HOST.CONF(5)              (2019-03-06)               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 con-
          figuration information.  The following keywords are recog-
          nized:

          trim This keyword may be listed more than once.  Each time
               it should be followed by a list of domains, separated
               by colons (aq:aq), semicolons (aq;aq) or commas (aq,aq), 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.

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          RESOLV_REORDER
               Overrides the reorder command.

          RESOLV_ADD_TRIM_DOMAINS
               A list of domains,  separated by colons (aq:aq), semi-
               colons (aq;aq) or commas (aq,aq), 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 (aq:aq), semi-
               colons (aq;aq) or commas (aq,aq), 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 recog-
          nized:

          order
               This keyword specifies how host lookups are to be per-
               formed.  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:

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          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 any-
               thing 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), hosts(5), nsswitch.conf(5),
          resolv.conf(5), hostname(7), named(8)

     COLOPHON
          This page is part of release 5.10 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/.

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