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
   considered  a  trusted  source  of DNS information (e.g., DNSSEC AD-bit
   information 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
          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 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.

          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:


          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)).

                 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. This may  not  be  the
                 total time taken by any resolver API call and there is no
                 guarantee that a single  resolver  API  call  maps  to  a
                 single  timeout.   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.

                 Some programs behave strangely when this option is turned

          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) zone; when  this  option
                 is  set (no-ip6-dotint), reverse IPv6 lookups are made in
                 the 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
                 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)
                 Sets RES_SNGLKUPREOP in _res.options.  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.

          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),       resolver(3),       host.conf(5),      hosts(5),
   nsswitch.conf(5), hostname(7), named(8)

   Name Server Operations Guide for BIND


   This page is part of release 4.09 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


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