resolver configuration file
is a set of routines in the C library
that provides access to the Internet Domain Name System (DNS).
configuration file, /etc/resolv.conf,
contains information that is read
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
The keyword and value must appear on a single line, and the keyword
(for example, nameserver) must start the line. The value
follows the keyword, separated by whitespace.
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.
Beginning with SCO OpenServer Release 5.0.6, the /etc/resolv.conf file
is created only if nameserver addresses and the system domain
name were entered during initial installation. Configuration
option values placed in /etc/resolv.conf during
hostresorder local bind
The configuration keywords are:
Internet address (in dot notation) of a name server that the
should query. Up to
(currently 3) name servers may be listed, one per keyword.
If there are multiple servers, the
library queries them in the order listed.
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.)
Local domain name.
Most queries for names within this domain can use short names
relative to the local domain.
entry is present, the domain is determined from the local hostname returned by
the domain part is taken to be everything after the first
Finally, if the hostname does not contain a domain part, the root
domain is assumed.
The domain and search
keywords are mutually exclusive.
If more than one instance of these keywords is present,
the last instance wins.
Search list for hostname 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
keyword with spaces or tabs separating the names.
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.
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.
Specifies database search order for the gethostbyname and
These lookup routines can search
any or all of the following databases: DNS,
the Network Information Service (NIS), and the file
hostresorder keyword value can include any or all of the
represent these databases: bind, nis, or local.
Tokens must be separated by white-space and/or a slash (/). If the
slash is used, a query will stop even if the database being queried
does not have the information. Otherwise, the next database specified
will be searched.
For example, the following line specifies a lookup order of NIS,
and finally the /etc/hosts file. All databases will be tried until a
match is found.
hostresorder nis bind local
The following example, however, specifies that the query should be
terminated if the answer is not found in either NIS or DNS.
hostresorder nis bind / local
The default behavior, if the keyword hostresorder is not specified
or a value is not specified, is to search all three databases, regardless of
failure, in the following order: bind, nis, and local.
following statement in /etc/resolv.conf is equivalent to the default
hostresorder bind nis local
Note that searching /etc/hosts prior to either of the other databases
can lead to inconsistent results if the hosts file contains out-of-date
information. However, avoiding network traffic during hostname lookup
can be useful in situations where only demand-dial serial lines are
available, since it can eliminate time and expense when resolving local
Note also that if neither DNS nor NIS is currently
/etc/hosts will be searched even if local is not specified on the
hostresorder line or even if local is preceded by a slash.
This is so
that lookups can be satisfied even when the TCP/IP protocol stack is
not fully initialized.
The user can override the default search order by setting the
environment variable HOSTRESORDER. For example:
HOSTRESORDER="nis bind local"
Allows addresses returned by
to be sorted.
is specified by IP address netmask pairs. The netmask is
optional and defaults to the natural netmask of the network. The IP
and optional network pairs are separated by slashes. Up to 10 pairs may
be specified. For example:
sortlist 184.108.40.206/255.255.240.0 220.127.116.11
Allows certain internal
variables to be modified.
The syntax is:
is one of the following:
Sets RES_DEBUG in
Sets a threshold for the number of dots which
must appear in a name given to
initial absolute query
will be made. The default for
n is 1,
meaning that if there are
dots in a name, the name will be tried first as an absolute name before any
elements are appended to it.
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>).
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>).
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.
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.
Sets RES_USE_INET6 in _res.options. This has
the effect of trying a AAAA query before an A query inside the
function, and of mapping IPv4 responses in IPv6 "tunnelled form"
if no AAAA records are found but an A record set exists.
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 in this section.
``Configuring the Domain Name Service'' in the Networking Guide
© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003