resolver

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.  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.

       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  sepa-
              rated  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  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)).

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

COLOPHON
       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
       http://www.kernel.org/doc/man-pages/.

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