resolver - resolver configuration file
The resolver is a set of routines in the C library that provide access
to the Internet Domain Name System. 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 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
constructed from the domain name.
The different configuration options are:
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(); 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
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 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.
e.g. sortlist 188.8.131.52/255.255.240.0 184.108.40.206
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
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 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
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.
gethostbyname(3), hostname(7), named(8),
Name Server Operations Guide for BIND
4th Berkeley Distribution 1993-11-11 RESOLVER(5)
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