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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.  The configuration file is considered a trusted
       source of DNS information (e.g., DNSSEC AD-bit information  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 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 set to '.', the root domain is  considered.   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

       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 (effective only if glibc was built with debug
                     support; see resolver(3)).

                     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. This may
                     not be the total time taken by  any  resolver  API  call  and  there  is  no
                     guarantee  that  a  single  resolver  API  call  maps  to  a single timeout.
                     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  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.

                     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.   Since  glibc  2.25,  this   option   is   deprecated;
                     applications should use getaddrinfo(3), rather than gethostbyname(3).

                     Some programs behave strangely when this option is turned on.

              ip6-bytestring (since glibc 2.3.4)
                     Sets RES_USEBSTRING 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  (which  is  the  default), then nibble format is used.  This option was
                     removed in glibc 2.25,  since  it  relied  on  a  backward-incompatible  DNS
                     extension that was never deployed on the Internet.

              ip6-dotint/no-ip6-dotint (glibc 2.3.4 to 2.24)
                     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.   These  options  are  available  in  glibc
                     versions  up  to 2.24, where no-ip6-dotint is the default.  Since ip6-dotint
                     support long ago ceased to be available on the Internet, these options  were
                     removed in glibc 2.25.

              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 performs 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  hardware mistakenly sends back only one
                     reply.  When that happens the client system will sit and wait for the second
                     reply.  Turning this option on changes this behavior so that if two requests
                     from the same port are not handled 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

       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.

       Lines that contain a semicolon (;) or hash character (#) in the first column  are  treated
       as comments.


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


       gethostbyname(3),  resolver(3),  host.conf(5),  hosts(5),  nsswitch.conf(5),  hostname(7),

       Name Server Operations Guide for BIND


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