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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.

       If this file doesn’t exist the only name server to be queried  will  be
       on  the  local machine; 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 (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 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.

                     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  maximum
                     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.  Measured in seconds,
                     the default is RES_TIMEOUT (currently 5, see <resolv.h>).
                     The  maximum  value for this option is silently capped to

                     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 maximum
                     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.

                     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

              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.

       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


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