RESOLV.CONF(5)						      BSD 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 infor-
     mation.

     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 constructed 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 num-
		 ber 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     lookup	 This keyword is now ignored: its function has been superseded by features of nsswitch.conf(5).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 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	 Sortlist allows addresses returned by gethostbyname 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, ie.

		 sortlist 130.155.160.0/255.255.240.0 130.155.0.0

     options	 Options allows certain internal resolver variables to be modified.  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)).

		 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.

		 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 over-
     ride.

     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,
     separated 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.

BSD
								  August 21, 2010							       BSD