RESOLVER(5) BSD File Formats Manual RESOLVER(5)
resolver -- 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 constructed 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 max-
imum 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 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 spa-
ces 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 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. E.g.,
sortlist 22.214.171.124/255.255.240.0 126.96.36.199
options 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.
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 different 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 giving 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 over-
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.
/etc/resolv.conf The file resolv.conf resides in /etc.
gethostbyname(3), resolver(3), hostname(7)
Name Server Operations Guide for BIND.
The resolv.conf file format appeared in 4.3BSD.
December 25, 2013 BSD