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.\"	@(#)hostname.7	6.4 (Berkeley) 1/16/90
.\"
.Dd February 16, 1994
.Dt HOSTNAME 7
.Os
.Sh NAME
.Nm hostname
.Nd host name resolution description
.Sh DESCRIPTION
Hostnames are domains.
A domain is a hierarchical, dot-separated list of subdomains.
For example, the machine
.Dq Li monet ,
in the
.Dq Li Berkeley
subdomain of the
.Dq Li EDU
subdomain of the Internet Domain Name System would be represented as
.Pp
.Dl monet.Berkeley.EDU
.Pp
(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
.Xr 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 administrator to override these
recommendations and to have the resolver behave the same as earlier,
non-RFC 1535 resolvers.
.Pp
The default method (using RFC 1535 guidelines) follows:
.Pp
If the name consists of a single component, i.e. contains no dot, and if the
environment variable
.Dq Ev 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.
.Pp
If there is at least one dot in the name, then the name is first tried
.Dq as-is .
The number of dots to cause this action is configurable by setting the
threshold using the
.Dq Li ndots
option in
.Pa /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
.Li ndots
option), without further processing.
.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
.Pa /etc/resolv.conf
file or the
.Dq Ev LOCALDOMAIN
environment variable is used, then the
search list of domains contains only the full domain specified by the
.Li domain
option (in
.Pa /etc/resolv.conf )
or the domain used in the local hostname.
For example, if the
.Dq Li domain
option is set to
.Li CS.Berkeley.EDU ,
then only
.Li CS.Berkeley.EDU
will be in the search list, and this will be the only
domain appended to the partial hostname.
For example, if
.Dq Li lithium
is the name to be resolved, this would make
.Li lithium.CS.Berkeley.EDU
the only name to be tried using the search list.
.Pp
If the
.Li search
option is used in
.Pa /etc/resolv.conf
or the environment variable
.Dq Ev LOCALDOMAIN
is set by the user, then
the search list will include what is set by these methods.
For example, if the
.Dq Li search
option contained
.Pp
.Dl CS.Berkeley.EDU CChem.Berkeley.EDU Berkeley.EDU
.Pp
then the partial hostname (e.g.,
.Dq Li lithium )
will be tried with
.Em each
domain name appended (in the same order specified); the resulting hostnames
that would be tried are:
.Bd -literal -offset indent
lithium.CS.Berkeley.EDU
lithium.CChem.Berkeley.EDU
lithium.Berkeley.EDU
.Ed
.Pp
The environment variable
.Dq Ev LOCALDOMAIN
overrides the
.Dq Li search
and
.Dq Li domain
options, and if both
.Li search
and
.Li domain
options are present in the resolver configuration file, then only the
.Em last
one listed is used (see
.Xr resolv.conf 5 ) .
.Pp
If the name was not previously tried
.Dq as-is
(i.e., it fell below the
.Dq Li ndots
threshold or did not contain a dot), then the name as
originally provided is attempted.
.Sh ENVIRONMENT
.Bl -tag -width "/etc/resolv.conf  "
.It Ev LOCALDOMAIN
Affects domains appended to partial hostnames.
.It Ev HOSTALIASES
Name of file containing
.Pq Ar host alias , full hostname
pairs.
.El
.Sh FILES
.Bl -tag -width "/etc/resolv.conf  " -compact
.It Pa /etc/resolv.conf
See
.Xr resolv.conf 5 .
.El
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr resolv.conf 5 ,
.Xr mailaddr 7