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     Hostnames are domains.  A domain is a hierarchical, dot-separated list of
     subdomains.  For example, the machine ``monet'', in the ``Berkeley'' sub­
     domain 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 admin­
     istrator to override these recommendations and to have the resolver be­
     have 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-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
     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 (de­
     fault:  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'' en­
     vironment 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 hostname(1) and resolver(5)).
     For example, if the ``domain'' option is set to CS.Berkeley.EDU, then on­
     ly 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 search list.

     If the search option is used in /etc/resolv.conf or the environment vari­
     able ``LOCALDOMAIN'' is set by the user, then the search list will in­
     clude what is set by these methods.  For example, if the ``search'' op­
     tion contained

     the resolver configuration file, then only the last one listed is used
     (see resolver(5)).

     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 original­
     ly provided is attempted.


     LOCALDOMAIN         Affects domains appended to partial hostnames.

     HOSTALIASES         Name of file containing (host alias, full hostname)


     /etc/resolv.conf    See resolve(5).

     HOSTALIASES         Name of file containing (host alias, full hostname)


     gethostbyname(3),  resolver(5),  mailaddr(7),  named(8).

4th Berkeley Distribution      February 16, 1994                             2

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