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NAME | DESCRIPTION | ENVIRONMENT | FILES | NOTES | SEE ALSO | COLOPHON |
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host.conf(5) File Formats Manual host.conf(5)
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.
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.
gethostbyname(3), hosts(5), nsswitch.conf(5), resolv.conf(5),
hostname(7), named(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 host.conf(5)
Pages that refer to this page: gethostbyname(3), hosts(5), resolv.conf(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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