RESOLV.CONF(5)            NetBSD File Formats Manual            RESOLV.CONF(5)

NAME
     resolv.conf -- resolver configuration file

DESCRIPTION
     The resolv.conf file specifies how the resolver(3) routines in the C
     library (which provide access to the Internet Domain Name System) should
     operate.  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 informa-
     tion.

     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 host name, and the domain search path is con-
     structed from the domain name.

     The different configuration options are:

     nameserver  IPv4 address (in dot notation) or IPv6 address (in
                 hex-and-colon notation) of a name server that the resolver
                 should query.  Scoped IPv6 address notation is accepted as
                 well (see inet6(4) for details).  Up to MAXNS (currently 3)
                 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 host name returned by gethostname(3); the domain part
                 is taken to be everything after the first `.'.  Finally, if
                 the host name does not contain a domain part, the root domain
                 is assumed.

     lookup      This keyword is now ignored: its function has been superseded
                 by features of nsswitch.conf(5).

     search      Search list for host-name lookup.  The search list is nor-
                 mally determined from the local domain name; by default, it
                 begins with the local domain name, then successive parent
                 domains that have at least two components in their names.
                 This may be changed by listing the desired domain search path
                 following the search keyword with spaces or tabs separating
                 the names.  Most resolver queries will be attempted using
                 each component of the search path in turn until a match is
                 found.  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 1024 characters.

     sortlist    Sortlist allows addresses returned by gethostbyname 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, ie.

                 sortlist 130.155.160.0/255.255.240.0 130.155.0.0

     options     Options allows certain internal resolver variables to be mod-
                 ified.  The syntax is:

                 options option ...

                 where option is one of the following:

                 debug           enable debugging information, by setting
                                 RES_DEBUG in _res.options (see resolver(3)).

                 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, mean-
                                 ing 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.

                 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 (see <resolv.h>).

                 attempts:n      sets the number of times the resolver will
                                 send a query to its name servers before giv-
                                 ing 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.

                 no-check-names  sets RES_NOCHECKNAME in _res.options, which
                                 disables the modern BIND checking of incoming
                                 host names and mail names for invalid charac-
                                 ters such as underscore (`_'), non-ASCII, or
                                 control characters.  This is the default.

                 check-names     clears RES_NOCHECKNAME in _res.options, which
                                 enables the modern BIND checking of incoming
                                 host names and mail names as described above.

                 edns0           attach OPT pseudo-RR for ENDS0 extension
                                 specified in RFC 2671, to inform DNS server
                                 of our receive buffer size.  The option will
                                 allow DNS servers to take advantage of non-
                                 default receive buffer size, and to send
                                 larger replies.  DNS query packets with EDNS0
                                 extension is not compatible with non-EDNS0
                                 DNS servers.  The option must be used only
                                 when all the DNS servers listed in nameserver
                                 lines are able to handle EDNS0 extension.

                 inet6           enable support for IPv6-only applications, by
                                 setting RES_USE_INET6 in _res.options (see
                                 resolver(3)).  The option is meaningful with
                                 certain kernel configuration only and use of
                                 this option is discouraged.

                 insecure1       Do not require IP source address on the reply
                                 packet to be equal to the servers' address.

                 insecure2       Do not check if the query section of the
                                 reply packet is equal to that of the query
                                 packet.  For testing purposes only.

                 no-tld-query    sets RES_NOTLDQUERY in _res.options.  This
                                 option causes res_nsearch() to not attempt to
                                 resolve a 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.

     The domain and search keywords are mutually exclusive.  If more than one
     instance of these keywords is present, the last instance will override.

     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.

     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, sepa-
     rated by white space.

FILES
     /etc/resolv.conf  The file resolv.conf resides in /etc.

SEE ALSO
     gethostbyname(3), resolver(3), nsswitch.conf(5), hostname(7), named(8),
     resolvconf(8)

     Paul Vixie, Kevin J. Dunlap, and Michael J. Karels, Name Server
     Operations Guide for BIND, CSRG, Department of Electrical Engineering and
     Computer Sciences, University of California, Berkeley, Release 4.9.4,
     http://www.dns.net/dnsrd/docs/bog/bog.html, July 16, 1996.

HISTORY
     The resolv.conf file format appeared in 4.3BSD.

NetBSD 7.0                     September 9, 2012                    NetBSD 7.0

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