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       hosts_options - host access control language extensions


       This  document  describes optional extensions to the language described
       in the hosts_access(5) document. The extensions are enabled at  program
       build  time.  For  example,  by editing the Makefile and turning on the
       PROCESS_OPTIONS compile-time option.

       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 pro-
       cessed  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.


       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  sup-
	      ported  on systems with older syslog implementations. The sever-
	      ity option can be  used  to  emphasize  or  to  ignore  specific



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

       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

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

	  ALL: .bad.domain: DENY

       Notice the leading dot on the domain name patterns.


       spawn shell_command
	      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  regu-
	      lar "finger" command, to limit possible damage from data sent by
	      the finger server. The "safe_finger" command is part of the dae-
	      mon  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 ban-
	      ners 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	commu-
	      nicate  with  the  client process; UDP requires other I/O primi-


	      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 deliv-
	      ered data after the server process closes a connection.


       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 ser-
	      vices  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 spec-
	      ified a compile-time defined default value is taken.


	      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  implementa-
	      tions that run all services with root privilege. The second form
	      is useful for services that need special group privileges  only.


       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.


       hosts_access(5), the default access control language


       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



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