The resolver routines are 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
computer, 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 as follows:
Internet address (in dot notation) of a name server that the
resolver routines should query. Up to MAXNS (currently
three) name servers can 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 computer. (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 is made).
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(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 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 can 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 might be slow and will
generate a lot of network traffic if the servers for the listed
domains are not local; 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(3) to be
sorted. A sortlist is specified by Internet Protocol (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 can be specified.
sortlist 126.96.36.199/255.255.240.0 188.8.131.52
Allows certain internal resolver variables to be
modified. The syntax is option... where option is one
of the following:
Sets RES_DEBUG in _res.options.
Sets a threshold for the number of dots that must appear in a
name given to res_query() 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 takes precedence.
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 previously in
The keyword and value must appear on a single line, and the
keyword (such as nameserver) must start the line. The value follows
the keyword, separated by white space.