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
(resolv.conf) contains information that is read by the resolver routines
the first time they are invoked by a process. The file is designed to be
read by humans and contains a list of keywords with values that provide
various types of resolver information.
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 every-
thing after the first dot (.). Finally, if the host name does not
contain a domain part, the root domain is assumed.
search Search list for host name lookup. By default, the search list
consists of the local domain name. You can change this system
wide by specifying search keyword followed by a list of domain
names separated by spaces or tabs, or on a per-process basis by
specifying the LOCALDOMAIN environment variable followed by a
list of domain names separated by spaces or tabs.
Most resolver queries will be attempted using each component of
the search path in turn until a match is found. This process
might be slow and 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 Enables the sorting of addresses returned by gethostname. A
sortlist comprises pairs of IP addresses and optional netmasks;
each IP address and optional netmask are separated by a slash
character (/). The netmask is optional, and defaults to the
current network netmask. You can specify up to 10 pairs. For
sortlist 188.8.131.52/255.255.240.0 184.108.40.206
Option for host name lookup. By default, when a host name con-
tains one or more dots (.), resolver looks up the name as given
before applying the search list. The n specifies the minimum
number of dots a name must contain in order to be looked up as
given before applying the search list. The range is from 0 to
You can change the resolver options system wide by specifying
options keyword followed a list of resolver options separated by
spaces, or on a per-process basis by specifying the RES_OPTIONS
environment variable followed by a list of resolver options
separated by spaces.
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 (for
example, nameserver) must start the line. The value follows the keyword,
separated by white space.
Functions: dn_comp(3), dn_expand(3), gethostbyname(3), res_init(3),
res_mkquery(3), res_query(3), res_search(3), res_send(3).