resolver -- resolver configuration file


keyword value ...


The resolver is a set of routines in the C library (resolver(SLIB)) that provides access to the Internet Domain Name System (DNS). The resolver configuration file, /etc/resolv.conf, 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. 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 installation are:

   nameserver primary_ip_address
   nameserver secondary_ip_address
   hostresorder local bind
   search domain_name

The configuration keywords are:

Internet address (in dot notation) of a name server that the resolver should query. Up to MAXNS (currently 3) 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.)

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 hostname returned by gethostname(SLIB): 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.

NOTE: 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 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.

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 gethostbyaddr routines (see gethostbyname(SLIB)). These lookup routines can search any or all of the following databases: DNS, the Network Information Service (NIS), and the file /etc/hosts. The hostresorder keyword value can include any or all of the tokens that 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, DNS, 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. Therefore, the following statement in /etc/resolv.conf is equivalent to the default behavior:
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 names.

Note also that if neither DNS nor NIS is currently available, /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 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 network. The IP address and optional network pairs are separated by slashes. Up to 10 pairs may be specified. For example:

Allows certain internal resolver variables to be modified. The syntax is:

options option

where option is one of the following:

Sets RES_DEBUG in _res.options.

Sets a threshold for the number of dots which must appear in a name given to res_query (see resolver(SLIB)) 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.

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 gethostbyname(SLIB) 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.



See also

gethostbyname(SLIB), hostname(ADMN), named(ADMN), resolver(SLIB)

``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