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HOST.CONF(5) Linux System Administration HOST.CONF(5)
NAME
host.conf - resolver configuration file
DESCRIPTION
The file /etc/host.conf contains configuration information specific to the resolver li-
brary. It should contain one configuration keyword per line, followed by appropriate con-
figuration 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 vari-
able 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 fa-
cility. The default value is off.
spoof Valid values are off, nowarn, and warn. If this option is set to off, spoofed ad-
dresses 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 fa-
cility. If this option is set to nowarn, the resolver library will attempt to pre-
vent 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 5.05 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/.
Linux 2019-03-06 HOST.CONF(5)
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