hostname(5)hostname(5)NAMEhostname - host name resolution description
Hostnames are domains. A domain is a hierarchical, dot-separated list
of subdomains. For example, the machine in the subdomain of the subdo‐
main of the Internet Domain Name System would be represented as
(with no trailing dot).
Hostnames are often used with network client and server programs, which
must generally translate the name to an address for use. (This task is
usually performed by the library routine
When NIS or the host table is being used for hostname resolution, the
hostname is looked up without modification. When DNS is used, the
resolver may append domains to the hostname.
The default method for resolving hostnames by the Internet name
resolver is to follow security recommendations. Actions can be taken
by the administrator to override these recommendations and to have the
resolver behave the same as earlier, non-RFC 1535 compliant resolvers.
The default method (using RFC 1535 guidelines) follows:
If the name consists of a single component, i.e. contains no dot, and
if the environment variable is set to the name of a file, that file is
searched for a string matching the input hostname. The file should
consist of lines made up of two strings separated by white-space, the
first of which is the hostname alias, and the second of which is the
complete hostname to be substituted for that alias. If a case-insensi‐
tive match is found between the hostname to be resolved and the first
field of a line in the file, the substituted name is looked up with no
If there is at least one dot in the name, then the name is first tried
as is. The number of dots to cause this action is configurable by set‐
ting the threshold using the option in (default: If the name ends with
a dot, the trailing dot is removed, and the remaining name is looked up
(regardless of the setting of the 'ndots' option) and no further pro‐
cessing is done.
If the input name does not end with a trailing dot, it is looked up by
searching through a list of domains until a match is found. If neither
the search option in the file or the environment variable is used, then
the search list of domains contains only the full domain specified by
the domain option (in or the domain used in the local hostname (see
resolver(4)). For example, if the option is set to CS.Berkeley.EDU,
then only CS.Berkeley.EDU will be in the search list and will be the
only domain appended to the partial hostname, lithium, making the only
name to be tried using the search list.
If the search option is used in or the environment variable, is set by
the user, then the search list will include what is set by these meth‐
ods. For example, if the option contained
then the partial hostname (e.g., lithium) will be tried with each
domain name appended (in the same order specified). The resulting
hostnames that would be tried are:
The environment variable overrides the and options, and if both options
are present in the resolver configuration file, then only the last one
listed is used (see resolver(4)).
If the name was not previously tried ``as is'' (i.e., it fell below the
threshold or did not contain a dot), then the name, as originally pro‐
vided, is attempted.
was developed by the University of California, Berkeley.
SEE ALSOnamed(1M), gethostbyname(3N), gethostent(3N), resolver(4), RFC 1535.