host.conf(5)                                            File Formats Manual                                            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 configu‐
       ration 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 appro‐
              priate 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 ap‐
              pears in the /etc/hosts file, instead of only the first.  This is off by default, as it may cause a substantial  per‐
              formance 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 be‐
       ginning 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  warn‐
              ings  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.

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

Linux man-pages 6.03                                         2023-02-05                                                host.conf(5)