NAME
resolver — resolver configuration file
DESCRIPTION
The
resolver
is a set of routines in the C library (see
resolver(3N))
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.
If the only name server to be queried is on the local machine,
then this file is not always necessary. The domain name could be
determined from the host name (see
hostname(1)),
if it has been set as a fully qualified domain name.
Recognized configuration options include:
- nameserver
Internet (IP) address, in dot notation, of a name server
that the resolver should query.
Up to
MAXNS
(currently 3) name servers can 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: Try a name server; if the query times out,
try the next and continue until all name servers have been tried,
then repeat trying all the name servers
until a maximum number of retries have been 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 host name returned by
gethostname()
(see
gethostname(2));
the domain part is interpreted as everything after the first dot
(.).
Finally, if the host name does not contain a domain part,
the root domain is assumed.
- search
Search list for host-name lookup.
If the search option is not used the search list will contain
only the the local domain name.
The search list can 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 generates a lot of network traffic
if the servers for the listed domains are not local,
and that queries 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.
The first domain in the search list must be the local domain for
short names to work properly in various files (such as
.rhosts
and
inetd.sec)
- options
Options allows certain internal resolver variables to be modified.
The syntax is
where currently the option supported is the following:
- ndots:n
Set a threshold for the number of dots
which must appear in a name given to
res_query
(see
resolver(3N))
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
domain
and
search
keywords are mutually exclusive.
If more than one instance of these keywords is present,
the last instance overrides.
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.
Note that the resolver routine
res_init()
silently ignores errors when reading this file (see
resolver(3N)).
EXAMPLES
A typical
resolv.conf
file resembles the following:
domain div.inc.com
nameserver 15.19.8.119
nameserver 15.19.8.197
WARNING
In order to reduce situations that may cause connections to unintended
destinations, the administrator should carefully select which domains
are put in the search list in the resolv.conf file. HP recommends that
the possible domains for the search list be limited to those
domains administered within your trusted organization. For more
information on the security implications of search lists please see
RFC
1535,
located in
/usr/share/doc.
AUTHOR
resolver
was developed by the University of California, Berkeley.
FILES
- /etc/resolv.conf
Resolver configuration file.
