Ubuntu Manpage: host.conf - resolver configuration file

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 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.

COLOPHON