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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 made.)

       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 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 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 default.

              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

       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.


       /etc/resolv.conf, <resolv.h>


       gethostbyname(3), resolver(3), hostname(7), named(8)
       Name Server Operations Guide for BIND


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