Hostnames are domains. A domain is a hierarchical, dot-separated list
of subdomains. For example, the machine
Li monet ,
in the
Li Berkeley
subdomain of the
Li 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 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 administrator to override these recommendations and to have the
resolver 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 the
environment variable
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.
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
Li ndots
option in
/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
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
Ev LOCALDOMAIN
environment 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
Li domain
option is set to
CS.Berkeley.EDU ,
then only
CS.Berkeley.EDU
will be in the search list, and this will be the only
domain appended to the partial hostname. For example, if
Li 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 variable
Ev LOCALDOMAIN
is set by the user, then
the search list will include what is set by these methods. For
example, if the
Li search
option contained
CS.Berkeley.EDU CChem.Berkeley.EDU Berkeley.EDU
then the partial hostname (e.g.,
Li lithium )
will be tried with
each
domain name appended (in the same order specified); the resulting hostnames
that would be tried are:
-literal -offset indent
lithium.CS.Berkeley.EDU
lithium.CChem.Berkeley.EDU
lithium.Berkeley.EDU
The environment variable
Ev LOCALDOMAIN
overrides the
Li search
and
Li domain
options, and if both
search
and
domain
options are present in 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
Li ndots
threshold or did not contain a dot), then the name as
originally provided is attempted.
