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 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 hostname
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 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 hostname returned by
gethostname(2);
the domain part is taken to be everything after the first aq.aq.
Finally, if the hostname 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
Sortlist allows addresses returned by
gethostbyname(3)
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 specified.
E.g.,
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options
Options allows certain internal resolver variables to be modified.
The syntax is
optionsoption...
where option is one of the following:
debug
sets
RES_DEBUG
in
_res.options.
ndots:n
sets a threshold for the number of dots which
must appear in a name given to
res_query(3)
(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.
timeout:n
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>).
attempts:n
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 <resolv.h>).
rotate
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.
no-check-names
sets
RES_NOCHECKNAME
in
_res.options,
which disables the modern BIND checking of incoming hostnames and
mail names for invalid characters such as underscore (_), non-ASCII,
or control characters.
inet6
sets
RES_USE_INET6
in
_res.options.
This has the effect of trying a AAAA query before an A query inside the
gethostbyname(3)
function, and of mapping IPv4 responses in IPv6 "tunneled form"
if no AAAA records are found but an A record set exists.
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 options.
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.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
