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HOST.CONF(5) Linux System Administration HOST.CONF(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 (':'),
            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 gethostby-
            name(3) 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 over-
     ride 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 com-
            mand.

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  implementa-
     tion.    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, 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  secu-
            rity of rlogin and rsh.  It works as follows: after performing a
            host address lookup, the resolver library will perform  a  host-
            name  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.