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). 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 everything
after the first dot (.). Finally, if the host name does
not contain a domain part, the root domain is assumed.
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
The search list is currently limited to six domains
with a total of 256 characters. 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 example:
sortlist 18.104.22.168/255.255.240.0 22.214.171.124
Option for host name lookup. By default, when a
host name contains 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 15.
You can change the resolver options system wide by
specifying options keyword followed a list of
resolver options separated by spaces, or on a perprocess
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),
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