resolv.conf manpage

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       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.

       On a normally configured system this file should not be necessary.  The
       only name server to be queried will be on the local machine; the domain
       name is determined from the host name 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

       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 host  name  returned  by
	      gethostname();  the  domain part is taken to be everything after
	      the first '.'.  Finally, if the host name	 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	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. 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() (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 <resolv.h>).

		     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>).

	      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	host  names  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() function, and of mapping  IPv4	 responses  in
		     IPv6  ''tunnelled form'' if no AAAA records are found but
		     an A record set exists.

       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.

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

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

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