The resolver is a set of routines in the C library that provide access to
the Internet Domain Name System (DNS). 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
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 Name server IP address
Internet address (in dot notation) of a name server that
the resolver should query. Up to MAXNS (currently 3, see <resolv.h>)
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
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 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 spaces or tabs separating the names. Resolver
queries having fewer than ndots dots (default is 1) in them will be
attempted using each component of the search path in turn until a match is
found. For environments with multiple subdomains please read options
ndots:n below to avoid man-in-the-middle attacks and
unnecessary traffic for the root-dns-servers. 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 separated by slashes. Up to 10 pairs may be
sortlist 126.96.36.199/255.255.240.0 188.8.131.52
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 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.
sets the amount of time the resolver will wait for a
response from a remote name server before retrying the query via a
different name server. Measured in seconds, the default is RES_TIMEOUT
(currently 5, see <resolv.h>).
sets the number of times the resolver will send a query to
its name servers before giving up and returning an error to the calling
application. The default is RES_DFLRETRY (currently 2, see
sets RES_ROTATE in _res.options, which causes round
robin selection of nameservers from among those listed. This has the
effect of spreading the query load among all listed servers, rather than
having all clients try the first listed server first every time.
sets RES_NOCHECKNAME in _res.options, which disables
the modern BIND checking of incoming host names and mail names for invalid
characters such as underscore (_), non-ASCII, or control characters.
sets RES_USE_INET6 in _res.options. This has the
effect of trying a AAAA query before an A query inside the
gethostbyname() function, and of mapping IPv4 responses in IPv6
``tunnelled form'' if no AAAA records are found but an A record set
The domain and search keywords are mutually exclusive. If more
than one instance of these keywords is present, the last instance wins.
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 domains.
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 above under
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.