resolver(4) Kernel Interfaces Manual resolver(4)
NAME
resolver, resolv.conf - resolver configuration file
SYNOPSIS
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:
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 mul-
tiple 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 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.
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 setting it through the (see resolver(3N)) environment variable. Whenever an invalid value is speci-
fied 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 setting it through the (see resolver(3N)) environment variable. Whenever an invalid value is speci-
fied 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 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 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 multiple 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 modified.
The syntax is
where currently the option supported is the following:
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 sepa-
rated 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 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.
AUTHOR
was developed by the University of California, Berkeley.
FILES
Resolver configuration file.
SEE ALSO
named(1M), resolver(3N), gethostent(3N), hostname(5), RFC 1535.
resolver(4)
