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RESOLVER

Section: 1993 (5)
Updated: November
Index

NAME

resolver - resolver configuration file

SYNOPSIS

/etc/resolv.conf

DESCRIPTION

Theresolveris a set of routines in the C library (resolv(3))that provide access to the Internet Domain Name System.The resolver configuration file contains information that is readby the resolver routines the first time they are invoked by a process.The file is designed to be human readable and contains a list ofkeywords with values that provide various types of resolver information.

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 host name,and the domain search path is constructed from the domain name.

The different configuration options are:

nameserver: Internet address (in dot notation) of a name serverthat 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 serversuntil a maximum number of retries are made).
domain: Local domain name.Most queries for names within this domain can use short namesrelative to the local domain.If no domain entry is present, the domain is determinedfrom the local host name returned bygethostname(2);the domain part is taken to be everything after the first `.'.Finally, if the host name does not contain a domain part, the rootdomain is assumed.
search: Search list for host-name 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 pathfollowing the search keyword with spaces or tabs separatingthe names.Most resolver queries will be attempted using each componentof the search path in turn until a match is found.Note that this process may be slow and will generate a lot of networktraffic if the servers for the listed domains are not local,and that queries will time out if no server is availablefor one of the domains.
: The search list is currently limited to six domainswith a total of 256 characters.
sortlist: Sortlist allows addresses returned by gethostbyname to be sorted.A sortlist is specified by IP address netmask pairs. The netmask isoptional and defaults to the natural netmask of the net. The IP addressand optional network pairs are separated by slashes. Up to 10 pairs maybe specified.
: e.g. sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options: 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 whichmust appear in a name given to res_query (see resolver(3))before an initial absolute query will be made. The default forn is ``1'', meaning that if there are any dots in a name, the namewill be tried first as an absolute name before any search listelements are appended to it.

The domain and search keywords are mutually exclusive.If more than one instance of these keywords is present,the last instance wins.

The search keyword of a system's resolv.conf file can beoverridden 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 beamended on a per-process basis by setting the environment variable``RES_OPTIONS'' to a space-separated list of resolver optionsas explained above under options.

The keyword and value must appear on a single line, and the keyword(e.g. nameserver) must start the line. The value followsthe keyword, separated by white space.

FILES

/etc/resolv.conf

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