host.conf(5) File Formats Manual host.conf(5)
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 do-
main 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 de-
fault, 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 gethostby-
name(3) is performed. Reordering is done for all lookup meth-
ods. The default value is off.
ENVIRONMENT
The following environment variables can be used to allow users to over-
ride the behavior which is configured in /etc/host.conf:
RESOLV_HOST_CONF
If set, this variable points to a file that should be read in-
stead 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 com-
mand.
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 implementa-
tion. A new command spoof and a new environment variable RE-
SOLV_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 secu-
rity of rlogin and rsh. It works as follows: after performing a
host address lookup, the resolver library will perform a host-
name 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), host-
name(7), named(8)
Linux man-pages 6.7 2023-10-31 host.conf(5)
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