resolv.conf(4tcp)

Synopsis

Description

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 different configuration keywords are:

domain

Local domain name. Most queries for names within this domain can use short names relative to the local domain. If no domain entries are present, the domain is determined from the local host name returned by gethostname(3C); the domain is taken to be everything after the first ``.''. Finally, if the host name does not contain a domain part, the root domain is assumed.

nameserver

Internet address (in dot notation) of a name server that the resolver should query. Up to three name servers may be listed; in that case 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).

options

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

options option ...

where option is one of the following:

debug

Sets RES_DEBUG in _res.options.

ndots:n

Sets 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.

search

Search list for host-name lookup. The search list is normally determined from the local domain name; by default, it begins with the local domain name, then successive parent domains that have at least two components in their names. For example:

search subnet1.company.com company.com

This default order 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.

sortlist

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 net. The IP address and optional network pairs are separated by slashes. Up to 10 pairs may be specified, for example:

sortlist 130.155.160.0/255.255.240.0 130.155.0.0

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 the options keyword description.

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 white space.

Lines beginning with a ``;'' or ``#'' are treated as comments and ignored.

Files

References

Examples

domain		yoyodyne.com
nameserver	128.212.64.5
nameserver	128.212.64.2