RESOLVER(5) BSD File Formats Manual RESOLVER(5)NAMEresolver — resolver configuration file
The resolver(3) is a set of routines in the C library which 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 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 multi‐
ple 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(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
search Search list for host-name lookup. The search list is nor‐
mally 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 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.
sortlist 18.104.22.168/255.255.240.0 22.214.171.124
options Options allows certain internal resolver variables to be mod‐
ified. 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”, mean‐
ing 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
timeout:n sets the initial amount of time the resolver
will wait for a response from a remote name
server before retrying the query via a differ‐
ent name server. The resolver may wait longer
during subsequent retries of the current query
since an exponential back-off is applied to the
timeout value. Measured in seconds, the
default is RES_TIMEOUT, the allowed maximum is
RES_MAXRETRANS (see <resolv.h>).
attempts:n sets the number of times the resolver will send
a query to each of its name servers before giv‐
ing up and returning an error to the calling
application. The default is RES_DFLRETRY, the
allowed maximum is RES_MAXRETRY (see
no_tld_query tells the resolver not to attempt to resolve a
top level domain name, that is, a name that
contains no dots. Use of this option does not
prevent the resolver from obeying the standard
domain and search rules with the given name.
Options may also be specified as a space or tab separated
list using the RES_OPTIONS environment variable.
The domain and search keywords are mutually exclusive. If more than one
instance of these keywords is present, the last instance will override.
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, sepa‐
rated by white space.
/etc/resolv.conf The file resolv.conf resides in /etc.
SEE ALSOgethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND.
The resolv.conf file format appeared in 4.3BSD.
BSD September 9, 2004 BSD