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The file specifies
how the routines in the C library (which provide access to the Internet
Domain Name System) should operate. 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: Internet
address (in dot notation) of a name server that the resolver should query.
Up to (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 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). Local
domain name. Most queries for names within this domain can use short names
relative to the local domain. If no entry is present, the domain is determined
from the local host name returned by 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. This keyword is used by the library routines
and It specifies which databases should be searched, and the order to
do so. The legal space-separated values are use the Domain Name server
by querying the search for entries in talk to the YP system if is
running If the keyword is not used in the system’s file then the assumed
order is Furthermore, if the system’s file does not exist, then the only
database used is Search list for host-name lookup. The search list is
normally determined from the local domain name; by default, it begins with
the local domain name, then successive parent domains that have at least
two components in their names. This may be changed by listing the desired
domain search path following the 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 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
seperated by slashes. Up to 10 pairs may be specified, ie. Options allows
certain internal resolver variables to be modified. The syntax is: where
option is one of the following: sets RES_DEBUG in _res.options. sets a
threshold for the number of dots which must appear in a name given to res_query
(see 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 and keywords are mutually exclusive. If more than one instance
of these keywords is present, the last instance will override. The keyword
of a system’s file can be overridden on a per-process basis by setting the
environment variable to a space-separated list of search domains. The
keyword of a system’s file can be amended on a per-process basis by setting
the environment variable to a space-separated list of resolver options
as explained above. The keyword and value must appear on a single line,
and the keyword (e.g. must start the line. The value follows the keyword,
separated by white space.
The file resides in
The
file format appeared in
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