👤
Home Man
Search
Today's Posts
Register

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:
Select Section of Man Page:
Select Man Page Repository:

Linux 2.6 - man page for resolver (linux section 5)

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 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 set to '.', the root domain is  considered.	If  no	domain	entry  is
	      present,	the  domain  is  determined  from the local hostname returned by gethost-
	      name(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 sep-
	      arated 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 (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 ele-
		     ments 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 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.

	      no-check-names
		     sets RES_NOCHECKNAME in _res.options, which disables the modern BIND  check-
		     ing  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.

	      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 exten-
		     sions 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).

	      single-request-reopen (since glibc 2.9)
		     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 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 send-
		     ing 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 3.55 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)


All times are GMT -4. The time now is 04:17 AM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
×
UNIX.COM Login
Username:
Password:  
Show Password