RESOLV.CONF(5)                                               Linux Programmer's Manual                                              RESOLV.CONF(5)

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 configura-
       tion 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  con-
              sidered.   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.

       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 dif-
                     ferent 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 gethostby-
                     name(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  mistak-
                     enly  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.

       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 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

       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

4th Berkeley Distribution                                           2017-09-15                                                      RESOLV.CONF(5)