172 lines
5.2 KiB
Groff
172 lines
5.2 KiB
Groff
.\" Copyright (c) 1987 The Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms are permitted
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.\" provided that the above copyright notice and this paragraph are
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.\" duplicated in all such forms and that any documentation,
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.\" advertising materials, and other materials related to such
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.\" distribution and use acknowledge that the software was developed
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.\" by the University of California, Berkeley. The name of the
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.\" University may not be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.\"
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.\" @(#)hostname.7 6.4 (Berkeley) 1/16/90
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.\"
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.Dd February 16, 1994
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.Dt HOSTNAME @DESC_EXT_U@
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.Os BSD 4
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.Sh NAME
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.Nm hostname
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.Nd host name resolution description
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.Sh DESCRIPTION
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Hostnames are domains. A domain is a hierarchical, dot-separated list
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of subdomains. For example, the machine
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.Dq Li monet ,
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in the
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.Dq Li Berkeley
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subdomain of the
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.Dq Li EDU
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subdomain of the Internet Domain Name System would be represented as
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.Pp
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.Dl monet.Berkeley.EDU
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.Pp
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(with no trailing dot).
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.Pp
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Hostnames are often used with network client and server programs,
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which must generally translate the name to an address for use.
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(This task is usually performed by the library routine
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.Xr gethostbyname @LIB_NETWORK_EXT@ . )
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The default method for resolving hostnames by the Internet name resolver is
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to follow RFC 1535's security recommendations. Actions can be taken
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by the administrator to override these recommendations and to have the
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resolver behave the same as earlier, non-RFC 1535
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resolvers.
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.Pp
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The default method (using RFC 1535 guidelines) follows:
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.Pp
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If the name consists of a single component, i.e. contains no dot, and if the
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environment variable
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.Dq Ev HOSTALIASES
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is set to the name of a file,
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that file is searched for a string matching the input hostname. The file
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should consist of lines made up of two strings separated by white-space, the
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first of which is the hostname alias, and the second of which is the complete
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hostname to be substituted for that alias. If a case-insensitive match is
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found between the hostname to be resolved and the first field of a line in
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the file, the substituted name is looked up with no further processing.
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.Pp
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If there is at least one dot in the name, then the name is first tried
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.Dq as-is .
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The number of dots to cause this action is configurable by setting the
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threshold using the
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.Dq Li ndots
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option in
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.Pa /etc/resolv.conf
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(default: 1). If the name ends with a dot, the trailing dot is
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removed, and the remaining name is looked up (regardless of the setting of
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the
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.Li ndots
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option), without further processing.
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.Pp
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If the input name does not end with a trailing dot, it is looked up by
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searching through a list of domains until a match is found. If neither the
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search option in the
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.Pa /etc/resolv.conf
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file or the
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.Dq Ev LOCALDOMAIN
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environment variable is used, then the
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search list of domains contains only the full domain specified by the
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.Li domain
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option (in
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.Pa /etc/resolv.conf )
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or the domain used in the local hostname (see
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.Xr hostname @CMD_EXT@
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and
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.Xr resolver @FORMAT_EXT@ ) .
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For example, if the
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.Dq Li domain
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option is set to
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.Li CS.Berkeley.EDU ,
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then only
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.Li CS.Berkeley.EDU
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will be in the search list, and this will be the only
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domain appended to the partial hostname. For example, if
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.Dq Li lithium
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is the name to be resolved, this would make
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.Li lithium.CS.Berkeley.EDU
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the only name to be tried using the search list.
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.Pp
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If the
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.Li search
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option is used in
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.Pa /etc/resolv.conf
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or the environment variable
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.Dq Ev LOCALDOMAIN
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is set by the user, then
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the search list will include what is set by these methods. For
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example, if the
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.Dq Li search
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option contained
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.Pp
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.Dl CS.Berkeley.EDU CChem.Berkeley.EDU Berkeley.EDU
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.Pp
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then the partial hostname (e.g.,
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.Dq Li lithium )
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will be tried with
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.Em each
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domain name appended (in the same order specified); the resulting hostnames
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that would be tried are:
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.Bd -literal -offset indent
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lithium.CS.Berkeley.EDU
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lithium.CChem.Berkeley.EDU
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lithium.Berkeley.EDU
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.Ed
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.Pp
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The environment variable
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.Dq Ev LOCALDOMAIN
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overrides the
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.Dq Li search
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and
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.Dq Li domain
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options, and if both
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.Li search
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and
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.Li domain
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options are present in the resolver configuration file, then only the
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.Em last
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one listed is used (see
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.Xr resolver @FORMAT_EXT@ ) .
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.Pp
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If the name was not previously tried
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.Dq as-is
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(i.e., it fell below the
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.Dq Li ndots
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threshold or did not contain a dot), then the name as
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originally provided is attempted.
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.Sh ENVIRONMENT
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.Bl -tag -width "/etc/resolv.conf " -compress
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.It Ev LOCALDOMAIN
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Affects domains appended to partial hostnames.
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.It Ev HOSTALIASES
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Name of file containing
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.Pq Ar host alias , full hostname
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pairs.
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.El
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.Sh FILES
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.Bl -tag -width "/etc/resolv.conf " -compress
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.It Pa /etc/resolv.conf
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See
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.Xr resolve @FORMAT_EXT@ .
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.It Ev HOSTALIASES
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Name of file containing
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.Pq Ar host alias , full hostname
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pairs.
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.Sh SEE ALSO
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.Xr gethostbyname @LIB_NETWORK_EXT@ ,
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.Xr resolver @FORMAT_EXT@ ,
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.Xr mailaddr @DESC_EXT@ ,
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.Xr @INDOT@named @SYS_OPS_EXT@ .
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