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HOSTNAME

Section: Environments, Tables, and Troff Macros (7)
Index
BSD mandoc
BSD 4

NAME

hostname - host name resolution description

DESCRIPTION

Hostnames are domains. A domain is a hierarchical, dot-separated listof subdomains. For example, the machine ``monet , ''in the ``Berkeley''subdomain of the ``EDU''subdomain of the Internet Domain Name System would be represented as

monet.Berkeley.EDU

(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 routinegethostbyname(3).)The default method for resolving hostnames by the Internet name resolver isto follow RFC 1535's security recommendations. Actions can be takenby the administrator to override these recommendations and to have theresolver 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 theenvironment variable ``HOSTALIASES ''is set to the name of a file,that file is searched for a string matching the input hostname. The fileshould consist of lines made up of two strings separated by white-space, thefirst of which is the hostname alias, and the second of which is the completehostname to be substituted for that alias. If a case-insensitive match isfound between the hostname to be resolved and the first field of a line inthe 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 thethreshold using the ``ndots''option in /etc/resolv.conf(default: 1). If the name ends with a dot, the trailing dot isremoved, and the remaining name is looked up (regardless of the setting ofthe ndots option), without further processing.

If the input name does not end with a trailing dot, it is looked up bysearching through a list of domains until a match is found. If neither thesearch option in the/etc/resolv.conffile or the ``LOCALDOMAIN ''environment variable is used, then thesearch list of domains contains only the full domain specified by the domainoption (in/etc/resolv.conf or the domain used in the local hostname (see hostname(1)andresolver(5)).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 onlydomain appended to the partial hostname. For example, if ``lithium ''is the name to be resolved, this would makelithium.CS.Berkeley.EDU the only name to be tried using the search list.

If the search option is used in/etc/resolv.confor the environment variable ``LOCALDOMAIN ''is set by the user, thenthe search list will include what is set by these methods. Forexample, if the ``search ''option contained

CS.Berkeley.EDU CChem.Berkeley.EDU Berkeley.EDU

then the partial hostname (e.g., ``lithium ) ''will be tried with eachdomain name appended (in the same order specified); the resulting hostnames that would be tried are:

lithium.CS.Berkeley.EDUlithium.CChem.Berkeley.EDUlithium.Berkeley.EDU

The environment variable ``LOCALDOMAIN ''overrides the``search ''and ``domain ''options, and if both search and domainoptions are present in the resolver configuration file, then only the lastone listed is used (seeresolver(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 asoriginally provided is attempted.

ENVIRONMENT

LOCALDOMAIN: Affects domains appended to partial hostnames.
HOSTALIASES: Name of file containing(host alias , full hostname)pairs.

FILES

/etc/resolv.conf: Seeresolve(5).
HOSTALIASES: Name of file containing (host alias , full hostname)pairs.

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