# host.conf

## 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 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 (':'), 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.
    


## 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_.
    


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