The
resolver
is a set of routines in the C library
Xr resolve 3
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 machine,
the domain name is determined from the host name,
and the domain search path is constructed from the domain name.
The different configuration directives are:
-tag -width "nameser"
Li nameserver
Internet address (in dot notation) of a name server that the
resolver
should query. Up to
MAXNS
(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).
Li 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 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.
Li 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.
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.
Li 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 specified. For example:
-literal -offset indent
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
Li options
Allows certain internal
resolver
variables to be modified.
The syntax is
Li options Ar option ...
where
option
is one of the following:
-tag -width "ndots:n "
Li debug
sets
RES_DEBUG
in
_res.options .
Li ndots: Ns Ar n
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.
Li timeout: Ns Ar 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
(see
<resolv.h> ).
Li attempts: Ns Ar 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
(see
<resolv.h> ).
Li 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.
Li no-check-names
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.
Li inet6
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 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
Ev 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
Ev 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.