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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 configuration file contains infor-
       mation  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.

       On  a  normally configured system this file should not be necessary.  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 mul-
              tiple subdomains please read options ndots:n below to avoid  man-in-the-mid-
              dle  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 syn-
              tax is

                     options option ...

              where option is one of the following:

              debug  sets RES_DEBUG in _res.options.

              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 maximum
                     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.   Measured  in seconds, the default is RES_TIMEOUT (currently
                     5, see <resolv.h>).  The maximum 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 appli-
                     cation.   The  default is RES_DFLRETRY (currently 2, see <resolv.h>).
                     The maximum 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.

              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 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.

              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
                     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)
                     The resolver uses the same socket for the A and AAAA requests.   Some
                     hardware mistakenly only sends back one reply.  When that happens the
                     client sytem 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-pro-
       cess  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.,  name-
       server)  must  start  the  line.  The value follows the keyword, separated by white
       space.

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

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



4th Berkeley Distribution         2009-03-01                    RESOLV.CONF(5)

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