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.
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)