host.conf - resolver configuration file
The file
/etc/host.conf contains configuration information specific to
the resolver library. It should contain one configuration keyword per line,
followed by appropriate configuration information. The following keywords are
recognized:
- trim
- This keyword may be listed more than once. Each time it
should be followed by a list of domains, separated by colons (':'),
semicolons (';') or commas (','), with the leading dot. When set, the
resolver library will automatically trim the given domain name from the
end of any hostname resolved via DNS. This is intended for use with local
hosts and domains. (Related note: trim will not affect hostnames
gathered via NIS or the hosts(5) file. Care should be taken to
ensure that the first hostname for each entry in the hosts file is fully
qualified or unqualified, as appropriate for the local installation.)
- multi
- Valid values are on and off. If set to
on, the resolver library will return all valid addresses for a host
that appears in the /etc/hosts file, instead of only the first.
This is off by default, as it may cause a substantial performance
loss at sites with large hosts files.
- reorder
- Valid values are on and off. If set to
on, the resolver library will attempt to reorder host addresses so
that local addresses (i.e., on the same subnet) are listed first when a
gethostbyname(3) is performed. Reordering is done for all lookup
methods. The default value is off.
The following environment variables can be used to allow users to override the
behavior which is configured in
/etc/host.conf:
- RESOLV_HOST_CONF
- If set, this variable points to a file that should be read
instead of /etc/host.conf.
- RESOLV_MULTI
- Overrides the multi command.
- RESOLV_REORDER
- Overrides the reorder command.
- RESOLV_ADD_TRIM_DOMAINS
- A list of domains, separated by colons (':'), semicolons
(';'), or commas (','), with the leading dot, which will be added to the
list of domains that should be trimmed.
- RESOLV_OVERRIDE_TRIM_DOMAINS
- A list of domains, separated by colons (':'), semicolons
(';'), or commas (','), with the leading dot, which will replace the list
of domains that should be trimmed. Overrides the trim command.
- /etc/host.conf
- Resolver configuration file
- /etc/resolv.conf
- Resolver configuration file
- /etc/hosts
- Local hosts database
The following differences exist compared to the original implementation. A new
command
spoof and a new environment variable
RESOLV_SPOOF_CHECK
can take arguments like
off,
nowarn, and
warn. Line
comments can appear anywhere and not only at the beginning of a line.
The
nsswitch.conf(5) file is the modern way of controlling the order of
host lookups.
In glibc 2.4 and earlier, the following keyword is recognized:
- order
- This keyword specifies how host lookups are to be
performed. It should be followed by one or more lookup methods, separated
by commas. Valid methods are bind, hosts, and
nis.
- RESOLV_SERV_ORDER
- Overrides the order command.
Since glibc 2.0.7, and up through glibc 2.24, the following keywords and
environment variable have been recognized but never implemented:
- nospoof
- Valid values are on and off. If set to
on, the resolver library will attempt to prevent hostname spoofing
to enhance the security of rlogin and rsh. It works as
follows: after performing a host address lookup, the resolver library will
perform a hostname lookup for that address. If the two hostnames do not
match, the query fails. The default value is off.
- spoofalert
- Valid values are on and off. If this option
is set to on and the nospoof option is also set, the
resolver library will log a warning of the error via the syslog facility.
The default value is off.
- spoof
- Valid values are off, nowarn, and
warn. If this option is set to off, spoofed addresses are
permitted and no warnings will be emitted via the syslog facility. If this
option is set to warn, the resolver library will attempt to prevent
hostname spoofing to enhance the security and log a warning of the error
via the syslog facility. If this option is set to nowarn, the
resolver library will attempt to prevent hostname spoofing to enhance the
security but not emit warnings via the syslog facility. Setting this
option to anything else is equal to setting it to nowarn.
- RESOLV_SPOOF_CHECK
- Overrides the nospoof, spoofalert, and
spoof commands in the same way as the spoof command is
parsed. Valid values are off, nowarn, and warn.
gethostbyname(3),
hosts(5),
nsswitch.conf(5),
resolv.conf(5),
hostname(7),
named(8)