resolver(4)resolver(4)NAME
resolver, resolv.conf - resolver configuration file
SYNOPSISDESCRIPTION
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:
Internet (IP) address, in dot notation, of a name server
that the resolver should query. Up to (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
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).
Local domain name.
Most queries for names within this domain can use
short names relative to the local domain. If no
entry is present, the domain is determined from
the local host name returned by (see gethost‐
name(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.
Retransmission timeout. It is interpreted during
the (see resolver(3N)) call. It has higher
precedence than setting it through the (see
resolver(3N)) API and lower precedence than set‐
ting it through the (see resolver(3N)) environ‐
ment variable. Whenever an invalid value is
specified for a message is flagged in The default
value is 5000 milliseconds.
To use the option, add the following name-value
pair in
Example: To set the retransmission value to
6000, use:
Number of retries. This is interpreted during
the (see resolver(3N)) call. It has higher
precedence than setting it through the (see
resolver(3N)) API and lower precedence than set‐
ting it through the (see resolver(3N)) environ‐
ment variable. Whenever an invalid value is
specified for a message is flagged in The default
value is 4.
To use the option, add the following name-value
pair in
Example: To set the number of retries to 6, use:
Search list for host-name lookup.
If the search option is not used the search list
will contain only the local domain name. The
search list can be changed by listing the desired
domain search path following the 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 avail‐
able 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 and
Causes addresses returned by
gethostbyname(3N) to be sorted in accordance with
network numbers specified in the address list.
This option allows you to specify which subnets
and networks for to prefer if it receives multi‐
ple addresses as the result of a query from the
domain nameserver. The syntax is
A sortlist is specified by IP address netmask
pairs. The netmask is optional and defaults to
the netmask of the net. The IP address and
optional netmask pairs are separated by slashes.
Up to 10 pairs may be specified. The pairs are
separated by blank spaces.
The following sortlist directive sorts the
address on 128.32.42 subnet,
The argument after the slash is the subnet mask
for the subnet in question. To prefer an entire
network, you can omit the slash and the subnet
mask:
sorts any addresses in a reply that match these
arguments into the order in which they appear in
the sortlist, and appends those addresses that do
not match to the end.
Options allows certain internal resolver variables to be modi‐
fied.
The syntax is
where currently the option supported is the fol‐
lowing:
Set a threshold for the number of dots
which must appear in a name given
to (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 and keywords are mutually exclusive. If more than one instance of
these keywords is present, the last instance overrides.
The keyword of a system's file can be overridden on a per-process basis
by setting the environment variable to a space-separated list of search
domains. The keyword of a system's file can be amended on a per-
process basis by setting the environment variable 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
(for example, must start the line. The value follows the keyword, sep‐
arated by white space.
Note that the resolver routine silently ignores errors when reading
this file (see resolver(3N)).
EXAMPLES
A typical file resembles the following:
WARNINGS
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 file. HP recommends that the possi‐
ble domains for the search list be limited to those domains adminis‐
tered within your trusted organization. For more information on the
security implications of search lists please see RFC 1535.
AUTHOR
was developed by the University of California, Berkeley.
FILES
Resolver configuration file.
SEE ALSOnamed(1M), resolver(3N), gethostent(3N), hostname(5), RFC 1535.
resolver(4)