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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.

       On a normally configured system this file should not be necessary.  The
       only name server to be queried will be on the local machine; the domain
       name is determined from the host name 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 host  name  returned  by
              gethostname();  the  domain part is taken to be everything after
              the first ‘.’.  Finally, if the host name  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  allows  addresses  returned  by  gethostbyname  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. E.g.,

              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() (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.

                     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>).

                     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>).

              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  host  names  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() function, and of mapping  IPv4  responses
                     in  IPv6  ‘‘tunnelled form’’ if no AAAA records are found
                     but an A record set exists.

       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