MirBSD manpage: resolv.conf(5), resolv.conf.tail(5), resolver(5)

RESOLV.CONF(5)               BSD Reference Manual               RESOLV.CONF(5)


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


     The resolv.conf file specifies how the resolver(3) routines in the C li-
     brary (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 pro-
     cess. The file is designed to be human readable and contains a list of
     keywords with values that provide various types of resolver information.

     If dhcp(8) is used to configure the network, the DHCP server will normal-
     ly provide information such as nameserver addresses. In this case, it is
     not normally necessary to provide a resolv.conf file. However the DHCP
     network configuration script, dhclient-script(8), will overwrite entries
     in resolv.conf. In order to force options to be passed to the resolver(3)
     routines, the file resolv.conf.tail may be created. This file is appended
     to the generated resolv.conf file by dhclient-script(8), ensuring options

     A hash mark '#' or semicolon ';' 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.

     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 list-
                 ed. 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 ".". 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  Use the Domain Name server by querying named(8).
                 file  Search for entries in /etc/hosts.

                 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 data-
                 base 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 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    Allows addresses returned by gethostbyname(3) to be sorted. A
                 sortlist is specified by IP address netmask pairs. The net-
                 mask 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, e.g.:


     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.

                 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 are 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      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 (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 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 or resolv.conf.tail 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 or resolv.conf.tail 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

     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 whitespace.




     gethostbyname(3), resolver(3), hosts(5), hostname(7), dhclient-script(8),
     dhcp(8), named(8)

     Name Server Operations Guide for BIND.


     The resolv.conf file format appeared in 4.3BSD.


     Due to resolver internal issues, getaddrinfo(3) may not behave as lookup
     suggests. Consequently, userland programs that use getaddrinfo(3) may
     behave differently from what lookup says.

MirBSD #10-current               May 10, 1991                                2

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