.\" $OpenBSD: hostname.7,v 1.2 1997/03/12 10:42:14 downsj Exp $ .\" Copyright (c) 1987 The Regents of the University of California. .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms are permitted .\" provided that the above copyright notice and this paragraph are .\" duplicated in all such forms and that any documentation, .\" advertising materials, and other materials related to such .\" distribution and use acknowledge that the software was developed .\" by the University of California, Berkeley. The name of the .\" University may not be used to endorse or promote products derived .\" from this software without specific prior written permission. .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .\" .\" @(#)hostname.7 6.4 (Berkeley) 1/16/90 .\" .TH HOSTNAME 7 "February 16, 1994" .UC 5 .SH NAME hostname \- host name resolution description .SH DESCRIPTION Hostnames are domains. A domain is a hierarchical, dot-separated list of subdomains. For example, the machine \fImonet\fP, in the \fIBerkeley\fP subdomain of the \fIEDU\fP subdomain of the Internet Domain Name System would be represented as .br \fImonet\fP.\fIBerkeley\fP.\fIEDU\fP .br (with no trailing dot). .PP 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 .IR gethostbyname (3).) The default method for resolving hostnames by the Internet name resolver is to follow \s-1RFC\s+1 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-\s-1RFC\s+1 1535 resolvers. .PP The default method (using \s-1RFC\s+1 1535 guidelines) follows: .PP If the name consists of a single component, i.e. contains no dot, and if the environment variable ``\s-1HOSTALIASES\s+1'' 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. .PP 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 ``\fIndots\fP'' option in .I /etc/resolv.conf (default: \fI1\fP). 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. .PP 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 .I /etc/resolv.conf file or the ``\s-1LOCALDOMAIN\s+1'' environment variable is used, then the search list of domains contains only the full domain specified by the domain option (in .IR /etc/resolv.conf ) or the domain used in the local hostname (see .IR hostname (1) and .IR resolver (5)). For example, if the ``\fIdomain\fP'' option is set to \fICS.Berkeley.EDU\fP, then only CS.Berkeley.EDU will be in the search list and will be the only domain appended to the partial hostname, for example, ``\fIlithium\fP'', making \fIlithium.CS.Berkeley.EDU\fP the only name to be tried using the search list. .PP If the search option is used in .I /etc/resolv.conf or the environment variable, ``\s-1LOCALDOMAIN\s+1'' is set by the user, then the search list will include what is set by these methods. For example, if the ``\fIsearch\fP'' option contained .br \fICS.Berkeley.EDU CChem.Berkeley.EDU Berkeley.EDU\fP .br then the partial hostname (e.g., ``\fIlithium\fP'') will be tried with each domainname appended (in the same order specified). The resulting hostnames that would be tried are: .nf \fIlithium.CS.Berkeley.EDU\fP \fIlithium.CChem.Berkeley.EDU\fP \fIlithium.Berkeley.EDU\fP .fi .PP The environment variable ``\s-1LOCALDOMAIN\s+1'' overrides the ``\fIsearch\fP'' and ``\fIdomain\fP'' options, and if both search and domain options are present in the resolver configuration file, then only the last one listed is used (see .IR resolver (5)). .PP If the name was not previously tried ``as is'' (i.e., it fell below the ``\fIndots\fP'' threshold or did not contain a dot), then the name as originally provided is attempted. .SH SEE ALSO .IR gethostbyname (3), .IR resolver (5), .IR mailaddr (5), .IR named (8)