RESOLVER(5) BSD Programmer's Manual RESOLVER(5)NAMEresolver - resolver configuration file
SYNOPSIS
/etc/resolv.conf
DESCRIPTION
The resolver is a set of routines in the C library (resolve(3)) that
provide access to the Internet Domain Name System. The resolver configu-
ration 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 information.
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
Internet address (in dot notation) of a name server that the
resolver should query. 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(2); 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.
search Search list for host-name lookup. The search list is normally
determined from the local domain name; by default, it contains
only the local domain name. 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 un-
til 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 256 characters.
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. For example:
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 .
ndots:n sets a threshold for the number of dots which must ap-
pear 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 ab-
solute name before any search list elements are ap-
pended to it.
The domain and search keywords are mutually exclusive. If more than one
instance of these keywords is present, the last instance wins.
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 under
options.
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
SEE ALSOgethostbyname(3), hostname(7), named(8), resolver(3), resolver(5).
``Name Server Operations Guide for BIND''
4th Berkeley Distribution November 11, 1993 2