HOSTNAME(7) Miscellaneous Information Manual HOSTNAME(7)
hostname -- host name resolution description
Hostnames are domains. A domain is a hierarchical, dot-separated list of
subdomains. For example, the machine ``monet'', in the ``Berkeley''
subdomain of the ``EDU'' subdomain 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 gethostbyname(3).) The default
method for resolving hostnames by the Internet name resolver is to follow
RFC 1535's 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 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 ``HOSTALIASES'' 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-
insensitive 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 further processing.
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
setting the threshold using the ``ndots'' option in /etc/resolv.conf
(default: 1). 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), without further processing.
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 /etc/resolv.conf file or the ``LOCALDOMAIN''
environment variable is used, then the search list of domains contains
only the full domain specified by the domain option (in /etc/resolv.conf)
or the domain used in the local hostname. For example, if the ``domain''
option is set to CS.Berkeley.EDU, then only CS.Berkeley.EDU will be in
the search list, and this will be the only domain appended to the partial
hostname. For example, if ``lithium'' is the name to be resolved, this
would make lithium.CS.Berkeley.EDU the only name to be tried using the
If the search option is used in /etc/resolv.conf or the environment
variable ``LOCALDOMAIN'' is set by the user, then the search list will
include what is set by these methods. For example, if the ``search''
CS.Berkeley.EDU CChem.Berkeley.EDU Berkeley.EDU
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 ``LOCALDOMAIN'' overrides the ``search'' and
``domain'' options, and if both search and domain options are present in
the resolver configuration file, then only the last one listed is used
If the name was not previously tried ``as-is'' (i.e., it fell below the
``ndots'' threshold or did not contain a dot), then the name as
originally provided is attempted.
LOCALDOMAIN Affects domains appended to partial hostnames.
HOSTALIASES Name of file containing (host alias, full hostname)
/etc/resolv.conf See resolver(5).
gethostbyname(3), resolver(5), mailaddr(7)
NetBSD 6.1.5 February 16, 1994 NetBSD 6.1.5