man > HOST.CONF(5)

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 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 lo‐
              cal 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  lo‐
              cal 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 re‐
              order 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 meth‐
              ods.  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.  Over‐
              rides 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  pre‐
              vent  hostname  spoofing  to enhance the security of rlogin and rsh.  It works as fol‐
              lows: 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  facil‐
              ity.  The default value is off.

       spoof  Valid  values  are  off,  nowarn, and warn.  If this option is set to off, spoofed ad‐
              dresses 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  facil‐
              ity.   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  facil‐
              ity.  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 6.7                          2023-10-31                                 host.conf(5)