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RESOLVER(5)                 BSD File Formats Manual                RESOLVER(5)

     resolver - DNS client


     The resolver is the DNS client used on most Linux and BSD systems. It comes with
     glibc.  Its configuration file /etc/resolv.conf (note the spelling) determines the
     DNS servers to use, and various other options - see below.

     Almost all machines have a DNS server set up in this file - if it doesn't exist, the
     system will assume there's a DNS server running on the local machine, and work out
     the search path from the machines domain name.

     The config file is read the first time the DNS client is invoked by a process.

     The different configuration options are:

     nameserver  IP address of a DNS server to use. Multiple name servers may be listed,
                 each on their own line. The resolver will use them in order listed - if
                 the first server times out answering the query, the next server will be
                 tried, and so on. If the resolver runs out out of name servers, the first
                 server will be queried again, until a maximum number of retries are made.

                 The maximum number of DNS servers to use is set by MAXNS (see <resolv.h>

     search      Domain(s) to use for DNS lookups when no domain is specified. List each
                 domain following the search keyword with spaces or tabs between them.
                 Each possible domain will be checked in order until a match is found.
                 Note that this process may be slow (queries will time out if no server is
                 available for a domain) and will generate a lot of network traffic if the
                 servers for the listed domains aren't local.

                 The search list is currently limited to six domains with a total of 256
                 characters.  If search isn't specified, the search list will be deter-
                 mined from the local domain name (whatever comes after the first dot). If
                 the host name doesn't contain a domain, the root domain is used.

                 By default, it search contains only the local domain name.

     domain      Local domain name. You can use this instead of the search option to spec-
                 ify a single domain to check if a hostname isn't specified. Most people
                 just use search instead (that option lets you use multiple servers,
                 domain doesn't). You can't use domain and search at the same time -
                 they're mutually exclusive.

                 If domain isn't specified, the domain will be determined from the local
                 domain name (whatever comes after the first dot). If the host name
                 doesn't contain a domain, the root domain is used.

     sortlist    Sorts addresses returned by the gethostbyname system call.  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.
                 For 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.

                 ndots:n   sets a threshold for the number of dots which must appear in a
                           name given to res_query() (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.

                           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.  Measured in seconds, the default is
                           RES_TIMEOUT (see <resolv.h> ).

                           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 (see
                           <resolv.h> ).

                 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.

                           sets RES_NOCHECKNAME in _res.options, which disables the modern
                           BIND checking of incoming host names and mail names for invalid
                           characters such as underscore (_), non-ASCII, or control char-

                 inet6     sets RES_USE_INET6 in _res.options.  This has the effect of
                           trying a AAAA query before an A query inside the gethostbyname
                           function, and of mapping IPv4 responses in IPv6 ''tunnelled
                           form'' if no AAAA records are found but an A record set exists.

                 ip6-dotint / no-ip6-dotint
                           sets / clears the RES_NOIP6DOTINT bit in _res.options, which
                           when set (ip6-dotint) will enable reverse IPv6 lookups to be
                           made in the (deprecated) zone; when clear (no-
                           ip6-dotint), reverse IPv6 lookups are made in the zone
                           by default.

                           sets RES_USEBSTRING in _res.options.  This causes reverse IPv6
                           lookups to be made using the bit-label format of RFC 2673; if
                           not set, then nibble format is used.

     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

     /etc/resolv.conf <resolv.h>

     gethostbyname(3), hostname(7), named(8), resolver(3), resolver(5).  "Name Server
     Operations Guide for BIND"

4th Berkeley Distribution        June 23, 2004       4th Berkeley Distribution

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