.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.TH host.conf 5 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
host.conf \- resolver configuration file
.SH DESCRIPTION
The file
.I /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:
.TP
.I trim
This keyword may be listed more than once.
Each time it should be
followed by a list of domains, separated by colons (\[aq]:\[aq]), semicolons
(\[aq];\[aq]) or commas (\[aq],\[aq]), 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:
.I trim
will not affect hostnames gathered via NIS or the
.BR 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.)
.TP
.I multi
Valid values are
.IR on " and " off .
If set to
.IR on ,
the resolver library will return all valid addresses for a host that
appears in the
.I /etc/hosts
file,
instead of only the first.
This is
.I off
by default, as it may cause a substantial performance loss at sites
with large hosts files.
.TP
.I reorder
Valid values are
.IR on " and " off .
If set to
.IR 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
.BR gethostbyname (3)
is performed.
Reordering is done for all lookup methods.
The default value is
.IR off .
.SH ENVIRONMENT
The following environment variables can be used to allow users to
override the behavior which is configured in
.IR /etc/host.conf :
.TP
.B RESOLV_HOST_CONF
If set, this variable points to a file that should be read instead of
.IR /etc/host.conf .
.TP
.B RESOLV_MULTI
Overrides the
.I multi
command.
.TP
.B RESOLV_REORDER
Overrides the
.I reorder
command.
.TP
.B RESOLV_ADD_TRIM_DOMAINS
A list of domains,
separated by
colons (\[aq]:\[aq]), semicolons (\[aq];\[aq]), or commas (\[aq],\[aq]),
with the leading dot,
which will be added to the list of domains that should be trimmed.
.TP
.B RESOLV_OVERRIDE_TRIM_DOMAINS
A list of domains,
separated by
colons (\[aq]:\[aq]), semicolons (\[aq];\[aq]), or commas (\[aq],\[aq]),
with the leading dot,
which will replace the list of domains that should be trimmed.
Overrides the
.I trim
command.
.SH FILES
.TP
.I /etc/host.conf
Resolver configuration file
.TP
.I /etc/resolv.conf
Resolver configuration file
.TP
.I /etc/hosts
Local hosts database
.SH NOTES
The following differences exist compared to the original implementation.
A new command
.I spoof
and a new environment variable
.B RESOLV_SPOOF_CHECK
can take arguments like
.IR off ", " nowarn ", and " warn .
Line comments can appear anywhere and not only at the beginning of a line.
.SS Historical
The
.BR nsswitch.conf (5)
file is the modern way of controlling the order of host lookups.
.P
In glibc 2.4 and earlier, the following keyword is recognized:
.TP
.I 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
.IR bind ", " hosts ", and " nis .
.TP
.B RESOLV_SERV_ORDER
Overrides the
.I order
command.
.P
.\" commit 7d68cdaa4f748e87ee921f587ee2d483db624b3d
Since glibc 2.0.7, and up through glibc 2.24,
the following keywords and environment variable
have been recognized but never implemented:
.TP
.I nospoof
Valid values are
.IR on " and " off .
If set to
.IR on ,
the resolver library will attempt to prevent hostname spoofing to
enhance the security of
.BR 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
.IR off .
.TP
.I spoofalert
Valid values are
.IR on " and " off .
If this option is set to
.I on
and the
.I nospoof
option is also set,
the resolver library will log a warning of the error via the
syslog facility.
The default value is
.IR off .
.TP
.I spoof
Valid values are
.IR off ", " nowarn ", and " warn .
If this option is set to
.IR off ,
spoofed addresses are permitted and no warnings will be emitted
via the syslog facility.
If this option is set to
.IR 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
.IR 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
.IR nowarn .
.TP
.B RESOLV_SPOOF_CHECK
Overrides the
.IR nospoof ", " spoofalert ", and " spoof
commands in the same way as the
.I spoof
command is parsed.
Valid values are
.IR off ", " nowarn ", and " warn .
.SH SEE ALSO
.BR gethostbyname (3),
.BR hosts (5),
.BR nsswitch.conf (5),
.BR resolv.conf (5),
.BR hostname (7),
.BR named (8)