015-05-07			RESOLV.CONF(5)