HOST.CONF
Section: Linux System Administration (5)
Updated: 2019-03-06
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NAME
host.conf - resolver configuration file
DESCRIPTION
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.
ENVIRONMENT
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.
FILES
- /etc/host.conf
-
Resolver configuration file
- /etc/resolv.conf
-
Resolver configuration file
- /etc/hosts
-
Local hosts database
NOTES
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.
Historical
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.
SEE ALSO
gethostbyname(3),
hosts(5),
nsswitch.conf(5),
resolv.conf(5),
hostname(7),
named(8)
COLOPHON
This page is part of release 5.11 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- DESCRIPTION
-
- ENVIRONMENT
-
- FILES
-
- NOTES
-
- Historical
-
- SEE ALSO
-
- COLOPHON
-
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