RESOLVER(5) BSD File Formats Manual RESOLVER(5)
resolver -- resolver configuration file
The resolver is a set of routines in the C library (resolve(3)) 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 vari-
ous 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 directives are:
Internet address (in dot notation) of a name server that the resolver should query.
Up to MAXNS (see <resolv.h>) 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 deter-
mined 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 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.
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 18.104.22.168/255.255.240.0 22.214.171.124
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.
sets the amount of time the resolver will wait for a response from a
remote name server before retrying the query via a different name server.
Measured in seconds, the default is RES_TIMEOUT (see <resolv.h> ).
sets the number of times the resolver will send a query to its name
servers before giving up and returning an error to the calling applica-
tion. The default is RES_DFLRETRY (see <resolv.h> ).
rotate sets RES_ROTATE in _res.options, which causes round robin selection of
nameservers from among those listed. This has the effect of spreading
the query load among all listed servers, rather than having all clients
try the first listed server first every time.
sets RES_NOCHECKNAME in _res.options, which disables the modern BIND
checking of incoming host names and mail names for invalid characters
such as underscore (_), non-ASCII, or control characters.
inet6 sets RES_USE_INET6 in _res.options. This has the effect of trying a AAAA
query before an A query inside the gethostbyname function, and of mapping
IPv4 responses in IPv6 ``tunnelled form'' if no AAAA records are found
but an A record set exists.
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
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.
gethostbyname(3), hostname(7), named(8), resolver(3), resolver(5). ``Name Server Operations
Guide for BIND''
4th Berkeley Distribution November 11, 1993 4th Berkeley Distribution