resolver(4) Kernel Interfaces Manual resolver(4)
NAME
resolver - resolver configuration file
SYNOPSIS
/etc/resolv.conf
DESCRIPTION
The resolver is a set of routines in the C library that provide access to the Internet Domain Name System. The resolver configuration file
(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 read by humans and contains a list of keywords with values that provide various types of resolver information.
The different configuration options 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 host name returned by gethostname(); the domain part is
taken to be everything after the first dot (.). Finally, if the host name does not contain a domain part, the root domain is assumed.
Search list for host name lookup. By default, the search list consists of the local domain name. You can change this system wide by speci-
fying search keyword followed by a list of domain names separated by spaces or tabs, or on a per-process basis by specifying the LOCALDO-
MAIN environment variable followed by a list of domain names separated by spaces or tabs.
Most resolver queries will be attempted using each component of the search path in turn until a match is found. This process might
be slow and 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. Enables the sorting of addresses returned by
gethostname. A sortlist comprises pairs of IP addresses and optional netmasks; each IP address and optional netmask are separated
by a slash character (/). The netmask is optional, and defaults to the current network netmask. You can specify up to 10 pairs.
For example: sortlist 130.155.160.0/255.255.240.0 130.155.0.0 Option for host name lookup. By default, when a host name contains
one or more dots (.), resolver looks up the name as given before applying the search list. The n specifies the minimum number of
dots a name must contain in order to be looked up as given before applying the search list. The range is from 0 to 15.
You can change the resolver options system wide by specifying options keyword followed a list of resolver options separated by spa-
ces, or on a per-process basis by specifying the RES_OPTIONS environment variable followed by a list of resolver options separated
by spaces.
The domain and search keywords are mutually exclusive. If more than one instance of these keywords is present, the last instance will
override.
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.
FILES
/etc/resolv.conf
RELATED INFORMATION
Functions: dn_comp(3), dn_expand(3), gethostbyname(3), res_init(3), res_mkquery(3), res_query(3), res_search(3), res_send(3).
Files: hostname(5).
Commands: named(8). delim off
resolver(4)