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NAME

       resolv.conf - resolver configuration file

SYNOPSIS

       /etc/resolv.conf

DESCRIPTION

       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,  and  the  search
       list contains the local domain name determined from the hostname.

       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.)

       search Search list for host-name lookup.
              By  default, the search list contains one entry, the local domain name.  It is determined from the
              local hostname returned by gethostname(2); the local domain name is taken to be  everything  after
              the first '.'.  Finally, if the hostname does not contain a '.', the root domain is assumed as 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.

              If there are multiple search directives, only the search list from the last instance is used.

              In glibc 2.25 and earlier, the search list  is  limited  to  six  domains  with  a  total  of  256
              characters.  Since glibc 2.26, the search list is unlimited.

              The  domain  directive  is  an obsolete name for the search directive that handles one search list
              entry only.

       sortlist
              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:

                  sortlist 130.155.160.0/255.255.240.0 130.155.0.0

       options
              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)).

              ndots:n
                     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.

              timeout:n
                     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.

              attempts:n
                     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.

              no-check-names
                     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)  ip6.int  zone;  when  this  option  is  set
                     (no-ip6-dotint),  reverse  IPv6  lookups  are  made in the ip6.arpa 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 resolutions.

              no-reload (since glibc 2.26)
                     Sets  RES_NORELOAD  in _res.options.  This option disables automatic reloading of a changed
                     configuration file.

       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
       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.

FILES

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

SEE ALSO

       gethostbyname(3), resolver(3), host.conf(5), hosts(5), nsswitch.conf(5), hostname(7), named(8)

       Name Server Operations Guide for BIND

COLOPHON

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