Linux man pages

Section (5) host.conf

Linux manual pages	Section 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 (_zsingle_quotesz_:_zsingle_quotesz_), semicolons (_zsingle_quotesz_;_zsingle_quotesz_) or commas (_zsingle_quotesz_,_zsingle_quotesz_), 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 (_zsingle_quotesz_:_zsingle_quotesz_), semicolons (_zsingle_quotesz_;_zsingle_quotesz_) or commas (_zsingle_quotesz_,_zsingle_quotesz_), 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 (_zsingle_quotesz_:_zsingle_quotesz_), semicolons (_zsingle_quotesz_;_zsingle_quotesz_) or commas (_zsingle_quotesz_,_zsingle_quotesz_), 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.

COLOPHON

This page is part of release 5.04 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/.

Copyright (c) 1997 Martin Schulze (joeyinfodrom.north.de)
Much of the text is copied from the manpage of resolv+(8).

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2003-08-23 Martin Schulze <joeyinfodrom.org> Updated according to glibc 2.3.2