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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  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 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 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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