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NAME

       hosts_options - host access control language extensions

DESCRIPTION

       This  document  describes  extensions  to  the  language  described in the hosts_access(5)
       document.

       The extensible language uses the following format:

          daemon_list : client_list : option : option ...

       The first two fields are described in the hosts_access(5) manual page.  The  remainder  of
       the  rules is a list of zero or more options.  Any ":" characters within options should be
       protected with a backslash.

       An option is of the form "keyword" or  "keyword  value".  Options  are  processed  in  the
       specified  order.  Some  options are subjected to %<letter> substitutions. For the sake of
       backwards compatibility with earlier versions, an "=" is  permitted  between  keyword  and
       value.

LOGGING

       severity mail.info

       severity notice
              Change  the  severity level at which the event will be logged. Facility names (such
              as mail) are  optional,  and  are  not  supported  on  systems  with  older  syslog
              implementations. The severity option can be used to emphasize or to ignore specific
              events.

ACCESS CONTROL

       allow

       deny   Grant (deny) service. These options must appear at the end of a rule.

       The allow and deny keywords make it possible to keep all access  control  rules  within  a
       single file, for example in the hosts.allow file.

       To permit access from specific hosts only:

          ALL: .friendly.domain: ALLOW
          ALL: ALL: DENY

       To permit access from all hosts except a few trouble makers:

          ALL: .bad.domain: DENY
          ALL: ALL: ALLOW

       Notice the leading dot on the domain name patterns.

RUNNING OTHER COMMANDS

       aclexec shell_command
              Execute,  in  a  child  process,  the specified shell command, after performing the
              %<letter> expansions described in the hosts_access(5) manual page.  The command  is
              executed  with  stdin,  stdout  and stderr connected to the null device, so that it
              won't mess up the conversation with the client host. Example:

                 smtp : ALL : aclexec checkdnsbl %a

              executes, in a background child process, the shell command  "checkdnsbl  %a"  after
              replacing %a by the address of the remote host.

              The  connection will be allowed or refused depending on whether the command returns
              a true or false exit status.

       spawn shell_command
              Execute, in a child process, the specified  shell  command,  after  performing  the
              %<letter>  expansions described in the hosts_access(5) manual page.  The command is
              executed with stdin, stdout and stderr connected to the null  device,  so  that  it
              won't mess up the conversation with the client host. Example:

                 spawn (/usr/sbin/safe_finger -l @%h | /usr/bin/mail root) &

              executes,  in  a  background child process, the shell command "safe_finger -l @%h |
              mail root" after replacing %h by the name or address of the remote host.

              The example uses the "safe_finger" command instead of the regular "finger" command,
              to  limit  possible  damage  from data sent by the finger server. The "safe_finger"
              command is part of the daemon wrapper package; it is a wrapper around  the  regular
              finger command that filters the data sent by the remote host.

       twist shell_command
              Replace  the  current  process by an instance of the specified shell command, after
              performing the %<letter> expansions described in the hosts_access(5)  manual  page.
              Stdin,  stdout  and  stderr  are  connected to the client process. This option must
              appear at the end of a rule.

              To send a customized bounce message to the client instead of running the  real  ftp
              daemon:

                 in.ftpd : ... : twist /bin/echo 421 Some bounce message

              For an alternative way to talk to client processes, see the banners option below.

              To  run  /some/other/in.telnetd  without  polluting  its  command-line array or its
              process environment:

                 in.telnetd : ... : twist PATH=/some/other; exec in.telnetd

              Warning:  in case of UDP services, do not twist to commands that use  the  standard
              I/O  or  the  read(2)/write(2) routines to communicate with the client process; UDP
              requires other I/O primitives.

NETWORK OPTIONS

       keepalive
              Causes the server to periodically send a message to the client.  The connection  is
              considered  broken  when  the  client does not respond. The keepalive option can be
              useful when users turn off their machine while it is still connected to  a  server.
              The keepalive option is not useful for datagram (UDP) services.

       linger number_of_seconds
              Specifies  how long the kernel will try to deliver not-yet delivered data after the
              server process closes a connection.

USERNAME LOOKUP

       rfc931 [ timeout_in_seconds ]
              Look up the client user name with the RFC 931  (TAP,  IDENT,  RFC  1413)  protocol.
              This  option is silently ignored in case of services based on transports other than
              TCP.  It requires that the client system runs an RFC 931 (IDENT,  etc.)  -compliant
              daemon,  and  may  cause  noticeable delays with connections from non-UNIX clients.
              The timeout period is optional. If no timeout is specified a  compile-time  defined
              default value is taken.

MISCELLANEOUS

       banners /some/directory
              Look  for a file in `/some/directory┬┤ with the same name as the daemon process (for
              example in.telnetd for the telnet service), and copy its contents  to  the  client.
              Newline characters are replaced by carriage-return newline, and %<letter> sequences
              are expanded (see the hosts_access(5) manual page).

              The  tcp  wrappers  source   code   distribution   provides   a   sample   makefile
              (Banners.Makefile) for convenient banner maintenance.

              Warning: banners are supported for connection-oriented (TCP) network services only.

       nice [ number ]
              Change  the  nice  value  of the process (default 10).  Specify a positive value to
              spend more CPU resources on other processes.

       setenv name value
              Place a (name, value) pair into the process environment. The value is subjected  to
              %<letter>  expansions  and  may contain whitespace (but leading and trailing blanks
              are stripped off).

              Warning: many network daemons reset their environment before spawning  a  login  or
              shell process.

       umask 022
              Like  the  umask command that is built into the shell. An umask of 022 prevents the
              creation of files with group and world write permission.  The umask argument should
              be an octal number.

       user nobody

       user nobody.kmem
              Assume  the privileges of the "nobody" userid (or user "nobody", group "kmem"). The
              first form is useful with inetd implementations that run  all  services  with  root
              privilege.  The  second  form  is  useful  for  services  that  need  special group
              privileges only.

DIAGNOSTICS

       When a syntax error is found in an access control rule,  the  error  is  reported  to  the
       syslog daemon; further options will be ignored, and service is denied.

SEE ALSO

       hosts_access(5), the default access control language

AUTHOR

       Wietse Venema (wietse@wzv.win.tue.nl)
       Department of Mathematics and Computing Science
       Eindhoven University of Technology
       Den Dolech 2, P.O. Box 513,
       5600 MB Eindhoven, The Netherlands

                                                                                 HOSTS_OPTIONS(5)