# HOST.CONF - resolver configuration file - man(5) - [phpMan] [_host.conf_(5)](https://www.chedong.com/phpMan.php/man/host.conf/5/markdown) File Formats Manual [_host.conf_(5)](https://www.chedong.com/phpMan.php/man/host.conf/5/markdown) ## 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 domain name from the end of any hostname resolved via DNS. This is intended for use with lo‐ cal hosts and domains. (Related note: _trim_ will not affect hostnames gathered via NIS or the [**hosts**(5)](https://www.chedong.com/phpMan.php/man/hosts/5/markdown) 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 lo‐ cal 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 re‐ order host addresses so that local addresses (i.e., on the same subnet) are listed first when a [**gethostbyname**(3)](https://www.chedong.com/phpMan.php/man/gethostbyname/3/markdown) 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 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. Over‐ rides 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)](https://www.chedong.com/phpMan.php/man/nsswitch.conf/5/markdown) 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 pre‐ vent hostname spoofing to enhance the security of **rlogin **and **rsh**. It works as fol‐ lows: 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 facil‐ ity. The default value is _off_. _spoof_ Valid values are _off_, _nowarn_, and _warn_. If this option is set to _off_, spoofed ad‐ dresses 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 facil‐ ity. 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 facil‐ ity. 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)](https://www.chedong.com/phpMan.php/man/gethostbyname/3/markdown), [**hosts**(5)](https://www.chedong.com/phpMan.php/man/hosts/5/markdown), [**nsswitch.conf**(5)](https://www.chedong.com/phpMan.php/man/nsswitch.conf/5/markdown), [**resolv.conf**(5)](https://www.chedong.com/phpMan.php/man/resolv.conf/5/markdown), [**hostname**(7)](https://www.chedong.com/phpMan.php/man/hostname/7/markdown), [**named**(8)](https://www.chedong.com/phpMan.php/man/named/8/markdown) Linux man-pages 6.7 2023-10-31 [_host.conf_(5)](https://www.chedong.com/phpMan.php/man/host.conf/5/markdown)