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

       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 gethost‐
              name(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 sep‐
              arated 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 ele‐
                     ments are appended to it.  The value for this option is silently  capped  to

                     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 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  check‐
                     ing  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)  ip6.int
                     zone; when this option is set (no-ip6-dotint), reverse IPv6 lookups are made
                     in the ip6.arpa 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 exten‐
                     sions 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)
                     The  resolver  uses the same socket for the A and AAAA requests.  Some hard‐
                     ware 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.

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

       This  page  is  part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

4th Berkeley Distribution                   2014-02-22                             RESOLV.CONF(5)

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