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

NAME
     resolv.conf, resolv.conf.tail -- resolver configuration files

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.  If the resolv.conf file does not exist, only the local host
     file /etc/hosts will be consulted, i.e. the Domain Name System will not
     be used to resolve hosts.

     The file is designed to be human readable and contains a list of keywords
     with values that provide various types of resolver information.  A
     resolv.conf file is not required for some setups, so this file is
     optional.  It can be created manually, and is also created as part of the
     OpenBSD install process if use of the DHCP protocol is specified for any
     interface or if any DNS nameservers are configured.

     If dhclient(8) is used to configure the network it will normally over-
     write the resolv.conf file with updated information such as nameserver
     addresses, losing any previous values the file contained.  In order to
     force options to be passed to the resolver(3) routines, the file
     resolv.conf.tail may be created manually.  This file will be appended to
     the generated resolv.conf file by dhclient, ensuring options remain.  If
     no updated information is available to dhclient, and resolv.conf.tail is
     not present, then resolv.conf will not be modified by dhclient.

     On a machine whose network connection does not change frequently (such as
     a desktop machine on a local-area network), the resolv.conf.tail file
     should not be necessary.  However the resolv.conf.tail file may be useful
     on notebooks, to search multiple domains, to refer to hard-coded informa-
     tion in local files, or otherwise override the defaults.

     A keyword and its values must appear on a single line, and the keyword
     (e.g. nameserver) must start the line.  The value follows the keyword,
     separated by whitespace.  A hash mark (#) or semicolon (;) in the file
     indicates the beginning of a comment; subsequent characters up to the end
     of the line are not interpreted by the routines that read the file.

     The configuration options (which may be placed in either file) 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).  A non-standard port may also be spec-
                 ified, with the host address enclosed in square brackets.
                 For example:

                       nameserver [10.0.0.1]:5353
                       nameserver [::1]:5353

                 Up to MAXNS (currently 3) name servers may be listed, one per
                 line.  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 name servers until a maximum
                 number of retries are performed.)

     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 dot.  Finally, if
                 the host name does not contain a domain part, the root domain
                 is assumed.

     lookup      This keyword is used by the library routines gethostbyname(3)
                 and gethostbyaddr(3).  It specifies which databases should be
                 searched, and the order to do so.  The legal space-separated
                 values are:

                       bind  Query a domain name server.
                       file  Search for entries in /etc/hosts.
                       yp    Talk to the YP system if ypbind(8) is running.

                 If the lookup keyword is not used in the system's resolv.conf
                 file then the assumed order is bind file.  Furthermore, if
                 the system's resolv.conf file does not exist, then the only
                 database used is file.

     search      Search list for hostname lookup.  The search list is normally
                 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 follow-
                 ing 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 avail-
                 able for one of the domains.

                 The search list is currently limited to six domains with a
                 total of 1024 characters.  Only one search line should
                 appear; if more than one is present, the last one found over-
                 writes any values found in earlier lines.  So if such a line
                 appears in the resolv.conf.tail file, it should include all
                 the domains that need to be searched.

     sortlist    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 sepa-
                 rated by slashes.  Up to 10 pairs may be specified.  For
                 example:

                       sortlist 130.155.160.0/255.255.240.0 130.155.0.0

     family      Specify which type of Internet protocol family to prefer, if
                 a host is reachable using different address families.  By
                 default IPv4 addresses are queried first, and then IPv6
                 addresses.  The syntax is:

                       family family [family]

                 A maximum of two families can be specified, where family can
                 be any of:

                       inet4     IPv4 queries.
                       inet6     IPv6 queries.

                 If only one family is specified, only that family is tried.

     options     Allows certain internal resolver variables to be modified.
                 The syntax is:

                       options option ...

                 Where option is one of the following:

                 debug      Print debugging messages, if libc is compiled with
                            DEBUG.  By default on OpenBSD this option does
                            nothing.

                 edns0      Attach an OPT pseudo-RR for the EDNS0 extension,
                            as specified in RFC 2671.  This informs DNS
                            servers of a client's receive buffer size, allow-
                            ing them to take advantage of a non-default
                            receive buffer size, and thus send larger replies.
                            DNS query packets with the EDNS0 extension are not
                            compatible with non-EDNS0 DNS servers, so the
                            option must be used only when all the servers
                            listed in nameserver lines are able to handle the
                            extension.  By default on OpenBSD this option does
                            nothing.

                 inet6      Enables support for IPv6-only applications, by
                            setting RES_USE_INET6 in _res.options (see
                            resolver(3)).  Use of this option is discouraged,
                            and meaningless on OpenBSD.

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

                 ndots:n    Sets a threshold for the number of dots which must
                            appear in a name given to res_query(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 abso-
                            lute name before any search list elements are
                            appended to it.

                 tcp        Forces the use of TCP for queries.  Normal behav-
                            iour is to query via UDP but fall back to TCP on
                            failure.

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

ENVIRONMENT
     LOCALDOMAIN     A space-separated list of search domains, overriding the
                     search keyword of a system's resolv.conf or
                     resolv.conf.tail file.

     RES_OPTIONS     A space-separated list of resolver options, overriding
                     the options keyword of a system's resolv.conf or
                     resolv.conf.tail file.

FILES
     /etc/resolv.conf
     /etc/resolv.conf.tail

SEE ALSO
     gethostbyname(3), resolver(3), hosts(5), hostname(7), dhclient(8),
     nsd(8), unbound(8)

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

BSD                             April 25, 2017                             BSD