hostname(5)

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().)

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 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 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 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) and no further processing 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 /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 (see resolver(4)). For example, if the domain 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 lithium.CS.Berkeley.EDU the only name to be tried using the search list.

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 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 LOCALDOMAIN overrides the search and domain 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 ndots threshold or did not contain a dot), then the name, as originally provided, is attempted.

AUTHOR

hostname was developed by the University of California, Berkeley.

SEE ALSO

named(1M), gethostbyname(3N), gethostent(3N), resolver(4), RFC 1535.