RESOLV.CONF

Section: File Formats (5)
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BSD mandoc
BSD 4

NAME

resolv.conf - resolver configuration file

DESCRIPTION

The resolv.conf file specifies how the resolver(3) routines in the C library (which provide access to the Internet Domain Name System) should operate. 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.

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 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).

domain

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 host name returned by gethostname(2); the domain part is taken to be everything after the first `.'. Finally, if the host name does not contain a domain part, the root domain is assumed.

lookup

This keyword is used by the library routines gethostbyname(3) and gethostbyaddr(3). It specifies which databases should be searched, and the order to do so. The legal space-separated values are

bind: use the Domain Name server by querying the named(8)
file: search for entries in /etc/hosts
yp: talk to the YP system if ypbind(8) is running

If the lookup keyword is not used in the system's resolv.conf file then the assumed order is bind file Furthermore, if the system's resolv.conf file does not exist, then the only database used is file

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. 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.

sortlist

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 seperated by slashes. Up to 10 pairs may be specified, ie.

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 which must appear in a name given to res_query (see resolver(3)) 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 domain and search keywords are mutually exclusive. If more than one instance of these keywords is present, the last instance will override.

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.

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

FILES

/etc/resolv.conf: The file resolv.conf resides in /etc

HISTORY

The resolv.conf file format appeared in BSD 4.3