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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 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,  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
              will fail.  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  host‐
              name 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.

SEE ALSO
       gethostbyname(3), hosts(5), nsswitch.conf(5), resolv.conf(5), hostname(7), named(8)

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-07-23                               HOST.CONF(5)

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