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RESOLV.CONF(5)		     BSD Reference 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 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
     remain.

     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.:

		 sortlist 130.155.160.0/255.255.240.0 130.155.0.0

     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
     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,
     separated by whitespace.

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

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

     Name Server Operations Guide for BIND.

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

BUGS
     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.

MirOS BSD #10-current		 May 10, 1991				     2
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