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 configura-
       tion 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  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 following 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 available 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 separated 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 modified.  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 dif-
		     ferent 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 gethostbyname(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 format 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 performs 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).

       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 3.44 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					    2012-11-11							    RESOLV.CONF(5)