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 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.

       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 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.

              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.   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 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 for-
                     mat 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.

       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 vari-
       able 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.

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 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



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